Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 201206
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
406 AM PST SAT DEC 20 2014

.SYNOPSIS...
A more active weather pattern will continue into early next week.
Another wet and robust weather system will bring snow to the
mountains with mainly rain for the valleys today and tonight.
Temperatures are expected to remain above average through early
next week...before dropping down toward normal values by Christmas
Day.


&&

.DISCUSSION...
Today through Sunday...The well advertised storm system with the
deep moisture tap into an atmospheric river is upon the Pac NW
this morning. Expect a wet 24-36 hours for all locations. The deep
moisture tap will sag south tonight. This will decrease the
chances of precipitation in the lee of the Cascades, but southwest
flow will keep high pops across the Cascades...the northern
mountains and the Panhandle mountains through the night. As the
moisture continues to drop into Oregon on Sunday the remainder of
the forecast area will see decreasing chances of precipitation.

As mentioned above this will be a very wet storm. The heaviest
precipitation will fall on the Cascades where 1.5-3.0 inches and
possibly more can be expected by Sunday afternoon. 1.0-2.0 inches
are possible for the Blue mountains and and the central Idaho
Panhandle. Around .50-1.0 for the northern mountains and .50-.75
for the Basin, Palouse and the West Plains.

The tricky part will be snow levels. With this much moisture
and warm air advection snow levels will rise dramatically through
the day. This morning snow levels are around 3000 feet across the
south and as low as 1500-2000 feet across the north. By this
afternoon snow levels will be 6000-7000 feet across the south and
2500-3500 feet across the north. So mostly rain in the valleys and
snow in the mountains. For the Cascades 1-2 feet of snow will be
possible, with 4-8 inches for the northern mountains and the
higher elevation of the central Panhandle. An area of concern will
be the Cascade valleys that are starting out a little colder and
will be slow to warm up. I still think that these valleys will
turn over to rain by mid morning, but before that 3-8 inches of
snow will be possible. Advisories have been issued to cover this
and will be continued.

Winds will be on the increase tonight and Sunday. The surface
gradient increases to around 10 mb between KPDT and KGPI with 850
mb winds 40-50 kts. Not sure that there will be sufficient mixing
to get these winds to surface. We normally need cold air advection
and that will not be the case Sunday morning. But it will still be
enough for breezy to windy conditions tonight and Sunday. With
gusty winds and strong warm air advection temperatures will be in
the 40s today and and 40s to 50s on Sunday. Tobin

Sunday night through Tuesday...The weather pattern over the
region will be in transition from a very moist Pacific flow to a
ridging pattern with a drier overland northwest trajectory. The
greatest chance of precipitation will be over the Cascade crest
and the southeast zones as the moisture fetch sinks south.
Westerly flow will keep a rain shadow over the east slopes of the
Cascades and the basin while enhancing precip chances for the
rising terrain east of the basin. The shift to northwest flow will
act to lower snow levels but precip coverage will be decreasing as
well.

As the ridge builds over the region the threat of low stratus and
fog will build as well. There will be ample low level moisture
available so the deciding factor for fog will be the amount of
clearing Monday morning as again Monday night. Downsloping breezy
winds across the western valleys and into the western basin should
mitigate the fog threat Sunday night but these winds will diminish
by Monday night. Tuesday will see an increase in precip chances as
Pacific moisture starts to stream into the region ahead of the
next trough. Temperatures will be warm enough for a valley
rain/mountain snow event.

Tuesday night through Friday...The large scale pattern will
transition from flat westerly flow aloft to a more amplified
pattern with a Pacific trough transiting the region during peak
holiday travel times. Medium range models had been moving toward a
shared solution but now that the 00Z run is available, the ECMWF
has flipped back to the more southern track with precipitation.
With this level of uncertainty, no changes will be made to the
extended forecast period at this time. High levels of uncertainty
remain as to snowfall probabilities for much of the Inland
Northwest. The southeast zones carry a higher degree of
confidence in accumulating snowfall down to about 2000 ft
elevations but the fact that this is a Day-5 forecast carries its
own uncertainty. /Kelch

&&

.AVIATION...
12Z TAFS: Precipitation from the next Pacific storm is already
pushing through KEAT and KMWH as of 11z...and the next 24 hours
will be a very wet period for eastern Washington and north Idaho.
Expectations are for -ra to begin at the eastern TAF sites around
14z but intensify after 16z...then linger through 12z Sunday.
There may be some brief breaks and periods of heavier rain over
the next 24 hours with widely varying cigs/vsby. Precipitation
may begin mixed to begin, but will quickly turn over to rain. Mainly
light SE-E winds today will change over to SW between 03-06z and
increase to around 10-12 kts with gusts 20-25kts for KPUW through
the night.


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        41  40  47  34  40  32 /  90 100  30  30  20  10
Coeur d`Alene  39  37  46  33  40  31 /  90  90  40  50  30  20
Pullman        43  40  51  38  45  34 /  90 100  80  50  20  10
Lewiston       44  42  56  41  49  36 /  80 100  70  70  20  10
Colville       38  35  41  29  37  29 / 100 100  20  30  10  20
Sandpoint      37  35  41  32  37  30 / 100 100  60  60  30  20
Kellogg        38  35  43  34  38  30 /  90 100  90  90  40  20
Moses Lake     42  41  50  35  44  34 / 100  80  10  10  10  10
Wenatchee      38  37  46  35  42  33 / 100  90  30  20  10  20
Omak           37  34  40  29  37  29 / 100  80  10  10  10  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...Winter Weather Advisory until 11 PM PST this evening for East
     Slopes Northern Cascades.

&&

$$




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