Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 212150
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
250 PM PDT TUE OCT 21 2014

.SYNOPSIS...
A cold front exits to east this evening, leaving only some isolated
showers in the Idaho Panhandle. From late Tuesday night through
Wednesday night, the next frontal wave and round of precipitation
comes through. The active pattern continues into early next week,
with the potential for more widespread rain late Friday into
Saturday, then again late Monday into next Tuesday, as moisture
associated with former typhoons makes its way toward the Pacific
Northwest.

&&

.DISCUSSION...
...A MOIST PACIFIC STORM WITH SUB-TROPICAL CONNECTIONS TO BRING
HEAVY RAINS ACROSS THE INLAND NORTHWEST WEDNESDAY INTO THURSDAY...

Tonight through Wednesday: Satellite imagery shows a deep upper
level low pressure system circulating in the Gulf Alaska. Water
vapor imagery shows a deep fetch of moisture extending out along
40N to an area of sub-tropical convection in the central Pacific.
Model guidance indicates P-wats of up around 1.25 inches directed
toward the Pacific Northwest. Much of this moisture content will
be squeezed out along the windward slopes of the northern Cascade
Mtns, but more than enough of this moisture is expected to
translate east of the Cascades for soaking rains across much of
the region beginning tonight.

The warm front shows up rather nicely on satellite, which
currently is just off the WA/OR coast. There is a good amount of
enhancement on both the water vapor and IR satellite imagery in
the warm sector of the low behind the leading edge of the warm
front. This will be the area to keep an eye on trough tonight for
periods of moderate to heavy rain. Much of this rainfall will
remain along the Cascade crest this evening into the early
portions of the overnight period. This moisture will begin to
push east of the Cascade Mtns early Wednesday morning. Strong
isentropic ascent will occur across the northwestern portion of
the forecast area with much of the precip falling northwest of a
line from about Ritzville to Sandpoint. Much of the southeastern
portion of the forecast area will have to wait for rain until the
cold front pushes further onshore after tomorrow afternoon.

*Rain Amounts: Easterly to southeasterly winds at low levels will
 result in good upslope flow into the East Slopes of the Northern
 Cascades over into the Okanogan Highlands. These areas are
 expected to see a range of 0.75-1.50 inches of rainfall over a 24
 hour period tonight through Wednesday. Higher amounts closer to 2
 inches will be possible up near the Cascade crest. The Wenatchee
 Area, Waterville Plateau and Okanogan Vly will see rainfall
 amounts closer to a half of an inch over this period; and the
 Moses Lake Area, Upper Columbia Basin into the Northeast Mtns
 will see rainfall amounts between 0.10 to around 0.25 inches
 through Wednesday.

*Impacts: There is a small chance for some rises to small streams
 and mud slides due to heavy rain. The primary area of concern
 will be along the East Slopes of the Northern Cascade Mtns and
 primarily in or near recent burn scars. Rainfall intensity will
 likely not be strong enough for Flash Flooding concerns; however,
 rainfall on recent burn scars will likely lead to more rapid
 runoff than normally observed in these kind of events.

*Snow: Snow levels will be up around 6,500 feet through this
 period. This will result in snow only up over the highest peaks
 and primarily over the peaks in the Cascades.

*Winds: A southeasterly surface pressure gradient will tighten tonight
 into Wednesday. Winds at 850 mbs will increase up to around 30-35
 kts. Since the region will be exclusively in the warm sector
 during this period, mixing will be limited somewhat; this will be
 especially so in locations where it is raining. A better chance
 for better mixing will be across the basin over into the Central
 Panhandle Mtns where the rain will take a bit longer to form.
 This is expected to result in breezy southerly winds with
 sustained speeds of around 15-20 mph and gusts up to around 25-30
 mph.

*Temperatures: Expect cooler temperatures with the rain to across
 the northwest portion of the forecast area on Wednesday. High
 temperatures will be more mild across the southeast portion of
 the forecast area and about 5 to 8 degrees above normal. /SVH

Wednesday night through Friday night: An active weather pattern
continues with a parade of systems tapping some subtropical
moisture. First from Wednesday evening through Thursday morning a
cold front pushes across the Cascades and advances toward Idaho
overnight into Thursday morning. Driven east by a mid-level
disturbance and a 130kt jet stream, this system is featuring
modest lift. The subtropical moisture plume directed into the
region ahead of this boundary pushes PWATs up toward 0.60 to 1.0
inches (or 150-240% of normal). This combination of lift and
moisture will bring likely rain between Wednesday night and early
Thursday morning across a large portion of eastern WA and north
ID. The lowest threat will be toward the L-C Valley and Camas
Prairie, especially before Thursday morning, and chances will
begin to wane in the lee of the Cascades late in the evening to
overnight.

