Derecho

On Friday, an intense line of thunderstorms moved across the Midwest and Mid-Atlantic, producing very strong winds and widespread damage to trees, power lines, and homes.  The storms developed in the Chicago area and tracked rapidly east-southeastward along a weak stationary front, moving across northern Indiana, much of Ohio, West Virginia, and then into Washington, DC.  The stationary front separated very hot, humid air to the south from (slightly) less hot, drier air over the Great Lakes.  The first image is storm reports from Friday, clearly showing the path of this storm.  Wind gusts between 70 and 95 mph were common along this path; several people were killed (mostly in car-related accidents with trees) and millions were without power.  Long-lived thunderstorm complexes producing straight line winds are called derechos.  These systems are not uncommon in the summer months, usually over the northern United States or southern Canada.  This one was particularly notable though for its track through heavily populated areas, and that it cut power to millions during an intense heat wave.

The second picture is a series of radar images as the derecho moved across Indiana and Ohio.  Note how over time, the line of storms expands and takes on a arc-like shape.  The strongest winds are usually found near the center, or apex, of the bow.  Some weather and media sources have started referring to this system as a 'land hurricane', because wind gusts were equivalent to minimal hurricane strength.  Really though, the term 'land hurricane' is inappropriate for this system.  This name only serves to confuse the general public and implies that Friday's event was a new phenomenon that has not occurred before.  Hurricanes are large, slow moving systems that feed off warm ocean waters.  Hurricanes do not form, and are not associated with, frontal boundaries.  The derecho that occurred on Friday was extremely fast (moving at 75+ mph) and developed along a stationary front, the interface between two dissimilar air masses.  Clearly, the comparison is bad at best.

Unusual warmth continues for central U.S.

After a record warm spring across much of the central, southern, and Mid-Atlantic regions, record heat continues into early summer.  A very strong upper-level ridge of high pressure has remained the dominant feature over the central and eastern U.S., producing very warm temperatures.  This ridge has also suppressed thunderstorm activity, which is resulting in worsening drought conditions for much of that region.  Large wildfires are now burning near populated areas of Colorado, including Fort Collins, Boulder, and Colorado Springs.  Both today and yesterday, Denver tied its all-time record high temperature of 105º - to tie that record once is rare enough, but two days in a row is extremely unusual.  Later this week, temperatures are expected to climb to or exceed 100º across a wide swath of the country from Texas northward to Michigan and east to the Atlantic.  100º temps are quite rare for the northern Midwest, especially so early in the summer season.

One factor resulting in the hot temperatures is very dry ground/soil in the central U.S.  Dry ground warms and cools much faster than wet ground (and water surfaces).  However, the lack of rainfall and dry ground also results in lower humidity levels.  So, while air temperatures may be near 100º, humidity levels will not be terribly high.  This summer is showing some similarities to the summer of 1988, which was a very hot, dry season for the central U.S.

The tropics continue to be unusually active, with four names storms already occurring in the Atlantic.  Currently, Tropical Storm Debby is crossing Florida (see satellite image on right), producing very heavy rains and a few tornadoes.  The silver lining there is much of Florida was experiencing drought conditions, so Debby's rains are not entirely unwanted.  Debby will continue tracking northeastward away from Florida and across the Atlantic.

A strong upper-level trough of low pressure has been anchored along the West Coast, resulting in cooler-than-normal temps and unusually heavy rains for June in the Pacific Northwest.  This trough has helped to mix out the marine layer and fog for the past several days, so locations along the coast (like San Francisco) have had more sun recently than is typical for June.

Eclipse Update

Just a reminder about the eclipse - will start around 5:30 p.m. on Sunday here in San Francisco and last a couple hours.  The peak of the eclipse, which in SF will mean about 80% of the sun is covered by the moon, will only last a few minutes though.  Skies will be clear inland, but there may be fog near the coast.  It would be nice to see the eclipse out over the water, but we probably know until Sunday where the fog will be.  Mount Tam in Marin might be a good spot for viewing!

