Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 250449 AAA
AFDPQR

Area Forecast Discussion...UPDATE
National Weather Service Portland OR
949 PM PDT Sun Mar 24 2024

Updated Aviation discussion.

.SYNOPSIS...Showery conditions continue today with another round
of steady, light rain and non-impactful Cascade snow on Monday.
A stronger lower pressure system will approach the Pacific Northwest
Tuesday night, leading to the potential for heavy rain, strong
winds, and hazardous marine conditions Wednesday into Thursday.
Cascades will see higher snow amounts as well, but impacts remain
minimal for the passes in the long term.

&&

.SHORT TERM...Sunday through Tuesday...Ensemble guidance indicates
northwest flow aloft will continue today into Tuesday as low
pressure deepens to our east and weak ridging builds over the
eastern Pacific. A round of light showers are moving southeast
through NW Oregon and SW Washington today as a weak shortwave moves
over the region. QPF are expected to be light with less than 0.1
inch for locations west of the Cascades and up to 0.15-0.25 inch for
the Cascades and adjacent foothills. Snow levels will be around 3500-
4500 feet through tonight, and with showers remaining light, snow
amounts will be limited to a dusting up to 3 inches at pass level
and 3-5 inches for elevations above 5000 feet. Skies will remain
mostly cloudy with high temperatures in the mid to upper 50s in the
lowlands and upper 30s to upper 40s in the mountains.

A slightly more enhanced shortwave originating from a parent low off
the Gulf of Alaska is slated to move through the region tomorrow,
which will bring another round of widespread showers across the
region. Additional QPF amounts will be slightly higher with 0.1-0.2
inch for the lowlands and 0.2-0.5 inch for the coast, Coast Range,
and Cascades. The 48 hour probability of rain amounts at or above
0.5 inch from 5 am today to 5 am Tuesday is only 25-45% for the high
elevations and less than 5-10% for the lowlands. Temperatures will
be a bit cooler as the shortwave brings some cooler air from the
north. Daytime temperatures are expected to peak in the low 50s
across the lowlands and low 30s to low 40s in the mountains. This
will also lower snow levels to around 3000-3500 feet tomorrow
morning. However, as showers won`t be very robust once more, snow
amounts will once again be low with 1-5 inches at pass level, a
dusting to a couple inches below 4000 feet, and 5-9 inches above
5000 feet.

Tuesday will see scattered showers with mostly dry conditions by the
afternoon for the lowlands. Temperatures will rise back to the upper
50s for the lowlands and upper 30s to upper 40s for the mountains as
shortwave ridging builds over the PacNW. However, widespread rain
will return late Tuesday night as a much more robust weather system
begins knocking on the door. More details about this system in the
long term discussion below. -HEC

.LONG TERM...Wednesday through Saturday...The latest WPC cluster
analysis remains in good agreement of a another upper level low
deepening over the NE Pacific and pushing multiple rounds of surface
fronts through the PacNW Wednesday into Thursday. GFS, Euro, and
Canadian ensembles all show a potentially sub-980 mb surface low
offshore of Vancouver Island late Tuesday night into Thursday
morning. Widespread rain will begin pushing into the region late
Tuesday night ahead of a stronger surface front that is forecast to
pass through the region Wednesday morning. This first front looks to
tap into some Pacific moisture with both the GFS and Euro ensembles
indicating IVT values around 250-450 kg/ms as the front passes
through the region quickly on Wednesday. Periods of moderate to
locally heavy rain are forecast Wednesday into Thursday, heaviest on
Wednesday. No major flooding impacts are expected at this time as
rain will be fairly stratiform and seasonal behind the Wednesday
front. Fortunately, with rivers levels starting off low, there is a
less than 5% chance of coastal rivers, Willamette mainstem rivers,
and Willamette tributaries rising above action stage. QPF
probabilities are listed below.

Breezy winds are also possible, with the frontal passage on
Wednesday. The coast and higher terrain could see gusts up to 40-50
mph at times with gusts up to 25-35 mph in the lowlands. Cascade
snowfall will increase, though as this is a warmer system, snow
levels will quickly rise to 4500-5500 feet by Wednesday morning.
Current forecast indicates around 1-5 inches from 4000-4500 feet,
including pass level, with up to 8-12 feet above 4500-5000 feet.
Snow levels will lower to around 3500-3500 feet Thursday behind the
passage of the cold front. However, precipitation will be lighter at
this time with only an additional 1-5 inches expected on Thursday.
Wind and snow probabilities are also listed below.

