Thunderstorms in NorCal

Last night and this morning featured unusual weather across northern California, as widespread thunderstorms developed over the Sierra, parts of the Central Valley, and the North and East Bay regions.  Thunderstorms are relatively common over the Sierra during summer, but are highly unusual for the lower elevations, especially during the summer months.  Thunderstorms typically require a combination of moisture, instability, and lift, and we rarely have these ingredients in combination over northern California.  However, a weak upper-level low pressure system over the region helped create an environment favorable for thunderstorms.  The cool air aloft associated with the upper level low helped increase instability - temperature decreasing rapidly with height.  Southeasterly winds at the mid-levels of the atmosphere helped draw moisture into the region from the south.  And, the upper low itself acted as a trigger, or lift, to generate thunderstorm activity.

The thunderstorms we experienced were highly unusual in that in some locations, a deep marine layer was present at the surface.  The marine layer is very stable and not conducive for thunderstorm activity.  However, the thunderstorms last night developed above the marine layer - these are referred to elevated thunderstorms, and are relatively common in the central and eastern U.S. during springtime.  Thunderstorms pull in warm, moist air as inflow, which rises, cools, and condenses as clouds and rain.  In an elevated thunderstorm, this inflow originates well above the surface, whereas with most thunderstorms, the inflow originates near the surface.

While the atmosphere was more humid than typical for summer in northern California, it was still relatively dry, and thus the storms that developed produced little rain.  Most locations received a tenth of an inch or less.  The storms did produce frequent lightning though, and according to CalFire, the lightning sparked numerous small fires across the region.  The image on the right from the Sacramento NWS office shows that nearly 7000 lightning strikes occurred across northern California and western Nevada with these storms.  The upper level low responsible for this active weather is already moving east, which will return us to our typical June pattern with a persistent marine layer near the coast and warm, sunny conditions inland.

Tornado Tragedy in Oklahoma

A large, violent tornado struck the Oklahoma City metro area this afternoon, causing widespread destruction and devastation through the town of Moore.  The radar image on the right shows the supercell thunderstorm responsible for this tornado, featuring an incredibly pronounced hook structure.  The ball of reflectivity in the center of the hook is a debris ball - debris lofted into the air from the tornado.  This particular storm went from an ordinary cumulus cloud to a monster supercell in only 30 minutes.  However, the NWS office in Norman, OK, issued warnings at least 15 minutes before the tornado even touched down.  Even with the warnings, this was still a near worst case scenario, with an EF4 to EF5 tornado plowing through a heavily populated area.  Most homes and businesses in the direct path of the tornado were simply wiped off the ground, leaving only the concrete foundation.  Worst of all, this tornado completely destroyed an elementary school with dozens of children inside.  At latest check, the death toll is 51 with 20 of those coming from the elementary school.  Incredibly, some debris from the storm has been reported on the ground as far east as Missouri.

Sadly, Moore, OK, is no stranger to tornadoes.  A violent EF5 tornado ripped through this suburb on May 3, 1999, and followed an eerily similar path (see image at right).  I don't know the exact chances of this happening statistically, but the odds are astronomically small.  Another severe weather outbreak appears likely on Tuesday, focused a little further east.  There will be a little less jet stream energy to work with tomorrow, which will hopefully keep storms from reaching the intensity seen today.

Tornado Outbreak in Kansas, Oklahoma

Severe thunderstorms broke out across the Plains this afternoon, producing several strong tornadoes, hail to baseball size, and damaging winds in several states.  Hardest hit were Oklahoma and Kansas, where several tornadoes caused considerable damage and a few fatalities.  The image on the right shows the preliminary storm reports received today.  Large metropolitan areas were affected by tornadoes during this event, including Oklahoma City, Wichita, and Des Moines.  This is typically a worse-case scenario.  Fortunately, the National Weather Service did an excellent job in predicting these storms well in advance, allowing residents to seek safe shelter.