Solar Eclipse - May 20

Later this month, a solar eclipse will take place, visible across the northern Pacific and the western United States.  This will be the first solar eclipse visible from the continental U.S. since 1994, showing how rare these events can be.  This particular solar eclipse is called an annual solar eclipse.  This means that at no point will the sun be totally obscured by the moon; instead, the outer edge of the sun will be visible as the moon passes in front of the sun.  This is because the moon will be relatively far from Earth, and thus appear smaller relative to the sun.  In a total solar eclipse, the entire disk of the sun is obscured by the moon, and occurs when the moon is relatively close to Earth.

The May 20 eclipse will peak along the path shown on the map (on the right), so far northern California into Nevada will see the moon directly over the sun.  The path is quite narrow, which illustrates the rarity of any one location experiencing the peak of an eclipse.  Here in San Francisco, the eclipse will start around 5:15 p.m. local time and peak around 6:30 p.m., so the sun will be low in the western sky.  At its peak, over 80% of the sun will be obscured by the moon as viewed from San Francisco - definitely not an event to miss!  Of course we also need clear skies to see this event, and often in the late evening we have fog rolling into the city.  Going just a few miles inland though should offer good viewing without fog, though.  Here's some more detailed information on the upcoming eclipse.

A total solar eclipse will occur on August 21, 2017 with a long, narrow path of totality across the United States, from Oregon southeastward all the way to South Carolina.  All of North America will see at least a partial eclipse during this event.

In the meantime, another nice weekend coming up for the Bay Area - mostly sunny with highs in the 70s near the coast and bays and 80s inland.

Unusual weather continues...

So far, April has brought record heat to the western U.S. with much cooler, wetter weather in the East, especially when compared to the record heat there back in March.  A strong upper-level ridge of high pressure brought sunny, warm weather to the immediate West Coast this past weekend.  San Francisco didn't break any records with highs in the 70s.  Many locations in the Central Valley, the Southwest, and the Pacific Northwest did break daily record highs over the past few days though, and some locations in the Great Basin even set new record highs for all of April.  This ridge is slowly moving east, and brought more record heat today for the Rockies and Plains.

Meanwhile, a strong trough of low pressure has been parked over the Northeast, resulting in cool temperatures and over the weekend produced heavy rain along the coast.  Over parts of western New York and Pennsylvania, heavy, wet snow fell, with over a foot in some locations.  For some parts of the Northeast, this was the biggest snowfall of this otherwise mild winter season.

Also incredible is that after the record warmth across the Midwest back in March, April is now on track to be cooler than March was.  Such an occurrence is extraordinarily rare, but will likely happen this year in Chicago, Milwaukee, and other nearby locations.  The last time this happened in Chicago was 1907.  And, this April will likely be 2º to 3º above normal for Chicago but will still be cooler than March, demonstrating how unusually warm March 2012 was.

In San Francisco, we've obviously cooled down since this weekend as a low-pressure system approaches from the Pacific.  Today was still fairly nice though, with mild temperatures and not much wind because onshore pressure gradients have weakened compared to Sunday and Monday.  As a quick measure of onshore gradients, we look at the pressure difference between San Francisco and Sacramento.  Outside of winter, SF normally has higher pressure and Sacramento has lower pressure (due to stronger heating in the Valley).  The approaching low pressure system offshore though has disrupted the usual onshore gradient, keeping the gradient almost neutral today.  So, we've had a relatively mild evening (image on right) without the usually-strong westerly winds.  Anyway, the bulk of the rain with the approaching system will be focused on southern California, but we will see some showers and possibly a thunderstorm in the Bay Area Wednesday afternoon and night.  Then, this weekend looks sunny and seasonable, with highs in the low 70s for downtown SF but cooler near the beaches with onshore flow.

