NWS OPC Forecast valid 00Z Sat 3/23 (5pm PDT Fri 3/22). Cold front is passing through Bay Area, immediately followed by low pressure system and associated trough that will bring convective lifting and the possibility of thunder storms. This is not expected to be a big rainmaker overall, but there is a possibility of locally heavy rain, moderate to high winds, small hail, and some thunder and lightning.
Possibility of t-storms, especially on Saturday.
Forecast for 00Z Saturday (5pm PDT Friday). The cold front is expected to pass through the Bay Area late Friday, with the trough behind it. Note the large storm in the mid-Pacific that may impact us by the middle of next week.
New storm system arrives on Friday.
Surface map forecast for 12Z (5am PDT) Friday Mar 22. Source: NWS Ocean Prediction Center (OPC):
From latest AFD:
.SYNOPSIS...
Issued at 159 AM PST Sat Mar 2 2024
A cold front is moving through the Bay Area early this morning.
Showers and strong westerly wind will continue Saturday before
tapering off Sunday. Cold temperatures will persist through
Monday.
&&
.SHORT TERM...
(Today and tonight)
Issued at 159 AM PST Sat Mar 2 2024
The surface cold front is currently moving through the Bay Area.
Doppler radar velocity data and higher elevation anemometers are
indicating winds are gusting as high as 55 mph as the frontal
boundary passes. Rain showers continue, although the coverage has
decreased overnight. The post-frontal environment will likely
continue to support this lull in precipitation Saturday morning,
and there will even be some sunshine. By Saturday afternoon, the
combination of surface heating and cold middle levels will
decrease the stability and will likely lead to a final round of
strong showers and possible low-topped thunderstorms. Winds will
subtly shift from SW to westerly behind the front, and remain
strong through the day.
&&
.LONG TERM...
(Sunday through Friday)
Issued at 159 AM PST Sat Mar 2 2024
High pressure will quickly build behind the front and the gradient
will relax by Sunday. There may still be a few showers, but they
will be lighter than Thu-Sat. Winds will also be lighter on
Sunday, but there will still be a moderate westerly breeze in the
afternoon. The biggest impact will shift from wind and rain to
cold morning temperatures. The low temps on Sunday and Monday
morning are expected to range from mid-30s to low-40s.
A weak system will move through in the middle of the week. This
will keep the cooler temperatures, cloud cover, and light rain
chances in the forecast through Tuesday or Wednesday. High
pressure finally rebuilds late week. Thursday and Friday look to
be clear and calm, with more normal temperatures. For example, the
Thursday high temperature at Santa Rosa is expected to climb back
to the mid-60s for the first time in a week.San Fransisco Peninsula Coast CA-Santa Cruz Mountains CA- Santa Clara Valley Including San Jose CA- San Francisco Bay Shoreline CA- 529 PM PST Fri Mar 1 2024 ...A STRONG THUNDERSTORM WILL IMPACT NORTHWESTERN SANTA CRUZ... SOUTHEASTERN SAN MATEO AND NORTHWESTERN SANTA CLARA COUNTIES THROUGH 600 PM PST... At 528 PM PST, Doppler radar was tracking a strong thunderstorm 10 miles west of Boulder Creek, or 18 miles southwest of Saratoga, moving northeast at 35 mph. HAZARD...Wind gusts up to 40 mph and pea size hail. SOURCE...Radar indicated. IMPACT...Gusty winds could knock down tree limbs and blow around unsecured objects. Minor hail damage to vegetation is possible. Locations impacted include... San Jose, Sunnyvale, Santa Clara, Redwood City, Mountain View, Palo Alto, Cupertino, Campbell, Menlo Park, Saratoga, Los Gatos, Los Altos, East Palo Alto, Los Altos Hills, Atherton, Woodside, Fruitdale, Boulder Creek, North Fair Oaks, and Stanford. PRECAUTIONARY/PREPAREDNESS ACTIONS... If outdoors, consider seeking shelter inside a building.
From AFD:
.SYNOPSIS...
Issued at 112 AM PST Fri Mar 1 2024
Expect a nasty couple of days ahead as a second cold front moves
through the Bay Area. Strong winds will pick up this afternoon.
Periods of rain will persist through Saturday with cold weather
sticking around into next week.
&&
.SHORT TERM...
(Today and tonight)
Issued at 112 AM PST Fri Mar 1 2024
The rain forecast is holding steady and gauges overnight are more
or less agreeing with the expected accumulation so far. There are
some exceptions where stronger rain bands were able to persist for
several hours. These include over 1.5" at Ben Lomond and similar
totals throughout the higher peaks in the Santa Cruz Mountains.
The Santa Lucias have also over performed, with several gauges
already over 2". Otherwise, the forecast has been very good so
far. With roughly another 1-2" expected over the next 48 hours,
there are still no major flooding concerns expected this weekend.
That being said, the rain combined with a cold wind will not make
for great picnic weather.
Strong SW winds will increase through the day ahead of the
frontal passage tonight. Gusts to gale force are expected,
especially as the frontal boundary moves through in the afternoon
and early evening. After the front passes, winds will shift to
westerly, but remain strong through the night. A wind advisory is
in effect for the majority of the area from noon today through 10
AM Sunday.
&&
.LONG TERM...
