Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 200937

Area Forecast Discussion
National Weather Service Portland OR
237 AM PDT Mon May 20 2024

.SYNOPSIS...Stratus across portions of the region this morning
with low temperatures falling to the mid 30s for some areas in
the southern Willamette Valley and upper Hood River Valley.
Dry and mostly sunny today. A weather system will bring more
widespread but generally light rain across the region Tuesday.
Showers with a thunderstorms possible on Wednesday. Will
maintain unsettled with near to slightly below average
temperatures for rest of the week into the weekend.


.SHORT TERM...Monday through Wednesday...Satellite imagery early
Monday morning shows stratus forming along the western slopes
and foothills of the Coast Range and Cascades. HREF and sounding
guidance indicates this stratus could spread up the Columbia
River Valley as well as back build from the eastern Cascades
into portions of the Willamette Valley, including into the
Portland metro area. Stratus will dissipate through the morning
hours and should be mostly clear by midday. Temperatures as of
2 AM remain fairly mild with 40s across the lowlands and the
upper Hood River Valley and 30s in the higher elevations of the
Coast Range and Cascades. Will still keep the Frost Advisory in
place through 8 AM as dew point temperatures have fallen into
the upper 30s in the lowlands, and HREF indicates a 20-50%
chance of temperatures falling to or below 35 degrees this
morning for the southern Willamette Valley south of Salem.

For the rest of today, expect mostly clear skies and dry weather
under a period of brief upper level ridging. Temperatures will
be warmer than previous days, warming to the mid to upper 60s
across the interior lowlands. However, probabilities of reaching
70 degrees for the Portland metro area have decreased to 30-45%
based on latest NBM guidance, likely due to the lingering
stratus. However, if stratus is not as thick as forecast or does
not last as late into the afternoon, then would not be surprised
if some metro locations reach the low 70s this afternoon.

Ensembles indicate the next frontal system is slated to move
through NW Oregon and SW Washington Tuesday. This front and
associated upper trough are moving in from the northwest, so it
will bring some colder air along with it as well as a band of
widespread stratiform rain across the region. The Willamette
Valley will likely be rain shadowed due to the direction of the
flow with higher precipitation amounts in SW Washington
(including lowlands), the Coast Range, and Cascades. NBM
indicates only a 20-40% probability of 0.25 inch or higher rain
amounts in 24 hours in the central and southern Willamette
Valley ending 5am Wednesday. These probabilities increase to
40-50% in the Portland/Vancouver metro area, and 75-90% in the
lowlands north of Clark County. The Coast Range and Cascades
north of a line from Tillamook to Government Camp are forecast
to receive the highest QPF, anywhere from 0.75-1.25 inch. NBM
indicates 50-70% probability of QPF greater than 0.75 inches for
these areas through Tuesday night. Snow levels will remain
above 5500-6000 feet through Tuesday, so snow at or below pass
level is not expected. As the front will move through the
region throughout the day, daytime temperatures in central
Oregon may not fall as much as areas to the north. Expect
lowland highs in the mid 50s to mid 60s, warmest in Lane County.

Ensemble guidance remains in good agreement (85% of WPC 500mb
clusters) that a closed upper low tracking south along the BC
coast Monday and Tuesday will continue the southern track along
the northern Washington coast Tuesday night before moving
southeast into eastern Oregon through the day Wednesday. This
pattern will allow onshore flow to continue with another round
of showers. Some uncertainty remains on exactly how deep into
Oregon the low will track with some clusters hedging the low
mostly along the Washington/Oregon border with others pushing it
farther east into east central Oregon. However, this would only
affect the amount of precipitation expected but not the overall
impacts. Generally expecting light precipitation less than
0.15 inch in the lowlands with upslope flow producing 0.25-0.75
inch over the higher terrain. However, there could be widespread
thunderstorm chances in the afternoon and evening hours depending
on how deep the trough moves into Oregon as the cold core moves
aloft. Heavier showers and small hail would be likely in this
scenario. -HEC

.LONG TERM...Thursday through Sunday...Thursday is looking
mainly dry as brief ridging builds in the eastern Pacific.
However, ensembles are beginning to show the potential for a
weak shortwave to form on the back end of the closed low on
Thursday, which would keep light shower chances continuing,
mainly for the higher elevations. WPC 500 mb clusters continue
to indicate a general trough pattern Friday into the weekend,
though specific details on timing and location of weather
systems and any potential precipitation amounts are still very
uncertain at this time. One aspect that does seem a little more
certain is temperatures are expected to remain fairly mild
through the weekend. NBM indicates a 75-95% chance that interior
lowland temperatures will warm back into the 60s and remain in
the 60s through the weekend. -HEC


.AVIATION...Current satellite imagery early this morning shows a
low stratus deck continuing to thicken along the western slopes of
the Cascades and Cascade foothills due to persistent upslope flow.
High resolution guidance remains confident in this layer back
building further west into the north Willamette Valley and
Portland/Vancouver metro by 12-14z ushering in high-end MVFR cigs
before breaking up by 18-21z Mon returning conditions to VFR.
Elsewhere, (excluding KAST where MVFR cigs are also anticipated
this morning) expect generally VFR conditions with scattered
cloud cover as the region maintains north/northwest flow aloft.
Winds will be mostly calm to start the day then pick up to 5-10
kt with occasional gusts to 18-22 kt possible during the
afternoon into the early evening along the coast and in the
Willamette Valley before decreasing again overnight.

PDX AND APPROACHES...Headed through this morning guidance
continues to suggests a 70-90% chance of MVFR cigs between roughly
~12-18z due to low clouds in the Cascade foothills shifting
westward overhead. Otherwise, once this layer breaks up 18-20z
expect predominately VFR conditions with scattered cloud cover.
Winds will be northwesterly and around 5-10 kt although occasional
gusts around 15-20 knots will be possible around 22-03z. -Schuldt


.MARINE...Small Craft Advisory conditions will persist for the
outer waters today with N-NW wind gusts near 20-22 knots to start
before the concern shifts to steeper seas of ~7-8ft at 7-8
seconds. Fortunately, calmer conditions return tonight into early
Tuesday morning, albeit briefly, before the next frontal passage
arrives on Tuesday bringing rain and a burst of northwesterly
winds. Confidence is high (70-80%) wind gusts reach into the 20-25
knot range for the outer waters north of Cape Foulweather Tuesday
evening/night in the post-frontal environment  additional Small
Craft Advisories may be needed. A northwesterly swell of 8 to 10
feet at 9-10 seconds then arrives Wednesday morning as winds begin
to subside. Looking ahead, calmer winds/seas return Thursday
followed by the potential for yet another weather disturbance
moving into the region late Friday/Saturday continuing the rather
progressive and active weather pattern. -Schuldt



OR...Frost Advisory until 8 AM PDT this morning for ORZ116>118-121-


PZ...Small Craft Advisory until 5 AM PDT early this morning for

     Small Craft Advisory until 11 PM PDT this evening for PZZ271>273.


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