Area Forecast Discussion
Issued by NWS Portland, OR

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649
FXUS66 KPQR 151039
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
338 AM PDT Tue Oct 15 2019

.SYNOPSIS...Much of today will be dry across southwest Washington and
northwest Oregon as high pressure lingers and slides east of the
Cascades. The dry weather will come to an abrupt end tonight and
Wednesday as the first in a series of several frontal systems moves
onshore, spreading rain and locally breezy conditions across the
forecast area. Active pattern will persist through the upcoming
weekend, with periods of rain and occasionally breezy conditions
across much of the Pacific Northwest.

&&

.SHORT TERM...Today through Thursday...High pressure is in the
process of moving east of the Cascades early this morning, allowing
modest offshore flow to develop mainly through the Columbia River
Gorge. The departing high pressure and offshore flow should keep the
forecast area mostly dry with seasonable temperatures today, but big
changes are brewing offshore as the first in a long series of frontal
systems expected to impact the Pac NW approaches. This will mark the
beginning of a prolonged period of active and wet weather that should
last through the upcoming weekend.

The first front has access to subtropical moisture, with NESDIS Total
Precipitable Water (TPW) imagery suggesting a plume of TPW over 1.25"
streaming NE from Hawaii to near 50N/130W, with the deepest moisture
axis approaching 1.50" TPW along and just ahead of the front.
Initially, this atmospheric river looks to focus on Vancouver Island
today as weak waves of low pressure develop along the front and slow
it down. However, a strong and zonal North Pacific jet stream near
150 kt has developed south pf the Aleutians and will eventually push
the frontal zone onshore tonight and early Wednesday, with a second
cold front following right on its heels later Wednesday. A second
impulse of moisture, this time partially taken from the remnants of
Typhoon Hagibis, will become incorporated into one or both of the
fronts while it pushes across SW Washington/ NW Oregon Wednesday.
This could lead to two rounds of intense, but brief rounds of
rainfall spreading east across the forecast area Wednesday into early
Wednesday evening.

Models show a decent southwesterly low-level jet of 35-45 kt
developing along and ahead of the frontal zone while it moves
onshore; this should provide significant orographic enhancement of
rainfall for the Coast Range and Cascade Foothills. Overall, much of
our coast and higher terrain, except perhaps the Lane County
Cascades, stand to see 0.75 to 2 inches of rain by early Thursday
morning. Some spots especially prone to enhancement from S-SW flow,
such as the Willapa Hills and S WA Cascade foothills, could receive
as much as 3 inches. Amounts will be considerably lighter for the
inland valleys, where only 0.25 to 0.75 inch is expected due to the
fast-moving nature of the frontal system(s) Wednesday.

Winds will be significant along the coast, but probably not to the
point of necessitating a High Wind Warning. Initially, coastal
communities will not feel much wind due to easterly pressure
gradients keeping the strongest wind offshore. However, by Wednesday
morning the stronger winds should begin affecting the beaches and
headlands with gusts 35-45 mph and local gusts perhaps up to 50 mph.
As the frontal zone moves onshore, a brief burst of stronger gusts
25-35 mph will push inland, affecting coastal communities and
possibly inland areas as well.

Southerly pressure gradients open up behind the front Wednesday night
and Thursday, leading to cool and blustery conditions with gusts
25-35 mph likely for much of the forecast area. Meanwhile, cool air
aloft will lead to marginally unstable conditions, so there should be
plenty of showers Wed night/Thu. With fairly strong low-level winds
veering with height and modest instability coming into play, will
need to watch for the potential of rotating cells Thursday afternoon
which may produce waterspouts near the coast and a couple funnel
clouds inland if low-level lapse rates are sufficient. Research
suggests we have entered our peak coastal waterspout/tornado season,
which runs from Sep 15 to Dec 31, and this is the type of pattern
that can produce them.Weagle

