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000
FXUS66 KOTX 202131
AFDOTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
231 PM PDT Mon May 20 2013

.SYNOPSIS...
The mild spring weather will come to an end. A cold front will
sweep through the region Tuesday with showers likely, a chance of
thunderstorms, breezy winds and cooler temperatures. Cool and
showery conditions will prevail through much of the week, but will
some improvements for the holiday weekend.

&&

.DISCUSSION...
Tonight: High pressure will weaken overnight as mid and high level
clouds increase across the region. Overnight lows will be mild as
winds turn out of the south to southeast and generate weak warm
air advection.

Tuesday: A deep Pacific low will swing toward the WA coast by
Tuesday morning and push a strong cold front across the Cascades
by late morning and reaching the Idaho by late afternoon. Deep
southerly flow aloft will over the region with the front taking on
a slight negative tilt through the day. There is the possibility
of some elevated convection ahead of the front Tuesday morning as
seen by the steeper mid level lapse rates from Wenatchee to Omak.
But once the front pushes through this area by midday, wet, cool
and stable conditions will develop rather quickly. The front will
march across the rest of eastern Washington while low level wind
profiles show increased directional shear, while the speed shear
increases aloft. Both the NAM and GFS point to a swath of
increased surface based instability from Repubic, to Colville and
Newport to Sandpoint Tuesday afternoon ahead of the front. The
main threat will be large hail with gusty winds, and may even see
a few strong longer lived thunderstorms. Temperatures will range
greatly across the region from low to mid 60s in the Cascades to
mid to upper 70s in north Idaho. /rfox.

Tuesday night through Friday...Confidence is high that the Inland
Northwest will be under a cold and wet weather pattern during the
mid-week period. Models are in very good agreement on the large
scale pattern of a closed upper low settling over the region. This
system will benefit from strong upper level dynamics courtesy of a
90kt jet circling the low, placing much of the forecast area under
strong DifQ forcing. There will be plenty of moisture advection on
southerly flow ahead of the cold front, and this moisture will
continue to wrap around the low as it hovers over the region.

The area of highest concern will be the northeast quadrant where a
long-lived deformation zone will set up from Tuesday night through
Wednesday. A model comparison of mid level deformation zones leads
to higher confidence of where the heaviest precip will fall. For
those areas that show good model agreement, PoPs and QPF have been
increased. This shows the focus squarely on the northeast zones.
This is reinforced by areas of strong upper level forcing around the
periphery of the low along with a persistent theta-e ridge over
northeast.

QPF...We could be looking at an inch to an inch and a half of liquid
falling over the northern third of the forecast area over a 36 to 48
hour period. Around a half to three-fourths of an inch can be
expected for the highway 2 corridor with about a quarter inch for the
basin.

Temps/Snow levels...One saving grace will be that a good deal of the
QPF will be falling as snow in the higher mountains Wednesday. The
NAM shows cooler at 850 mb than other models but it has also been
more consistent, and now the 12Z ECMWF is trending cooler than its
previous run. Also taken into consideration is that with heavy rain,
there is the possibility of the column cooling further to trigger a
change-over to heavy snow. Snow levels across the Idaho panhandle
have been lowered to around 2500-3000 ft for Wednesday into
Wednesday night. There is a risk of accumulating snow for the high
mountains but also for an inch or two for the Idaho passes. The
mid to low level snow pack will not last long as temperatures will
return to near seasonal normals by the weekend.

As the system meanders around the Pacific Northwest it will start to
fill on Thursday and the best forcing will shift away from the
forecast area, into southern Oregon and southeast Idaho. We will
still be in a moist and modestly unstable environment, so if we see
any breaks in cloud cover we could see some convection firing up
over the southeast zones during the afternoon and evening hours
Thursday and Friday but confidence is rather low for now. /Kelch

Saturday through Monday: The longwave upper level trough will
linger over the Pacific Northwest and British Columbia over the
weekend and into early next week. Does this mean that the holiday
weekend will be a washout? Not necessarily. There is decent
agreement between the GFS, ECMWF and Canadian models that the
upper trough will become sheared apart by Saturday with one upper
low over the Canadian Rockies and another upper low digging into
northern California. It is tough to go with a dry forecast
Saturday and Sunday given the upper trofiness over the region, but
it does appear that the best forcing over the weekend will be both
north and south of our region. Showers in this regime will
probably be favored over the mountainous zones with the bulk of
the showers occurring during the afternoon and evening hours
(during peak heating). There is loose agreement between the 12Z
operational models that the aforementioned northern California low
will lift into Idaho on Monday. If this occurs, there will be more
widespread showers and possibly thunderstorms to round out the
holiday weekend on Monday. /GKoch

&&

.AVIATION...
18Z TAFS: High pressure will be over the TAF sites today with
light winds and VFR conditions. The patchy fog has lifted and
a few cumulus build ups will over the ridges to the north and east
of KGEG. Mid and high clouds will be on the increase tonight. A cold
front will reach the Cascades by 18z. Light showers are possible
ahead of the front in the KEAT vicinity before 18z. /rfox.

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        51  75  41  52  36  60 /   0  20  90  90  40  40
Coeur d`Alene  46  77  42  49  36  59 /   0  30  90  90  50  40
Pullman        48  73  39  51  35  60 /   0  10  80  80  30  40
Lewiston       52  81  45  59  40  67 /   0  10  80  70  30  40
Colville       45  78  43  55  37  64 /   0  50  90  80  60  50
Sandpoint      44  76  43  51  36  58 /   0  30  90  90  80  50
Kellogg        50  76  41  45  34  54 /   0  30  90  90  70  60
Moses Lake     51  73  43  63  40  68 /   0  50  50  30  30  30
Wenatchee      53  66  43  59  43  64 /   0  60  50  40  40  40
Omak           48  71  42  60  38  65 /   0  80  80  70  50  50

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.
&&

$$