177
FXUS66 KPQR 010420 AAA
AFDPQR

Area Forecast Discussion...Update
National Weather Service Portland OR
919 PM PDT Tue Apr 30 2024

.UPDATE...Latest KRTX Doppler Radar imagery shows snow showers
becoming more confined to the Clackamas, Marion, and Linn Cascades
this evening, but they are dwindling a little slower than expected.
We were able to expire the Winter Wx Advisory for the S WA Cascades
on time at 6 PM, but debris from convective activity over the
Willamette Valley and Cascade foothills has maintained the snow
showers into this evening between Mount Hood and the Three Sisters
area. It appears most of this activity is aiming for the Marion/Linn
Cascades, so we extended the advisory above 3000 ft for that zone
until midnight. By then, subsidence associated with high pressure
building in from the Pacific should be sufficient to bring an end to
the showers. Those travelling across the Cascades should know that
roads that were wet during the daylight hours are likely becoming icy
tonight as temperatures fall.

The Frost Advisory looks to be in good shape for some of our outlying
valleys; skies are clearing across much of our interior lowlands.
Given the short nights this time of year, frost should mostly impact
outlying valleys and perhaps some suburbs Wednesday morning.

Aviation and marine sections have also been updated below.  Weagle

&&

.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
suggests higher 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 predominantly VFR throughout the region
as almost all precipitation has moved off to the east over the
Cascades, and threat of thunderstorms has ended. The rest of the
TAF period remains essentially dry barring brief drizzles due to
weak high pressure building in. Winds will remain weak and
variable for the rest of the night, with a slight increase in
southerly winds after sunrise on Wednesday, though winds still
generally remain under 6-8 kt.

Uncertainty remains regarding the possibility of fog later Tuesday
night. High resolution model guidance continues to trend towards
increased probability of fog developing in the northern inland
terminals, with around a 40% probability of LIFR visibility
thresholds currently. However, other guidance is keeping the
possibility of fog closer to 10-20%, with the final outcome
heavily dependent on exactly which areas see clearing. For now,
going with around a 25% chance of dense fog at KPDX, KHIO, KTTD,
and KUAO. Additionally, there`s around a 20% probability that
some clouds ahead of the next system move in around 12z Wed,
bringing a broken deck that could reduce ceilings to MVFR. Have
not included in the TAFs due to low probability. /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...Conditions are currently VFR, but two
possibilities exist that could deteriorate conditions throughout
the TAF period. Chances are around 25% for dense fog to form in
the latter half of Tuesday night (11z to 17z Wed) that could
potentially reduce visibilities down to LIFR thresholds.
Additionally, some clouds ahead of the next front on Wednesday
could cause ceilings to drop to MVFR between 12-16z Wed, dropping
ceilings to MVFR thresholds. These situations will likely be
mutually exclusive; if cloud cover develops, then fog is unlikely
to develop or become dense. /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 midnight PDT tonight for ORZ127.

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

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

$$

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