Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Spokane Washington
430 am PDT Tue Jul 26 2016
a couple upper systems cross the region, bringing a chance for
showers and thunderstorms across the north today and Wednesday.
Warm weather will continue through the end of the week, with
highs commonly in the 90s. A cool down is expected by Sunday.
Gusty winds will accompany this cool down, especially across the
Columbia Basin and Cascade gaps.
today through wednesday: the inland northwest will see continued warm
weather, locally breezy conditions, and some threat of showers and
thunderstorms. The main shower/T-storm threat will be across the
north. Yet the forecast challenge is how far south will that
threat extend? The broad-scale pattern (5-wave chart) shows a
ridge over the western US, supporting the above normal temperatures.
However WV/infrared shows a progressive flow and the ridge is littered
with shortwave disturbances.
One shortwave has been tracking along the Canadian border
overnight, producing a few showers across far northeast Okanogan
to northern Stevens County. That will lose its influence but a
second shortwave is moving onshore. This will be main weather
maker today and this evening as it tracks from west to east. It
already helped produce a shower over northern Chelan County
earlier. A deformation axis also arches across the northern
mountains this morning, from near Lake Chelan to west-central
Montana. Models show its western side edges northeast with the
incoming shortwave so that its axis stretches from northeast
Okanogan County/Central Ferry County to west-central Montana this
afternoon and evening. This is important because areas near and to
the north of it be a potential Focal Point.
Yet will the shortwave and deformation axis bring enough lift?
Also have sufficient moisture and instability to work with to
produce showers? The bulk of the lift appears will be across the
north but areas along the upper I-90 corridor will be close.
Moisture appears will sufficient with pwats rising to an average
0.80-1.0 inches (or 150-175% of normal). However instability will
be evolving. This morning all models show an area of elevated
instability (cape and unstable total totals) across the northern
mountains and near to just west of the Waterville Plateau. This is
where the main shower and isolated thunderstorm chances will be
found this morning.
This afternoon the surface-based instability blossoms. Models
show cape values between 500-1000 j/kg across north-central to
northeast Washington and the Idaho Panhandle, with pockets of higher cape.
This includes much of the upper Columbia Basin through the Palouse
and the Spokane and Coeur D'Alene area. Forecast soundings shows
little cap/cin this afternoon with afternoon highs approaching
their convective temperatures. Scattered showers and thunderstorms
will expand throughout those northern mountains. Some may contain
heavy rain amounts, especially further northeast where storm
motion looks slower. We will have to watch for localized flooding,
especially in and around the burn scars. Some hail may also be
found. As for the remainder of the cwa, given all the factors
above I did add some isolated shower/thunderstorm to the upper
Columbia Basin through the Spokane/c'da area and central
Panhandle. Radar trends will need to be monitored for this
afternoon and the forecast will be updated should coverage needs
be increased or decreased. In these latter areas I cannot rule out
a brief heavy downpour either. If nothing else, i'm expecting more
clouds than recent days. Going into later this evening, after 8
PM, the threat of showers and thunderstorms will shift to mainly
the far northeast mountains and wane overnight.
A secondary issue of the day will be locally breezy conditions.
This morning a modest northwest to southwest gradient will lead to
some drainage and channeling near the Lee of Cascades and Okanogan
Valley. Speeds of 10-15 mph with gusts to 20-25 mph will be
possible early, especially in the Okanogan Valley. Speeds abate
some through late morning and early afternoon, but the passing
shortwave and increased gradient near the Cascades late this
afternoon and evening will once again increase The Gap winds,
especially near places like the Wenatchee River Valley and
Ellensburg area with speeds around 10-20 mph with gusts of
25-30 mph. Speeds start to abate late evening into the overnight.
Wednesday the next shortwave, currently out in the Pacific
approaching 140w, moves in. However this feature right now looks
somewhat weaker and the deformation axis will be shifted further
north toward the Canadian border. Instability blossoms around the
north mountains again in the afternoon and the threat of showers
and thunderstorms with it. Pwats are little lower with the average
around 130-150% but we will still have to watch for any locally
heavy rain and localized flooding. Meanwhile higher confidence
exists the the remainder of the area will remain dry. /J. Cote'
Wed nt through tues nt: we kept the fcst dry for this six day
period, with slowly warming high temps through Fri. The warmest
weather should peak Fri with nearly all of ern WA and north Idaho well
into the 90s under a pronounced thermal ridge surging NE ahead of
an approaching broad offshore British Columbia trough. This trough will help to
lower temps to normal values late weekend into next week, but this
transition to cooler weather will accompany gusty winds across the
Columbia Basin as well. And with a dry frontal passage combined
with the gusty winds, any new or recent fire starts will be in a
favorable fire wx environment. Bz
12z tafs: a disturbance crosses the Pacific northwest between this
morning and evening. Isolated shra/tsra this morning across the
northern counties will increase in coverage this afternoon and
evening. Some may cross geg/sff/Coe taf sites after 21-23z, with
the risk ending between 01-03z. Yet the better threat is expected
to remain north of here. Breezy conditions are expected early
this morning near eat (and omk) before abating toward 16-18z.
Winds then increase again near eat toward 00-02z with the typical
drainage winds enhanced by the passing disturbance. Speeds then
abate here overnight. /J. Cote'
Preliminary point temps/pops...
Spokane 91 63 91 63 91 66 / 20 20 10 10 0 0
Coeur D'Alene 90 59 90 59 91 62 / 20 20 10 10 0 0
Pullman 90 53 90 53 91 56 / 10 0 0 0 0 0
Lewiston 98 66 97 67 97 68 / 10 0 0 0 0 0
Colville 91 56 92 57 93 58 / 40 30 20 10 0 0
Sandpoint 86 54 86 53 87 56 / 30 30 20 10 0 0
Kellogg 88 55 88 55 87 56 / 20 10 0 10 0 0
Moses Lake 96 62 95 63 97 66 / 10 0 0 0 0 0
Wenatchee 93 67 94 68 97 71 / 10 0 0 0 0 0
Omak 94 66 94 66 95 68 / 40 10 10 10 0 0