Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 042132

Area Forecast Discussion
National Weather Service Spokane WA
232 PM PDT MON MAY 4 2015

A strong cold front is expected tonight with cooler and breezier
weather in the forecast for Tuesday. Spotty showers will be
possible Tuesday and Wednesday as well as a chance of thunderstorms
on Wednesday. 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. Temperatures could moderate a
bit by the weekend as another weak disturbance approaches the
Inland Northwest.


Tonight through Tuesday Night: A deep upper level low pressure
system resides just off of Vancouver Island in the eastern
Pacific. Swinging around the base of the low is a cold front with
a clearly defined dry slot along the back edge of the front
depicted on the water vapor satellite imagery. The cold front is
expected to cross the Cascade Mtns this evening and then push into
the ID Panhandle tonight. Not much in the way of moisture will
ride up with the front, but showers are expected in the Cascades
and across the extreme southeast portion of the forecast area.
Showers that develop across northeast OR and into the southern and
central ID Panhandle will tap into weak instability. It is enough
instability that thunderstorms will be a possibility, but better
chance will lie southeast of the region.

Another impact from the cold front will be breezy winds overnight
into Tuesday. A good amount of cold air advection with the front
should prevent the boundary layer from decoupling completely. The
strongest winds will be felt through the Cascade gaps with gusts
to around 25-30 mph possible. Winds are expected to be less gusty
of around 15-25 mph as they push across the basin.

The upper level low will begin to split as it pushes across
western WA and into BC on Tuesday. The southern branch of the low
will move into southwestern WA while the northern branch slides
into southern BC during the afternoon on Tuesday. This will place
the cold pool aloft across the northwestern portion of the
forecast area. This is where the best chance for showers and
thunderstorms is expected, which will include the east slopes of
the Cascades over to the Northeast Mtns. The Waterville Plateau
and western portion of the basin will also see a chance for some
showers and thunderstorms. Drier air across the southeastern
portion of the forecast area will make it difficult for showers to

Much cooler temperatures can be expected on Tuesday with highs
dropping back down into the 60s. /SVH

Wednesday...The models are in good agreement showing a deep low
pressure system dropping out of B.C. Tuesday night and finally
closing off over OR/CAL Wednesday night and Thursday. 500Mb temps
of -29C will slide slowly south through the Inland Northwest
Tuesday through Wednesday. The cold pool will keep the atmosphere
unstable enough to for widespread rain and mountain snow showers
through Wednesday night. Afternoon surface heating will increase
surface based and mid level cape, and with lapses rates increasing
to 8C/km there will be a chance for afternoon and evening
thunderstorms. The GFS/EC keep the thunderstorms across the
western half of the forecast area, while the NAM looks like there
could be thunderstorms just about anywhere in the warning area.
Previous forecasts leaned in the direction of the NAM and see no
good argument to change that thought. Then only thing that is
really missing is a deep moisture source but the deep instability
should overcome that. And while localized heavy rain is always
possible under a heavy shower, QPF is generally under a tenth of
an inch through this period. Temperatures on Wednesday will be the
chilliest of the week and 3-5 degrees below average. Afternoon
winds will not be as strong but still in the 5-15 mph range.

Thursday through Saturday the models are in decent agreement. The
closed low will continue to move south into southern Cal/Nevada,
meanwhile a low pressure system in the central Canadian plains
will retrograde back towards the Continental Divide. This will put
the Inland Northwest in between weather systems for a mainly dry
forecast and a bit of a warming trend. Some showers were kept in
the orographically favored Idaho Panhandle, but these should be
very spotty. Temperatures will rebound by several degrees on
Thursday and warm back up to near normal. Temperatures will
then increase back into the mid 60s to mid 70s Friday and Saturday.

Sunday and Monday another upper level low pressure system will
move into the eastern Pacific off of the Washington coast. This
where timing differences of the models increase. Southerly flow
ahead of the upper low pressure system should tap into deeper sub-
tropical moisture and increase the chances for showers both Sunday
and Monday. Tobin


18z TAFS: VFR conditions are expected at all TAF sites through 18Z
Tuesday. A cold front will sweep across the region during the
evening hours. This will produce wind gusts of 25-30 kts at KEAT
after 00Z this afternoon. Breezy winds will then spread across the
basin through the evening and reach KGEG, KSFF, KCOE and KPUW
between 05-08Z. Showers ahead of the cold front will primarily
remain southeast of KPUW. There is a small chance for thunderstorms
near KLWS, but confidence is too low to include in the TAFs. /SVH


Spokane        41  61  36  60  39  66 /   0   0  10  30  20  10
Coeur d`Alene  41  60  35  60  36  65 /  10  10  10  30  20  20
Pullman        40  60  34  58  36  64 /  10  10  10  20  20  20
Lewiston       48  66  39  64  41  69 /  20  10  10  20  20  20
Colville       42  65  38  63  37  70 /   0  10  30  50  30  10
Sandpoint      38  61  32  60  33  65 /  10  10  10  40  20  20
Kellogg        41  59  35  58  36  62 /  20  30  10  30  20  20
Moses Lake     41  66  38  65  39  72 /   0   0  20  20  10   0
Wenatchee      46  64  43  64  42  73 /   0  10  30  30  10  10
Omak           38  64  36  65  36  71 /  10  20  20  40  20  10



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