Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 190321 AAA
AFDPQR

Area Forecast Discussion UPDATED
National Weather Service Portland OR
820 PM PDT Mon Mar 18 2024

EVENING UPDATE, for aviation weather.

.SYNOPSIS...Unseasonably warm weather through Tuesday with strong
high pressure aloft. Gradual return to more typical March weather by
midweek as high pressure weakens, allowing a weak low pressure
system to brush the coast with some light rain as early as Wednesday
morning. A significant cooling trend is expected to start Wednesday
with a more substantial systembringing cooler and moister weather
across the region towards Friday/Saturday and will likely linger
into the following week.

&&

.DISCUSSION...Tonight through Sunday...Unseasonably warm
temperatureswill continue through the middle of the week as a ridge
of high pressure continues to be the dominant synoptic feature
across the Pac NW. Daytime highs will be in the low 60s to mid 70s
on Tuesday thanks to a slightly stronger onshore push. Overnight
lows will also be on the warm side and be in the mid to upper 40s
across most of the forecast area, with slightly cooler temperatures
expected for the Cascades. Expect a return of fog along the coast
tonight along with some patchy fog within the Willamette Valley. The
cooling trend will continue through the week along with some light
precipitation on Wednesday and a stronger signal for precipitation
by Friday through next Sunday.

Now, with the general overview done, lets get into the details. A
pattern change is expected as deterministic, ensemble and high
resolution models all have a very broad/disorganized area of low
pressure moving into the eastern Pacific Ocean and pushing the ridge
pf high pressure axis east of the Cascades over the next 24 hours.
As this happen expect a stronger onshore push, will will be the
harbinger of a cooler and wetter pattern for the remainder of the
week and into next Sunday. What is causing this change is a weak,
upper level low embedded within this broad area of low pressure.
Models are showing much cooler 850 mb temperatures being brought
into the Pac NW. Comparing the 850 mb temperatures over the next 48
hours, will help clarify what is currently in the forecast. For
example, models have 850 mb temperatures around 11C to 13C today
(Monday), 7C to 11C Tuesday and 3C to 5C by Wednesday. This cooling
trend will translate into daytime highs in the the upper 60s to low
70s for inland locations, mid 50s to low 60s for the OR/WA coast and
Coast Range and low to mid 60s for the Cascades for Tuesday.
Wednesday, mid to upper 50s for inland locations, upper 40s to mid
50s for the OR/WA coast, Coast Range and Cascades for Wednesday.
Also, with this upper level low passing, precipitation chances will
also increase. But, looking at deterministic model QPF spread, this
is shaping up to be a high PoP/low QPF event. As we move beyond the
middle of the week and through next Sunday, the WPC 500mb clusters
are in good, general agreement that broad troughing pattern across
the western half of CONUS. This means that a cooling trend is
expected, with snow levels falling towards 4000-3000 ft by
Friday/Saturday and yes there is some precipitation associated with
the low towards the end of this week.
While overall QPF values will likely change, if the pattern holds
rain and snow will return to the region. /42

&&

.AVIATION...High pressure remains over the region, maintaining
mostly clear skies across the interior. However, marine stratus will
quickly spread along the coast this evening, with that stratus
drifting inland into the Coast Range valleys and up the Columbia
River for a while overnight. Not sure if this stratus will punch
into the Willamette Valley. However, with light winds and cool air
mass, should see patchy fog and areas of low stratus in the interior
from KPDX area southward to KEUG, with lowest flight conditions
being south of Salem towards KEUG. Will trend 06Z TAFs as such.

On Tuesday, stratus will persist along the coast, but lift to lower
MVFR by midday (18Z-20Z). Farther inland, any stratus or fog that
does form will dissipate by 18Z for the most part, with VFR
afterwards under mostly clear skies.

* Note: The observations out of KONP are generally limited. As such,
  ammendments are not expected.

PDX AND APPROACHES...VFR tonight and most of Tue, as high pressure
remains over the region. However, weak onshore flow in the low
levels should pull in enough marine air from the coast to provide a
period of IFR or low MVFR stratus between 12Z and 18Z Tue. Suspect
will be rather scattered to broken in nature.           /Rockey
&&

.MARINE...We`ll continue to see an large area of high pressure
sit over the Pacific Northwest today with northerly winds over the
waters persisting through tonight into Tuesday. Over the next
several days, the high pressure overhead will weaken and shift
eastward before we transition to a more active weather pattern
Wednesday through the end of the week. Mid period westerly swell
around 4 to 6 ft expected to last into mid-week. /Schuldt

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

www.weather.gov/portland

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