Another tornado outbreak

Yesterday, atmospheric conditions were particularly favorable for large, destructive tornadoes across the Central Plains.  In brief, the system that brought thunderstorms to San Francisco on Thursday moved across the Rockies and into the Plains yesterday, where it encountered a warm, humid air mass.  This system had a strong jet stream aloft and good directional shear - that is, winds from different directions at different heights in the atmosphere.  In this case, south and southeasterly winds at the surface brought in warm, moist air; these winds veered to the southwest and west higher aloft, and brought in much drier air.  This change in wind direction with height is a critical ingredient in supercell thunderstorms and tornadoes.  The thunderstorms with this system formed along a dryline - similar to a front, but very shallow and separates warm, humid air to the east from dry, hot air to the west.  Drylines are focal points for convergence and rising motion, making them a trigger for cloud and thunderstorm development.  The top picture is a radar image of one of the tornadic supercell thunderstorms, this one over south central Kansas.  The radar itself is located in the black dot at the top-right of the image, in Wichita (KICT).  Green indicates light precipitation; red indicates heavy (and probably large hail).  Notice the hook-like appendage at the southwest corner of the storm - the tornado is located there, south of the heavy rain and hail.  The second figure shows radar velocities at the same time.  Doppler radars have the ability to detect whether cloud/rain drops are moving toward or away from the radar.  Green/blue areas indicate motion toward the radar (toward Wichita) and red is motion away.  The figure shows a textbook velocity "couplet" with this storm - the dark green and blue area next to the red/white/purple area.  This indicated rotation and likely a tornado.  Here's a link to video of some of the tornadoes.

Several tornadoes occurred across Oklahoma, Kansas, and Nebraska.  Some of the tornadoes were quite large and long tracked; fortunately, this part of the country is sparsely populated, so injuries and fatalities were relatively low considering the number and size of these storms.  But as important was excellent forecasting by the National Weather Service (NWS) and the Storm Prediction Center (SPC).  On Friday, 24 hours before the event, SPC issued a High Risk severe weather outlook for the central Plains.  High Risks are rarely issued, and are almost never issued a day in advance.  This gave the region a good heads-up that tornadoes were likely.  Then, on Saturday during the event, the local NWS offices in the region did a great job in issuing tornado warnings.  The map on the right shows the observed tornadoes (dots) and the tornado warnings issued by NWS (red).  Most all of the tornadoes had warnings associated with them, with only a few false alarms (when warnings were issued and no tornado was reported).  Hats off to those forecasters.

Unfortunately there were five fatalities, all in Woodward, Oklahoma.  A tornado struck the town in the middle of the night and the tornado sirens weren't working - they were damaged in a thunderstorm earlier that same day.  The situation could have been much worse though, because larger population centers, like Wichita, Kansas, were also hit, but without fatalities. 

We have very benign weather for the next week in San Francisco.  There will be occasional fog especially near the coast, but otherwise mostly sunny all week with highs in the 60s and by Friday/Saturday into the 70s.

Thunderstorms in San Francisco!

We've had quite an eventful couple of days weatherwise across California.  Yesterday, a storm system moved through with mostly light to moderate rain in the Bay Area.  Daytime heating though helped fuel several thunderstorms in the Central Valley, one of which produced a tornado near Stockton (picture on right).  The tornado was rated an F1 (on a scale of F0 to F5, F5 being the strongest) with winds estimated near 90 mph; one garage was destroyed while the tornado was briefly on the ground.  Tornadoes are rated based on the damage they produce.  Tornadoes are generally rare in California and when they do occur, they are usually weak and short lived, and occur in smaller thunderstorms compared to the large, supercell thunderstorms that produce violent tornadoes east of the Rockies.  I'll write more about that in a future post.

Then, last night, another system quickly moved through with a brief period of heavy rain, gusty winds, and a few lightning strikes in the San Francisco area.  A stronger system is now impacting our region and produced a strong line of thunderstorms that crossed through San Francisco earlier this evening.  Those storms produced a lot of lightning and thunder - probably the most I've seen in my ~6 years on the West Coast.  It wasn't quite a Midwestern thunderstorm, but it was quite impressive for San Francisco.  Rain will continue on and off tonight into tomorrow morning.  Then, Saturday will be partly sunny but cool, followed by warmer temps Sunday and Monday.

