617
FXUS66 KPQR 030427
AFDPQR

Area Forecast Discussion...Updated Aviation
National Weather Service Portland OR
926 PM PDT Thu May 2 2024

.SYNOPSIS...Dry conditions with partly sunny skies through this
evening between systems. Heavier rain arrives Friday into
Saturday, but we are not expecting any widespread hydrological
concerns at this time. Cool, wet, unsettled weather likely
continues through the middle of next week.

&&

.SHORT TERM...Through Saturday Night...We are starting to see a
few breaks of sunshine across northwest Oregon and southwest
Washington this afternoon as skies begin to partially clear
from the north behind a departing shortwave trough exiting east
of the Cascades. Short term guidance is hinting at a slight
chance of thunderstorms over the south Washington Cascades
through early this evening as a trailing disturbance clips the
northern edge of the area, but for the most part expect any
convective activity to remain north of the area. Expect the rest
of the area to remain dry into Friday morning, with high level
cloud cover starting to filter back into the region later this
evening ahead of the next system dropping out of the Gulf of
Alaska.

That next system is readily apparent on water vapor satellite
imagery this afternoon as a developing low centered near 52N
144W. The low will continue to amplify as it digs southeastward
to a position several hundred miles off the Oregon Coast by
Friday evening, sending a surface front across the area Friday
afternoon into early Saturday. The forecast remains on track
with respect to expected rainfall amounts, which continue to
look impressive by early May standards but are not expected to
pose any widespread hydro concerns at this time. Underlying
metrics have not changed much as IVT values briefly max out
around 250-400 kg/ms Friday evening, with precipitable water
values around 1 inch running two standard deviations above
normal. The progressive nature and track of the system still
look to be the saving grace for more widespread hydro issues as
rain will transition to showers by Saturday afternoon as the
low passes south towards the California border. All told, QPF
amounts are still holding around 1-1.5 inches for the
interior lowlands, and 1.5-2 inches for the coast and mountains
from late Friday morning to Saturday afternoon, with HEFS
probabilistic guidance still showing a less than 10 percent
chance to reach action stage on all area rivers and less than a
5 percent chance on most of them. Snow levels will drop back to
around 3000 feet as the colder air arrives on Saturday, bringing
snow back to the Cascade passes. May need to consider another
round of Winter Weather Advisories on Saturday as the event
draws closer, with another 5-8 inches of snow looking possible
from roughly Santiam Pass down through Willamette Pass through
Saturday night. Finally, a few thunderstorms may be possible
across the southern half of the area on Saturday afternoon as
the upper low clips the area and colder air arrives aloft. /CB

.LONG TERM...Sunday through Thursday...Little change in the
forecast through mid to late next week as WPC ensembles
continue to depict upper level troughing keeping an active,
cool, and showery pattern in place across the Pacific Northwest
through Wednesday or Thursday. Still do not see any signals for
particularly notable QPF in any one period beyond Saturday, but
area rivers will likely continue to run high for this time of
year as episodic bouts of rain continue across the region.
Temperatures will continue to run a bit below normal, with
daytime highs struggling to get out of the 50s through Thursday.
The bulk of the model guidance does continue to hint at high
pressure towards the end of next week, so there may be some
light at the end of the tunnel for warmer and drier conditions
returning beyond next Thursday. /CB

&&

.AVIATION...The next front is beginning to approach the area, with
southern coastal areas (KONP) already seeing pre-frontal cloud
cover, bringing ceilings to high end MVFR thresholds. The front
properly moves ashore around 20z Friday, reaching the Willamette
Valley around 22z Fri to 0z Sat. Model guidance indicates around a
60-70% chance of high end MVFR conditions at that time at all
terminals. The southern coast (KONP) sees the most precipitation
trained there, and around a 70% chance of IFR conditions is expect
at that time.

Winds remain fairly variable and light (<5 kt) until the front
comes in properly, but once the front arrives, winds will take on
a stronger southerly character. Winds at all terminals will begin
to gust to around 22-24 kt from the south around 20-22z Fri, then
gradually turn west and weaken through the rest of the TAF
period. Snow will continue over the Cascades with this frontal
passage as temperatures are cold enough to allow for it.

PDX AND APPROACHES...VFR conditions continue, with very little
cloud cover until around 13z as clouds ahead of The next front
move in. A high-end VFR overcast deck will begin to develop at
that time. The front pushes in right around 00z Sat, and around a
50-60% chance of high end MVFR ceilings begins to develop shortly
afterwards at 03z Sat. Winds will pick up around 22z shortly ahead
of the front as well, with southerly gusts around 10-12 kt at that
time. /JLiu

&&

.MARINE...Weak high pressure over the waters will erode tonight as
a strengthening low pressure system drops down from the Gulf of
Alaska. This low is preceded by a cooler front which will cause
winds to shift to the south. Winds along the coast will not be as
high as the outer waters as they are not aligned directly to the
coastline. Winds will be in the 15 to 20 kt with gusts to 25 kt
range from 10-60 NM. The coastal waters will hover right around 20
kt. The north Oregon and central Oregon inner waters may still
see isolated gusts up to 25 kt Friday afternoon. Due to a lack of
widespread increased winds, have decided not to issue a small
craft advisory for the inner waters. However, there remains a
slight probability in the case the low tracks a little further
north than currently project. Will add, that the rain will be
quite heavy so visibility may be difficult at times. Seas will
rise to around 6 to 8 ft at 12 to 14 seconds due to the amplified
wind wave. Not expecting to exceed 10 ft. -Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 8 AM to 11 PM PDT Friday for
     PZZ271>273.

&&

$$

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