Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 231152

Area Forecast Discussion
National Weather Service San Francisco Bay Area
452 AM PDT Mon Apr 23 2018

.SYNOPSIS...Dry and seasonably warm conditions will continue
today. A cooling trend is forecast from Tuesday through the end of
the week as an area of low pressure offshore slowly approaches the
coast. This low pressure may produce scattered showers from late
Friday into Saturday, mainly across the north.


.DISCUSSION...As of 3:30 AM PDT Monday...Skies remain mostly
clear early this morning, except for thin high clouds over the San
Francisco Bay Area and patchy low clouds around Monterey Bay.
Onshore gradients are weak, and so patchy low clouds will likely
remain confined to coastal areas through mid morning. Weather
conditions today are expected to be much like yesterday as an
upper ridge persists over California for one more day. Expect
mostly sunny skies today along with light winds. Afternoon highs
will once again be about 10 degrees warmer than normal.

Early morning satellite shows an upper trough deepening over the
eastern Pacific along 150W. Models agree that a cutoff low will
develop near 40N/140W by late tonight. This low is then forecast
to slowly approach the northern California coast through the
remainder of the work week, resulting in a cooling trend starting
tomorrow and continuing through the week. Temperatures are
forecast to cool to near normal are slightly below normal by
midweek. Night and morning low clouds are expected to become more
extensive by Tuesday morning, but will then likely mix out later
in the week.

The models typically struggle with predicting the movement of
cutoff lows, especially during the spring months. With the
upcoming cutoff low the models generally agree that the low will
drift towards the CA/OR border through early Friday, but
thereafter the track of the low becomes less certain. Previous
runs of the ECMWF indicated the low would move quickly off to the
ENE, which would keep all shower activity to our north. But the
latest ECMWF shows the low shifting towards the southeast once it
moves inland, thereby bringing shower chances to at least the
northern portion of our forecast area from late Friday into
Saturday. The GFS, on the other hand, maintains the low`s ENE
trajectory and keeps our area mostly dry. Yesterday, these two
models were predicting the opposite, with the GFS offering the
wetter solution and the ECMWF the drier. For now will maintain
shower chances at the end of the week, but have confined those
chances mostly to the northern portion of our area.

In the longer range, the models forecast a trough to move in from
the northwest late in the weekend and early next week, which will
keep temperatures on the cool side of normal through the end of
April and into the first couple of days of May. But once again,
differences regarding the track this trough takes means
uncertainty regarding precipitation chances.


.AVIATION...As of 4:48 AM PDT Monday...for 12Z TAFs...Mainly
clear skies will prevail today with the exception of high clouds
continuing to stream across the region. Patchy fog expected over
Monterey Bay terminals this morning with clearing by noon. Light
winds forecast through this morning with a slight seabreeze
kicking in this afternoon.

Vicinity of KSFO...VFR conditions will prevail through the period
with mainly high clouds. Light winds will increase to around 10
to 20 kt after 21z this afternoon.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR/LIFR cigs anticipated this morning
with clearing expected around 19z. Light and variable winds
will continue through this morning with moderate onshore flow in
the 10 to 15 kt range anticipated in the afternoon.

&& of 2:48 AM PDT Monday...Northwesterly winds will
ease through much of the week as the pressure gradient weakens
across the coastal waters. Light to moderate seas will prevail
through the forecast period.





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