Another look at March

One last post (I think) about the record warm March for much of the country.  After some more data analysis, NOAA released its March weather summary for the U.S., confirming the incredible warmth over the Midwest and East.  The map on the right shows how March 2012 compared to state-level temperature rankings for the past 118 years.  Notice all the states - 25 in total - with the number 118.  That indicates those states had their record warmest March, a really incredible number.  Many other states nearly set new records for March temperatures.

The other latest weather story is from Texas.  Last week, a cluster of severe thunderstorms raked northern Texas, producing several tornadoes (17 in all) in the Dallas-Fort Worth area.  Miraculously, no one was killed in these storms, despite some of the tornadoes being long-lived and tracking through populated areas.  In addition to the tornadoes, very large hail, up to baseball size, was reported with several storms, including near DFW airport.  The National Weather Service produced a nice summary of the event with photos.

Closer to home, we're amid a week of April showers in the Bay Area.  A large trough of low pressure over the eastern Pacific will gradually move across California this week with cool temperatures and occasional showers.  Wednesday, expect a few showers and possibly a thunderstorm (if the sun comes out and we get some surface heating).  Late Thursday into Friday, the next system will move in with more rain.  By Sunday and Monday though, high pressure will build in with more sun and warmer temps.

Look back at March 2012

As I noted in previous posts, March 2012 was incredibly warm across the central and eastern U.S.  Now that the dust has settled, we can take a look back at what might have caused this early spring heatwave and just how infrequent such an event is.  The NOAA Earth System Research Laboratory released a preliminary analysis of the episode.  Here are the highlights:

• Over 7000 daily record highs were set across the country
• Anomalously strong and persistent southerly flow transported warm air poleward from the Gulf of Mexico into central Canada
• Warming was intense throughout the troposphere, indicative of deep layer meridional transport
• Lack of snow cover was likely not a major factor, except perhaps the northern Upper Midwest
• Signals of the upcoming heat wave were detectable well in advance of the event
• Global warming associated with greenhouse gas (GHG) emissions contributed ~ 5 to 10% of the intensity of the heat wave (preliminary estimate)

This last bullet is perhaps most interesting.  Assessing the influence of global warming on individual weather events is quite difficult, and since global warming came to the forefront of the atmospheric sciences, I have seen few weather events described as, in part, enhanced by global warming.  As the NOAA analysis points out, it's highly unlikely that warming due to GHG emissions caused the heat wave, but it is quite likely that it enhanced the magnitude of the event.  Studies also show that warming associated with GHG emissions  does increase the chance that extreme weather events, such as heat waves, will occur, and perhaps enhance them when they do occur.  Climate researchers often use an analogy of "loading the dice" to describe this.  The take home message:  The March heat wave was extraordinarily rare, but these types of events may occur more frequently as uncontrolled GHG emissions continue.

Back here on the West Coast, we finally had a month with above-normal rainfall in San Francisco.  The downtown observation site reported 5.71" of rain in March, which was 2.45" above normal.  We're still way below normal for the season though.  The Northwest has been extremely wet this March, with Portland and Spokane recording their wettest March on record.  Coming up, we have a decent chance for at least light rain in San Francisco on Tuesday night, followed by a warm up into the weekend.

Unprecedented warmth in the Midwest

The Midwest/Great Lakes region continues to bask in historic warmth for the middle of March.  Temperatures have been in the 70s and 80s across a wide area from Missouri to Minnesota and east to the Mid-Atlantic.  The culprit is a highly amplified upper-level pattern over the country.  A strong ridge has persisted and strengthened from the Southeast northward to Ontario, with a deep trough over the Rockies.  That trough brought us all the rain last week and snow into Arizona and northern Mexico.  Also of note is the wind flow from Texas due north to central Canada.

Just how warm has it been?  Numerous locations are on track for their warmest March on record.  The graph on the right is from the Chicago National Weather Service office, showing the average March temperature and the previous 4 warmest Marches on record.  2012 is on pace to blow away the old record.  Tomorrow's forecast high for Chicago is 85º - this is warmer than Chicago's average high in the middle of July.  This is indeed a historic event, and as such, it's difficult to predict what's coming next.  Much of weather forecasting is based on a phenomenology of what's happened in the past, but we don't really have much to compare these warm temperatures to.  While good for outdoor spring/summer activities, this unusual warmth could have implications on things like crops and lake water levels.

