FXUS66 KOTX 202323

Area Forecast Discussion
National Weather Service Spokane WA
423 PM PDT Tue Mar 20 2018

After a quiet and seasonably cool night tonight, a strong and
moist weather system will bring widespread rain and high mountain
snow Wednesday afternoon through Thursday evening to the Inland
Northwest. A cold front late Thursday will bring gusty winds and
deliver significantly cooler conditions from Friday through the
weekend with the threat of rain and snow showers each day.



...Small stream flooding and mudslides possible Wednesday night
through Thursday...

Tonight through Thursday...The Pacific satellite imagery this
afternoon reveals three features of note which will interact and
produce a prolonged round of wet and active weather across the
region over the next couple days. First a closed low is visible
circulating far off the California coast and directing the
second feature...a deep plume of subtropical moisture into
California itself. the third player is a new and aggressively
digging Gulf of Alaska trough visible descending down the Alaska

There is good model agreement and consistency over multiple days
now in a mating of the southern upper low and the digging
northern trough into a monster upper low pressure spinning off the
northwest US coast for the remainder of the work week. This
feature will direct southerly flow in the mid levels and aloft
into the forecast area beginning Wednesday. Some of the moisture
from the atmospheric river impacting California will be drawn up
in this southerly flow and fuel a long run period of mainly
valley/basin rain and mountain snow initially over a warm front
draped across the region on Wednesday night and then along a
strong cold occluded front passing through the forecast area on
Thursday. Low level easterly flow feeding into surface low
pressure offshore will be quite favorable for upslope enhancement
into the Cascades during this period. Snow levels will be rising
to the 5000 foot level or above until the cold front passage.

So...All of this adds up to a significant prolonged rain
event...augmented by rain on melting low and mid elevation snow
particularly in the Cascades and mountains north of the basin over
the next 48 hours. Current expectations are easily 3/4 to
slightly over 1 inch of rain in the Cascades valleys and
midslopes...with one or two feet of snow accumulation above 5500
feet. A general 3/4 to around 1 inch of rain in the Okanogan
Highlands and northeast Washington mountains and far north Idaho
is also a good bet.

Tonight will be benign and seasonably cool as all of the players
evolve and organize offshore. Wednesday will bring increasing and
thickening clouds with an increasing chance of light stratiform
rain developing during the afternoon. Wednesday night and Thursday
will be the prime periods of widespread rain and high mountain
snow...with an added minor threat of a few weak/brief
thunderstorms ahead of the front in the Idaho panhandle Thursday.
As the front passes trough the forecast area from west to east on
Thursday rain will begin to taper off but breezy to windy
conditions will develop across the region...gust potential on the
exposed terrain of the eastern basin may reach 35 to 40 mph late
Thursday afternoon. This potential will need to be monitored for
potential wind advisory highlights as it develops.

The potential impacts from this rain and snow melt include a very
real chance of small stream flooding and mudslides in steep
terrain. An Areal Flood Watch will be issued to highlight these
threats for the mountain zones where the risk is most acute...but
it should be noted any small streams anywhere in the region may
display rapid rises with local ponding in low spots and minor wash
outs on slopes and secondary roads. Main stem rivers are low
enough right now to be able to handle this event with noticeable
but non-threatening rises. /Fugazzi

Thursday night through Tuesday: The passage of a strong cold front
Thursday evening will bring cool, breezy and showery conditions to
the Inland NW for much of the period. This front coupled with a
shortwave aloft will sweep through extreme eastern Washington and
north Idaho Thursday evening and bring an end to the abundant
precipitation. Snow levels will drop behind front, giving the
lower slopes a little light snow before the drier air arrives. 850
winds kick up to 30-40kt behind the front, leading to breezy and
gusty winds Thursday evening especially across the Columbia Basin
and Palouse region. Weak instability coupled with the frontal
passage could lead to isolated thunderstorms across the southern
Panhandle. Winds and showers will decrease overnight as an upper
level trough moves into the region. A weak wave in the flow will
brush through north central Washington Friday morning increase
snow showers with light accumulations. Temperatures and snow
levels will rise into the mountains by afternoon, while breezy
winds redevelop. Weak instability remaining over north Idaho may
lead to graupel showers and a possible thunderstorm Friday
afternoon. The upper level trough remains over the region through
the weekend with diurnally driven spring-time convection
especially in the mountains. Showers may be a mix of rain and snow
at times, although accumulations will be minor. Temperatures will
be cooler than normal with occasional breezy winds in the
afternoon. The trough slowly exits by Monday leaving northwest
flow aloft. A weak shortwave brushes by across southern BC with
light orographic precipitation for Monday into Tuesday while
temperatures gradually moderate closer to normal. /rfox.


00Z TAFs: VFR conditions will prevail at all TAF sites through
18Z Wednesday. High and mid level clouds will stream up from the
south and thicken and lower in preparation for an onset of light
rain from an approaching Pacific storm system near the end of the
TAF period.


Spokane 34 51 41 53 32 46 / 0 30 50 80 70 20
Coeur d'Alene 33 48 38 51 31 44 / 0 30 50 80 100 30
Pullman 37 54 43 53 32 44 / 0 30 50 80 70 50
Lewiston 40 60 46 61 36 52 / 10 20 30 60 70 40
Colville 33 51 39 52 32 47 / 0 30 80 100 100 40
Sandpoint 32 47 38 47 32 43 / 10 30 50 90 100 40
Kellogg 32 48 36 50 30 41 / 0 30 40 70 100 50
Moses Lake 34 57 42 59 32 53 / 0 20 60 70 10 20
Wenatchee 36 51 40 53 31 48 / 0 30 90 80 20 60
Omak 35 49 41 53 31 48 / 0 60 100 100 20 60


ID...Flood Watch from Wednesday evening through Thursday evening for
Northern Panhandle.

WA...Flood Watch from Wednesday evening through Thursday evening for
East Slopes Northern Cascades-Northeast Mountains-Okanogan
Highlands-Okanogan Valley-Wenatchee Area.


Back to Main Forecast Discussion Page