Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 060949

Area Forecast Discussion
National Weather Service Spokane WA
249 AM PDT WED 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 CA/OR border through Tuesday while the ECMWF takes it into NW
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/WA 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


06Z TAFS: An upper level low pressure system will slowly drift
south across western WA tonight. Best chance for showers this
evening and overnight will be across the Cascade Mtns and Okanogan
Highlands. There is also a chance for thunderstorms across these
areas during the afternoon/evening hours Wednesday, mainly along
the crest and near the Canadian border. Gusty winds will diminish
tonight, then kick up again Wednesday after 17Z. /EK


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



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