Area Forecast Discussion
Issued by NWS Spokane, WA

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223
FXUS66 KOTX 302332
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
432 PM PDT SAT APR 30 2016

.SYNOPSIS...
An isolated shower threat wanes this evening. High pressure
builds Sunday and Monday, resulting in a warming trend, with
temperatures climbing into the mid 70s to lower 80s. High pressure
shifts east toward the middle of next week, allowing a chance for
showers and thunderstorms to return.


&&

.DISCUSSION...
Tonight through Monday night: The Inland NW dries out and warms up,
until isolated shower chances start back into a small portion of
the region late Monday.

Tonight the area remains in a northeast flow. Low pressure tracks
south of the Blues this evening and toward the OR/CA border by
Sunday morning. Meanwhile the deformation axis lingering on its
backside stretches and weakens. All this points to the threat of
showers dwindling with the loss of heating. Clouds will also ebb
away. However with precipitation moistening up the boundary layer,
I did introduce the threat of patchy fog overnight/early Sunday
morning toward the L-C Valley and in the sheltered northern
valleys. There could also be some shallow fog near Spokane River
and Long Lake areas but the risk is remote.

Sunday into Monday high pressure flops over Pacific Northwest
into the Northern Rockies. The flow gradually turns easterly and
then south to southeasterly. A modest northeast to southwest
gradient continues through Sunday, which will continue to provide
some breezy northeast winds. Also under this set-up milder air
starts into the region, with temperatures pushing into the 70s to
low 80s, especially by Monday.

Lastly, later Monday into Monday night a mid-level disturbance
starts to move up from the south. With an increase in some
mid-level moisture and pockets of convective instability, expect
some increased cloud cover for Monday afternoon and night. However
there appears to be too much CIN to produce any real significant
threat of showers, except for a couple spots. Those spots include
the Blue Mountains/Camas Prairie and the higher elevations around
the Cascades. Even then at this time if anything occurs it looks
like isolated. /J. Cote`


Tuesday and Wednesday...Model guidance is in good agreement with
the overall synoptic pattern. This pattern will feature the
eastward shift of the upper level ridge and its eventual
replacement by moist southerly flow ahead of a deepening east
Pacific trough. Precipitable water values are expected to near
0.75 inches by Tuesday afternoon in the Cascades while the
surface-850 mb flow begins to turn to the south or southeast. This
should bring the initial chance of precipitation to this
area...while farther east conditions will be considerably more
stable given the positioning of the upper level ridge over western
Montana. Model soundings on Tuesday afternoon reveal enough
instability to support scattered showers and even a few
thunderstorms near the Cascades. Synoptic scale forcing looks
quite weak aside from some weak upper level divergence associated
with upper level southerly jet.

For Tuesday night through Wednesday night the chances for showers
and thunderstorms will increase across the entire forecast area as
the offshore trough shifts onto the coast but digs south toward
California. The persistent southerly flow will bring more
moisture into the region with precipitable water values nearing an
inch over a good portion of eastern Washington. This is well over
the 90th percentile and could support widespread precipitation as
the instability increases rapidly. By Wednesday afternoon, we
could be dealing with CAPE values in excess of 1000 j/kg, which is
fairly substantial for this region, however other convective
parameters, namely shear look paltry. Given this scenario we fully
expect to see numerous showers and some thunderstorms, but nothing
too substantial at this point. Suspect the main impact will be the
possibility of some moderate to locally heavy rains. Model total
rainfall and some model ensembles suggest the best potential for
heavy rains will occur near the Cascades. These rains will be hit
or miss rather than widespread, however when combined with
significant runoff from mild temperatures (more on that later),
there is some risk of hydrological issues. An river statement has
already been issued to address some action stages on a few of the
local rivers (including the Entiat, Methow, Stehekin, and
Okanogan).

By Thursday and Thursday night the upper level trough is expected
to dig even further into southern California with the upper level
flow turning from southerly to more of an east to southeast regime
aloft and the low-level thermal ridge shifting to the WA/ID border
as a marine push or weak cold front moves through. This will
result in increasing stability over most of the forecast area,
save extreme eastern Washington and the Idaho Panhandle where
thunder will remain possible. Model CAPEs again could exceed 1000
j/kg but shear and forcing look even weaker than on Wednesday so
nothing extraordinary is anticipated. Meanwhile a much lower
chance of precipitation is expected in the lee of the Cascades.

By Friday into Saturday...the upper level low is expected to
meander into the desert SW while warmer air aloft wraps across
most of our forecast area as another strong ridge folds over the
Cascades. This should lower the chances for precipitation even
further, save the SE part of the forecast area.

Temperatures will likely peak on Tuesday with widespread highs in
the upper 70s to mid 80s with subtle cooling after that. As the
ridge returns by next weekend we should see temperatures climb
back into the mid 70s to lower 80s. These temperatures are around
10 to 15 degrees warmer than normal for this time of year. fx


&&

.AVIATION...
00Z TAFS: Leftover Showers and isolated thunderstorms producing
local MVFR ceilings continue to diminish this evening with
majority of activity gone after 03Z Sunday. Some patchy fog near
sheltered mountain valleys and near KLWS otherwise IFR conditions
to prevail tomorrow. /Pelatti

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        46  74  50  78  52  80 /  10   0   0   0   0   0
Coeur d`Alene  43  75  46  78  48  80 /  20   0   0   0  10   0
Pullman        42  71  45  76  48  78 /  30   0   0  10  10  10
Lewiston       46  76  49  81  51  84 /  20   0   0  10  10  10
Colville       42  80  44  81  45  83 /  20   0   0   0   0  10
Sandpoint      39  73  41  77  42  78 /  10   0   0  10   0  10
Kellogg        40  71  42  77  43  79 /  20   0   0  10   0  10
Moses Lake     44  79  46  83  48  84 /  10   0   0   0   0  10
Wenatchee      49  79  52  82  54  82 /   0   0   0   0  10  10
Omak           45  78  48  81  50  82 /   0   0   0   0  10  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.
&&

$$



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