Flooding in Idaho, April 22 to 30, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
436 PM PDT Sun Apr 22 2012

.SYNOPSIS...
The combination of warm temperatures and precipitation will lead
to river flooding in parts of North Idaho, with rises on most
streams and rivers across the Inland Northwest. This warming trend
will persist through the middle of next week. Moist southerly flow
tonight night through Tuesday and Wednesday will combine with
these warm temperatures to produce a chance of afternoon and
evening thunderstorms into much of next week.

&&

.DISCUSSION...

Tonight through Tuesday: Mild weather with shower and thunderstorm
chances increasing each day as weak waves slide up from the south
ahead of the approaching long-wave trough. WV/IR satellite shows
a weak vorticity maxima coming up from the south-southwest this
evening. It exited northern California earlier today. Models are
tracking it relatively well, especially the GFS, bringing it
toward the region through this evening into the overnight. LAPs
and SPC Analysis has been indicating increasing CAPE across Oregon
into eastern Washington and the central ID Panhandle. However
convective inhibition (CIN) has accompanied it. The CIN has been
eroding across portions of the higher terrain south of the region.
Yet the question remains will it erode enough and/or will the
incoming vorticity maxima be strong enough to kick off any
showers and thunderstorms?

With all this said, the passing vorticity max does not look very
strong. Showers, if any, are expected to be isolated to scattered
in nature. This means some may see nothing while some just 5 miles
away could see a storm. The overall best threat remains over the
southeast forecast area, near the Blues and Camas Prairie and
secondarily onto the Palouse and central Panhandle. After dark,
there is continued elevated instability across the southeast half
of the CWA with negative dtheta/dz lapse rates around -0.5 to
-3.0 degrees K/km. The vorticity max still passing will keep the
threat of showers going through the night or perhaps a passing
sprinkle away.

Between Monday and Tuesday a more organized shortwave disturbance,
with a cluster of vorticity maxima, is expected to cross portions
of the forecast area. Currently south of California and west of
Baja, this disturbance is projected to be picked up by the
southerly flow in advance of the offshore trough. It tracks
through the Desert Southwest and Great Basin before moving over
much of central and eastern Washington and northern Idaho by late
Monday into Tuesday. The initial leading and weaker vorticity
maxima cross the southeast to eastern forecast area late Monday
into Monday night. These will interact with the renewed and
stronger convective instability before sunset, to renew the threat
of showers and thunderstorms first across the far southeast CWA.
The threat expands over much of northeast Washington and northern
Idaho by later Monday night into Tuesday morning.

By late Tuesday morning into the afternoon the brunt of the
vorticity max coming in from near California and the Baja passes
near the southeast CWA, while a second disturbance and jet streak
passes the northern forecast area. Coinciding with some afternoon
heating and convective instability, this period looks to have the
best overall chances for showers and thunderstorms across the CWA.
The highest threat will be over the Idaho Panhandle and the
eastern third of Washington. /J. Cote'

Tue Nt through Thurs Nt: The wettest part of the fcst will be
confined to the Wed through Thurs Nt time span as the slow-to-
eject offshore upper trof and cold front moves inland. The
toughest part of the fcst, which will have significant impact on
pcpn amnts, will be deciding how much of the deeper/rich moisture
now over the Great Basin and four corners region to our south gets
advected north ahead of the trof. This plume, containing pwat
values as high as one inch, is handled similarly in both the GFS
and ECMWF, streaming north into Ern Wa beginning Wed ahead of
strengthening large-scale forcing for ascent just downstream of
the ejecting vort max. This will combine with moderate instability
to a potentially very wet fcst for the region. We've adjusted the
pcpn fcst and now have amnts between 1 to 2 inches in the Idaho
panhandle and also near the Cascade crest. This is not a big
change from the previous fcst. There will be a thunder threat
each day that will produce banded localized very heavy amnts that
could have major affects on the smaller streams and creeks.

