Tigard, Oregon 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

315
FXUS66 KPQR 111810 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1009 AM PST Wed Dec 11 2024

...Updated aviation discussion and WWA...

.SYNOPSIS...High pressure over the region weakens today with
stratus encompassing the area. Some patchy dense fog this
morning but will dissipate with incoming wind shifts as a front
nears the coastline. A low pressure system moves inland today
bringing widespread rain and breezy winds. A series of fronts
push inland through the week before a much more robust front
arrives Friday into Saturday.

&&

.SHORT TERM...Now through Saturday...High pressure weakens
today, but the overnight inversion has allowed for fog and low
stratus to form. Observations this morning show dense fog in
the southern Willamette Valley along I-5 and the generally to
the west in Benton County. Fog is patchy but given the coverage
have issued a Dense Fog Advisory which will end this morning.

The high pressure of Tuesday will erode today as a low pressure
system drops down from the Gulf of Alaska. The overall system
has slowed slightly with a shortwave trough first moving in this
morning followed by a closed low pressure system late today or
early Thursday morning. As the shortwave through moves in it
will increase mixing and lift any remaining fog. Rain chances
too increase today though it will initially be showery with
varied accumulation. Winds will remain easterly which will keep
high temperatures cooler. Some models are suggesting that this
trough will dig further south while others take on a more
northerly solution. If the northerly solution is favored, the
easterly winds will be stronger and keep the area cooler as it
ushers in cold air from east of the Cascades. If the southerly
shift manifests then the temperatures will be more moderated. An
example of this is in Portland where the 90th percentile max
temperature is 53 deg F, and the 10th percentile is 43 deg F. A
similar spread is forecast through the interior lowlands. For
coastal sites less variation is expected as winds are generally
southerly.

Late tonight a low will move inland. There continues to be
discontinuity in this system like the shortwave trough of today
as there is both a possible northerly, and southerly solution.
The GFS takes on the northerly track while most other
deterministic models are tracking towards the OR/CA border. Have
trended towards that solution for sensible weather features.
Rain will intensify into Thursday. One forecast challenge will
be any possible freezing rain at the mid-elevations. Right now
it looks like the foothills of the Cascades in the Gifford-
Pinchot NF and the Upper Hood River Valley have around a 30%
chance of freezing rain with minimal accumulations. This is due
to the precipitation timing with the cold air pool in the area.
Confidence in this pattern is low though in regards to the
formation of the freezing rain, and will really depend on just
how strong the onshore flow becomes. Overall, looking at fairly
typical weather on Thursday.

Friday brings the initial stages another active weather system.
This system features a coupled low at the surface and aloft,
and a digging trough within the jet stream. Rain will continue
to fall through the day intensifying in the evening into
Saturday. Unfortunately the consistency in models does not get
better with this system and in fact gets worse. Looking the
ensemble low center outputs, there is a swath of options that
stretch from British Columbia to northern California. Because of
this difference, confidence is low in the dominate weather
features. At this point wind is of the greatest concern due to
an intense jet stream at the upper and low levels. At 250 mb
(~35,000 ft) the jet streak, with speeds of near 120 kt, and at
500 mb speeds are around 60 kt. This coupled jet stream enhances
convection at the surface and thus the strength of the system.
A coastal jet could form Friday night with strong winds along
the coast. This does appear to be more of a marine event instead
of a land wind issue at this point. Saturday is a mixed bag of
possibilities but there is high confidence that it will be wet
and breezy.

To quickly summarize: breezy Wednesday, wet Thursday, wet and
windy Friday, and chaotic Saturday. -Muessle


.LONG TERM...Saturday Night through Tuesday...Rain to continue
through Saturday as a transient ridge begins to build on Sunday.
This break will be short lived though as there is a signal for
another round of rain on Monday with the passage of another
frontal system. Due to the inconsistencies in the short term
forecast, those issues are compounded in the long term forecast
so confidence is quite low in the overall outcome. Cluster
analysis depicts more of a zonal flow pattern on Sunday with
less direction on Monday and Tuesday. There are nearly equal
chances for dry and wet days. I will mention that the GEFS tends
to be wetter on Monday/Tuesday in the ensembles likely due to
the main low moving more northward, and northwest
Oregon/southwest Washington sitting in the most active portions
of the front.
-Muessle

&&

.AVIATION...Satellite imagery and terminal observations as of
1730z Wed depict LIFR CIGs and a mix of LIFR/IFR/MVFR VIS across the
Willamette Valley due to a strong subsidence inversion. The coast
remains clear due to offshore flow, and breezy easterly winds
through the Gorge have been keeping conditions VFR in the eastern
Portland Metro Area. Expect conditions to gradually improve to VFR
in the Valley early this afternoon as mixing increasing with daytime
warming. Will see high clouds as the next system approaches.

A frontal system will arrive this afternoon with rain spreading from
south to north and reaching terminals by 22z Wed to 00z Thu.
Will see CIGs across all terminals fall to low-end VFR by this
evening, with potential for MVFR cigs late tonight into early
Thursday morning. Probabilities for MVFR CIGs vary across terminals,
but are generally between 40-70% from 12-18z Thu. Winds turn more
southerly this evening for most places, however easterly wind gusts
up to 30 kt continue through the Gorge and east Portland Metro
(including KTTD) as KTTD-KDLS pressure gradients tighten.

PDX AND APPROACHES...Predominately VFR thresholds through late
Wednesday night. Frontal system arrives, returning rain and lower
CIGs this evening. Guidance suggests a 60-80% chance of MVFR CIGs or
lower between 12-18z Thu. Easterly winds generally around 10 kt with
gusts increasing to 20-25 kt after 03-06z Thu.     -Alviz

&&

.MARINE...Winds have turned southerly across the waters due to an
approaching frontal system. Expect southerly to southeasterly
wind gusts up to 25-30 kt across the waters through Thursday
afternoon. Therefore, a Small Craft Advisory has been issued for
all of the waters including the Columbia River Bar through 4 PM
PST Thursday. Seas remain around 8-9 feet at 15 seconds as this
system pushes through, but will gradually build to 11-15 feet at
15 seconds on Thursday as a westerly swell follows this system.

Another low pressure system will bring continued active weather
toward the end of the week, with increasing confidence for another
round of gusty winds and building seas. The most impactful time
frame is currently forecast between early Friday morning through
Saturday afternoon. Guidance suggests a 70-80% chance of Gale
force wind gusts, a 80-90% chance of seas exceeding 15 feet, and a
40-50% chance of seas exceeding 20 feet by Saturday.-Alviz/HEC

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 4 PM PST Thursday for PZZ210-251>253-
     271>273.
&&

$$

www.weather.gov/portland

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