994
FXUS66 KOTX 151017
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
317 AM PDT Thu Aug 15 2024

.SYNOPSIS...
The next weather system will move into the Inland Northwest
from the southwest this afternoon. Thunderstorms Thursday
afternoon into Thursday night will bring the potential for gusty
outflow winds. This will be followed by wetter showers and
thunderstorms on Friday particularly across the Idaho Panhandle
and northern mountains. More showers and thunderstorms are
possible late Saturday afternoon and night for central WA.

&&

.DISCUSSION...

Today through Friday night: A low pressure circulation will bring
an increasing chance for showers and thunderstorms to the Inland
Northwest today into tonight. The center of the low is currently
moving into southern Oregon with a diffluent wind flow pattern
aloft over eastern Oregon. The prognoses is for the center of the
low to move into northern Oregon this afternoon with favorable
dynamics aloft rotating into eastern Washington and the Idaho
Panhandle at this time. The upper level wave takes on a negative
tilt as it does so, which should add in strengthening upward
motions particularly over the Idaho Panhandle and into extreme
eastern Washington late this afternoon into this evening. The low
continues its northward march into eastern Washington tonight
precipitation morphing into more of a stratiform deformation band
across the northern panhandle and into northeast Washington. There
will be embedded convective elements that will be present with
isolated thunderstorms remaining possible, but overall,
precipitation looks to take on more of a steady moderate
precipitation event overnight. The low then looks to begin its
exit into southeastern BC late Friday. Moisture wrapping around
the low will develop the Cascades and northern mountains. The
atmosphere will be fairly moist throughout the atmospheric column
across these areas by Friday afternoon. Sun breaks will heat the
surface with covective temperatures quickly being met and
convection expected to contain periods of heavy rainfall. This is
multifaceted low pressure system with multiple hazards that will
be possible beginning this afternoon and continuing into Friday.

* Today`s Convection: Mid levels of the atmosphere will destabilize
  through the morning into the afternoon out ahead of the advancing
  low. Altocumulus Castellanus (ACCAS) clouds are expected to
  develop across the region and any one of these will be capable of
  developing into a quick shower. Best instability with the NAM
  indicating up to 200-400 J/kg of MUCAPE will be over southeast
  Washington into the southern to central Idaho Panhandle. This is
  where the best chances for elevated showers and thunderstorms are
  expected with a 30-50% chance expected. This includes places like
  Pullman/Moscow, Lewiston-Clarkston Valley, Camas Prairie, and
  Silver Valley; however, all areas will see at least a 10-20%
  chance for at least showers as the afternoon progresses.

* Evening and Overnight Convection: Surface based convection looks
  to fire off over northeast Oregon and in the southern Idaho
  Panhandle as this mid level band clears the area by the late
  afternoon hours. This convection looks to become strong with 500-
  1000 J/kg of surface based CAPE to work with. Model guidance has
  also trended up with shear profiles on the order of 20-30 kts in
  the 0-6 km layer. This is marginal at best for seeing severe
  thunderstorms. DCAPEs though look to be between 800-900 J/kg for
  the Camas Prairie, into southeast Washington, and across much of
  the Central Panhandle Mountains. The concern will be for
  strengthening outflow winds from convection with the advancing
  cold pool. The question remains though if the gust front will
  outrun the convection or sustain it, and it becomes stronger as it
  advances north and westward. Movement of storms will be a little
  unusual in the sense that they will be moving off of the higher
  terrain the Panhandle (east to west) toward communities located in
  the Palouse region and Spokane/Coeur d`Alene areas. Wind gusts in
  the neighborhood of 40-50 mph will be possible with stronger
  convective cells. Convection then looks to transition into more of
  a stratiform band of precipitation that will deliver primarily
  beneficial moderate rainfall across the Northern Panhandle into
  the Northeast Mountains and Okanogan Highlands.

