Severe Thunderstorm Warning 25, NWS Spokane, WA

Area Forecast Discussion
National Weather Service Spokane WA
351 PM PDT Tue Jun 27 2023

.SYNOPSIS...
Scattered showers and thunderstorms will continue this afternoon,
then gradually decrease after dark, before developing again
Wednesday afternoon. Some of the thunderstorms will produce
locally heavy rain, hail, and frequent lightning. High pressure,
with drier air, builds over the Pacific Northwest Thursday into
early next week. This will lead to little to no potential for
precipitation. Highs are forecast to rise into the upper 80s to
mid 90s over the weekend into the July Fourth holiday.

&&

.DISCUSSION...

This evening through Friday: Storms continue to form across the
region today. Storms have formed over the higher terrain in the
Idaho Panhandle and eastern Washington, as easterly flow is
forced up the mountainous regions by the upper-level low still
positioned near the Washington-Oregon border. There is
considerable solar heating that can initiate vertical motion
across the area that's reinforced by the upper level divergence
from the low. Guidance has storms continuing to develop through
the early afternoon, but there is little vertical wind shear
(0-6km is near 14 knots) so most storms will be short-lived and
most will die off as quick as they form, though some stronger
storms cannot be ruled out. Forecasted soundings show an
enhanced-v signature, which indicates that dry microbursts and
very gusty winds are possible today where storms are able to grow.
This will likely be a larger threat over the higher terrain in
the Idaho Panhandle. Later this evening the HRRR and other CAMs
models depict a vorticity maxima (created by the upper-level low)
moving across the area, along with an MCS from convection last
night over Montana and Wyoming. These two features will make
organized convection possible for a short window between 6pm-10pm
if it realized. Confidence that this will take place is low
(20-30% chance). The organized convection would be able to move
across eastern Washington and bring with it heavy rains, gusty
winds and frequent lightning. The freezing layer is forecasted to
be around 3 kilometers and CAPE levels between -20 and -30C are
low, so large hail is unlikely. Convection potential should taper
off as the sun sets near 9 pm.

Tomorrow, convection is possible in the higher terrains again, but
should be much weaker than today as the low ejects to the east and
there is less forcing over the area. ECMWF ensemble guidance is
signaling PWAT values 130-140% of normal tomorrow, so brief
downpours will be possible with the storms that do form. The NBM
gives a 40% chance for rain in southeastern Washington and the
lower Idaho Panhandle. Thursday is more of the same with
thunderstorm chances over the mountains in the afternoon, though
chances stay low near 20% with the loss of the low’s forcing. The
HREF one hour QPF shows the probability of rain rates >1 inch/hr
is zero so the likelihood for prolonged heavy rain and flooding is
minimal. High temperatures will be in the upper 90s in the
valleys and 70s in the mountains with relative humidity values
near 40%. /Butler /Sinowitz

Friday to Tuesday: The pattern looks much calmer; temperatures
will be warm to hot each afternoon from Thursday to Tuesday, with
highs in the upper 80s in the valleys and low 70s in the higher
terrain. Some areas could reach into the lower to middle 90s. The
ECMWF ensemble gives the deeper Columbia Basin and L-C Valley
90-100% probability of exceeding 90 degrees Friday through
Tuesday; the GEFS holds off slightly, with probabilities near
80%. So what is causing the warming? Low pressure ejects to our
east Thursday night to be replaced by a strong ridge.  The ridge
and its associated high pressure system over the Desert Southwest
will stop any significant moisture advection into the region,
keeping the area dry over the extended period. The GEFS and
European ensembles have PWATs between 40-60% of normal, much less
than our current situation. Overall the MSLP gradients are pretty
low area wide with the ridge, so winds will not be terribly high
but will still be in the low-grade breezy category with speeds
around 10-15 mph in the afternoon hours from Saturday onward, with
higher gusts. The higher speeds are more likely toward the Cascade
gaps and the western basin. The hotter temperatures and low RH
values during this period (around 15-25%) will help finer fuels
(like grasses) to dry out. This may be of concern for the fire
weather community going into the 4th of July holiday, but
confidence that critical thresholds will be met is low. /Solveig
/Sinowitz

&&

.AVIATION...
00Z TAFs:  Cells over the high terrain of north Idaho and
northeast Washington will have a 40 percent chance of spreading
into Coeur d'Alene, Spokane, and Lewiston before 4Z. Localized
wind gusts to 35 mph, hail, and heavy rain cores will accompany
the strongest cells before convection weakens between 02-04z.
Winds will be light with VFR conditions after 4Z. /Butler

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        57  85  58  89  61  91 /  20  10  10   0   0   0
Coeur d'Alene  56  83  57  87  61  90 /  30  20  10   0   0   0
Pullman        54  80  55  85  58  89 /  40  20   0   0   0   0
Lewiston       63  88  63  93  66  98 /  50  30   0   0   0   0
Colville       51  84  53  88  57  90 /  20  30  10  10   0   0
Sandpoint      56  83  56  87  59  88 /  30  30  20  10  10   0
Kellogg        57  77  59  82  63  86 /  40  50  20  10   0   0
Moses Lake     58  91  59  93  60  96 /  10  10   0   0   0   0
Wenatchee      64  90  64  92  64  94 /  10  10   0   0   0   0
Omak           59  89  61  92  62  95 /  20  20  10   0   0   0

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...Flood Watch until 9 PM PDT this evening for Central Chelan
     County-Okanogan Highlands-Okanogan Valley-Upper Columbia
     Basin-Wenatchee Area-Western Chelan County-Western Okanogan
     County.

&&

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