Area Forecast Discussion
Issued by NWS Spokane, WA

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241
FXUS66 KOTX 180128
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
528 PM PST Tue Jan 17 2017

.SYNOPSIS...
A prolonged period of active weather continues at least through
Thursday. The region will be overspread by mountain snow and
valley and basin freezing rain and sleet before transitioning to
continued mountain snow and valley rain Wednesday into Thursday.
Localized flooding will also be possibility. The weather will
remain active but not as cold through next weekend.

&&

.DISCUSSION...
Early evening update: Quick update sent to lower snow levels this
evening along the East Slopes of the Cascades where Leavenworth
cameras show the wintry mix has changed over to snow.
Precipitation intensity is likely helping to cool the column
enough for this changeover. Cameras in west portions of Wenatchee
show a similar story although the warm layer is too close to
freezing to guarantee that evening precip remains this way so
forecast continues to call for a wintry mix with significant
accumulations of snow, sleet, and freezing rain. As precipitation
continued to move east latest guidance suggests mainly snow north
of a line Lake Chelan to Grand Coulee to Chewelah to Bonners
Ferry, a freezing rain/snow mix for southern Stevens and Pend
Oreille counties, and mostly freezing rain for the Columbia Basin,
Spokane, and Coeur D`Alene areas. If precipitation stays mostly
freezing rain then its possible some of the freezing rain
advisories may need to be upgraded to ice storm warnings. The
latest NAM for Spokane suggest significant ice accumulation while
the GFS has the column just cold enough for snow. GFS however is
too cold with the 00z observed sounding at NWS Spokane still
showing a small melting layer. With the strong SW flow regime
forecast still favors the milder NAM. For the Pullman/Lewiston
areas temperatures have warmed to near freezing but pockets of
below freezing temperatures could result in freezing rain for some
with highlights possible.

A very complex weather situation tonight so stay tuned for future
updates. JW

&&

.AVIATION...
00Z TAFS: The low level inversion over the region locking in
residual arctic air in the boundary layer will be gradually eroded
tonight and Wednesday...but this process will result in a
widespread freezing rain at all TAF sites as a very moist fetch
isentropically enhances into precipitation. KLWS is above freezing
at 00Z but is expected to wet bulb back below freezing as rain
begins tonight...and then rise back above freezing late tonight.
The KGEG area TAF sites will experience a significant period of
-FZRA tonight with between a tenth and a quarter of an inch of ice
accumulation by 12Z Wednesday. KEAT will likely remain below
freezing with periods of -FZRA leading to ice accumulations of
over a half an inch by the end of the TAF period. After
temperatures rise above freezing rain will continue with MVFR and
IFR ceilings through 00Z Thursday. /MJF


&&

WIDESPREAD FREEZING RAIN TONIGHT THROUGHOUT THE COLUMBIA BASIN
AND ADJACENT VALLEYS

Tonight...Quite a mess is expected tonight across most of the
forecast area. The in-situ cold fossilized arctic boundary layer
air mass in the Columbia Basin will be overrun by a deep
atmospheric river tonight and through Wednesday. Satellite clearly
indicates this moisture surge plowing into the Oregon and
Washington coasts this afternoon...and will overspread the inland
northwest this evening.

The problem is getting a grip on the nature and timing of the
imminent erosion of the low level cold air. The moisture surge
will begin precipitating over the forecast area from west to east
between 4PM and 10PM...enveloping the region in almost universal
mountain snow and basin freezing rain. Current observations
suggest only Lewiston is above freezing...and Pullman is getting
close. These locations may get simple warm rain tonight although
as precipitation begins surface temperatures may wet bulb back
below freezing before rising again late tonight.

The deep basin and Cascades lee zones will be confidently freezing
rain after an initial hour of possible sleet or light snow. The
Ice Storm Warning for these areas still looks quite certain to
verify over the next 24 hours.

The mountains ringing the basin are also confidently likely to
pick up heavy snow accumulations...as is the Methow Valley and
northern valleys near the Canadian border which will be the last
areas to experience the mid level warm intrusion. The Winter Storm
Warnings for these areas also looks good...with a multitude of
precipitation types in the southern valleys opening into the
basin.

