Area forecast discussion
National Weather Service Spokane Washington
457 am PDT Thu may 23 2013
Synopsis...
cool, showery weather will continue over the inland northwest due
to a lingering upper low. Snow will also be possible over the
mountains. Conditions could moderate a bit by the weekend, as the
low moves out of the area, but it will be replaced by another cool
low sometime early next week.
&&
Discussion...
today and tonight: an upper low provides continued hit-and-miss
shower chances and cool temperatures. The low was centered just
south of Seattle this morning. As a shortwave pivots around its
northwest side today, the low is expected to retrograde toward the
mouth of the Columbia River. This will keep the inland northwest
in a south to southeasterly flow and in the path of weaker
vorticity maxima. One vort Max strings by northeast Washington and
northern Idaho early today and a second slips by the Cascades later
this afternoon and evening. Finally, a third slips across Oregon
toward the clearwaters late this afternoon and evening too. With
the afternoon heating, models depict increased convective
instability. Agreement on where this instability will lay is
highest over the Cascades to Waterville Plateau and secondarily
across the lower Columbia Basin into the Blue Mountains. One to
two hundred j/kg of cape is depicted.
The weak forcing from the passing vort Max, coupled with the
south/southeast flow, will support the highest shower chances
across the Cascades, Wenatchee area and Waterville Plateau.
Secondary chances will be found near the Blue Mountains. Given the
unstable nature of the upper trough a slight chance of
thunderstorms will be found here this afternoon and early evening
too. Tertiary chances will be found in northern mountains, largely
due to the orographics. The lowest threat will be of the upper
Columbia Basin through the central Panhandle, including the
Spokane/c'da area and Palouse as models continue to entrain a fair
amount of dry air into southeast Washington and the Idaho
Panhandle. By tonight waning instability and exiting vorticity Max
will diminish the threat of showers throughout much of the region.
The main exceptions will be near the Cascades and near the
Blue Mountains as the main shortwave pivots around from the
northwest to the south side of the low and begins to push swing
across Oregon overnight into Friday morning). /J. Cote'
Friday through Wednesday...models are in good agreement for Friday
in maintaining a deep upper low pressure over the forecast area.
While the main deformation axis with widespread showers will have
pivoted north and east and be well clear of the region...the
forecast area will reside underneath the destabilizing cold pool
aloft in the center of the upper low. Thus...scattered showers are
a good bet and just about all zones will be susceptible. In
addition...if any sunbreaks occur...which is a distinct
possibility...the Prospect for at least isolated thunderstorms
will be very real...again just about anywhere. Temperatures will
remain well below normal through Friday.
From Saturday Onward model agreement decreases somewhat...especially
with sub-synoptic details...however the general model consensus is
to weaken or eject the upper low north into Canada and leave a
distinct but weaker troffy pattern over the northwest US. This is
good news for Holiday weekend outdoor planning as it implies that
there are likely to be considerable periods of dry or minimal
shower activity and the main shower activity expected each
afternoon and evening will be concentrated over the mountains
ringing the Columbia Basin. This is not to say that random showers
will not prowl the basin from time to time...just that the
activity will not be as sure a bet as when the upper low was
sitting right over the forecast area.
For the new work week the latest GFS and European model (ecmwf) both hint at the
arrival of a new and deeper trough into the region on or about
Tuesday or Wednesday...with the potential for a well defined and
moist frontal surface leading to more widespread showers.
Through this period temperatures will start out well below normal
on Friday and gradually increase day by day to around seasonal
normals by Sunday. /Fugazzi
&&
Aviation...
12z tafs: low level moisture will promote areas of low clouds and
patchy fog this morning around north taf sites...especially the
kgeg-kcoe corridor. At the same time an unstable upper low
remains in the region. This unusual set-up degrades confidence in
the persistence and timing of low clouds and fog. However expect
IFR conditions through 15-17z, before mixing mixing and increased
instability with the nearby low helps lift the clouds. The
afternoon will bring an increased threat of rain showers and
perhaps some thunderstorms. The overall best threat will remain
closer to Cascades, near the center of the low, I.E. Near keat.
The overall threat wanes after dark with the loss of daytime
heating, but a few showers will still remain possible near the
Cascades and Blue Mountains. /J. Cote'
&&
Preliminary point temps/pops...
Spokane 60 41 60 42 64 45 / 20 20 40 30 20 10
Coeur D'Alene 59 39 59 41 64 43 / 20 20 50 40 20 20
Pullman 60 38 56 40 64 42 / 20 20 50 20 20 20
Lewiston 66 45 63 45 71 49 / 20 10 50 20 20 20
Colville 63 38 67 42 69 43 / 30 30 50 50 40 30
Sandpoint 57 36 62 40 63 40 / 30 20 60 50 30 30
Kellogg 55 39 56 41 60 44 / 20 20 60 50 40 40
Moses Lake 66 43 68 46 72 47 / 20 20 30 20 10 10
Wenatchee 62 44 66 48 69 49 / 40 30 40 20 10 10
Omak 64 38 68 42 69 43 / 30 30 40 30 30 20
&&
Otx watches/warnings/advisories...
Idaho...freeze warning until 7 am PDT this morning for central Panhandle
mountains-Idaho Palouse-Lewis and southern Nez Perce
counties.
Washington...freeze warning until 7 am PDT this morning for Washington
Palouse.
&&
$$
