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Area forecast discussion 
National Weather Service Spokane Washington
420 am PDT Tuesday Apr 28 2015

Synopsis...
high pressure will begin to get pushed east today, producing mild
and dry conditions. An approaching cold front will lead to an
increase in winds later today through Wednesday, along with a
chance for showers and possibly a few thunderstorms mainly tonight.
A few showers may linger over the mountains Wednesday and Thursday.
This will be followed by a general drying trend into the weekend.



&&

Discussion...
today through Wednesday...satellite imagery shows a low pressure
system just off the central British Columbia coast as of 2 am with a weak cold
front extending south from the low off the Washington and Oregon
coast. There is good model agreement that this cold front will
pass through tonight.

Ahead of the front prefrontal southwest winds and 850mb
temperatures ranging from 12-17c (warmest over north idaho) will
result in a mild day with widespread Valley High temps in the 70s
with low 80s for the Lewiston-Clarkston area. As the cold front
moves into the this afternoon and then across central Washington
by early this evening, models show the best shower threat near the
Cascade crest. Moisture will be a limiting factor with this front.
Downslope westerly winds off the Cascades should keep showers just
along the immediate crest. However enough instability will be
present where isolated showers can not be ruled out over the
higher terrain of the Cascades and the Okanogan Highlands. Breezy
winds through the Cascade gaps and windy conditions on the ridge
tops can be expected behind the front this evening.

Then as the front tracks into eastern Washington and north Idaho
overnight into early Wednesday morning showers coverage will
increase due to favorable jet support...low level upslope
flow...as well as strong forcing and mid level instability.
Isolated thunderstorms are possible with the front. Drier air
behind the front will spread across all of central and eastern
Washington early Wednesday reaching north Idaho by noon. Models are
not showing much Post-frontal shower activity, with the best
chances near the Canadian border and Cascade crest. There will be
just enough instability near the Canadian border around Northport
and Metaline Falls where an isolated late afternoon thunderstorm
can not be ruled out. Jw

Wednesday night and Thursday night...the upper level trough will
flatten out and shift east towards the Continental Divide, with
the flow becoming more zonal (west to east). Enough instability
will remain across the north Panhandle to hold onto to some
isolated showers and thunderstorms through sunset. The flow then
becomes westerly and allows drier air to move into the region
overnight. Some orographically enhanced showers may linger through
the afternoon on Thursday near the Canadian border, but these
should be very spotty with light amounts of rain and high
mountains snow. Temperatures will warm by a few degrees. Winds
will remain out of the southwest but should subside nicely as the
surface gradient decreases.

Thursday night through Sunday night high pressure will strengthen
in the eastern Pacific resulting in a weak and flat trough
dropping south through b.C. To about the Canadian border. The
models are back up to their old tricks with some major timing
differences on just when to push this trough out of the area.
Consensus will be to stay away from a GFS solution which will mean
the trough will linger through Sunday night, and this solution is
accepted. Moisture and dynamics are very weak with this trough. We
may see some mid level clouds and possibly a few mountain
sprinkles or light showers at times for the northern mountains,
but I can't get very excited about this pattern. Temperatures will
increase due to more sunshine, and while temperatures will remain
on the warm side of normal I do not foresee any Big Warm up.

Monday and Tuesday the weak ridge will push into the region for
warm and dry conditions. Temperatures will be 5-8 degrees above
normal. The past 2 or 3 model runs have been showing some very dry
air coming into the region with this weak ridge and we will have
to keep an eye on the relative humidity over the weekend. There
hasn't been a lot of moisture of late and low relative humidity
will continue to dry conditions. Fortunately we are not looking at
any wind events with the ridge. Stay tuned. Tobin

&&

Aviation...
12z tafs: dry conditions are expected through this afternoon ahead
of a cold front. The front will pass through keat this afternoon
and across the remainder of the area tonight. East winds this
morning will switch to southerly this afternoon...and then west-
southwest behind the front for most areas. Showers are expected
with the front for the eastern taf sites. Isolated thunderstorms
are also possible...but confidence is low whether not any of the
taf sites will see a storm and thus left mention out of the taf's.
Jw

&&

Preliminary point temps/pops...
Spokane 75 45 61 39 62 40 / 0 50 20 10 10 10
Coeur D'Alene 76 42 60 36 61 37 / 0 50 40 10 10 10
Pullman 75 42 59 36 60 37 / 0 20 30 10 10 10
Lewiston 82 48 66 41 67 42 / 0 20 30 10 10 10
Colville 78 46 67 37 67 37 / 0 60 20 10 10 10
Sandpoint 75 44 61 33 61 35 / 0 50 50 10 10 10
Kellogg 73 41 57 35 58 36 / 0 20 50 10 10 10
Moses Lake 78 42 68 39 70 41 / 10 20 0 10 10 10
Wenatchee 74 48 68 44 70 44 / 10 10 0 10 10 10
Omak 77 42 69 36 68 39 / 10 10 10 10 10 10

&&

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

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