195
FXUS66 KOTX 241033
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
333 AM PDT Tue Sep 24 2024

.SYNOPSIS...

A ridge of high pressure will deliver above normal temperatures
through Wednesday. Strong winds accompany a cold front passage
Wednesday afternoon and evening. Areas of blowing dust along with
showers and thunderstorms will be possible with the frontal
passage. Temperatures return to near normal by Thursday with
another cold front passage Friday.

&&

.DISCUSSION...

...ROBUST COLD FRONT TO BRING STRONG WINDS, BLOWING DUST, AND SHOWERS
AND THUNDERSTORMS....

Today: A strong upper-level ridge is anchored over the Northwest this
morning and will bring one more day of unseasonably warm
conditions. Temperatures will be warming into the mid to upper
80s, roughly 5 degrees warmer than Monday. These readings are
15-18 degrees warmer than our 30-year averages for this time of
year. Stable weather conditions under the ridge will deliver
mostly sunny skies outside some passing mid and high clouds. Winds
will generally be light for most locations and influenced by the
terrain. In the Columbia Basin and Okanogan Valley, southerly
winds will increase near 10 mph with gusts of 15-18 mph.


Wednesday and Wednesday night: The weather will become much more active
Wednesday with the passage of a robust midlevel trough and
associated cold front. This will be a late afternoon and evening
frontal passage allowing Wednesday to become just as warm. Winds
will increase throughout day, becoming strong just ahead and with
the frontal passage between 1PM and 8PM. There is a 80% chance or
greater for sustained winds to reach 20 mph or greater and 5-15%
chance for sustained winds of 30 mph. Wind gusts of 35 to 45 mph
will be common. Direction will be from the southwest then west
with the frontal passage. There are several concerns coming from
the frontal passage including 1) fire weather, 2) blowing dust, 3)
wind related impacts such as tree damage and power outages, and
4) showers and thunderstorms.

Fire Weather: Sustained winds of 20-30 mph combined with gusts
around 40 mph raises moderate to high concern for rapidly
spreading grass fires. Fire danger indices are not as high as
those observed in July and early August but many areas between
Spokane and Moses Lake have not received much in the way of
rainfall over the last 30 days. Consequently, any new fire starts
could be fanned by the strong winds Wednesday afternoon leading to
rapid fire growth. Humidity levels are expected to rise with the
frontal passage due to developing showers/thunderstorms and push
of cooler, marine air. Would not rule out a few hours with
observations flirting with critical fire weather conditions but
given the shift toward cooler conditions, higher humidity, and
showers... will hold off on any fire weather highlights. Hot, dry,
windy does argue against this with points in the Basin near the
90-95th percentile based on climatology which is dropping fast
this time of year.

Blowing Dust: Fields are being worked this time of year as an effort
to harvest summer crops or prepare for fall seeding. The incoming
winds will come with speeds we have identified to be problematic
based on previous events. Greatest concern for visibility
restrictions to a mile or less will be across the Columbia Basin
potentially impacting I-90, Highway 2, and several north to south
state routes. Areas of haze or blowing dust will be likely for
Idaho and Northern WA as well but given the distance from
potential dust sources, come with lower confidence for visibility
below two miles.

Wind Impacts: A 10-12 mb pressure gradient across the forecast area
leads to high confidence for windy conditions. The sustained
winds of 20-30 mph will create challenging conditions for high
profile vehicles on north to south highways. Gusts of 40-45 mph
will be capable of isolated tree damage and power outages. These
gusts will not only impact the Basin but also the northern
mountains and mountain valleys due to strong mechanical mixing
along the front and unstable conditions in the lower atmosphere
given the timing aligning with peak heating. Several ensemble
models indicate some of the highest anomalies for these typically
sheltered areas due to the amount of members supporting the wind
gusts. Area lakes will become choppy Wednesday afternoon and
continue into the evening.

Showers and thunderstorms: This aspect of the forecast carries
moderate uncertainty, especially in the lower Basin. Majority of
the hi- res models support bands of showers and embedded
thunderstorms coming with and ahead of the frontal passage. Pops
have trend up in the last 12 hours. Rainfall amounts will be
extremely light in areas around Moses, Othello, Ritzville,
Wenatchee, and Omak while a 10-20% chance for a tenth of an inch
of rain will be possible in the mountains. Also have increased
the risk for thunderstorms. CAPE values of 200-500 J/kg will
support isolated to scattered thunderstorms given the robust
lifting mechanism. CAPE alone is not enough warrant strong storms
but a strong jet within the 850-700mb layer will be capable of
getting mixed down with downdrafts and could result in isolated
outflow winds near 45 mph or stronger. These will be capable of
tree damage and isolated power outages. /sb

Thursday through Monday: Breezy conditions Thursday remain in place
in the wake of Wednesday evenings/night cold front passage. In fact
the progressive zonal flow/flat ridging remaining in place will
allow even more weak disturbances, including yet another frontal
feature, to pass through Friday as well. The moisture feed extending
into Friday`s weak feature passage elongates and weakens some as it
moves thru thus the pops and QPF associated with it for Friday
remain rather low and are also paired up with a mention of breezy
conditions. Remainder of the weekend and on into Monday the airmass
remains rather dry coupled with flat ridging with weak disturbance
overtopping it so rather than producing any sensible precipitation
there is a gradual cooldown after temperatures rebound and peak on
Saturday. As earlier mentioned a 15 to near 20 degree drop in high
temps remains in place for Thursday. /Pelatti

&&

.AVIATION...
06Z TAFS: Scattered high, thin clouds are advertised by the High
Resolution Ensemble Forecast (HREF) through the day Tuesday and
into the evening. The majority of these clouds will be at or above
20 thousand feet. Winds will generally 10 mph or less.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence
for VFR conditions at all TAF sites. /GKoch

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        86  59  85  47  70  48 /   0   0  30  50   0  10
Coeur d`Alene  83  57  83  48  68  47 /   0   0  20  60   0  10
Pullman        83  56  83  47  68  46 /   0   0  20  60   0   0
Lewiston       90  63  90  55  76  53 /   0   0  20  60   0   0
Colville       83  47  83  39  69  38 /   0   0  30  40   0  10
Sandpoint      78  53  79  47  66  44 /   0   0  30  70  10  10
Kellogg        81  61  81  49  66  51 /   0   0  30  70  10  10
Moses Lake     86  56  85  44  72  47 /   0   0  10  10   0   0
Wenatchee      85  62  80  47  72  54 /   0   0  40  20   0  20
Omak           87  58  84  43  72  49 /   0   0  30  10   0  20

&&

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

$$