Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 240037
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
437 PM PST Tue Jan 23 2018

.SYNOPSIS...
An active weather pattern is expected through the week. A wet and
mild weather system will bring significant precipitation today
into Wednesday and again next weekend with a mix of rain and snow
across the lower elevations and snow in the mountains. Difficult
travel conditions are expected in the mountains at times.

&&

.DISCUSSION...
Tonight: The warm front continues is progression north into
southern WA this afternoon. It is raining in Moses Lake and the
area of rain will spread north and east through the afternoon and
evening hours. Looking at area cameras across portions of the
Waterville Plateau and Okanogan Valley, it is snowing, but not
really accumulating on the roads. Temperatures are above freezing
around the Spokane area. Dew points are right around freezing.
Precip may start off as snow or a wet rain/snow mix in the lower
elevations. The snow will change over to rain within an hour or
two of the onset. Road temperatures are around 40, so am not
expecting accumulations on most roads. Untreated roads and grassy
surfaces may see some light accumulations through the evening
commute. There is also the potential of it raining so hard that
temperatures lower for it to change back to snow for a bit in the
evening. Given the warm front moving north through the night have
low temperatures the same or just a degree or two lower than our
high for today. To the north, snow will continue through the
evening but then many of the valleys will transition to a rain
snow mix overnight. Locations near and along the Canadian border
will likely stay all snow.

Wednesday through Thursday: The warm front will keep slowly
pushing north and changing many valleys to a rain/snow mix or
rain. Precipitation will continue through the day for most
locations. By Wed evening a cold front will move across the region
and briefly put to an end precipitation for most locations.
Thursday the trough moves inland with instability snow showers
expected across much of the region in the afternoon. /Nisbet

Thursday night-Friday Night...Most of this period will be
dominated by an unstable air mass associated with a very slow
moving upper level trough. The core of the trough will remain off
the coast through this period however it will bring at least two
shortwave disturbances through the region each of which will
likely trigger numerous snow showers. The first disturbance is
expected to begin over south-central Washington on Thursday and
track into NE WA and most of the Idaho Panhandle during the
evening. The next significant disturbance is scheduled to move NE
across the region on Friday. The atmosphere will be unstable
enough to generate snow showers along and ahead of each
disturbance`s path, however cold air advection in the lee of the
Cascades between 850-700 mbs will generally result in downslope
flow and little chance of precipitation for the lower Wenatchee
Valley, Okanogan Valley and western Columbia Basin. Meanwhile the
same west to southwest flow through this layer will contribute to
good orographic ascent for the eastern third of Washington and
most of the Panhandle. Consequently we will see a large gradient
in the chance of precipitation from west to east across the
forecast area with likely (or higher) pops in north ID and slight
chance pops for the aforementioned sites in the lee of the
Cascades. Unlike the current event with complex snow levels and
precipitation types, this period should be rather straight forward
with all snow. Amounts will generally be hit or miss, however
we`d expect to see the heaviest amounts over the Idaho mountains.
For now we`d expect to see anywhere from 4-8 over the Idaho
Mountains between Thursday and Friday evening but again it won`t
be widespread and in the valleys amounts will likely be quite
variable. By late Friday night we will transition from the cold
and unstable air mass to a more stabilized and warmer one as the
next atmospheric river make headway into the SE portion of our
forecast area (Lower Columbia Basin, Wenatchee area, Cascades
south of Lake Chelan). This should initially result in an
increasing chance of snow for these areas. Temperatures during
this period should show a cooling trend as skies could actually
clear at times in the wake of each shortwave. This clearing is
most likely to occur in the lee of the Cascades and will likely
allow min temperatures to dip into the teens in the Methow Valley
and near the Republic area. Temperatures could get even cooler
than forecast if the clearing occurs through much of the night and
couples with light winds and the fresh snow cover. fx

Saturday through Tuesday: Models remain consistent in depicting a
rather warm and wet weekend and beginning to the next workweek as a
quasi-stationary subtropical moisture plume continues to stream
moisture up into Eastern Washington and Northern Idaho. As such
forecast temperatures will show a warming trend and rises in snow
levels that should peak Sunday/Monday. The combination of mild
temperatures, snow melt, and periods of moderate rainfall will raise
concern for potential flooding and rock slides in steep terrain. The
moisture plume eventually exits to the east Tuesday leaving a post
frontal conditionally unstable airmass over Eastern Washington and
Northern Idaho. This supports a more snow showery precipitation type
near and over the more orographically favored mountains of North
Idaho, Northeast Washington, and within the close proximity of the
lee of the Cascade Crest along with cooler temperatures and lower
snow levels. /Pelatti
&&

.AVIATION...
00z TAFS....This remains a very low confidence forecast as
widespread precipitation overtakes all of the TAF sites. The
trickiest forecast will be for the GEG-COE corridor due to the
fickle nature of the precipitation type. Initially all sites in the
corridor will begin as rain, however we fully expect at least COE
and GEG to change to all snow sometime between now and 03z before
changing to rain again sometime overnight. Once the changeover
occurs, the snow could be heavy at times and quick accumulations are
possible. However based on the amount of time it will be snowing
before changing to back to rain we suspect accumulations will
generally be limited to 2 inches or less (likely much less).
Conditions during this time will degrade from MVFR to IFR. Steady
precipitation is likely to continue through tomorrow morning with
another round of precip in the afternoon. PUW is the only puzzle as
they could possibly see a rain snow mix shortly after the onset of
precip. Odds suggest a better chance of all rain there though and we
forecast accordingly. For the other sites, confidence is higher in
regards to precip type. EAT will remain as all snow with IFR
conditions through the evening. The precip there will taper off
after midnight. MWH will see all rain with a slightly later ending
time. fx


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        35  40  30  36  29  36 / 100 100  80  40  50  60
Coeur d`Alene  34  38  30  36  28  34 / 100 100  90  30  60  80
Pullman        37  42  31  39  29  37 / 100 100 100  30  50  60
Lewiston       41  48  34  43  32  42 /  80  70  70  30  30  40
Colville       33  38  30  36  28  35 / 100  90  90  30  60  60
Sandpoint      33  36  31  35  28  33 / 100 100 100  40  70  70
Kellogg        33  38  29  34  27  32 / 100  90 100  30  70  90
Moses Lake     37  47  29  41  29  42 /  90  70  10  30  20  10
Wenatchee      33  39  25  35  26  36 /  90  50  10  40  20  10
Omak           32  36  25  33  25  34 / 100 100  80  40  20  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Weather Advisory until 9 AM PST Wednesday for Coeur
     d`Alene Area.

     Winter Storm Warning until 4 AM PST Thursday for Northern
     Panhandle.

     Winter Storm Warning until 4 PM PST Wednesday above 3500 feet
     for Central Panhandle Mountains.

WA...Winter Storm Warning until 4 AM PST Thursday for East Slopes
     Northern Cascades.

     Winter Weather Advisory until 4 AM PST Wednesday for Waterville
     Plateau-Wenatchee Area.

     Winter Weather Advisory until 9 AM PST Wednesday for Spokane
     Area.

     Winter Storm Warning until 4 AM PST Thursday for Northeast
     Mountains-Okanogan Highlands.

     Winter Weather Advisory until 4 AM PST Wednesday for Okanogan
     Valley.


&&

$$



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