084
FXUS66 KOTX 101757
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
1057 AM PDT Sat Aug 10 2024

.SYNOPSIS...
Showers and thunderstorms will expand across the region this
afternoon. Lightning from thunderstorms will be capable of
starting new fires. Thunderstorms will also come with brief heavy
rain and gusty outflow winds. There is a growing threat for
nocturnal thunderstorms over southeast Washington into the Idaho
Panhandle Sunday night into Monday morning. Thunderstorms will
also be possible in the afternoon and again on Tuesday. Most of
them will occur over the northern third of eastern Washington
into the northern half of the Idaho Panhandle. Quieter weather
looks likely for the middle of the week but thunder chances will
rise once again by the end of the work week. Daytime temperatures
will begin the week slightly warmer than normal but should drop to
average or slightly below by midweek.

&&

.DISCUSSION...
Morning update: adjust sky cover to decrease it some in the
immediate term, then gradually increase it across the region for
the rest of the day as convection starts to expand and the threat
of showers/t-storm chances start to increase with the upper
trough. /Solveig

Previous Discussion:

Today and Tonight: A broad upper level trough of lower pressure will
be over the Inland Northwest today. A shortwave disturbance will
also swing through on the backside of this trough that is expected
to force convection over the Cascades, northern mountains, and more
so into the basin compared to yesterday. Instability parameters
support the potential for scattered thunderstorms primarily over the
higher terrain, and more isolated in nature over the Columbia Basin.
Showers and thunderstorms will be relatively slow moving at 10-15
mph to the east. Brief heavy rain over burn scars and in steep
terrain will be a threat for flash flooding and/or debris flows.
Pwats are modest at best between 0.8 to 0.9 inches. There will be
rain under the cores, but not expected to be torrential. It will
take a low probability set of circumstances for a slow moving
thunderstorm and training storms over a susceptible are for a
flash flood or debris flow to occur. Probability is so low that a
flood watch isn`t warranted.

Another threat will be for new fire starts. Lightning is a
certainty, although coverage is of some question. I`m not
convinced that lightning density rises to the level of being
abundant enough for a red flag warning. Thunderstorms will also
contain rainfall as previously stated and this moisture will help
to extinguish new fires. Confidence isn`t high enough for a red
flag warning for lightning today. Like with convection yesterday,
these storms have a high potential for outflow winds with gusts up
to around 40 mph possible. Convection will decrease into the
evening with the loss of surface heating, but I expect isolated
showers to continue over extreme eastern Washington and into the
Idaho Panhandle overnight.

Sunday and Sunday night: Another shortwave disturbance will round
the backside of the trough over BC late Sunday. There will be
isolated thunderstorms mainly over the Cascades and northern
mountains of eastern Washington and the Norther Panhandle Sunday
afternoon. What looks more concerning is the potential for nocturnal
thunderstorms Sunday night into Monday morning. The NAM tends to
depict elevated convection well, and has had a good track record
recently. This mesoscale model shows an area of 100-300 J/kg of
MUCAPE being drawn up across southeast Washington into the Central
Panhandle Mountains out ahead of that shortwave that settles in over
western Washington and forms a closed low circulation. The position
of where this low sets up will determine where the axis of best
elevated convection occurs. Either way, the synoptic scale set up
looks favorable for high based thunderstorms that will be moving
quicker and contain less rainfall. Lightning with this batch of
convection will likely be more efficient at causing new fires.
Instability is on the lower end for scattered thunderstorms though,
so there is question of how much lightning will occur. Upper level
dynamics looks good though with a diffluent 500 mb wind flow
pattern. This will lead to diverging motions aloft with favorable
positive vorticity advection as well.

Temperatures will remain steady with highs ranging from the mid 80s
to mid 90s. Temperatures look to remain more mild Sunday night with
increased cloud cover due to the elevated convection and lows will
struggle to cool below 60 degrees across the forecast area. /SVH

