Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 031143

Area Forecast Discussion
National Weather Service Spokane WA
443 AM PDT TUE MAY 3 2016

High pressure will begin to shift east today. Expect one more day
of warm and dry conditions today. By late today and lasting until
Friday a slow moving weather disturbance will move through the
region. This will result in a cooler, unsettled pattern with a
good chance for showers and thunderstorms with locally heavy rain
by Wednesday and Thursday. Improving conditions will begin again
late on Friday, lasting into the weekend.



Today through Wednesday: The weather pattern over the Inland NW will
gradually become more active as the dome of high pressure
continues to slide into central MT and southerly flow draws a rich
moisture plume into the region. The moisture will arrive along the
Cascade Crest today then begin to spread into Ern WA and N ID on
Wednesday. Consequently, the threat for showers and thunderstorms
will follow suite. Temperatures will remain quite toasty for May
standards with afternoon highs 15-20 degrees above normal today
and only cooling a handful of degrees Wednesday. The threat of
thunderstorms and showers capable of moderate rainfall amounts
will be the main focus through this period with confidence levels
near to below average regarding the threat to burn scars.

* Thunderstorm Potential: The main focus for thunderstorms this
  afternoon and evening will be along the Cascade Crest and points
  just slightly east. The combination of afternoon CAPE on the
  order of 400-800 J/kg and PWATS climbing near 0.70" does raise
  concern for cells capable of heavy downpours. Any storms that
  develop will track south to north around 20-25 mph so one storm
  will not pose as much of a threat but if several cells were
  track over the same area...this training effect could lead to
  hefty rainfall amounts. Prior to 8PM...the main forcing will
  come from converging terrain driven winds and this generally
  carries lower confidence. The threat for showers and storms will
  increase further into the evening hours as a 850-700mb frontal
  boundary arrives. The burn scar that will be most susceptible to
  heavier showers late afternoon and evening will be the Wolverine
  Complex impacting locations between Holden Village and Lucerne.

  The air mass will continue to slowly moisten
  across Ern WA and N ID on Wednesday but it looks like the region
  will lack a strong trigger to initiate afternoon convection.
  Just about every location across the CWA supports enough
  afternoon instability to support a thunderstorm but whether
  parcels can break through the CIN layer and develop into
  thunderstorms is highly uncertain. The main threat for any
  storms will come in the afternoon following spotty light showers
  in the morning and attm, there is loose agreement that the
  northern mountains and Cascade Crest will carry the highest
  chances. A much better opportunity arrives with a wave
  approaching from the south Wednesday night. /sb

Wednesday night through Monday...This period will consist of a wet
showery period Wednesday night and Thursday followed by a drying
trend Friday through Sunday...with a possible breezy dry cold
front by next Monday.

The stormy period will be compliments of a moist southerly flow
visible on satellite as a deep fetch of Pacific moisture moving up
ahead of a highly meridional trough just off the coast this
morning. By Wednesday evening models are in good agreement in
depicting this deep moisture feed laying along the Cascades with
very slow movement eastward as the parent trough keeps
digging...eventually pinching off into a lazy and broad closed low
over California. A surface thermal trough will set up over the
Columbia Basin on Tuesday and Wednesday. This moisture and low
level heating will lead to an unstable air mass. Triggering
mechanisms besides orography will be provided by a weak wave
transiting up out of Oregon Wednesday night and pulling the
moisture axis eastward across the forecast area on Thursday...into
the surface thermal trough. All of this adds up to a very showery
and potentially thundery pattern Wednesday evening through Thursday
for just about the entire forecast area. The Major evolving
concern will be the potential for debris flows and flash floods on
recent burn scars in the Cascades and Okanogan area. This
potential will be closely monitored and further analyzed through
the next 24 to 36 hours for the possibility of issuing a Flash
Flood Watch for these areas.

A cooling trend will begin from west to east with the passage of
this system...with Wednesday and Thursday`s high temperatures
probably still above normal but moderated off of the peak
temperatures of Tuesday.

After Thursday a drying trend will commence...still with some
lingering minor showers and perhaps a stray thunderstorm on Friday
concentrated over mountains and the southeastern a
flop-over upper ridge noses into the region over the low pressure
far to the south. This will keep the forecast area in a quiet
weather zone but with breezy north to northeast winds ushering in
some dry Canadian continental air through the Okanogan Valley and
Purcell Trench Friday and Saturday. Temperatures in this dry air
mass under clearing skies will begin to march upward again for the
weekend cresting once again around 10 degrees above normal.

Far in the extended forecast models are more ambiguous but seem to
be hinting at a dry cold front passage on or about Monday as the
polar storm track dips south and flattens the upper ridge. In
addition to breezy conditions...this time out of the
west/northwest...temperatures should moderate back toward normal
for the start of the new work week. /Fugazzi


12Z TAFS: Southerly flow sandwiched between high pressure to the
east and an approaching trough will allow mid and high clouds to
stream into Central WA today then Ern WA and N ID Wed. Combined
with afternoon instability...there is a modest chance for showers
and thunderstorms 22-04Z along the Cascade Crest. Spotty light
elevated showers will expand toward KEAT/KOMK 04-12Z but given the
dry subcloud layer, main impacts through 12Z will be incr ceilings
8-10K ft AGL and spotty sprinkles. /sb


Spokane        81  54  78  55  68  50 /   0   0  20  50  60  30
Coeur d`Alene  82  49  78  52  69  49 /   0   0  20  50  60  30
Pullman        79  49  76  51  67  47 /   0   0  20  50  50  40
Lewiston       85  53  82  56  72  51 /   0  10  20  50  50  40
Colville       87  45  79  49  70  45 /   0   0  30  50  70  30
Sandpoint      79  44  77  48  70  44 /   0   0  20  40  60  30
Kellogg        80  45  79  46  69  44 /   0   0  20  40  50  50
Moses Lake     85  53  79  53  72  47 /   0  10  20  50  50  20
Wenatchee      83  56  76  55  70  52 /  10  20  30  60  50  20
Omak           84  53  77  54  69  48 /   0  20  40  60  60  20



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