Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 02/22/20
Area Forecast Discussion
National Weather Service San Francisco Bay Area
416 PM PST Fri Feb 21 2020
.SYNOPSIS...A slight chance of light rain showers for the Central
Coast into Saturday morning as a system to our south impacts
southern California. High pressure redevelops by early next week
bringing a continued dry and warm conditions regionwide.
.DISCUSSION...as of 03:00 PM PST Friday... An atypical weather
pattern for late winter is in place over the region this afternoon.
A 555dm 500mb upper low is positioned approximately 425 miles
SSW of San Francisco while a low amplitude, progressively tilted
high pressure ridge is beginning to build into the state from the
northwest. Moist S/SW flow on the E/NE flank of the upper low has
continued to advect in mid to mainly upper level moisture over the
southern counties (Monterey, San Benito, Santa Cruz, and southern
Santa Clara). KMUX radar has showed increasing reflectivity
signatures off the Big Sur coast and eastward into the Central
Valley in an area of elevated diffluence on the NE periphery of
the upper low. So far, we have not received any reports of any
precipitation making it to the ground and instead have mainly had
eye witness reports of virga (ie rain falling but evaporating
before it makes it to the surface) -- which is not too surprising
with these higher based showers. Forecast models have steadily
backed off on coverage, probability and amounts of any
precipitation expected in the southern counties over the last 12
to 24 hours, with the latest Euro ensemble showing mostly dry
members even as far south as Big Sur station. Latest HRRR hourly
output keeps the area mostly dry as well, with some weaker wrap
around showers arriving late tonight into Saturday morning mainly
over southern Monterey and southern San Benito counties.
Otherwise, temperatures are running much warmer than the same time
yesterday in response to warm air advection (+1-2C/prev 24hr at
850mb) aloft. At the surface, this has translated to temperatures
running roughly 3-7 degrees over yesterday`s readings, which puts
the area at around 6-14 degrees above seasonal normals for today. As
of 2pm, Santa Rosa was 74F, which is +10F over 64F yesterday, and
Monterey was 73F which is +8F over 65F yesterday. Intermittent cloud
coverage has resulted in a mixed bag in day over day temperature
changes however with a few pockets under thicker cloud cover near
or even cooler than this time yesterday. Temperatures are forecast
to relax by 5-10F regionwide this weekend as low pressure passes
to the north and onshore flow picks back up.
Dry conditions persist next week as well. A broader, higher
amplitude high pressure system will then develop early next
week, pushing 850mb temperatures up to 12-15C from Tuesday
through Thursday -- resulting in further dry, unseasonably warm
weather for the region. Some inland areas could approach or
surpass 80F on Wednesday or Thursday of next given recent model
trends. This dry weather opens the door for many areas to receive
no precipitation for the month of February, including downtown
San Francisco. In all the years that weather observations have
been taken in downtown San Francisco, this has only occurred once
before -- in 1864.
.AVIATION...as of 4:15 PM PST Friday...Upper level low dropping
south off the coast into the southern California offshore waters.
This will bring a chance of showers to the MRY Bay Area and
possibly as far north as SJC but cloud bases will remain in the
Vicinity of KSFO...VFR with mid to high cloudiness. Winds
remaining under 10 kt.
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...VFR. A chance for showers between 07Z and
15Z with clouds bases getting as low as 5000 ft.
.MARINE...as of 04:02 PM PST Friday...An upper level low will move
to near Point Conception by early Saturday morning then quickly
move eastward during the weekend. The low will bring a chance of
showers over the southern coastal waters late tonight and
Saturday. A strong high pressure system will build over the
eastern Pacific late in the weekend and next week. Northwest
winds will increase over the northern waters by early tonight and
increase to the south on Saturday. Expect moderate period
northwest swell through early next week. A long period northwest
swell is likely mid next week.
.Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
SCA...Pt Arena to Pigeon Pt 10-60 nm
SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 10 PM
PUBLIC FORECAST: DRP
AVIATION: W Pi
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