Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 221138
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
437 AM PDT Fri Mar 22 2024

.SYNOPSIS...Clouds and rain will increase today as weak low pressure
developing near Cape Mendocino lifts north through the region.
Stronger low pressure will develop offshore tonight, maintaining some
precipitation through Saturday. Cool air aloft may lead to a couple
thunderstorms this afternoon and Saturday, with the best chance south
of Salem. An upper trough dropping down from British Columbia will
bring cooler temperatures Saturday night and Sunday, with snow levels
lowering to the Cascade passes in light precip. Strong low pressure
systems will develop over the NE Pacific beginning Tuesday, likely
leading to more substantial precipitation and the potential for high
surf and/or coastal flooding mid to late next week.

&&

.SHORT TERM...Now through Sunday...Mild temperatures overnight
continue into this morning as southerly flow maintains subtropical
air and high snow levels across the region. Latest GOES-West
satellite imagery reveals developing weak low pressure near Cape
Mendocino in response to an upper level disturbance. This will likely
lead to a wet and dreary day for the CWA today as a beneficial weak
atmospheric river interacts with the weak low and its associated
occluding frontal system. The heaviest rain will focus well south of
our CWA though, with NBM deterministic QPF suggesting 0.25-0.50" of
QPF for most of our lowlands and 0.50-1.00" of QPF for the higher
terrain through early Saturday morning. Snow levels will remain above
5000 ft through most of this precip, so impacts will be minimal for
the Cascade passes. Given the subtropical influence, inland valleys
should reach the lower 60s today with plenty of 50s and 40s for the
higher terrain.

Stronger low pressure is forecast to develop over the southern OR
offshore waters tonight, lifting north a bit Saturday morning before
eventually being shunted southward and inland by a cold front and
upper trough dropping down from western Canada. Cool air aloft
associated with this system will enhance instability, and if enough
sunbreaks occur, SBCAPEs of up to 500-1000 J/kg could support some
thunderstorm development Saturday afternoon. We will maintain a
thunder mention for areas Wilsonville southward based on higher-end
NBM SBCAPEs, but our hunch is that deformation clouds/precip will
keep the low levels too saturated for thunder either today or
Saturday. The cooler air mass and stubborn cloud cover will probably
hold temps in the 50s for most of the lowlands Saturday and Sunday.

As low pressure and its associated upper trough move onshore, snow
levels will lower to around Cascade pass level Sat night/Sunday. That
said, most guidance only shows light precipitation by then, so
advisories are not anticipated for the Cascades. Overall through the
weekend, expect a dusting to 3 inches of snow for the Cascade passes,
mainly falling Sat night/Sunday. The higher ski resort elevations
should see a little more snow, perhaps on the order of 3 to 5 inches
by the time Sunday night arrives.  Weagle

.LONG TERM...Monday through Thursday...The majority of ensemble
members from the WPC cluster analyses suggest cool, northwesterly
flow aloft, maintaining showery conditions. Showers will briefly
change to widespread rain as a shortwave moves through the region
Monday afternoon. Rain will initially be light, with NBM guidance
suggesting chances of 24-hr QPF exceeding 0.5 inch around 40-50% for
the Coast Range and Cascades and 10% chance for the Coast and
Willamette Valley for both Monday and Tuesday.

Looking farther west into the Pacific, a Madden-Julian Oscillation
fueled jet stream is pushing 200 knots along 35-40N just west of the
International Dateline. This has implications down the road for us as
it will usher in a more active and progressive pattern, with strong
and rapid cyclogenesis occurring over the NE Pacific, and possibly
multiple times next week.

