Active Tuesday, Strong Thunderstorms Possible

Active Tuesday, Strong Thunderstorms Possible

 MARCH 31, 2014

A cold upper low off the northern Californian coast will slowly work southeast along the coast overnight tonight into Tuesday, spiraling bands of convective clouds and precipitation into northern California beginning late tonight into Tuesday morning. Snow levels have already dropped off to 2000 – 3000ft across the sierra, and may flutter down lower at times through Tuesday, especially with convection, which is expected to be widespread.

IR satellite, mosaic radar with precipitation-typing, LSRs, METARs, and MSLP analysis from 8:55pm Monday.

Short range high-resolution models such as the 4km NAM indicate potential for at least partial clearing by mid to late-morning, and indicates pockets of strong surface heating with temperatures rising into the upper 50s to locally low 60s across the Sacramento and northern San Joaquin valley. These surface temperatures would support some very steep low-level lapse rates, with lapse rates remaining steep into the mid-levels given -30 or so 500mb temperatures aloft associated with the upper-level and surface low. Dewpoints are lacking given the pretty cold environment, however there’s enough low-level moisture to promote surface-based CAPE values between 200 – 400 j/kg according to the RAP and 4km NAM, which combined with decent buoyancy via strong diurnal heating would promote a risk of strong thunderstorm development itself.

0z 4km NAM’s forecast MLCAPE valid around 2pm Tuesday, indicating widespread modest values.

Upper jet of 100 – 120kts swings around the low into central California, with weaker upper level winds in northern California Tuesday. However, a band of modest upper-level winds wrapping directly around the low supports a bit of upper-level wind dynamics. Perhaps a bit more important, directional shear will be quite favorable give the low offshore, including locally breezy south surface winds, and westerly to southwesterly mid/upper level winds. Hodographs do show some small clockwise hooks and loops, in coordination with with pockets of 0 – 3km storm relative helicity (SRH) values in the 75 – 150 M2/S2 range… locally higher in favored terrain.

4km NAM’s forecast 0 – 3km SRH and 10-meter wind speed/directional arrows.

NAM’s forecast sounding for the Sacramento area with hodograph, valid Tuesday afternoon for the Sacramento area.

With that said, if clearing does occur in parts of the valley Tuesday morning and afternoon between convective bands, sufficient instability could evolve and support the development of strong enough thunderstorms to work with available shear. Main threats from thunderstorms Tuesday will be hail given the low freezing level, and yet again the threat for funnel clouds or weak brief tornadoes. I currently believe the highest threat appears to be from Stockton north to Redding given lift decreases a bit as you work your way further south.

Another 6 – 12″ of snow may fall Tuesday across the sierra/mountains above 4000ft, with locally upwards of 2 – 4″ between 2500 – 3500ft… locally some accumulation down to 2000ft. In the lower elevations, additional rainfall amounts between a few hundredths and perhaps more than a half inch are possible depending on convective/thunderstorm coverage. Thunderstorms would be rather slow moving (possibly as slow as 5 – 10mph in general) Tuesday, and could drop significant amounts of rain and hail over impacted areas.

Occluded convective band rotates in during the evening, which itself may roll through the valley and mountains with some thunderstorms. Low dives south through the morning hours Wednesday, shunting lift and moisture to our south. Weak trough may bring a round of light precipitation to northern California Thursday night into Friday.

Warming is expected by weekend into next week, with 80s possibly returning to the valley by early next week.