Flood watch until 10am Wednesday and possibility of t-storms

February 20, 2024 admin
Excerpt from today’s AFD:
.SHORT TERM...
(Today and tonight)
Issued at 120 AM PST Tue Feb 20 2024

Upper troughing encompassed much of the northeast Pacific earth this
morning with a closed upper circulation just offshore from the N
CA/OR coast. A 160 kt basal jet was located along the equatorward
side of the upper trough and was pointed into SoCal as of this
writing. An occluding surface low was noted off the NW CA coast,
with low-level troughing extending SSW from it. Showers and
thunderstorms were ongoing ahead of the trough around 250 nm
offshore from the Central Coast, with additional showers ongoing
across the Bay Area. For today expect the upper trough to slowly
progress through the region. As temperatures aloft drop this will
steepen lapse rates and increase instability with forcing for ascent
enhanced via the approaching surface trough. Expect an uptick in
shower coverage later this morning with shower and storm potential
peaking this afternoon and evening. Several southwest to northeast
bands of (mainly) light showers were noted early this morning, and
some of the convection allowing models suggest similar
banding/training of showers is possible later today. HREF ensemble
mean probabilities for seeing 1"+ in a 3 hour period peak at 10% as
early as 18Z/10AM for the North Bay this morning and spread through
the remainder of the Bay Area between 21Z/1PM and 00Z/4 PM before
reaching the Central Coast between 03Z/7PM and 06Z/10 PM. Given this
setup we`ll need to keep an eye on flooding potential yet again
today. For most areas this means primarily minor urban and small
stream flooding. However flash flooding is a possibility if any
heavier rain bands are able to train over an urban or
hydrologically sensitive area over a prolonged period.
Thunderstorms are a hazard today as well and given lowered
freezing levels lightning, small hail, and a few stronger wind
gusts are the primary concerns. Thunderstorm potential should be
rather limited early this morning but should increase into the
15-25% chance range over the Bay Area by this afternoon and
perhaps the Central Coast by this evening. Additional rain amounts
for most areas will be in the 0.25"-0.5" range, with around 1" in
the orographically favorable coastal ranges. Localized heavier
amounts remain possible for localized areas that receive
thunderstorms however. Temperatures today will top out in the 50s
(North Bay) to lower 60s elsewhere. Lows tonight will dip into the
mid to upper 40s.