Area Forecast Discussion
Issued by NWS Spokane, WA

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149
FXUS66 KOTX 252145
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
245 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 SE 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...
18Z TAFS: VFR conditions are expected at all TAF sites through 18z
Friday. An upper trough over the region combined with daytime
heating will produce scattered showers and isolated thunderstorms
mainly over the northern mountains...Cascades...and Idaho
Panhandle. HRRR suggests storms likely staying away from the TAF
sites...which makes sense with the current cloud shield this
morning over the TAF sites limiting convective potential this
afternoon. Although there is a small chance that thunderstorms
over the northern mountains could produce a weak outflow boundary
towards KGEG/KSFF/KCOE between 0-3z this evening. JW

&&

.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...
ID...None.
WA...None.
&&

$$



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