333 
FXUS66 KOTX 180013
AFDOTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
455 PM PDT Wed Apr 17 2013

.SYNOPSIS...
A warming trend will occur through Thursday, but another weather
system is expected to bring valley rain and high mountain snow on
Friday...with showers lingering both Saturday and Sunday. A cold
front is expected to pass through Northeast Washington and North
Idaho Sunday for cooler temperatures. A warming and drying trend
is likely beginning next Monday as high pressure begins to build
over the region.

&&

.DISCUSSION...

Tonight and Thurs: Our biggest challenge will be timing how
quickly the clouds increase overnight along and ahead of the warm
front currently approaching the Pacific NW coast. Clouds have been
slow to thicken, but are expected to move across Ern Wa and N Id
overnight and into Thurs. These inbound mid and high clouds
certainly will play a major role in overnight low temps, as well as
high temps tomorrow. With a rather dry air mass in place now and
through tonight, based on the latest sfc obs and fcst soundings,
it's going to take more much synoptic-scale forcing for ascent
(especially isentropic ascent over the warm front) to give us any
chance of pcpn. And given the slow expected saturation, combined
with stable short wave ridging, it's really not until midday Thurs
that our pcpn chances increase significantly and pose a light rain
threat for the lower elevations. A few hundredths of pcpn in the
Cascades and N Id will be likely however. bz

Thursday night through Saturday: The Inland Northwest becomes wet.
A frontal wave (now pushing toward 140W) arrives late Thursday
night. Deep lift within the leading isentropic ascent and LFQ of
the jet develops in earnest late Thursday night into Friday
morning. The lift persists into the afternoon with the incoming
cold front and mid-level shortwave and a second low (now near
southeast Alaska) dropping through British Columbia. While the
Pacific low weakens as it moves inland that B.C. low strengthens.
During the system's progression inland it taps subtropical
moisture. PWATs rise to 130-180% of normal (or the 0.50 to 0.75
inch range) ahead of the cold front. Precipitation chances
increase to likely for a good part the Inland Northwest by early
Friday and PoPs remain high through the day. The highest
precipitation amounts focus at the Cascade crest and the Idaho
Panhandle southwest into the Blue Mountains. Yet some modest
amounts (one-quarter to one-third inch) are also projected over
the eastern third of Washington. A west-northwest flow will,
however, lead to a modest rain shadow in the lee of the Cascades.
This area will have a better chance of remaining dry. I kept a
slight chance of thunderstorms across east Washington and north
Idaho through early evening, with some low-grade CAPE and negative
LIs on the northeast side of the jet. The afternoon passage of the
front would also work to bring some thunder risk.

By Friday evening the cold front moves east, but through the night
and continuing into Saturday an inverted surface trough lingers
near the Idaho-Montana border. In addition a secondary shortwave
approaches from British Columbia and northeast Washington and
northern Idaho remains in favorable lift in the diffluent LFQ of
the jet. The core of the deep moisture plume does shift south, but
PWATs remain respectable between 95-120% of normal (or the 0.25 to
0.45 inch range). So precipitation chances will remain high across
Idaho as well as the eastern third of Washington, particularly
closer to Idaho border, and at the Cascade crest. However
precipitation amounts are not expected to be as high for much of
the region compared with those generated by the primary system.
The northwesterly flow focuses some of the higher amounts over the
central Panhandle.

Snow levels, based on 850mb temperatures rising to between 3 and 
6 C and wet-bulb zero heights, are projected to be around 
5000-6500 feet ahead of the cold front. They drop to around
4000-5000 feet behind the cold front and locally near 3500 closer
to Cascades through Okanogan Highlands Saturday morning. Lastly,
the dynamic front and mixing along it in proximity to the jet will
make for breezy conditions. At this time, speeds are not expected
to warrant any highlights. Yet gusts between 20 and 30 mph are
possible, with locally higher amounts in the exposed terrain.

Temperatures are projected to be milder. However persistent cloud
cover and precipitation on Friday, especially over the Panhandle,
will keep temperatures from reaching their full potential. This
means values near to a degree or so under seasonal norms. Milder
temperatures are expected toward the lee of the Cascades and
deeper Columbia Basin thanks to down-sloping and more sun. /J.
Cote'

Saturday night through Sunday night: Another shortwave upper level
trough of lower pressure will push across the region out of
British Columbia. This will bring a chance for more unsettled
weather through the weekend. Best chances will be mainly be across
the Idaho Panhandle. This disturbance will usher in some colder
Polar Continental air, which will result in dropping snow levels
across the Idaho Panhandle. Mid slopes across the ID Panhandle mountains
stand a good chance of picking up some light snowfall. This could
result in some accumulating snowfall over Lookout Pass Saturday
night into Sunday. I cannot rule out some accumulating snowfall
in the valleys (especially above 2500 feet), but the timing is not
good for valley accumulations as it looks like the coldest air
will not move in until during the day on Sunday when insolation
will help to warm up temps.

We could see a brisk northerly breeze as this system passes
through for Sunday with below normal temperatures. The northerly
breeze will result in a wind chill factor making it feel even
colder than the ambient temperature. Sunday night is expected to
be our coldest night of the period. Our official start of the
growing season for low elevation areas including the Wenatchee
Area, Moses Lake Area and L-C Valley is April 23 based off of the
average last freeze for the season. At the moment, These areas
are not expected to see widespread freezing temperatures, but I
wouldn't be surprised if our low temps are not cold enough as we
should see clearing skies and weakening winds. Some pockets of
freezing temps will be possible across these areas.

Monday through Wednesday: High pressure offshore looks to push
into the region for next week. This will result in a decreasing
chance for precip and a warming trend. However, the 00Z GFS and
ECMWF model guidance brings in the ridge a bit quicker than their
ensemble means, so there is some uncertainty with how warm we
will actually get and when we will see the warmest temps. However,
we should see above normal temps by around mid week. /SVH

&&

.AVIATION...
00Z TAFS: A deep fetch of Pacific moisture will begin to spill into 
the Inland NW during then next 24 hours.  Look for a gradual 
lowering of cigs aft 12z Thursday...lowering btwn 6-8k agl aft 18z 
Thursday and possibly down to 5k agl arnd 00z. The low level air mass 
is very dry and pcpn will mainly fall in the form of sprinkles 
through 00z and incr in intensity thur nt and fri.  /sb

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        30  55  41  56  38  57 /  10  20  50  80  30  30 
Coeur d'Alene  28  54  38  53  36  56 /  10  20  60  80  40  40 
Pullman        30  53  39  55  37  55 /   0  20  60  90  50  30 
Lewiston       34  59  43  60  42  61 /   0  10  30  70  30  20 
Colville       31  60  40  61  35  62 /  10  10  50  90  40  30 
Sandpoint      27  52  36  51  35  55 /  10  20  50  90  50  50 
Kellogg        28  48  36  49  36  50 /  10  20  70 100  60  60 
Moses Lake     35  64  43  68  40  67 /  10  10  20  20  10  10 
Wenatchee      40  59  44  66  41  62 /  10   0  20  20  10  10 
Omak           33  62  40  66  37  65 /  10  10  20  20  10  10 

&&

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

$$