Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 281753

Area Forecast Discussion
National Weather Service Spokane WA
1053 AM PDT Tue Sep 28 2021

Scatter showers with isolated afternoon thunderstorms today.
Temperatures will also be cooler temperatures with breezy
conditions. Areas of fog in the mountain valleys with some frosty
temperatures for Wednesday morning. Then a weaker weather system
arrives on Thursday with mainly mountain showers. Temperatures
returning to near normal toward the end of the week.


Today through Wednesday: The leading edge of the upper level
trough of lower pressure is beginning to push into the Idaho
Panhandle at 2:30 AM. Light to moderate stratiform rain will
continue for Lewiston, Pullman/Moscow, St. Maries, Coeur d`Alene,
Sandpoint, and Bonners Ferry for another 3 to 6 hours or so before
this band shift east of the area. Cold temperatures aloft will
then move in overhead with temperatures up at 500 mbs dropping
down to around -24 to -25 Celsius making the atmosphere
conditionally unstable. Diurnal heating will continue to
destabilize the atmosphere with scattered showers expected to
develop across eastern Washington and the Idaho Panhandle by the
afternoon. Last night`s rainfall will supply moisture in the
boundary with surface dew points expected to be in the low 40s.
The NAM model shows surface based CAPEs of up to around 500 J/kg
to work with. The 12Z operational Canadian solution depicts
similar CAPEs for this afternoon. The 12Z GFS solution appears to
have dew points that are too low resulting in less CAPE and I am
largely discounting this. This gives me high confidence that we
will see convection bubbling up by mid afternoon into the early
evening. Thunderstorms will likely be short-lived and collapse
quickly after producing some lightning strikes. I expect just some
occasional cloud to ground lightning with stronger cells expected
to produce brief heavy rain, and small hail.

Winds will be breezy today behind the front. Wind gusts will range
between 20-30 mph across the exposed areas of the basin to the
Spokane Area and Palouse. It will feel a bit chilly outside as
well with the cloud cover, passing showers and breezy winds. High
temperatures will be in the 50s to low 60s. The colder
temperatures will also result in lowering snow levels with highest
peaks above 5,000 feet seeing the potential for a little bit of
accumulating snow. Best potential for high elevation snow will be
early this morning near the Cascade crest.

Winds will weaken heading into the overnight hours with convective
clouds dissipating as well. The surface will radiate out with
areas of fog expected to develop in the mountain valleys,
especially those that received a healthy dose of rainfall. This
includes valleys tucked up near the Cascade crest, as well as,
those mountain valleys in northeast Washington and in the Idaho
Panhandle. Low temperatures will be dropping down into the mid 30s
for areas across the Palouse and over the northern portions of the
Upper Columbia Basin. Temperatures dipping down into the lower
30s is expected for those northern mountain valleys. Anyone
looking to extend the growing season into October should protect
sensitive vegetation to frost damage. /SVH

Wednesday night through Monday: Shortwave ridging of Wednesday shows
signs of getting displaced to the east with the approach of yet
another weather system thus pops show a ramp up over the Washington
Cascades Wednesday night that continues a slow eastward spread
Thursday over most of Eastern Washington and parts of North Idaho.
Pops remain moderate with fairly light QPF as it appears longwave
ridging to the west causes the moisture feed into the weather system
to weaken. A weakly conditionally unstable airmass is left behind in
the wake of this system passage on Friday allowing for some showery
precip to linger. In addition the airmass is fairly moist so a
mention of morning fog and even frost shows up from time to time.
This scenario should allow for the second dip of morning low
temperatures to bottom out again Saturday morning but generally
about 2 to 5 degrees warmer in comparison to the cooler lowest seven
day morning lows expected to occur Wednesday morning. The remainder
of the weekend is marked by a warming trend and a dry forecast due
to the influence of ridge amplification over the intermountain west
which is compensating for a trof digging southward in the Gulf of
Alaska. Ensembles suggest by Monday ridging in the area may
reoriented in such a manner as to allow the southwest flow on the
west edge of it to have slightly increased capability of steering
weak disturbances up into the Cascades again which is marked mainly
by a slight increase of sky cover rather than pops for Monday.


18Z TAFS: Stratiform precip is moving out of the ID panhandle and
scattered showers and isolated thunderstorms will develop this
afternoon. Confidence is low of showers/thunder occurring over a
specific TAF location. Breezy winds with gusts to between 20-30
kts in the afternoon and widespread mountain obscurations will
occur. Low stratus is expected to form aft 12Z with marginal VFR
to IFR conditions expected. Ceilings will gradually lift aft 16Z.


Spokane        59  40  59  43  68  44 /  40  20   0   0  20  40
Coeur d`Alene  56  41  58  40  68  45 /  80  30   0   0  10  40
Pullman        55  35  60  39  70  41 /  60  20   0   0  10  30
Lewiston       63  45  67  46  76  52 /  70  10   0   0   0  20
Colville       61  32  59  36  64  36 /  40  20  10  10  40  50
Sandpoint      52  37  56  38  61  41 / 100  40  10  10  20  50
Kellogg        52  42  57  44  68  48 / 100  40   0   0  10  40
Moses Lake     64  37  62  43  70  42 /  10   0   0  10  20  30
Wenatchee      62  43  60  51  68  49 /  20   0  10  30  50  30
Omak           65  39  60  46  66  44 /  20   0  10  20  50  40




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