Drought Update

The rainy season is quickly ending in California, and unfortunately, the drought has only continued to worsen over the course of the winter.  Lowland rainfall and mountain snowpack for winter 2013-14 are well below normal across all of the state, only adding to the precipitation deficits from the previous winter.  The latest US Drought monitor depicts the dire conditions, with extreme drought stretching from San Francisco southward to Santa Barbara and inland across the San Joaquin Valley.  The California Department of Water Resources just completed its final Sierra snow survey of the season, and found that the current snow water content was only 18% of normal for May 1.  For the northern Sierra region, the snow water content was just 7% of normal.  Snow water content is also well below normal across almost all of the southwestern US.  Late-season storms helped bring the snowpack to near- of above-normal conditions for the Pacific Northwest and northern Rockies.

A look at cumulative observed and average rainfall in San Francisco over the past four years clearly shows the mounting precipitation deficits.  For reference, San Francisco normally receives about 23 inches of rain per year, the vast majority of which falls between November and March.  Frequent storms brought above-normal precipitation to San Francisco and most of California during the 2010-2011 winter season, followed by slightly below-average precipitation during the 2011-2012 winter season.  Frequent storms and heavy precipitation returned during December 2012, but since then, the precipitation deficits have increased dramatically.  We ended the 2012-2013 winter season with about 50% of normal rainfall in San Francisco, and are now on track to end the 2013-2014 winter season with about 50% of normal rainfall.  With no significant precipitation on the horizon through mid-May, it is very likely that most areas in California will receive no substantial precipitation until this fall.  This all points to a summer of potentially extensive and numerous wildfires, poor air quality from wildfire smoke, and mandatory water restrictions in many areas.  An animation of the weekly US Drought Monitor since 2011 depicts the development and intensification of drought in California, going from no drought at the start of 2011 to extensive drought for much of the state today.

There is a dim light at the very end of the tunnel, as forecasts call for the development of El Nino conditions in the Pacific Ocean.  Many of California's wettest winters occurred during El Ninos.  However, it remains to be seen how strong the upcoming El Nino will be.  El Nino is a warming of the Pacific waters west of equatorial South America, but its atmospheric component is measured by differences in air pressure between Darwin, Australia, and Tahiti (in the Central Pacific).  Historically, moderate to strong El Ninos are typically associated with above-normal precipitation in California, but weak El Ninos have shown little correlation with precipitation here.

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.