Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 291059
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
359 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) Low clouds continue to
stream inland through the coast range gaps. They will continue to
largely fill the I-5 corridor this morning and will result in
another 3-5 degree drop to temperatures today inland. Tricky area
will be the Columbia River Gorge and just how far east the cooler
marine air will influence conditions today. Feel the surface thermal
trough will still have some influence for the central gorge with the
Hood River area still making it into the upper 80s today but then
rapidly cooling this evening as a westerly marine push dominates.

On the larger scale, the approaching trough will stay offshore today
but there is a southern piece off the northern California coast
which has undergone cyclogenesis spinning up into a nearly mesoscale
closed low. That appears to have taken enough energy to now confine
the thunderstorm threat closer to Willamette Pass. Most of the
cooler air aloft will remain over the ocean and this leave a
generally more stable air mass over the area. Additionally, the
cooler inland temperatures will make it harder for surface parcels
to overcome a midlevel cap sitting around 750 mb. 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...High IFR CIGs around 900 ft will fill most of the
interior tonight and early Monday starting in the south and
working northward as well as spreading up the Columbia River to
KPDX. Clouds will dissipate over the interior through midday Mon
while coastal areas are likely to remain cloudy.

KPDX AND APPROACHES...CIGs at 800 to 900 ft through much of
Monday morning, dissipating by around 18Z for SKC the rest of
the day. Bowen

&&

.MARINE...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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