Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1102 PM PDT Wednesday Apr 1 2015
Synopsis...an upper level ridge will build over the area Thursday
bringing drier and slightly warmer weather through Friday. An upper
level trough will bring increasing clouds to the area Saturday
with a chance of rain mainly over the North Bay on Sunday. Cool
weather is expected for next week with a slight chance for rain
Discussion...as of 9:00 PM PDT Wednesday...an upper level
disturbance brought breezy conditions to the area today. High
temperatures today were a few degrees cooler than yesterday but
were still near normal.
A cool and drier airmass will settle over the area tonight as an
upper level ridge starts to build off the coast and surface high
develops over the Great Basin. Winds are expected to decrease
late tonight and may lead to patchy frost in the coldest wind-
sheltered areas. There will still be enough winds in the North
Bay to keep most areas in the upper 30s to low 40s but a few of
the wind-sheltered areas may drop into the middle 30s and there may
be very patchy frost in the early morning hours. Winds in
southern Monterey and San Benito counties are expected to decrease
enough for temperatures to drop to the low 30s in the coldest
spots. Given that it is now April and there may be some budding
agricultural crops...a frost advisory has been issued for these
areas from 3 am to 8 am Thursday morning.
The upper high and offshore flow will lead to slightly warmer
temperatures over the area Thursday. The ridge flattens out on
Friday but west to southwest flow will keep temperatures about the
same. An upper level low from the Gulf of Alaska drops into the
Pacific northwest coast Saturday and moves inland Sunday. The
European model (ecmwf) NAM and GFS models show most of the moisture concentrated
from the North Bay northward but the 18z GFS brought light
amounts of rain down to the mry Bay area. As a result the forecast
calls for a chance of rain showers in the North Bay Sunday and a
slight chance over the sfo and mry Bay areas. Temperatures will be
cooler this weekend due to the increased clouds and lowering 500
mb heights. It is too early to determine rainfall amounts but they
will not be very large.
Medium range models differ in terms of rain chances for next
week. European model (ecmwf) indicates chance of rain midweek and next weekend
while GFS has chance of rain next weekend only.
Aviation...as of 10:45 PM PDT Wednesday...a cold and dry upper
level trough will move east reaching the Great Basin after midnight.
Plenty of drying is forecasted behind the trough within the middle-upper
levels of the atmosphere tonight which is much more ideal for surface
wind de-coupling. The acv-sfo and uki-sts pressure gradients are
7.1 mb and 2.9 mb respectively but despite a strong north-S gradient
not seeing a whole lot of wind at ksts right now...north-northwest wind at 13
knots. Other area terminals are reporting less wind. Moderate to
high confidence VFR for the period.
Vicinity of ksfo...VFR. Westerly winds have decreased appreciably
through the evening. Recent observation show 260 degrees at 8 knots. A
weaker sfo-SAC gradient is predicted for Thursday afternoon per
the 00z NAM model...but still could see a few middle-late afternoon
west-northwest gusts approaching 20-22 knots.
Sfo bridge approach...similar to ksfo.
Monterey Bay area terminals...winds have subsided appreciably
through the evening. VFR.
Marine...as of 8:33 PM PDT Wednesday...strong and gusty northwest
winds will for much of the forecast. Local gale force gusts will
be possible...especially the Big Sur coast. The gusty winds will
cause steeper wind waves and hazardous conditions.
... Advisory...southern Salinas valley and San Benito
County and interior Monterey County.
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm until 5 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm until 5 am
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm
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...rough bar advisory for sf bar until 3 am
Small Craft Advisory...Monterey Bay until 11 PM
Public forecast: west pi
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