(Saturday through Thursday)
Issued at 112 AM PST Fri Mar 1 2024
Saturday will feel like the Pacific Northwest. In addition to
periods of rain, it`s going to start cold and won`t warm up very
much. The official forecast for San Francisco has a low of 46F
with the high only reaching 53F. While that`s nowhere near the
daily record (lowest max temp) of 45F set in 1896, it`s 9 degrees
below normal and more in line with the average March high temp in
Seattle (54F). The majority of the rainfall looks to be in the
morning, with only scattered showers in the afternoon. Winds will
also be lighter than Friday, but there will still be a stiff
westerly breeze through the day, especially along the Central
Coast. The first real snow of the year is expected in the
mountains as the thickness finally drops low enough during the
tail end of the multiday rain event. There`s some instability,
but the air mass will be drying out in the post-front environment.
The 06Z NAM point sounding for Santa Rosa valid at 10AM Saturday
brings the freezing level to 1465` with 142 J/kg of CAPE. The
problem is the precipitable water is only 0.38" at that time, and
continuing to decrease through the day. A few inches are expected
in the higher peaks of the Mayacamas and Santa Lucias, around an
inch in the Diablo Range, and a possible dusting in the Santa Cruz
Mountains. To be clear, the snow levels will remain above 2,000
ft and will not impact the main mountain passes.
The rain will really taper off Sunday, but the cold temperatures
will linger through the week. Monday morning looks to bring the
coldest temperatures with most areas dropping into the mid to
upper 30s before the sun comes up. The remainder of the week will
have lows in the upper 30s to mid 40s and highs in the upper 50s.
Some light rain is possible mid week. The uncertainty is much
higher than normal beyond Thursday with the global models all over
the place by next weekend.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.Excerpt from recent AFD:
.SHORT TERM...
(Today and tonight)
Issued at 1233 AM PST Tue Feb 27 2024
High pressure will prevail at the surface and aloft, with dry
conditions, northerly breezes, and cooler temperatures. A few higher
clouds may filter in at times, but overall look for plenty of
sunshine and an absence of coastal stratus after this morning.
Highs each afternoon will reach the upper 50s to lower 60s, with a
few of the warmer inland places (Santa Clara and southern Salinas
Valleys) perhaps nearing the 70 degree mark. Lows will be chilly
and in the 30s (inland) to lower 40s for tonight, with mostly 40s
for tomorrow night.
&&
.LONG TERM...
(Wednesday through Monday)
Issued at 1233 AM PST Tue Feb 27 2024
An unsettled weather pattern will develop as a low pressure system
slowly migrates from the Gulf of Alaska towards the West Coast. The
pattern looks rather dreary with cloudier conditions, gusty winds,
and periods of moderate rainfall. Light rain should first reach the
North Bay by Thursday/Thursday night, with showers overspreading the
remainder of the area on Friday into the weekend. Most of the
rainfall looks to occur on Friday into Saturday, with showers
lingering into early Sunday. Deterministic models, their ensembles,
and the consensus blended guidance all remain in relatively good
agreement regarding rain totals, and no major changes have occurred
since the previous forecast. Storm total amounts still look to
average 1-2" for most locations, with perhaps another inch or so in
the coastal ranges. This suggests that rain-related impacts from
this system should be much more modest as compared to previous storm
systems this winter. Breezy to gusty westerly winds are also
expected with peak speeds generally between 25-35 mph. This will be
a colder system and snow levels this weekend will lower to between
2,000 feet in the North Bay to 3,000 feet for the Central Coast.
This suggests some higher elevation snowfall is possible above these
elevations. The greatest potential for this appears to be across the
Santa Lucias where cold temperatures and precipitation should best
overlap. But can`t rule out light accumulations for some of the
highest peaks in the Mayacamas and Diablo range as well, depending
upon how things play out. At this point it appears snow levels will
remain high enough such that no impacts are anticipated for any of
the passes or major travel corridors.
Colder temperatures return for early next week with frost and freeze
potential for inland areas on Sunday and Monday nights.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.
CAC081-200030-
/O.CON.KMTR.SV.W.0001.000000T0000Z-240220T0030Z/
San Mateo CA-
402 PM PST Mon Feb 19 2024
...A SEVERE THUNDERSTORM WARNING REMAINS IN EFFECT UNTIL 430 PM PST
FOR CENTRAL SAN MATEO COUNTY...
At 401 PM PST, a severe thunderstorm was located near Woodside, or
near San Carlos, moving northeast at 35 mph.
HAZARD...60 mph wind gusts.
SOURCE...Radar indicated.
IMPACT...Expect damage to roofs, siding, and trees.
Locations impacted include...
San Mateo, Redwood City, Menlo Park, Foster City, Burlingame, San
Carlos, Belmont, Hillsborough, Atherton, Woodside, North Fair Oaks,
Highlands-Baywood Park, Highlands, and Emerald Lake Hills.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
Remain alert for a possible tornado! Tornadoes can develop quickly
from severe thunderstorms. If you spot a tornado go at once into the
basement or small central room in a sturdy structure.
For your protection move to an interior room on the lowest floor of a
building.
&&
LAT...LON 3759 12234 3759 12232 3758 12231 3756 12225
3755 12224 3754 12220 3751 12221 3752 12220
3751 12217 3751 12214 3739 12232 3742 12238
TIME...MOT...LOC 0001Z 210DEG 32KT 3745 12232
TORNADO...POSSIBLE
HAIL THREAT...RADAR INDICATED
MAX HAIL SIZE...<.75 IN
WIND THREAT...RADAR INDICATED
MAX WIND GUST...60 MPH
$$
McCorkle