Area Forecast Discussion
Issued by NWS Great Falls, MT
FXUS65 KTFX 220450
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1140 PM MDT Thu Aug 21 2014
AVIATION SECTION UPDATED
Update forthcoming. Have made minor adjustments to the PoP grids.
Have lowered Pops for the remaining hours until midnight. Water
vapor imagery shows a slot of drier air moving across the area.
Convective activity over southwest Montana has decreased.
Precipitation should increase again after midnight as increasing
southwest flow aloft ahead of the upper trof over the Pacific
Northwest brings additional moisture and instability to the area.
Temperatures overnight look good.
Current radar imagery showing isolated to scattered showers across
the region, with a few brief and weak thunderstorms from KHLN to
KBZN. Should see most of the convective activity dissipate over
next hour or so, then next redevelop between 09 and 12Z as
upper-level trof over WA/OR slowly comes eastward. Forecast models
continue to indicate periods of -RA/-SHRA at all local terminals
through the day tomorrow, but confidence is only moderate at best on
precipitation coverage given poor model precip verification past 24
hrs. Ceilings will be a mix of VFR/MVFR/ localized IFR conditions
as ceilings gradually lower with the widespread precipitation. Fog
with IFR ceilings is also possible at KHVR/KLWT on Fri morning.
.PREV DISCUSSION...Issued 256 PM MDT Thu Aug 21 2014
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.
.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