596
FXUS66 KOTX 251029
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
329 AM PDT Wed Sep 25 2024

.SYNOPSIS...

A ridge of high pressure will deliver above normal temperatures
through today. Strong winds accompany a cold front passage
this 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. Above average temperatures
return Saturday into early next week, with Saturday the warmest
day of the weekend. Some freezing overnight lows are possible
again heading into next week.

&&

.DISCUSSION...


...STRONG COLD FRONT WILL PASS THROUGH THE INLAND NORTHWEST THIS
AFTERNOON AND EVENING DELIVERING BREEZY/WINDY CONDITIONS AND
POSSIBLY SOME STRONG THUNDERSTORMS...

Today and This evening...The stage is set for some impactful
weather later today and tonight care of a strong upper level
trough and cold front. As of 2 am the trough was located just west
of 130W and moving slowly eastward. This matches up well with the
latest suite of model data which brings the cold front to the
Cascades shortly after midday and into eastern WA/N ID after 5pm.
So what will this front bring is the big question.

*Winds:This will likely be the biggest impact from the cold front.
 Model data has been consistent on producing an 850 mb
 southwesterly jet of 30-40 knots over the Inland NW this
 afternoon, with even stronger speeds over south-central WA and
 then spreading into north ID early this evening. How much of this
 mixes down to the ground is questionable, however downward
 momentum associated with cold air advection in the wake of the
 front, should help mix it down to the ground. Wind gusts of 35-45
 mph look like a good possibility over much of eastern WA/N ID
 after 2pm and continuing into the evening. Whats most impressive
 about this is the HREF mean wind gusts produce the strongest
 speeds over extreme NE WA and N ID, which are typically locations
 which are sheltered from the strongest wind speeds due to complex
 terrain. However if theres terrain alignment with the SW winds
 aloft some channeling is certainly possible. This area is also
 highlighted by the European Extreme Forecast Index which shows an
 extreme event is possible so even if we dont see the winds
 materialize in the valleys, there will likely be some tree damage
 over the mountains and possible power outages. The winds will
 also likely stir up a considerable amount of blowing dust over
 the Columbia Basin where freshly plowed fields will combine with
 exceedingly dry soils. Over the past 90 days much of s of Grant,
 Adams, Lincoln, and western portions of Spokane and Whitman
 Counties have seen less than 25% of the normal precipitation and
 according to the Climate Toolbox some locations havent seen a
 wetting rain (more than 0.10) of rain in over 114 days! So the
 stage is certainly set for the potential of blowing dust given
 the wind forecast and open agriculture fields over portions of
 the Columbia Basin. Its always hard to tell how low the
 visibility will get, however we will certainly monitor the
 possibility of areas falling below a quarter mile late this
 afternoon and dust storm warnings may need to be issued.

*Thunderstorms...This is the secondary concern. Before the front
 arrives we expect to see another unusually warm day for this time
 of hear. Highs are expected to surge into the mid 80s to around
 90 over most valley locations which is about 15F warmer than
 normal for this time of year. This heat combined with dewpoints
 in the mid 40s to lower 50s will result in impressive instability
 for late September. Most of the model solutions are forecasting
 MUCAPE values of 250-500 j/kg right ahead of the cold front,
 which in an of itself isnt terribly impressive. Whats more
 impressive is the forecast 0-6 km shear values. From NE WA into
 north Idaho theres a good chance of seeing shear values in
 excess of 50 kts, which is a usual indicator for the potential
 for organize severe convection given sufficient CAPE values. In
 our case, the CAPE values look a little low for severe
 thunderstorms, however we cant entirely rule it out, especially
 given DCAPE values in excess of 1000 j/kg, which can support
 severe winds due to strong downdrafts. The best chance of severe
 weather if any occurs will span from Ferry County east to the
 ID/MT border and south toward the Clearwater Mountains.

*Fire concerns...Typically this setup would be conducive for
 extreme fire weather conditions. Given the aforementioned
 prolonged dry weather, the fuels are bone dry over portions of
 eastern WA and the strong wind forecast can fan flames should any
 develop. The Hot, Dry, Windy Index is also hinting at a 95th or
 greater percentile event, due to the combination of unusual heat
 and winds. However the one thing not favorable for this scenario
 is the presence of relatively moist air mass. Given the forecast
 dewpoints (in the 90-95th percentiles), most of the RH values
 will exceed 25% which is just a bit too high for meet our red
 flag warning criteria. Nonetheless, if fires get established,
 extra control measures may be necessary.

