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Area forecast discussion 
National Weather Service Spokane Washington 
435 PM PDT Thursday Mar 18 2010 


Synopsis... 
after a very weak disturbance clips northern Idaho tonight...high 
pressure will build into the region Friday and Saturday for dry 
conditions. Temperatures will warm several degrees on Saturday as 
the ridge moves overhead. The next system will arrive on Sunday 
and Monday bringing a chance for mostly rain in the valleys and snow 
in the mountains. 


&& 


Discussion... 
tonight and Friday...water vapor satellite shows a vorticity maximum will 
move southeast along the British Columbia and Alberta border into 
northwest Montana late this afternoon and early evening. With this 
feature moving east of north Idaho...the best instability associated 
with a cold pool of -34c aloft will also slide to the east keeping 
the best threat for showers east of the area. Still can not rule 
out an isolated shower clipping the Idaho/Montana border...otherwise 
cumulus associated with afternoon heating over the northern and 
eastern mountains will dissipate after sunset. With dew points in the 
lower to middle 20s and mostly clear skies overnight radiational 
cooling will again be efficient for another cool night similar to 
last night. On Friday upper level high pressure will begin to 
build in resulting in dry conditions. However ridge axis remains 
off the coast with 850mb temperatures similar to today so no big 
temperature changes are expected Friday. Jw 


Friday night to Sunday...the inland northwest remains under high pressure 
and a dry northerly flow until the system developing near 35n/145w comes 
in. Friday night to Saturday a surface high slowly shifts east-southeast...the 
middle-level ridge axis near the coast migrates toward Idaho and the north-northwest 
flow turns west. Through much of this period there is at least weak 
subsidence and the atmosphere remains relatively stable and dry. 
It is not until Saturday afternoon that some low-grade instability 
approaches the western County Warning Area...with neutral or weak lift with the 
approach of the developing low. Expect occasional periods of 
higher clouds and some afternoon cloud build up near the 
Cascades...but otherwise quiet weather. Temperatures are expected to 
be above normal... supported by 850 temperatures pushing into the 
middle-single digits Saturday afternoon. Saturday night into Sunday the 
low slides north-northeast toward British Columbia. In conjunction...a warm front lifts across 
the Cascades Saturday night quickly followed by a cold front that 
tracks across the inland northwest Sunday. Most deeper moisture and lift is 
tied to the cold front. Expect increasing clouds through the inland 
northwest Saturday night...with increasing rain and High Mountain snow 
chances over the Cascades. Rain chances will expand east through 
Sunday. A GFS/European model (ecmwf) blend is favored...placing the highest 
precipitation amounts over the Cascades. Expect breezy conditions 
with the passing front...along with cooler temperatures Sunday 
afternoon relative to Saturday. /Jcote 


Sunday night through Tuesday...shortwave trough will track 
across the region bringing elevated precipitation chances and 
cooler temperatures. There is fair agreement amongst the forecast 
models but still some uncertainty in regards to what degree the 
low will retrograde into the base of the offshore ridge leading to 
slight timing differences when the shortwave clears the region 
late Tuesday. Otherwise...even the slight differences within the 
guidance yields very little difference for the upcoming weather 
across inland northwest. 


As the cold pool moves across the region...just about everywhere 
will have a fair chance for showers as the atmosphere becomes 
increasingly unstable under a -30 celsius cold pool. The heaviest 
and most organized precipitation will set up along a weak area of q-vector 
convergence and associated higher Theta-E axis wrapping up the 
eastern flank of the shortwave which will be stretched across the 
Idaho Panhandle down into NE Oregon. Not only will the moisture 
content be exceptionally higher...but the relatively 
warmer...moist low-level air will result in steeper 700-500h lapse 
rates and feed off low-level forcing associated with west/northwest 
orographic flow into the aforementioned area. For this 
reasoning...quantitative precipitation forecast amounts could be on the order of 0.20" - 0.60". 
The main challenge will be how fast snow levels fall. It is a 
given that as the trough moves across the area...the column 
cools...and snow levels will be falling. Initially near 3000-3500' 
Sunday night staying slightly elevated along the Theta-E axis then 
down to 2000' by Monday night so depending in the transition 
through the day Monday...there is a moderate potential for snow 
accumulations and winter highlights across the Camas 
Prairie...Palouse...into the central Panhandle mountains 
additionally...this will also bring potential for moderate 
snowfall along the Cascade crest as 850-700h flow remains west/northwest 
under the dome of instability. With some spillover possible into 
the upper East Slope valleys. For the remainder of the 
region...iso-sct showers will be most prominent in the afternoon hours 
with some surface heating...but will also focus along mesoscale 
vorticity maximums which typically spin-up in the core of these 
troughs...but are poorly modeled days in advance. 


Although very marginal...there is a slight potential for an 
isolated thunderstorm Monday afternoon as lifted indice's fall -1 to 0...lapse 
rates increase near 7... marginal thermodynamic profiles 
setup with skinny cape profile roughly from 800-500mb or within 
the -5 to -25 celsius layer. As in most Spring- cases...the 
column may be too cool to get sufficient charge separation but its 
Worth keeping an eye on...specifically across the Idaho 
Panhandle. 


Tuesday night through Thursday night...high pressure will return 
Tuesday night and Wednesday with a drying trend and slow warming 
of temperatures. Another wave will ride along the top of the ridge 
Wednesday nt and Thursday however there is quite a bit of model 
spread to what degree this wave will impact the Pacific northwest. In 
collaboration with neighboring offices... have decided to make 
little changes to the forecast which continues to call for climatology 
probability of precipitation and near seasonal temperatures. /Sb 


&& 


Aviation... 
shortwave ridging will continue to build into the inland northwest 
this evening through tomorrow resulting in continued VFR 
conditions at all taf sites through 00z Saturday. Lingering 
instability over the Idaho Panhandle will keep the threat for 
isolated showers through 02z mainly vicinity kcoe then sky clear skies will 
return region-wide. Light northerly gradients this afternoon will 
gradually become NE and light overnight...anticipate a few gusts 
to 20kts across kmwh-kgeg-kcoe before the decoupling of the bl 
after 03z. Sb 


&& 


Preliminary point temps/pops... 
Spokane 27 49 29 57 34 53 / 0 0 0 0 0 20 
Coeur D'Alene 25 50 27 58 32 53 / 0 0 0 0 0 20 
Pullman 22 50 29 60 35 55 / 0 0 0 0 0 20 
Lewiston 31 54 32 65 40 60 / 0 0 0 0 0 20 
Colville 23 53 26 58 31 57 / 0 0 0 0 0 30 
Sandpoint 28 48 26 53 29 50 / 10 0 0 0 0 20 
Kellogg 26 48 28 57 33 52 / 10 0 0 0 0 30 
Moses Lake 28 57 30 61 36 59 / 0 0 0 0 0 10 
Wenatchee 33 54 32 60 38 55 / 0 0 0 0 10 20 
Omak 25 57 30 60 33 57 / 0 0 0 0 10 20 


&& 


Otx watches/warnings/advisories... 
Idaho...none. 
Washington...none. 
&& 


$$ 

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