As for California, last week's series of systems brought us much needed rain, with over 10 inches falling in some of the higher elevations of the Bay Area.  For San Francisco, we're up to about 50% of normal for rainy season precipitation.  And, the Sierra received several feet of snow.

Rain moving through; very warm Midwest/East

The first storm system in a series is moving through the Bay Area right now.  It brought light to moderate rain for much of the region with rainfall totals from near a half inch in San Francisco to over two inches in the North Bay, with generally a quarter inch or less south of SF.  These totals are a bit less than was forecasted but still respectable.  The latest radar image shows just a few light areas of rain slowly shifting southward across the area with the bulk of the rain remaining over far northern California.

One area of note though was the South Bay.  San Jose still has only reported a trace of rain from this system, despite radar imagery showing rain over the region for much of the day (not at present though).  A likely reason for this was strong southerly winds for much of the day.  Southerly flow enhances rain in parts of the North Bay mountains and the southward facing Santa Cruz mountains.  However, San Jose is north of the Santa Cruz mountains, so San Jose may have been in the rain shadow.  I haven't looked at enough data to support this hypothesis, but it is possible.  Also, the San Francisco/Monterey area radar is located on Mount Ununhum in the Santa Cruz mountains at near 3000 feet elevation, and thus is detecting rain/clouds well above sea-level.  So while it may have shown rain over the South Bay for much of the day, that precipitation was likely well above the surface and not reaching the ground.

We have a weaker system coming in Thursday afternoon and then a stronger, colder system for Friday and Saturday.  That one will bring much lower snow levels and still some possibility for a few thunderstorms. 

Meanwhile, unseasonably warm temperatures will cover much of the Midwest and East this week, with highs well into the 70s and even 80s.  These temperatures are extraordinarily warm for mid-March, over 30 degrees above normal in some spots.  Chicago, as an example, is expecting highs in the low 80s for Wednesday, which is more typical of June.  The average high for Wednesday is only 46º.  This will certainly be a new daily record, but this may go down as one of the warmest March weeks on record for many locations.  The table below shows a few cities and forecast highs for Wednesday.

Wet week ahead for Bay Area

After a fairly average weekend in the Bay Area, with periods of sun, clouds, drizzle, and onshore winds, we have a much more active weather pattern in store this week.  As I alluded to in earlier blogs, several signs were pointing to a wet period in mid-March, and it's about to start.  In fact, this may very well be the wettest week of the winter season.  Very moist air from the subtropics is moving northeastward across the eastern Pacific, currently headed for the Pacific Northwest associated with a strong upper-level trough developing off the coast.  This system brought thunderstorms, heavy rain, and large hail to Hawaii a few days ago.  This trough will gradually sink southeastward across California this week bringing periods of rain, sometimes heavy.  Rain will begin in the Bay Area on Monday night as a warm front moves through.  As warm air moves in from the southwest, it is forced up and over cooler air at the surface.  This results in clouds and precipitation.  Rain will generally be light to moderate into early Tuesday before a strong cold front moves in with a period of heavy rain and wind late Tuesday.  The first image is a model 12-hour precipitation forecast for the day Tuesday, indicating at least an inch of rainfall for much of the area.  The other computer models are in agreement as well, which gives us more confidence in the forecast.  Another cold front will drive through Thursday into Friday with more rain.

The parent upper-level trough will then settle over California this weekend as indicated by the second image.  This will bring in much colder air, especially aloft, and will result in very low snow levels - perhaps lower than 2000 feet at times.  With such cold air aloft, combined with the relatively strong March sun, the atmosphere will become quite unstable (by California standards).  This may result in a few thunderstorms in the Bay Area, but it's still too early to say for sure.  Regardless, we can expected perhaps 2 to 4 inches of rain in the Bay Area this week with several feet of snow in the Sierra.  This is very good news considering the lack of rain so far this winter!

