983
FXUS66 KPQR 291830 AAA
AFDPQR

Area Forecast Discussion...Updated
National Weather Service Portland OR
1130 AM PDT Mon Apr 29 2024

Updated Aviation...

.SYNOPSIS...Relatively colder weather systems will move through
northwest Oregon and southwest Washington today and tomorrow.
Expect lowland rain showers, Cascade snow showers, and the
potential for thunderstorms with these systems. Potential
hazards with any thunderstorms or heavy showers include
lightning, small hail, heavy rain, and gusty winds. Conditions
become dry and warm up on Wednesday. Another system arrives on
Thursday, with unsettled weather potentially continuing into the
weekend.

&&

.SHORT TERM...Now through Wednesday...As of 3 AM PDT, an upper
level trough is beginning to enter the Pacific Northwest. The
associated surface cold front is now moving inland, with radar
imagery showing increasing shower activity across northwest
Oregon and southwest Washington. Expect a continuation of
lowland rain showers and Cascade snow showers as the system
pushes through today. Snow levels will gradually fall this
morning and settle between 2500-3500 ft today. Thus, the Winter
Weather Advisory remains in effect for our entire Cascades
through this afternoon, with the heaviest snowfall expected
above the passes. Forecast QPF today will be around 0.15-0.40
inch for the coast and inland valleys. Expect an additional 2-8
inches of snow in the Cascades, except up to 12 inches above the
passes.

Note that there will be a 15-25% chance of thunderstorms across
northwest Oregon and southwest Washington today. Potential hazards
with any thunderstorms or heavy showers include lightning, small
hail, heavy rain, and gusty winds. Cold air will filter in
aloft as the upper trough progresses through the region between
now and 11 AM - 1 PM today. This will create a more unstable
airmass, as HREF shows CAPE values increasing to around 150-300
J/kg today. The threat for small hail will also increase as
models show 500 mb temperatures falling to -30 to -35 deg C and
lapse rates steepening to around -8 deg C/km. The upper trough
doesn`t dip too far south, so locations north of Lincoln
City/Salem have the highest probabilities (20-25%) for
thunderstorms today.

The upper level trough is expected to exit our area and push
east of the Cascades after 1-3 PM today. Expect mostly light and
decreasing shower activity late afternoon through this evening.
However, the Cascades could maintain a 15-20% chance of
thunderstorms this evening as the system exits. This relative
lull will be short-lived: another low pressure system will drop
down from the northwest tonight into Tuesday. This low is
forecast to make landfall over the north Oregon coast. Similar
to today, we`ll also have a 15-25% chance for thunderstorms
across the region. Forecast QPF will be around 0.25-0.50 along
the coast and inland valleys. Meanwhile, the Cascades would see
another round of snowfall, albeit it doesn`t look advisory
worthy at the moment. Snow levels fall to around 2000 ft
tonight, with an additional 2 to 6 inches of snow for the
Cascades in a 24 hour period ending at 11 PM Tuesday.

With these cooler systems in place today and tomorrow, high
temperatures will be around 10 degrees below normal. Expect
today and tomorrow`s highs to be in the low to mid 50s along
the coast and inland valleys, 40s in the Coast Range and Willapa
Hills, and 30s in the Cascades (sub-freezing highs for the
crest and highest peaks of the Cascades).

Tuesday night to Wednesday, conditions begin to dry up as the
second system pushes east and upper level high pressure builds.
We could see some clearing Tuesday night - this could lead to
radiational cooling and the potential for frost development.
Currently, it looks like the highest frost potential will be in
the Coast Range, Willapa Hills, and Upper Hood River Valley.
Based on the latest NBM, these locations of a greater than
70-90% chance of Wednesday morning lows dropping below 37
degrees. For the Willamette Valley, that probability is around
40-60%. Seasonable high temperatures will likely return
Wednesday, with a 60-80% chance of highs exceeding 60 degrees
for inland valleys.-Alviz

&&

.LONG TERM...Thursday through next Sunday...WPC cluster
analyses have around 65% of ensemble members suggesting an
additional trough pushing into the Pacific Northwest Thursday.
In this case, we would see another round of light precipitation.
The other 35% of ensembles members suggest ridging, which would
maintain drier conditions. Current NBM probabilities for QPF
exceeding 0.25 inch ending 5 PM Thursday is less than 20% for
inland valleys and 40-50% for the Coast Range and Cascades.

Friday looks relatively drier as the majority of WPC clusters
suggest transient ridging; however, the ridging doesn`t look
particularly strong. This may allow for some shortwave
troughing to push through and bring some precipitation. The
majority of clusters still show precipitation on Friday, despite
them also showing ridging. With this uncertainty, NBM PoPs of
25-50% across the region Friday look reasonable.

Saturday to Sunday, the majority of the clusters are showing a
troughing pattern returning. However there is uncertainty with
the strength of the next trough. About half of the ensemble
members show weak troughing moving into the Pacific Northwest,
while the other half show a deeper trough developing over the NE
Pacific. The members that show the deeper trough have it
progressing southward toward southern Oregon and northern
California by Sunday, and becoming negatively tilted. In either
scenario, unsettled weather with showery activity would
continue for our area.-Alviz

&&

.AVIATION...Scattered showers ongoing across the region as of 18z
Monday in broad onshore flow, producing variable VFR/MVFR
conditions at all terminals as showers move through. Expect
showers to diminish in coverage with improvement more solidly
towards VFR after 20z as an upper level disturbance passes east of
the Cascades, with showers continuing to produce mountain
obscurations into the afternoon. There is 10-20 percent chance of
thunderstorms across the area through 00z Tue, with the best
chances north of a KONP-KSLE line. Based on recent observational
trends, chances were not high enough to warrant inclusion of TS
in the TAFs. Winds out of the W-SW at around 10 kt through the
afternoon. Another disturbance arrives after 06z Tue, bringing
increased shower activity and a return of MVFR conditions 08-10z
Tue as winds shift southerly through the end of the period.

Notes: The ASOS at KTTD is only available via the dial in option.
The AWOS at KMMV is still out of commission, but a tentative work
around has been implemented. There is no estimated time of
full repair and restoration for either of these locations.

PDX AND APPROACHES...Variable VFR/MVFR conditions expected to
improve to VFR with cigs above FL040 after 20z Mon as showers
diminish in coverage. Can`t rule out a thunderstorm near the
terminal through 00z Tue, but chances are small enough  to not
include in the TAF. Conditions trend back to MVFR after 10z Tue as
another approaching disturbance brings increased shower activity.
W-SW winds 5-10 kt this afternoon, diminishing in the evening and
then turning southerly and increasing back to 10 kt after 10z
Tue. /CB

&&

.MARINE...Active weather continues into midweek. A surface front
moving through the waters on Monday will bring another round of
elevated winds along with steep and choppy seas through at least
Monday afternoon. Generally westerly winds with gusts up to 25 kt
and seas of 6 to 9 ft at 10 to 13 seconds through this time frame.
Therefore have maintained the current suite of Small Craft
Advisories.

A series of weak fronts will move through the waters Monday night
into Tuesday and again late Tuesday into Wednesday, bringing
elevated winds and seas at times. Some models are suggesting that
small craft conditions could develop with each frontal passage.
Will continue to monitor the situation as it develops. -HEC/42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory until 3 PM PDT this afternoon for
     ORZ126>128.

WA...Winter Weather Advisory until 3 PM PDT this afternoon for WAZ211.

PZ...Small Craft Advisory until 11 PM PDT Tuesday for PZZ210-251>253-
     271>273.
&&

$$

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