Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Spokane Washington
336 PM PST sun Dec 21 2014

Synopsis...
the active weather from the past few days quiet down Monday.
Expect dry conditions and cooler but still above normal
temperatures tonight through Monday night. Light mountain snow and
valley rain or snow will return late Tuesday. An organized storm
system will impact the region around Christmas eve. This will
bring the potential for moderate to heavy snow in the mountains
and light accumulations to the valleys.

&&

Discussion...
tonight through Tuesday...the forecast area is entering a quiet
break period tonight which will last through Monday night before
things get active again during the day on Tuesday.

The Pacific satellite image reveals the deep and strong fetch of
Pacific moisture...the same feed that brought a general 1/2 to
over 1 inch of rain to many lower elevations yesterday and up to a
foot of heavy wet snow to the mountains and some of the northern
Cascade valleys...is now directed into Oregon and has essentially
moved out of the forecast area. The northern fringe of this axis
is still producing mountain showers over the Cascades and the
Idaho Panhandle mountains but the basin has benefited from a rain
shadow in fast westerly mid level flow today.

Models are in decent agreement in creating a building ridge just
off the northwest coast which will keep the moisture feed buckles
a directed to the south of the forecast area and allow a weak
bleed of dryer Canadian Continental air to invade from the north.
This will eventually end any lingering shower activity tonight
and Monday and allow some sunshine and decreasing winds over the
region. Winds will probably remain elevated enough to keep fog to
a minimum Monday morning...but by Tuesday morning a placid high
pressure to the east will once again allow pooling of moist low
level air in the deep basin and banked against the Cascades for a
higher threat of morning fog and low clouds.

During the day on Tuesday the deep Pacific moisture feed will once
again become favorably directed into the northwest coast...with
the deepest axis clipping the northern forecast area as a weak
warm front lifts through the region. This will promote increasing
probabilities of light mountain snow and valley rain or snow by
Tuesday afternoon mainly north of Interstate 90 with light
accumulations possible in the valleys near the Canadian border and
the Cascades. /Fugazzi

..moderate threat for snow to impact mountain passes Wednesday...

Tuesday night through thursday: a wet storm system continues to be
advertised via the latest forecast models as a trof of low pressure
drops out of the Gulf of Alaska and interacts with rich, narrow plume
of moisture. There is still a split in model camps with the
GFS/Canadian in one corner and European model (ecmwf)/sref/NAM in the other. Over
the last 24 hours...the European model (ecmwf) has trend northward ever so slightly
and now lines decently with the NAM/sref and at this time, this is
the path our forecast has taken. However, we cannot completely
ignore the GFS/Canadian given its consistency over the last 2-3
days...and all guidance will be monitored closely. Generally
speaking, the plume of subtropical moisture will sag into the
region starting Tuesday night with precipitation developing over
the northern mountains/valleys then sagging south through the day
on Wednesday and finally departing to the southeast Thursday
morning. Snow levels will start off low in the north with light
valley snow possible but look to surge between 4-6k ft during the
heaviest precipitation before crashing back near valley floors
Wednesday evening/Thursday morning. A few light snow showers will
linger over the Idaho Panhandle Thursday morning but for the most
part, drier conditions coupled with seasonal temperatures will
return Christmas day.

* Snow: most mountain locations will receive some snow. The heaviest
mountain snow look to be along the Cascade crest and Idaho
Panhandle where 6-12 inches will be possible. Confidence is
moderate to high but actual snow levels will fluctuate some so
particular accumulations at any level below 6000 feet carry
moderate to low confidence. A few inches of snow will be
possible in the valleys of northeastern Washington and northern Idaho
including Bonners Ferry, Sandpoint, Metaline Falls, and Ione.
Confidence is low at this time due to uncertainty with the
timing of rising snow levels. 2-4 inches will be possible in the
blue mtns, Camas Prairie, and benches surrounding the central
Panhandle mtns of Idaho Wednesday night into Thursday morning
following the transition from rain to snow. This includes
locations like Anatone, Winchester, Nez Perce, Deary, and Santa
to name a few. Confidence is moderate for at least an inch but
lower for additional accumulations due to the timing of the
changeover. Trace to one inch will be possible in the valleys
east of a line from Republic to Pomeroy with the exception of
the already mentioned locations. Best chances for any
accumulations will be above 2000 feet but this also carries low
confidence with snow levels falling as precipitation ends.

