425
FXUS66 KPQR 230446
AFDPQR

Area Forecast Discussion...UPDATE
National Weather Service Portland OR
946 PM PDT Wed May 22 2024

Update to aviation discussion.

.SYNOPSIS...Low pressure is mostly stationary spinning right
along the Oregon and Washington border. This low position has
maintained showers throughout the region - especially areas that
are being orographically lifted by terrain. Weak ridging forms
tomorrow while troughing comes right back in on Friday. Minimal
change in overall sensible weather through the week.

&&

.SHORT TERM...Now through Friday...The pattern has been
unchanging today as low pressure remains nearly stationary over
the Oregon and Washington border. This low will be slower to
move as it loses momentum and begins to fill. The showers ended
up being much more robust than previously thought. Looking at
the overall pattern from the upper most levels to the surface,
there was a notable coupled pattern with a jet streak set up
right over the region, a vorticity maximum at 500 mb, and a
trough at the surface. Showers were robust at times with amounts
ranging from 0.01-0.25 inch in the last 6 hours. Wind too has
been more robust than previously thought. This is evident in the
observed pressure gradient between Troutdale and the Dalles
which is around 4-5 mb. In some of the more exposed westerly
slopes, gusts as high as 50 mph were observed. These conditions
are not expected to persist overnight though as conditions ease.

Speaking of tonight, weak ridging will cause conditions to
improve and clear slightly. Some forecast soundings are
suggesting a weak inversion forming right around sunrise. While
this is definitely possible, the amount of cloud cover today
will make chances limited. Areas in the southern Willamette
Valley and the central Cascade foothills have around a 30%
chance for patchy fog tonight. Transitioning into Thursday, mild
conditions prevail as the ridge shifts inland. More zonal flow
follows. Previous models were suggesting a more amplified ridge,
however, because the low today was so slow and didn`t dig
southward, there isn`t enough momentum to truly build the ridge
to warm/clear things out even more. Temperatures will see very
little change over the next few days. One thing to note though
are overnight temperatures, especially in the valleys of the
Coast Range and the Cascade foothills. High resolution models
are showing around a 40% chance of low temperatures below 40
degrees F both early Thursday morning and Friday morning. No
frost is expected as winds will be too elevated, but something
to take note of - especially if skies clear.

On Friday yet another low pressure system will drop down from
Canada which will bring another round of showers. This appears
to be yet again another high probability low accumulation type
event, but still something to note. So if you`re wanting to do
some yard work and rain is an issue, Thursday may be the better
option if you cannot hold off until early next week. -Muessle

.LONG TERM...Saturday through Wednesday...Stronger ridging in
the long term but even then, onshore flow is the most dominate
feature through the weekend. Will see some residual showers on
Saturday but they will slowly subside by Sunday afternoon. The
modeled clusters have much to be desired in regards to
consistence and locking onto an overall pattern. It`s pretty
nondescript at this point with a flattened weak ridge popping
up in most outputs. Sunday will likely be more of a transition
day as a stronger ridge over the Pacific shifts eastward. This
ridge will become the dominate feature on starting Monday.
Nearly all models are showing a strong trend towards a high
pressure solution which would bring clearer skies and warmer
temperatures in general. Temperatures will slowly rise during
the day but, even though the air will be warmer in general, the
clear skies will cause temperatures overnight to fall. In
regards to temperature, have maintained a strong push towards
the NBM with a bit of climatological models included as these
quick transitions are hard to capture in models.

  -Muessle


&&

.AVIATION...VFR prevailing this evening but with spots of MVFR at
the coast. Predominant CIGS will fluctuate between 4500-5500 ft.
MVFR conditions likely to become the dominant flight category
along the coast around 06Z-16Z Thursday. Inland expected to be
VFR through about 11Z then about a 30-60 percent chance for MVFR
cigs through 18Z. There after probabilities drop rapidly from
south to north.

PDX AND APPROACHES...VFR conditions expected through about 11Z Thu
then chances for MVFR increase. HREF indicates40-60 percent
chance for cigs 2000-3000 ft through about 19Z. In this weather
pattern, appears the clouds will backbuild from the Cascades to
the terminal area. But can`t discount some MVFR cigs on western
approaches as well. Evening westerly winds 8 to 10 kt will ease
overnight and pickup again later Thu afternoon. /mh

&&

.MARINE...North/northwesterly flow will continue to produce
breezy winds through the overnight hours with gusts up to 25
kt across all waters. General seas 10 to 14 ft through the
overnight hours, with seas slowly subsiding towards 5 to 7 ft by
Thursday. Expect steep and choppy conditions through early
Thursday morning across all waters. Therefore have adjusted the
current suite of Small Craft Advisories. Thursday, a pattern
change is expected as a shortwave ridge, brings a brief reprieve
to winds and seas across all waters. However, another low swings
into the region from the NNW Friday and Saturday bringing a
return of active weather. Seas build towards to 7 to 9 feet at 8
to 9 seconds on Saturday before subsiding towards 3 to 5 feet by
Sunday. /42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 3 AM PDT Thursday for PZZ210-251>253-
     271>273.
&&


$$

www.weather.gov/portland

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland