Weather Service Report

FXUS66 KOTX 021141

Area Forecast Discussion
National Weather Service Spokane WA
441 AM PDT WED SEP 2 2015

A weak slow moving cold front will bring light showers, breezy winds
and cooler temperatures to the Inland Northwest through mid week.
This cooler and showery weather pattern will linger into the
holiday weekend, especially for southeast Washington and the
Idaho Panhandle. Drier and warmer weather is expected by early
next week.


Today and tonight: A threat of showers, breezy conditions and
some thunderstorms across the north will be the main impacts in
the forecast. There could even bee some high elevation snow
tonight, but it is not expected to have any real impacts. First
this morning a cold front is pushing across eastern WA. It will
continue into the Panhandle and southeast WA this morning. The
parent upper trough begins to pivot toward the coast between late
morning and afternoon with a 100kt+ jet streak rounding its east
side bisecting the state of WA. A threat of showers will be found
within this set-up. However a limiting factor will be the dry slot
nosing in with that jet streak.

The overall best chance of precipitation will be near the Cascade
crest, the northern mountains, as well as the Blues through
central Panhandle. The late morning to afternoon also comes with
increasing instability, especially across the north and especially
toward the Canadian border. This will support the threat of
isolated to locally scattered thunderstorms. The best agreement
with respect to instability lays across areas north to northeast
of Republic such as Curlew, Danville and Laurier. Models show
SBCAPE values between 100 to 400 J/kg. Nothing too extreme, but
this is where confidence is highest. A chance will be found east
into northern Pend Oreille and Boundary counties as well.

The upper Columbia Basin, the Spokane/C'dA area, Palouse and L-C
valley will also have some threat of showers. However overall
precipitation amounts in these locations looks rather light, with
models generally showing trace amounts to less than 0.05 inches.
The further away from the Idaho Panhandle one travels into the
Columbia Basin the thought is sprinkles will be more likely and
the threat appears will decrease in these areas from the west as
the day progresses. A possible foil to this thinking is a slower
intrusion of the dry slot and the front becoming hung up farther
west than models suggest. Additionally, some guidance suggests
some spotty shower activity in some weak instability just behind
the front and near the jet streak.

Going into this evening and overnight the broader precipitation
threat will wane with the loss of daytime heating. There will be
two general locations where the threat will continue throughout
the night. The first will be from the Blues through the higher
Palouse and central Panhandle in the vicinity of the stalled front
and the moisture that converges along it. The highest risk in this
region will be from the Camas Prairie to southeast Shoshone county
(southeast into the Clearwaters). The second area will be near the
Cascades into northern Ferry County, closer to the parent trough
and continuing elevated instability. Several shortwave are
embedded in the flow around that trough and one pivots across the
Cascades and north-northeast through tonight. There may even be
enough instability to keep at least an isolated thunderstorm
threat going through the night too but the risk it too remote at
this time to include.

However we will be seeing the potential for high elevation snow.
Most of the day and evening snow levels remain generally above
6000 feet and in most cases above 7000 feet. Yet late tonight into
the overnight snow levels drop to between 5500 to 6000 feet near
the Cascades through central WA. Still this is generally above
most passes, save for perhaps near the North Cascades Highway on
SR20 and any higher elevation areas.

Winds will be breezy and gusty with this system as well. A
tightening gradient, increased mixing with the front and we have
some ingredients for the winds increasing. They should be on the
increase this morning, with the most notable gusts in the
afternoon, before abating in the evening.

Temperatures are expected to be below normal today and tonight.
The main note regarding temperatures is the potential for some
patches of near freezing temperatures in some of the more
sheltered valleys across northeast WA/north ID. Even if it doesn't
drop to freezing some plants may be sensitive to temperatures
below 40. Temperatures may even be colder going into Friday
morning. /J. Cote'

Thursday through Tuesday...Latest models are converging on a
solution which drops the next upper level closed low down coastal
Washington and Oregon and into the Great basin during the Thursday
through Saturday period then kicks it eastward through Idaho and
into Montana on Sunday. Ramifications of this consensus include a 
smaller chance of precipitation for the rest of the work week as
the upper low drags a very dry surface Canadian continental
air mass into the region through the northern gaps starting late
Thursday. This argues for breezy and dry conditions in the
Okanogan Valley Friday and Saturday although temperatures will
remain below normal. Still...even in this cool air mass the RH may
be low enough and winds breezy enough to flirt with critical
values for Red Flag conditions in this fire ravaged area. 

The other ramification will be a smaller chance of any
precipitation on Thursday and Friday than previously forecast. The
Cascades look to have the best chance for showers both days with a
small chance of some afternoon and early evening thunderstorms as
well as the cold pool aloft transits southward through the
western zones.

Later on for the holiday weekend there is some uncertainty as
model details begin to diverge. The ejecting upper low will pass
to the south of the forecast area through central Idaho Saturday
with the ECMWF in particular forming a deformation region of
potential showers or even light stratiform rain over the
southeastern zones and possibly penetrating well into the entire
Idaho Panhandle and eastern Washington. The GFS is not as
aggressive with this scenario...however given the upcoming end of
summer outdoor activity holiday it is prudent to at least begin to
introduce this potentially wet scenario to the forecast. 

Farther out the models come into loose agreement with passing the
weakening upper low well to the east on Sunday maybe with some
residual showers in the panhandle mountains but otherwise dry.
Confidence is low farther out. There may very well be a weak
follow-on short wave on or about Monday for a threat of mountain
showers followed by a short wave ridge Tuesday for drying and
warming. /Fugazzi


12Z TAFS: A cold front pushes into eastern WA this morning, stalls
over near the central Panhandle to southeast WA, while an upper
trough moves. This will bring a threat of showers to most TAF
sites, with the best risk along the stalled front near PUW/LWS
later this afternoon/evening. The best overall risk for showers
will be near the Cascades and mountains, with the potential for
some thunderstorms in the northern mountains. The front will bring
breezy conditions, with speeds increasing through the morning and
early afternoon, before abating after 01-03Z. /J. Cote'


Spokane        68  45  66  40  68  45 /  20  10  10  10  10  10 
Coeur d'Alene  68  44  65  39  67  44 /  30  20  10  10  10  10 
Pullman        66  41  66  38  68  41 /  30  20  10  10  10  10 
Lewiston       72  50  71  44  73  50 /  30  20  20  10  10  20 
Colville       70  40  68  37  70  40 /  30  10  10  20  10  10 
Sandpoint      67  40  65  37  67  42 /  50  20  10  20  10  10 
Kellogg        65  44  63  36  67  41 /  60  30  20  20  20  20 
Moses Lake     73  43  71  42  70  46 /  10   0  10  10  10  10 
Wenatchee      72  51  69  47  70  50 /  20   0  10  10  10  10 
Omak           73  43  68  41  68  43 /  20  10  20  20  10  10 




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