Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 251750
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1049 AM PDT Mon Mar 25 2024

.SYNOPSIS...Seasonably cool, cloudy, and showery conditions
continue today and Tuesday with light snow in the High Cascades.
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. Warmer and drier conditions may return by next
weekend.

&&

.SHORT TERM...Through Wednesday Night...Radar shows spotty,
light shower activity ongoing across the region this morning as
as transient shortwave ridging is displaced east of the
Cascades by a weak upper level disturbance embedded in a broad
onshore flow regime. This will maintain seasonably cool, cloudy,
and showery conditions today, characterized by highs in the low
50s across the lowlands of northwest Oregon and southwest
Washington. Shower activity will be enhanced by a weak front
passing through the coastal waters later this morning and moving
inland during the afternoon. Model consensus generally caps QPF
amounts around a tenth of an inch in the Willamette Valley
through this evening, with closer to a quarter inch along the
coast and a quarter to a half inch in the higher terrain. The
high Cascades could pick up another 1 to 3 inches of snow in the
passes through this evening as snow levels reside around 3000
feet, with locally higher amounts for some of the ski areas
above 5000 feet. Do not expect any thunderstorms today as
widespread cloud cover limits instability. Benign conditions
persist behind the front as onshore flow maintains scattered
showers tonight into early Tuesday. Shower activity will
diminish through Tuesday afternoon as another shortwave ridge
crosses overhead, which should allow high temperatures to
rebound into the upper 50s on Tuesday.

More active weather returns overnight Tuesday into Wednesday
morning as strong low pressure develops over the NE Pacific,
anchored by an intense upper level jet and an approximately 980
mb surface low well offshore. This will send a strong front
through the coastal waters early Wednesday which will push
inland during the day accompanied by heavier rain and breezy
conditions. Expect the strongest winds to remain confined to
the coastal waters and the immediate beaches and headlands as
the latest model trend has been to keep the surface low farther
offshore and maintain a slight offshore component to the
pressure gradient. Expect a few gusts as high as 50 mph on those
beaches and headlands during the day on Wednesday with south
winds gusting in the 25-35 mph range through Wednesday
afternoon. Model QPF amounts are holding steady from previous
forecast cycles, depicting anywhere from 1.0-1.5 inches of rain
along the coast and Coast Range, with closer to a half inch
expected through Wednesday afternoon in the interior valleys.
Do not anticipate any significant hydro concerns as guidance
continues to depict a less than 5 percent chance for mainstem
rivers and tributaries to rise above action stage. Precipitation
will become more showery in nature behind the front as the
upper trough moves closer to shore and steepen mid level lapse
rates, yielding a slight chance of thunderstorms across much of
the area west of the Cascades later Wednesday into Wednesday
night. Meanwhile, snow will mainly be confined to the passes and
above as snow levels briefly rise to around 5000 feet ahead of
the front, eventually falling back to around 3000 feet behind
the front and allowing a few more inches of accumulation through
Wednesday night. /CB

.LONG TERM...Thursday through Sunday...Active weather continues
into Thursday as the vertically stacked low pressure system
gradually fills in and weakens offshore while moving closer to
the Washington Coast. Will have to pay attention to any smaller
scale disturbances rounding the base of the upper trough, but
this pattern will generally keep breezy and widespread showery
conditions over the region through much of the day on Thursday,
with, favorable orographic support in strong southwest flow
keeping the heaviest precipitation focused along the Coast and
Coast Range. Conditions look to improve by Friday as an intense
shortwave diving into the backside of the trough refocuses the
upper low farther south off of California, with a new dominant
surface low forming over the CA coastal waters. This will result
in weaker upper level flow over the Pacific Northwest and a
more benign showery pattern into Friday.

Looking into next weekend, WPC ensemble clusters show strong
agreement on upper level ridging beginning to poke into the
Pacific Northwest from the Pacific as the upper trough
continues to sink south towards southern CA. This is reflected
in a drier forecast with NBM temperatures well into the 60s
across the area next Saturday and Sunday, with NBM 75th
percentile guidance showing temps in the low 70s. So, after
another period of stormier weather mid to late week, it appears
that warmer and sunnier conditions may be on the horizon as we
head into the beginning of April. /CB

&&

.AVIATION...High resolution models are in good agreement conditions
will deteriorate into a mixture of low end VFR and MVFR conditions
across much of northwest Oregon and southwest Washington between
now and 02z Tuesday as a weak front slides across the region. Low
level cold air advection behind the front should result in
conditions trending more into VFR thresholds although low
probabilities (10-20% chance) of brief dips into MVFR conditions
will persist overnight. Low level cold air advection begins to
ease between 12-18z Tuesday so high resolution model guidance
suggesting increasing probabilities for MVFR conditions returning
looks on track.

* 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...High resolution models are in good agreement
conditions will deteriorate into a mixture of low end VFR and MVFR
conditions across much of northwest Oregon and southwest
Washington between now and 02z Tuesday as a weak front slides
across the region. Low level cold air advection behind the front
should result in conditions trending more into VFR thresholds
although post frontal showers will still bring a low probability
(10-20% chance) for MVFR conditions at any given hour overnight.
/Neuman


&&

.MARINE...Overall expect active conditions to develop by the
middle of the week as several weather disturbances take aim at the
region. Today, a weak front traverses across the coastal waters
bringing a brief burst of breezy southwesterly winds this morning
before shifting northwesterly this afternoon. It still appears
these winds approach Small Craft Advisory criteria south of Cape
Falcon(20-25 knots), although confidence in widespread frequent
gusts exceeding the ~22 knot threshold is still low(20-40%) at
this time. As such, given the very marginal nature of the
conditions Small Craft Advisories were not issued but anticipate
at least a few isolated gusts nearing 25 knots. Otherwise,
conditions remain rather benign through Monday night and Tuesday
as a transient ridge of high pressure leads to light to moderate
westerly breezes and a slowly building westerly swells to 7-9
feet.

Come the middle of the week our focus shifts to the arrival of a
strong low pressure system into the NE Pacific accompanied by a
frontal boundary slated to move overhead Tuesday night into
Wednesday morning. Both deterministic and ensemble guidance
continue to indicate a high probability(60-95%) for Gale Force
winds during this time period. With little change in the overall
setup and projected wind speeds/gusts over the last several model
runs, a Gale Watch has been issued for the outer waters, inner
waters, and Columbia River Bar Tuesday night through Wednesday
morning. Thursday into Friday the now vertically stacked upper-
level low will gradually shift eastward towards the Washington
coastline. We’ll have to keep an eye on potential smaller
disturbances swinging around the larger low feature which could
result in an increase to already near to low-end gale force winds
focused over the outer waters - one of which traverses just to our
south. A large westerly swell is also forecasted to arrive
Wednesday night into Thursday, pushing combined seas near 15-20
feet before decreasing late in the week. Mariners should expect
hazardous marine conditions from Tuesday night into Friday before
conditions improve over the weekend. /Schuldt

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Gale Watch from late Tuesday night through Wednesday morning for
PZZ210.

     Gale Watch from late Tuesday night through Wednesday morning for
PZZ251>253.

     Gale Watch from Tuesday evening through Wednesday morning for
     PZZ271>273.

&&


$$

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