054
FXUS66 KPQR 302257
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
357 PM PDT Tue Apr 30 2024

.SYNOPSIS...A cool late season storm will move east of the area
overnight and result in showers and thunderstorms decreasing
this evening. Very brief drying and warming arrives on
Wednesday before the next system arrives Wednesday night/Thursday
morning. Another wetter system will arrive Friday night/Saturday
with unsettled weather continuing into early next week.

&&

.SHORT TERM...Now through Thursday...Satellite and radar show
scattered showers that have bubbled up as daytime heating has
warmed inland locations under a cold air mass aloft. Isolated
and short lived thunderstorms will primarily confined to the
eastern Willamette Valley late this afternoon and early this
evening. Between freezing levels near 3000 feet and surfaced
based CAPE values of 500 J/kg as analyzed by the RAP, small hail
will accompany the stronger showers and thunderstorms. Models
are in good agreement an upper level shortwave trough and an
attendant surface low pressure will shift east of the region
overnight. This will decrease showers and thunderstorm chances
from west to east through this evening.

The current Winter Weather Advisory for the Cascades between
the South Washington Cascades and Marion/Linn County Cascades
remains in effect through this evening, as convection may bring
heavy snow showers at times. Those traveling through the passes
(highways 20/22/26) today should prepare for winter weather
conditions. No plans to extend this Winter Weather Advisory, but
be prepared for wet surfaced to freeze overnight across the
mountain passes/Cascades.

Tonight into tomorrow (Wednesday), conditions dry as this
system pushes east and upper level high pressure builds. We will
see some cloud breaks tonight, with low stratus and chances of
fog (20-30%) in the Coast Range and valleys. With clouds
breaking, radiational cooling can occur and thus cause frost
development. NBM probabilities for Wednesday morning`s lows
below 36 degrees are around 60-80% in the Coast Range and
Willapa Hills, and 10-20% in the Willamette Valley. HREF
suggestshigher chances in the Tualatin Valley and mainly
Yamhill and Polk counties so have opted to issue a Frost
Advisory for these areas. Some uncertainty remains with frost
development because there remains differences between models in
how much clearing will take place overnight versus stratus
backbuilding off the Cascades and Coast Range.

Temperatures warm up slightly on Wednesday, with highs in the
lower/mid 50s along the coast and Coast Range, upper 50s/low 60s
for inland valleys, and 40s in the Cascades. The next system
looks to arrive Wednesday night/Thursday morning, bringing
another round of precipitation, including 1 to 4 inches of snow
across the Cascades above 4000 feet.
-JH

.LONG TERM...Friday through Monday...The WPC clusters suggest
weak shortwave ridging will traverse the PNW early Friday. While
the day appears to start off dry, NBM PoPs will climb to 80-90%
by Friday evening as the next warm front pushes towards the
region. The NBM and our official forecast currently have between
0.50 and 1 inch of rain falling Friday night into early
Saturday in the Willamette Valley and 1 to 2 inches in the Coast
Range and Cascades. The EPS generally shows between 1 to 1.5
inches in the Willamette Valley and 2 to 3 inches in the Coast
Range and Cascades. Given reservoirs are near full for water
supply, the highest end scenario does bring some concern to
hydrology issues along the Willamette River. Fortunately this
still appears to be a less than 10% chance of occurring, but we
will need to monitor if more ensemble guidance falls in line
with the bulk of the EPS data.

WPC cluster guidance suggests a low probability of less than 20%
that the Saturday storm system drops far enough south that we
dry out Sunday. The rest of the ensemble guidance generally
suggests we will remain under the influence of the Saturday
storm system and keep rain chances going into Sunday. There is
good agreement systems that another storm system will enter the
PNW Monday into Tuesday and keep the area cool and wet.
-JH

&&

.AVIATION...Conditions are continue to produce a mix of VFR and
MVFR conditions, but these trend towards VFR as high pressure
continues to build in throughout the afternoon. Clearing is
starting to occur in the Willamette Valley, allowing for a
possibility of a short-lived thunderstorm or two throughout the
afternoon before 02z Wed. However, latest high-resolution model
guidance is trending towards instability being weaker than
previously forecast, and chances have likely decreased to 15-20%,
weaker in going southward where temperatures aloft are warmer
aloft.

Precipitation ends at all terminals by 04z Wed, and the rest of
the TAF period sees dry conditions due to weak high pressure
building in. Southerly winds at southern terminals are currently
gusting to around 20-24 kts, but will decrease rapidly once the
area begins to dry out. In the latter half of Tuesday night, high
resolution model guidance points to a 30-40% chance of IFR
conditions due to fog or low clouds forming. However, other
statistical model guidance has considerably lower chances. For
now, guessing that chances will be around 25-30% chance of IFR
conditions across inland terminals at that time. /JLiu

Notes: The ASOS at KTTD is only available via the dial in option.
There is no estimated time of full repair and restoration for
this location.

PDX AND APPROACHES...Current MVFR conditions will give way to
predominantly VFR around 22z Tue as the main band of rain passes
to the east. Afterwards there will be a period of a few stray
showers, but these will be isolated. Instability looks weaker than
previously forecast, with threat of a stray mid/late afternoon
thunderstorm having dropped to 15-20%. All precipitation ends 04z
Wed, and VFR conditions with weak variable winds are expected
afterwards. High resolution model guidance points towards fog or
low stratus being a possibility 11-16z Wed, with around a 25-30%
chance of IFR thresholds being met at that time. /JLiu

&&

.MARINE...Observations from buoy 46050 show wind gusts continuing
to barely meet Small Craft Advisory criteria, with gusts just
barely above 20 kt. Seas remain around 8-10 ft at 11 seconds through
the afternoon, slowly decreasing going into the evening. High
pressure continues building, and over Tuesday night, winds will
become very weak and seas drop to 4-6 ft by the end of the night. More
active weather then returns later Wednesday into Thursday as
another low approaches the waters. /JLiu

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Frost Advisory from midnight tonight to 8 AM PDT Wednesday for
     ORZ104-106-109-114.

     Winter Weather Advisory until 6 PM PDT this evening for ORZ126-
     127.

WA...Frost Advisory from midnight tonight to 8 AM PDT Wednesday for
     WAZ203.

     Winter Weather Advisory until 6 PM PDT this evening for WAZ211.

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

&&

$$

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