Debris Flows and Flooding in Washington, April 16 to 19, 2018

Area Forecast Discussion
National Weather Service Spokane WA
506 AM PDT Mon Apr 16 2018

.SYNOPSIS...
Widespread this morning with the potential for moderate rainfall
and significant high mountain snow. Cooler air and gusty winds
will arrive this morning as a strong cold front pushes through
the region late tonight with falling snow levels. The colder
temperatures will result graupel with afternoon showers today.
Tuesday will still see a mix of rain and snow showers while the
rest of the week will be drier with occasional showers.

&&

.DISCUSSION...
Today through Tuesday: A cold upper level low pressure system will
move in over the region this afternoon. There is a cold front push
up across northern OR early this morning at 2:00 AM. It is progged
to sweep across eastern WA and the ID Panhandle through the
morning. There is strong dynamics along the front combined with
steep mid level lapse rates will result in more moderate to
locally heavy rain out ahead of the front. Strong cold air
advection with the front may also result in a brief period of rain
switching over to snow on the Palouse, to the higher benches
around Spokane, into the valleys of the Central Panhandle Mtns and
on the Camas Prairie. The Camas Prairie may see a quick inch of
slush snow, but much of the rest of the lower elevations will see
non-accumulating snow if at all.

Winds will become gusty with frontal passage across the basin and
into the Spokane Area and Palouse. Wind gusts of up around 35 mph
will be possible. Breezy to windy conditions will then continue
into the afternoon hours. There will be a brief break from the
precip in the mid to late morning hours behind the front. However,
showers will return in the afternoon over much of the region as a -30
Celsius cold pool at 500 mb moves in over the region. This will
destabilize the atmosphere with graupel showers mixed in with the
rain showers through the afternoon. Some of the stronger cells
that pulse up may also spit out a couple lightning strikes. Lifted
Condensation Levels (LCLs) will be on the colder side, so
separation of charges will likely limit our thunderstorm
potential for today.

Rivers and small streams will be on the rise after our wet night.
No flooding is anticipated though. Large puddles in low lying
areas will be likely and commuters should be aware of this
possibility while heading out this morning. Soils are quite
saturated after multiple rounds of heavy rain events in the past
month. Debris flows or mud flows will continue to be a risk in
steep terrain, especially in the east slopes of the northern
Cascades.

The upper level low will begin to make its exit tomorrow. It will
send across some more energy overnight Monday though. Precip will
be focused mainly into the Central Panhandle Mountains. Snow
levels will be much lower with snow possible down into the
valleys. Moderate snow accumulations will be possible above 3000
feet and highlight will may be needed later today for the
potential of accumulating snow for travel over Lookout Pass.

Pressure gradients remain tight for Tuesday over the region with
another windy day in store. A chance of showers is expected on
Tuesday, but not expected to be as unstable. Best chances for
showers on Tuesday will be in the ID Panhandle. /SVH

Wednesday through Sunday: Tuesday night showers over the mountains
dissipate and drier weather is expected to continue into Wednesday
as the ridge axis moves into eastern Washington overnight. Partly
cloudy skies will stick around Wednesday as a weakening low digs
well to our south, keeping most of its energy and showers in
Oregon and California. It looks to split off a smaller, weak
disturbance that, if it stays far north enough, could bring
scattered shower chances on Wednesday afternoon and evening. Any
shower chances quickly diminish overnight as the parent low moves
further into the desert southwest. A stronger ridge lingering over
the region for most of Friday and Saturday will bring the most
pleasant weather of the week with dry conditions, sunny skies, and
temperatures warming by several degrees into the mid and upper
50s. Saturday's system is trending weaker with the past several
model runs, and there is potential that it will not dip as far
south as previously though, which would limit precip chances.
Right now the best chance for rain/snow showers on Saturday look
to be in the Cascades and NE Washington/N Idaho mountains. Winds
will be breezy as that surface low passes us to the north with
gusts of 25-30 mph possible in the Columbia Basin. Cautiously
optimistic that Sunday into next week will bring a more prolonged
period of dry and warm weather to the region. /bw

&&

.AVIATION...
12Z TAFS: A surface low is moving up the Okanogan Valley early
this morning with a cold front extending out over the Spokane Area
and to Pullman/Lewiston in eastern WA. Strong upward motion along
the cold front will result in moderate to heavy rainfall in the ID
Panhandle. KCOE will see IFR conditions through at least 13Z
before tapering off to showers. All other TAF sites will see the
potential for MVFR cigs through mid morning. Cigs will then lift
in the afternoon, but showers will fill back in over the region
behind the cold front as the cold upper level low takes residence
over north-central WA. Showers will consist of a mix of rain and
graupel. Gusty southwest winds today to between 25-30 kts. /SVH

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        47  35  49  32  50  33 /  80  60  30  10  20  10
Coeur d'Alene  46  34  47  31  49  31 / 100  70  80  10  20  10
Pullman        47  33  47  32  50  33 /  80  60  60  10  20  10
Lewiston       53  39  53  36  56  37 /  80  30  30  10  20  10
Colville       49  36  53  31  53  31 / 100  60  60  20  20  10
Sandpoint      44  35  44  32  47  31 / 100  80  80  30  20  10
Kellogg        42  33  42  30  46  30 / 100  80  90  20  20  10
Moses Lake     55  40  57  35  56  35 /  40  30  10  10  20  10
Wenatchee      55  38  55  37  54  36 /  40  20  10  10  30  10
Omak           55  38  58  34  55  34 /  70  40  10  10  30  10

&&

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

&&

Hazardous Weather Outlook
National Weather Service Spokane WA
619 AM PDT Mon Apr 16 2018

IDZ002-026-WAZ032>036-041-043-044-171330-
Coeur d'Alene Area-Lewiston Area-
Lower Garfield and Asotin Counties-Washington Palouse-
Moses Lake Area-Upper Columbia Basin-Spokane Area-Wenatchee Area-
Okanogan Valley-Waterville Plateau-
619 AM PDT Mon Apr 16 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...TODAY AND TONIGHT

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY

No hazardous weather is expected at this time.

IDZ001-003-004-027-WAZ031-037-038-042-171330-
Northern Panhandle-Idaho Palouse-Central Panhandle Mountains-
Lewis and Southern Nez Perce Counties-Northeast Blue Mountains-
Northeast Mountains-Okanogan Highlands-
East Slopes Northern Cascades-
619 AM PDT Mon Apr 16 2018

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

.DAY ONE...TODAY AND TONIGHT

Snow levels will drop today behind a cold front passage. Another
round of precipitation tonight will result in accumulating snow
into Tuesday morning above 3000 feet.

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY

No hazardous weather is expected at this time.