Area forecast discussion
National Weather Service Spokane Washington
237 PM PDT sun may 19 2013
Synopsis...
evening showers will begin to wind down as the sun sets and a
ridge heads into the region. By Monday the ridge will become
firmly established over much of the region providing a dry and
warm day before a strong storm system enters the region on
Tuesday. Tuesday's weather system will bring a good threat of showers
and thunderstorms followed by significantly cooler temperatures.
Cool and showery weather will envelop the region from mid- week
into the Holiday weekend.
&&
Discussion...
tonight and Monday...elongated upper level trough remains fixed
over the eastern half of the forecast area late this afternoon
per the latest water vapor loops. The best moisture per satellite
looks like it extends east of a line from Omak to the Moses Lake.
This region correlates fairly well with the best region of
instability per the rapid refresh model which shows MLCAPE values
between 100-300 j/kg mainly over the NE corner of Washington and
extending across most of the Idaho Panhandle. Showers and even a
thunderstorm or two will likely continue into the early
evening...especially over the NE quarter of Washington and the
northern half of the Panhandle...however since there is very
little if any trigger in the mid/upper atmosphere evident on
satellite things should taper off rapidly during the evening.
After the threat wanes...the focus will turn to an upper level
ridge axis meandering into the region from the SW. Plenty of dry
air is evident across of much of Oregon and western Washington and
this will track into the region later tonight and Monday. The
decreasing moisture will contribute to a drier forecast while
increasing stability will also work against any convection. The
one exception could occur tomorrow afternoon near the
Montana/Idaho border where residual instability and moisture could
trigger a shower during the afternoon. Model soundings also
suggest a small possibility of thunderstorms...but better chances
will likely occur east of the border as the ridge works into Idaho
during the day. Fx
Monday night through wednesday: we will transition from a dry and
above average temperature regime to a very wet and below average
temperature regime.
Monday night through Tuesday morning: a very strong closed upper level
low pressure system will move south down the coast line out of the
Gulf of Alaska. This will put the inland northwest under southerly
flow ahead of the cold front. The forecast will remain dry with
the exception of the Cascade crest where starting Tuesday
morning they may start to see rain. Temperatures will remain at or
above average.
Tuesday afternoon through wednesday: the low will push inland and
the chances of widespread rain will spread from west to east
starting Tuesday afternoon. Tuesday afternoon there may be enough
instability for an isolated thunderstorm ahead of the front across
extreme eastern Washington and north Idaho. The front will push through early
Wednesday morning. Widespread rain can be expected everywhere. Have
increased chance of precipitation as well as how much liquid
precipitation we are expecting. Temperatures will plummet from
Tuesday to Wednesday. A 10 degree drop in temps across the
Cascades and a 20 to 25 degree drop in temps across eastern Washington and
north Idaho can be anticipated. Winds will be breezy from the
southwest 10 to 15 mph with gusts around 20 mph. Snow levels will
lower to about 4k ft by Wed with snow expected in the mountains. /Nisbet
Wednesday night through Sunday...confidence is high that the region
will be under a very slow moving upper level low pressure center
that will bring an abundance of clouds, unseasonably cool daytime
temperatures along with a good chance of precipitation for the
inland northwest during the extended forecast period. If there were
to be any breaks in the overcast during daytime heating, and this is
fraught with uncertainty, some convection would be possible. For
now, confidence is too low to add thunder to the weather grids.
/Kelch
&&
Aviation...
18z tafs: VFR conditions expected to affect all taf sites through
the period...although brief MVFR cigs will be possible through 20z
at Coe due to the presence of convective cloud bases just below
030. There will be a threat of -shra at puw...Coe...geg...and sff
however the limited coverage suggests that if an Airport is hit
the wet weather would be brief. Do not believe there will be any
thunder at any of the sites as best instability will remain over
NE Washington. Skies will clear overnight at all sites with the upper
level ridge moving overhead. Fx
&&
Preliminary point temps/pops...
Spokane 46 72 51 76 44 51 / 10 0 0 20 90 90
Coeur D'Alene 45 71 46 76 45 49 / 10 0 0 10 60 90
Pullman 42 71 48 75 42 50 / 10 0 0 10 70 60
Lewiston 48 79 52 81 49 57 / 0 0 0 10 60 60
Colville 44 78 45 77 46 55 / 20 0 0 40 90 80
Sandpoint 42 72 44 76 45 52 / 20 10 0 20 70 80
Kellogg 41 69 50 76 45 47 / 20 10 0 10 60 90
Moses Lake 48 80 51 73 47 60 / 0 0 0 40 50 30
Wenatchee 49 78 53 67 46 57 / 0 0 0 50 50 30
Omak 43 78 47 70 46 58 / 0 0 0 70 90 60
&&
Otx watches/warnings/advisories...
Idaho...none.
Washington...none.
&&
$$
