450
FXUS66 KOTX 111220
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
520 AM PDT Sun Aug 11 2024

.SYNOPSIS...
Several more days of showers and thunderstorms over portions of
the region. Focus for thunderstorms will be over the Cascades, and
northern mountains, as well as over extreme southeastern
Washington into the southern Panhandle Sunday night through Monday
night. Lightning from thunderstorms will be capable of starting
new fires. Thunderstorms will also come with brief heavy rain and
gusty outflow winds. Quieter weather looks likely for the middle
of the week, but thunder chances will rise once again by the end
of the work week and into the weekend. Daytime temperatures will
begin the week slightly warmer than normal but should drop to
average or slightly below by midweek.

&&

.DISCUSSION...

Today through Monday: A shortwave trough of lower pressure will drop
south from the Vancouver Island today and swing across the northern
Cascades to the Okanogan Highlands into tonight. A second shortwave
disturbance will take a similar track for Monday afternoon into
Monday evening. The disturbance for today looks more potent and will
feature a robust vorticity maximum, and is apparent by darkening on
water vapor satellite over this area. The NAM has been performing
well in modeling the instability parameters more accurately
depicting dew point temperatures in the mid 40s to around 50, and
near the Cascade crest, up into the mid to upper 50s. This
equates to around 500-1000 J/kg of surface based CAPE over the
mountains of the northern Washington Cascades to the Okanogan
Highlands for both today and again on Monday. There`s enough
instability at mid levels up to around 200 J/kg of MUCAPE for the
threat of lightning to continue through tonight as well. Lightning
flashes are expected to be more frequent in nature in the
afternoon and evening, and more likely to be isolated overnight
into the morning hours on Monday. Downdraft CAPEs of 1000-1500
J/kg will result in the potential for strong outflow winds.
Conditions look similar to the past couple of days and we`ve
received reports of 40+ mph winds with wind damage from convective
cells. Damaging winds are isolated in nature and have come with
very localized microbursts. There will be the threat of brief
heavy rain as well, but the threat for flooding type impacts looks
minimal as storms will have some motion to them off to the
northeast on the order of 10-15 mph. The potential for new fire
starts and fanning of flames from outflow winds will be the higher
risk over the next couple of days. See the fire weather section
below for additional details including a red flag warning that
will be in effect today through Monday.

Monday night through Tuesday night: A trough of lower pressure will
swing through on Tuesday. There is an axis of mid level instability
extending from northeast Oregon into the southern Idaho Panhandle
for Monday night into Tuesday morning. An axis of isolated to
scattered high based thunderstorms is expected over extreme eastern
Washington into the Idaho Panhandle. These thunderstorms will be
moving more quickly than storms expected over the northwest portion
of the forecast area. The faster storm motion will result in
relatively drier storms with new fires possible from lightning
strikes. Thunderstorms are once again expected over the northern
Cascades (primarily north of Lake Chelan) into the Okanogan
Highlands on Tuesday. Thunderstorm coverage looks to be less than
what is expected today and Monday. Similar threat can be expected
though with convection with lightning strikes and gusty outflow
winds.

The ejecting trough will increase the westerly winds. Expect winds
to increase through the Cascade gaps late Monday afternoon, and
looks to remain breezy across the Columbia Basin into Tuesday.
Exposed area across the basin are expected to see sustained winds of
up to 15 mph and gusts up to around 25 mph. The trough pushing
through on Tuesday will drop temperatures from the upper 80s to low
90s Sunday and Monday into the 80s regionwide for Tuesday. /SVH

Wednesday and Thursday... The ensemble models are in very good
agreement for this period with a weak shortwave ridge expected
over the Inland NW on Wednesday followed by a rebuilding trough
off the coast or just inland and the return of mid-level southerly
flow. This regime should deliver dry weather on Wednesday with
increasing moisture on Thursday. Whether or not this increased
moisture will deliver precipitation is questionable. The base of
the trough and the bulk of the synoptic forcing will generally lie
south of the Inland NW in the morning, but then there are hints
it will eject northeast in the afternoon. If this comes to
fruition, we should see an increasing chance of showers and
thunderstorms over SE WA and NC ID by afternoon as well as a
smaller chance near the Cascades. Temperatures will be around to
slightly cooler than normal on Wednesday with highs primarily in
the 80s while on Thursday there could be some lower 90s over the
LC Valley and lower Columbia Basin.

