Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Eureka California
339 PM PDT Tuesday Sep 30 2014
Synopsis....a drying and warming trend will begin today and
continue through the end of the week as a strong ridge of high
pressure builds across the region.
Short term...in the wake of the cold front that moved passed the
area last night gusty winds have set up along the coast and skies
have mostly clear. Tonight with clear skies and cool temperatures
aloft surface temperatures will drop to some of the coldest so far
this fall. Gusty winds will persist over the ridgetops...but the
interior valleys should decouple tonight once the sun GOES down.
Most valleys will drop into the 40s but Trinity County should see
a few of the coldest valleys drop into the middle 30's which will
allow some frost to form. Confidence is low on this as dewpoints
have not dropped yet. With the drier air and subsidence dewpoints
are expected to drop rapidly this afternoon and evening. Also the
winds will need to decouple in the valleys and this cause a few
areas to stay warmer than expected. Went ahead and issued the
frost advisory to get the word out even though confidence is low.
Along the coast there may be a few areas of patchy fog...although
this is expected to be limited to around Humboldt and Arcata Bay
as well as the Eel River Valley. High pressure and offshore flow
should keep the rest of the coast clear.
Offshore flow and high pressure should persist through Thursday.
Temperatures inland will warm a bit each day. Thursday morning
will likely be a few degrees warmer than Wednesday morning as the
airmass aloft warms significantly. Mkk
Long term (fri through tue)...upper level ridging will dominate the
weather across northwest California through the weekend with dry
conditions...mostly clear skies...and above normal temperatures.
The warmest day looks to be Friday with high temperatures across
interior valleys a good 5 to 10 degrees above normal for this time
of year. Temperatures will gradually cool towards more seasonal
values by Monday and Tuesday. Models continue to indicate dry
conditions through at least the middle of the coming week as
ridging persists over the West Coast. Stp
Aviation....morning cloud cover quickly scattered making for a
mostly sunny day across northern California. Afternoon winds became
gusty as the offshore ridge and thermal trough along the California
coast increased the pressure gradient. Due to this setup, offshore
flow will develop overnight keeping northwest California mostly clear
through tomorrow morning. Some ground fog may develop around
Humboldt Bay early Wednesday morning otherwise expect VFR to prevail
through the taf period. The only aviation concern is the potential
for low level wind shear as the winds decouple overnight making
winds light near the surface and gusty aloft. \Kml
Marine...northerly winds increased substantially this afternoon as
an offshore ridge interacted with a thermal trough along the
California coast and strengthened the pressure gradient. The 18z
ascat pass showed gale force winds south of Cape Mendocino. There
was only a sliver of ascat data available for the northern waters
which showed winds in lower to middle 20 knots range. The hazardous seas
warning was continued for the northern outer waters as gale force
winds are expected to remain marginal. Never the less, seas will
remain steep and winds will be gusty through Wednesday afternoon.
Conditions will ease starting Thursday through the weekend as an
approaching low pressure system causes the gradient to relax. Over
the weekend two separate long period wave systems will impact the
coastal waters...one out of the northwest and the other out of the SW.
Otherwise the forecast remains well on track with good consistency
in the models and little changes were made in the extended
Fire weather...offshore flow resulting from building high
pressure in the Pacific northwest and a strengthening thermal
trough over central California has resulted in gusty
northeasterly winds in the higher elevations of northwest
California and this is expected to continue through Wednesday
morning. Additionally...the northeast flow will allow much drier
air to filter into the area. This will result in poor relative humidity
recoveries on ridgetops each night and low relative humidity values across the
interior falling into the teens or possibly single digits during
the afternoon hours Wednesday through the end of the week. With
recent rainfall likely reducing the receptiveness of fuels...the
potential for rapid fire growth is currently reduced. Thus only a
headline for the wind and low relative humidity values will be included in the
fire weather forecast at this time. Mkk/rpa
Northwest California coastal waters...
Small Craft Advisory until 3 am PDT Wednesday for pzz450-455.
Small Craft Advisory until 9 am PDT Thursday for pzz470.
Hazardous seas warning until 3 am PDT Thursday for pzz470.
Small Craft Advisory until 3 PM PDT this afternoon for pzz475.
Gale Warning until 9 am PDT Wednesday for pzz475.
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