The strongest tornadoes were just east of Oklahoma City and just west of Wichita, and caused widespread damage to several mostly rural communities.  The radar image on the right shows a storm approaching Wichita.  This particular type of storm is called a supercell.  Supercells are typically isolated, large, rotating storms that often produce large hail and sometimes tornadoes.  The supercell near Shawnee is near-textbook in appearance, with a hook-like structure on the southwest side of the storm.  The tornado was located in the small notch inside the hook, north of Clearwater, moving east-northeast.  North of the tornado, in the area of intense radar returns, was heavy rain and large hail.  The image below the radar capture is a picture taken by storm chaser Juston Drake of a violent tornado near Shawnee, Oklahoma - truly frightening.

Conditions today were very favorable for supercell thunderstorms, with warm, moist air moving north from the Gulf of Mexico, cooler air moving southeast from the northern Rockies, a surge of dry, unstable air aloft from the southwest, and a strong jet stream overhead.  The supercells that developed today remained isolated for several hours, allowing them to spawn several tornadoes along their paths.  This isolation is important for persistent, strong tornadoes, because when on its own, the supercell does not have to compete with other storms for instability and moisture.

Unfortunately, a significant severe weather threat exists again on Monday, and to a lesser extent, Tuesday, covering some of the same areas that were affected today.

Atmospheric CO2 levels reach 400 ppm

The big weather/climate news this past week has been a measurement of CO2 in Earth's atmosphere of 400 ppm.  This measurement represents a daily average concentration and was taken at Mauna Loa, Hawaii.  This reporting station has the longest continuous record of CO2 measurements on Earth (see image on right, courtesy UC San Diego).  Climate scientists have long expected this day to come as unchecked, anthropogenic (human) burning of fossil fuels puts ever more carbon dioxide into the atmosphere.  The reading of 400 ppm is not meaningfully different from a climate standpoint compared to the 399 ppm measured in days prior.  Climate scientists also typically consider the annual average CO2 concentration as opposed to a single daily average, because atmospheric CO2 levels are typically highest in May - this is because plants are the main remover (or sink) of atmospheric CO2, and vegetation in the expansive Northern Hemisphere forests helps remove atmospheric CO2 during the summer months.

This 400 ppm observation is, however, very symbolic of a new normal in atmospheric CO2 concentrations and how humans and all other life will adapt.  CO2 levels this high are estimated to not have occurred on Earth for at least the past 3 million years, and certainly not during any time since humans first existed.  It's also estimated that, 3 million years ago, average planetary temperatures were about 5ºF higher and sea levels were 30 feet higher than they are now.  Why the difference?  There are other factors that control temperature and sea levels, and, the rate of increase in CO2 since the Industrial Revolution has been so rapid, that global temperature and sea level have not had enough time to fully respond.

What is clear is that climate change is a reality that mankind and all planetary life will have to adapt to in order to survive.  Even if humans can cut all CO2 emissions to zero, the excess CO2 that has built up over the past century will remain in the atmosphere for hundreds to thousands of years.  This is because CO2 is a very stable gas, meaning it does not break down easily, and the natural processes to remove CO2 work on very long time scales.  And at the moment, CO2 emissions continue to increase uncontrollably as world governments make little progress in agreeing to emissions reductions.  China is currently the biggest emitter of CO2, followed by the United States.  CO2 emission by the US are actually slowly decreasing, largely due to a gradual switch from coal to gas fired power generation and a steady improvement in automobile fuel economy, but emission from the US are still very high.  And, the rest of the developing world, such as south Asia, Africa, and Latin America, will likely have huge increases in CO2 emissions as their economies grow.  There is also little chance of coming up with a climate change silver bullet - both in ceasing CO2 emissions by developing alternative energy sources, and removing the excess CO2 that already resides in our atmosphere.  It is clear that climate change is no longer an issue of prevention, but an issue of triage, and has been for some time.

