Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 201223 RRA

Area Forecast Discussion
National Weather Service Spokane WA
517 AM PDT Tue Mar 20 2018

Tuesday will be the first official day of spring and will feature
the mildest and driest weather of the week. A vigorous cold front
is expected to bring widespread precipitation as early as
Wednesday afternoon for the far northern portions of eastern
Washington and north Idaho and a little bit later for the
remainder of the Inland Northwest. The wet weather will likely
continue at times through Thursday and will be accompanied by
gusty winds late int the day. The cold front will deliver
significantly cooler conditions from Friday through the weekend
with the threat of rain and snow showers each day.


Today and tonight...The Inland Northwest will lie between two
distinct systems today. One plume of moisture and clouds will
remain fixed over southern Oregon while a weaker system sweeps
slowly SE out of BC. This secondary system is the main one of
concern for us, but it may not do much as it is expected to shear
apart along the WA/BC border. We suspect the main impact will be
an influx of clouds through the day and perhaps a small chance of
showers in the afternoon. Atmospheric lapse rates this afternoon
compared to yesterday suggest increasing stability and likely
less forcing as well. Of all models, the 00z GFS is the wettest,
by a large margin, however at least if places precipitation across
the area where it would be most likely to occur, the northern
third of the region.

Wednesday...The weather will become increasingly active as a deep
upper level low drops southward through the Gulf of Alaska and
backs the mean mid/upper level flow to more of a southerly
orientation. This in turn leads to an invasion of moisture from
the south with precipitable water values nearly doubling from
their current values. This will lead to a cloud filled day as well
as an increasing chance of precipitation. By afternoon a chance of
light precipitation will impact the area however the best chances
will likely occur over eastern Okanogan and Chelan Counties due to
the development of a SE winds at 850 mbs leading to pooled moisture
and weak upslope flow. Precipitation amounts on Wednesday will
generally be a couple hundredths or less, however locally heavier
amounts are possible in the aforementioned areas. Temperatures on
Tuesday and Wednesday should be the warmest of the week over much of
the forecast areas with highs in the upper 40s and 50s. It`s hard to
determine which the warmer day will be since at 850 mbs there is no
question Wednesday will be warmer, however the sunshine potential
will be much lower than today. The threat of afternoon rains will
also temper this warming aloft. fx

Wednesday night and Thursday: Models continue to show a large closed
low sending a wet cold front across the region from west to east on
Thursday. Ahead of the front moist southwest flow Wednesday night
will lead to increasing chances for precipitation with steady rain
and mountain snow over North Central WA where the best isentropic
lift will occur along with easterly upslope flow into the Cascades.
As the cold front front passes on Thursday snow levels will range
from 4-6k feet along and ahead of the front when the bulk of the
precipitation falls. The high snow levels should keep precipitation
as rain for most of the mountain passes until after the cold front
passes. This looks to occur late Thursday afternoon in the Cascades
and Thursday evening over the Blue Mountains into the Idaho
Panhandle. Drier air moving in behind the front should result in
only a brief period of snow over the mountain passes before snow
comes to an end. Thus no highlights are expected at this time due to
snow. Regarding rainfall amounts models continue to show 0.75-1.00
inches for the Okanogan/Methow Valleys...0.50-0.75 inches for NE WA
into the Idaho Panhandle...and 0.25-0.50-inches for the Columbia
This wet weather along with continued snow melt for portions of the
northern mid slope/plateau areas as well as the Methow Valley could
trigger a few additional slides. Lastly...breezy winds are still
expected with this system in the Columbia Basin...Spokane area, and
Palouse due to strong cold advection with the cold front allowing
wind gusts of 30-35 mph to mix down. Windy conditions are expected
in the mountains as 850mb winds increase to 25-40 kts.

Friday through Monday: A cool...unstable...showery air mass will
promote mostly snow and graupel showers Friday and Saturday as a
cold upper trough becomes established over the region with 500mb
temperatures as cold as -38C. Models shows a moist wave embedded
within this trough tracking across the Cascades on Friday, and then
the Blue Mountains, Camas Prairie, into the Central Panhandle
Mountains Saturday morning which could lead to light to moderate
snow accumulations and travel impacts. Sunday and Monday forecast
becomes more uncertain with some model solutions showing more
pronounced waves in northwest flow bringing stratiform rain or snow.
Morning time would favor more snow than rain in the valleys while
afternoon timing would result in mostly valley rain/mountain snow.
Too far out and too much inconsistency to have much confidence in
this part of the forecast. Although cool and unsettled weather is
likely into early next week.  JW


12z TAFs: All TAF sites will see VFR conditions through this period.
There could still be some patchy fog this morning but it won`t occur
near any of the forecast sites. The main trend through the day will
revolve around increasing clouds from a disturbance over southern
BC. Most of these clouds will be above 090 but lower clouds will
likely form during afternoon heating. Supect brief cigs will be
possible from this lower deck but again VFR conditions will prevail.
Expect high clouds to fill in over most sites overnight with a
lesser fog threat than this AM. fx


Spokane        51  32  53  42  54  31 /   0   0  20  60  80  90
Coeur d`Alene  48  30  50  39  51  30 /  10   0  30  60  80 100
Pullman        50  35  54  44  54  32 /   0   0  20  50  90  80
Lewiston       55  38  62  47  61  37 /   0   0  10  30  60  80
Colville       52  31  54  39  53  31 /  10   0  30  80  80  80
Sandpoint      46  31  48  38  48  31 /  10  10  30  60  80 100
Kellogg        45  30  48  37  50  29 /  10   0  20  40  80 100
Moses Lake     59  31  59  42  59  31 /   0   0  10  50  60  50
Wenatchee      57  35  53  39  54  31 /   0   0  20  80  70  20
Omak           53  34  53  40  53  31 /   0   0  40 100  90  70




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