589 
FXUS66 KPQR 190446 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
946 PM PDT Mon Oct 18 2021

Updated aviation discussion

.SYNOPSIS...After a brief break in the wet weather through Tuesday,
expect a very active weather pattern to bring valley rain
and high mountain snow every 12-24 hours through early next week.


&&


.SHORT TERM...THIS EVENING THROUGH THURSDAY...Clouds have been 
slow to erode in the southern Willamette Valley and Cascades so far 
this afternoon, and it's doubtful whether they'll clear before the 
day's over, but the radar remains clear and observations have been 
coming in dry across the board the past several hours. The next 
timeframe that area-wide rain will be likely begins tomorrow 
evening. Between now and then, on the back side of the cut-off low 
that brought rain to parts of the region yesterday, the overall 
pattern is tranquil in nature, with weak pressure gradients that 
will favor light winds. The area is likely to be cooler than normal 
today with highs generally in the 50s across the board in the 
lowlands. It will be a little warmer tomorrow in warm air advection 
ahead of the next approaching trough, though there is some 
uncertainty in how warm due to uncertainty in how cool it will get 
tonight. Sky guidance is overwhelmingly indicative of clearing late 
this evening through the overnight, with hints that radiational fog 
and/or low stratus will develop in the early morning - mainly across 
the central and western Willamette Valley. If this does occur, the 
very presence of fog would imply maximum surface cooling overnight - 
with temperatures falling to the dewpoints (currently forecast to be 
in the 30s early tomorrow morning) in a layer at/very near the 
surface. But then the fog/low stratus itself could hinder surface 
heating for part of the day, depending on the depth of this 
saturated layer (the deeper it is, the longer it takes for the sun 
to "burn" it off), and a later burn off time means surface heating 
starts later; this, in turn, means that there is less time for the 
surface to heat before the sun begins to go down and outgoing 
longwave radiation from the surface becomes greater than the 
incoming shortwave that it's receiving; all this is to say 
forecasting tomorrow's high is tricky.

What's a little easier is assessing the time of arrival for 
precipitation, which looks to arrive first in the Florence area 
during the 4-7 PM timeframe tomorrow before quickly overspreading 
the remainder of the area over the successive several hours. A weak 
surface low will then move along the coast, dragging with it a cold 
front that will bring an end to the steady rain and introduce a 
threat for showers - especially along the coast and in the Coast 
Range - Wednesday. NBM continues to suggest the possibility of 
thunder over the waters Wednesday morning, and the SREF doesn't 
disagree, so for now that has been left in the forecast. The low 
will also yield some gusty winds Wednesday morning. As is usually 
the case in these scenarios, the coast and Coast Range will receive 
the brunt of the impact from these gusty winds, but the interior 
lowlands and even the Cascade Foothills are looking gusty too. The 
latest runs of the HRRR and ARW are suggestive of gusts to around 35 
to 40 mph (perhaps higher in spots) in the southern and central 
Willamette Valley, 40 to 45 mph in the Coast Range and Cascade 
foothills, and 45 to 50 mph along the shoreline (with gusts in the 
high Cascades at least that high). For now, the forecast is a
conservative blend of high resolution models, but it could certainly
be a bit gustier if the higher end of guidance shakes out.

The most favored timeframe for a break in precipitation looks to be 
late Wednesday evening through around lunchtime on Thursday. 
Thursday evening an occluded front from the bombed out low in the 
Gulf of Alaska nears the area to bring another round of widespread, 
gentle rain to all of NW Oregon and SW Washington from Thursday 
evening through Friday night. -Bumgardner


.LONG TERM...FRIDAY THROUGH SUNDAY...If the ECMWF and GFS 
are right, the deep cut-off low in the Gulf of Alaska gets picked up 
by another upper trough swinging down from the northwest, and 
subsequently merges with another surface low which will bring in 
another cold/occluded front during the Saturday/Saturday night time 
period. Ahead of the front, the area should experience another round 
of rain; behind it, the pattern turns showery again.

Forecasting becomes exceptionally difficult beyond Saturday, as the 
pattern stays active, but each ensemble member seems to have a 
different depiction of the placement and strength of any storm 
system to approach our area on and beyond Sunday. In the left exit 
region of a ~150 knot 200-300mb jet extending across central/ 
northern CA into northern NV, strong cyclogenesis is possible 
in/near the Pacific Northwest and the adjacent eastern Pacific 
between Saturday night and late Monday. If an intense enough system 
can approach our area with the proper orientation of its pressure 
field, a high wind event could unfold, but given the model 
disparities at this time range it just wouldn't make sense to make 
consequential changes to what the NBM has populated in its forecast. 
Models' forecasts of dynamic processes favored for cyclogenesis 
(including 500mb vorticity advection; thermal advection; adiabatic 
cooling and heating due to vertical motions; diabatic heating and 
cooling provided by earth's surface, solar heating, and latent heat 
release by condensation in clouds; etc.) will be analyzed in the 
coming days to determine whether anything consequential will arise 
from this pattern. -Bumgardner


&&


.AVIATION...Expect mainly VFR conditions over the next 24 hours
as light offshore flow develops tonight. The exception will be if 
patchy fog is able to form in the valley after 09-12z. MOS 
guidance continues to suggest the TAF sites most likely to be 
affected by IFR conditions are KEUG, KSLE, and KHIO between 10Z-
15Z Tuesday. Overall confidence is low though, especially since 
mid and high level clouds ongoing across the area may keep
temperatures warm enough to prevent fog from forming.

For detailed regional Pac NW aviation weather information, go 
online to: https://weather.gov/zse

KPDX AND APPROACHES...VFR conditions are expected to continue
through Tuesday evening. -TK

&&

.MARINE...High pressure will push inland tonight with winds
becoming southeasterly across the coastal waters. A strong front
will approach the region later Tuesday into Wednesday. Expect
winds to gradually increase ahead of the front overnight. Winds
will continue to increase Tuesday as the front enters the coastal
waters with a period of Gale force winds expected mainly for the
outer waters Tuesday afternoon through Tuesday night. The 
strongest wind is expected west of about 35 NM. Expect periods 
of enhanced southerlies to then persist through the end of the 
week as another front moves through the waters Thursday into 
Friday.

Meanwhile, seas will be dominated by a decaying westerly swell
through the beginning of the week as it subsides from around 10
feet this afternoon to 5-6 feet on Tuesday morning. The next 
front will bring an uptick in short period seas Tuesday into 
Wednesday, peaking around 12 ft nearer to shore and 20 feet 
beyond 35 NM early Wednesday. Seas settle some later Wednesday to
around 8 to 10 ft as a fresh westerly swell arrives. DH/MH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 11 AM PDT Tuesday for coastal waters
from Cape Shoalwater WA to Florence OR from 10 to 60 nm.

     Gale Warning from 11 AM Tuesday to 5 AM PDT Wednesday for 
     coastal waters from Cape Shoalwater WA to Florence OR from 10 to
60 nm.

     Small Craft Advisory from 8 PM Tuesday to 5 PM PDT Wednesday 
     for coastal waters from Cape Shoalwater WA to Florence OR out 10
nm.

&&

$$