Much needed rain in California

An upper-level low pressure system is slowly moving across California and generating scattered areas of showers and thunderstorms in parts of the state.  Rainfall overall has not been heavy or widespread, but any rain is beneficial after a very dry winter.  Showers have been most extensive over southern California, the Sierra Nevada, and parts of the Central Valley.  Downtown Los Angeles received over 2/3 of an inch of rain on Monday - this is actually double the normal amount of rain they receive during the entire month of May.  One storm up in Lassen County produced 65 mph winds.  Localized heavy rain also fell in the Sacramento region, with estimates of over an inch in some areas as the thunderstorms are moving very slowly through the region.  Much of the Bay Area, however, has seen little if any rain out of this system.

Counter-clockwise flow around the upper-level low has resulted in warm, offshore winds in the Pacific Northwest.  Similar to the Bay Area, the marine layer offshore the Pacific Northwest coast plays a huge role in determining temperatures.  Surface winds there are typically onshore, which results in relatively cool temperatures and occasional low clouds and fog, especially along the coast.  However, when offshore flow develops, the Pacific Northwest can really heat up, even out to the coast.  Seattle reached 87º on Monday, smashing the old record high of 79º for the date.  Seattle was also warmer than almost all other areas of the country on Monday, including all of Florida, Hawaii, and Texas - only Phoenix was as warm as Seattle today.  Quite a rare and impressive feat. 

This upper-level low is now gradually moving east and will move into Nevada on Tuesday.  The system will weaken as it crosses the Rockies but will help to generate thunderstorms, possibly severe, in the southern Plains states by Wednesday and Thursday.  The set-up will not be ideal since the upper-level support with this system will be weakening, but the combination of increasingly humid air moving northwestward from the Gulf of Mexico and considerably drier air moving eastward out of New Mexico will set the stage for intense thunderstorms in western Texas, Oklahoma, and Kansas.  The cooler temperatures aloft associated with the weakening upper-level low will help to destabilize the atmosphere and promote convection.

The upper-level low that brought record cold to the South is still spinning over the region.  The system is now pulling in moisture off the Atlantic and Gulf, resulting in very heavy rain in the southern Appalachians.  The image on the right shows precipitation from yesterday (using ground observations and radar estimates).  Some areas in western North Carolina received several inches of rain, resulting in flooding concerns.  The upper low is gradually shifting northeastward, putting Virginia and the Mid-Atlantic states at risk for heavy rain today.  The east facing slopes of the Appalachians will be at greatest risk as east-southeasterly upslope flow will locally enhance rainfall rates. 

Unusual weather pattern continues across U.S.

The storm system that brought record late-season snows to parts of the Plains and Midwest continues to live on, and is currently drifting eastward across the southern U.S.  Temperatures are too warm for snow at this point, but prolonged heavy rain will affect parts of Tennessee, Alabama, and Georgia early this week.  The big story with this system continues to be nearly-unprecedented cold temperatures for this time of year.  Numerous locations across Texas, Arkansas, Louisiana, and Mississippi set records this morning not only for daily lowest temperature, but for lowest temperature recorded in the month of May.  Houston and College Station both fell to 42º this morning, easily breaking their old records for coldest May temperature.

The culprit behind the record cold is a cut-off low-pressure system drifting across the U.S.  This system broke away from the main steering flow, or jet stream, mid last week, and now without a steering mechanism, this system can only crawl along.  The entire upper-level weather pattern over North America is essentially "blocked", with the main jet stream well to the north in Canada, and several strong lows and highs staggered across the United States (see image on right).  The big doughnut over the South is the cut-off low causing record cold and heavy rain.  Another cut-off low is located over northern California, which will bring cooler temps and scattered thunderstorms early this week - welcome relief to the recent spate of hot temperatures and wildfires (assuming the thunderstorms themselves do not start additional wildfires).