The strong jet stream appears poised to push a series of powerful
fronts across the Pac NW beginning Tuesday with the pattern
potentially lasting into late week. 00z WPC cluster analyses strongly
favor this pattern, which adds confidence. Initially, an easterly
component to the MSLP gradients will probably rob some of the wind
potential and even some of the QPF from the first of these fronts
Tuesday night. However given the momentum associated with the strong
jet stream and increasingly unstable air mass as cool air air moves
in aloft, the frontal systems to follow will likely be more effective
at mixing stronger winds down to the surface, especially along the
coast. With the fast jet stream, the timing of any particular frontal
system remains uncertain, but the message here is that the confidence
is high that considerably more active weather will affect the region
mid to late next week.

Marine impacts are discussed in the marine discussion below, but the
rapid cyclogenesis and the track of the resulting lows appear poised
to support a dynamic fetch for wave generation in the north-central
and NE Pacific as the lows strengthen and race E-NE. With limited
wave data, it is difficult to come up with hard probabilistic numbers
for the potential of coastal flooding and/or high surf, but the
pattern appears to suggest a considerable likelihood of both
potentially as early as Wednesday.  Weagle/JH

&&

.AVIATION...Patchy dense fog developed overnight as holes developed
in the high cloud cover, allowing for enough radiational cooling to
develop some patchy IFR/LIFR clouds given our moist low-level air
mass. Suspect the fog will become less widespread as high clouds
thicken aloft over the coming hours, re-radiating warmth back down to
the surface and disrupting the radiational cooling. Otherwise, cigs
will mostly be VFR through this morning, but increasing rain will
likely lead to increasing MVFR and possibly even some localized IFR
later this afternoon into tonight. Winds will be light through today,
but S-SE winds will likely strengthen just above the surface at KEUG,
potentially leading to some LLWS this evening.

* 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...Mostly VFR through early afternoon, then rain
will lead to lowering cigs and likely some MVFR. That said, hi-res
guidance only shows a 20% chance of MVFR, but given the rain lifting
northward it appears the chances are higher than that. Also keeping
an eye on some patchy river fog along the Columbia, which will
probably tease KPDX with IFR fog/low clouds until 15z.Weagle

&&

.MARINE...Complex low pressure developing over the NE Pacific will
have waves of low pressure rotating around it as it wobbles toward
the coast through Sunday. One such wave of low pressure is developing
near Cape Mendocino but should move quickly inland and have minimal
impact to the coastal waters. Second sfc low will develop further
offshore this afternoon/evening, with some models suggesting it will
develop enough and approach close enough to support gales along the
southern/central OR coast (in the outer waters). Confidence was too
low for gales, so we handled this with a Small Craft Advisory today
evolving into a Hazardous Seas Warning for PZZ272/273 from 8 PM
tonight to 11 AM Saturday. Offshore MSLP gradients ahead of the low
will likely keep the strongest winds/seas out of the inner waters.

Beginning Tuesday, the weather pattern turns much more dangerous over
the coastal waters, even for larger vessels. A powerful Pacific jet
stream will push a series of strong Pacific frontal systems across
the Pac NW waters beginning Tuesday. Strong, sub-980 mb low pressure
appears likely to develop over the NE Pac Tuesday, with another
developing closer to the coast Wednesday. The track of these systems
is such that there could be a dynamic fetch, which really tends to
pile up seas as the lows undergo rapid development. Latest WCWave
data show seas easily exceeding 20 ft by Wednesday night, and the
seas will be fairly long in period and energetic. We went with the
WCWave output beginning Wednesday, so these higher seas are not
showing up in the CWF just yet. Astronomical tides will also be high
due to the full moon, so the combination of these tides and the
powerful swell may spell trouble for coastal flood concerns by the
end of next week.  Weagle

&&

OR...None.
WA...None.
PZ...Small Craft Advisory from 5 PM this afternoon to 5 PM PDT
     Saturday for PZZ251.

     Small Craft Advisory from 11 AM this morning to 5 PM PDT
     Saturday for PZZ252-253-271.

     Small Craft Advisory until 8 PM PDT this evening for PZZ272-273.

     Hazardous Seas Warning from 8 PM this evening to 11 AM PDT
     Saturday for PZZ272-273.

&&

$$

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