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 40992380