From Friday into the weekend, there is a strong signal that an
unusually deep 500 mb low will drop down from the Gulf of Alaska
and take residence somewhere off the West Coast. The situational
awareness tables suggest the low will be in the tenth percentile
(or stronger) for lows this time of year which is a far cry from
the weather pattern weve seen for much of this summer.
Unfortunately although the notion of the low is consistent, where
it takes residence and how quickly it heads inland is anything
but. Thus this will be a low confidence forecast in terms of when
and where we could see thunderstorms however there is no arguing
this pattern is conducive for an unsettled weather pattern. Taking
a look at the operational models is interesting to say the least
although this practice isnt a great idea 5-7 days out due to high
variability of specifics. In any event the 8/11 00z GFS run shows
a strong 563 dm negatively tilted trough sweeping NE through the
region Friday night which could potentially trigger widespread
showers and thunderstorms especially given the forecast of 50 or
more knots of 0-6 km shear. However the corresponding EC runs keep
the low off the coast through the entire weekend which would
result in a drier and significantly warmer forecast. Breaking up
the ensemble runs into clusters suggest about 40-50 percent of the
runs bring the low inland which would result wet weather for much
of the area with highs only in the 70s for the weekend. Meanwhile
another 30 percent of the runs leave the low well off the coast
which would keep highs in the 80s to lower 90s. The other runs are
somewhere in between. In any event, stay tuned as we get closer
to this portion of the forecast and hopefully we will see a little
better agreement between the ensembles. fx


&&

.FIRE WEATHER...
A red flag warning will be issued for the east slopes of the
central to northern Cascades of Washington, Methow Valley, and
Okanogan Valley/Highlands. This will be for abundant lightning
and gusty outflow winds from thunderstorms, and will be in effect
from 1 PM today through 1 PM Monday. Uncertainty remains on
lightning coverage over this portion of the forecast area.The
combination of favorable upper level dynamics, instability, and
available moisture is expected to result in widely scattered
thunderstorm development with frequent cloud to ground lightning
strikes. Density of lightning strikes will be highest during the
afternoon and evening hours with a lull expected late tonight into
Monday morning; however, a redevelopment of thunderstorms with
abundant lightning is expected Monday afternoon. There will be a
mix of wet and dry thunderstorms across these areas. The frequency
of lighting is expected to be enough for new fire starts despite
wetting rains that occurs under the cores of thunderstorms. Gusty
outflow winds of 40+ mph is also expect from thunderstorms that
may fan flames and result in the rapid spread of any new or
ongoing fires.

A secondary threat for lightning will be over southeast fire
districts and into north Idaho both tonight, but more so Monday
night with high based nocturnal thunderstorms. These thunderstorms
will be fast moving and more likely to contain dry lightning.
There is low confidence on coverage of lightning with these
thunderstorms. /SVH

&&

.AVIATION...
12Z TAFS: An area of low pressure will drop south across the
Vancouver Island today with diffluent upper level flow over the
northwest portion of the forecast area. Surface heating will
destabilize lower level of the forecast area with widely scattered
thunderstorms over the Cascades that will include across the
Methow Valley State Airport (KS52) and Omak Airport (KOMK). A
frontal boundary at mid levels will slide in over southeast
Washington to the northern Idaho Panhandle with a band of mid
level clouds forming this afternoon. Mid level instability will
increase across far southeast Washington into the southern Idaho
Panhandle tonight. This is expected to produce ACCAS clouds across
KLWS and KPUW. Convection with isolated thunderstorm activity is
expected to continue in the vicinity of KS52 and KOMK overnight.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Confidence is high
for VFR conditions. General haziness from regional fires may
result in brief MVFR conditions with reduced visibility,
particularly in the Methow Valley and possibly at KPUW. Confidence
is high for thunderstorms near the Cascades this afternoon into
this evening.

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

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        89  62  88  57  80  54 /  10  10  10   0  10   0
Coeur d`Alene  86  61  86  55  79  52 /  10  10  10  10  10   0
Pullman        86  57  84  52  75  48 /  10  10  20  20  20   0
Lewiston       96  67  93  63  86  60 /  10  20  20  20  20   0
Colville       88  51  87  48  81  44 /  20  30  20  10  10  10
Sandpoint      84  57  83  53  78  50 /  10  10  10  10  10   0
Kellogg        84  64  84  58  75  54 /  20  10  20  20  10   0
Moses Lake     91  61  90  56  84  53 /   0   0  10   0   0   0
Wenatchee      91  67  88  62  84  60 /  10  10  10  10   0   0
Omak           93  64  91  61  89  59 /  20  20  30  20  20  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...Red Flag Warning from 1 PM this afternoon to 11 PM PDT Monday
     for East Portion of North Cascades National Park/Lake
     Chelan National Recreation Area (Zone 697)-East Washington
     Central Cascades (Zone 696)-East Washington North Cascades
     (Zone 698)-Methow Valley (Zone 704)-Okanogan Highlands and
     Kettle Mountains (Zone 699)-Okanogan Valley (Zone 703).

&&

$$