Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service San Francisco Bay area
1037 am PDT Monday Sep 22 2014

Synopsis...two more days of near normal temperatures and coastal
low clouds in store for the forecast area. A Pacific storm system
will bring rain to our area by midweek.

&&

Discussion...as of 09:15 am PDT Monday...made a few minor changes
to the forecast this morning with respect to cloud cover based off
the latest visible satellite. Should continue to see the stratus
burn-off through the morning hours and give way to mostly sunny
skies inland by this afternoon. Otherwise...the forecast looks on
track for today and will work on the details of the midweek
system set to bring rainfall to the region...especially across the
North Bay southward to the Santa Cruz Mountains.

&&

Previous discussion...as of 3:00 am Monday...unlike the past several
days the overnight satellite product is a bit different early this
morning with plenty of breaks off the coast and clouds in the bays
and adjacent valleys. Last few frames do show some filling in from
the waters so will be interesting to see how things look by day
break. Could actually see sunrise near local beaches -- especially
for parts of Monterey County south of Point Sur. Temperatures will
be similar to yesterday with highs in the 60s to middle 70s at the
coast with middle 70s to lower 80s in most urban spots. Tuesday looks
to be a near repeat of today with little change in the surface
pattern and both 500 mb heights and 850 mb temperatures virtually
unchanged.

All attention continues to focus on Wednesday and Thursday as a
Pacific storm system will advance to the pacnw first on Tuesday
then down into the northern half of California. Rain will enter
northern California early on Wednesday and then spread to the
south through the day. Although there remains some disagreement
between the speed and coverage of the rain, there is overall
fairly good agreement that the associated cold front will advance
to the North Bay Wednesday afternoon or evening and then continue
to progress to the south and east during the overnight hours and
into Thursday. Due to the rain and clouds temperatures will cool
to below normal values inland by Thursday.

The event is still a few days out, so unfortunately we still have
a limited number of more detailed models for quantitative precipitation forecast although general
indications are more than half an inch could fall by Thursday
afternoon over the North Bay with closer to .1" - .25" around sf
Bay as well as the Santa Cruz Mountains. Other spots including
the Santa Clara Valley and Monterey will likely pick up less
rainfall. Possible that higher amounts could occur with precipitable water
amounts forecast to be as high as 1.65" Wednesday night. Models
have also continued to indicate some instability aloft, so added
a slight chance for thunderstorms as well especially over the
northern waters and the North Bay. With noting that the overall
trend over the past couple of days has been for a stronger and
wetter system.

There had been a large discrepancy between the model solutions
for Thursday night into the weekend which thankfully seems to be
resolved. Latest trends are now more in line with the operational
European model (ecmwf) which has the longwave trough overhead for a much longer
period as a ridge builds across the central Continental U.S.. this will
allow US to keep at least a chance of showers going across part
of our County Warning Area into the weekend along with temperatures generally
cooler than normal (especially for inland spots).

Looking into next week, quiet weather returns as the main storm
track shifts up to the north.

&&

Aviation...as of 10:30 am PDT Monday...the marine layer remains
around 2000 feet this morning. Northwest flow is scouring stratus
from the Bay area while also driving it further into the Santa
Clara Valley and Monterey peninsula. Expect clearing at all but
mry before morning is out. VFR this afternoon with mostly light
winds. Some patchy stratus to stay along the coast today and
likely to return to the terminals tonight.

Vicinity of ksfo...becoming VFR momentarily. Onshore winds to
around 15 knots this afternoon. Stratus to return tonight late
evening.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...sns to clear in the next hour as
ceilings erode in the Salinas valley. Northwest flow to maintain ceilings at mry
into early afternoon. Then VFR through middle to late afternoon.

&&

Climate...autumn officially begins (autumnal equinox) at 7:29 PM
PDT today.

&&

Marine...as of 09:15 am PDT Monday...light to moderate
northwesterly winds will continue along the central coast as high
pressure remains over the eastern Pacific. A cold front will will
move through the region middle week bringing a chance for showers and
possibly thunderstorms. In addition...northwest swell will build
and impact our area starting on Wednesday.

&&

Mtr watches/warnings/advisories...
...

$$

Public forecast: rgass
aviation/marine: ac

Visit US at weather.Gov/sanfrancisco

Follow US on facebook, twitter, and youtube at:
www.Facebook.Com/nwsbayarea
www.Twitter.Com/nwsbayarea
www.Youtube.Com/nwsbayarea

National Weather Service Glossary of Abbreviations