Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS66 KMTR 082357

Area Forecast Discussion
National Weather Service San Francisco Bay Area
357 PM PST Thu Dec 8 2016

.SYNOPSIS...Look for scattered showers to continue into Friday.
Widespread rain will return starting late Friday as a front
approaches our region. Rain will changer over to showers late on
Saturday with drier conditions forecast for Sunday. More systems
are on tap starting the middle of next week.

&& of 2:00 PM PST Thursday...Widespread rainfall has
fallen across our region today with amounts ranging from less
than a tenth for some inland locations to well over 2 inches along
the coastal ranges. Highest value recorded so far is 3.37" in the
Big Sur region. For metro locations amounts range from just a few
hundredths up to around an inch and a quarter.

Based off of radar and reports at the surface, rainfall is
continuing to gradually progress to the south just just isolated
to scattered activity from San Francisco to the north. The various
models indicate showers continuing into Friday due to the very
moist flow forecast to remain overhead. However, additional
amounts through tomorrow afternoon should be light -- generally
less than 1/4".

Our next round of substantial widespread rainfall is forecast to
return starting late on Friday as a slug of moisture around 1200
miles WNW of San Francisco will become entrained in the flow and
move to the West Coast. By the time it makes it to our area, PW
values could exceed 1.30" at the coast with numbers closer to the
1.10" to 1.20" inland. Although there is some disagreement
whether or not the rain will move through the northern half of the
CWA Friday evening or closer to Saturday morning, all of the
models do indicate widespread rainfall with an associated frontal
boundary near the Bay Area. Rain will spread to the south and
eventually impact the entire region. Rain will again taper off to
showers later on Saturday with just isolated showers expected on

Rainfall totals from now through Saturday are similar to the
values that we received from yesterday into today. In general,
1/2" to 1.25" for most urban spots with 1.5" to locally more than
3" for the coastal ranges. Rainfall should be the heaviest late
Friday into the first half of Saturday, although amounts are not
expected to lead to hydro issues except for less than ideal
driving conditions. Pre-frontal south to southeast winds will also
increase late Friday into Saturday. Urban locations will see gusts
to 25 MPH Saturday morning with higher elevation spots likely to
see local gusts to 35 MPH.

Unsettled weather will remain in the cards through the end of
next week as the zonal flow keeps the moist air in place. CPC 6 to
10 and 8 to 14 day outlooks (out to December 22nd) both favor
above normal rainfall. In fact, their outlooks are trending wetter
compared to the previous versions.


.AVIATION...As of 3:55 PM PST Thursday...Moist onshore flow will
continue through the forecast period. Most shower activity will be
confined to the coastal mountain ranges, although isolated showers
may impact the terminals from time to time. More widespread rain
is due in by late Friday. In the meantime, look for continued
widespread MVFR/IFR ceilings along with locally reduced
visibilities. Winds will mostly be light southerly. 

Vicinity of KSFO...MVFR ceilings forecast through the evening
hours. IFR ceilings possible later tonight and into Friday
morning, along with MVFR visibilities. Showers expected in the
vicinity of the terminal through much of the forecast period and a
few brief light showers may reach the terminal from time to time.
More widespread rain will likely develop late in the forecast
period. Winds light from the south and southeast through the
period. Forecast confidence is low to moderate.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR ceilings forecast through the
period with MVFR visibility possible later tonight and into Friday
morning. Rain showers in the vicinity overnight, becoming more
likely by late Friday morning. Winds light from the south and
southeast. Forecast confidence low to moderate.

&& of 09:41 AM PST Thursday...A warm front will continue
to move through the region this morning. Strong and gusty south
to southeast winds will gradually weaken today and veer west. The
strongest winds are expected along the coast today. A dominate
westerly swell train will move across the waters today through the
remainder of the work week.





National Weather Service Glossary of Abbreviations