Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 270339
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
838 PM PDT Fri Aug 26 2016

.SYNOPSIS...A pattern change is beginning this weekend that will
bring an end to the hot weather, dropping temperatures closer to
seasonal normals this weekend and possibly below normal next week,
with a few showers possible at times. The upper ridge over the
northeast Pacific of the past few days is being replaced by a broad
trough along the B.C. coast this weekend with westerly flow aloft in
the Pacific Northwest. The trough will probably dig south near or
just off the coast next week, with a few showers around Tuesday and
again later in the week.
&&

.SHORT TERM...(Tonight through Monday)...A pattern change is in the
works tonight as the flow aloft is turning more northwesterly tonight
and westerly on Saturday as the upper ridge in the northeast Pacific
of the past few days is giving way to an upper trough dropping south
along the B.C. coast. This is resulting in a southerly low stratus
and fog surge up the coast this evening. This surge should affect the
whole coast by later tonight, then be reinforced by westerly onshore
flow later tonight and Saturday. The models indicate some drizzle and
sprinkles are possible along the coast tonight and Saturday morning.
Some of the low clouds may spread inland into the coastal gaps early
on Saturday but don`t believe the coverage will be very much yet
inland.

After another day today which saw records at Astoria, Salem, and
Eugene, with 90s in the valleys and at Astoria and Tillamook, temps
will drop closer to seasonal normals on Saturday. Thus the heat
advisories will be ending this evening.

As this trough develops along the B.C. coast tonight and Saturday,
the models show a weak boundary pushing into the north coast Saturday
afternoon and evening. This will spread some clouds inland across the
north part of the forecast area as well, but precipitation inland is
looking unlikely. However, this will likely lead to greater coverage
of morning clouds inland on Sunday morning. Nevertheless, the models
are also indicating that the threat of any precipitation Sunday is
looking minimal.

The models are then saying the upper trough will dig south just off
the coast Sunday night and Monday. The resulting southwest flow aloft
across the area will maintain the onshore flow with night and morning
low clouds. The threat of precipitation Monday is looking minimal as
well, except perhaps a small chance along the north coast. Temps will
remain near or perhaps slightly below seasonal normals. Tolleson
&&

.LONG TERM...No Changes. Previous discussion follows...
(Monday night through Friday)...The trough that models dig off the
Pacific NW coast over the weekend is expected to linger through much
of the week. Models indicating a SW marine push Monday night
preceding a weakening cold front expected to push inland Tue and Tue
night, which is a bit slower than earlier model runs so will need to
delay the spread of pops onto the coast more towards Tuesday morning.
In addition to the weak cold front and northern shortwave pushing in
across the north Tue for a chance of showers across the NW part of
the forecast area, a second shortwave passes to the south of the
area. GFS is the furthest north with this feature, and warrants
carrying a slight chance of thunder in the Lane county Cascades as
the southerly flow preceding it potentially spreads instability up
from the south.

Another shortwave digging down the backside of the trough Wednesday
holds the main trough axis offshore, reducing pops for Wednesday,
although the southerly flow in the GFS continues a threat for thunder
over the Oregon Cascades. With models diverging some through the rest
of the week but troughiness remaining, will hang on to low pops
through the rest of the week. High temps through the extended period
likely to remain below normal.
&&

.AVIATION...Marine stratus will continue to spread north overnight
bringing widespread IFR/LIFR conditions to the coast. LIFR stratus
has already returned to KONP this evening, with the stratus layer
starting to approach KTMK. Expect the stratus layer to reach KAST
between 05-07Z tonight. There could be a few hours of vsbys below 1SM
tonight as the layer settles but suspect it will be patchy at best.
Conditions likely improve to MVFR Sat afternoon along the north
coast, but expect IFR cigs to continue on the central coast.

The interior will generally remain VFR under sporadic high clouds.
However, the south valley could see some patchy low stratus early Sat
morning as the marine layer ties to filter inland through the coastal
gaps under southwesterly flow. Could also see some patchy low stratus
along the Columbia River Sat morning, but confidence is low.
Otherwise, conditions remain VFR tonight and Sat.

KPDX AND APPROACHES...VFR through Sat under sporadic high clouds.
There is a slim chance that some patchy low stratus may develop along
the Columbia River Sat morning which could bring a few hours of MVFR
cigs, but confidence is low. /64
&&

.MARINE...No changes. Previous discussion follows...A low pressure
system aloft will bring an end to the north winds and usher in a
period of benign marine weather that will continue into next week.
There is a slight chance of 20 knot winds briefly ahead of a few
fronts which may cross the waters early next week, but overall winds
will remain below 20 knots. In addition, seas will remain 6 feet or
less this weekend and drop below 5 feet early next week. /Bentley
&&

.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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