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Area forecast discussion 
National Weather Service Spokane Washington
247 am PDT Sat Sep 5 2015

Synopsis...

A low pressure system will bring widespread rain and much cooler
temperatures to the Idaho Panhandle and portions of eastern
Washington Saturday. Light showers may linger mainly across the
mountains Sunday and Monday. A drying and warming trend is
expected next week.



&&

Discussion...
today through sunday: rainy and cool conditions today will
give way to isolated showers and somewhat milder conditions
Sunday. From this morning to evening low pressure centered near
the Washington/or/ID border tracks through Idaho and into western Montana. The
trowal and moisture wrapped around the low will help bring rain
across the eastern third of Washington and Idaho through the early afternoon,
before the focus for precipitation starts to shift toward far
northeast Washington and the Idaho Panhandle this evening. More widely
scattered to isolated showers will be a threat toward the western
Columbia Basin and Cascades today. However a second shortwave will
be dropping in from the northwest and approaching the Cascades
late tonight into Sunday morning, bringing a slightly better
chance of showers there. That shortwave then pivots southeast
across the remainder of Washington and north Idaho during the day Sunday.
This relatively quick moving feature will have some moisture to
work with, but it doesn't appear to be very deep. Instability
exists but it is limited. In tandem with some moisture still
wrapping around the backside of the exiting low, look for a threat
of isolated showers over most locations. As for thunderstorm
chances over the next 36 hours, there will be some risk but it
will be limited. The best chances will be near the Cascades and
over far north and eastern Washington and Idaho today, with the threat waning
this evening after sunset, before being renewed across far
northeast Washington and the northern Panhandle Sunday afternoon.
Temperatures will be held back today across the eastern third of
Washington and north Idaho under the rain and clouds, with many areas not
getting out of the 50s. Milder readings are expected toward the
western Columbia Basin and Cascades which will have more sunshine
and less precipitation. By Sunday temperatures moderate across the
region as the cold low exits. However values will still remain
some 10 degrees below normal. /J. Cote'

Sunday night through Wednesday night...although this period won't
be as wet or cool as the beginning of the forecast it might not be
as dry as it previously appeared either. The prevailing synoptic
pattern for this period will feature a relatively Flat Ridge off
the coast with west to northwest upper level flow over the inland
northwest. This places the upper level jet somewhere near the
Washington/British Columbia border through much of this period which results in
several weak upper level disturbances tracking through the
northern portions of the forecast area. Beginning on Sunday night,
we should see a brief drying trend develop after the deep upper
level low currently over the region shifts into Saskatchewan.
There could still be some light showers during the evening mainly
over the north Idaho Panhandle but little else. Then by Monday
shortwave number one swings into the region from the northwest.
The European model (ecmwf) and Canadian have consistently taken this feature into
southeast Washington by midday, whereas the latest GFS leaves it near the
NE corner of Washington. The 18z GFS showed this farther south
track and will be the favored solution. Moisture with this system
won't be terribly deep, nor will the potential instability,
however given the orographic ascent expected to develop,
especially over extreme NE Washington and the Idaho Panhandle, we
will bring the chances of rain farther south than was previously
anticipated. Most of the rain chances will occur north of I-90. In
the Lee of the Cascades this system won't do a whole lot as the
mean mid- level flow will favor moderate downslope regime. The
next shortwave is expected to cross southern British Columbia on Tuesday. Model
agreement with this feature is quite good, unlike yesterday where
the GFS had the feature much farther north, which left the
forecast area largely bereft of precipitation. Now most of the
model solutions paint some light to locally moderate precipitation
extending from the north Cascades to the Idaho Panhandle north of
I-90. The heavier precipitation amounts are legitimate since this
system will be briefly accompanied by a narrow precipitable water
plume nearing an inch. Precipitation amounts could near a quarter
inch across extreme NE Washington and the northern tip of the Idaho
Panhandle with similar amounts possible near the Cascade crest.
Meanwhile very little if any rain will fall in the Lee of the
Cascades or the Okanogan Valley due to continued downslope flow
off the Cascades. This feature should push east of the area on
Tuesday evening. Wednesday will feature the last in the series of
shortwave troughs. This one will be fast moving and fairly weak.
Model cross sections show fairly limited moisture with most of it
located above 12k feet so other than passing clouds we don't
expect much precipitation at this point. Temperatures are still
expected to warm through this period, however the trend will be
slower than previously expected. Highs on Sunday will still be
cooler than normal for this time of year with readings in the mid
60s to mid 70s. By Wednesday temperatures should climb about 10
degrees resulting in highs in the mid 70s to mid 80s, which is
right around normal for this time of year. Fx

Thursday through saturday: models are in very good agreement for
this period. A dry zonal pattern is expected to dominate this
period with some weak ridging build into the region. This will
keep any chance of rain for the region close to non existent.
Temperatures will continue to creep warmer with highs expected in
the upper 70s to low 80s. Overnight lows will range from mid 40s
to mid 50s. /Jdc

&&

Aviation...
06z tafs: showers will drift north into extreme southeast Washington and
central Panhandle of Idaho tonight along with isolated
thunderstorms possible but confidence is low that any ts will
affect a particular taf site. The wettest interval will be
overnight tonight and into tomorrow as a wet frontal zone stalls
over eastern Washington and north Idaho from about 09z on bringing
MVFR conditions to taf sites except keat and kmwh. Kmwh will be on
the western edge of the precipitation shield and may see very
little rain. As the low moves east Sat afternoon, any breaks in
cloud cover may allow convection to develop, with isolated
thunderstorms possible across the forecast area. Cascade Gap winds
will increase after 18z Sat with gusts of 25 to 30 kts possible at
keat. /Ek

&&

Preliminary point temps/pops...
Spokane 54 44 64 44 67 48 / 100 50 10 10 10 10
Coeur D'Alene 53 43 63 43 65 45 / 90 70 20 10 20 10
Pullman 53 41 62 41 66 45 / 80 60 20 10 10 10
Lewiston 60 48 69 47 74 51 / 70 60 10 10 10 10
Colville 59 44 67 43 70 46 / 90 50 20 20 20 10
Sandpoint 55 43 62 40 65 43 / 100 70 20 20 20 10
Kellogg 53 42 61 41 63 41 / 90 80 30 20 20 10
Moses Lake 72 48 73 47 75 50 / 70 20 20 0 0 0
Wenatchee 73 53 73 52 76 53 / 20 10 20 10 0 0
Omak 73 47 74 46 76 50 / 20 20 20 10 10 10

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...none.
&&

$$

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