751
FXUS66 KOTX 162332
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
432 PM PDT Fri Aug 16 2024

.SYNOPSIS...
Thunderstorms will continue into the evening across the northern
mountains with concerns for flash flooding over burn scars near
the Cascades. More showers and thunderstorms are expected
Saturday evening and night for central Washington with the
possibility of strong outflow winds, especially over the southern
half of eastern Washington. Temperatures will be quite warm  on
Saturday with dry and breezy northeast winds across the Columbia
Basin, West Plains, and southeast Washington. Unsettled weather
will continue near the Cascades on Sunday with drier weather
expected for Monday and Tuesday. Wetter and unsettled weather
returns by midweek.

&&

.DISCUSSION...
Tonight...Interesting forecast through this evening as
slow moving shortwave trough pivots slowly northward into extreme
SE BC. The presence of this feature combined with orographic
enhancement from southerly winds and MUCAPE values of 500-1000
j/kg should keep thunderstorms going through the evening. Although
these storms wont be severe, due to limited shear (less than 10
kts) the main threat will continue to be locally heavy rains and
the possibility of flash flooding. This is a direct result of
precipitable water values in excess of 150% of normal for this
time of year and storm motions of less than 10 mph. So far the
heaviest rainfall per radar estimations has occurred over western
Okanogan County. We will continue with our flash flood watch
through 11 pm after which the threat of thunderstorms will taper
off rapidly.

While tonight`s weather will prove interesting the weather for
Saturday and Saturday night will be potentially even more
dynamic. The culprit will be a strong upper level low currently
poised off the WA/OR coast between 130-135w. All the models dig
this low southward overnight and into Saturday morning before
pivoting it north-northeast during the late evening and overnight
hours. Initially this amplifies a strong shortwave ridge over the
Inland NW which will bring generally sunny skies and increasing
amounts of east-northeast winds. High temperatures will surge into
the mid 80s and 90s for all valley locations and could even touch
triple digits in the LC Valley, Snake River Valley and the lower
Columbia Basin. According to the NBM probabilities theres a 60%
chance of exceeding 100F at the Lewiston Airport and a higher
chance in downtown Lewiston with a 10-20% chance over SW Whitman
and extreme southern Grant Counties. In addition to the heat, much
of the Columbia Basin will see NE winds of 10 to 15 mph with gust
of 20-25 mph possible during the hours of peak heating. There
will also be relative humidity values dropping into the upper
teens to mid 20s This will lead to enhanced fire weather
conditions but will likely be too marginal to issue a fire weather
watch.

Saturday evening/night should be when most of the thunderstorm
action develops as the low ejects NE and brings enhanced upper
level difluence combined with strong synoptic scale forcing and
MUCAPEs from 500-1000 j/kg. This will also be accompanied by 0-6km
shear values of 40-50 kts which is more than enough to bring the
possibility of severe weather. However the main threat will lie
from the Cascades westward and thus we dont foresee terribly
strong thunderstorms over our portion of eastern WA and north
Idaho. Much more concerning will be the potential of strong
outflow winds developing near the Columbia Gorge late in the
evening and overspreading the Columbia Basin, Palouse, Blues, and
perhaps into the Spokane area overnight. Some of the finer
resolution CAM models are forecasting outflow winds out of the
southwest of 40-50 mph and perhaps locally stronger close to the
WA/OR border. We shall see. Whether or not these winds actually
materialize is questionable however the DCAPE values in excess of
1500 j/kg certainly supports this notion. If the winds do occur
they could cause areas of blowing dust, an abrupt wind shift, and
the possibility of local tree or property damage. Most of the
potential impacts in the Inland NW will occur after 9pm, which is
fairly unusual for a severe/strong weather event. Nonetheless we
should not disregard what some of the CAMs models are depicting.
As far as other threats, much of the area near the Cascades will
see another round of precipitation and locally heavy rains.
However unlike todays threat of heavy rain, tomorrows will
feature much faster moving storms and thus a smaller possibility
of flooding rain. Consequently we wont issue a flash flood watch
at this time. fx

Sunday through Friday: The majority of the extended forecast will
see low pressure to our west along the coast and a ridge just to the
east of the Inland Northwest. Chances for showers and strong
thunderstorms will linger into Sunday, but look to be confined to
the Cascades at that point. Forecast then turns drier for Monday
through Tuesday. Chances for storms return Wednesday and Wednesday
night as a shortwave impacts the region. Enough lingering moisture
(and uncertainty in the finer details of the weather pattern) means
we are keeping slight chances for mountain showers in the forecast
for Thursday and Friday. Daytime temps near daily normals through
much of the extended, though Monday looks the warmest with temps
around 5 degrees above average. Best chances for gusty winds will be
near any thunderstorms that develop through the period, though
locally breezy winds are anticipated Sunday across central
Washington. /KD

&&

.AVIATION...
00Z TAFS: Rain showers will continue across portions of northern
WA, extreme eastern WA and the ID Panhandle, with isolated
embedded thunderstorms possible through the evening.
Thunderstorms are not expected to impact TAF sites, but could go
over smaller fields like Winthrop, Omak, Republic and Colville.
Clouds will clear south to north through the late afternoon and
evening hours. Saturday will be relatively quiet through the
afternoon. An incoming system will begin to impact the MWH area
with gusty winds near the end of the TAF period.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence
for VFR conditions for the TAF period. /JDC


-----------------------

Confidence descriptors:
Low - Less than a 30 percent chance
Moderate - 30 to 70
percent chance
High - Greater than a 70 percent chance

For additional probabilistic information for NWS Spokane airports,
please refer to the Aviation Dashboard on our webpage:
https:/www.weather.gov/otx/avndashboard

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        55  90  59  86  58  89 /   0   0  10   0   0   0
Coeur d`Alene  53  88  58  84  57  88 /   0   0  10   0   0  10
Pullman        52  90  53  83  53  84 /   0   0  10   0   0   0
Lewiston       62  99  65  92  64  92 /   0   0  10   0   0   0
Colville       45  87  48  85  47  90 /  20   0  20   0   0   0
Sandpoint      50  84  56  83  53  87 /  10   0  10   0   0  10
Kellogg        56  86  59  83  61  88 /  10   0  10   0  10  10
Moses Lake     55  94  59  86  57  89 /   0   0  30   0   0   0
Wenatchee      63  91  66  85  64  88 /   0   0  40  10   0   0
Omak           58  93  63  91  60  93 /  20   0  30  10   0   0

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...Flash Flood Watch until 11 PM PDT this evening for Central
     Chelan County-Okanogan Highlands-Okanogan Valley-Upper
     Columbia Basin-Wenatchee Area-Western Chelan County-Western
     Okanogan County.

&&

$$