Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 211215

Area Forecast Discussion
National Weather Service Spokane WA
514 AM PDT THU AUG 21 2014

Cooler temperatures and the occasional threat of showers and
thunderstorms will linger the rest of the week and into the
weekend. Thunderstorms will be slow moving with the cores of these
storms containing very heavy rainfall. This may lead to some rapid
runoff in steep terrain and possibly flash flooding or debris
flows, especially on burn scars along the east slopes of the
Cascades with Friday having the best potential. Warmer and drier
weather is expected toward the middle portion of next week.


Thursday and Friday: Unsettled weather will be found across the
Inland Northwest as low pressure continues to settle into the
region. Models remain in decent agreement with the large scale
features and confidence remains above average. For today, look for
widely scattered showers and thunderstorms to once again develop
across most mountainous zones on the northern and eastern
periphery of the Columbia Basin. One change we should see this
afternoon will be storms expanding west into the East Slopes of
the Cascades, Waterville Plateau, and Wenatchee/Okanogan Valleys.
This is largely in response to the arrival of a potent shortwave
swinging into the large-scale trof. Not only will this bring a
means of forcing to the Cascades just prior to sunset, but also
lead to deepening surface low over SE WA. This will allow the the
axis of deeper moisture and instability that has been anchored
over the northern mountains to get pulled south into the East
Slopes and eventually Columbia Basin into the overnight hours.
Consequently, precipitation chances will focus over the mountains
north and east of of the Basin this afternoon then expand west
and south this evening and overnight...with nearly all locations
looking at the least, a small threat for showers and isolated
storms. The only exception will be in southeastern WA which looks
to remain dry slotted until latter Friday.

A closed low pressure center will migrate east/southeast along
the WA/OR border on Friday with numerous showers and storms
developing/rotating around its periphery. This generally
encompasses the entire Inland NW. The threat for thunderstorms
will likely decrease for the Idaho Panhandle, at least northern
portions where clouds will be more prevalent and precipitation
processes will become a bit more stable. Outside this steadier
cloud band, just about any location will be suceptible to
isolated or scattered thunderstorms pending where the best
sunbreaks occur.

As the upper low tracks along the WA/OR border Friday afternoon,
steering flow for storms will generally be less than 10 mph and
cyclonically around the low. This will mean storms will trickle
west to southwest across the Okanogan Highlands and East Slopes.
Atmospheric moisture has increased from values looked at yesterday
and now indicate PWATS near 0.80" in these areas. The depth of the
instability, is the most uncertain parameter given the messy cloud
coverage expected but pockets of SB CAPE ranging from 700-1200
J/kg raises a lot of concern for heavy rain over the burn scars. A
flash flood watch has been issued for Friday afternoon to address
these concerns. /sb

Fri Nt through Mon: We`re still seeing a clear westward trend in
run-to-run model guidance in the path of the deep upper low that
digs south through BC and into Central Wa. Cyclogenesis well to
the south ahead of the upper low quickly produces a significant
band of pcpn across SE Wa and the Idaho Panhandle Fri Nt through
Sat Nt. This band of pcpn, associated with a well- defined trowal,
will be the strongest pcpn-producing feature we`ll have to deal
with...which is a recent change from what model guidance has been
showing the last few days. That said, we again trended to add more
pcpn across the SE Wa and North Idaho zones...especially the
central Idaho Panhandle. A second strong wave embedded in the NW
steering flow quickly moves SE into the region Sunday Nt with yet
another round of showers and embedded thunder. The latter feature
doesn`t produce significant cyclogenesis as the Sat wave, and also
moves through the area quicker, leading to lesser amnts of pcpn.
These waves will continue to keep the fcst relatively cool. bz


12Z TAFS: Low pressure over the region will continue to bring isold
to scattered showers and thunderstorms across the northern
mountains...which will expand south toward the Hwy 2 corridor aft
23z. Conditions more favorable this aftn for storms to spread into
the Cascades and vcnty KEAT. Impacts from convection today will
include: outflow winds up to 35 mph, small hail and heavy rain.
A disturbance passing through overnight will keep a threat of
showers and a few thunderstorms across the region depsite the loss
of daytime heating. /sb


Spokane        77  55  74  52  76  54 /  20  30  40  30  10  10
Coeur d`Alene  76  53  71  52  74  53 /  40  40  50  30  10  10
Pullman        77  51  75  50  74  49 /  10  20  20  40  30  10
Lewiston       84  57  79  57  80  56 /  10  10  20  40  30  10
Colville       78  51  77  50  79  49 /  70  60  60  30  10  10
Sandpoint      71  49  68  49  74  46 /  60  60  70  30  10  10
Kellogg        70  50  65  49  70  47 /  40  30  70  50  40  20
Moses Lake     85  57  82  55  82  56 /  10  20  30  30  10   0
Wenatchee      83  58  80  59  83  58 /  10  20  50  30  10  10
Omak           83  56  79  55  83  55 /  30  60  50  30  10  10


WA...Flash Flood Watch from Friday afternoon through Friday evening
     for East Slopes Northern Cascades-Okanogan Valley-Wenatchee


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