867
FXUS66 KPQR 170936
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
236 AM PDT Fri May 17 2024

.SYNOPSIS...A weak cold front that brought very light
precipitation has shifted eastward with only a few lingering
showers trailing. Today will be mostly dry with onshore flow.
Another shortwave trough pushes over the forecast area on
Saturday, but yet again, precipitation is minimal. Potential for
more active weather starting Monday.

&&

.SHORT TERM...Now through Sunday...Fairly benign weather for
the next few days as the pattern appears to be mostly "rinse and
repeat". Radar shows a few lingering showers behind the cold
front, but it produced very little rain. Areas that received
the highest amounts were over the Mt Hood Natl Forest at just
around 0.25 inch in the last 6 hours as of 3 AM. The rain shadow
was in full effect with the valleys only seeing a trace of
rain. Zonal flow remains today with little to no actionable
sensible weather. Temperatures will be mild in the upper 60s...
similar to Thursday. Onshore winds will slowly ease as the
pressure gradient lessens so will see less windy conditions
within the Columbia River Gorge and the ranges today.

Moving into Saturday, another shortwave will advect over the
Pacific Northwest. This trough has a low centered over the
Alaska Panhandle and will shift southeastward through the day.
The main band of energy is just to the north over central
Washington so we will see more of the southern band of
precipitation. Looking at another 20-30% chance of any
measurable precipitation on Saturday. Potential for slightly
cooler temperatures as a -2 degree C airmass moves in aloft at
850 mb (~5,000 ft). Again, minimal concerns. -Muessle


.LONG TERM...Sunday through Thursday...Zonal flow to weakly
ridged pattern incoming on Sunday as the trough from Saturday
exits, and the next, potentially more robust longwave trough
drops down from the Gulf of Alaska. This incoming system has the
potential to bring more rain than we have seen as of late.
Looking at the long range ensemble clusters at 500 mb, there
still is quite a bit of variation both in the intensity and the
timing. Tuesday through Wednesday are the general targets for
the precipitation and wind, but two of the ensemble outputs are
showing almost a weak ridge. If this were to manifest, then
rain would be much less and temperatures would be higher. The
temperature spread has become much narrower in regards to the
25th-75th percentile, but the 10th to 90th is still quite large
on Monday and Tuesday. Some of the global long range models are
suggesting warmer temperatures on Tuesday than on either Monday
or Wednesday. This is due to more of a ridging pattern within
the GFS. The mean temperature in Portland is 73 degrees F, 70 F
in Hood River, 61 F in Tillamook, and 70 F in Eugene.

On Wednesday, the front will be the most robust with cooler
temperatures (potentially) and more precipitation. Probabilities
are around 60% for measurable rain, with chances for over 0.25
inch over the Cascades by Wednesday afternoon. There continues
to be broad variability in the long term pattern so have used
the grand ensemble in order to capture most of the pattern
outcomes. Lingering showers on Thursday which are dependent on
the timing and location of the front. -Muessle


&&

.AVIATION...At 09z Friday, a weak front had pushed east of the
Cascades and was moving into central Oregon. Radar observations
showed rapidly decreasing showers across western OR/WA in the wake
of the frontal passage while satellite observations depicted
clearing skies over most of the Willamette Valley. As such, VFR
flight conditions are expected to continue over the next 24 hours
for inland terminals (only exception being KEUG where a localized
MVFR stratus deck has developed and will most likely linger over
or near the terminal through roughly 15z Friday).

Cigs along the north coast have lifted to VFR thresholds while
MVFR cigs remained along the central coast. Cannot rule out
LIFR/IFR cigs at KONP through 15z Friday (20-25% chance), however
probabilities for VFR cigs increase rapidly after 15z.

PDX AND APPROACHES...Expect VFR flight conditions through Friday
evening as skies begin clearing out towards 12z Friday in the wake
of a cold frontal passage. Winds will generally remain out of the
northwest sustained between 5-10 kt.  -TK

&&

.MARINE...Northwest winds have increased to around 15-25 kt over
most of the waters early Friday morning in the wake of a frontal
passage. However, winds will begin decreasing after sunrise. Seas
will be rather steep and choppy on Friday due to a fresh
northwest swell around 7 to 9 ft with a dominant wave period
between 7 and 9 seconds.

High pressure over the upcoming weekend will result in a typical
summertime northerly flow regime with wind gusts peaking in
strength during the afternoon/evening hours each day. The
strongest winds will occur over the southern waters where marginal
small craft advisory level wind gusts to 25 kt are possible
(20-40% chance on Saturday increasing to a 30-60% chance on
Sunday).

The next frontal passage of any significance is set to arrive
Monday night into Tuesday morning, bringing rain and a shift to
westerly winds. However, wind gusts look to generally stay around
or below 20 kt as the pressure gradient associated with this
frontal passage appears rather weak. The exception to that is
over the northern outer waters where isolated small craft advisory
level gusts to 25 kt are possible (20-50% chance).  -TK


&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 8 AM PDT this morning for PZZ210.
     Small Craft Advisory until 8 PM PDT this evening for PZZ251-
     252-271-272.
     Small Craft Advisory until 11 AM PDT this morning for PZZ273.
&&

$$

www.weather.gov/portland

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