Monday...The unsettled weather will likely continue into the
beginning of the new work week as there is good model consistency
is showing a shortwave trough ejecting northeast through the
Inland NW ahead of western WA trough. High level Total Totals
suggest there will be two main areas of convection. The first area
will occur near the north Cascade/Okanogan Highlands. This region
will largely depend on the ascent associated with the ejecting
shortwave combined with good mid-level instability. The second
areas will likely occur over SE WA into NC ID...under a burgeoning
low-level Theta-e ridge axis combined with good mid-level
instability and weaker lifting associated with the ejecting
trough. This setup is similar to what the region experienced on
the overnight and morning hours on August 5th and could result in
considerable lightning. Not surprisingly, the NBM underplays this
event, much like it has for most of the past week during non-
surface based convection. We have adjusted the forecast
accordingly. By afternoon, the models are decent agreement that
the aforementioned shortwave trough will migrate into NW MT with
the deeper trough moving onshore Cascades. This setup will still
allow convection to develop with the best chances reserved for the
northern third of eastern WA and the northern half of the ID
Panhandle. These thunderstorms will primarily form in an
atmosphere with 120-140 percent of the normal precipitable water
and that suggests they wont be dry. The movement will be fast
enough (15-20 mph) that the flash flooding risk wont be great,
however the motion will be about half that value near the north
Cascades in western Okanogan County so that bears watching. With
the deeper trough heading inland, the temperatures west of the
Cascades should drop resulting in a strong cross-Cascade
temperature gradient. This suggests we should see some gap winds
develop by late afternoon or evening.

Tuesday...All of the ensemble runs take to the upper level trough
and push it eastward through the day while digging it slightly
southward. This should place most of the upper level energy south
of our forecast area and thus wed expect to see a quieter day for
convection. The CAPE values also are quite a bit less than what
we should see on Monday, also supporting a quieter day.
Nonetheless it wont necessarily be dry. The best chances again
will lie over the northern third of WA into the ID Panhandle. The
cross-cascade thermal gradient will remain tight and thus we could
be looking at more gusty winds in the lee of the Cascades.

For Wednesday and Thursday there is decent agreement between the
ensembles with longwave troughing rebuilding off the coast and
weak southwest flow over eastern WA and N ID. This pattern will
show little support for synoptic scale forcing and deep
instability and thus the thunderstorm chances will be small.
However watch for Thursday if the offshore trough gets close
enough there could be some weak offshore flow into the Cascade
Crest combined with uncapped instability.

By Friday the odds will increase for renewed thunderstorm chances
as the offshore trough heads eastward. How far east is the
question. Nearly a third of the ensemble runs push the trough east
of the Cascades and this could lead to an active thunderstorm
day, but the majority of the models keep the trough west of the
Cascades.

Aside from the thunderstorm chances, we are likely looking at near
normal to slightly above normal temperatures to begin the week and
then bottoming out on Wednesday or Thursday. That equates to highs
in the mid 80s to mid 90s on Monday dipping into the 80s for most
valley locations on Wednesday and then slowly warming thereafter.
fx

&&

.AVIATION...
18Z TAFS: Showers and thunderstorms will increase in coverage this
afternoon. Development of convection will favor the Cascades and
northern mountains of eastern Washington early this afternoon, but
will move off of the higher terrain and across the basin after
22Z. Wenatchee, Omak, Ephrata, and Moses Lake airports are
expected to see impacts from convection with lightning, brief
heavy rain, and gusty outflow winds up to 40 kts possible. There
is potential for convection to move into the Spokane-Coeur d`Alene
corridor and Palouse region in the evening after 00Z. Convection
will wane with the loss of surface heating with lingering showers
expected overnight.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Confidence is high
for VFR conditions except if a thunderstorm moves over an airport.
In this scenario, IFR to MVFR conditions with reduced visibility
is possible. There is a 20-30% chance for thunderstorms at KEAT
and KOMK and KCOE. Chances for thunderstorms decrease to around
15-20% at KEPH-KMWH-KGEG-KSFF and around 10% at KPUW and KLWS.

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

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        90  60  90  64  87  56 /  20  20   0  20  20  10
Coeur d`Alene  87  57  87  61  85  55 /  20  30  10  20  10  10
Pullman        87  56  86  60  83  51 /  20  20   0  20  20  10
Lewiston       97  67  96  67  93  62 /  20  20   0  20  20  20
Colville       89  49  88  55  87  47 /  30  30  10  10  20  10
Sandpoint      85  55  84  59  84  52 /  30  30  10  10  20  10
Kellogg        85  60  84  63  83  57 /  30  30  10  10  20  20
Moses Lake     93  60  91  63  89  55 /  20  20   0   0  10   0
Wenatchee      92  68  91  66  89  62 /  30  10   0   0  10   0
Omak           91  62  94  64  92  61 /  30  20  10  10  20  10

&&

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

$$