Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 220955
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
254 AM PDT WED OCT 22 2014

.SYNOPSIS...
Through Wednesday night, a very moist frontal boundary will
produce heavy precipitation for the Cascades and possibly the
mountains of North Idaho. Though the heavy rains end Thursday
Night, the pattern will continue to be active into early next
week. This includes the potential for widespread rain Friday into
Saturday, then again Monday into  Tuesday, as moisture associated
with former typhoons makes its way toward the Pacific Northwest.

&&

.DISCUSSION...
Today through Tonight...Wet period awaits most of the Inland Northwest
during this period. The latest satellite images are showing a
very moist occluded front extending from deep upper level low over
the eastern Gulf of Alaska south-southwest through Vancouver
Island and the Olympic Peninsula. The blended precipitable water
product shows a very impressive atmospheric river streaming from
west of the Dateline all the way to the Washington coast.
Precipitable water values within this front ranged from 1 to 1.3
inches...which is about 2 standard deviations above normal for
this time of year. This moisture will eventually push east of the
Cascades...but the question is when. Over the past several hours
the eastward progression of the front has slowed...likely in
response to a shortwave trough forming near 42/135. This should
delay the onset of the precipitation in the lee of the Cascades.
00Z models aren`t depicting this slowing trend well....however the
new 06z NAM is doing much better. Through late morning...most of
the precipitation will occur west of a line from Colville .to
Rtzville.. By late afternoon...that line will extend from Kellogg
to the Blue Mountains. Once the precipitation begins...it will
continue through much of the night for most of the night as low-
level moisture transport and isentropic ascent look very
formidable. The only drying trend during this period will occur
after midnight in the lee of the Cascades...as the mid-level flow
trends to the west and the precipitable water plume shifts south
and east of that area. QPF amounts during this period will be
impressive with the heaviest amounts expected over the northern
Cascades. Values ranging from 1-1.5 inches will be possible...with
amounts around an inch extending across the northern mountains of
Washington. Despite the lofty precipitation amounts...hydro
problems if any...will be minor due to relatively dry in situ
conditions and low river levels...combined with a slow but steady
stratiform precipitation regime (as opposed to a rapid convective
event). If problems were to occur...it would likely be near the
Cascade burn scars. We will continue to issue hydrological
statements to address this issue. The location with the lightest
precipitation amounts will be over the LC Valley...due the the
late onset of the precipitation (last area for front to arrive)
and persistent southerly downslope flow off the Blues.

Aside from the widespread threat of precipitation...we are looking
at another warm and perhaps windy day. Until the front and
precipitation arrives...most of the area will be subject to
moderate south-southeast winds...combined with warm 850 mb
temperatures. Between late afternoon and early evening...850 mb
temps surge above 10c over the SE quarter of Washington and the
southern Idaho Panhandle. Enhanced mixing potential will allow
temperatures to surge into the mid to upper 60s (if not slightly
warmer) across SE Washington and adjacent portions of Idaho.
Meanwhile temperatures will be significantly cooler across the NW
portions of the forecast area. With the rain likely to develop
early...the impacts of diurnal heating will be largely mitigated.
Highs in these areas won`t be much warmer than the current
temperatures which suggests readings in the mid 50s. fx

Thurs through Sat: For Thurs, we slowly progressed the frontal
boundary and associated rich moisture plume through southeast Wa
and the Cntrl Idaho Panhandle. The tricky part will be determining
how far east across the Cascades and Upper Columbia Basin to bring
the post-frontal dry slot as well as steep mid-level lapse rates.
It`s possible we`ll see some cloud breaks Thurs afternoon that
would help to enhance any sfc-based instability and produce
localized heavier embedded showers. This will be especially true
for the Cascades as an inbound vort max reaches NW Wa by
afternoon. We increased winds as well for Thurs as the vertical
profile becomes nearly unidirectional from the SW in the dry slot,
with 850mb winds around 30 kts. Once the vort max moves into Srn
BC, rapidly rising hts aloft ahead of a warm front over Oregon
will help to quickly end the persistent heavy pcpn threat Thurs
Nt. Concerning this warm front, we made some significant changes
to the timing of the pcpn as this front moves north across Ern Wa.
This includes slowing down the onset of light rain, keeping Nrn
Wa and much of the N Idaho Panhandle near BC dry for Fri.bz

Sunday through Wednesday: The models are indicating a dry period
on Sunday followed by an extended period of rainshowers through
midweek with multiple systems passing through the region. The
temperatures are expected to be near the season normals with highs
around mid 50s and lows around 40. /JDC

&&

.AVIATION...
06Z TAFS: 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. Light rain will
spread east to KMWH around 16Z, then to KGEG-KCOE after 20Z. KPUW
and KLWS may not see precipitation until after 06Z Thursday. 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. /EK

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        54  48  59  43  56  44 /  60 100  50  20  40  60
Coeur d`Alene  53  45  58  41  55  41 /  40 100  70  30  40  60
Pullman        63  47  59  45  58  47 /  30  80  60  30  50  50
Lewiston       67  51  63  48  61  46 /  10  40  60  40  60  30
Colville       50  45  57  41  59  41 / 100 100  60  30  20  60
Sandpoint      54  43  55  38  57  38 /  60 100  90  50  20  60
Kellogg        53  45  54  41  53  42 /  20  90  80  60  40  60
Moses Lake     60  48  64  43  58  45 /  90  90  20  10  50  60
Wenatchee      56  48  61  44  55  47 / 100 100  30  10  60  70
Omak           57  46  59  40  58  44 / 100 100  40  20  20  60

&&

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

$$




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