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Area forecast discussion 
National Weather Service Spokane Washington
408 am PST Friday Nov 28 2014

Synopsis...
a rainy and breezy weather pattern will envelope the region
going today, but snow levels will remain above pass levels for
motorists traveling. The arrival of a very cold and windy Arctic
front Friday night into Saturday will bring winter driving
conditions to the mountain passes and much colder weather for the
weekend. Cold conditions, with a few minor snow bearing weather
disturbances, is forecast for the new work week.

&&

Discussion...

Today through saturday: the biggest change to the previous fcst
was the issue a winter wx advsy for snow for Saturday for the
cntrl Idaho Panhandle mtn zone (mostly Shoshone county) for
significant snow accumulations down to the valley floors. This
could be a potentially busy I-90 travel day back home following
Thanksgiving. The Wind Advisory for today has not changed. Very
mild temps and windy conditions are still expected today as we
remain in the pre-Arctic cold front passage expected Fri nt. A
nearly unidirectional vertical SW wind profile... with 40-50kt
aloft...will help to provide very efficient mixing and Transfer
of these higher speeds around 5k ft aloft to the sfc. Snow levels
will remain very high today, but falling quickly to the sfc as the
surge of very cold air behind the Arctic front works its way from
north to south across ern WA and north Idaho Sat. The most tricky
part of the fcst will be snow amnts. This front and associated
upper wave will move through quickly and help to limit snow
accumulations. Offsetting this somewhat will be respectable lg-
scale forcing for ascent with the wave as well as a strong lower
tropospheric upslope component of flow into the Idaho Panhandle
Sat. Also, swe ratios (once the cold air mass fully arrives) will
be at least 20-30:1, promoting heavier accumulations. And even
moderately gusty winds would produce blowing snow and restricted
visibilities where snow is expected. We'll have to watch the Camas
Prairie of Idaho for significant snow accumulations Sat nt as the
low-level flow veers to the northwest and enhances the upslope flow.
Confidence is not great as far as snow accumulations for the
Spokane area Sat. The majority of model guidance (which we've
favored) shows the best pcpn threat ending very close to when the
cold air moves into the area. We'll be watching the scenario
closely, and adjust the snowfall fcst accordingly and as quickly
as possible as soon as these snow ingredients allow US to fine
tune the accumulations better. Bz

Saturday night through Monday...the region will transition to a
cold and dry weather pattern as a polar front swings south across
the inland northwest. Gusty north winds will continue to be
channeled down the Okanogan Valley Saturday night. The combination
of very cold temperatures and windy conditions will result in wind
chill temperatures of -15 to -20 f overnight. The strong surface
pressure gradient fostering the elevated winds will slacken Sunday
as the low pressure center moves off to the southeast and weakens.
This will allow winds to diminish and become more terrain driven.
High temperatures Sunday will be 12 to 15 degrees below seasonal
normals. While most valley locations will be in the low to mid
20s, some of the northern valleys will remain in the teens during
the day, then drop back near zero Sunday night.

The large scale pattern will become more westerly by Monday. Without
the reinforcing cold north winds, temperatures will start to
moderate somewhat. A closed low off the California coast will drift east,
acting to push the frontal boundary back north as a warm front.
The 00z run of both the GFS and the European model (ecmwf) now keep this front
mostly to our south with just the southeast zones under the threat
of precipitation from this feature. The warm front will battle
with the deep cold air in place. Some warm air may push in aloft
but for now wet bulb temperatures remain below freezing throughout
the day Monday so precip would likely fall as snow.

Monday night through Thursday...the extended forecast has been
trended toward the European model (ecmwf) which builds a foldover ridge over the
region in response to the California offshore low. This is a drier
forecast than the GFS which brings the moisture stream farther
north in more zonal flow. There will be a very gradual warmup
since winds will be light and mainly terrain driven with limited
mixing potential. The ridge does not hang around for long. It will
be pushed east once the coastal low moves inland during the mid-
week period, bringing the next round of precipitation to the
region. /Kelch

&&

Aviation...
12z tafs: a developing storm system will bring rain chances over
the next 24 hours. This pcpn has already started around puw/lws.
Look for VFR conditions, with local MVFR/IFR cigs possible in
heavier precipitation as afternoon progresses into evening.
Strengthening winds this morning, topping lights sfc winds, may
lead to areas of low level wind shear. By late morning/afternoon some winds gusts
between 30 and 45 kts will be possible.

Note: the visible sensor at eat is out of service so amendments to
visible are not scheduled. The offending sensor replacement is on
order and may be returned to service by Saturday.

&&

Preliminary point temps/pops...
Spokane 50 26 27 10 23 13 / 70 80 20 0 0 0
Coeur D'Alene 49 29 29 12 23 13 / 90 100 20 10 0 0
Pullman 53 31 32 12 25 17 / 90 100 50 10 0 0
Lewiston 57 37 38 20 28 20 / 60 90 60 20 0 0
Colville 47 22 23 3 21 9 / 70 60 10 0 0 0
Sandpoint 45 26 27 8 20 8 / 90 100 40 10 0 0
Kellogg 44 29 30 10 21 11 / 100 100 70 10 0 0
Moses Lake 55 27 29 8 25 13 / 30 20 10 0 0 0
Wenatchee 49 29 30 13 23 12 / 30 10 10 0 0 10
Omak 46 17 19 1 18 8 / 30 10 10 0 0 0

&&

Otx watches/warnings/advisories...
Idaho...Winter Weather Advisory from 4 am to 4 PM PST Saturday for
central Panhandle mountains.

Wind Advisory from 1 PM this afternoon to midnight PST tonight
for Idaho Palouse.

Washington...Wind Advisory from 1 PM this afternoon to midnight PST tonight
for lower Garfield and Asotin counties-Spokane area-upper
Columbia Basin-Washington Palouse.

&&

$$

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