Flash Flood in Washington, May 20, 2018

Area Forecast Discussion
National Weather Service Spokane WA
437 PM PDT Sun May 20 2018

.SYNOPSIS...
A risk for scattered showers, along with a few thunderstorms,
will linger over the region into Monday. A significant warming
trend is forecast this week. A broader area of showers and
thunderstorms is possible late Wednesday into Thursday. Friday
should be a dry day, before shower and thunderstorm chances
increase over the Memorial Day Weekend.

&&

.DISCUSSION...
Tonight and Monday: The upper level trough will push into
California tonight. As it does, it is sending some pieces of
energy up in the southerly flow into Oregon, Idaho and Washington.
Showers and isolated thunderstorms have developed across portions
of eastern WA and north central ID this afternoon. These areas of
showers are moving pretty slow, about 10-20 mph toward the north.
Expect some moderate to heavy rain with showers or thunderstorms
that do develop. The high resolution models have a pretty good
handle on stuff that is developing across SE WA and NC ID. They
show it shearing apart and weakening btwn 6-8pm. Showers will be
possible through the evening and overnight hours across the Camas
Prairie and into the Blue Mountains as energy continues to rotate
around as the trough pushes into southern CA. The high resolution
models do not have a good handle however on the showers that are
moving up the eastern Columbia Basin around Ritzville. Would
suspect that they will decrease after 6pm as well. Monday the
ridge axis will begin to push into Washington. This will bring
some warming temperatures and drier air to central WA. The Idaho
Panhandle will remain unsettled as afternoon showers and
thunderstorms will be possible. We will start our warming trend
with temperatures 4-8 degrees above average; or valley
temperatures in the mid 70s to around 80. /Nisbet

Monday night through Friday: A rex block pattern sets up
initially with a closed low over the southwest US and a blocking
high over SE BC and Alberta. This should leave our region mostly
dry Monday night and Tuesday except for a threat of isolated
showers and thunderstorms over the higher terrain of SE Washington
into the Idaho Panhandle where some lingering moisture and
instability will lie. On Wednesday the high moves east while the
low over the southwest US lifts north over Idaho. Mid level
moisture and instability increases over the region with surface
based CAPE of 1000-1500 J/KG. Latest guidance leans towards
afternoon convection in the Cascades, SE WA, N Idaho especially
over the higher terrain with the chances for showers and
thunderstorms expanding north and west in the evening to include
the Columbia Basin and Spokane/Coeur d'Alene area. Elevated
instability lingers overnight into Thursday morning over Eastern
WA/N Idaho for a continued chance of showers. Convection should
become focuses mainly over the higher terrain of N Idaho Thursday
afternoon with the exiting wave. Friday should be a dry day as
upper ridge moves in. Temperatures will remain well above normal
through this forecast period.

Memorial Day Weekend: Current model solutions show an increasing
threat of showers and thunderstorms over the Memorial Day Weekend.
On Saturday a large closed low sits off the northern California
coast with southerly flow aloft ahead of it over the region. There
may be enough moisture/instability ahead of the low for a
shower/thunderstorm threat Saturday but confidence is low.
Saturday night/Sunday the low ejects northeast with a strong wave
tracking through. Showers and thunderstorm chances increase along
with winds as well as significant cooling by the end of the
Memorial Day Weekend. It is too early to tell degree and
intensity of convection but something we will be monitoring for
the next several days.  JW

&&

.AVIATION...
00Z TAFS: An unstable upper trough will provide a threat of isold
to sct -shra and maybe an isold -tsra over the Columbia Basin
through the central and southern Panhandle, with mainly isold -shra
toward the northern mountains. These best threat around TAF sites
will be around PUW/LWS, with smaller chances near COE/SFF/GEG/MWH/EAT.
These will be capable of heavy downpours and small hail. The
threat wanes at most TAF sites after 02-03Z with the loss of
daytime heating. The threat continues through the night into early
Monday near PUW/LWS in proximity to an embedded shortwave and
lingering instability. Primarily VFR conditions are expected, but
brief MVFR conditions possible in any heavier cells. /J. Cote'

&&

.HYDROLOGY...
Mountain snow melt issues continue to promote mainstem flooding
on a couple rivers. Temperatures will warm through the week to
well above normal for this time of year resulting in additional
snow melt and high levels on most area rivers/streams.

Flood warnings remain in effect for the following rivers and
lakes

Lake Pend Oreille AFFECTING Bonner County
Pend Oreille River below Albeni Falls AFFECTING Bonner and Pend
Oreille Counties
Okanogan River near Tonasket AFFECTING Okanogan County

An Areal Flood Warning continues over Okanogan County for the
lake shore of Lake Osoyoos...and the possibility of recent
rainfall runoff flooding on the Salmon Creek from Conconully to
Okanogan.

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        54  76  56  82  60  82 /  10   0   0   0   0  10
Coeur d'Alene  53  75  52  82  55  82 /   0  20   0  10  10  20
Pullman        53  70  51  79  55  79 /  20  10   0  10  10  20
Lewiston       58  76  57  84  61  83 /  20  10  10  10  10  20
Colville       51  78  49  88  55  88 /  10   0   0   0   0  10
Sandpoint      50  75  48  81  55  81 /   0   0  10  10  10  20
Kellogg        50  71  48  79  51  79 /  50  50  10  10  10  30
Moses Lake     53  80  51  86  58  89 /   0   0   0   0   0   0
Wenatchee      54  80  56  86  61  88 /   0   0   0   0   0  10
Omak           54  79  54  86  59  87 /   0   0   0   0   0  10

&&

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

&&

Hazardous Weather Outlook
National Weather Service Spokane WA
412 PM PDT Sun May 20 2018

IDZ001>004-026-027-WAZ031>038-041>044-212315-
Northern Panhandle-Coeur d'Alene Area-Idaho Palouse-
Central Panhandle Mountains-Lewiston Area-
Lewis and Southern Nez Perce Counties-Northeast Blue Mountains-
Lower Garfield and Asotin Counties-Washington Palouse-
Moses Lake Area-Upper Columbia Basin-Spokane Area-
Northeast Mountains-Okanogan Highlands-Wenatchee Area-
East Slopes Northern Cascades-Okanogan Valley-Waterville Plateau-
412 PM PDT Sun May 20 2018

This Hazardous Weather Outlook is for portions of North Idaho...
North Central Idaho...Central Washington...East Central
Washington...North Central Washington...Northeast Washington and
Southeast Washington.

.DAY ONE...THIS AFTERNOON AND TONIGHT

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

There is a potential for widely scattered thunderstorms
Wednesday. At this time, the Idaho Panhandle and southeastern WA
carry the greatest risk but isolated storms are expected to expand
into the remainder of Eastern Washington. Heavy rainfall will be
a concern with these storms. Widespread thunderstorms will be
possible again on Sunday over the Holiday weekend. Those with
outdoor plans are urged to monitor the upcoming forecasts.