Shower/Thunderstorm Threat Today (Thursday)

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Some, if not quite a few of you readers probably know that last night’s cold front busted from Sacramento northward, with the bulk of the frontal precipitation occurring in the San Joaquin valley, west slope of the sierra, and southern foothills. However, we’re not done with precipitation just yet; in fact, additional precipitation chances are expected to be higher from Sacramento northward this afternoon and evening as the trough’s base works its way overhead tonight.

January 30th

Offshore, a upper-level low low shows up well behind a band of convective precipitation, indicative of decent lift and some notable instability. Between the cold front and this upper low, a dry slot is present, allowing clouds to break up across portions of northern California, thus allowing some warming of the surface. This subtle to moderate warming surface temperatures beneath -20 to -25c 500mb temperatures will promote steep lapse rates from the low to mid-levels, including 7.5 – 8.5 c LL lapse rates and 7 – 7.5c mid-level lapse rates this afternoon and early evening. In addition, decent low-level moisture combined with the cooler air aloft will promote SBCAPEs of 200 – 500 j/kg on a widespread scale from Sacramento to Redding in the valley, coastal mountains, motherload, and northern sierra/northern mountains this afternoon. Some bullish models indicate SBCAPE values upwards of 700 – 1100 j/kg, with mixed-layer CAPE values of 200 – 600 j/kg also possible, which is typically harder to come by in northern California.

RAP's forecast MLCAPE for the early evening, generating some rather impressive values in the valley.

RAP’s forecast MLCAPE for the early evening, generating some rather impressive values in the valley.

This amount of buoyancy would be well-sufficient for some thunderstorm development this afternoon, but the question is will lift be adequate to get some convection blooming. Depending on the exact track and timing of the upper-low, there may or may not be enough. Currently, short range and high resolution models such as the 4km & 12km NAM, SPC WRF, RAP,  indicate enough lift to get storms going across the western central valley later this afternoon, continuing into the evening as they track east to southeast into the evening, remaining energized by remnant instability and the base of the trough cutting through northern California. While the main upper low itself offshore is tracking southwest, a smaller, mesoscale convective vortex may form along a band of convection in the coastal mountains later this afternoon into the evening, tracking across the Sacramento valley and into the adjacent foothills and mountains through the rest of the evening and overnight. MCVs such as these are quite well known to continue supporting convection into the overnight hours, and while they don’t develop as often in California as they do in the central part of the country, they do indeed develop. Convergence will also be a big player in convective development this afternoon and evening to, and it does appear that at least some moisture convergence will be present across portions of the Sacramento valley.

12z 4km NAM's forecast radar reflectivity for around 9pm this evening.

12z 4km NAM’s forecast radar reflectivity for around 9pm this evening.

Forecast radar reflexivity for around 6pm via the 4km SPC WRF, indicating storms similar to that of the 4km WRF, but at a more favorable time of the day.

Forecast radar reflexivity for around 6pm via the 4km SPC WRF, indicating storms similar to that of the 4km WRF, but at a more favorable time of the day.

Shear wise, while the upper low is tracking south, away from northern California, leftover jet streak winds in the upper levels combined with winds generated from the convection itself could promote effective shear values of up to 20 – 25kts locally in parts of the valley and mountains this afternoon and evening, with pockets of 0 – 3km storm relative helicity of 50 – 150 M2/S2 possible as well. Given the mesoscale nature of speed and directional shear, forecasting strong t0 severe storms is difficult and rather uncertain… however, given that uncertainty some stronger storms, if any convection even is to develop, cannot be ruled out, even a few hours after sunset. The most probable capability thunderstorms would have today would more likely be hail, gusty winds, and heavy rain or snow if storms are to develop or push into the sierra today/tonight. While a couple days ago this setup looked more promising as to the tornadic potential from storms, I hate to completely rule it out. If the HRRR were to be correct in indicating 0 – 3km SRH values of 50 – 150 M2/S2, it can’t be ignored or pushed to the side. I wouldn’t put anything on it, but just something to keep in mind if strong/robust thunderstorms were to erupt in the valley this afternoon into the early evening.

RAP's forecast 0 - 3km storm relative helicity for the early evening.

RAP’s forecast 0 – 3km storm relative helicity for the early evening.

Convection may linger into the early morning hours Friday, but besides that the lower elevations will clear out through the day. Cold air aloft combined with some heating (once again) may promote enough instability over the sierra Friday afternoon to aid in the development of some more showers (snow over 4000ft or so), but the lower elevations should remain dry aside from perhaps some clouds here and there.

Keep an eye on the Facebook page for rapid updates if thunderstorms do develop, though if you don’t use Facebook I’ll try to populate the comments of this post with some information as well.

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