Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 252148

Area Forecast Discussion
National Weather Service Spokane WA
248 PM PDT SAT APR 25 2015

Showery, cool conditions will linger through this evening, as an
upper low edges across the Inland Northwest. This will be followed
by high pressure, with warmer temperatures and mostly dry
conditions, for the beginning of next week. The threat for showers
will increase beginning late Tuesday, lasting through the end of
next week.


Tonight through Sunday night: Showers associated with an upper
low dissipate tonight, followed by high pressure and mostly dry
weather Sunday. This afternoon the upper low was centered over the
northern Cascades. This evening that low moves into the Columbia
Basin, stretches southeast and weakens overnight, and shifts
across north Idaho into western Montana through Sunday. Showers
will continue to be a threat early this evening under the cold
pool and instability with the upper low. These will be most
numerous within the deformation axis across across northeast WA
and north ID before 8 PM, before a ridge of high pressure and mid-
level warming starts in from the west. These showers may contain a
mix of rain, snow and graupel/small hail, along with brief gusty
winds. A few thunderstorms may be embedded in the showers too, but
the risk will be limited. By the overnight the main shower chances
will be across the ID Panhandle and near the Canadian border in
the deformation axis, while the rest of the area will see decreasing
clouds. By Sunday the region will come more under the influence of
the upper ridge. However the mid-level trough axis and northwest
flow will be close enough to keep an isolated shower threat alive
across the northeast WA and north ID mountain zones. By and large,
however, the day will be dry. Sunday night a warm front lifts in
with the building ridge. This will mainly bring some high clouds
to the region. The Cascades, however, will be brushed by some
deeper moisture and threat of showers again. /J. Cote`

Monday through Friday...The week will begin on a dry and warm note
as strong upper level ridging builds over the Inland NW. The
ridge will push a weak atmospheric river northward into extreme NW
Washington and SW BC. The big question with this pattern whether
or not precipitation from this river will intercept the far
northern Cascades. Whereas the latest NAM and previous GFS runs
said that would be the case the majority opinion is no. The axis
of atmospheric river is expected to get pivoted more north-south
as the next trough moves off the Washington coast. By Tuesday
models are coming into better agreement that the offshore trough
will weaken and push into SW BC by midday and toward the BC/AB
border by late in the afternoon. The main impact of this passing
trough will be the push of a cold front through the
region...beginning in the Cascades late in the day and moving into
the Idaho Panhandle by Wednesday morning. Model cross-sections
suggest the moisture along the front will not be sufficiently deep
to produce precipitation as it track through the area. We suspect
most of the precipitation threat will be reserved for locations
near the Cascades Crest late Tuesday afternoon shifting over SE
Washington and the Panhandle by Tuesday night and Wednesday. While
the precipitation won`t be necessarily widespread, the winds
behind the front will be. 850 mb winds peak around 20-30 MPH with
the passing front and suspect we will see the possibility of some
of these winds reaching the ground as the front moves through. The
timing of the front makes the Tuesday afternoon high temperature
forecast somewhat difficult as the thermal ridge will shift from
the Cascades on Monday and into western Montana by Tuesday. This
passage is faster than advertised by previous runs and suggests it
may not get as warm as previously thought. Even so most high
temperatures will reach the 70s which is well above normal for
this time of year.

From Wednesday and beyond the forecast details get quite uncertain
as there are many significant model differences. These
differences all revolve around the next trough moving toward the
coast. All the models show this as being a deeper trough than the
Tuesday afternoon/night system however the speed at which this
moves in is questionable. The previous ECMWF brought the trough in
on Wednesday while the GFS and Canadian are much slower. If the EC
is correct we`d see another potentially wet day for parts of the
region. While the other models would give us a dry day. Thursday
would seem to be the most likely day for the trough to move
inland. The pattern is somewhat similar to what we are seeing
today however its somewhat warmer and less conducive to graupel
showers. If things pan out like the GFS suggests we`d see a small
shot of thunderstorms as well. By Friday most of the models take
the trough to the east with moist zonal or westerly flow moving
in. Confidence on rainy days is now, however we are confident the
weather should be significantly cooler than the first couple days
of the week. fx


18Z TAFS: An upper low will bring a threat of isolated to
scattered showers this afternoon and evening, before the threat
wanes and shifts toward the mainly the ID Panhandle after 01 to
03Z. Some isolated thunderstorms are possible, but the risk of
any passing a TAF sites is too low to mention in a TAF. Brief MVFR
conditions are possible over GEG to COE, and possibly PUW, area.
The main threat in showers will be brief downpours, small
hail/grapel, but brief gusty winds are possible. Overnight into
Sunday morning will be mainly dry with VFR conditions, though
there is some small risk for low clouds/patchy fog in the outlying
areas. /J. Cote`


Spokane        35  59  39  67  45  74 /  40  10   0   0   0   0
Coeur d`Alene  35  59  35  67  41  73 /  60  10  10   0   0   0
Pullman        33  57  35  67  43  71 /  30  10   0   0   0   0
Lewiston       39  63  40  72  45  79 /  30  10   0   0   0   0
Colville       33  62  36  70  41  75 /  60  10  10   0   0  10
Sandpoint      33  58  32  66  36  72 /  60  20  10   0   0  10
Kellogg        36  56  35  66  40  71 /  70  20  10   0   0   0
Moses Lake     34  64  40  72  45  76 /  20   0  10   0   0   0
Wenatchee      40  66  45  73  48  72 /  20  10  10   0   0  10
Omak           32  65  39  72  42  73 /  30  10  10   0   0  10



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