Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Spokane Washington
455 PM PDT sun Sep 21 2014

warmer-than-normal temperatures expected through at least Tuesday,
as Summer-like temperatures struggle to break free. A pattern-
change will occur by mid-week, and more early autumn-like weather
will be common over the inland northwest to finish out the week. A
slight warming and drying trend is expected to occur next weekend.


tonight and monday: our upper level ridge will be shunted eastward
over the next couple of days. The "rex block" set up with high
pressure over the northern rockies and low pressure over the Great
Basin will erode as a longwave trough off the coast slowly
advances toward the Pacific northwest. We will be monitoring mid
and high level moisture associated with the Great Basin low
tonight into Monday as it is advected northeastward. At this time,
the GFS, NAM and European model (ecmwf) produce some shallow, high-based
instability from early morning Monday through Tuesday which may be
sufficient for light showers or sprinkles. Theta-E lapse rates
between 700-500mb suggest a small chance for precipitation over
the Cascades around sunrise then spreading eastward during the day
and evening on Monday. With cloud bases at or above ten thousand
feet there may be more virga on Monday than precipitation reaching
the ground. The best surface based instability looks to be over
central and northeast Oregon Monday afternoon and evening where a
few stronger showers and thunderstorms may develop. It is not out
of the question that a stray thunderstorm could develop over The
Blues of southeast Washington. Temperatures will remain well above
average for mid to late September, but should be 2 to 4 degrees
cooler Monday than Sunday due to more cloud cover. /Gkoch

Tuesday through Wednesday night: a slow-moving trough will be
inching toward the inland northwest, transitioning our flow to a
more moist South-West to southerly flow pattern by Wednesday.
There are a few challenges with this storm system. One will be
precipitation chances and cloud cover, which will then affect how
warm our temperatures get. The first shortwave rotates through
eastern Washington/Idaho Panhandle Tuesday, but doesn't have whole
lot of upper level support. Much of the lift associated with the
trough will be hanging back over western Washington into the
Cascades. But pwats approach or even exceed one inch, so any
shower that can develop will be wet, just maybe not widespread.
A weak warm front will move north as well on Tuesday and continue
into Wednesday over eastern portions of the region (far East
Washington/Idaho panhandle). This will keep temperatures continued
warm, but increasing cloud cover for much of basin and points west
will keep temperatures cooler there.

Precipitation chances on Wednesday and Wednesday night were
confined over the Cascades, with smaller chances further east
into the basin. Mostly dry weather is expected over the Idaho
Panhandle, until the trough finally moves onshore and moves over
the region (late in the week). Again, temperatures over the
precipitation-free regions will remain on the warm side of
normal, while cooling temperatures are expected further west as
the cooling affects of the trough move further east. Precipitation
chances will still remain highest over the Cascades through
Wednesday night. TY

Thursday through Sunday...longwave pattern change well underway at
the beginning of this forecast interval as the jet stream lingers
overhead in a south to north orientation along the eastern edge of
a large and narrow trof with an axis offshore. By late Saturday
the large low pressure area is showing signs of splitting with a
large portion of if forming a large closed low to the south of
Washington and north Idaho. A col or Saddle Point is whats left
over Washington and north Idaho for Saturday that elongates and
weakens Sunday which leaves potential for the low to the south to
eject mesoscale shortwaves north/northwest and influence southeast
Washington and southern Idaho. There is also potential for the
trailing edge of the northern branch of the exiting jet
stream/baroclinic band to act as a focus for minor showers to the
extreme north near the British Columbia border. That would leave
the Spokane/coeur d' Alene corridor in between and closer to the
bisecting axis of the col/Saddle Point and as such that locations
will hold on to lower pops in comparison to locations to the north
and south of the corridor. With such a seasonable trof overhead
forecast high temperatures show a pronounced drop and are in line
with normals for this time of year while the expectation there
will be enough lingering cloud cover, lower level moisture, and
mixing to keep overnight lows from dropping too drastically. It
should be fairly breezy and gusty with the jet stream overhead
Thursday and Friday with some decrease in wind and gusts Saturday
and Sunday. /Pelatti


00z tafs: middle-level moisture associated with an upper low in the
Great Basin will move northeastward tonight into Monday. The best
combination of middle-level moisture and shallow instability for
early morning showers looks to be over the Cascades, but it is
hard to get more excited than vicinity showers for the Wenatchee
taf. With cloud bases at or above ten thousand feet, any
precipitation amounts will be light. Smoke or haze is tough to
forecast. Pullman and Lewiston taf sites may need amendments as
these sites could experience visibility reductions to 4 miles
until middle day Monday. An increase in west winds Monday afternoon
should produce enough mixing to disperse the smoke originating
near Grangeville. /Gkoch


Preliminary point temps/pops...
Spokane 57 83 58 80 56 81 / 0 0 20 20 10 10
Coeur D'Alene 52 83 55 79 55 82 / 0 0 20 20 10 10
Pullman 53 83 55 82 55 84 / 0 0 20 20 10 10
Lewiston 61 86 61 86 61 87 / 0 10 20 10 0 0
Colville 51 84 52 79 52 81 / 0 10 30 20 10 20
Sandpoint 47 79 50 77 50 79 / 0 0 20 30 10 10
Kellogg 55 81 55 79 55 81 / 0 0 20 20 10 0
Moses Lake 58 87 56 82 58 81 / 0 10 10 10 10 20
Wenatchee 63 85 61 79 61 77 / 10 10 10 10 20 30
Omak 55 86 56 79 57 76 / 0 20 20 20 20 40


Otx watches/warnings/advisories...


National Weather Service Glossary of Abbreviations