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Area forecast discussion 
National Weather Service Spokane Washington
439 am PDT Tuesday Jul 22 2014

Synopsis...
a strong storm system will approach the region today and pass
through on Wednesday. The threat for thunderstorms will increase
across the southeast today then spread region-wide tonight and on
Wednesday. Some storms could be strong and wet bringing the
potential for strong winds, hail, and heavy rainfall. Temperatures
will rebound by the weekend and be accompanied by dry conditions.

&&

Discussion...

Today and tonight...longwave pattern change underway with longwave
ridge pushed away to the east as longwave trof off the coast moves
further east and exerts more influence over the Pacific northwest.
The result is a more progressive and active southwest to northeast
flow between the offshore trof and the intermountain West Ridge
which is more favorable for allowing showers and thunderstorms to
form in Oregon and move north and northeast into eastern
Washington and northern Idaho today and tonight. With this in mind
sky grids have considerable cloud cover to account for the
expected convection which also supports forecast Max temps on the
cool side of normal and overnight lows on the warm side of normal.
Precipitation amounts are expected to be light due to both the
typically spotty nature of convection and that the lower levels of
the atmosphere remain quite dry as there has not been recent
widespread rainfall for quite a number of days now. /Pelatti

Wednesday through Friday...models continue to be in good
agreement overall in depicting the arrival and passage through the
forecast area of an anomalously strong (for this time of year)
upper low pressure Wednesday and Wednesday night. The latest GFS
and ec agree well with each other...while the NAM is in the
ballpark with the other guidance with a slightly more northerly
track as it ejects inland. Overall...ramifications of this
evolution should not create significant differences in sensible
weather for the region as this system arrives and passes.

Two regimes appear to impact the forecast area during this time
frame. The first will be over the Cascades east slopes and perhaps
the deep basin/Okanogan Valley tucked against the Cascades where
the heavy moisture ahead of the incoming low will focus starting
Tuesday night and maximizing on Wednesday. This region will
probably wind up the wettest area as a result of strong
differential vorticity dynamics and jet divergence in the
immediate vicinity of the upper low center...with instability
generated by the inter-trough cold pool aloft. Showers will be
frequent over this area...and perhaps constant near the
crest...interspersed with occasional thunderstorms which have the
potential to lay down some heavy rain swaths under their cores.
This will benefit the fire fighting efforts in these areas with
much needed rain but may also prove to be a double edged sword
with the very real possibility of flash flooding and debris flows
on burn scars if any particular spot is run over by multiple storm
cores. A general blend of all the available model quantitative precipitation forecast guidance
suggests storm total rain fall approaching or exceeding 1 inch
near the crest and tapering down to a couple tenths of an inch in
the Wenatchee-Waterville Plateau-Omak corridor. This estimate does
not account for the potential for heavy storm cores or training
cells which could produce locally higher amounts anywhere in the
area.

The second threat regime will be over the eastern Columbia Basin
and Idaho Panhandle where the cold front associated with the upper
low may promote a late afternoon/early evening line of organized
convection with gusty winds...copious lightning...hail and short
bursts of heavy rain moving from southwest to northeast...with a
breezy and showery regime through the rest of the night. The
potential for this event and the severity of the storms will be
somewhat dependent on how warm it gets during the relatively benign
daytime heating period over the eastern basin. Fewer clouds
allowing more solar heating will aggravate the situation with
higher values of surface based instability when the surface front
chugs through late in the day.

At this time...confidence is high for the wet regime expected over
the Cascades...and moderate for the strong evening squall line
potential over the east.

From Thursday through Friday a drying trend will commence with
lingering showers and isolated thunderstorms over the northeast
Thursday and perhaps over the far north Cascades. Thursday night
the upper low will move out of the region into the Canadian
prairie and a zonal-ish weak ridging regime will begin.
Temperatures Wednesday and Thursday will be heavily dependent on
cloud cover and precipitation areal extent...with the safe bet on
well below average for a change...with a noticeable warming trend
on Friday. /Fugazzi

Friday night through Wednesday morning: a gradually building
ridge will continue to dry things out and warm things up into next
week. Saturday will be the most normal day of the far extended
forecast, with zonal flow over the inland northwest. Temperatures
will be right about where they should be (80s to low 90s) with
light winds. But Sunday, the ridge begins to nudge northward into
the northern rockies, with warming 850 mb temperatures each day
over the region. We will be right back into the "hot" category by
Monday and Tuesday (and even a few days after?). very dry air at
the surface will lead to low relative humidity values during the afternoon. These
conditions will not be good news on the fire weather front. TY

&&

Aviation...
12z tafs: as a low pressure system moves closer to the region the
low level flow will become more from the south and southwest. This
change will allow for moisture and energy to stream across the
aviation area for the next 24 to 48 hours. Regardless of the light
showers and thunderstorms expected today and some wildfire smoke
lingering in spots VFR conditions are expected to prevail.

&&

Preliminary point temps/pops...
Spokane 78 60 83 52 72 53 / 20 40 50 70 20 10
Coeur D'Alene 78 58 84 53 71 51 / 30 40 40 80 40 10
Pullman 79 60 84 49 71 47 / 40 40 40 70 10 10
Lewiston 87 63 90 58 80 56 / 40 40 40 60 10 0
Colville 78 56 81 51 74 49 / 20 30 60 80 50 10
Sandpoint 74 56 85 51 68 46 / 30 40 50 80 50 20
Kellogg 78 56 85 52 67 51 / 50 40 40 80 40 10
Moses Lake 83 65 83 56 80 56 / 20 30 50 40 10 0
Wenatchee 79 62 75 59 77 59 / 40 30 70 40 10 0
Omak 83 58 77 55 79 53 / 30 20 80 60 50 10

&&

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

$$

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