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000
FXUS66 KPQR 160443 AAB
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
942 PM PDT Mon Apr 15 2024

Updated aviation discussion...

.SYNOPSIS...Onshore flow will maintain plenty of clouds and cooler
temperatures across the region through Tuesday, along with a few
showers. High pressure builds across region for Wednesday to Friday,
with return of dry weather and warming temperatures. Bit more
unsettled for next weekend, with more clouds and perhaps even some
showers.

&&

.UPDATE...Tuesday night through Wednesday morning...Confidence has
increased for widespread frost across northwest OR and southwest WA
Tuesday night through 9am Wednesday. High pressure, clear skies, and
light winds will bring a perfect setup for radiational cooling,
allowing temps to dip into the low to mid 30s across all of northwest
OR and southwest WA. The exception looks to be the immediate
coastline where temps will likely stay in the upper 30s, but you
won`t have to go far inland for temps to dip into the mid 30s. In
fact, some inland coastal communities could fall below freezing (best
chance at Tillamook where the EPS mean is around 30 degrees for
overnight lows; seems reasonable given the setup). Given the
widespread frost expected, decided to issue a Frost Advisory for
southwest WA and northwest OR from the coast to the Cascades (except
for the high Cascades as we do not issue frost/freeze headlines for
these zones). The only exception is the Upper Hood River Valley zone
where a Freeze Warning has been issued for temps between 29-32F (this
zone looks to be coldest). Anyone with outdoor vegetation that is
sensitive to cold temperatures should bring their plants indoors if
possible, or take other appropriate actions to prevent frost damage.
-TK

&&

.SHORT TERM...Monday afternoon through Wednesday...Radar imagery
shows showers across NW Oregon and SW Washington this afternoon,
mainly north of Salem and over the Cascades as a weakening upper
trough continues slowly pushing into the region today, though main
core of the system remains well to the north over north Washington
into southern British Columbia. The system offshore is not all that
strong with rather zonal flow aloft off the Pacific into the Pacific
NW today into tonight. Rainfall amounts will be very meager with up
to 0.1 to 0.2 inch over the south Washington Cascades into the far
north Oregon Cascades. Likely to see only a trace to perhaps up to
0.05 inch for inland lowlands. Onshore flow will maintain plenty of
clouds today, with some gaps in the clouds from time to time.
Temperatures will be cooler today with highs in the mid to upper 50s
in the lowlands, slightly below mid-April normals.

Overall, not a lot of change for the region for Tuesday, as will
maintain moderate onshore flow with mostly zonal upper level flow.
The shower potential will be more limited to SW Washington as well
as the higher terrain north of Tillamook to Portland to Mt Hood
line. Surface ridge axis will be shifting northward slowly during
the afternoon. As such, would expect to see clouds breaking apart,
mainly over western Oregon. Would not be surprised if Salem
southward through the Eugene/Springfield area becoming partly to
mostly sunny in the afternoon.

High pressure offshore continues to strengthen on Tuesday night into
Wednesday. Air mass will be drier Tuesday night, and with clearing
skies and light winds, temperatures will drop into the lower to
middle 30s over most of the region away from the immediate coast.
Most locations are likely to see frost Tuesday night into Wednesday
morning. Otherwise, after a chilly start, should end up with rather
pleasant Wednesday, with sunny skies and a warming trend beginning
with temperatures 55 to 60 along the coast and 60 to 65 for most of
the interior lowlands east of the Coast Range/Willapa Hills. -
Rockey/HEC

.LONG TERM...(THURSDAY THROUGH SUNDAY)...WPC 500 mb cluster
analysis indicates that ensembles are in agreement that upper level
ridging will continue building over the eastern Pacific and shift
east to just off the West Coast Thursday into Friday. At the
surface, high pressure strengthens off the coast with a thermal
trough building north from California into western Oregon. Due to
this, sunny skies and a warming trend will continue across NW Oregon
and SW Washington. Inland valley temperatures will inch towards the
70s once again with NBM indicating 55-75% of temperatures exceeding
70 degrees on Thursday and a 70-85% probably of the same on Friday.
This pattern will also result in increased surface pressure
gradients over the Cascades and Columbia River Valley, producing
increased offshore (easterly) winds for areas near the Columbia
River and Cascades, strongest through the Columbia River Gorge. NBM
indicates roughly 50-70% probability of wind gusts exceeding 30 mph
Thursday and Friday for the Cascades and locations north of Salem.
Additionally, NBM indicates a 60-80% probability for wind gusts to
exceed 45 mph for the Columbia River Gorge Friday.

Beyond Friday, ensembles are still uncertain at the exact pattern.
About 30% of the 500 mb clusters indicate ridging will continue
through the weekend, which would allow for warm and dry conditions
to continue. About 50% of the clusters indicate ridging will begin
to weaken or even become more zonal flow, which would allow
temperatures to cool somewhat, potentially back to seasonal normals
(low 60s for inland locations, upper 50s for the coast). The other
20% or so of the clusters indicate weak troughing will impact the
region, bringing near to below normal temperatures and light
showers. No matter which solution winds up occurring, no impactful
weather is expected in the extended period. -HEC

&&

.AVIATION...VFR conditions prevail through the TAF period with mid
to high level cloud cover tonight. Winds will be light and from the
north/northwest through Tuesday morning. Onshore flow pressure
gradients tighten slightly early Tuesday afternoon, leading to
breezier NW winds with gusts up to 20 kt along the coast and parts
of the Willamette Valley. Skies begin to clear up more Tuesday
evening as high pressure builds and the air mass above dries up.

PDX AND APPROACHES...VFR thresholds prevail through the TAF period.
NW winds will be around 5-10 kt, strongest in the afternoon/evening.
                                                        -Alviz

&&

.MARINE... Late tonight through early Tuesday the region gets
brushed by an upper-level trough dropping into northeast Washington.
Midweek and continuing into the weekend, guidance continues to show
a ridging pattern developing which would likely lead to continued
N/NE surface flow. Friday and continuing into the weekend with seas
around 4 to 6 feet at 13 to 15 seconds. /42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Frost Advisory from 11 PM Tuesday to 9 AM PDT Wednesday for
ORZ101>120-122>125.

     Freeze Warning from 11 PM Tuesday to 9 AM PDT Wednesday for
ORZ121.

WA...Frost Advisory from 11 PM Tuesday to 9 AM PDT Wednesday for
WAZ201>210.

PZ...None.

&&

$$


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