Area forecast discussion
National Weather Service Spokane Washington
455 PM PDT Mon may 20 2013
Synopsis...
the mild Spring weather will come to an end. A strong 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 Washington 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 Republic, 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 quantitative precipitation forecast 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.
Quantitative precipitation forecast...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
quantitative precipitation forecast 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 European model (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, European model (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...
00z tafs: a deep area of low pressure will track into the region
during the next 24 hrs. Mid and high clouds will be on the increase
tonight and showers are expected to develop along btwn keat-komk
during the early morning hrs with the slight potential for an
elevated T-storm. More alarming to the aviation community will
be the passage of a strong cold front through the region through
the aftn hrs. The front is expected to cross through keat/kmwh
arnd 18z and kgeg-kcoe-kpuw-klws near 21z. There is a decent
potential that thunderstorms will erupt along the front upon
reaching the Washington/Idaho border...some which may become strong or severe
over NE Washington and nrn Idaho. Gusty west/SW winds are expected behind the
front along with steady pcpn aft 00z. /Sb
&&
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...
Idaho...none.
Washington...none.
&&
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