Area forecast discussion...corrected
National Weather Service Eureka California
1130 PM PDT Monday may 20 2013
Synopsis... much colder air will arrive for the rest of the week
in the wake of a cold front. Areas along the coast will notice
little difference in temperatures but interior areas will be 15 to
20 degrees colder than Monday. Showers will be possible through
Thursday north of Cape Mendocino.
&&
Update...updated the marine grids to increase winds in the
northern outer waters. The RUC/rap and the hrr3km seem to show
gale force winds over the northern outer waters. This is a very
dynamic system with strong winds aloft. Opted to issue a Gale
Warning for the northern outer waters starting Tuesday morning when
the surface low moves by just the north of the area. This system
will also bring strong waves. The Swan model shows them peaking
around 13 feet in the outer waters. These may spread into the other
waters...however currently they are just below the thresholds for
hazardous seas so have issued small craft advisories. Mkk
&&
Previous discussion... /issued 407 PM PDT Monday may 20 2013/
Discussion...
in the near-term localized stratus between the Klamath and Eel River
Delta may tend to gradually increase in coverage this evening as the
flow veers and becomes more directly onshore this evening. This will
bring increasing moisture into the lowest 5-10kft of the atmosphere
overnight as a tightly wound area of low pressure approaches from
the northwest. While this system lacks much in the way of a return
flow ahead of it...increasingly moist onshore flow and instability
with colder air aloft arriving in the wake of the front will be
enough to keep showers going for much of the Tuesday through
Thursday time period. It will be tricky timing individual
disturbances rotating around the parent upper-low...but the best
chances of showers look to be Tuesday morning...and during the day
Wednesday and Thursday. Actually Tuesday morning may be more of a
period of light rain and drizzle...with clearing skies and widely
scattered shower activity during the afternoon as the lower
atmosphere becomes more unstable and well-mixed.
Snow-levels will fall to as low as 2500 feet across much of
northwest California especially north of Mendocino County. Not much
in the way of precipitation is forecast by the various model
guidance...but as is often the case with these dynamic systems...the
models are probably underdone. That said, with the best shot at
snowfall occurring during the daytime after a period of warm
temperatures...it will be tough to see much accumulation on roadways
and thus no impacts are expected to the major passes. Snow showers
will be possible over some of the 299 passes especially Wednesday
morning but expect little to no accumulation below 3500 feet. Above
that...would not be surprised to see a few inches in spots.
Depending on how skies clear Wednesday and Thursday
morning...freezing temperatures will be a distinct possibility for
the interior. Issued a freeze watch for interior mendo and Trinity
for Wednesday morning...but actually Thursday morning looks like the
best shot at a widespread freeze. Frost advisories may be needed for
the northcoast interior as well.
Looks like there will be a lull in the weather for later Thursday
through Friday night...until another trough arrives for the weekend.
This system does not look as moist or as dynamic...but with warming
low-level temperatures ahead of it there may be thunderstorms over the
interior mountains Saturday afternoon. Aad
Aviation...almost a clear northwest California as of 21z except for a few
pockets of stratus north of Cape Mendocino. I expect there is still
time for the remaining stratus to completely scour out, though,
models and guidance hint at a return of coastal clouds after the sun
sets this evening. The approach of the upper level trough will allow
ceilings to expand overnight into the MVFR range for coastal
airfields tonight. High cloud cover will flow over region but will
otherwise remain VFR for inland airfields. Kml
Marine...winds and seas will decrease overnight as the pressure
gradient weakens due to an approaching upper level trough. Hazards
will drop in the evening as conditions lie down, though, extensions
for small crafts may be needed. Either way there will be a short
period where winds and seas will briefly decrease...very early
Tuesday morning. Winds will begin increasing in the outer waters
Tuesday afternoon as the upper level trough draws a tightened
pressure gradient into the coastal waters. High pressure will hang
out over the eastern Pacific while the upper level trough will
linger over the Pacific northwest over the next few days. This will
maintain fresh winds in the outer waters and steep seas into the
middle of the week. Kml
&&
Eka watches/warnings/advisories...
California...freeze watch from late Tuesday night through Wednesday morning
caz004-076.
Northwest California coastal waters...Small Craft Advisory from 2 PM Tuesday to 2 PM PDT Wednesday
pzz450.
Gale Warning from 6 am to 2 PM PDT Tuesday pzz470.
Small Craft Advisory until 3 am PDT Tuesday pzz455-475.
&&
$$
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