Wet, windy, and cold Thursday through Saturday
There is some uncertainty about the timing and amount of precip expected with this storm. There is concern about wet and windy conditions, and the possibility of significant snow accumulation in the mountains. Stay tuned for updates, and stay safe!!
From AFD:
.SYNOPSIS...
Issued at 157 AM PST Thu Feb 29 2024
A large, cold low pressure system will result in rain, colder than
normal temperatures and gusty winds today and into the weekend. High
elevation snow in the Bay Area is likely this weekend. The pattern
will remain mostly active and unsettled through the middle of next
week.
&&
.SHORT TERM...
(Today and tonight)
Issued at 318 AM PST Thu Feb 29 2024
The extent of arctic air coupled to anomalously deep 500 mb
heights in a planetary scale long wave trough along western North
America is evident on infrared satellite imagery. There`s a very
large field of cellular clouds, a signature of instability, latent
and sensible heat transfers are occuring from sea surface up
through the atmosphere upstream over the Pacific all the way to
the Aleutian Islands. Condensation (clouds) releases sensible heat
furthering instability, the colder surrounding environmental air
does additional work to force relatively milder air parcels
upwards and precipitation often ensues, including when air parcels
are forced upwards in the eventual ascent over mountains hence the
additional cooling (decreasing pressure, increasing volume) and
increasing precipitation rates.
The northern hemispheric pattern is active, a large dynamic
rotation rate, the western North America long wave is one of
several. Winter time synoptic pattern recognition for times of
west coast cold air intrusions with little over water trajectory
quite often require a sharp 500 mb unstable ridge over Alaska, the
ridge is more to the west this time; it`s the short wave troughs
within the long wave trough doing all the work in this case. The
overall slow progression of the long wave trough will bring an
extended period of colder than normal temperatures, rain, rain
mixed with snow over our mountains, including measurable snow on
the highest peaks in the Bay Area. Higher mountain peaks of the
west will get multiple feet of snow; this is a dangerous scenario,
with major to extreme conditions, if you haven`t already please
check with your local NWS office(s) for the latest.
For our forecast area the amount of rain we get from this strong
mid-high latitude influence will be because of less water vapor
availability mixed with a relative lack of fully developing smaller
scale low pressure systems (short wave troughs) within the slowly
eastward moving long wave trough. If the meso-scale models don`t
have a good handle on the smaller scale lows then rainfall rates
may end up higher in our forecast area, but as noted previously
there`s been good model agreement so far. Gradually increasingly
colder air aloft will move across our forecast area mainly from
the west and west-northwest, taking a strong over sea trajectory
there will be sea surface based air mass modification, but also
upward heat and moisture fluxes, westward facing slopes will
benefit the most in precipitation.
By later today the initial cold front will arrive, southwesterly
prefrontal winds today breezy to gusty and rain developing across
much of the forecast area. Light measurable rain of a few one
hundredths has already started over Sonoma and Marin counties
early this morning. Later today, tonight post frontal winds will
become westerly to west-northwesterly in direction.
&&
.LONG TERM...
(Friday through Wednesday)
Issued at 318 AM PST Thu Feb 29 2024
A high surf advisory is in effect for the entire coastline 10 am
Friday to 4 pm Saturday. Large break waves along west-facing
beaches will result in dangerous surf conditions and localized
beach erosion. Never a good idea to turn your back on the ocean,
large waves can sweep across the beach without warning, pulling
people into the sea. Coastal flooding is not expected since
observed and forecast high tides are running lower.
Otherwise plan on chilly, wet weather this weekend along with
periodic breezy to gusty winds. Of course will have to keep a
close eye on radar, satellite (plus model forecasts) over the
weekend if any potential last minute changes in rainfall and/or
wind forecasts. A break in the rain arrives either late in the
weekend or early next week. At and beyond 120 hours (5 days) model
solutions diverge, disagreement appears centered on the location
of the long wave trough, thus if the long wave trough stalls then
additional eastern Pacific low pressure system(s) will have an
opportunity to develop next week. March can be a powerhouse with
regard to precipitation, for many locations in our forecast area
long term climatology shows it`s normally the fourth wettest month
of the year.