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Area forecast discussion
National Weather Service Spokane Washington
454 PM PDT Thu may 25 2017

Synopsis...
after this evening's showers and thunderstorms, strong high
pressure will build over the region for this weekend. Expect dry
and warm weather for Memorial Day. Look for afternoon
temperatures in the 80s by Sunday, Monday and Tuesday. Our warm
weekend will likely cause rises on rivers in central and north
central Washington next week. A return to cooler and unsettled
weather looks to arrive late next week.

&&

Discussion...
tonight and Friday...an upper trough will remain over the region
through tonight before beginning to shift east on Friday. A
combination of daytime heating and cold air aloft associated with
the trough will lead to continued development of scattered
showers and isolated thunderstorms into early this evening. This
will occur mainly over the Cascades, northern mountains, and Idaho
Panhandle where sun breaks have been the most prevalent. Cloud
cover has limited heating across the Columbia Basin, Spokane area,
and over much of southeast Washington and the Lewiston area. This should
result in dry conditions through tonight although can not rule out
a shower coming off the northern mountains towards the Highway 2
corridor from Wilbur to Airway Heights. Overall convection will
decrease after sunset but just enough mid level moisture and
instability will linger overnight for the potential for isolated
showers for the Idaho Panhandle. With the departing upper trough
and drier air aloft moving into central and eastern Washington on
Friday...skies are expected to be mostly clear with warmer
temperatures. Residual showers are expected over the Idaho
Panhandle with a slight chance of thunderstorms for the Camas
Prairie and central Panhandle mountains. Jw

Friday night through tuesday: warm and mostly dry weather can be
expected for the Holiday weekend. An upper level ridge will
strengthen over the region, while a trough digs over the eastern
Pacific and slowly presses toward the coast. The main story will
be warming temperatures, some of the warmest of the year so far.
Anticipate daytime highs in the 80s to lower 90s for Sunday,
Monday and even Tuesday. This would be above normal by about 15
degrees! Little factoid, the last time we had a Memorial Day this
warm was in 1995.

Despite the strong ridge of high pressure, there will be subtle
changes in the pattern that will lead to convection. For Saturday,
this should not be a problem with plenty of dry air aloft and a
good cap in place. Mountain convection looks to begin to bloom
over the Cascades Sunday afternoon and early evening. Moisture at
700mb will increase in the weak southwest flow into the Cascades
while instability increases and the cap eases along the periphery
of the ridge aloft. Light upslope flow into the east slopes of the
Cascades will aid in the development of a few showers and
possible thunderstorms. By Monday, there looks to be another
round of convection, targeting the Cascades and parts of northern
Idaho mountains with a weak cap and orographic lift. Then Tuesday,
more convection in the mountains is possible but this round may
have more support and better instability. GFS seems to be the more
aggressive model with this scenario while the ecwmf is weaker and
slower.

This warming will lead to increased mountain snow melt and rises
on area rivers and streams. A couple rivers may still be on the
rise from the previous warm spell, but most river levels have
fallen. River models indicate rises increasing up by early next
week. In addition to rising water levels, remember water
temperatures will be quite cold with the recent snow melt. /Rfox.

Tuesday evening through Friday...just how long the ridge holds
onto the area is questionable. The last few runs of the GFS have
shifted the ridge axis into western Montana by Tuesday evening,
while the ec is a bit slower. This model doesn't nudge the ridge
east of the area until Wednesday afternoon. The Canadian is
somewhere in between. For now we will keep the mild and dry
weather largely intact through Tuesday evening and then we will
introduce the chance of precipitation through the remainder of the
period. Model agreement is better by Thursday and Friday will all
solutions having a trough of some sorts over the region. This will
result in cooling temperatures with highs retreating into the 70s.
Given the strength of the ridge breakdown, we'd expect to see some
thunderstorms, especially over the mountains. And if the GFS is
correct some of these storms could be stronger than just Ordinary
thunderstorms, but that's a long way out so there could be lots of
changes before then. Fx

&&

Aviation...
00z tafs: convection over the Okanogan Highlands and northeast
mtns of Washington will be capable of occasional lightning
strikes, wind gusts to 25 mph and brief heavy rain. Much of the
activity through the rest of this evening is expected to be around
airfields near Colville, Republic and Omak. Thunderstorms will be
moving south into a more stable environment with the expectation
that they will weaken as they push off of the higher terrain.
There will be a small possibility for a weak outflow from the
northwest into kgeg and ksff between 02-04z. The upper low
pressure system helping to generate this convection will exit the
region to the south with skies clearing overnight. /Svh

&&

Preliminary point temps/pops...
Spokane 47 74 51 78 55 82 / 20 0 0 0 0 0
Coeur D'Alene 44 73 47 77 49 81 / 20 10 10 0 0 10
Pullman 44 70 45 76 47 80 / 10 10 0 0 0 0
Lewiston 48 77 50 82 53 87 / 10 10 10 0 0 0
Colville 45 76 48 81 52 82 / 20 0 0 0 0 0
Sandpoint 42 71 43 76 46 78 / 20 10 10 0 0 10
Kellogg 41 71 43 76 46 79 / 20 20 10 0 0 10
Moses Lake 47 82 50 86 52 89 / 10 0 0 0 0 0
Wenatchee 51 81 55 84 57 88 / 10 0 0 0 0 0
Omak 49 81 50 85 52 87 / 20 0 0 0 0 10

&&

Otx watches/warnings/advisories...
Idaho...none.

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