Area forecast discussion
National Weather Service Spokane Washington
430 am PDT Mon may 20 2013
Synopsis...
high pressure will provide mainly dry, warm conditions today. A
strong storm system enters the region on Tuesday and Wednesday,
with a good threat of rain, and the potential for thunderstorms.
The system will also usher in significantly cooler temperatures.
Cool, showery weather will continue into the weekend.
&&
Discussion...
today and tonight: the inland northwest will be mainly dry and
mild under the influence of high pressure, while a pattern-changing
trough approaches the coast. The flow transitions from northwest
to southwest by late this afternoon as the mid-level ridge axis
shifts into central Idaho and north California. Lingering low-grade convective
instability is depicted around the Panhandle mountains this afternoon.
However broad-scale subsidence with the ridge should keep the threat
of showers in the mountains, if any occur at all. Otherwise the
higher clouds that are reaching over the Cascades this morning
will continue to spread in, coupled with some afternoon cumulus
build-up. The developing southwest flow and subsidence will
promote milder temperatures today as compared to Sunday. Look for
temperatures near to slightly above normal. Tonight low pressure
and a cold front comes toward Cascades while the long-wave trough
deepens and takes on a more meridional (north-south) orientation.
This will bring increasing rain chances to the Cascades overnight
and primarily near the crest. Elsewhere clouds will begin to
thicken and lower from the west. /J. Cote'
Tuesday and Tuesday night: a strong low pressure system will finally
push into our region on Tuesday bringing with it the almost certain
chance of widespread rain for all of the inland northwest. This system
has been slowly moving down from the Gulf of Alaska and once in
place near the area will persist for quite some time. Quite a slug
of moisture looks to push inland with the initial barrage with the
brunt of the rainfall occurring near the crests of the Cascades
and later pushing further into the forecast area to bring lighter
amounts east of the crests.
Timing on this system looks to be in good agreement amongst the
models with all models showing the initial rainfall reaching the
Cascade crests by Tuesday morning and pushing further east
throughout the day Tuesday and into the evening for the Idaho
Panhandle. Ahead of the strong cold front will be some instability
that could trigger some thunderstorms for the eastern part of Washington
and the Idaho Panhandle in part due to the strong daytime heating due
to the southerly flow and an increased moisture push associated
with the incoming system. Once sunset occurs and overnight cooling
begins most all convection will come to an abrupt end...but the
rain showers will persist into the overnight hours. Snow levels remain
high until the cold front passes behind the initial rain which will
lead to a transition from rain to snow in the high elevations of
mainly the Cascades. With a bulk of the precip falling as rain we
will see an immediate rise in the local streams and rivers which
will be something we will continue to monitor. Although temps drop
after the passing of the cold front along with snow levels...the
lower snow levels will lead to less direct runoff in turn allowing
some storage of water and allowing streams to not rise as rapidly
as if all quantitative precipitation forecast fell as rain. Overall this time frame will be
characterized as a transition period from warm and dry to wet and
cool for the entire region. /Fliehman
Wednesday through Sunday...models continue to be very
insistent...in good agreement and consistent over multiple
previous runs...in parking a deep upper level low smack dab over
the forecast area through most of the extended period. Thus
confidence is high in anticipation of a cool and periodically wet
forecast from mid-week Onward through at least early next weekend.
The main uncertainties concern exactly where and when areas of
extra dynamic and deformation lift/focus will wind up...and thus
delineating periods of dense/frequent showers or downright
stratiform steady rain from periods of merely hit-and-miss or
intermittent showers. At this time it appears the Tuesday night
through Thursday morning time frame will be the wettest period
with the potential for a good deformation band slowly pivoting
from south to north through the entire region...potentially
bringing 3/4 of an inch or more of rain to the valleys of the
northern zones and Cascades. Even higher quantitative precipitation forecast is likely in the
mountains...but much of this will be locked up as a few inches of
wet snow above 4000 to 5000 feet through the period and
unavailable to swell rivers with mountain run-off.
There may be a few other periods of steadier showers over some
areas of the forecast area...but beyond Thursday the sub-synoptic
model details are suspect and inconsistent with each other in
nailing this down. Still...with this upper low taking up
residence it is certain that just about all zones will be at risk
for at least scattered showers each day through the end of the
week. The potential for thunderstorms is also possible...but this
will depend mainly on sun breaks allowing the generation of sb cape
during the afternoon hours and this Prospect is too ephemeral to
nail down at this time.
The other main impact of this pattern will be high temperatures
well below normal for this time of year...while overnight lows
will probably remain only slightly cooler than normal given a
moist low level air mass and heavy clouds reducing radiation
potential. /Fugazzi
&&
Aviation...
12z tafs: high pressure moves across the region today. Patchy fog
will be found across NE Washington/nrn Idaho through 15-17z, including in the
vcnty of the kgeg/ksff/kcoe taf sites. Otherwise dry/VFR
conditions are expected. Mid and high clouds increase from the
west late tonight into Tuesday morning as low pressure and a cold
front approach. /J. Cote'
&&
Preliminary point temps/pops...
Spokane 71 51 75 43 51 38 / 0 0 10 80 90 40
Coeur D'Alene 71 46 77 45 48 39 / 0 0 10 70 90 50
Pullman 72 48 74 42 50 37 / 0 0 0 70 70 30
Lewiston 79 52 81 48 58 41 / 0 0 0 60 70 30
Colville 78 45 78 46 56 40 / 0 0 40 80 80 60
Sandpoint 71 44 77 45 52 39 / 10 0 20 70 80 70
Kellogg 70 50 76 44 46 37 / 10 0 10 60 90 70
Moses Lake 79 51 74 46 61 41 / 0 0 40 50 30 30
Wenatchee 77 53 67 46 58 43 / 0 0 50 60 40 40
Omak 78 47 71 46 60 40 / 0 0 70 90 70 50
&&
Otx watches/warnings/advisories...
Idaho...none.
Washington...none.
&&
$$