NBM probabilities:

48-hr QPF 1+ inch ending 5 AM Fri:
- Coast: 65-90%
- Inland Valleys: 25-45%
- Coast Range/Cascades: 70-90%

48-hr QPF 2+ inches ending 5 AM Fri:
- Coast: 5-20%
- Inland Valleys: Less than 5%
- Coast Range/Cascades: 40-60%

48-hr snow 6+ inches ending 5 AM Fri:
- Cascade passes: 30-50%

48-hr snow 8+ inches ending 5 AM Fri:
- Cascade passes: 15-25%

24-hr max wind gusts 40+ mph ending 11 PM Wed:
- Coast: 60-80%
- Inland Valleys: 5-20%
- Coast Range/Cascades: 50-70% (for the highest terrain)

Beyond Thursday, there remains uncertainty with the fate of the
upper trough. All clusters on the latest WPC cluster analysis show
the parent low deepening over the eastern Pacific towards California
just off of the West Coast Friday into Saturday. However, there are
uncertainties on the exact track of the low. Additionally, about 45%
of the members indicate the trough could split with the parent low
pushing south toward California and a weaker low remaining over the
PacNW. If this solution pans out, lighter scattered showers will
continue into Friday and potentially Saturday. However, if the low
doesn`t split and just pushes south into California, NW Oregon and
SW Washington could see mostly dry conditions Friday into Saturday.
For now going with the NBM forecast indicating widespread showers
Friday with showers continuing mainly for the higher terrain on
Saturday. -HEC/Alviz

&&

.AVIATION...Showers decreasing this evening with VFR inland and
MVFR at the coast. Next front spreads rain across the area Monday
morning. Front moving at a good pac reaches the coast around
midday and over the inland valleys early in the afternoon. MVFR
becoming widespread after 12Z Mon, likely lasting to around 00Z
Tue. After that should see gradually improvement. However with
cloud breaks and the surface ridge shifting over the inland
valley, may see patchy IFR fog or clouds developing overnight.

* Note: The observations out of KONP are generally limited to wind
  and pressure at this time. Periodically cloud cover, vis, and
  present wx are reported but are unreliable.

PDX AND APPROACHES...VFR conditions currently and guidance suggest
around 80 percent chance for cigs below 3000 ft developing 15-18Z
Mon and around a 40 percent chance for cigs 1000-2000 ft. MVFR
conditions decreasing around 00Z Tue.
/mh

&&

.MARINE...Expect relatively benign conditions through tonight as
winds back more westerly later today and a modest westerly swell
moves through the waters at 5 to 7 ft. A weak front will move
across the coastal waters on Monday, bringing a brief period of
breezy southerly winds ahead of the front Monday morning. A period
of low end Small Craft Advisory conditions will be possible,
mainly south of Cape Falcon, from late morning into early
afternoon as winds gust up to 20-25 kt. Otherwise, conditions
remain on the quiet side through most of the day on Tuesday as
transient high pressure yields light to moderate westerly breezes
while the westerly swell slowly builds to around 8 ft by Tuesday
afternoon.

Expect a much more active pattern during the latter half of the
week as strong low pressure develops over the NE Pacific,
anchored by an intense upper level jet and a potential sub 980 mb
surface low offshore of Vancouver Island by early Wednesday. This
will send a strong front through the coastal waters Tuesday night
into Wednesday morning, with guidance continuing to indicate a
high probability of Gale Force winds through Wednesday morning.
Models diverge somewhat on how the pattern evolves beyond
Wednesday, but several solutions depict another strong low
approaching the OR and WA coastal waters and sending another Gale
Force front into the region on Thursday. Meanwhile, a large
westerly swell is forecast to arrive Wednesday into Thursday,
which is likely to push combined seas into at the least the mid
teens and possibly into the low 20s if a dynamic fetch scenario
materializes with the potential second arriving low on Thursday.
Regardless, mariners should expect hazardous marine conditions
from Tuesday night into the end of the week as stormy conditions
return to the waters of the NE Pacific. DH/CB

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...None.

&&


$$

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