Also on the weekend forecast map, notice the strong upper-level ridge over the eastern United States.  Many locations there, even the northern states, may have temperatures in the 70s.  That would be extraordinarily warm for this time of year, considering that often such locations (Great Lakes and the Northeast) are covered with snow in early to mid-March.  It is not uncommon though to have one side of the country cool and wet while the other is unseasonably warm.  In fact, this is often the case.  Strong troughing over the western U.S. forces strong, downstream ridging over the eastern U.S. in response, and vice versa.  This is why it may often seem that during periods of sun and warmth here, the Midwest and East are getting severe thunderstorms and/or snowstorms.

More Midwest/East tornadoes

Friday, March 2, 2012 will long be remembered in the Ohio Valley for its outbreak of severe thunderstorms and tornadoes.  Dozens of tornadoes were reported across a large area from Indiana southward to Alabama, some large, and resulting in about 40 fatalities.  A particularly strong, long-tracked tornado carved a long path across southern Indiana into far northern Kentucky, narrowly missing Cincinnati.  The image on the right is a compilation of large hail, damaging wind, and tornado reports from March 2.

This event was a classic setup for severe weather.  A low-pressure system rapidly developed over Missouri and accelerated northeastward toward Michigan.  Ahead of this system, warm, moist air from the south surged northward.  And, a strong jet stream aloft resulted in strong vertical shear - that is, winds changing in speed and direction with height above the ground.  This is an essential ingredient for tornadoes.  And, much colder air rushed in behind this system.  Heavy, wet snow blanketed the region from Chicago and Milwaukee and eastward across Michigan.  I'll talk more details about the meteorological factors behind severe thunderstorms as the spring continues.

Conversely, we have had continued benign weather here in the Bay Area.  Temperatures were well above normal this weekend, with highs in the 70s and even a few 80s well inland.  This will quickly change Monday and Tuesday as a cold front moves through.  Precipitation will be light, but it will become quite windy, especially on Tuesday.  Thursday, Friday, and possibly Saturday look nice though.  After that, indications are still there that the middle of March could be wetter than normal across northern and central California.  This is still a long way out in terms of reliable forecasting, but it would be welcome precipitation after a very dry winter.

Heavy Sierra Snow, Tornadoes East

In my previous post, I talked about the dearth of snow in the Sierra.  In timely fashion, a strong storm system is bringing heavy snow to the Sierra, with several feet of accumulation expected.  This is a cold, moist storm that originated over the Gulf of Alaska.  While not a big rain producer at lower elevations (only a quarter to half inch in the Bay Area), the dynamics and moisture profile associated with this system favors heavy snow in the Sierra.  As the system moved in, strong, moist west-northwesterly mid- and upper-level winds impinged on the Sierra.  This is evident in the map on the right, which shows wind up around 30,000 feet.  As this flow hit the Sierra, it was forced to rise and inevitably cools.  As the air cools, it is unable to hold as much moisture, so that moisture condenses out in the form of clouds and snow.  This also favors the heaviest snow on the western slopes of the Sierra up to the crest, with less snow on the eastern side.  Snow levels will be as low as 2000-3000 feet, with snow accumulations up to 5 feet for locations above 7000 feet elevation.  The snow will continue on and off through Thursday before a warmer, drier air mass moves in this weekend.  The copious snow will improve the water outlook picture somewhat as we head into the spring.  And, I think there are still signs of a more active weather pattern for northern California around mid-March.

Meanwhile, on the same upper-level wind map above, there is a strong jet stream over the southern United States.  This is associated with another storm system that produced severe thunderstorms with tornadoes across parts of the Mississippi Valley.  Unfortunately, many of the tornadoes occurred overnight and early this morning.  This is extremely dangerous since most people are sleeping without any idea a tornado is approaching, and since it is dark, tornadoes are difficult to see and track.  One tornado that moved across southern Illinois, causing several fatalities, has already been rated EF-4 (on a scale of 0 to 5), with winds of 175 mph.  It is not unusual to have tornadoes in late February, but this time of year they are more common across the Deep South as opposed to Missouri and Illinois.

