Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 282342

Area Forecast Discussion
National Weather Service Spokane WA
442 PM PDT Sun Aug 28 2016

Dry and sunny conditions with warmer temperatures are expected
Monday. Tuesday will bring some clouds to filter the sun, but
it will continue to be dry and warm. The weather pattern gradually
changes through the work week. An upper level trough will move
into the region, ushering in cooler temperatures and showery
conditions for late in the week and into the holiday weekend.


Tonight through Tuesday: A fairly quiet and benign late summer
weather regime will continue through the next couple days with
some significant changes for later in the work week. Satellite
indicates a digging upper level trough off the coast with westerly
flow or shallow ridging over the forecast area this afternoon.
Some mid and high level moisture is caught in this flow which will
promote patches of transient cirrus from time to time
tonight...with an otherwise quiet overnight period.

Monday will be warmer and largely sunny over the region as the
upper level trough digs aggressively offshore and allows
increasingly southerly flow aloft to develop over the forecast
area. This flow will pump warm air into the region and allow high
temperatures on Monday to soar back above normal...mid 80s to mid
90s will be common throughout the populated valleys and the
Columbia Basin for another taste of high summer hot and dry

Monday night and Tuesday will bring some changes as the
offshore trough begins to eject inland. There will be no cold
front yet...but the approach of the trough will shift the surface
thermal trough eastward from the basin leading to a moderation of
a few degrees over Monday`s high temperatures...and the first
indications of a more active weather period in the form of
increased ACCAS and mountain cumulus filtering the sun as mid
level moisture works with some very weak elevated instability
ahead of an approaching kicker wave in the southerly flow. The
dynamics associated with this wave...the missing ingredient for
actual thunderstorms and precipitation...will probably hold off
until Tuesday night...making Tuesday another dry...warm and benign
day across the region. /Fugazzi

Tuesday night through Wednesday night: The broad area of low pressure
currently off the BC Coast will settle off the WA Coast then
slowly begin to inch inland. As this occurs, the compact
circulation currently noted at 35N/140W will get absorbed into the
system and slung inland across Ern WA and N ID Tuesday night into
early Wednesday morning. This is well agreed upon amongst the
forecast models relative to timing but the exact track of the
moisture and how much precipitation or lightning comes out of this
feature still carries high uncertainty. NAM/GFS indicate the
potential for 100-300 J/kg of elevated CAPE. When examining model
soundings under these pockets of instability, the moisture
associated with the elevated convection is confined to levels
above 600mb. This suggest the instability is largely centered
within temperatures colder than -8C and theoretically should
struggle to initiate charge separation, ie. lightning. Lapse rates
600mb and above are nearly moist adiabatic but do steepen
700-600mb where models indicate little to no moisture. In these
marginal scenarios, typically strong forcing can overcome the lack
of moisture but in this case, the forcing will be strongest upon
moving on to the Oregon Coast then weaken while making the track
through the Inland NW and into Canada. Additionally, below 600mb,
the air mass will remain very dry. Putting the pieces together, I
have moderate confidence for elevated showers producing very light
precipitation amounts (0.05" or less) but low to very low
confidence whether lightning will accompany this activity. I would
not rule out some gusty winds as the rain falls into the dry sub-
cloud. The placement of this activity could vary from as far west
as the Hwy 97 corridor to the Montana/Idaho Border with Sunday`s
models trending further west than previous runs.

The aforementioned wave will lift north out of the region Wednesday
morning and much of Ern WA and N ID will experience a dry day
coupled with seasonal temperatures. The trof of low pressure will
begin to inch inland and a shortwave pivoting within will usher a
cold front into the Cascades by late afternoon into toward the
WA/ID border overnight. Scattered light showers will accompany the
front but precipitation amounts will be very light with most
locations looking at amounts between a trace and 0.03". High
temperatures will be warming into the upper 70s to lower 80s. /sb

Thursday to Sunday: The Inland NW`s weather pattern will be unsettled
as the eastern Pacific trough and its embedded shortwaves migrate
inland. Yet aside from the Cascades and northern mountains the
opportunity for significant amounts or at prolonged periods of
precipitation is limited. Temperatures are projected to be cooler
than average. In fact there could be some rather chilly nights,
especially toward Saturday morning. Depending on how the pattern
evolves when some spots could see lows under 40 degrees by then.

On Thursday models suggest a shortwave and front pushes across
the eastern CWA early. It is tied to some elevated instability but
it isn`t that strong and moisture and lift is limited and narrow.
Expect some clouds and a slight chance of showers or sprinkles
here early. At the same time a stronger shortwave closer to the
center of the upper trough pivots across the Cascades. This is
tied to some deeper moisture and, by afternoon, better SBCAPE. So
a better threat of showers and thunderstorms will be found here,
with smaller chances across the northern mountains largely closer
to the Canadian border. The main impact elsewhere by afternoon
will be an area of middle to high clouds and marginally breezy
conditions over the eastern Columbia Basin.

From Thursday night into Friday models diverge a bit. Some
solutions bring weak ridging, leaving the main shower threat near
the Cascades until Friday afternoon when some risk returns to the
northeast mountains too. Yet other solutions don`t indicate the
weak ridging. Instead they bring another wave around the base of
the parent trough, pivot it into the southeastern CWA and interact
with the stalled boundary here. So in addition to the shower
chances near the Cascades, this results in shower chances from the
Blue Mountains to the Central Panhandle Thursday night into Friday
morning. Then a generalized shower and thunderstorm threat
develops around the mountains Friday afternoon. This latter
solution is preferred.

Then Friday night into Saturday morning the shower risk decreases.
Then Saturday afternoon the next shortwave moves in with the
parent trough on its heels, before it starts to shift east Sunday
and the region transitions to a northwesterly flow. Shower and
thunderstorm chances will be found around the mountains Saturday
and mainly toward the northeast WA and ID Panhandle mountains
Sunday as that flow changes. /J. Cote`


00Z TAFS: Mid to high clouds will continue to pass through the
region throughout the overnight period. Otherwise winds will be
light generally around 10 mph or less. Could see locally breezy
conditions (15-20 mph) briefly accompany sprinkles or light
showers in and around the North Cascades and Okanogan Valley
between 1400-1800 PDT. This carries low confidence but a
possibility as midlevel showers evaporate into the dry subcloud
layers. winds will continue to weaken throughout the day. /JDC


Spokane        58  90  61  86  59  81 /   0  10  10  10  20  10
Coeur d`Alene  53  89  56  86  56  81 /   0  10  10  10  20  10
Pullman        53  90  54  83  55  79 /   0  10  10  10  20  10
Lewiston       62  96  63  89  61  85 /   0  10  10  10  20  10
Colville       50  90  52  88  55  83 /  10  10  10  10  20  10
Sandpoint      48  86  50  86  54  80 /  10  10  10  10  20  10
Kellogg        52  87  54  86  54  81 /   0  10  10  10  20  10
Moses Lake     55  91  57  86  56  82 /   0   0  10   0  10  10
Wenatchee      60  90  61  86  59  79 /   0   0  10  10  10  10
Omak           54  90  55  90  57  84 /  10  10  10  10  10  10



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