Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 222057
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
157 PM PDT Fri Mar 22 2024

.SYNOPSIS...Clouds and rain will continue through Saturday as a
frontal system pushes through the region. Cool air aloft may
lead to a couple thunderstorms this afternoon and Saturday. An
upper trough dropping down from British Columbia will bring
cooler temperatures Saturday night and Sunday, with snow levels
lowering to the Cascade passes, bringing light snow. Strong low
pressure systems will develop over the NE Pacific beginning
Wednesday, likely leading to strong winds and more substantial
precipitation as well as the potential for high surf and/or
coastal flooding mid to late next week.

&&

.SHORT TERM...Friday afternoon through Sunday...Visible
satellite imagery early this afternoon depicts a low pressure
system continuing to spin over the eastern Pacific off the
southern Oregon coast. Pockets of brief clearing are being
observed behind the passage of a weak occluded front that moved
through this morning. A stronger surface cold front has
developed over the waters as the low strengthens offshore. This
low will continue to strengthen over the 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. The cold front is forecast to move
inland ahead of the low, pushing northeast through the region
tonight into Saturday morning. This is expected to bring another
couple of rounds of widespread bands of rain with deterministic
QPF suggesting an additional 0.25-0.5 inch of rain for most
areas west of the Cascades and 0.30-0.75 inch of rain for the
Cascades through early Saturday morning. Snow levels will
remain above 5000 ft through most of this, so impacts will be
minimal for the Cascade passes. Given the subtropical influence,
inland valleys should reach the upper 50s to lower 60s today
with plenty of 50s and 40s for the higher terrain.

Cool air aloft associated with this system will enhance
instability. If enough sunbreaks occur, SBCAPEs of up to
200-500 J/kg could support some thunderstorm development
in the afternoon and evenings hours today and Saturday. Chances
today are waning as high clouds have remained fairly thick with
only a little bit of clearing here and there. However, CAMs and
soundings suggest the best timing for thunderstorms today are
around 4-6 pm PDT. We will maintain a thunder mention for areas
southern Columbia County WA and southward based on higher- end
NBM and NAM SBCAPEs, but our hunch is that deformation
clouds/precipitation will keep the low levels too saturated for
thunder either today or Saturday. However, can`t rule out small
hail or graupel with any stronger rain cells that form. 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 Saturday
night into 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 Saturday
night into Sunday. The higher ski resort elevations should see
a little more snow on the order of 3 to 5 inches by the time
Sunday night arrives. -Weagle/HEC

.LONG TERM...Monday through Thursday...The majority of ensemble
members from the WPC cluster analyses suggest cool, northwesterly
flow aloft through Tuesday, maintaining showery conditions.
Showers will briefly change to widespread rain as a shortwave
moves through the region Monday afternoon and evening. 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 10-20% chance for the Coast and Willamette
Valley by Tuesday morning.

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
potentially powerful fronts across the PacNW beginning
Wednesday with the pattern potentially lasting into late week.
00z WPC cluster analyses strongly favor this pattern, which adds
confidence. Strong winds and periods of moderate to heavy rain
will be possible at times along with snow in the Cascades. 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/HEC

&&

.AVIATION...A low pressure system offshore will maintain showery
conditions with moist, southerly flow aloft through tonight.
Predominately VFR cigs expected, although heavier showers may
reduce visibility to MVFR at times. Guidance suggests generally
less than a 20% chance of MVFR conditions through 00Z Sat. Chances
for IFR and MVFR do increase this evening into Sat morning to
around 30-40% as the next round of heavier rainfall is expected
to lift north across the area. Surface winds will generally be
light through tonight.

* 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...Predominately VFR expected through Saturday
with periods of light rain. Chances for MVFR increase to around
20-40% between 00-06Z Sat as a band of heavier rain is expected.
East to southeast winds less than 8 kt. /DH

&&

.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 surface low developing offshore
this afternoon/evening will approach close enough to support
gales along the southern/central OR coast (in the outer waters)
tonight. Confidence is 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 possible to develop over the NE Pac late Tuesday, moving
closer to the coast Wednesday into Thursday. The track of this
system is such that there could be a dynamic fetch, which really
tends to pile up seas as the lows undergo rapid development.
Latest data show seas building to around 20 ft by Wednesday
night. Seas will be fairly long in period and energetic.
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/DH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 5 PM this afternoon to 5 PM PDT
     Saturday for PZZ251.
     Small Craft Advisory until 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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