Dry winter continues

Our very dry winter continues in northern California.  Sounding like a broken record now, a strong ridge of high pressure remains quasi-stationary over the eastern Pacific into California, keeping us in dry northerly flow.  Storm systems have continued to track from the Pacific Northwest southeastward into the Great Basin, but have mostly stayed to our northeast.  And, these systems are not big rainmakers anyway, as they are coming from a northerly (dry and cold) trajectory instead of a more favorable west or southwesterly trajectory (moist and warm).


So far, San Francisco has received just 6.86 inches of rain so far this water year, or about 40% of normal.  The Sierra snowpack also remains well below normal, as shown in the first image.  Much of the west (and especially the Sierra) has below-normal snow water contents, with the exception of the northern tier states.  This is troublesome heading into the spring, because the majority of our precipitation in California falls from November through February.  If not for the precipitation surpluses of last year, we would be in considerable trouble from a water resource perspective.  The second image is the U.S. Drought Monitor, a depiction of drought status across the country updated weekly.  Most of California is in a Moderate drought, but this is likely to degrade further unless we get substantial spring storms.  Elsewhere across the country, we find comparably much worse drought conditions over Texas - much of that is carryover from 2011, which was extremely hot and dry in that area.  There are some subtle signs that we could see increased precipitation here in California in March.   Let's hope so!

Quick update - rain delay

It's finally raining in San Francisco after much delay.  The system that impacted the Pac NW with heavy rain, ice, and snow yesterday finally slid south and brought light rain to the Bay Area, which will continue on and off overnight.  Rainfall totals will remain below a quarter inch in SF though, which is less than previously expected.  A considerably stronger system will move in Friday afternoon and night, with up to an inch of rain possible along with gusty winds, so evening plans tomorrow should include an umbrella.  Much of Saturday and Sunday will likely be dry with only passing showers, so weather impacts for the 49ers game should be minor.  A third system will move in Sunday night, with moderate rains possible.

After that, unfortunately, it appears the persistent ridge of high pressure that was over the West Coast for most of the past few months will rebuild.  This will send heavy rains northward, leaving us with dry and warmer conditions likely through the end of January.  So, it does appear that this transition to a wetter pattern across California will be quick.

Rain still on the way

The major change in the large-scale weather pattern is fortunately still on track (and actually is already underway).  Today, a cold upper-level trough moved into the Pacific Northwest, bringing rain and even snow down to low-elevation cities like Seattle and Portland.  This system also cooled our temperatures dramatically as onshore winds increased.  This system has also dislodged a stubborn ridge of high pressure that has been present much of this winter along the West Coast.  This will allow a parade of storm systems to impact northern California starting mid-week.

Monday, Tuesday, and much of Wednesday will still be dry in the Bay Area.  Despite mostly sunny skies though, the air mass aloft is much cooler, and cannot support the temperatures in the 60s to near 70 we have been enjoying lately.  The first in a series of cold fronts will impact our area late Wednesday, with rain developing in the North Bay and spreading south into San Francisco after dark.  Then, a couple more cold fronts will move in, with the strongest one expected late Friday into early Saturday.  This strongest front will bring rain even to southern California.

What is uncertain though is where the heaviest rain will be and how much will fall.  The models have been fairly consistent in producing the bulk of the heavy rain north of San Francisco, from Sonoma County northward into Oregon.  The image shown is model estimated cumulative rainfall from now until just after midnight on Saturday.  The model depicts a tight precipitation gradient from north to south across the Bay Area, with 2 to 5 inches across the North Bay, 1 to 2 inches around San Francisco, and perhaps only an inch or less across the South Bay and southward.  This creates a very difficult forecast - 5 inches of rain has considerably different ramifications than 1 inch, so nailing down exactly where the rainfall bulls-eye will be key.

