Heavy Rain in Montana, August 21 to 22, 2014

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
545 PM MDT Thu Aug 21 2014

AVIATION SECTION UPDATED

.DISCUSSION...
Tonight through Saturday...The main message continues to be cooler
and wet. An upper trough off the Pacific Northwest coast will dig
southeast and develop into an upper low over Washington and Oregon
by Friday morning. The upper low will then gradually move east and
end up near the triple point between Idaho/Wyoming/Montana by late
Saturday afternoon. A series of weather disturbances ahead of the
upper low will affect the forecast area later tonight into Saturday.
Models are not in good agreement with the details of these
disturbances and also associated precipitation patterns. Thus
confidence in rainfall amounts is only in the low to moderate range.
The current best estimate on total rainfall amounts tonight through
Saturday night is 1 to 2 inches over southwest Montana and 2 to 3
inches over the plains with in excess of 3 inches for some mountains
of the Rocky Mountain Front and central Montana. As for the snow
situation over the Rocky Mountain Front guidance suggests snow
levels will lower to around 8000 feet by Saturday morning but it is
not unusual for them to be lower than what guidance suggests. So
have gone with snow levels near 7000 feet over the Rocky Mountain
Front. Expect that Logan Pass will see a trace of snow late Friday
night and Saturday morning with a small chance of 1 to 2 inches.
Above 7000 feet expect 2 to 4 inches but amounts a little in excess
of a half-foot could occur. There will be isolated thunderstorms
over central/southwest Montana Friday and possibly Saturday but they
will not be severe. Inherited forecast cool temperatures look pretty
good so did not make any major changes. Blank

Saturday Night through Thursday...A slow moving upper level trough
will remain over the region through the weekend until at least
Tuesday. Models are in decent agreement on details through Monday
morning keeping a good likelihood of precipitation over the region
Saturday evening then a chance of showers into Monday. Beyond
Monday, model solutions have come into better agreement with the 12Z
runs, as the ECMWF is more in line with the more consistent GFS
solution. The upper trough will move east out of the area and leave
Montana under a generally dry northwest to westerly flow aloft. This
will allow temperatures to warm back closer to normal through
Thursday with very little chance of showers/thunderstorms.
mpj/Coulston

&&

.AVIATION...
UPDATED 2345Z.
A broad upper level trough of low pressure will remain over the
Pacific Northwest and Northern Rocky Mountains through Friday
morning. Isolated to scattered showers, with a few short-lived
embedded thunderstorms, will continue to pop-up across the region
(mainly over southwest MT from KHLN to KDLN) through 02-03Z then
dissipate tonight.  Precipitation redevelops during the overnight
period as next disturbance traveling through the broad trough pushes
into western MT. Rain should become more widespread tomorrow but
confidence is moderate at best as forecast models have been
struggling with precipitation coverage last 24 hrs. VFR conditions
expected away from any of this evening's -SHRA/-TSRA, with MVFR
possible under any stronger cells.  Ceilings will be trending lower
overnight with broad MVFR conditions looking more likely from
KHLN/KLWT north to the US/Can border starting around 09-12Z.
Waranauskas

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  51  62  52  55 /  60  90 100 100
CTB  49  56  49  53 /  60  90 100 100
HLN  53  65  52  59 /  60  90 100 100
BZN  51  67  49  62 /  60  80  80  50
WEY  45  61  40  58 /  50  50  40  50
DLN  48  65  45  60 /  50  80  70  50
HVR  55  65  55  59 /  50  80 100 100
LWT  50  60  51  53 /  60  90 100 100

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

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