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Area forecast discussion 
National Weather Service Spokane Washington
421 am PDT Thu Jul 28 2016

Synopsis...
hot and mostly dry weather is expected Thursday and Friday. A
cold front arrives this weekend, bringing breezy conditions and
cooler weather for the start of next week. A few showers or
thunderstorms will be possible over the northern counties early
next week.




&&

Discussion...
today through friday: hot, mostly dry conditions will dominate.
Some threat of showers and thunderstorms will linger near the
Cascades and Canadian border in the later afternoon and evening,
but anything that develops is expected to remain isolated. The
inland northwest is under a dirty ridge and westerly flow. One weak
shortwave will be strung across the region late this afternoon and
evening. A second approaches late Friday. As for moisture: the
atmosphere remains relatively juicy, with pwats over the next
couple days averaging between 0.60 to 0.90 inches (or about
100-150% of normal). As for instability: areas of SBCAPE expand
around the Cascades and northern mountains each afternoon. These
aforementioned shortwaves along with some upslope flow may produce
enough lift to work with that moisture and instability to provide
that isolated shower/T-storm threat. Otherwise some afternoon
cumulus develop, especially around the mountains, and perhaps some
thin cirrus. Aside from some increased speeds near the Cascade
gaps during the late afternoon and evening, winds will be largely
light. Temperatures will remain above seasonal norms, with highs
averaging in the 90s. /J. Cote'

Friday night through saturday: changes are expected to develop
during this part of the forecast as all model solutions depict an
upper level low dropping steadily southward out of Canada. This
will push the upper level jet into the northern Cascades by
Saturday afternoon with it dipping toward the I-90 corridor by
Sunday afternoon. The advance of the low and upper level jet will
push a cold front through the region on Saturday. The air mass
behind the cold front will be quite dry with precipitable water
values possibly falling to half of what they were today or even
lower. This translates to values ranging from 0.25-0.50 inches
across most of the inland northwest. With values this low, the
main story won't be a chance of showers or thunderstorms, although
there will be a small chance near the Canadian border on Sunday.
Rather than main focus will be around fire weather concerns as the
low relative humidity values will combine with breezy conditions
on Saturday. It's interesting to note that as the cold front
advances across the area, we will see the sea level pressure drop
significantly to around 1003 mbs by Saturday afternoon. Although
this may not sound low, it's actually unusually low for this time
of year and is in the first percentile for late July-early August.
This alone is not significant, however the strengthening west-east
pressure gradient will be. The gradient will contribute to the
potential for breezy to windy conditions which will combine with
the drier air mass. While we have no doubt the relative humidity values behind
the front will drop well into the teens, we are a little less
confident in the strong wind potential. The surface pressure
gradient will likely be sufficient for 15-20 mph sustained winds
at least over the western part of our forecast area during the
afternoon however the 850 mb winds really won't help much as they
are in the same ballpark and thus the mixing potential won't
likely aid in strengthening the surface winds. Looks like we could
be dealing with borderline red flag conditions for Saturday with
the best chances of hitting criteria likely impacting the Cascade
gaps and the lower Columbia Basin. The pressure gradient is
expected to drop off on Sunday as will the winds however the dry
air will remain fixed over the region. As for temperatures it
looks like Saturday will remain quite warm for this time of year
with highs likely remaining in the 90s for most locations. By
Sunday we should see readings cool to a more seasonable 80s

Monday and tuesday: there isn't great consistency between the
models as they all differ on the next upper level low heading into
the region. The ec has been the deepest and it brings increased
moisture and precipitation chances into the Cascades as early as
Monday afternoon and across the northern half of the region on
Tuesday. The GFS is slower and the Canadian model leaves the next
upper level low off the coast for the entire period. The ec is
consistent with its ensemble mean and hence should not be
disregarded. Consequently we have raised pops at least across the
northern portions of the forecast area from Monday afternoon
through Tuesday. Looks like the atmosphere will be unstable enough
to support thunderstorms at least near the Canadian border. There
may even be a small chance of nocturnal thunder near the Cascades
late Monday night but confidence is too low to place in grids and
forecast for now. This next low will likely lower temperatures
further with highs ranging from the mid 70s to mid 80s on Tuesday.
Fx

Tuesday night through thursday: models still differ in the
details of the exit of our early week trough. The European model (ecmwf) has been
the most inconsistent, flip flopping between holding the trough
around for Tuesday night or building in weak shortwave ridging
quicker. For this reason, a GFS deterministic and GFS ensemble
compromise were used for this forecast. A small chance of showers
across NE Washington and north Idaho Panhandle for Tuesday night seems
reasonable as enough of a 500 mb cold pool remains over the area.
Otherwise, expect zonal flow or weak ridging to build in for the
rest of the extended, leading to drying and warming conditions for
the second half of Wednesday and into Thursday. High temperatures
Thursday should return to near normal for this time of year after
a few days of being below normal. /Kalin

&&

Aviation...
12z tafs: a westerly flow and weak disturbance may bring isold
-shra/-tsra north of Lake Chelan/Methow Valley and near the
northern mountains. Some afternoon cumulus is expected too, largely
near the mountains. Marginally breezy conditions are expected
near omk and eat early today and near eat late this afternoon and
evening. Otherwise dry/VFR conditions prevail. /J. Cote'



&&

Preliminary point temps/pops...
Spokane 93 68 95 65 90 59 / 10 10 0 0 0 0
Coeur D'Alene 92 63 93 62 90 56 / 10 10 0 0 0 0
Pullman 92 59 94 57 90 51 / 0 0 0 0 0 0
Lewiston 99 70 101 68 98 62 / 0 0 0 0 0 0
Colville 95 60 95 57 93 53 / 10 10 0 10 10 10
Sandpoint 88 55 89 56 87 51 / 10 10 0 10 10 10
Kellogg 90 58 90 60 87 53 / 10 10 10 0 0 0
Moses Lake 98 66 100 63 93 58 / 0 0 0 0 0 0
Wenatchee 98 72 99 68 91 63 / 0 10 0 0 0 0
Omak 97 69 98 66 94 60 / 10 10 0 0 0 0

&&

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

$$

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