.LONG TERM...Thursday night through Monday. If the forecast seems
fairly similar each day for the short term period, then the long term
period will likely begin with more of the same. Guidance continues to
suggest a fairly wet week as a series of disturbances will enhance
showers at times in the moist mid and upper level flow. This will
generally result in the highest amounts of precipitation in the
terrain of the Coast Range and Cascades, but with each round of
enhancement, there will be the potential for stronger periods of
rain. Given the potential rainfall through the rest of this week,
will need to monitor both potential rainfall amounts and how much of
a break there is between each round of rain, as this will be
important in determining what, if any, hydrologic issues may develop.
One other note is that a slight chance of thunderstorms was
introduced into the forecast both Thursday evening and Friday
afternoon/evening, with forecast model soundings suggesting decent
instability each day. The timing of each upper level disturbance, and
the resulting cold pool aloft, moving across the region may
significantly alter these chances in the coming days but there was
enough of a signal in the guidance to include at this time.
Otherwise, temperatures will remain rather steady-state during the
long term period, not fluctuating by more than a couple of degrees as
this wet pattern remains persistent through the period.   Cullen


&&


.AVIATION...Predominant VFR across the forecast area as of 09Z.
Patchy LIFR noted in the south Willamette Valley, particularly
K77S. Also, IFR to LIFR stratus along the south Oregon coastline
to near KOTH. The 00Z HREF shows this stratus pushing north to
KONP by about 15Z. Meanwhile, an advancing frontal system will
likely result in MVFR conditions developing along the north
Oregon and south Washington coast late this morning. Cannot rule
out localized IFR or worse conditions in the interior valleys in
through 18Z, but thickening high cloud cover may be a limiting
factor. Expect MVFR along the coast this afternoon through
tonight. Inland areas to improve or remain VFR 18Z today through
Tue night.

KPDX AND APPROACHES...VFR at the terminal and vicinity at 09Z and
this to continue through Tue night. Cannot rule out localized IFR
or worse conditions through 17Z, primarily in the Tualatin River
valley. KTTD-KDLS gradient expected to peak near -4 mb this
morning, for gusts 20-25 kt at KTTD. Weishaar

&&

.MARINE...Major pattern change in store for the coastal waters
beginning today. A frontal system near 130W early this morning
will continue to march east today and tonight. The frontal speed
will slow as it moves inside 125W and become more parallel to the
mid and upper level flow. Expect small craft advisory level wind
speeds to develop in the outer waters around sunrise and spread
to the inner waters mid to late morning. Most, if not all, high-
resolution guidance in good agreement indicating 25-30 kt
boundary layer wind speeds in most of the waters by Tue
afternoon.

Models have been consistent suggesting gale force gusts
developing over the outer waters late Tue afternoon or early
evening. Will go ahead and issue a gale warning for the outer
waters valid 02Z Wed through 00Z Thu and for the inner waters 06Z
Wed through 00Z Thu. It appears the peak wind period will be Wed
morning through mid-afternoon. Still looking at 25-30 kt gusts
Wed evening through Thu evening. Longer range operational models
show more variance late this week through the weekend and have
gone with a blended approach for wind speeds.

Seas will begin to build this afternoon and continue to amplify
through Thu. Peak wave heights will be near 20 ft late Wed night
through Thu. Forecast periods are such that high surf conditions
could be met. Seas to remain in the teens Thu night through at
least Sat night. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 8 AM this morning to 7 PM PDT this
     evening for coastal waters from Cape Shoalwater WA to
     Florence OR from 10 to 60 NM.

     Gale Warning from 7 PM this evening to 5 PM PDT Wednesday for
     coastal waters from Cape Shoalwater WA to Florence OR from
     10 to 60 NM.

     Small Craft Advisory from 11 AM this morning to 11 PM PDT this
     evening for coastal waters from Cape Shoalwater WA to
     Florence OR out 10 NM.

     Gale Warning from 11 PM this evening to 5 PM PDT Wednesday for
     coastal waters from Cape Shoalwater WA to Florence OR out
     10 NM.

&&

$$

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



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