Meanwhile, two strong high pressure systems are located over the Pacific Northwest and the Great Lakes.  These regions are experiencing sunny skies with above-normal temperatures.  Locations such as Seattle, Detroit, and Toronto will be warmer compared to many cities further south such as San Francisco, Atlanta, and Memphis early this week.  The position of these lows and highs relative to one another is causing the atmospheric "blocking".  There are two main types of blocking patterns - a Rex Block, which is a high pressure system over (poleward of) a low pressure system, and an Omega Block, which consists of a high pressure ridge flanked by a low pressure system on either side and looks somewhat similar to the Greek letter Omega (Ω).  A schematic of these blocking patterns is shown on the right.  Both patterns are stable patterns, meaning they can remain in place for a long

time.  For example, in the Rex Block pattern, the circulation around the high to the north opposes feeds into the circulation of the low to the south, and vice versa.  Thus, as long as the high and low are of relatively equal strength, there is little movement to the west or east.  Right now, we have forms of both blocking patterns over the U.S., with a large pseudo-Omega block (dashed blue circle) consisting of a high pressure ridge over the Plains flanked by the cut-off lows over the Southeast and California, and then the two cut-off lows are themselves part of Rex Blocks (dashed red circles) with the high pressure systems over the Pacific NW and the Great Lakes.  This pattern will be very slow to break down and likely won't happen until late in the week when forecast models indicate a strong jet stream over the north Pacific will break into the Rex Block on the West Coast.

Heat, Cold, and Snow Records

Just a quick update - new records were set across the country today but for different reasons.  Here in California, it was the heat, with several daily high temperature records set in the Bay Area:
92º in Oakland and San Jose
89º at San Francisco Airport
88º at downtown San Francisco
87º at Monterey

Tomorrow and especially the weekend will be cooler for the Bay Area.  In the Front Range of the Rockies, daily record lows were set when skies cleared overnight after a fresh coating of snow:
19º in Denver
9º in Cheyenne

The most notable records set, however, were for snow in the Plains and Midwest.  Minnesota, Wisconsin, and Iowa likely set new all-time state records for May snowfall, with at least 10-12" of heavy, wet snow in some locations in those three states.  The measurements are currently unofficial but should be certified in coming days.  Locations further south are also seeing snow, such as Kansas City and Omaha, where accumulating snow has not occurred in May for over 40 years.  Parts of Oklahoma and the Texas Panhandle also reported trace amounts of snow.

Early Fire Season for California

As I mentioned in my previous post, a combination of warm to hot temperatures, gusty offshore winds, low humidity levels, and abnormally dry vegetation would result in a high fire risk across much of California this week.  Several fires have already broken out in parts of northern California, namely in Sonoma County (north of San Francisco) and in the foothills of the Sierra well north of Sacramento.  These fires are not very large and are nearly contained, but it is quite early for numerous wildfires to develop - normally the fire season in California starts in June or July and peaks in September.  Smoke from the Panther Fire in the Sierra foothills north of Sacramento was clearly evident on satellite imagery this afternoon, with a plume of smoke being blown westward with the offshore winds.  The fires north of San Francisco are smaller and not producing enough smoke to be visible in this image.


The heat will continue across much of California for the next two to three days, with the warmest temps likely occurring at the coast on Thursday and inland on Friday.  A Heat Advisory is even in effect for the Bay Area, where afternoon temperatures may approach 90º in San Francisco and into the mid-90s inland.


The other big weather story is the very late season snow storm moving through the Rockies and Upper Midwest.  The Denver area received several inches of snow earlier today, and now areas from Nebraska north to Minnesota and Wisconsin are seeing snow or a rain/snow mix.  The snowiest May in Minneapolis was in 1946 when 3" was reported.  For the current storm system, some areas of eastern Minnesota may receive up to 6" of snow, which would easily break most May snowfall records for the region.  Radar imagery this evening from the Twin Cities shows that the heaviest band of snow is (so far) remaining just southeast of Minneapolis and the airport there (where the official snow measurements are taken).  The snow is generally moving to the northeast, so it remains to be seen whether this record can be broken.  Regardless, some areas will receive several inches out of this.  These late season, record type events are extremely difficult to forecast as well since there is so little precedent for them.  Forecasters weight climatology heavily when making most weather forecasts, so highly abnormal events give forecasters little to work with except the computer model runs.