The front moves out of the region during the evening and overnight
hours and the winds will begin to steadily taper off. This should
eliminate the chances of blowing dust and any other wind impacts.
The last location to see precipitation will be over the ID
Panhandle. There is also a chance of seeing a post frontal Puget
Sound Convergence Zone late this evening toward midnight which
could bring some showers to western Chelan County. fx


Thursday to Friday: The Inland NW will remain in a progressive
pattern. Wednesday`s system will be exiting, while the next will
be approaching the Pacific Coastline and moving inland Thursday
night into Friday. However that next system largely tracks north
into British Columbia. Shower chances end near the ID/MT border
early Thursday, leaving otherwise dry conditions with a few
clouds. The next system will bring increasing clouds from the west
Thursday night, with rain chances coming to the Cascades and near
the immediate Canadian border. Expect some breezy conditions in
the afternoon hours both days, with gusts near 15-20 mph locally
up to 30 mph around the Okanogan Valley and Waterville Plateau
Thursday afternoon. Highs will be in the mid 60s and 70s on
Thursday and 70s to near 80 on Friday. Lows will be in the 40s to
low 50s, with a few 30s in the valleys Friday night into Saturday
morning.

Saturday to Tuesday: The progressive pattern continues, with
limited precipitation chances and fluctuating temperatures. A
shortwave ridge migrates across the region early Saturday, while
the next shortwave trough moves into the region later Saturday
into Sunday. However a lack of moisture will only bring limited
shower chances near the Cascade crest and northeast mountains,
closer to the Canadian border. However look for increased cloud
cover, especially later Saturday into early Sunday. Weak ridging
returns Monday before another trough approach Tuesday, but that
trough will only bring limited shower chances near the Cascade
crest too, with transient cloud cover. Highs will be in the
mid-70s to mid-80s Saturday, then cool to mainly the upper 60s and
70s. Lows will be in the 40s to low 50s, with a few upper 30s in
the sheltered valleys Sunday morning. Thereafter lows are forecast
to be in the 30s and 40s, with some upper 20s in sheltered
northeast valleys Monday and Tuesday morning, i.e. a freeze or
frost will be possible again. /Solveig

&&

.AVIATION...
06Z TAFS: Quiet weather tonight, before a strong cold front
brings westerly winds and chance for showers and thunderstorms
Wednesday afternoon and early evening. Wind gusts in excess of 30
mph are expected. Areas of blowing dust could impact visibility
across the Basin impacting MWH-GEG-SFF-COE-PUW. The chance of
thunderstorms is 10-20 percent so did not include a mention in any
of the TAF sites. A band of rain is expected to develop over SE
Washington into North Idaho Wednesday evening as the front
encounters the higher terrain.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: There is high
confidence of VFR conditions for all TAF sites through 20z
Wednesday. After that blowing dust is expected to lower visibility
down to MVFR across the Columbia Basin, with localized
restrictions down to IFR. Confidence is low with degree of
restrictions at KMWH/KGEG/KSFF/KPUW.  JW

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        85  46  69  50  75  46 /  20  30   0   0   0   0
Coeur d`Alene  83  47  69  49  74  47 /  10  40   0   0   0   0
Pullman        83  46  67  47  72  46 /  20  50   0   0   0   0
Lewiston       90  56  76  55  80  54 /  20  60   0   0   0   0
Colville       84  36  68  40  74  35 /  30  40   0  10   0   0
Sandpoint      79  45  66  45  71  42 /  10  60   0  10   0   0
Kellogg        82  49  66  51  70  50 /  10  70   0   0   0   0
Moses Lake     85  44  71  48  77  45 /  20  20   0   0   0   0
Wenatchee      80  49  70  53  74  50 /  40  10   0  20   0   0
Omak           83  43  71  49  77  47 /  30  20   0  10   0   0

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Wind Advisory from 2 PM this afternoon to 11 PM PDT this evening
     for Central Panhandle Mountains-Coeur d`Alene Area-Idaho
     Palouse-Lewiston Area-Northern Panhandle.

WA...Wind Advisory from 2 PM this afternoon to 11 PM PDT this evening
     for Lower Garfield and Asotin Counties-Northeast Blue
     Mountains-Northeast Mountains-Okanogan Highlands-Okanogan
     Valley-Wenatchee Area.

     Blowing Dust Advisory from 2 PM this afternoon to 11 PM PDT this
     evening for Moses Lake Area-Spokane Area-Upper Columbia
     Basin-Washington Palouse-Waterville Plateau.

&&

$$