* Friday`s Convection: Models indicate 500-1000 J/kg of surface
  based CAPE from the east slopes of the northern Cascades and
  across the northern mountains for Friday afternoon. Dew points
  increase into the lower to mid 50s across these areas: Methow
  Valley, Okanogan Valley, Republic, Northport, etc. Model soundings
  show a moist, skinny CAPE profile, and weak steering flow.
  Thunderstorms that develop are expected to be tied to the terrain
  and will slow moving or nearly stationary, particularly in the
  Cascades around and north of Lake Chelan. The concern will be for
  flash flooding and debris flows in and around burn scars, as well
  as in steep terrain.

Convection will dissipate Friday evening leaving a fairly moist
boundary layer in the mountain valleys. The added moisture from
rainfall looks to result in fog forming. Friday will be cooler with
high temperatures cooling from the 80s to low 90s on Thursday into
the mid 70s to low 80s for Friday. /SVH

Saturday: We will see mostly sunny skies to start the day. Clouds
will begin to increase in the afternoon across central WA. The
trough will push a wave up into central WA and an associated
surface low. Models are indicating enough instability for showers
and thunderstorms to move into portions of central WA during the
late afternoon and evening hours, and then keep pushing north
through the night to the Canadian border. Areas of main concern
are the east slopes of the Cascades, Wenatchee area, the
Waterville Plateau and the Okanogan and Methow Valleys. NBM
probability of precip was pretty good, but have `massaged` a few
locations to have higher chances Saturday evening/night. There is
a 20-30% chance of thunderstorms for the aforementioned areas. The
chance of showers extends further east into eastern WA and
portions of north ID, and that is possible, but the best moisture
and instability for showers/thunder threat remains further west.

Sunday through Thursday: The Inland Northwest will remain
sandwiched between a trough along the west coast and a ridge over
the rockies/plains region. There is a 20-30% chance of residual
showers in the Cascades on Sunday, but then after that the
forecast is mostly dry. The next chance of showers is
Wednesday/Thursday with a 20% chance mainly across the mountains.
Wednesday 60% of ensembles are showing the trough having a more
direct impact on the region, but by Thursday that goes down to a
50% chance. Temperatures will fluctuate in the 80s to around 90,
with a trend towards upper 70s and low 80s perhaps by Thursday.
Sunday afternoon and evening will see southerly wind gusts up to
20 mph...locally 25 mph across central WA. The remaining period
will see afternoon/evening gusts to about 15 mph for those
typically breezier locations in central WA. /Nisbet

&&

.AVIATION...
06Z TAFS: The Inland Northwest will experience clear skies and
light winds for the remainder until 15-18z Thursday. An upper
level disturbance moving up from Oregon will bring increasing
chances for showers and thunderstorms late in the afternoon and
into the evening. PROB30 thunderstorms and VCSH has been added to
the Lewiston, Pullman, Coeur d`Alene, and Wenatchee TAFs.
Convective development may be as early as 21z over the mountainous
terrain around Lewiston and Wenatchee between 21-00z with
increasing chances after 02-04z for places like Pullman and Coeur
d`Alene. /GKoch

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: There are timing
differences between members of the High Resolution Ensemble
Forecast (HREF) on the evolution of thunderstorm clusters emerging
from northeast into the Lewiston and Pullman areas. Hopefully, we
will be able to add more details to timing as the high resolution
models resolve the Oregon disturbance. Wind gusts of 40 mph or
more will be a possibility with these storms if they arrive prior
to 04z at LWS and PUW while the air mass is still warm. /GKoch

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

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        87  58  80  55  90  58 /  20  60  30  10  10  20
Coeur d`Alene  85  57  78  53  87  57 /  20  80  50  10  10  20
Pullman        80  52  78  51  90  53 /  20  70  20   0  10  10
Lewiston       88  63  86  62  98  63 /  20  70  20  10  10  10
Colville       88  49  77  44  87  47 /   0  50  60  30  10  30
Sandpoint      84  54  74  51  84  54 /  20  70  80  20   0  20
Kellogg        83  57  74  57  86  59 /  40  80  70  10  10  10
Moses Lake     87  57  83  55  92  56 /  20  20  10   0  20  40
Wenatchee      87  62  82  62  89  63 /  20  20  10  10  30  60
Omak           93  62  86  58  92  61 /   0  20  40  20  10  50

&&

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

$$