018
FXUS66 KPQR 132158
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
258 PM PDT Mon May 13 2024

.SYNOPSIS...High pressure ridging returns Tuesday, maintaining
warm, dry, and mostly clear conditions through at least Thursday.
Uncertainty remains with whether or not this trend continues
over the weekend.

&&

.SHORT TERM...Tonight through Wednesday...Satellite imagery as
of 230 PM PDT shows mostly clear skies across northwest Oregon
and southwest Washington. An exception would be the south
Washington and far north Oregon coast, where marine clouds have
been persistent today. Decent onshore flow has been keeping
temperatures relatively cool compared to the last few days.
We`re still on track to have highs this afternoon peak in the
low 70s for interior valleys and upper 50s/low 60s for the coast
and higher terrain.

Tonight, we could see marine stratus develop again, but it`s
not certain for all coastal locations. Models are showing a
thermal trough forming along the southern Oregon and northern
California coast. The influence of this thermal trough could
extend into the central Oregon coast. If this happens, then
offshore flow would develop and inhibit low stratus formation
along the coast (mainly south of Manzanita). The south Washington
coast and far north Oregon coast look more likely to keep an
onshore component to the wind, however, it will ultimately
depend on how far north the thermal trough extends.

An upper level ridge over the northeast Pacific will shift
toward the U.S. West Coast and return a warming trend through
mid-week. Expect mostly clear skies and dry conditions. High
temperatures will gradually increase beginning tomorrow, peaking
on Wednesday. NBM probabilities for exceeding 75 degrees on
Tuesday across the Portland/Vancouver metro are around 60-80%
and 30-50% for the central and southern Willamette Valley. On
Wednesday, highs for interior valleys will approach 80, as NBM
shows high confidence (80-90% chance) for temperatures exceeding
75 degrees for interior valleys. During the day Tuesday and
Wednesday, thermal troughing remains confined to the southern
Oregon coast, so development of breezy east flow looks unlikely
over our area. We`ll mostly maintain northerly/northwesterly
winds. In addition, models showing KTTD-KDLS pressure gradients
of +3 to +5 are another indicator that we`ll maintain an
onshore wind component.      -Alviz

&&

.LONG TERM...Wednesday Night through Sunday...WPC cluster
analyses are in agreement of upper level ridging continuing over
the Pacific Northwest through Thursday. This will maintain dry
and warm conditions under predominately clear skies. On Friday,
the majority of clusters show this ridge beginning to flatten,
returning mostly zonal flow. Only 20% of members show weak
troughing returning on Friday from Canada, but even then
precipitation looks minimal. On the flip side, only about 10% of
members show the ridge building further. In this case, we would
likely see a return of cooler temperatures if the ridge breaks
down by the end of the week.

Uncertainty in the upper level pattern over the Pacific
Northwest continues into the weekend, however, one thing the
clusters all have in common is a high pressure ridge over the
northeast Pacific Ocean. Most of the uncertainty relates to how
much of an influence this ridge has on our area. Clusters still
suggest a 50% chance of ridging and a 50% chance of zonal flow
or troughing going into the weekend. Thus, NBM does show around
a 9 to 11 degree spread between the 25th and 75th percentiles of
high temperatures over parts of the Willamette Valley.

If models trend toward a troughing/zonal flow pattern, we`ll
likely see temperatures cool and potentially a slight chance
(15-24%) of PoPs. If models trend toward keeping the ridge,
then expect the forecast to maintain warmer, drier, and clearer
conditions.      -Alviz

&&

.AVIATION...The previous front has passed through the area, though
some marine stratus clouds remain at the northern coastal
terminals (KAST), continuing to reduce cigs to MVFR thresholds.
Daytime heating should burn off cloud cover by mid/late afternoon
(00z Tue), after which a period of VFR thresholds is expected.

Further inland, VFR conditions with essentially clear skies
continue throughout the rest of Monday day and night as high pressure
continue to impact the area. Gustier winds will be possible from
the north, with gusts up to 15-25kt possible at most terminals.

Later Monday night, model guidance is trending towards the
thermal trough developing around the coast around 09-15z Tue,
resulting in easterly flow and clear skies through the rest of the TAF
period. If this falls apart, we could see some marine clouds
continue to reduce visibilities back to MVFR at the coast, around
a 40% chance of MVFR cigs. The northern coast sees better chances
of MVFR cigs, closer to 70%. /JLiu

PDX AND APPROACHES...VFR with northerly winds. Breezy northerly
winds continue as high pressure redevelops behind the front.
Slight chance (30%) for MVFR CIGS around 15z Tue due to the
thermal trough shifting inland, but still predominantly expecting
VFR. /JLiu

&&

.MARINE...Post-frontal conditions continue to support breezy northerly
winds that will intensify as high pressure builds over the area
and a slight thermal trough forms over the coastline. Winds are
currently gusting to about 15-20 kt, but will ramp up again in
the late afternoon/early afternoon. From there, winds will
increase further to 20 to 25 kt with gusts up to 30 kt starting
around sunset on Monday. The winds will start picking up in the
inner waters out to 10 NM, then spread north and west. Small
Craft Advisory remains on track to start around 1600 through
Tuesday.

In regards to seas, nothing overly remarkable as the
primary swell is from the northwest, and the wind wave from the
north. Wind waves will range from around 4 to 6 feet. Combined
seas will be 6 to 8 ft through Tuesday, though conditions ramp up
mid-week. -Muessle/JLiu

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 5 PM PDT Tuesday for PZZ251>253-
     271>273.
&&

$$

www.weather.gov/portland

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