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Area forecast discussion 
National Weather Service Spokane Washington
240 am PDT Friday Sep 4 2015

Synopsis...
a cool and showery weather pattern is expected for the remainder
of the work week as a cold storm system moves by the area.
Widespread rain is expected in the Panhandle and portions of eastern
Washington on Saturday with a smaller chance of precipitation for
Sunday and Monday. Drier and warmer weather is expected to return by
early next week.

&&

Discussion...
today through saturday: shower chances linger around the
mountains before a broader rain threat expands in tonight into
Saturday. Today low pressure migrates south from western Washington to
western or. The associated deformation axis along the Cascades and
shortwave rounding its east side will work afternoon instability
to bring a threat of showers over the Cascades, as well as the
Blue Mountains to central Panhandle mountains. A few shower may
also form over the northern Panhandle mountains but the risk here
will be low.

This evening into the overnight the low pivots east-northeast
toward the or/WA/ID border. Moisture wrapping into a developing
trowal in tandem with the low and shortwave rounding it will bring
expanding rain into southeast Washington and lower elevations of the
central and southern Idaho Panhandle this evening. Overnight the rain
threat will expand west into the Columbia Basin and Waterville
Plateau and north into northeast Washington and north Idaho.

By Saturday much of the region will have some threat of rain. Yet
the area which will have the steadier and broader threat of
precipitation will be within the vicinity of the trowal and
activity elsewhere will be more scattered on its fringes. Models
disagree precisely where that feature will lay, with some pushing
it further west into the Columbia Basin than others. If this
verifies it would allow for some decrease in the precipitation
across the southeast Saturday afternoon.

As for thunderstorm between tonight and Saturday there will be
some risk. Tonight, especially overnight, models indicate a ribbon
of elevated instability expanding into southeast Washington and the Idaho
Panhandle. This is represented by some high level total totals
between 35 and 40 c. So some embedded thunderstorms are possible
here. By Saturday afternoon some SBCAPE develops with the best
agreement across the Idaho Panhandle, northern mountains and
Cascades. But some models show some convective instability across
the Columbia Basin too. Overall there will be a threat of isolated
to locally scattered thunderstorms but nothing too organized is
expected, given the lack of significant shear or deep instability.

Temperatures will remain below normal today and Saturday, with the
coldest average temperature expected on Saturday with the trowal
and steadier precipitation threat in the region. Some locations may
not get out of the 50s or lower 60s Saturday afternoon. /J. Cote'

Saturday night through Monday...looks like the weather will remain
on the cool and wet side during this period as the medium range
models are fairly consistent on ejecting saturdays low into
Montana only to be swiftly replaced by another low dropping down
from the northwest. Given this pattern, the best chances of
precipitation on Saturday evening will be fixed over the Idaho
Panhandle due to the placement of the upper level cold pool.
Potential instability parameters suggest there will be a small
chance of thunderstorms early in the evening, however warming
aloft will eliminate any chance of thunderstorms during the late
evening and overnight hours. For Sunday the next low is expected
to cross the Cascades via SW British Columbia with the upper level
jet extending from Seattle to Mullan Pass per the 00z GFS. If this
verifies, any chance of precipitation will generally occur over
the northern half of the forecast area, or generally north of
Highway 2 in Washington and over the northern half of the Idaho
Panhandle. The Canadian and ec show a slightly different scenario
with the Canadian showing a much weaker shortwave trough remaining
over BC, while the ec takes this second low and pushes it to the
Washington/or border. Suffice it to say confidence is high there will be a
secondary low, however where it GOES is questionable. No matter
what the case, this second low will be weaker than the Saturday
system and will contain less potential instability. It will also
contain mid-level westerly flow Sans warm air advection. This
means there will be a sizable rain shadow in the Lee of the
Cascades. Even where the rain is expected to occur, precipitation
amounts will be light.

For Monday night through Tuesday night...any semblance of a low
temporarily moves out of the region with either zonal flow or weak
ridging replacing it. This suggests dry weather will prevail and
temperatures will begin to warm. Speaking of temperatures,
readings for the latter half of the weekend will remain
significantly cooler than normal with highs in the mid 60s to mid
70s or nearly 10 degrees below normal. By Tuesday the jet will
shift north of the Canadian border with temperatures surging
toward normal values for the first time in nearly a week. Fx

Wednesday through friday: a dry zonal pattern is expected to
dominate this period with some weak ridging build into the region.
This will keep any chance of rain for the region close to non
existent. Temperatures will continue to creep warmer with highs
expected in the upper 70s to low 80s. Overnight lows will range
from mid 40s to mid 50s. /Jdc

&&

Aviation...
06z tafs: a low pressure system over western Washington will drop
south into Oregon tonight. This will result in isolated showers
along with a slight chance of thunderstorms mainly in the Cascades
and near the Canadian border. Convection is expected mainly over
the high terrain with the taf sites remaining dry with a
continuation of VFR conditions. Winds will shift to the north
overnight with some gusty winds expected down the Okanogan Valley
(komk) and less so down the Purcell Trench (kcoe). The next
precipitation expected to affect the taf sites will begin after
00z Saturday. /Ek




&&

Preliminary point temps/pops...
Spokane 70 48 57 46 66 45 / 0 60 70 30 10 10
Coeur D'Alene 69 48 56 46 65 44 / 0 70 70 40 10 10
Pullman 69 46 55 45 64 43 / 10 70 70 40 10 10
Lewiston 74 52 59 48 72 50 / 10 70 60 30 10 10
Colville 72 49 61 44 67 41 / 0 40 70 40 20 20
Sandpoint 68 47 58 45 64 40 / 10 60 70 60 20 20
Kellogg 65 46 55 42 63 40 / 30 60 70 60 30 20
Moses Lake 74 53 72 48 75 48 / 0 20 40 10 10 10
Wenatchee 73 53 72 51 75 51 / 20 10 20 10 10 10
Omak 72 49 74 45 76 45 / 20 10 20 10 10 10

&&

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

$$

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