Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 161235 CCA

National Weather Service Eureka CA
535 AM PDT Wed Aug 16 2017

.SYNOPSIS...High pressure building over the west coast will bring
warming to the interior for the rest of the week. An upper level
trough will knock down temperatures slighlty this weekend.


.DISCUSSION...Upper level ridging and associated subsidence will
build toward the west coast today resulting in a warming trend
that will last through at least Friday. Temperatures initially
will rise about 5 degrees above normal today. The ridge will nose
over the area on Thursday and high temperatures across the
interior valleys will warm another 5-10F. The heat risk will be
moderate across central and eastern Trinity county as well as
northeastern Mendocino county where high temperatures are expected
to peak around 100-105F. People should take the necessary
precautions to avoid heat related issues. Drink plenty of water
and stay cool.

The north coast should see warming also as an offshore wind flow
pattern suppresses the marine layer and limits the extent and
duration of stratus and fog. Lower 70`s for Eureka seems unlikely
with a distinct onshore component in the afternoon. The warming
air aloft will most likely result in a stronger but lower marine
inversion. Thus, suspect clouds confined to mostly the coastline.
The Mendocino coast on the other extreme probably will not see
very much clearing or warming today and Thursday as stratus
lingers along the coastline. High resolution models continue to
indicate a southerly wind reversal taking shape today. The
southerly flow will keep stratus locked onto most of the Mendo
coast through the day. Marine air will trickle into the Anderson
valley and perhaps into the Russian river valley reducing the
potential for 100F heat. It is not clear that stratus will round
the Cape Thu or Fri. Diurnal local stratus will still form north
of the Cape, especially around Humboldt bay and in the delta.

The warming trend will level off or reverse this weekend as an
upper level trough approaches the northwest California coast.
The trough will weaken and split with a cut-off low forming
nearby the forecast area by late in the weekend. We may be in
store for another round of interior showers and thunderstorms
early next week. The latest GFS continues to indicate wrap around
moisture as well as instability on Tuesday. The threat for
interior showers and wet storms may continue into Wed.


.AVIATION...The aviation forecast is a bit tricky this morning as
stratus is not as expansive as it has been the past couple of
nights. Satellite imagery shows that south of Cape Mendocino stratus
is hugging the coast a seeping through portions of the Noyo and
Navarro River valleys. North of the Cape, status is quite sparse.
Portions of the Eel River Valley, Mad River Valley, and Klamath
Rivers are showing stratus as well as Crescent City and Brookings.
Winds have been relatively calm for the past couple of hours so I
don`t think the stratus is going anywhere fast. The good news is
that since the stratus isn`t as deep we should see a good amount of
clearing today. IFR to LIFR conditions are forecast for coastal
terminals north of Cape Mendocino through mid to late morning with
clearing conditions expected for the afternoon. Interior areas will
continue to see VFR conditions prevail.


.MARINE...North winds will increase today as high pressure takes
control of the forecast and tightens the pressure gradient over the
waters. The strongest winds will be found over the northern outer
waters (zone 470) with gale force gusts (35 to 40 kt) starting
this afternoon and then continuing through Thursday morning. A
Gale Warning has been issued in response to this with Small Craft
Advisories happening over the other marine zones. Looking past
Thursday morning, winds are forecast to hold at Small Craft
Advisory levels into Friday before increasing over the weekend.

In addition to the persistent northerly flow regime, steep waves are
forecast to respond to the winds over the forecast period. The
largest waves are expected to develop on today and then again over
the weekend. A moderately sized northwest swell will propagate
through the waters over the next couple of days.


.FIRE WEATHER...RH`s were in the lower to mid 30`s across the
higher elevations of zones 203 and 204 early this morning.
Easterly winds so far have been gusting to around 25 mph in these
zones. Even across the higher elevations (about 2500 feet) of
zones 201, 211 and 212 early morning RH`s were running in the
upper 30`s and 40`s. Thus far the easterly winds have been light
in these zones, though suspect the SchoolHouse RAWS will pick up
toward daybreak. The offshore wind flow should let up by early
afternoon, then kick back in tonight. Thus expect a near carbon
copy tonight with slightly lower overnight RH`s as the air mass
continues to warm through the night. For now will continue to
headline the forecast to call attention to this critical fire
weather situation. The gusty offshore winds and poor overnight
RH recoveries will probably repeat for the next 2 to 3 nights,
especially for zones 203 and 204.

A fairly strong ridge aloft will nose into the area today bringing
warmer temperatures and lower daytime RH`s. The ridge will build
over the area on Thu through Fri and high temperatures will
likely peak around 100-105F in the hottest interior valleys. Min
RH`s will bottom out around 10-25% each day for the next several
days. This is not too uncommon for mid to latter August. Main
impacts will be ongoing fires where perimeters poke into higher
elevations which may be influenced by the east winds and low
overnight humidities.

An upper level trough should knock down temperatures this weekend.
This trough may also generate a few mountain showers. Next week is
looking much more interesting for storms. The GFS continues to
show a cut-off low forming off the coast early next week. The
track and evolution of the upper low are still very uncertain as
the GFS changes around ever so slightly from one run to the next.
The GFS does indicate wrap around moisture and southeasterly flow
on Tuesday which seems to be the best day for wet storms at this
time. For now will indicate only a slight chance for Tuesday based
on the last several runs of the GFS. The ECMWF was not in perfect
sync with the GFS, but also indicates a trough over the area with
the best chances of storms east of NW California.


Small Craft Advisory until 5 AM PDT Thursday for PZZ450.
Small Craft Advisory until 5 AM PDT Friday for PZZ475.
Gale Warning from 2 PM today to 5 AM PDT Thursday for PZZ470.



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