Hydrology/river rises: Fcst temps in the 70s and near 80 today
through Tues, combined with the heavy pcpn threat (the lesser
threat Tues, and a much greater threat mid to late week), will
produce another round of rises on creeks and mainstem rivers
especially for N Id. A number of rivers will go above action
stage soon (our threshold BELOW the flood stage that prompts us to
monitor the river level and trends closely). Flood stages will be
reached for the St Joe River and Coeur D'alene river. bz

Friday through Sunday...Eastern Washington and Northern Idaho
starting out the period in the cool conditionally unstable air-mass
to the north of the jet stream Friday. There is a big
kicker/disturbance/trof passing through this air-mass which will
utilize this instability to kick off convective showers,
especially over the more orographically favored areas Friday. By
late in the evening Saturday however most of this cool trof and
its associated unstable air-mass has moved east and away from the
area and is replaced with very weak and flat ridging moving in
from the west. This allows for decreasing pops Saturday. However
by Sunday there appears to be a disturbance with reasonable amount
of moisture feeding into it that over-tops this flat ridge so pops
ramp back up again. Difficult to give a good forecast trend in
temperatures based on this trof/flat ridge scenario other they are
likely to remain on the cool side of what would be considered
normal for this time of year. /Pelatti

&&

.AVIATION...
00Z TAFs: High pressure aloft and at the surface will persist
at the taf sites through the next 24 hours. VFR conditions will
prevail but anticipate an increase of mid and high level clouds
overnight as a weak shortwave sneaks under the ridge of high
pressure. There is a small chance for some elevated convection
firing from this moisture band overnight, but at this time
confidence is low on the placement and coverage if any convection
does develop. The weak wave and moisure will lift to the Idaho
panhandle by midday Monday. There is a small chance of surface based
showers and thunderstorms from the Blue Mts to the Idaho panhandle
by late Monday afternoon. /rfox.

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        52  79  54  73  48  70 /  10  10  20  40  20  20
Coeur d'Alene  50  79  52  73  46  71 /  10  10  20  50  20  20
Pullman        50  79  54  71  46  70 /  20  20  50  60  30  30
Lewiston       55  85  58  76  52  75 /  20  10  40  50  20  30
Colville       47  83  49  77  44  75 /   0  10  20  20  20  20
Sandpoint      46  77  48  72  43  70 /  10  10  20  50  30  20
Kellogg        47  77  47  68  43  70 /  20  20  40  60  40  30
Moses Lake     50  84  52  77  49  73 /  10  10  10  20  10  30
Wenatchee      52  83  55  76  50  72 /  10  10  10  20  10  30
Omak           48  83  48  77  45  72 /  10  10  10  20  10  20

&&

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

HAZARDOUS WEATHER OUTLOOK...CORRECTED
NATIONAL WEATHER SERVICE SPOKANE WA
554 AM PDT SUN APR 22 2012

IDZ026-027-WAZ032>035-041-043-044-231300-
LEWISTON AREA-LEWIS AND SOUTHERN NEZ PERCE COUNTIES-
LOWER GARFIELD AND ASOTIN COUNTIES-WASHINGTON PALOUSE-
MOSES LAKE AREA-UPPER COLUMBIA BASIN-WENATCHEE AREA-
OKANOGAN VALLEY-WATERVILLE PLATEAU-
554 AM PDT SUN APR 22 2012

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF NORTH CENTRAL
IDAHO...CENTRAL WASHINGTON...EAST CENTRAL WASHINGTON...NORTH
CENTRAL WASHINGTON AND SOUTHEAST WASHINGTON.

.DAY ONE...TODAY AND TONIGHT

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

IDZ001>004-WAZ031-036>038-042-231300-
NORTHERN PANHANDLE-COEUR D'ALENE AREA-IDAHO PALOUSE-
CENTRAL PANHANDLE MOUNTAINS-NORTHEAST BLUE MOUNTAINS-SPOKANE AREA-
NORTHEAST MOUNTAINS-OKANOGAN HIGHLANDS-
EAST SLOPES NORTHERN CASCADES-
554 AM PDT SUN APR 22 2012

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF NORTH IDAHO...
NORTH CENTRAL WASHINGTON...NORTHEAST WASHINGTON AND SOUTHEAST
WASHINGTON.

.DAY ONE...TODAY AND TONIGHT

HAZARDOUS WEATHER IS NOT EXPECTED AT THIS TIME.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

WARM TEMPERATURES OVER THE REGION WILL ACCELERATE MOUNTAIN SNOW
MELT THROUGH THE BEGINNING OF THE NEW WORK WEEK. THIS WILL LEAD TO
STRONG RISES ON RIVERS AND STREAMS DRAINING THE MOUNTAINS. MANY
RIVERS ARE EXPECTED TO REACH BANK FULL LEVELS...WITH THE COEUR
D'ALENE AND ST JOE RIVERS EXPECTED TO REACH FLOOD STAGE EARLY THIS
WEEK.