Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 291615
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
915 AM PDT Mon May 29 2017

.SYNOPSIS...Cooler for Monday, with a lot more in way of low clouds
in the morning into early afternoon. Approaching low pressure from
the west will bring increasing southerly flow aloft. This will
result in threat of late afternoon/early evening thunderstorms over
the central Oregon Cascades today. THe thunder threat spreads
further north Tuesday. Otherwise, back to typical late May/early
June weather, with partly to mostly cloudy skies and near seasonable
temperatures.
&&

.SHORT TERM...Today through Wednesday Night...Marine clouds filled
in the western valleys pretty solidly overnight, with 12Z SLE
sounding indicating the marine layer up to about 3K ft. With the
deep marine layer in place, and a continuation of onshore flow today
with the termal induced trough east of the Cascades, clouds will not
be in any hurry to burn off today. Breakout program suggest a
breakup of valley clouds around midday to early afternoon. Along the
coast models suggest clouds may linger through most of the day. Will
reduce afternoon high temps a few more degrees given the depth of
the marine layer and likelihood of clouds lasting past noon. Water
vapor pictures  showed a low cutoff off the north CA coast this
morning, moving slowly east. This feature would seem likely to cut
off the northward spread of moisture and instability past sw or
central OR today. Model soundings indicate a warm layer in mid
levels under 700 mb that should also act to inhibit low level based
convection today. Will limit any mention of thunderstorms to areas
generally south of Santiam Pass today, and also indicate a more
limited area with the slight chance for this evening.

Remainder of short term discussion unchanged...Later this evening
and overnight, models continue to show some form of light shower
activity across the region. Confidence is very low for precise
location thus have broadbrushed the threat. Amounts should remain
rather light, however.

For Tuesday, models are in much better agreement regarding the shape
and strength of the short-wave trough approaching the area. The
trough axis passes overhead Tuesday afternoon with the colder air
aloft holding off until after dark. This will greatly reduce the
potential for thunderstorms across the area and have reduced the
threat to the Cascades. Even then, do not have strong feelings they
will develop west of the Cascade crest. Best chance will be Tuesday
afternoon but feel most activity will develop over the east slopes
late Tuesday afternoon and evening leaving just modest shower
activity on the west side.

The longer wave upper trough will split apart Wednesday with another
segment of energy crossing south of the region and into northern
California with the primary low staying well northwest. This could
leave Wednesday largely dry as we sit in between these two segments.
There may be enough left of a trailing cold (cool?) front to bring
light rain Wednesday night, but it is already showing signs of
splitting apart on models which usually leads to little or no precip
in reality. /JBonk

.LONG TERM...(Thursday through Sunday) No changes. Bulk of
the previous discussion follows. Additional shortwaves will move
through the area Thursday and Friday which will increase shower
chances. The forecast becomes a lot more uncertain towards next
weekend as the ECMWF tries to cut off an upper low overhead while
the GFS strengthens a ridge into the area. Went with a consensus
blend today and will work to add more details this week as things
become more clear. /Bentley

&&

.AVIATION...Mix of mostly low MVFR/IFR cigs across the area. With
tops around 2500-3500 feet, expect cigs to burn off north to
south between 19Z-21Z today inland for VFR conditions through the
rest of the afternoon and this evening. Cigs along the coast
will likely persist through the day today, lifting to MVFR this
afternoon, then lowering back to IFR/LIFR tonight. Expect another
surge of marine stratus inland overnight tonight, perhaps a
couple hours earlier than last night due to stratus persisting
along the coast.

KPDX AND APPROACHES...Cigs around 1000-1500 ft this morning will
continue to slowly lift but keep MVFR cigs until stratus burns
off around 19Z-20Z. After that, expect VFR conditions through
tonight, with high IFR cigs returning Tuesday morning around
08Z-10Z. -McCoy

&&

.MARINE...No changes. Previous discussion follows...
Quiet conditions continue with winds below 20 kt through at
least the middle of this week and likely longer. Winds turn late
Tuesday from out of the northwest to out of the southwest then
south. Seas around 4 ft and will continue through at least the
middle of the week.

Next chance for winds above 20 kt will be next weekend as a
broad upper-level ridge builds over the NE Pacific, setting up a
summer-like gusty northerly wind pattern over our waters.
Bowen/McCoy

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.

&&

$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.



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