Scientific Forecaster Discussion
fxus66 kmtr 161752
Area forecast discussion
National Weather Service San Francisco Bay area
952 am PST Sat Feb 16 2019
Synopsis...cool weather conditions will persist along with
continued shower activity through the weekend. Accumulating snow
is likely above 3000 feet, with snow levels lowering to 2000 feet
or lower by Sunday morning. Overnight temperatures turn colder
early next week as precipitation ends and sky conditions clear.
A chance of showers will return by the middle of next week, with
below normal temperatures expected through the period.
Discussion...as of 08:46 am PST Saturday...rain showers have
diminished a bit in coverage over the past few hours as one
disturbance exits the region and another approaches northern
California. With this, do expect a brief decrease in shower
activity through the late morning/early afternoon before we
experience another uptick late this afternoon and through the
overnight. The ongoing forecast remains on track this morning with
no updates anticipated at this time. For full details, please see
the previous forecast discussion below.
Previous discussion...as of 03:55 am PST Saturday...showers
increased late this evening as a weak shortwave trough approached
the area. These showers have been moving quickly from northwest to
southeast, limiting accumulation. Temperatures above 3000 feet
are below freezing so these showers are producing accumulating
snowfall in the higher peaks. Cold air aloft is supporting weak
convection, however no lightning has been observed in the County Warning Area over
the past few hours. The stronger cells are producing brief heavy
rain and small hail. Hrrr indicates showers will decrease in
coverage later this morning, and this idea is supported by
satellite and radar with less coverage upstream to the northwest.
Any locally heavy showers will be brief and no significant rises
on creeks and streams are expected.
Another weak shortwave trough will drop into the region under
northwesterly flow this afternoon. Showers will increase somewhat
as the trough passes through the area. Have added a slight chance
of thunderstorms to the forecast for this morning and afternoon,
as cold air aloft and marginal instability may allow a few
lightning strikes to occur. Small hail will be possible with any
thunderstorms as well as the stronger showers which develop. Snow
levels Saturday will remain generally between 2500 and 3000 feet
with several inches of snow likely above 3000 feet. Temperatures
in the lower elevations will be cool with highs in the low to mid
50s. Northwest winds will be breezy from 15 to 25 mph, making
conditions feel even cooler. Showers will briefly decrease
On Saturday night, a stronger shortwave trough will move through
the area from the north-northwest. Showers will become widespread
late Saturday night, with showers reaching most locations by
sunrise Sunday. Showers will decrease in coverage from northwest
to southeast on Sunday morning, with a few scattered showers
possible in our southern areas and along the coast Sunday
afternoon. Colder air aloft will arrive as the upper low center
approaches Sunday morning. Snow levels will drop to 1500 to 2000
feet by Sunday afternoon, but these lower snow levels are expected
to lag behind most of the precipitation. Additional rainfall
totals through Sunday afternoon are expected to range from 0.25 to
0.5 inches in the urban and valley locations with up to an inch
in the higher terrain. Above 3000 feet, this precipitation will
accumulate as snow with 3-6 inches expected. The Santa Lucia
mountains may see up to a foot of additional snowfall due to the
higher terrain and northwesterly flow. Between 2000 and 3000 feet,
a couple inches of snow are possible with perhaps a dusting down
to 1500 feet on Sunday morning.
Drier northerly flow and a cold airmass will allow low
temperatures to become very chilly on Sunday and Monday night.
Lows will drop to near freezing in most inland valleys. Mid to
upper 30s are expected in sheltered areas along the sf Bay
shoreline and coast, with lows near 40 in the mildest spots such
as San Francisco. Despite mostly sunny skies on Monday and
Tuesday, highs will remain in the low to mid 50s.
A weak trough will move from north to south on Tuesday, but is
currently shown by the models to keep precipitation offshore.
Another cold trough will move in from the north towards the middle
of next week. There are differences in timing between the GFS and
European model (ecmwf) but both show additional showers and a reinforcing shot of
cold air by late Wednesday and Thursday. Increasing moisture will
keep lows from becoming quite as cold, but highs will remain well
below normal with low to mid 50s.
The upper low will slowly drift southeast, becoming centered to
our southeast on Friday. An upper ridge will then nose into the
West Coast. This will keep a cool northerly flow overhead. Under
this pattern, precipitation chances will be minimal on Friday and
Aviation...as of 9:50 am PST Saturday...scattered showers
continue to roll through the area. Hi-res models indicate an
increase in shower activity by late afternoon and again early
Sunday morning as a couple of upper level disturbances move
through. Cigs remaining VFR at 4000 ft or above.
Vicinity of ksfo...VFR with cigs remaining at or above 4000 ft.
On and off showers through Sunday morning. Westerly winds gusting
as high as 20-25 kt from 20z through 04z.
Sfo bridge approach...similar to sfo.
Monterey Bay terminals...VFR with scattered showers coming off the
Bay through Sunday morning.
Beaches...as of 9:50 am PST Saturday...latest buoy reports at 9
am indicated 13 to 16 foot swells with 16 second periods. A high
surf advisory is in effect through 9 PM Saturday for breakers as
high as 18 to 25 feet along west and northwest facing beaches.
Swells will slowly subside tonight.
Marine...as of 09:38 am PST Saturday...high pressure centered
1150 miles west of Cape Mendocino combined with low pressure over
the Pacific northwest are producing moderate northwest winds along
with large northwest swells over the coastal waters. Winds will
continue through early next week as the low slides into the Great
Basin. Swells will start to decrease tonight and Sunday.
... Surf advisory...caz006-505-509-529-530
coastal flood advisory...caz006-505-509-529-530
Small Craft Advisory...sf Bay
Small Craft Advisory...mry Bay
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...rough bar advisory for sf bar
Public forecast: rgass/St
aviation: west pi
marine: west pi
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