* Mountain passes: snow is likely to impact mountain passes along
I-90, Hwy 2, and Hwy 20 starting Tuesday night and continue
through Wednesday. For additional travel information: click the
seasonal links over the map on our home Page or visit the Washington or
Idaho dot sites.

Snoqualmie: will start off as rain Tuesday evening but should switch
to snow by early Wednesday morning. Amounts could be light to
moderate.

Stevens pass: will be a mix of wet snow and rain Tuesday evening but
should quickly switch over to snow overnight and remain snow on
Wednesday. Amounts could be moderate.

Lookout pass: will start off as snow Tuesday night, may briefly mix
with rain or become heavy wet snow overnight, then become all snow
for Wednesday. Amounts could be heavy.

Sherman pass: snow Tuesday night into Wednesday morning then tapering
off through the day. Amounts will not be heavy.

* Confidence levels: large changes are possible with the storm if
model trends switch toward the GFS/Canadian. Consequently,
confidence is moderate to low. Confidence is moderate to high
for mountain snow, and the threat (lack of) for significant
valley accumulations. If model trends go toward the
GFS/Canadian, winds will be stronger, the precipitation will be
slower to traverse the region, and snow levels will be higher.
/Sb

Thursday night to sunday: seasonably cold conditions and a few
opportunities for snow mark this period with the region in a
northwest flow. Thursday night into Friday one disturbance exits
and a shortwave ridge builds in. Snow shower chances will linger
across southeast Washington through the Idaho Panhandle mountains in the
northwest flow, but overall precipitation amounts look light. The
rest of the inland northwest is forecast to be dry. Look for some low
clouds, and maybe some patchy fog, across the eastern third of Washington
and north Idaho with the low level southwest flow and lingering bl
moisture. Sometime between Friday night and Sunday models bring
the next frontal wave and precipitation chances in. There are
disagreements over the timing. The GFS/Gem/dgex bring the feature
in Friday night and saturday; the European model (ecmwf) holds it off until
Saturday night into Sunday. Some favor is given to the quicker
solutions, which exhibit some agreement and run-to-run
consistency. The ECMWF, on the other hand, has exhibited wider
swings. Overall look for precipitation chances throughout next
weekend, which some of the broader chances on Saturday and
Saturday night. /J. Cote'




&&

Aviation...
00z tafs: overall a fast westerly flow will dominate the region
for the next 24 hours as surface high pressure builds in from the
west. This will promote some lingering valley rain and mountain
snow showers slopping over the Cascade crest and due to upslope
flow over the Idaho Panhandle. Early in the taf period a few
showers may survive to as far east as the keat taf site before
decreasing overnight. Showers may form near the kpuw and klws taf
sites between 00z and 03z tonight. Despite this threat all taf
sites should remain VFR this evening. Decreasing winds overnight
with a moist low level air mass may create a few hour period of
IFR or low MVFR stratus at the kgeg vcnty taf sites between 10z
and 18z Monday...otherwise VFR conditions are expected at all taf
sites. /Mjf



&&

Preliminary point temps/pops...
Spokane 32 40 29 41 32 37 / 0 0 0 20 60 80
Coeur D'Alene 32 40 29 40 32 37 / 10 10 0 20 70 80
Pullman 37 43 30 43 33 40 / 20 10 0 10 30 80
Lewiston 39 47 33 46 36 43 / 20 10 0 10 20 80
Colville 28 38 27 37 31 34 / 0 0 10 50 70 40
Sandpoint 30 37 27 36 33 35 / 10 10 10 30 70 70
Kellogg 32 37 27 37 33 35 / 50 50 0 30 70 80
Moses Lake 34 44 30 43 34 40 / 10 10 10 10 20 60
Wenatchee 35 43 33 41 34 39 / 20 10 10 30 20 50
Omak 28 37 27 35 30 34 / 20 0 10 40 40 30

&&

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

$$

National Weather Service Glossary of Abbreviations