Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 042345
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
445 PM PDT FRI SEP 4 2015

.SYNOPSIS...

A low pressure system will bring widespread rain and much cooler
temperatures to the Idaho Panhandle and portions of Eastern
Washington Saturday. Light showers may linger mainly across the
mountains Sunday and Monday. A drying and warming trend is
expected next week.

&&

.DISCUSSION...

...WET and MUCH COOLER for the eastern third of Washington and
North Idaho on Saturday...

Tonight through Saturday night...A wet period is in store for the
eastern third of Washington and North Idaho bringing much cooler
temperatures and some needed rain. A closed low over SW Washington
this afternoon will drop into Central Oregon this evening...NE
Oregon overnight...and then NW Montana by early Saturday evening.
There are several aspects of this system to talk about and we`ll
start with precipitation.

Precipitation: Models continue to show a band of stratiform rain
developing on the north side of this low track with a band of wrap
around rain as well on the back side of the low as it tracks
across the area. The low track will favor the highest rain totals
over the North Idaho Panhandle, NE Washington Mountains,
Spokane/Coeur D`Alene area, Washington Palouse, the possibly the
far eastern portions of the Columbia Basin. Most of the rain will
fall overnight tonight into early Saturday evening. Models details
differ on precise amounts...but general consensus favors around a
half inch of rain in these areas with local pockets up to an inch.
The Okanogan Highlands and Moses Lake areas should be on the
western fringes of this rain with little to no rain for the
valleys along the East Slopes of the Cascades as well as the
Okanogan Valley and Moses Lake area.

Wind: As the low drops into Oregon tonight surface pressure
gradients increase over northern Washington and near the Cascades
supporting breezy to locally windy conditions mainly on ridge tops
and through channeled valleys. 850mb winds out of the north of
20-30 kts tonight will especially favor the northern valleys and
then as 850mb winds turn to the west-northwest on Saturday the
focus of the strongest winds should shift to the East Slope
Cascade valleys such as the Methow, Lake Chelan, and Wenatchee
River Valley.

Temperatures: 850mb temps fall to 4-6C over far Eastern WA and
North Idaho on Saturday due to a rain cooled air mass. High temps
in the valleys will only reach the low to mid 50s with upper 30s
and 40s in the mountains. Snow levels are expected to fall over
Eastern WA/N Idaho to 6500-7000 feet.

Thunderstorms: Isolated thunderstorms are possible tonight around
the Spokane area, Palouse, Camas Prairie, Lewiston area, and
Central Panhandle Mountains as strong forcing and mid level
instability is present in these areas. Isolated thunderstorms are
also possible over the high terrain of the Cascades through early
this evening under the cold pool aloft with some strikes already
occurring along the west slopes. On Saturday it will remain
unstable with a 500mb cold pool of -20C over the region. If sun
breaks materialize a few short lived storms could develop.  JW

Sunday through Wednesday night...This interval starts out with a
general longwave trof lingering in the area but the feature
appears slightly less intense due to the exiting synoptic scale
low pressure system in Montana heading east and away from the
area. This this allows a bit of shortwave ridging to nest in the
general trof and gives it that slightly less intense look about
it. A second much weaker mesoscale disturbance passes through
Sunday night and exits Monday. The combination of the disturbance
exit and some lingering instability and northwest flow allow for
pops to persist a bit longer in the North Idaho Panhandle Monday
while locations further to the west clear out with the northerly
flow ahead of positively tilted ridging just to the west. This
shortwave passage may also allow for some breezy conditions and
shifting winds Sunday afternoon and evening. Then a positively
tilted ridge is overhead of the forecast area allowing for a dry
Monday night. This ridge flattens out a bit on Tuesday yet the
conditions are expected to remain dry. An almost flat zonal flow
void of shortwave migration remains for Tuesday night into
Wednesday that in turn gets amplified by yet more positively
tilted shortwave ridging moving in from the west allowing for a
dry Wednesday night. Forecast daytime high temperatures remain on
the cool side of what would be considered normal for this time of
year with a slight warming trend due to the transition from
longwave trof to more of a ridge. /Pelatti

Thurs through Saturday morning...With a broad upper ridge shown
by the majority of model guidance, expect well above normal high
temperatures in the 80s for most towns and no chance of pcpn. The
largest uncertainty is with the ECMWF showing a transient short-
wave trough moving across Srn Bc. The ensemble means still support
a dry fcst, though.bz

&&

.AVIATION...
00Z TAFS: Low pressure in close proximity to the forecast area
keeps both stratiform and convective cloud cover over next 24 hrs.
Showers will drift north into extreme southeast WA and central
panhandle of Idaho this evening along with isolated thunderstorms
possible but confidence is low that any TS will affect a
particular TAF site. The wettest interval will be overnight
tonight and into tomorrow as a wet frontal zone stalls over
Eastern Washington and North Idaho from about 09Z on bringing MVFR
conditions to TAF sites except KEAT. KMWH will be on the western
edge of the precipitation shield and may or may not see any rain.
As the low moves east Sat afternoon, any breaks in cloud cover may
allow convection to develop, with isolated thunderstorms possible
across the forecast area. /EK

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        46  53  43  66  45  68 /  80 100  50  10  10  10
Coeur d`Alene  46  53  43  65  44  67 /  80  90  70  10  10  10
Pullman        44  53  41  64  43  66 /  90  70  60  10  10  10
Lewiston       50  60  48  72  50  72 /  80  70  60  10  10  10
Colville       51  57  44  67  41  70 /  40  90  50  20  20  10
Sandpoint      47  54  45  64  40  66 /  60 100  60  20  20  20
Kellogg        46  52  43  63  40  63 /  70  80  70  30  30  20
Moses Lake     53  71  49  75  48  76 /  30  50  10  10  10  10
Wenatchee      53  72  52  75  51  78 /  10  10  10  10  10  10
Omak           52  73  46  75  45  78 /  10  10  10  10  10  10

&&

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

$$



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