Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 211207
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
507 AM PDT Mon Sep 21 2020

.SYNOPSIS...
Temperatures will be above average through Tuesday with a shot at
light rain showers over southeast Washington and into the Idaho
Panhandle Monday night. A moist and vigorous frontal system will
begin to move in Wednesday afternoon with widespread rain
Wednesday night into Thursday morning. This will be a sign of fall
with wet conditions along the front followed by breezy to wind
conditions behind the front for Thursday afternoon. Temperatures
will also cool below normal into Friday, with lingering shower
chances. Conditions may improve some toward next weekend.

&&

.DISCUSSION...

...WET AND BREEZY FALL STORM SYSTEM TO MOVE IN OVER THE INLAND
NORTHWEST WEDNESDAY INTO THURSDAY...

Today through Tuesday night: A couple of weak disturbances will
push across the Inland Northwest over the next 36 hours or so. The
first is still off the Northwest coastline. Water vapor satellite
imagery at 2:00 AM shows a circulation center at 44.5N/130W
associated with a modest vorticity maximum. It`s progged to track
across northwest Washington and will spread mid to high level
cloud cover, but lower levels will remain too dry for
precipitation. The second weak shortwave disturbance is a bit
further west out at 44N/139W. It will track across tonight into
Tuesday morning. Moisture will have increased a bit more and mid
levels look to destabilize a bit more as well. High level Total
Totals will increase to between 30-32 Celsius across southeast
Washington and into the Central Panhandle. The HREF model
guidance shows the potential for a strip of light showers
developing across this area and I increased precipitation chances
a bit more to account for this. Showers will be pretty light with
a couple hundredths of an inch of rain at best is generally
expected with these showers.

Wednesday through Thursday: Model guidance are all in good
agreement for a moderate atmospheric river event settling over the
Inland Northwest. P-wats will be at around 200% of normal with a
plume of 1.5 inches of P-wats directed into the Pacific Northwest.
The warm front will move across into eastern Washington and north
Idaho Wednesday afternoon. It will take a little bit of the
afternoon for the atmosphere to moisten up but not too long. By
the evening and overnight hours, mush of the Inland Northwest will
be within the warm sector with widespread light to moderate
rainfall. There will be a moderately strong upper level jet
directed over the region as well with westerly to southwesterly
winds resulting in favorable upslope flow into the western
Cascades and moderate to heavy rain slopping over the Cascade
crest. Favorable orographics are also expected across northeast
Washington and into the Idaho Panhandle. This will be mostly
beneficial rainfall and nothing out of the ordinary for our first
significant widespread rains across eastern Washington. It is a
sign of autumn and a transition out of the driest part of the
year. Heaviest precipitation will be up at the Cascade crest with
2-3 inches of rainfall possible. The higher elevations of
northeast Washington over the Idaho Panhandle will see 0.50 to
1.00 inches and locally up to 1.50 inches. The lee side of the
Cascades will see significantly less precipitation due to the rain
shadow, but even the western basin to Wenatchee and the
Waterville Plateau will see the potential for around a tenth of an
inch of accumulation. The Spokane Area and Palouse will see
accumulations of a quarter to a half of an inch come Thursday
afternoon.

There is some concern that moderate rainfall will result in mud
flows off of recent burn scars across the region. Much of the
rain will come via stratiform precip and doesn`t look to contain
the rain rates necessary for a significant flash flood threat.
Still, some mud flows out of the recently burned areas for 2020
will be possible. There does look to be an isolated thunderstorm
threat for Thursday afternoon as well. Thunderstorms will be
capable of heavier rain rates, but best chances for convection
looks to mostly be north and east of most of our larger burn
scars. Winds will also be quite breezy to windy. Southerly winds
with 15-25 mph gusts are expected on Wednesday then turning
westerly with gusts of 30-40 mph on Thursday. Strongest wind gusts
will be over the more exposed areas of the Columbia Basin. /SVH

Thursday night through Sunday: The active pattern continues as a
130kt upper level jet is aimed at the region with a mid level
wave tracking through Thursday night into Friday for another round
of showers and breezy conditions. There may even be enough
instability for isolated thunderstorms over the mountains of NE
Washington and North Idaho Friday afternoon. Deterministic models
as well as the ensemble means from the GFS, ECMWF, and Canadian
lift the jet north of the area into the weekend. This will likely
bring about a warming trend although precipitation chances carry
greater uncertainty as another round of Pacific moisture gets
directed mainly north of the Canadian border on Saturday but could
be close enough for some rain with the Cascades, northern
mountains, and Idaho Panhandle having the best chances. JW

&&

.AVIATION...
12Z TAFS: Patchy valley fog is expected through 16Z for the
mountain valleys of northeast Washington and in the central to
northern Idaho Panhandle. Expect this fog in the vicinity of Deer
Park to Sandpoint to Coeur d`Alene. A weak upper level
disturbance will spread some high clouds across the region today.
A second disturbance will produce light showers or sprinkles
Monday night from northeast Oregon to the Central Panhandle
Mountains. /SVH

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        75  48  74  50  70  54 /   0   0  10   0  40 100
Coeur d`Alene  75  47  74  49  70  52 /   0   0  20   0  30 100
Pullman        78  46  74  48  73  54 /   0  20  30   0  20  90
Lewiston       85  56  80  54  81  57 /   0  20  20   0  10  70
Colville       77  41  77  47  69  53 /   0  10   0   0  60 100
Sandpoint      71  44  72  48  67  52 /   0   0  10   0  30 100
Kellogg        74  53  71  51  71  50 /   0  10  30   0  20 100
Moses Lake     78  47  78  52  72  56 /   0   0   0   0  50  90
Wenatchee      76  54  76  56  67  53 /   0   0   0  10  70  90
Omak           76  49  77  53  65  52 /   0   0   0  10  80 100

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.

&&

$$


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