Continued warmth west, cool and wet east

We're starting a very warm week in California, and conditions will be about as summer-like as they get in San Francisco.  As a strong ridge of high pressure strengthens over the West Coast, temperatures will warm and offshore winds will keep the marine layer out of the Bay Area.  For much of the week daily high temperatures in San Francisco will reach the low- to mid-80s, well above normal, and many locations in the region will likely set new daily record highs over this period.  The image on the right shows forecast highs for Friday, but similar temperatures can be expected Wednesday and Thursday as well.  The dry, offshore winds will also result in fire concerns, and the NWS has issued a Red Flag Warning for the North and East Bay hills.  Any unattended fires could quickly get out of control given the hot, dry, windy conditions and abnormally dry vegetation for this time of year.

In the central and eastern U.S., temperatures have finally warmed up to near normal after a cool March and most of April.  However, the warm up is short lived as anomalously deep upper level trough moves into the region.  The images on the right are forecast 500 mb heights, winds, and vorticity for Tuesday night and then Friday morning.  On Tuesday, a strong trough is noted over the western Plains region.  However, instead of progressing quickly eastward as would typically be expected, this trough closes off and detaches from the steering flow (or jet stream).  Without the upper level steering, this closed low will be left to drift across the central Plains and will only slowly move east.  The system will bring unseasonably cold temperatures, with highs only in the 40s for many locations where average highs are well into the 60s.  The air will be cold enough that some forecast models also show the potential for accumulating snow over parts of the Rockies followed by the Upper Midwest (see final image).  To put this system in some historical perspective, the Minneapolis-St. Paul area has not seen measurable snow in May since 1991.

Assessing Record High Temperatures

I came across an interesting piece of information from the National Weather Service office in Minneapolis-St. Paul (MSP), showing that a new record low of 21 was set on April 20.  A single record temperature, high or low, is not necessarily notable, but part of what makes this story interesting is that the new record low set on April 20 was the first new record low set temperature set in the Twin Cities since August 2004.  For some perspective, the frequency in the occurrence of new daily record temperatures varies from location to location.  For stations with a long record history, setting new record temperatures is relatively rare.  MSP has a long record history, with observations dating back to 1871.  So, we wouldn't necessarily expect the Twin Cities to experience record temperatures very frequently.

What is really astonishing about this simple record low temperature is that while looking through daily records at Minneapolis-St. Paul since 2004, I found nearly 40 daily record high temperatures (see table).  In a static climate scenario, one would expect a roughly equal number of daily record high and low temperatures to be set over a given time period.  The fact that MSP has set nearly 40 record highs for just one record low since August 2004 is statistically quite unlikely.  Many of these record highs occurred in March 2012, an anomalously warm month for the Midwest that I have discussed in previous entries.  Data in the table at right are from the University of Minnesota Climatology Working Group.



Setting a single record high or low temperature it not especially useful in itself for assessing climate trends.  However, a series of records, or a much greater number of records in one direction versus another, is quite telling, and an increased frequency of record highs is expected under our current climate warming scenario.  If we plotted all of the observed high temperatures for a given date at a given site, we would expect a normal, or Bell shaped, distribution, with the majority of days clustered around the daily mean temperature (blue series on chart).  Record temperatures would be represented by the tails of this curve and would occur infrequently.  If we shift the distribution just a few degrees to the right, as is expected under a climate warming scenario, record high temperatures under the old scenario become much more common (red series on chart).  The chart is purposefully exaggerated for effect, but the unfortunately, depicts the pattern of record temperatures over recent years at MSP quite well.

Spring 2012 vs. 2013

A lot of attention this spring in terms of weather and climate has been focused on the relatively cold weather observed in the central and southern United States.  This pattern started in February and has persisted right up to mid-April, with a broad region experiencing below to well-below normal temperatures and uncharacteristically heavy snowfall for this late in the season, especially when compared to previous years.  The map on the right shows the temperature departure from normal for March 2013.  Areas in green, blue, and purple reported below normal temperatures (most of the central and eastern U.S. with the exception of New England).  Some areas of the northern Plains were over 10 degrees below normal for the month.  Areas of the southwest and New England were slightly above normal for the month.

