Flood Watch 2, NWS Spokane, WA

Area Forecast Discussion
National Weather Service Spokane WA
334 PM PDT Wed May 3 2023

.SYNOPSIS...
An unsettled and active pattern will continue through this week,
with a chance of showers and thunderstorms each day. Temperatures
remain above normal through Thursday, then cool into the 50s and 60s
Friday and stay in that range through the weekend.

&&

.DISCUSSION...

Today and Tonight: Analyzing the current satellite and radar
situation, the east central/eastern Washington and central Idaho
Panhandle areas are in a clear slot of sorts with very few clouds
outside of some cumulus developing along the higher terrain in
northern Washington and parts of the Idaho Panhandle. The 12z KOTX
sounding today looks very similar to yesterday, however a few things
are limiting our convection this afternoon. Precipitable water of
0.60" (which is about 90% of average on this date) and weak mean
flow (5-10 knots) with 800-1500 J/kg of CAPE is a fairly prime
environment for heavy rain producing storms. However, if we take a
look at the downdraft CAPE and subsidence under our weak ridge
pattern, the DCAPE values of 1000-1500 J/kg are nearly canceling
out our instability. Not to say it's not possible, but the
potential for strong storms today not as favorable as tomorrow.

Thursday: Tomorrow we will have a shortwave feature move north which
will bring additional moisture into the Camas Prairie and Idaho
Panhandle area. 200% of normal PWAT content will surge northward
during the afternoon hours when instability will be on the rise.
Clear skies early on will also help to prime the environment (erode
away any morning inversion) for steeper lapse rates. Forecast
soundings have an elevated amount of wind shear (surface to 500mb)
during the afternoon and evening hours from central Idaho to WA/ID
Palouse and into northeast Washington. Thunderstorm probabilities
tomorrow are 25-35% and given the shortwave moving through, our
subsidence will be much less of an impact to the convection.

Impacts: With weaker steering flow and increased wind shear along
with higher instability, the threat for heavy rain and gusty outflow
winds along with lightning will be the hazards to look for.
Afternoon and evening outdoor recreation will be a focus for the
thunderstorms.

Along with convection to consider, we also will be facing additional
warming, or another day today and tomorrow of warmer than normal
temperatures. This will continue to play into our rising rivers. The
rotating shortwave will continue to push northward Thursday bringing
additional precipitation to our area. Ensemble climate QPF begins to
show greater than 99th percentile for precipitation amounts as we
head into late this week with more stratiform rainfall expected.
/Dewey

Friday through Tuesday: As a shortwave trough approaches the West
Coast and collides with a ridge of high pressure over Central
Canada, a deformation band will develop along the collision zone
stretching from North Central Washington to the Central ID
Panhandle. The band will slowly move to the northeast over the
course of Friday into Saturday afternoon before weakening.

Due to the slow movement of the band and precipitable water
values around 200% of normal in the region of the band,
significant precipitation accumulation is expected for the
northeast corner of Washington and the Northern ID Panhandle as
the band passes through. The bulk of the rain will fall between
Friday morning and Saturday morning and will be of a widespread,
stratiform nature rather than isolated and convective like we've
been seeing. 0.5 to 1 inch of rain is expected throughout this
time period across Ferry, Stevens, Pend Orielle, Boundary, Bonner,
and Shoshone Counties with over an inch possible for areas of
higher terrain.

Substantial precip amounts after the warm temperatures and high
elevation snowmelt that has taken place the past few days will
increase flooding concerns for the counties listed above. On the
positive side, we'll see a 15- to 25-degree drop in temperatures
behind the deformation band with highs in the 50s across most of
the region. This should help slow additional snowmelt.

Ahead of the deformation band, warmer conditions will stick
around through Friday but will follow suit with the rest of the
region by Saturday. Cooler temperatures in the 50s and 60s will
persist through at least Tuesday of next week as we remain in an
unstable troughing pattern.

Sunday we'll see a brief break in precip with the exception of a
few diurnally driven showers for the Northern Mountains and the
Idaho Panhandle. Monday and Tuesday the trough will move onshore
and pass over our region bringing increased chances for precip
again.

After Tuesday model agreement begins falling apart. Some models
show a continuing troughing pattern, while others show a shortwave
ridge taking over. /Fewkes

&&

.AVIATION...
18Z TAFS: Expect a sunny VFR day today with mainly light winds.
Isolated afternoon/evening thunderstorms are possible but confidence
is too low to put in the TAFs. Winds are expected to pick up this
evening around 03Z for LWS with gusts of 15 to 20kts possible. Winds
will pick up for COE and EAT between 09Z and 11Z and may also have
gusts up to 20kts.

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        54  83  49  56  43  53 /   0  10  60  80  70  50
Coeur d'Alene  54  83  50  58  42  50 /   0  10  60  90  80  70
Pullman        52  75  46  53  42  52 /   0  30  80  90  60  50
Lewiston       57  81  55  61  48  59 /  10  40  90  90  70  40
Colville       52  88  51  64  43  51 /  10  10  40  80  80  80
Sandpoint      53  83  53  60  44  47 /   0  20  40  90  80  80
Kellogg        57  83  53  58  43  47 /  10  30  60  90  80  80
Moses Lake     56  82  50  59  44  61 /  10  10  70  70  40  20
Wenatchee      59  83  51  59  45  56 /  10  20  70  80  40  20
Omak           57  88  57  64  47  53 /   0  10  50  80  70  50

&&

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