Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 202346
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
446 PM PDT WED AUG 20 2014

.SYNOPSIS...
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.

&&

.DISCUSSION...
Tonight through Thursday night: A shortwave trough of lower
pressure is currently pushing across eastern WA this afternoon.
Numerous showers with embedded thunderstorms will continue to
develop into the evening hours along this wave. Some of this
convection could get quite deep late this afternoon with some
small hail and heavy downbursts possible, but these stronger cells
will collapse quite quickly due to a low shear environment. Much
of this activity is expected to wane as we head into the overnight
hours; however, some mid level instability will remain mostly
over the ID Panhandle into the Northeast Mtns, so a slight chance
of thunderstorms will continue to be possible into Thursday
morning.

We will then be watching a stronger shortwave disturbance dips
down the backside of the longwave trough along the BC coastline.
This disturbance is located off of northern BC and will begin to
dig into northwestern WA Thursday afternoon. Better dynamics is
expected with this wave and will work with about 500-1000 J/KG of
surface based CAPE across the northern mountains. This will result
in more showers and thunderstorms across this area for the
afternoon hours. The characterization of these thunderstorms will
not change much compared to yesterday and today; thunderstorms
will be slow moving, so heavy rainfall with localized low land
flooding will be a possibility (especially in steep terrain);
small hail will also be possible. We will have to watch the burn
scars closely for Flash Flooding as there should be a better
chance for thunderstorms in the Cascades on Thursday; however, it
looks like the best potential for Flash Flooding will be on
Friday.

There will also be a ridge of elevated Equivalent Potential
Temperature (Theta E) across the Panhandle and across the northern
mountains overnight on Thursday. Once the disturbance digs further
east Thursday night, we will see a chance for showers and
thunderstorms developing along this Theta E ridge into Friday
morning.

Temperatures will continue on their downward trend with the
longwave trough of lower pressure digging into the region.
Temperatures will be below normal on Thursday with highs generally
in the 70s to low 80s. /SVH

Friday: The computer models continue to advertise an active
thunderstorm day on Friday. A Flash Flood Watch has been issued
for the East Slopes of the Northern Cascades. Our primary concern
is locally heavy rainfall over the wildfire scars including the
Carlton Complex, Little Bridge Creek, and Chewaukum scars. So,
most of Chelan county is included as well as the western half of
Okanogan county.

By Friday, the shortwave trough currently along the coast of
northern British Columbia will be digging southward into northern
Oregon. The GFS, ECMWF and NAM forecast a tap of rich, deep layer
moisture wrapping around the low into the Idaho Panhandle,
northern Washington and into the Cascades. Precipitatble water
values will be above normal, but not as impressive as some of our
recent events. However, given the cooler resident air mass, it
will take less precipitable water to saturate the atmosphere than
other recent events with surface temperatures in the 90s. We are
more concerned with the increasing instability Friday under the
500mb cold pool and the weak storm steering flow. Cells that
develop won`t move much and should be efficient precipitation
producers. Ferry county may be added to the Flash Flood Watch if
the models continue to show a good convective signal in the
vicinity of the Devil`s Elbow burn scar north of Keller.

Friday Night and Saturday: Gusty north or northeast winds will
likely develop through the Okanogan Valley and north Idaho
Panhandle as low pressure moves out of Oregon into southern Idaho.
A push of cooler and drier air will reinforce the cool air mass
already in place. The best chance for wrap around showers on
Saturday should be over the southern/central Idaho Panhandle and
extreme southeast Washington under a decaying deformation zone.
/GKoch

Sunday through Wednesday...Another short wave trough over British
Columbia will drop into Northeast Washington and the North Idaho
Panhandle late Sunday into Monday. Models show sufficient
instabilty to trigger scattered showers and isolated
thunderstorms. North Central and southeast portions of Washington
looks to be more on the fringes of the lift and instability with
lower elevations likely remaining dry. As this trough exits, high
pressure gradually builds in Tuesday into Wednesday. This will
result in warmer temperatures under mostly clear skies. JW

&&

.AVIATION...
00Z TAFS: Thunderstorms continue across the northern mountains of
WA and north ID. These storms are expected to push into the
vicinity of KGEG, KSFF and KCOE aft 00Z. Impacts from convection
today will include: outflow winds up to 35 mph, small hail and
very heavy rain. Showers and thunderstorms will wind down after
06Z this evening with VFR conditions expected at all taf sites
for the next 24 hrs.


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        56  77  55  75  52  76 /  40  10  20  40  20  10
Coeur d`Alene  53  75  54  74  52  74 /  50  20  30  50  30  10
Pullman        51  78  48  76  50  74 /   0  10  20  20  40  30
Lewiston       59  83  57  82  57  80 /   0   0  20  20  40  30
Colville       52  79  52  79  50  79 /  50  70  60  60  30  10
Sandpoint      51  71  51  71  49  74 /  60  60  60  60  30  10
Kellogg        54  71  51  69  49  70 /  40  30  40  60  50  30
Moses Lake     56  83  56  83  55  82 /   0  10  10  30  30  10
Wenatchee      61  81  61  80  59  83 /   0  10  20  50  30  10
Omak           58  82  58  80  55  83 /  20  30  60  50  30  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...Flash Flood Watch from Friday afternoon through Friday evening
     for East Slopes Northern Cascades-Okanogan Valley-Wenatchee
     Area.

&&



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