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Area forecast discussion 
National Weather Service Spokane Washington
232 am PDT Friday Aug 22 2014

Synopsis...
Friday will bring another round of showers and thunderstorms to
the region with the potential for more flash flooding over the
Cascades burn scars. The threat of scattered showers and
thunderstorms will continue through the weekend. A drier and
warmer weather pattern is expected for the new work week.

&&

Discussion...
friday: a deep upper-level low pressure system will pass through the
region today delivering widely scattered showers and thunderstorms.
Storms containing heavy rainfall and very little movement will be
the main concern as the risk for flash flooding and debris flows
remains elevated over the east slopes and Okanogan Highlands.
Additional urban and rural areas could also experience localized
flooding impacts if a storm was to setup overhead. An isolated
stronger storm could also bring brief hail and gusty outflow
winds on the order of 40 mph.

2am satellite reveals the center of the low crossing over the
Columbia Gorge near The Dalles. The low will shift east toward
the Tri Cities this morning...then continue toward the or/WA/ID
border by midday before tracking into southeast Idaho this evening. With
the low overhead, an unstable air mass will be in place with
500mb temperatures cooling near -17c. Meanwhile, a tongue of
Richer moisture and instability...marked well by the 850-700mb
Theta-E progs will wrap around the eastern and northern periphery
of the low...wrapping back south on its western edge. This Richer
moisture in conjunction with little to no steering flow is a good
recipe for storms containing heavy rainfall. The good news is that
this lack of steering flow (storm movement) only resides through
late afternoon then as the system departs the east, increasing
northerly flow will give any lingering storms a push. Needless to
say, we saw what these storms are capable of Thursday afternoon
with numerous debris flows and mudslides within the Carlton
complex burn scar.

As of 2am...thunderstorms continue to go strong across the
northern mountains with additional development now over the lower
Idaho Panhandle. This activity is tied to the placement of the higher
Theta-east axis and lift along the northeastern periphery of the
upper low dropping into the pac northwest. Hrrr and local hi-res model
simulated radar suggest the larger area of showers/storms over
Ferry County will pivot to the west this morning, fall apart after
sunrise, allowing cells to redevelop after a few hours of
sunshine. Convective temperatures are only in the upper 60's to
lower 70's so it won't take much. Whether the cloud cover from
this activity completely falls apart or just thins will be a major
challenge related to storm intensity this afternoon. For NE Washington and
the Idaho Panhandle, looks as if isolated storms will be possible
through morning then afternoon heating and a few vorticity
maximums will promote widely scattered storms again after the
12pm-3pm timeframe...continuing through the evening. Slightly
deeper wind shear over the lower Idaho Panhandle could support a few
organized cells capable of hail and wind gusts in excess of 40 mph
but overall, the threat for severe storms (hail and damaging
winds) remains low region-wide.

Saturday: the upper-low will continue its March east Friday night
and Saturday into central Montana Saturday morning then North
Dakota by the evening. Look for drier north to northwest flow over
the Cascades and western basin Saturday morning to translate east
into the Idaho Panhandle by Saturday evening. The main
precipitation threat will concentrate over the Idaho Panhandle and
into western Montana. Some models are more aggressive with the
drying so we may be on the high end for precipitation chances in
Idaho, especially west of a line from Mullan to the blue mtns.
Given the lingering cold pool aloft, a few convective showers will
be possible in the mountains all the way to the Cascade crest but
these will be the weak and shallow variety producing little to no
rainfall.

We will also see breezy northerly winds as the low deepens today
then pulls out Saturday. Winds will be strongest through the
narrow Okanogan Valley and Purcell Trench. Winds sustained 10-20
mph with gusts to 35 mph are a strong possibility. Temperatures
will remain on the cool side of normal. /Sb

Saturday night through Wednesday...general agreement exists
between the latest medium range models with the overall field of
motion. The general troffy and unsettled pattern will persist
through Sunday featuring a continued threat of showers and at
least isolated thunderstorms mainly during the afternoon and
evening hours and concentrated over the northern mountains...with
a few showers persisting well into the night over the Idaho
Panhandle. Temperatures in this polar influenced air mass will run
at or slightly below normal through Monday.

Monday appears to bring a transition from a general trough to a
new ridging cycle...with only a few lingering showers over the
northeast and Panhandle while the remainder of the forecast area
dries out and stabilizes under the ridge driven warming aloft.

Tuesday through Wednesday bring high confidence of dry and
clear/partly cloudy conditions throughout the region as the polar
storm track retreats to the north with temperatures once again
cresting well above normal. /Fugazzi

Wednesday evening through Thursday night: starting Wednesday
evening the strength of the ridge will be on the decline as the
next trough of low pressure approaches the region. Models are
still wavering a bit on the breakdown, but one thing in common for
now is Wednesday will be the warmest day of the forecast period.
Considering model differences for the breakdown, the GFS continues
to be much more aggressive bringing a deep trough swinging
through late Thursday. This solution would support another round
of widespread rain as the system would have both mid latitude
Pacific moisture as well as topical moisture from decaying
Tropical Storm Lowell to work with. The Euro on the other hand
keeps the trough further north leading to a much drier zonal flow
pattern for the pac northwest. Either way, we are looking at breezy to
windy conditions for the later part of the week as cold air
advection coupled with a tightened pressure gradient increases the
winds. Both models have been fairly consistent in their respective
solutions, so the dgex was used to help lean in one models favor.
It more closely followed the Euro leading to higher confidence in
it.

So leaning towards the Euro, it delays the major threat of precip
beyond the reaches of this forecast so pops were kept relatively low.
Temperatures were also raised a bit Thursday to show the
transition to a more zonal pattern rather than the cold incoming
trough from the GFS. One thing to keep in mind is that when models
try to ingest this tropical storm moisture, it often leads to some
inconsistent runs. Knowing this, the extended will need to be
tinkered with throughout the next couple of days as models align
better. /Fliehman

&&

Aviation...
06z tafs: a somewhat moist and unstable air mass associated with
an area of low pressure will continue to influence the aviation
area for the next 24 hours. During the daytime hours surface based
convective showers and thunderstorms will occur, primarily over
and near mountain locations but most taf locations will be under
threat for a close miss or brief thunderstorm Friday afternoon and
evening. /Mjf

&&

Preliminary point temps/pops...
Spokane 75 55 75 54 79 56 / 40 40 10 10 20 30
Coeur D'Alene 73 54 73 51 77 53 / 50 50 10 10 20 30
Pullman 75 51 74 49 79 51 / 40 50 20 20 20 20
Lewiston 80 60 79 56 85 59 / 60 50 30 20 10 20
Colville 72 51 78 49 81 52 / 60 50 10 10 20 30
Sandpoint 69 50 71 46 75 49 / 70 50 10 20 20 30
Kellogg 68 52 67 47 72 53 / 70 60 40 20 30 30
Moses Lake 81 58 83 56 86 58 / 30 50 10 0 10 10
Wenatchee 80 63 82 58 84 64 / 40 50 10 10 20 10
Omak 79 59 82 53 85 58 / 50 40 10 10 20 10

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...Flash Flood Watch through this evening for east slopes northern
Cascades-Okanogan Valley-Wenatchee area.

&&

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