The northern and eastern basin are the most uncertain forecast
problems. Overnight low temperatures will probably be this
afternoon`s high temperatures in most locations...although a few
degrees of initial cooling during the precipitation onset is
possible due to wet bulb cooling. Thus the problem is figuring out
when any particular location east of a line from Coulee Dam to
Ritzville and Walla Walla will go above freezing and end the ice
accumulation threat. The current windy conditions off the Palouse
appear to be slowly infiltrating from southeast to northwest into
the eastern basin and if this keeps up there will be a scrubbing
sometime late this evening. The GFS scrubs the rising terrain of
the eastern basin soonest tonight while the NAM 4KM is the
slowest but still allows a rise above freezing overnight. Given
the stubbornness of the low level inversion the later solution is
probably the one to go with...and the Freezing Rain Advisory for
the Eastern Basin and the Idaho Panhandle mountains will be
extended to cover the Wednesday morning commute period. Even
though the temperature will be above freezing at most
locations...it will still be raining and road surfaces will still
be slowly responding to the warm up...making for a potentially
very slippery commute period with wet ice on the pavement of
primary roads and potentially soupy deep cottage cheese snow and
ice conditions on secondary roads. /Fugazzi

Wednesday through Friday: The wet and wintry weather will
continue across the Inland NW, especially through Thursday.
Wednesday morning a strong southwesterly flow and subtropical
moisture plume will continue to overrun the colder air at the
surface, leading to widespread precipitation. By afternoon
precipitation rates starts decline over the deeper Columbia Basin,
but the precipitation will continue around the western and
northern rim of the Columbia Basin, across the eastern Columbia
Basin, Idaho and the mountain zones. Between Wednesday night and
Thursday evening impulses will continue to ride in from the
Pacific while the parent upper trough starts to move inland. But
the core of the subtropical plume begins to migrate south. The
precipitation threat will remain high, especially around the
Cascades and northern and eastern CWA, but are the rates are
expected to decrease in comparison to Wednesday. This isn`t too
say there wouldn`t be some heavier bursts of precipitation in
there. Thursday night the main threat retreats into the mountains,
but smaller chances will linger around the north and eastern rims
of the Columbia Basin. Another system starts to edge in Friday,
redeveloping the threat of precipitation across the Cascades
through Waterville Plateau.

* Precipitation-type: the tricky part of the forecast remains what
  will happen with the thermal profile and associated
  precipitation-type. Wednesday morning a warm nose aloft will
  extend across much of the Columbia Basin, with 850mb
  temperatures up to around +1 to +3C. Models hold surface
  temperatures below freezing in the morning generally along and
  west of a line from Wilbur to Ritzville to Washtucna back into
  the Cascades. Freezing rain and the potential for significant
  icing remains a good threat roughly over the valleys of Chelan,
  Douglas, Grant and western Adams and Grant. However the threat
  of freezing rain may linger early on over the northeastern CWA
  and sheltered north and central Idaho Valley in the morning,
  especially if temperatures are slow to warm up as has been the
  case of the past few days. Even if it turns to all rain in these
  eastern portions, the ground temperatures are apt to remain
  below freezing so there could be some light icing in the
  morning. Meanwhile snow is more likely across the northern
  mountains and some of the valley closer to the Canadian border,
  including the Okanogan Valley. By Wednesday afternoon a freezing
  rain threat will continue across the lee of the Cascades onto
  the Waterville Plateau. Pockets of freezing rain may also linger
  into some of the sheltered northern mountain valleys, but
  otherwise the it turns to a valley rain/mountain snow event. By
  Wednesday night some of the threat of freezing rain starts to
  wane, though we may have look for some pockets of it lingering
  near the Cascades. Thursday precipitation looks like rain over
  the central and eastern valleys and Basin, snow in the
  mountains. Snow will be possible over the Cascades, Okanogan
  Valley and Waterville Plateau.

* Flooding: the incoming warm-up, some breezy conditions, combined
  with rain and snow melt will lead to the threat of flooding. A
  flood watch has been issued for a large portion of the CWA for
  Wednesday through Thursday night. This includes the threat of
  ice-jam flooding, flooding in poor drainage areas and areas near
  frozen soils.

