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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
331 am pst mon mar 8 2021

.synopsis...low pressure dropping south offshore will keep a threat
of showers over the region through wednesday. upper level high
pressure is expected to bring drier weather for much of thursday
through at least early saturday. a trough will drop south from
alaska to bring precipitation chances back in the forecast at some
point on saturday, then cross the region through monday.

&&

.short term...today through wednesday night...upper level low
pressure, now centered about 450 miles off the vancouver island
coast, has moved a (sarcastically) whopping 150 miles south over the
last 24 hours. this has resulted in significant timing changes in
the forecast for the next few days. to begin with, the initial cold
front from yesterday morning has since fully occluded, wrapped
around the low, and since dissipated. there is now an elongated area
of unstable clouds stretching from vancouver island and south to off
the golden gate that we suppose would best be called a developing
cold front. have seen plenty of lightning activity both along the
north end of the front off the olympic peninsula as well as west of
the front and rotating around the base of the closed low.

models seem to have adapted to this new orientation and slower
movement to the low. the new quasi-frontal boundary does appear to
stay positioned just off the coast per the models, keeping most of
the deeper convection with it. as such, am following their lead by
maintaining showers over the broad area and also introducing a
threat for thunderstorms mostly over our coastal waters today with
storms continuing into tonight. upper dynamic support for the over
land showers seems much weaker than yesterday with the expectation
that showers will be weaker and hail much more difficult to come by.
the exception may be the coast south of newport where the quasi-
front could drift slightly further east this afternoon and evening
to bring stronger showers and maybe a thunderstorm.

tonight will again see the land showers dissipate in the evening as
the sunsets, however, the upper low is currently progged to drift a
little closer to shore as it makes a roughly 300 mile move to the
south-southeast. this position again brings the cwa under better
diffluent flow aloft with general shower activity potentially
returning overnight along with a continued threat for nocturnal
thunderstorms over the waters and along the central coast as a
shortwave rotates northward overnight.

tuesday will see the upper low center drift to off the sw oregon
coast. usually a low becoming centered this far south means our area
is out of the deeper convection threat. am hesitant to fully buy in
on that notion though as the low also appears to be elongating to
the north as the center drifts south keeping diffluent flow
overhead. this may be a case where the central and south valley see
southerly flow and unstable conditions while the pdx metro and
northward areas start showery and then somewhat stabilize with time
tuesday.

wednesday has a new wrinkle as the upper low will now be much closer
to the south than was indicated 24 hours ago. additionally, an upper
trough with ties to the canadian arctic will be pushing southwest
across alberta. this will destabilize the region again as it ties in
to some of the lingering moisture rotating north around the still
nearby closed low. given multiple models have latched onto this,
felt compelled to expand the shower coverage wednesday. again,
showers likely dissipate near sunset.

back to this morning, areas of low stratus have developed from
radiational cooling given the breaks in higher clouds. suspect this
will only slightly delay the onset of showers today. however, do
have concern this stratus will settle closer to the surface and
result in patchy dense fog. will continue to monitor as adjust as
needed. do expect this scenario to potentially play out each night
through the next few nights, especially assuming showers continue
each day as expected. /jbonk

.long term...thursday through sunday...main change is again an
overall delay to the dry period which will now see some lower end
pops appear on a 24 hour delay to thursday. have added showers for
mainly lane county end of the forecast area, although, these showers
may not even make it to the afternoon. elsewhere, the dry period is
expected to begin as high pressure gets pinched over our area
between another trough dropping south from alaska and north of the
departing closed low to our south.

operational models are now all in agreement as the ecmwf compromised
toward the gfs solution with the approaching trough now slated to
arrive sometime saturday late afternoon or night. this trough will
be substantially cooler than what we`ve seen with the current upper
low. nbm snow levels based on wide ranging and tuned ensemble data
brought overnight snow levels as low as 500 feet msl. the operational
runs, however, barely fall below 1500 feet during overnight hours
sunday and monday. for now, have taken the nbm guidance but
artificially boosted the low end overnight values closer to 1500
feet while keeping the daytime 2000-2500 ft values as is.
regardless, the lowest snow levels are slated to occur after the
initial cold front and heaviest precipitation occurs. it should be
said, the cascades, higher foothills, and higher coast range appear
to possibly receive several inches of snow sometime between late
saturday night and sunday night, depending on the specific frontal
arrival timing. /jbonk

&&

.aviation...at 11z monday, high clouds with cigs above 10 kft
were occurring across northwest or and far southwest wa. patchy
fog and low stratus with cigs in the 300-500 ft range had formed
over the cowlitz valley where high clouds are thinner. low
stratus and fog has also moved down parts of the lower columbia
into vancouver and kpdx early this morning. fog and low stratus
should scatter out by 16z monday and give way to vfr flight conditions
through monday evening. the main exception will be with passing
showers expected this afternoon and evening, which will bring the
potential for brief periods of mvfr cigs and lowered
visibilities. there is also a slight chance of thunderstorms this
evening into tonight, mainly along the central or coast. if a
thunderstorm does affect a taf site, it would most likely be at
konp. stronger showers or storms will have the potential to
produce small hail and brief gusty winds. confidence in exact
timing for potential storms is too low to include a mention of it
in the 12z tafs at this time. model guidance suggests cigs will
lower to 2000-3500 ft monday night into tuesday morning, but
cloud cover should be scattered to broken rather than overcast.

for detailed regional pac nw aviation weather information, go
online to: https://weather.gov/zse

kpdx and approaches...fog early monday morning with broken cigs
around 300 ft, becoming vfr by 16z. vfr should then continue
through at least monday evening, with cigs potentially lowering
to mvfr monday night. scattered showers this afternoon and
evening will also bring the potential for brief periods of mvfr
cigs. small hail and brief gusty winds cannot be ruled out with
stronger showers. -tk

&&

.marine...westerly swell will remain elevated early this morning
between 12 and 18 ft, highest over the outer waters. expect seas
to gradually decrease during the mid to late morning hours, falling
to 10 to 12 ft by mid afternoon. despite falling seas, southerly
winds should increase a bit late this evening into tonight with
gusts to 30 kt expected over the outer waters and gusts to 25 kt
for the inner waters. therefore, will maintain a small craft
advisory for winds and seas through tuesday morning. in addition,
the threat for sneaker waves will remain high along the coastline
through monday afternoon.

model guidance suggests seas will fall to 7 to 9 ft tuesday night
with winds falling below 20 kt as weak high pressure begins to
settle in. high pressure is expected to strengthen wednesday into
friday, bringing a prolonged period of winds below 20 kt and seas
in the 5 to 9 ft range. benign conditions will continue into
saturday, before a cold front sweeps across the waters saturday
night into sunday. -tk

&&

.pqr watches/warnings/advisories...
or...none.
wa...none.
pz...small craft advisory until 10 am pst tuesday for coastal waters
     from cape shoalwater wa to florence or out 60 nm.

     small craft advisory until 6 pm pst monday for columbia river
     bar.

&&


$$

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