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Area forecast discussion 
National Weather Service Spokane Washington
502 am PDT sun may 24 2015

Synopsis...
today will be another warm day with temperatures in the upper 70s
to low 80s over much of the inland northwest. The arrival of a
cold front tonight will cool temperatures closer to average on
Memorial Day. The front will also bring an increasing chance of
showers and thunderstorms to the region. Some thunderstorms across
the northern mountains may become strong with hail and gusty
outflows winds possible. Slow moving afternoon thunderstorms
Monday, Tuesday, and Wednesday will be capable of producing
locally heavy rain. Next weekend looks to be quite warm with
Summer-like temperatures in the mid 80s to low 90s.

&&

Discussion...
Sunday through Monday night: the weather over the region will be
under the influence of an upper level low pressure system
currently residing over southwest British Columbia. This low will drop further
south today. Not much will change at the surface as much of the
region will continue to be moist at the surface. Dew point
temperatures are generally running in the 40s and low 50s across
the region. In fact, the little bit of drier air that crept its
way into the northern half of the region is already getting
supplanted by more moist air from the south. The 00z GFS solution
seems to dry out the boundary layer too much for this afternoon.
Some moisture should mix out as the inversion breaks; otherwise, I
do not see an air mass change and I prefer the 00z NAM depiction
of cape profiles across the region. The more unstable NAM has up
around 500-1000 j/kg of cape to tap into for this afternoon. The
mountains will see the best chances for breaking the capping
inversion with convection possible by the early afternoon hours.
This will include the northeast blue mtns and the Idaho Panhandle.
The low tracking down out of British Columbia will also provide some lift,
especially across the northern portion of the region. This weather
system is expected to produce more organized convection from the
east slopes of the northern Cascades over to the western basin and
into the Okanogan Highlands by the mid afternoon hours. Chances
for thunderstorms will then increase eastward across the basin,
into the northeast mtns by the late afternoon hours; this trend
will continue into the overnight hours where the Panhandle will
see a chance for widespread showers and a chance for thunderstorms
through the night.

There will also be some shear to work with this afternoon. Models
indicate 0-6 km unidirectional shear of up to around 30-35 kts in
the Cascades north of Lake Chelan over into the northeast mtns by
the evening. These northern mountain areas will see a chance for
stronger thunderstorms today. There will be the possibility for
organized convection with hail and gusty outflow winds as
potential threats. Thunderstorms will also bring the potential
for frequent cloud to ground lightning strikes and heavy
downpours. The good news is that these storms should be moving at
about 20 mph to the east and northeast. This is not a fast storm
motion, but will lesson the risk for flash flooding. With that
said, there will be the potential for heavy rainfall over some of
the 2014 burn scars in the Cascades and in the Okanogan Highlands
(most notably the Carlton complex and Devils Elbow burn scars).

Stronger storms with large hail and strong outflow winds will be
less of a concern on Monday as the upper level low shifts further
over the region. Storm motion will be a bit slower, so heavy
rainfall with flooding impacts is expected to be the primary focus.
This would include the potential for flash flooding and/or debris
flows. Thunderstorms will be possible over recent burn scars
across the east slopes of the northern Cascades and in the
Okanogan Highlands, so these areas will need to be monitored in
particular.

One more "hot" day is in store for today. High temperatures will
once again be up in the 80s for many locations. Temperatures will
cool a bit on Monday, but will only cool to right around seasonal
normals. /Svh

Tuesday through thursday: the Cascade burn scars stand a good shot
of afternoon and evening thunderstorms Tuesday through Thursday.
The medium range models produce moderate levels of surface based
instability over The Spine of the northern and central Washington
Cascades several days in a row this week. Tuesday could produce
the best coverage of showers and thunderstorms over the inland
northwest (and the cascades) as a slow moving, vertically stacked
low sets up over the Columbia Basin. The models are in good
agreement that the low will drift into the southern Idaho
Panhandle by Wednesday afternoon. By Thursday, remnants of the low
will likely be well south of our region, but residual low level
moisture combined with afternoon instability will once again
generate scattered afternoon showers and thunderstorms. The
synoptic set up Tuesday through Thursday is not our "text book"
heavy rain pattern which features southerly flow and high
precipitable water values. However, the slow progression of the
upper low through our region combined with weak storm steering
flow and moderate instability will likely yield cells capable of
heavy rainfall. It is tough to envision Big Burn scars like
Carlton (methow valley), Mills Canyon (south of lake chelan), and
Chiwaukum (hwy 2) not being tested this week. Elsewhere, the
thunderstorms Tuesday through Thursday don't appear to be much
more than our typical late Spring storms. Weak shear and modest
instability suggest the potential for locally heavy rain. The lack
of shear will probably limit large hail to relatively brief
pulses.

Friday and saturday: a warm high pressure ridge is expected to
build over the western U.S. Friday into next weekend. The 850mb
temperatures progged by the GFS, ECMWF, and Canadian models are
in the 18-20c neighborhood by Saturday. MOS guidance from the GFS
and European model (ecmwf) looks too cool, and raw model numbers look more
realistic given the warm air aloft. It looks like upper 80s and
lower 90s by Saturday. If early week rains pan out, residual
surface moisture could contribute to a "muggy feel" at least by
our region's standards next weekend. /Gkoch

&&

Aviation...
12z tafs: abundant boundary layer moisture is expected to result
in low cumulus development through the morning at the kpuw taf
site with MVFR cigs possible. An upper level low pressure system
will then push into the region out of southwestern British
Columbia this afternoon. This will result in scattered showers and
thunderstorms primarily east and north of the keat taf site. Best
chances for thunderstorms will be in the Cascade mtns north of
Lake Chelan over into the northern Panhandle. Thunderstorms will
develop over the Cascades early in the afternoon and then spread
eastward through the day into the overnight hours. There is
moderate confidence that at least isolated thunderstorms will
impact the kmwh, kgeg, ksff and kcoe taf sites. Confidence is low
that thunderstorms will impact keat, kpuw and klws taf sites. /Svh

&&

Preliminary point temps/pops...
Spokane 79 54 71 51 73 52 / 10 50 60 40 50 40
Coeur D'Alene 80 53 70 49 73 50 / 10 30 60 40 50 40
Pullman 74 50 70 45 71 47 / 10 20 30 10 30 30
Lewiston 81 54 77 52 78 55 / 20 20 30 10 30 30
Colville 83 53 72 50 76 50 / 40 70 70 60 70 40
Sandpoint 79 52 70 48 71 50 / 10 50 70 60 60 50
Kellogg 78 51 71 46 72 47 / 10 30 70 50 60 50
Moses Lake 83 55 79 54 80 53 / 10 20 10 20 40 20
Wenatchee 82 55 78 56 78 58 / 20 10 20 20 50 30
Omak 83 53 76 51 78 51 / 60 60 60 50 60 40

&&

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

$$

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