Thursday night the main threat retreats into the mountains, but
smaller chances will linger around the north and eastern rims of
the Columbia Basin. Another system starts to edge in Friday,
redeveloping the threat of precipitation across the Cascades
through Waterville Plateau. Thursday night snow is the more likely
precipitation-type, while Friday a rain/snow mix is more likely
in the valleys with snow in the mountains. An exception may be
near the Okanogan Valley/Cascade Valleys, Waterville Plateau. Some
snow will be possible there. /J. Cote`

Friday night through Monday: The longwave pressure pattern over
the west doesn`t change much. The big difference from now to this
weekend is that we don`t have a good sub-tropical moisture tap.
An upper level low remains over the Gulf of Alaska that gradually
begins to move into the region on Monday. A couple of weak
shortwave disturbances will round the upper trough Friday night
into Saturday. Dynamics appears to be weak with both and not much
in the way of precipitation is anticipated.

A shortwave disturbance on Sunday will give us our best shot at
precipitation over the weekend. There are model differences in
timing and strength, so finer details are still a bit fuzzy. Snow
levels will be lowering by this time with most areas expected to
see snowfall. The exceptions will be the lower portions of the
upper basin and into the L-C Valley where a rain or a rain/snow
mix looks more likely. The GFS model is considerably wetter when
compared to other operational medium range model guidance and
there is a considerable amount of variability in precipitation
amounts amongst the GEFS ensemble members. So, I am hesitant to
believe the 12Z GFS solution at this time and prefer more of a
ECMWF, Canadian and GEFS ensemble mean blend. A general 1-3 inches
of snowfall can be expected for most areas on Sunday.

By Tuesday, models are in good agreement with the flow pattern
turning north-northwesterly. This will be a drier flow pattern;
although, lingering snow showers can be expected for the southern
to central ID Panhandle. Temperatures will be on a cooling trend
from around normal for the weekend back to below normal with highs
around the upper 20s to lower 30s for early next week. /SVH

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        25  39  33  38  26  37 / 100 100 100  80  20  10
Coeur d`Alene  30  38  34  39  26  37 / 100 100 100  70  30  20
Pullman        33  40  33  40  28  37 / 100 100 100  70  30  10
Lewiston       36  43  37  42  30  39 / 100  90  90  60  20  10
Colville       31  35  33  39  29  38 / 100 100 100  60  30  20
Sandpoint      28  36  34  39  29  37 / 100 100 100  90  40  20
Kellogg        30  37  34  39  28  36 / 100 100 100  90  40  20
Moses Lake     20  36  29  38  22  36 / 100 100  80  30  10  30
Wenatchee      19  31  29  36  26  33 / 100  80  70  20  30  70
Omak           24  34  31  39  28  37 / 100 100  90  30  30  40

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Storm Warning until 8 AM PST Thursday for Northern
     Panhandle.

     Flood Watch from Wednesday morning through late Thursday night
     for Central Panhandle Mountains-Coeur d`Alene Area-Idaho
     Palouse-Lewis and Southern Nez Perce Counties-Lewiston
     Area.

     Freezing Rain Advisory until 10 AM PST Wednesday for Central
     Panhandle Mountains-Coeur d`Alene Area-Idaho Palouse.

WA...Winter Storm Warning until 8 AM PST Thursday for East Slopes
     Northern Cascades-Northeast Mountains-Okanogan Highlands-
     Okanogan Valley.

     Flood Watch from Wednesday morning through late Thursday night
     for Lower Garfield and Asotin Counties-Moses Lake Area-
     Northeast Blue Mountains-Okanogan Valley-Spokane Area-Upper
     Columbia Basin-Washington Palouse-Waterville Plateau-
     Wenatchee Area.

     Freezing Rain Advisory until 10 AM PST Wednesday for Spokane
     Area-Upper Columbia Basin-Washington Palouse.

     Ice Storm Warning until 10 AM PST Wednesday for Moses Lake Area.

     Ice Storm Warning until 4 PM PST Wednesday for East Slopes
     Northern Cascades-Waterville Plateau-Wenatchee Area.

&&

$$



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