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

fxus66 kpqr 010316

Area Forecast Discussion
National Weather Service Portland, OR or
815 pm pdt tue mar 31 2020

updated: aviation discussion.

.synopsis...a complex upper low will continue to impact the forecast
area through wednesday. a cool and unstable air mass will lead to a
threat of thunderstorms wednesday, with small hail possible in
heavier showers. the cool and unsettled pattern continues thursday.
another upper level low is forecast to slide south over the eastern
pacific friday, well away from the coast. meanwhile, unsettled
northwest flow aloft will continue over washington and northwest
oregon. there may be a brief dry period saturday, followed by
another round of cool and unsettled conditions early next week.


.short term...tonight through friday...early afternoon water vapor
imagery revealed a large, complex upper level trough over the pacific
northwest and southern british columbia. the trough contains multiple
500 mb low pressure centers. the most notable low center for the
pacific northwest was located over the north tip of vancouver island.
solar heating working on the cold and unstable air mass has resulted
in multiple reports of small hail since late morning. as of 20z
lightning detection has not picked up any lightning strikes or
flashes over the cwa. cascade odot web cams continue to show packed
snow over the major highway passes. snotel and nwac avalache sensors
suggest 3 to 8 inches of snow have fallen since midnight. highest
amounts appear to be in the lane county cascades. holland meadows
snotel (4930 ft) showed about 8 inches since 09z.

small-scale impulses coming from the vancouver island upper low will
continue to rotate through southwest washington and northwest oregon
for the next couple of days. there will be some degree of orographic
enhancement into the cascades due to west low and mid-level flow.
although not very robust, on the order of 15-20 kt at 850 mb, this
should maintain frequent snow shower activity through early evening.
the nbm and 4km uw-wrf gfs suggest a total of up to 6 inches for the
cascades from this morning to 00z wed. have extended the snow
advisory through the 01z wed, even though the expected additional
amounts are not up to advisory criteria. the major cascade passes
will continue to be snowpacked.

snow levels early this afternoon are around 2000 to 2500 feet, based
on the astoria profiler and other webcams. the nam indicates 500 mb
temps of -30 to -34c across the northern third of the cwa this
afternoon. un-modified model sounding for kast shows some skinny cape
through the afternoon, with low-level lapse rates 7-8c. these
moderate lapse rates combined with some surface heating will result
in a small possibility of thunderstorms, mainly for the north half of
the forecast area. a more common occurrence will continue to be small
hail or graupel in heavier showers. shower activity will wane after
sunset with the loss of solar heating. however, snow levels tonight
and wed will remain on the low side, down to 1500 feet tonight
through wed morning. the upper low slides south tonight and wed,
which will provide extra dynamics for another active shower day wed.
the 15z sref 12-hr calibrated thunderstorm probability shows a 30-40
percent core over the south washington and far north oregon waters
late wed morning, with the 10 percent line extending to the north
willamette valley. by 00z thu, the 30 percent line is over the north
willamette valley and clark county in southwest washington. the
href-calibrated 4-hr thunder probability is not as robust, barely
showing a 20 percent area in the north willamette valley wed
afternoon. however, with the nam showing 500 temps of -32 to -34c and
850 mb temps hovering around -4 to -5c, lower and mid-level lapse
rates should be sufficient enough for modest instability.

the upper trough complex gradually shifts east wed through thu, with
the upper trough axis centered over the northern rockies by 18z thu.
however, the 12z models are in general agreement depicting another
vorticity lobe near 45n 128w by 18z thu. this feature swings inland
thu afternoon. the south end of the forecast area will be in the left
exit region of a 120-140 kt jet streak. thus, will need to maintain
at least 30-50 percent pops for much of the area, with likely pops in
sw washington in the colder air aloft and closer to the main upper

a complicated upper level pattern develops thu night and fri. the pac
nw will remain under the influence of the cold upper trough during
this period. however, models show another closed 500 mb low near 45n
133w fri. this low seems to separate from the main flow. the low
becomes an open trough late fri as it heads toward northern
california. the 12z operational model runs show a developing surface
low west of florence fri afternoon. the bulk of the over-running
precipitation is forecast to spread into sw oregon and nw california.
cannot rule out some of this reaching the south end of the forecast
area. meanwhile, the cyclonic northwest upper level flow will
maintain precip chances for the north half of the cwa as well.

.long term...friday night through tuesday...there is the potential
for a brief dry period late fri night through sat, especially based
off the gfs. it depicts a weak 500 mb transitory ridge sat. however,
the ecmwf maintains zonal flow sat and a higher probability of some
qpf. the ecmwf ensemble mean maintains below-normal 500 mb heights
over the area with general troughiness over western oregon and
northern california. the relative measure of predictability (rmop)
valid 00z sun is more in line with the ecwmf, with zonal flow and
moderate predictability. several gfs ensemble members suggest some
degree of troughiness over the area sat.

bigger model differences crop up sun as the gfs shows a deep closed
low well off the north california coast, while the ecmwf indicates a
long-wave trough over the area. only a handful of the gfs ensemble
members resemble the 12z operational run. most members are closer to
the ecmwf. thus, there is reasonable confidence that sunday looks
cool and unsettled. there is hope for some improvement mon as the
ecmwf ensemble mean shows rising 500 mb heights and some degree of
500 mb ridging inside 130w. all in all, daytime temps will remain
below normal throughout the long range period. weishaar


.aviation...westerly winds and post-cold frontal showers will
continue into the overnight hours and through wednesday morning.
coastal sites should expect a slight chance of thunderstorms to
continue overnight. inland sights will get relief from
thunderstorms until roughly 18z wed when activity will start to
pick up again. vfr conditions are expected to continue through
07z wednesday throughout the forecast area. ceilings are expected
to lower to mvfr criteria after 07 to 09z wednesday. the system
is expected to be out of the area by 02/00z.

.pdx and approaches...vfr conditions will continue through 08z
wed. ceilings will drop to mvfr criteria after 09z with light
rain. thunderstorm activity will likely pickup after 18z
wednesday and continue through the evening. -bphillips


.marine...winds gusting to 20-25 kt range through the overnight
hours as a trough moves across the waters with the highest gusts
beyond 30nm. winds ease on wednesday as a weak low moves over
the northern or waters and then inland, which will cause the
winds to weaken and shift to a more northwesterly direction. high
pressure will strengthen offshore wednesday afternoon, but will
be weakened as a series of weak troughs drop south over the
waters through late in the week. models continue to show a
developing low undercutting this weak ridge and approach from the
west on friday. this has the potential to impact our waters, but
guidance continues to suggest that the low will continue a
southeastward track towards the oregon/california border.

seas around 10 to 12 ft and gradually drop below 10 ft thru
wednesday. seas will continue to subside to around 4 to 6 ft
through the latter part of the week. /42


.pqr watches/warnings/advisories...
pz...small craft advisory until 5 am pdt wednesday for coastal
     waters from cape shoalwater wa to florence or out 60 nm.




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

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