Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
250 PM PST sun Feb 1 2015
Synopsis...high pressure will maintain dry and mild weather
across the region through the middle of the week. A Pacific
weather system is expected to spread rain across our area starting
late Thursday and continuing into the weekend. The heaviest
amounts will be over the North Bay. Gusty winds are also likely
especially for northern coastal areas plus higher elevation spots
starting on Friday.
Discussion...as of 3:00 PM PST Sunday...although we are generally
sunny across the region today a reduced offshore surface component
has lead to some large drops in 24 hour temperatures with some
spots running 10 to 20 degrees colder. Highs will likely end up
generally in the 60s today. Outside of some potential fog over the
North Bay tonight no weather related issues are forecast. Most
spots will be in the 40s although cannot rule out some upper 30s
in sheltered North Bay spots.
The ridge of high pressure overhead will stay in control through
Wednesday with highs mostly in the middle 60s to the lower 70s. A few
middle 70s are possible in the warmest spots of Monterey County.
Most of the attention for the shift remains focused for Thursday
through the upcoming weekend as a wet west-southwest flow aloft
moves in as the ridge progresses well to the east. As the shift
occurs...moist Pacific air with precipitable water values approaching 1.5 inches will
stream into the coast and progress southward from the pacnw into
norcal starting on Thursday. The rain will then continue to
advance to the south and into our County Warning Area by Thursday night. By Friday
widespread rain will be across the North Bay with the big
question being just how far south the main band of rain will
advance. As the previous shift mentions any slight shift in the
focus of the main plume of moisture will greatly impact rainfall
amounts and coverage.
Rainfall amounts through Friday night from the rfc show as much as
2 to 4 inches over northern Sonoma County with generally 1 to 2
inches for the rest of the North Bay. Sf Bay region and SC
mountains are generally looking at a quarter to a half with
locally up to an inch. The rest of the area falls under a quarter
of an inch. Other guidance is more bullish with numbers. Cips
analog Page shows greater than a 60 percent chance of more than an
inch falling by Friday night across a good portion of the sf Bay
area. In addition...isentropic flow forecast to be 40 to 50 knots
throughout the 1.3 to 1.5 inch precipitable water region. Outside of rain
concerns...winds will also pick up as 925 mb speeds are forecast
to possibly exceed 60 knots. Biggest concern would be along the North
Bay coast plus higher elevation spots. Locations with
southwesterly exposures could really see gusty winds.
Rain will taper off during the day on Saturday before another
round is expected to move through late Saturday into Sunday as
another wave moves across. Similar to the first round...precipitable water values may
exceed 1.5 inches and isentropic flow is forecast to be in the 40
to 50 knots range. Could easily see a repeat of the rain values we
get for Thursday through Friday. Very difficult to try to Pin
Point locations for the second event as the margin of error for
the main focus of an atmospheric river 6 to 7 days out is quite
large especially considering the size of our County Warning Area.
Bottom line...wet weather is forecast to return to our area
starting the second half of Thursday. The best chance for rain
will be late Thursday through Friday along with late Saturday into
Sunday. At the current time the favored region is the North
Bay...however due to the even being a few days out there is a
potential for it to shift farther to the south.
Looking into next work week the European model (ecmwf) builds a ridge along the
coast which diverts the rain well to our north. The GFS builds a
weak ridge followed by another atmospheric river impacting norcal
and possibly our area late Tuesday through Wednesday.
Aviation...as of 9:49 am PST Sunday...for 18z tafs. Just
expecting high clouds today with VFR conditions at terminals with
light winds. Return of onshore flow will likely allow stratus to
reform by early Monday morning for Bay area terminals.
Vicinity of ksfo...nearby low clouds should erode through 20z with
only high clouds over the terminal this afternoon and evening. Low
clouds may return for Monday morning.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...just expecting high clouds this
afternoon and evening with no impact to terminals. Strong
southeast winds at ksns should ease by 20z.
Marine...as of 3:00 PM PST Sunday...light northwest wind and seas
through Tuesday under high pressure. On Wednesday winds will turn
southerly as a warm front approaches the region. Stronger
southerly winds will continue through the end of the week as the
front slowly passes north of the region while bringing periods of
rain and building seas. Winds will switch to SW later Friday into
Saturday behind the front.
Public forecast: Bell
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