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Area forecast discussion 
National Weather Service Great Falls Montana
535 am MDT Mon Sep 22 2014

Discussion...

As of ~1000z, cut-off mid-level low, within a much larger scale
upper-level ridge, was located along the Utah/Idaho border and was
slowly moving east-northeast as seen on WV imagery loop. Enough elevated
instability associated with the low for lightning strikes as far
north as northern Utah. Cyclonic flow around this low in the mid
levels will continue to advect higher mixing ratios into SW Montana
into this aftn. Initial band of rain showers have developed in
area of positive vorticity advection with -ra reported at West. Yellowstone and Dillon.

Today and tonight...the level of tropospheric moisture in this
system is quite impressive, most of which likely originated from the
east Pacific off the coasts of S California and northwest Mexico. Over SW Mt,
precipitable water is forecast to be near 1 inch with 0.50 - 0.75
inch values expected over north-central Montana. The 1 inch precipitable water values
across SW Montana are very anomalous for the end of sept., And are 3-4
Standard deviations above the mean or greater than the 99.5
percentile.

The high amount of moisture and weak mid-level steering flow
would point at the possibility for slow moving showers/storms and
the potential for localized flooding. However, the good news is
that weak/disorganized upper-level forcing for ascent will
inhibit widespread precip with this event. Nonetheless, isolated
areas in southern Beaverhead, Madison, and Gallatin counties may
experience hourly rainfall rates approaching or exceeding 0.50
inches/per hour during the morning and aftn hours. As usual,
valleys near mountainous terrain would be most prone to flash
flooding. Hi-res nmm redevelops isolated to sct slow moving
showers and storms this aftn and evening with a focus over the SW
mountains.

Tuesday and Wednesday...well above average temps will continue
along with mainly dry conditions as upper low pulls away. Even as
a large-scale trough approaches the Pacific NW, heights will
continue to rise to near 588 dam...1000-500mb thicknesses near 580
dam by Wednesday aftn. Highs over the lower elevations will range
from near 80f over the SW to the upper 80s close to 90f over
north-Central/Northeast portion of the forecast area. Uttech

Wednesday night through Monday...high pressure aloft will begin to
move slowly east of the area on Thursday as a deep upper level
trough off the Pacific northwest coast begins to move onshore.
The area will remain dry with temperatures similar to those
from Wednesday (15 to 20 degrees above normal).

Model differences occur as the trough moves onshore and toward
Montana. The GFS is about 24 hours faster than the European model (ecmwf) with the
progression of the trough. They both develop a cut-off low over
California/Nevada and start to lift it northeast over the Great
Basin. However, the European model (ecmwf) weakens it into an open wave trough that
continues to move slower than the other models and lingers over
Montana into Monday. The GFS keeps the feature as a cut-off low and
moves it east over the central rockies and out of the area Sunday
night into Monday. As a result, the models agree to initially bring
a chance of showers/thunderstorms into the area Thursday night into
Friday and continue it through Sunday. However, the placement of the
potentially heavier rainfall is not agreed upon, so have decided to
go with a broad-brushed high chance for showers/storms across the
area through the period. Again, as mentioned above, the GFS then
decreases the chance of precipitation significantly after Sunday,
while the European model (ecmwf) holds on to a good chance of showers into Monday.
Again, with this kind of discrepancy this late in the forecast
period, I have decided to keep the chance for showers higher than
climatological normals and adjust as necessary as the models
hopefully come into better agreement.

The other concern with this system will be temperatures. Forecast
model compromise brings a much cooler airmass into the area with the
cut-off low/upper trough. As the system brings increasing
clouds/showers and a cold front into the area Friday into Saturday,
temperatures will cool from mostly 70s on Friday (about 10 degrees
above normal) into the mid 50s to mid 60s on Saturday (about 5 to 10
degrees below normal). With the faster exit of the system by the
GFS, it begins warming temperatures on Monday. However, have
maintained the trend of a lingering system and kept temperatures
similarly cool through the end of the period. This may cause some
mountains to get a rain/snow mix during the overnight hours.
Coulston

&&

Aviation...
updated 1135z.
VFR conditions will generally prevail through the period, unless
otherwise noted. Moisture wrapping around a disturbance over southeast
Idaho/SW Montana/northwest Wyoming will bring areas of mid level cloudiness
as far north as a kgtf-klwt line, but scattered showers and isolated
afternoon/evening thunderstorms will be limited to southwest Montana
(including kbzn). Some of these storms may produce brief heavy rain
and brief MVFR conditions. The clouds/showers/storms will decrease
and move east out of the area with the disturbance after 02z. North
of the kgtf-klwt line, only high cloudiness will pass overhead.
Winds will mostly be light, but they may become breezy at times out
of the west this afternoon/evening along the east slopes of The
Rockies.
Coulston

&&

Preliminary point temps/pops...
gtf 82 53 83 55 / 10 0 10 10
ctb 83 46 81 48 / 0 0 20 10
hln 82 51 83 53 / 20 10 20 10
bzn 76 47 79 48 / 20 10 10 10
wey 62 36 68 37 / 60 30 10 0
dln 73 48 78 50 / 50 20 10 10
hvr 83 50 86 52 / 0 0 0 10
lwt 81 51 84 53 / 10 0 0 10

&&

Tfx watches/warnings/advisories...
none.
&&

$$

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