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.