Mesoscale Precipitation Discussion 0071
NWS Weather Prediction Center College Park MD
1220 PM EST Tue Feb 20 2024

Areas affected...Central California...

Concerning...Heavy rainfall...Flash flooding possible

Valid 201720Z - 202315Z

SUMMARY...Scattered slow-moving but locally intense downpours pose
.5-.75"/hr rates with up to 1.5" totals pose possible localized
flash flooding concerns, especially near urban settings.

DISCUSSION...GOES-W WV denotes a subtle inflection in the broad
southwesterly flow along the Central CA coast.  This is along the
western cyclonic portion, weakly diffluent edge of the 150kt jet
currently over south-central CA, providing sufficient upper-level
divergence to further amplify the wave.  Combined with sharpening
core of the upper-level trof near 41.7N 127.3W along with broad
CAA aloft is providing a steepening lapse rates increasing
available instability for convective development in the next few
hours.  MUCAPE to 750-1000 J/kg near coast remains to 250-500 J/kg
into the southern Sacramento Valley.  GOES-W GLM and Visible loop
shows recent convective development nearing the mouth of San
Francisco Bay,  given depth of modest moisture and 15-20kts of low
level moisture convergence (within .75" total PWats) suggest rates
of .5"/hr are possible within the core of the narrow updraft
cores.  Weak bulk shear to 30kts and favorable back-building flow
regime suggest some slow cell motions and perhaps weak rotation
for increased isallobaric moisture flux to potentially reach a low
end possibility of .75"/hr rates particularly near the instability
source for the next few hours before height-falls from the main
trof press this regime eastward after 00z. A spot or two of 1-1.5"
totals are not out of the realm of possibility.  Given the
rates/short-duration, localized flash flooding is considered
possible given well above normal run-off capability given
saturated ground conditions (per NASA SPoRT LIS). 

Further east...As cells translate through the Napa Valley/Santa
Cruz range toward the Sacramento Valley, cells will weaken with
diminished instability but some filtered insolation may help some
buoyancy to maintain weaker elevated cells.  Deep layer flow will
continue to support narrow banded-like features translating
eastward allowing for some training and increased duration.  With
reducing rates but increased duration streaks  .5-1.25" are
possible and may induce some nuisance flooding especially near
urban/poor draining locations. 

Gallina

ATTN...WFO...EKA...MTR...STO...

ATTN...RFC...CNRFC...NWC...

LAT...LON   39602185 39462115 38982095 38442103 37842113
            37182142 37002184 37032238 37432277 38362335
            38892319 39432248