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

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Area Forecast Discussion
National Weather Service Portland, OR or
409 am pdt tue sep 26 2023

.synopsis...an upper level trough is beginning to move into the
pacific northwest today, as monday`s cold front slowly moves east
across wa/or. the steadiest precipitation has moved into the
cascades, with post-frontal showers and a few thunderstorms likely to
continue throughout the day. stronger thunderstorms from late morning
through this afternoon may be capable of producing gusty winds and
small hail. the next low will develop rapidly offshore today, move
across the coastal waters tonight, then move onshore into washington
early wed morning for another quick shot of wind and rain. cool fall
weather will follow as an upper trough settles over the region late
week, with showery weather persisting.

&&

.discussion...today through next monday...forecast remains largely on
track as an active early fall weather pattern persists over nw oregon
and sw washington. since early sunday morning, the lowlands and
cascades have generally received around 1 inch of rain, except for
the hood river valley which has received about a half inch. the coast
range has seen more, with the most being in western lane county where
3 inches have fallen on the 3 buttes fire. we have yet to hear of any
debris flow issues, so thus far this appears to be a beneficial,
soaking rain. peak wind gusts with the front were generally in the
35-45 mph range along the coast, with the strongest gust being 55 mph
at the highly exposed weather station in garibaldi. largely easterly
mslp gradients kept winds modest inland, with peak gusts 25-35 mph
for the willamette valley.

the very first hints of a baroclinic leaf and developing low pressure
are presently occurring between 130w-140w, roughly due west of the
mouth of the columbia river. this is as the core of the jet stream
presently over the ne pacific noses east of 140w, with the
development occurring in the left-front quadrant of the jet. with a
sharpening baroclinic zone and strong q-vector convergence, the low
appears to be in a fairly ideal position to develop rapidly over the
coming 18-24 hours. forecast models do rapidly develop a low offshore
this afternoon and evening, but most are keeping it weak and in the
995-1005 mb range, peaking before making landfall along the wa coast.
still feel that given the ingredients in play, these models are
underestimating the potential for development and that the low will
eventually turn out to be in the 985-995 mb range. a couple of 00z
href members agree now, showing a 988 mb low.

regardless of strength and track, models show high confidence that
the resulting low pressure will be compact. therefore strength and
position will be even more important than usual for determining the
wind threat with this system. with models and ensembles still showing
a good deal of variation in strength/track despite being within 24-36
hours from now, we will need to monitor closely how the low
develops/tracks today in order to get more clarity in whether or not
damaging winds are possible late tonight/wed morning. for now, the
most likely scenario appears to be gusts 45-55 mph along the coast
and 30-40 mph for inland areas. href probabilistic guidance shows the
chances of exceeding these ranges at around 20 percent, which
essentially equates to the chance of needing a high wind warning
along the coast and a wind advisory for the i-5 corridor and
willamette valley. based on the thoughts above, would still say the
chances are actually closer to 30-50%, but this is still insufficient
to warrant issuing a warning. again, will need to monitor
satellite/radar trends closely today to determine whether or not
these probabilities are in the ballpark.

as for qpf, the deterministic output from nbm v4.2 appears
reasonable. with the deepest moisture having shifted east of the
area, widespread heavy rain is not expected, nor or major hydrologic
issues. for the southern half of the forecast area, 00z href means
are generally 0.25-0.75 inch, with 0.50-2.00 inches across the
northern half. lowest amounts will tend to be across the inland
valleys, with the higher amounts in terrain, especially the coast
range and particularly the willapa hills. href 90th percentile qpf
shows 2-3 inches for the willapa hills and s wa cascades over the
next 24 hours, which could threaten minor flooding on the grays
river. otherwise, this appears to be another beneficial rain event as
the region plays catch-up from the dry start to september.

once the low moves onshore wed morning, we will transition to a
cool/showery pattern that will persist as an upper level trough
settles into the pac nw for the remainder of the week. with 00z eps
850 mb temperature inter-quartile range around +2 to +4 deg c for the
late week period, we may flirt with wet snow reaching the cascade
passes, but accumulating snows appear reserved for the higher ski
resort elevations above 5500 ft or so. for the lowlands, it will feel
like true autumn weather with chilly nights/mornings warming well
into the 60s or lower 70s each afternoon. frost is not anticipated at
this time, but given the air mass it is not difficult to imagine a
situation where our cooler outlying valleys have frost if skies clear
out wed night or thu night. cluster analyses beyond thursday suggest
a high likelihood of another trough digging into the northwestern usa
by next weekend, but it remains uncertain whether we will be under
the trough and showery, or under cool but drier northerly flow on the
western periphery of the upper trough. either way, temperatures
appear likely to remain below normal through the upcoming weekend.
weagle


&&

.aviation...12z tafs: showers continue today in post-frontal air
mass. isolated thunderstorms are possible through the day,
generally west of the cascades. afternoon thunderstorm may produce
small hail. flight conditions generally vfr but expect occasional
mvfr cigs and visibilities with heavier showers.

next system spreads rain and another round of breezy condition
across the area as a fast moving low approaches the north oregon
coast or south washington coast late tonight or early wednesday
morning.

kpdx and approaches...showers continue primarily vfr conditions.
brief mvfr cigs & visibilities can be expected through the day.
winds under 10 kt out of the southeast this morning, then veer
to more southerly in the afternoon with gust 15-20 kt. winds then
ease and back to southeast in the evening, then gradually increase
overnight.

/mh

&&

.marine...a low near haida gwaii at about 975 mb will continue to
weaken as it drifts northward over the next couple of days. of
interest is a compact, fast moving system that will develop
offshore an approach wa/or tonight. models are not in great
agreement with the path of the low, but ensembles generally place
the low somewhere on the central wa coast to north oregon coast at
5 am wednesday. this low is expected to produce gale force winds
across the waters tonight into early wednesday morning.

a hazardous seas warning in effect for all zones through early
this evening before the gale warnings come into effect. seas are
in the 16 to 20 ft range with a period of 13-15 seconds as of 2
am. seas will gradually subside, but the next storm will increase
the wind waves again, though not too significantly due to its
fast motion.

the weather pattern remain active for the latter half of the week,
but overall systems are expected to be much weaker, with winds
turning westerly and seas subsiding below 10 feet late wednesday
afternoon.

&&

.pqr watches/warnings/advisories...

or...none.
wa...none.
pz...hazardous seas warning until 8 pm pdt this evening for coastal
     waters from cape shoalwater wa to florence or out 10
     nm-columbia river bar-coastal waters from cape shoalwater
     wa to florence or from 10 to 60 nm.

     gale warning from 8 pm this evening to 8 am pdt wednesday for
     coastal waters from cape shoalwater wa to florence or out
     60 nm.
&&

$$

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