The current spate of cold weather can be partially attributed to the current phase of the North Atlantic Oscillation, or NAO.  The NAO is technically the difference in sea-level pressure between the Icelandic Low and Azores High (two semi-permanent weather systems over the North Atlantic).  When these two systems are relatively strong, the difference in pressure between the two is greater, and this pressure differential generates a strong west-to-east jet stream across the Atlantic (see image below-left, courtesy of NC State University).  When this jet is stronger than normal, it generally keeps cold Arctic air over or near the polar regions; this is called the positive phase of the NAO.  When the two systems are relatively weak, the pressure differential decreases, and in turn, so does the strength of the west-to-east jet stream.  This allows cold Arctic air to move southward into the United States and Europe, and is called the negative phase of the NAO.  The image below-right (NOAA-CDC) shows the state of the NAO so far for 2013.  Since February, the NAO has been in its negative phase, which fits with the colder than normal temperatures observed over the U.S. and Europe.  The red lines are model predictions for the NAO over the next few weeks, and show a dramatic change to the positive phase, so temperatures may finally warm up for the central and eastern U.S. later this month.

It was a cold March for much of the lower 48 to be sure, but it was far from record breaking cold compared to the historical record.  So, any claim that this colder weather is evidence against the existence of climate change is severely in error.  In fact, one need only look back to March 2012 when temperatures were indeed record breaking - but were much, much above normal.  The map on the right shows temperature departures from normal for March 2012.  Note the huge area of above normal readings in the same regions that currently are running below normal.  Much of the Midwest was 10 to even 15 degrees above normal for the month, which easily made March 2012 the warmest March on record for the U.S.  The NAO was in its positive phase at that time, which may explain a portion of the warmer weather.  However, the unparalleled warmth of March 2012 likely had some contribution from human forced climate change, as I talked about in a post last year.  This type of extreme weather is expected with climate change - record warm temperatures will become more common and more extreme (like March 2012), and while periods of below normal temperatures will still occur (like March 2013), they will be less common and less pronounced.

       

Winter 2012-13 Recap

Winter 2012-13 was really a tale of two seasons for northern California.  November and December
were wetter than normal, when an active Pacific jet stream brought series of moisture-rich systems into the region.  January through March were much different, when a persistent upper-level ridge of high pressure over California deflected incoming storm systems well north of the state.  As a result, San Francisco recorded its driest January to March period on record with only 2.31 inches of rain, or over 9 inches below normal.  This record is also impressive due to the fact that records in San Francisco date back to 1850.  The heavier precipitation in November and December was very promising given the preceding very dry winter of 2011-12; unfortunately, this winter too will go down in the record books as another dry one for California. 

High elevation snow in the Sierra has been equally disappointing this winter.  Late March to early April is normally the peak in snow depth for California's mountains.  However, the most recent data show that California's snow pack currently holds only about 50% of normal for this time of year.  This is extremely concerning for agricultural and ecological concerns in the state, not to mention the upcoming fire season, especially following the dry winter last year.  Leading seasonal climate indicators for California weather, such as ENSO, have generally been neutral this season, with no preference toward unusually wet or dry conditions.  It's also implausible to pin the two consecutive dry winters on climate change, as California has experienced its share of dry winters in the historical record.  Climate research shows varying results in regards to future precipitation amounts for California, but the general consensus is for fewer but stronger winter time storm systems, which would likely result in more winters at both ends of the precipitation spectrum - either very dry or very wet.

Since the vast majority of California's precipitation falls between November and March, the state stands little chance of making a dent this spring in the current precipitation deficit.  Last year, despite the lack of rainfall, we were still able to benefit on the coattails of the wet 2010-11 winter season.  This year, California has no such luxury.