From Thursday morning to early Thursday evening the frontal
boundary stalls near the central ID/MT border through northeast
OR, just south of Lookout Pass to the Blue Mountains, with the jet
just north of the front. Regional PWATs drop to between 0.40 to
0.70 inches (or 100-170% of normal). This is still a decent amount
of moisture. The proximity of the front and the enhanced lift near
the LFQ of the jet will keep precipitation likely across the Idaho
Panhandle through the Blues and near the Cascade crest. Chances
will also remain moderately high across the eastern third of WA,
with the proximity of that LFQ of the jet, but will be waning in
the lee of the Cascades.

From late Thursday evening to Friday morning the atmosphere dries
out a little more, with the core of deeper plume of moisture
briefly sagging toward southern Oregon. This plus weak shortwave
ridging nosing in ahead of the next system will further decrease
the precipitation through this period. The best chances of
precipitation during this time frame will be near southeast WA and
the ID Panhandle mountains, as well as near the Cascade crest.

From Friday afternoon through Friday night the next low pressure
system deepens as it lifts north from the CA/OR coast toward the
WA/OR coast. With the increased southerly flow a warm front lifts
into the region, with the plume of subtropical moisture coming
back north with it. Models suggest this should evolve as a band of
precipitation which envelopes areas along the WA/OR border through
the Idaho Clearwaters by later Friday afternoon, then lifts north
across much of remainder of central and eastern WA and north ID
early Friday evening, before the focus lifts toward the northern
mountains and backs against the Cascades overnight into Saturday
morning. So some modest precipitation amounts are possible:
potentially between a tenth and a quarter of an inch, with local
amounts over a half inch to an inch toward the Cascades. This
could mean some slow rises on creeks and streams and other waterways
near the Cascades, with minor mudslides/debris flows. /J. Cote`

Saturday through Tuesday...A trough off the coast starts moving
in on Saturday with a decent cold front, gusty winds with near
or above normal temperatures and higher mountain snow. Sunday
will see a high pressure ridge building in for a brief break in
the weather. Then for Monday into Monday night, there will be the
next wet weather system on the way with Pacific moisture moving
in. After that it will be a fairly active weather pattern in the
future with multiple storm systems from the west especially for
the mountains but overall, no severe weather is expected for this
time of the forecast. /TC

&&

.AVIATION...
18Z TAFS: Low stratus will linger around the northern and eastern
basin, as well as, into the ID Panhandle through this afternoon.
This will impact the KGEG, KSFF, KCOE and KPUW TAF locations. The stratus
deck is expected to lift into a stratocumulus deck through the
afternoon with VFR cigs possible by the late afternoon hours. Low
level flow will back to out of the southeast overnight as a moist
Pacific storm system approaches the region. Strong warm air
advection should clear out the low clouds tonight with rainfall
forming northwest of a line from KMWH to KCQV by Wednesday
morning. KEAT will likely see light rain developing after 12Z with
cigs lowering to MVFR category. Low level wind shear will become a
concern as well with winds increasing significantly off the
surface and veering to out of the southwest; however, low level
wind shear is not mentioned in the TAFs as it does not look to be
strong enough. /SVH

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        43  59  48  59  43  56 /  10  60  90  50  20  40
Coeur d`Alene  40  57  45  58  41  55 /  10  40  90  70  30  40
Pullman        43  62  47  59  45  58 /   0  20  70  60  30  50
Lewiston       44  66  51  63  48  61 /   0  10  30  60  40  60
Colville       44  54  46  57  41  56 /  20 100 100  60  20  20
Sandpoint      40  55  43  55  38  55 /  10  60  90  70  40  20
Kellogg        38  57  45  54  41  53 /  10  10  80  80  60  40
Moses Lake     49  59  48  64  43  58 /  20  90  90  20  10  50
Wenatchee      48  55  48  61  44  55 /  70 100  80  30  10  60
Omak           46  52  45  59  40  55 /  80 100 100  40  10  20

&&

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

$$




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