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National Weather Service Area Forecast Discussion for Long Beach, WA USA

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Area Forecast Discussion
National Weather Service Portland, OR or
305 am pst wed feb 21 2018

.synopsis...cool weather stays over the region for this week under
a north flow aloft. a shortwave trough will provide another chance
for low-elevation snow wednesday night into early thu. additional
troughs will continue a threat for rain and low-elevation snow this
weekend into early next week.

&&

.short term...today through friday...yesterdays system was exiting
the forecast area early this morning, with radar indicating the
remaining areas of light snow generally limited to the oregon
cascades. today will be our "break" period between the shortwaves
dropping south through the region in northerly flow. the break
however will be short, as another short wave with origins in the
arctic of northern alaska, was seen in water vapor pictures moving
south out of the yukon into northern bc early this morning. this
yukon clipper is expected to bring mid level moisture with it as it
drops down the coast today and tonight. a weak surface low is
expected to develop in response to the trough and cold air running
over the warmer coastal waters by this evening, enhancing low level
onshore flow ahead (i.e. to the south) of the low. in addition to
injecting some low level moisture into the system, some moist low
level isentropic flow develops ahead of the low across western wa
and or, best seen along the 280k isentrope tonight. this increases
confidence in seeing some measurable precipitation out of this
system similar to the precipitation that developed tue during the
morning with similar low level isentropic lift, so will expand
categorical pops by late tonight to include the entire coast and
coast range, with likely pops reaching inland across the vally into
the cascade foothills. also similar to tue, snow levels may have a
little trouble dipping down to lowest elevations due to the onshore
flow ahead of the low, esp along the coast and in the southern half
of the willamette valley, although the later night arrival of the
precipitation may help to get the snow down to lowest elevations,
especially inland. general feel from spcs href ensembles is that
amounts from 1 to 4 inches are the most likely outcome with this
system, with heaviest amounts in the coast range due to the reliance
on the low level onshore flow for getting low level moisture and
lift into the system. at this point will hold off on any headlines,
as the coast is unlikely to see widespread accumulations due to
temps, expected coast range amounts seem likely to fall below
advisory criteria, and the willamette valley and i-5 corridor are on
the periphery of having enough confidence in seeing the measurable
precipitation needed for measurable snow.

gfs remains the fastest model in sending the upper shortwave and
surface low south of the area thu. past experience with these types
of weak surface lows that form in response to the cold air and warm
water interaction, is that a slightly slower movement and
dissolution like that seen in the nam may be the way to go. as such,
will be a little slower to to bring an end to the showers thu,
although by evening still expect to be able to relegate pops to the
cascades.

yet another system in the northerly flow aloft, drops down into the
region fri. models appear reasonably consistent in bringing this
system in more from the nnw, with a little better over water
trajectory and perhaps a little warmer of on air mass. as models
suggest, will spread pops in slowly fri, with the best pops still
over the nw half of the forecast area fri afternoon. will also
continue to indicate snow levels not quite so low as previous
systems, although still good for mixed precipitation below 1000 feet
in the inland valleys.

.long term...friday night through tuesday...no changes. previous
discussion follows.  stuck in a colder north to northwesterly flow
pattern that will continue to maintain pretty low snow levels
through the period.

the models show the system starting to drop down from the north on
friday will move through friday night, followed by trailing short
waves through saturday into saturday night. this series of short
waves, while quite cool, has a decent onshore flow with them with an
increasing southerly gradient across our forecast area. the net
result would be that this should begin to lift snow levels a bit off
the valley floors, probably closer to 1000 feet or so by later
friday night or saturday morning. the cascades, foothills and
coastal mountains could see pretty decent snow accumulations through
this time frame.

the next system drops in late sunday and sunday night, continuing
monday. the ecmwf has a surface low that would induce offshore flow
and probably drop snow levels back down near the valley floors, the
other models less certain of that. nevertheless, the snow levels
will continue to be quite low. precipitation is likely to ease by
tuesday but with snow levels continuing quite low. pt

&&

.aviation...drier air moving into the region should result in any
remaining flight restrictions turning vfr and remaining vfr into
early this evening. with that said, keug will likely stand the
best chance for mvfr and ifr conditions to hang around the
longest in the interim as gentle northerly flow upslopes into
the southern willamette valley. another weak surface low pressure
system will drop southeastward across the region tonight and
should result in snow spreading back across the area. this should
result in periods of ifr conditions at most taf sites after 06z
thursday.

pdx and approaches...drier air moving into the region should
result in predominantly vfr conditions until at least late this
evening. there is a good chance light snow will spread back over
the terminal and result in periods of ifr conditions after 06z
thursday. /neuman

&&

.marine...winds are dropping across the waters this morning as a
weak low pressure move inland. however, the next surface low
pressure will drop southeastward across the region wednesday
evening through early thursday. this should bring an additional
round of small craft advisory level wind gusts of 25 to 30 kt to
the waters. will let the current small craft advisories expire
later this morning before issuing any additional advisories to
cover this next round of wind.

another low pressure will drop southeastward out of the gulf of
alaska over the weekend. this should have a longer fetch of winds
that will likely drive seas well into the teens, but models have
wavered quite a bit on how high seas will peak and exactly when.
as a result, have dampened the peaks and troughs of model wave
height guidance and kept seas generally in the low to mid teens
in the extended for now. /neuman

&&

.pqr watches/warnings/advisories...
or...none.
wa...none.
pz...small craft advisory for winds until 7 am pst this morning for
     waters from cape shoalwater wa to florence or from 10 to
     60 nm.

&&


$$

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this discussion is for northwest oregon and southwest washington
from the cascade crest to 60 nautical miles offshore. this area is
commonly referred to as the cwa or forecast area.

Information courtesy of the National Weather Service - NWS Portland...
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