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Area forecast discussion 
National Weather Service Spokane Washington
446 am PDT Wednesday may 6 2015

a low pressure system will result in afternoon convection once
again today...mainly over the Cascades and Okanogan Highlands.
The weather will generally be dry and warm through the end of the
workweek, except for a small chance of mountain showers over the
Idaho Panhandle on Friday. Temperatures will moderate a bit by
the weekend as another weak disturbance approaches the coast. This
feature will likely pass into the area early next week with a
return to cooler temperatures and a chance of precipitation.


today through Thursday...there is good model agreement of a closed
low over central Washington early this morning dropping south into
Oregon this afternoon, then continuing its southward track into
central California on Thursday.

Today: the low will be in close enough proximity this afternoon
that afternoon/early evening convection is expected. The coldest
temps aloft will be over central Washington with uncapped cape
values of 200-500 j/kg over the Cascades and Okanogan Highlands
supporting the potential for isolated thunderstorms. Elsewhere the
dry low level air mass will keep shower coverage isolated...with
most towns remaining dry through the day.

Tonight: convection will die down after 6-8 PM with the loss of
daytime heating. As the low drops south a band of wrap around
moisture will clip the Camas Prairie tonight. There is some
guidance that says it could make it as far north as the Palouse
and central Panhandle mountains but the consensus is for the best
chances on the Camas Prairie.

Thursday: as the low drops further south on Thursday northerly
pressure gradients will increase. This will bring drier air into
the region with breezy winds down the Okanogan Valley. Increased
sunshine and warming 850mb temps to near 11c will allow valley
highs to climb back above normal with upper 60s to mid 70s
forecast. Jw

Thursday night through sunday: as the low is continuing to be
pushed further south and east, a ridge begins to fill into the
region. This leads to a warming and drying trend for this period.
Precip chances are near zero for the period. The dry upper levels
will allow for mostly sunny conditions during this period.
Temperatures are expected to be in the low 70s to low 80s for the
weekend. /Jdc

Monday through Wednesday...confidence in this period is fairly low,
especially during the Tuesday/Wednesday time frame. Odds are good
Monday will be the driest day during this period as most of the
model solutions maintain the semblance of a narrow ridge over
extreme western Montana with a deep closed low off the northern
California or southern Oregon coast. What happens after that
though is rather inconsistent. The general sense is the offshore
low will gradually get ejected from an upstream kicker moving
southeast through the Gulf of Alaska. How quickly the low will
eject is the big question. The GFS keeps the core of the low along
the California/or border through Tuesday while the European model (ecmwf) takes it into northwest
Oregon. Meanwhile the Canadian model keeps it down by the Bay
area. The differences between ensemble runs at 500 mbs show vast
differences so confidence is quite low. Not sure which solution to
buy off on but it would probably be best to keep the forecast
reflective of the ensemble means which suggests a compromise
between the fast ec and the slower GFS. Based on this trend we
will increase the precipitation chances over the southern third of
Washington as well as the southern Panhandle. By Wednesday there
is a little better consensus that the low will drift at least as
far north as the or/Washington border. This places a good portion of our
forecast area in the wraparound moisture north of the low
suggesting this will be the wettest day of the period as a
deformation band sets up. Whether this band impacts locations near
the Canadian border or along the Highway 2 corridor is
questionable, however we will go with climatological pops or
higher for Wednesday.

The problem with timing of the precipitation will also have
repercussions on the potential temperatures. Since Monday is
expected to be the driest day with the most potential sunshine we
made that the warmest day with highs in the 70s. Tuesday could be
close to that as well, with warmer readings in the north and
slightly cooler ones in the south. By Wednesday, temperatures
could drop significantly if the deformation band sets up over our
forecast area. We are going with highs in the mid 60s to lower 70s
for now, but that could prove way too warm depending on if and
where the deformation band sets up. Fx


12z tafs: a low pressure system over the region will result in
scattered showers and a slight chance of thunderstorms between
18z today - 03z Thursday mainly over the mountains across the
Cascades and Okanogan Highlands. All of the taf sites will likely
remain dry with VFR conditions through 12z Thu. Jw


Preliminary point temps/pops...
Spokane 61 41 69 42 69 41 / 20 10 0 0 10 0
Coeur D'Alene 61 40 68 39 68 39 / 20 10 0 0 10 10
Pullman 59 40 67 38 69 37 / 10 10 10 10 10 0
Lewiston 65 44 73 44 74 44 / 10 20 10 10 10 0
Colville 64 41 72 41 73 40 / 30 20 0 0 10 0
Sandpoint 61 36 67 36 66 34 / 20 10 0 0 10 10
Kellogg 59 39 66 37 66 37 / 10 10 10 10 10 10
Moses Lake 67 43 76 46 77 44 / 20 10 0 0 0 0
Wenatchee 65 44 75 48 78 48 / 20 10 0 0 0 10
Omak 65 42 74 42 75 42 / 20 10 0 0 0 0


Otx watches/warnings/advisories...


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