National Weather Service Raw Text Product

Displaying AFOS PIL: AFDMTR Received: 2017-12-07 06:22 UTC


721 
FXUS66 KMTR 070622
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1022 PM PST Wed Dec 6 2017

.SYNOPSIS...Locally gusty easterly winds are forecast in the San
Francisco Bay Area overnight, mainly in the Berkeley Hills and
also locally near the coast. Strong high pressure will remain 
entrenched along the west coast through the forecast period,
resulting in sunny days with above average temperatures and clear
and cool nights.

&&

.DISCUSSION...As of 8:55 PM PST Wednesday...Primary forecast 
concerns in the short-term center around locally gusty offshore 
winds tonight and Thursday morning, and potential for freezing 
temperatures in portions of our are late tonight and early 
Thursday. 

Strong surface high pressure persists over the Great Basin at at 8
pm this evening the pressure gradient from WMC to SFO stood at an
impressive 17 millibars. It should be noted that the bulk of that
gradient spanned the Sierra Nevada and only 1.3 mb of offshore 
gradient existed between SAC and SFO. In any case, locally gusty 
winds have already developed in portions of our area with winds 
currently gusting as high as 35 mph in the Berkeley Hills. Would 
expect winds in the hills to increase further overnight as the 
nocturnal inversion develops. The 00Z WRF does show an increase in
wind speeds overnight and into Thursday morning across the North 
and East Bay Hills as well as locally across southwest Marin 
County (near Stinson Beach) and along portions of the San Mateo 
County Coast. One complicating factor in the short-term wind 
forecast is the passage of an elongated vort max from east to west
across our forecast area by late this evening. After this feature
passes, the 00Z NAM shows 925 mb winds shifting to the south and 
decreasing. Thus, this does not have all the aspects of a standard
offshore wind event where 925 mb winds increase in the North and 
East Bay Hills through the night. The 00Z WRF forecasts localized 
pockets of strong and gusty winds overnight and into Thursday 
morning in the Berkeley Hills, across southwest Marin County, and
along portions of the San Mateo County Coast. A few locations may
reach wind advisory levels for a few hours, but this does not 
appear to be a widespread or prolonged wind event. Have therefore 
decided to refrain from issuing a wind advisory and instead have 
issued a Hazardous Weather Outlook which highlights the locally 
gusty winds. Winds will subside in all areas by Thursday 
afternoon.

8 pm temperatures are generally warmer compared to 24 hours ago
and surface dewpoints are generally higher. This would suggest
overnight lows tonight will not be as cold as last night,
particularly in areas where gusty offshore winds are able to mix
down into the valleys. However, some locations are currently a 
bit cooler and drier compared to 24 hours ago. Given the
conflicting trends in current observations, it might be best to
rely on model guidance which suggests that most locations will be
slightly warmer than last night. But even with slightly warmer
temperatures tonight, widespread freezing temps will still likely
occur in the interior valleys of southern Monterey county given 
that the southern Salinas Valley town of Bradley recorded a low of
only 20 degrees this morning. In other words, even with a slight 
moderation in temperatures tonight, freezing temps would still 
occur in southern interior Monterey County. Earlier this evening a
Freeze Warning was issued for the southern valleys of interior 
Monterey County, including the southern Salinas Valley. The freeze
warning is in effect from 3-9 am Thursday.

All models continue to maintain a high amplitude upper ridge along
the West Coast through at least the next 10 days. This means
continued dry weather with sunny mild days and clear and cool
nights. 

&&

.AVIATION...As of 10:22 PM PST Wednesday...VFR except thin patchy
fog is possible at KSTS early Thursday morning depending on wind 
behavior. Pilot reports indicate moderate to severe turbulence as 
low as 3,000-4,000 feet AGL over the Bay Area this evening. The 
WMC-SFO pressure gradient is strong at 16.3 mb and SAC-SFO gradient
is moderate at 2.2 mb. 

A stable, dry air mass with strengthening near surface temperature
inversions is tending to separate calm or light surface winds from
strong and gusty NE winds aloft. Strong and gusty winds aloft will
be primarily focusing over the Bay Area tonight and Thursday morning
with occasional mixing with surface winds particularly over the East
Bay and North Bay areas until 16z Thursday; low level wind shear is
continued until 16z in all Bay Area tafs except til 13z for KSJC as
vertical mixing increases between cold air drainage surface winds and
E-SE winds aloft, which should decrease the vertical shear.

Cold air drainage E-SE winds in the Salinas Valley will most likely
couple with increasing SE winds at 925 mb and 850 mb levels resulting
in gusty northern valley winds late tonight into Thursday morning.

Vicinity of KSFO...VFR. Low level wind shear til 16z Thursday.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR with clear skies. Generally light E-SE
winds at KMRY, but gusty E-SE winds northern Salinas Valley later
tonight and Thursday morning. 

&&

.MARINE...as of 9:27 PM PST Wednesday...High pressure continues along
the west coast. Gusty winds can be expected on the immediate coast,
especially near coastal gaps and passes, from Point Reyes through
Pigeon Point. Moderate period northwest seas.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Freeze Warning...CAZ516
             SCA...Pt Reyes to Pigeon Pt 0-10 nm

&&

$$

PUBLIC FORECAST: Dykema
AVIATION: Canepa
MARINE: AS

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