Mesoscale Precipitation Discussion 0063 NWS Weather Prediction Center College Park MD 1045 PM EST Sun Feb 18 2024 Areas affected...Northern and Central California Concerning...Heavy rainfall...Flash flooding possible Valid 190345Z - 190945Z SUMMARY...Heavy rainfall is expected overnight from bands of showers and thunderstorms with locally enhanced rainfall rates. Areas of flooding will be a concern, especially around the Bay Area tonight, and there may be at least a localized flash flooding threat in portions of the Sacramento Valley where any bands of convection train over the same area. DISCUSSION...A deep upper trough offshore of the West Coast continues to gradually shift off to the east which will be bringing a front inland across areas of central and northern CA overnight. Bands of heavy showers and a few thunderstorms are already impacting many coastal ranges of northern CA and also portions of the northern Sacramento Valley where recent dual-pol radar imagery is showing some localized concentration of stronger convection. Cloud top temperatures in the GOES-W IR satellite imagery are also cooling across the region, including areas just offshore, and thus suggestive of an uptick in convective organization and rainfall rate trends. In fact, the latest RAP analysis is showing a well-defined nose of instability ahead of the front, with MUCAPE values of 250+ J/kg, nosing across the Bay Area and into the Sacramento Valley. This coupled with the arrival of rather strong left-exit region jet dynamics with the aforementioned upper trough, and stronger low-level convergence/forcing with the cold front should maintain and expand the convective threat heading well into the overnight hours. The 12Z/18Z GFS solutions support a well-defined nose of stronger 850/700mb flow arriving across the Bay Area and through the Sacramento Valley by 06Z, with a southerly low-level jet forecast to peak at 50+ kts. This is expected to drive moisture flux anomalies of 3 to 4 standard deviations above normal which coupled with the available instability should drive locally enhanced rainfall rates. Already some of the more organized convective cells in the northern Sacramento Valley are focusing 1.0" to 1.5"/hour rainfall rates, and this should continue locally over the next several hours as the heavier rainfall threat gradually shifts off to the south and east with the arrival of the approaching front. The 18Z/00Z HREF guidance does support this along with recent runs of the HRRR guidance as well. Some of these heavier rains are likely to also spread into at least the windward slopes/foothills of the northern Sierra Nevada. Areas closer to the coast and near the Bay Area with the enhanced Pacific fetch of moisture will see occasionally heavy rain as well, and especially with the aid of orographic forcing. Going through the overnight hours, generally as much as 1 to 3 inches of rain will be possible for the coastal ranges, with potentially heavier amounts of 2 to 4 inches where any convection sets up and locally trains over the same area farther inland. Given the wet antecedent conditions, the additional rains heading into the overnight hours will result in a regional threat of flooding, especially around the Bay Area, and this will include a more discrete flash flood threat for portions of the northern Sacramento Valley. Orrison ATTN...WFO...EKA...MFR...MTR...STO... ATTN...RFC...CNRFC...NWC... LAT...LON 41162223 40072090 38982061 37612091 37022162 37002208 37222247 37942307 38512344 39272386 40022408 40992380Download in GIS format: Shapefile | KML
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