Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 022112

Area Forecast Discussion
National Weather Service Spokane WA
212 PM PDT MON MAY 2 2016

High pressure will begin to shift east tonight and Tuesday.
Expect one more day of warm and dry conditions on Tuesday. By
late Tuesday 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.


Tonight through Tuesday night: The axis of the ridge of high
pressure has moved into western Montana and SE B.C., meanwhile a
deep upper level low has moved inside 140W. This dance, as the
ridge shifts east and the upper low moves inland will play out
over the next several days. Through the short term the resultant
southerly flow will pump Pacific moisture into the region, mainly
along and west of the Cascades, and mainly above 700mb through
Tuesday mid-day or so. As the moisture deepens Tuesday afternoon
and evening it will move east of the crest, but mainly west of the
Columbia river along the east slopes of the Cascades. The surface
and mid levels will destabilize Tuesday and Tuesday night, but
there is no real strong forcing mechanism save for orographics and
the heating of the day. The chance for precipitation has been
bumped up along the crest just due to the shear magnitude of the
moisture advection, and while there is a chance for thunderstorms
the confidence in some moderate showers is much higher than
lightning. Temperatures on Tuesday will likely be the warmest of
the week with highs in the 70s to lower 80s. This may be mitigated
a bit by cloud cover, but the rapid snow melt due to the warm
temperatures of late will continue.

* Impacts: Low at this time. Rain fall amounts will only be
  around a tenth or so, but this will just help to moisten the
  soils for additional rain and snow melt for Wednesday and
  Thursday. Tobin

Wednesday through Monday: Shower and thunderstorm chances
migrate across the Inland NW through Friday, before drier weather
returns for the weekend. Between Tuesday night and Thursday a
stretching/splitting upper trough edges from the coastline to the
Cascades, before it starts to shift out Friday. This slow-moving
boundary and smaller-scale disturbances tracking up from the south
will work with instability and deepening moisture to bring the
shower and thunderstorm chances.

* Precipitation evolution: Wednesday the best forcing and highest
  threat of precipitation will be near the Cascades and northern
  mountains. Precipitation elsewhere will be more isolated to
  scattered in nature. Wednesday night the broader surface-based
  instability abates, but some elevated instability continues
  across south-central/southeastern WA into the lower Panhandle.
  At the same time a more robust vorticity max starts in from the
  south and the best surge of moisture starts in from the south.
  This will lead to the next round of precipitation developing
  from the south through the night into Thursday morning. Thursday
  the focus for instability begins to shift away from the Cascades
  but remains moderate elsewhere. With moisture remaining
  abundant, a deformation along the WA/ID wrapping back toward the
  Cascades and more viable vorticity lobes coming across the
  region, it looks like Thursday should have the broadest threat
  of showers and thunderstorms. At this time if there are
  organized/stronger thunderstorms it appears the best risk would
  be over southeast WA and the lower ID Panhandle Thursday
  afternoon. Thursday night into Friday the southern and dominate
  split of the upper trough sags into the Great Basin and the
  northern split moves into across the Canadian Rockies. This
  leaves a weaker deformation axis in the east and a southeastern
  flow that carries some weak vorticity lobes by the Blues and
  Camas Prairie. The highest shower chances will linger across
  southeast WA and the ID Panhandle. Some guidance, however, shows
  a threat of showers drifting as far west as the Okanogan
  Highlands and eastern Waterville Plateau. This doesn`t seem out
  of the question given the easterly flow so I expanded a slight
  threat here.

* Flooding risk: Through this time models depict abundant
  moisture nosing in, with PWATs rising to between 0.75 to 1.0
  inches Wednesday night into Thursday. This is about 200-300% of
  normal. So any showers or thunderstorms that develop could
  produce some heavy downpours. Given the flow pattern there is a
  risk of training showers, i.e. precipitation repeatedly going
  over the same spots. This could mean localized flooding,
  especially in burn scars or other more high-risk areas such as
  steeper terrain. The first threat will be near the Cascades
  Wednesday, then expand east Thursday before waning from the west
  Thursday evening.

* Winds: going into Thursday night and Friday the shifting
  pattern leads to an increasing gradient. Thursday night the
  gradient appears strongest from north to south, bringing some of
  the stronger winds to the Okanogan Valley through western
  Columbia Basin. Friday the stronger gradient comes in from
  northeast, expanding a broader stronger wind threat across north
  Idaho out through the Spokane area into the Columbia Basin. I
  raised speeds compared to the previous forecast. Average speeds
  10-20 mph with gusts to 30 mph. However localized specific
  details will differ.

From Saturday to Monday the Inland NW will be in a relatively
drier pattern, with the upper low well south of the region and the
jet stream well north of the Canadian border until about Sunday
night to Monday. Saturday winds will remain breezy to strong from
the northwest before abating through the later afternoon in
evening, as gradient start to slacken. An isolated shower threat
will linger near the central/southern ID Panhandle mountains, with
dry and mostly sunny weather elsewhere. Sunday looks mostly sunny
and dry. By Monday a system drops southeast from Canadian toward
the northern Rockies, bringing just a few more clouds and a slight
threat of showers near the northeast WA and ID Panhandle
mountains. Otherwise dry weather continues. /J. Cote`


18Z TAFS: Some high thin clouds may get over the strong ridge that
is over the region this afternoon and tonight...but should have
little effect on the TAf sites. Conditions will remain VFR
through Tuesday at 12z. Tobin


Spokane        52  81  54  78  54  67 /   0   0   0  20  50  70
Coeur d`Alene  47  82  49  79  51  69 /   0   0   0  20  50  60
Pullman        47  79  48  76  49  66 /   0   0  10  20  50  60
Lewiston       50  85  52  82  54  72 /   0   0  10  20  50  60
Colville       44  85  47  80  47  70 /   0   0  10  20  50  80
Sandpoint      41  79  45  77  47  69 /   0   0   0  20  30  60
Kellogg        44  80  45  80  45  69 /   0   0   0  20  40  60
Moses Lake     46  84  51  79  50  70 /   0   0  10  30  60  60
Wenatchee      53  82  56  76  53  69 /   0  10  20  30  50  50
Omak           49  84  52  77  52  69 /   0  10  20  40  50  60



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