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!