Area Forecast Discussion
Issued by NWS Spokane, WA

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683
FXUS66 KOTX 221133
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
432 AM PDT FRI MAY 22 2015

.SYNOPSIS...
The Memorial Day Weekend is expected to be warmer than average
with afternoon temperatures in the 70s and 80s. Look for scattered
showers and thunderstorms this weekend into next week. Some
thunderstorms will be capable of producing heavy downpours and
frequent lightning.

&&

.DISCUSSION...
Today through Saturday night: The weather pattern will see subtle
changes as we move into the holiday weekend, keeping a persistence
forecast with minor adjustments. The upper level pattern remain
diffuse and in chaos with broad trough across the western U.S.
Shortwaves embedded in the trough will provide the lift for
convection. While the main circulation lies over southern CA and
helps pump moisture up across the Rockies into the Pac NW, a
second circulation will be dropping down the BC coast. Even though
somewhat drier northeast flow resides over northeast Washington
and far northern Panhandle, pwats continue to be high, near an
inch, across the Cascades and southern Washington, similar to
yesterday, which will help fuel convection. In fact, yesterday`s
convection developed over southeast Washington and Blue mountains
and took a slow track westward toward the Cascades without any
real kicker. Anticipate similar conditions today, but possibly a
better chance of convection as the northern shortwave approaches.
As this feature arrives in northwest Washington this afternoon, it
will weaken but may provide enough lift to enhance convection in
the Cascades. Wet bulb zero heights will be approaching 10k while
cape values near 1000 j/kg in the Cascades, and even higher in the
Blue mountains by this afternoon. The instability values alone
would lean toward stronger thunderstorms will larger hail, but the
warmer atmosphere may lead to more heavy rain. Given the slow
steer flow and weak shear, would side with heavy rain and possible
flooding as the main impacts. Agree with the current flash
flooding watch for the northern Cascades given the many burn scars
for recent fires and any thunderstorm that sits over a burn area
too long may lead to issues. The Blue mountains area will most
likely see the stronger storms with both heavy rain and hail.
Anticipate the thunder threat to decrease by early evening and the
convection to wane overnight. By Saturday, the drier and more
stable over northeast Washington filters into the region as the
remnants of the shortwave side into Oregon. The bulk of mid/upper
level moisture sinks south of the region. May still see afternoon
convection, but current model runs paint less instability and
moisture over the Inland Northwest, although the areas of concern
will still be near the Cascade crest and the Blue mountains and
the southern ID panhandle. Temperatures were bumped on Saturday
following a more persistence forecast, although not as warm as
experienced yesterday. Expect the diurnally convection to decrease
by Saturday evening with light winds both at the surface and
aloft. /rfox

Sunday and Memorial Day: We should see a subtle change in the
pattern Sunday into Monday which has the potential to shift our
best chances for showers from our southern Panhandle and Cascade
zones to our northeast Washington and north Idaho zones. The
upper low which has been over southern California for the last
week is expected to migrate into the Central Rockies Sunday night
into Monday. A weak, slow moving trough is expected to descend
from British Columbia into western or central Washington Sunday
into Monday. This change in the upper pattern will bring a shift
in our storm steering flow from our current light easterly flow to
our more typical westerly flow by Sunday afternoon. Surface
dewpoints in northeast Washington and far north Idaho should
increase Sunday and Monday yielding higher surface based
instability in places like Colville, Republic, Sandpoint, and
Bonners Ferry. If the GFS, ECMWF and Canadian GEM hold true, our
best chance for showers and thunderstorms on Sunday afternoon will
be over the north Cascades into the Okanogan Highlands. For Monday
afternoon, the focus will shift into the Idaho Panhandle and
northeast Washington. The increased onshore flow Monday should
push high temperature down a few degrees and the increased west to
east pressure gradient should produce more wind (10-15mph out of
the west or southwest).

Tuesday through Thursday: Chances for showers Tuesday through
Thursday will largely vary on what happens to the upper low that
descends from B.C. on Monday. The GFS and Canadian models are very
slow to move this feature suggesting good chances for showers and
thunderstorms Tuesday, Wednesday, and Thursday. The ECMWF moves
the upper low out more quickly with less convection Wednesday and
Thursday. Our forecast was based on a troughy GFS ensemble mean.
/GKoch

&&

.AVIATION...
12Z TAFS: Scattered high based showers will continue to drift
west from the vicnity of KMWH toward the KEAT area this morning,
with mainly light rain expected. Convection will again develop
late today, after 20z. Expect a similar trend as experienced on
Thursday with activity starting in the Blue mountains and Camas
Prairie and than drifting west toward the Cascades. Today
anticipate more deveopment in the Cascades under the easterly
low level flow and the approaching shortwave from the northwest. The
HRRR shows convection will begin to fire over the high terrain of
the Cascades and Blue Mountains after 20z Fri. Gusty outflow
winds and brief heavy rain will be the main threats from
thunderstorms, which should decrease after 03z. /rfox.

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        82  53  80  53  79  53 /  10  10  10  10  20  30
Coeur d`Alene  81  52  79  53  79  52 /  10  10  10  10  20  30
Pullman        75  49  73  47  75  49 /  30  40  20  10  10  10
Lewiston       80  56  79  54  82  56 /  40  40  20  20  20  20
Colville       86  53  83  55  82  52 /  10  10  10  10  30  30
Sandpoint      81  50  80  48  77  50 /  10   0  10  10  20  20
Kellogg        80  50  79  49  78  50 /  20  10  30  20  20  20
Moses Lake     86  56  84  55  83  54 /  20  10  10  10  20  20
Wenatchee      84  59  83  58  82  57 /  50  30  20  10  20  20
Omak           86  53  85  54  83  53 /  20  20  10  10  30  20

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...Flash Flood Watch from 1 PM PDT this afternoon through this
     evening for East Slopes Northern Cascades.

&&

$$




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