The weather pattern looks to remain active into early next week before quieting down again.  A few inches of rain (and heavy snow in the mountains) will be much welcome.  But we must also keep in mind that such rainfall amounts are not unheard of for this time of year, and will not erase the huge deficits from November, December, and now the first half of January.

Finally, winter. Eventually.

It's been a long time in coming, but signs are pointing to a major change in the weather pattern across the northern Hemisphere.  So far this winter, the coldest air has been largely confined to the Arctic, with an absence of strong storm systems to pull that cold air southward.  Along the West Coast, a strong ridge of high pressure has been the dominant feature.  The first image is a computer model forecast for tomorrow (Jan 12), showing the position of upper-level features (about 5 to 6 km above the surface).  The colors indicate vorticity, or spin.  And in general, wind follows the black lines.  The ridge is noted by what looks like a "hill" in the black lines stretching from California northward almost to the North Pole.  This has steered incoming Pacific storm systems northward, usually into British Columbia, and leaving us with dry, mostly sunny weather under northerly winds.

Finally, this high pressure ridge is forecast to break down and shift northward starting this weekend.  The second image shows the forecast for next Wednesday (Jan 18).  The big high pressure ridge that was along the West Coast strengthened and moved northwestward (or retrogressed, because its position moved opposite the usual west to east motion).  It's forecast to be a huge high pressure system over eastern Russia.  While this happens, strong westerly flow from Asia undercuts the high pressure system, stretching from Japan eastward and aimed at the Pacific Northwest.  And, embedded in that westerly flow are several storm systems, noted by the vorticity (colored) regions.  The first one will move into Washington and Oregon on Wednesday, but will drag a cold front across northern California, and finally bring us some rain.  Then, several more systems will follow.  It's still way too early to tell how much rain we will get.  Much of the heaviest rain may be focused north of here, but we can at least bet on some decent rainfall totals, and accumulating snow in the Sierra above 4000 to 5000 feet.  It's also uncertain how long this new pattern will last.  We could be in for an active few weeks, or the atmosphere could reset back to a pattern similar to what we've had most of this winter.

Lake Effect Snow

Since our weather here in the Bay Area remains benign, let's look at some interesting weather occurring further east.  After a mild December, very cold air is finally making its way into the Midwest and East.  For many locations, strong winds with temperatures in the teens and 20s are expected, but only light snow.  The major exception will be locations downwind of the Great Lakes.

This time of year, the water temps of the Great Lakes are usually in the 30s and 40s.  When an Arctic air mass drops south over the lakes, with air temps in the teens and 20s, a large vertical temperature gradient is created, from the relatively warmer lake waters to the very cold air above.  This results in instability.  As the cold air moves over the lakes, it is heated and moistened from below and rises.  As this lake-modified air rises, it cools and releases its moisture in the form of clouds and precipitation, impacting areas downwind of the lake.  It's a relatively simple atmospheric process but the devil is in the details.  Forecasting lake effect snow with precision is difficult.  Lake snow tends to set up in narrow bands of heavy snow - in one location, whiteout conditions may be occurring, while a few miles away, the sun is shining.  Wind direction is the major determiner in what locations receive the most snow.  Usually for the Midwest and East, cold air comes from the west, northwest, or north.  Thus, the heaviest lake effect snows are usually found to the south, southeast, or east of the Great Lakes.  Cities like Marquette and Grand Rapids in Michigan, Cleveland, and Buffalo are all on the east or southeast sides of the Great Lakes, and typically receive much more snow than cities on the west sides of the lakes, such as Detroit, Chicago, and Milwaukee.

As I said earlier, a cold air mass is currently moving southeast over the lakes, triggering heavy lake snows along the Lake Superior and Michigan shores, and will develop along the eastern Great Lakes shortly.  The radar image shows the narrow bands of heavy snow (darker green).  This event is expected to produce about a foot of snow some spots, along with 40-50 mph wind gusts.  In the worst historical lake snow events, some locations receive several feet of snow - this tends to occur most along Lake Superior and downwind of Lake Ontario.

Still no major rains coming to California anytime soon.  If this dry pattern continues through January, we will risk a serious drought situation heading into the spring and summer.