Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Eureka California
755 PM PST Tuesday Nov 24 2015
Synopsis...an upper level trough will continue to bring showers
across portions of northwest California tonight. Dry weather will
return Wednesday with cold weather persisting through the week.
Update...showers are now limited to areas of Humboldt County
south of Eureka and in Mendocino County as a low pressure system
moves south over northwest California. Model guidance indicates
the showers will continue to shift south overnight in advance of
the low. Weak instability will allow some stronger showers to
develop with small hail and isolated lightning possible in
Mendocino County tonight.
As the upper low shifts south there will remain a slight chance of
showers elsewhere across the region...though most areas north and
east of the current area of showers will remain dry. Thus the
Winter Weather Advisory for the upper Trinity zone has been
cancelled early. The north coast interior advisory will likely
have to be cancelled early as well once showers move completely
out of Humboldt County in a few hours.
Previous discussion......issued at 310 PM PST...
Short term...the upper level trough moving over the area
continues to bring rain and snow showers to the area. The NAM is
showing another burst of showers late in the afternoon and
evening as the upper level low moves overhead. Snow levels are
currently around 2500 feet. These will continue to drop tonight as
the colder air filters in. They are expected to bottom out around
1500 feet...but accumulating snow will likely remain above 2000
feet. Hail has been reported in many of the showers along the
coast with some of the heavier showers. The biggest has been dime
sized...but in some places it has covered the roadway.
Temperatures will be chilly tonight once the showers taper off and
in between showers. Near freezing temperatures are expected at the
coast with some areas of freezing temperatures in the inland
Wednesday there may be a few lingering showers in the inland
areas...but little to no additional rain and snow accumulation is
expected. Highs will remain in the low 50s at the coast and in the
40s in the inland areas. The coldest night is expected to be
Wednesday night into Thursday morning. Temperatures will likely be
near to slightly below freezing at the coast. The inland areas
will see temperatures as low as 20 degrees in the cold spots that
remain clear. There are still some questions on low clouds and fog
in the interior valleys...but for now it looks like they will
remain clear. Offshore flow is expected to keep the coast mainly
clear. Have issued a freeze watch for the Mendocino coast and
Anderson Valley for Thursday morning. The coldest temperatures
will be in the Anderson Valley...but the coast may see some
frost. Elsewhere the growing season has ended and there has been a
killing freeze so will not be issuing any other freeze warnings.
Highs will warm into the upper 40s to low 50s for Thursday
afternoon with most areas being clear. Temperatures will warm
slightly for Friday and Friday night...but only by a couple of
degrees. Mainly clear skies are expected. Mkk
Long term (saturday through tuesday)...models continue to be in
decent agreement with maintaining a broad upper low over the Great
Basin on Sat...leaving northwest California in northeasterly cyclonic flow aloft.
Models continue to diverge on weak perturbations pin-Wheeling around
the backside of the low center...so maintained a slight chance of
showers...mainly across eastern Trinity and Mendocino counties.
Overall the weekend continues to look dry but cold as a light
offshore wind flow pattern holds over the region. Both the GFS and
European model (ecmwf) indicate a ridge transitioning over the area on Sunday as the
low heads eastward and another storm approaches 130w. Both models
indicate westerly flow undercutting the ridge early to middle next
week. However both diverge on the timing and track of a short wave
Monday into Tuesday. The European model (ecmwf) has a southerly track and takes the storm
into central/Southern California while the GFS maintains a northerly
track. At this point there is too much variability and spread to
nail things down...so will maintain average confidence precipitation
chances across much of the area. It will most likely remain fairly
cold with a slow warm up through the period. There is a potential
for weak over-running early to middle next before warmer air mixes into
the interior valleys.
Aviation...a cold and unstable air mass will continue to spread
across the region this evening generating occasional showers and
MVFR conditions. There is a possibility of thunderstorms some of
which may produce small hail and brief IFR conditions. Once the low
pressure system exits to our southeast, we expect to see clearing
skies overnight as light offshore winds develop at kcec and kacv.
At kuki gusty north-northwest winds should fall off overnight and become calm
late tonight. Tomorrow VFR conditions should prevail across northwest California
after morning low clouds and fog burn off in the interior river
Marine...northerly winds will ramp again tonight in the wake of
a Post frontal trough. The air mass will remain cold and unstable
and the potential for strong northerly gusts will persist tonight.
Gradients tighten up on Wednesday and north winds will once again be on
the uptick...mainly in the outer waters. Seas have been increasing
through the day...near 15 feet at 13 seconds. Offshore buoy #2 has
been up to 25 to 29 feet at 13 seconds as of 2 PM. Granted winds
were howling from the north-northwest at 33kt with gusts to 45kt. Ascat pass
from this morning indicated a broad swath of north winds around 40
knots over Oregon offshore waters. Most of the wave energy will
probably parallel the coastline tonight and remain primarily over
the outer waters. Seas may briefly hit 16 to 18 feet inside
10nm...but for now will hold off on hoisting a warning for seas.
Conditions should start to ease up on Thursday as an offshore wind flow
pattern develops. Wind should remain fairly low on Friday and
Sat...with mix bag of fairly small short and long period wave
groups. Another storm will begin to under-cut an offshore ridge
late in the weekend into early next week. Confidence on the timing
and magnitude is not high at this time.
Winter Weather Advisory until 2 am PST Wednesday above 2000 feet
Freeze watch from late Wednesday night through Thursday morning
Northwest California coastal waters...
Small Craft Advisory until 3 am PST Thursday for pzz455.
Gale Warning until 9 PM PST Wednesday for pzz470-475.
Small Craft Advisory until 3 am PST Thursday for pzz450.
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