Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 171036 CCA

Area Forecast Discussion...Corrected SYNOPSIS section
National Weather Service Great Falls MT
435 AM MDT Mon Jul 17 2017


A weak disturbance in the westerly flow aloft will bring another
round of isolated thunderstorms to Southwest Montana this afternoon
and evening. Another disturbance will then bring a slight chance of
thunderstorms to Central Montana Tuesday night. Otherwise, the high
pressure ridge will gradually rebuild into the region through mid
week, keeping the area mostly clear, and causing temperatures to
warm from near normal today to between 5 and 10 degrees above normal
by Wednesday. Area wildfires will also keep the area somewhat hazy.



Today through Wednesday...A weak frontal boundary moved south
over North Central Montana overnight, bringing higher dewpoints
across the area, as well as patchy fog along the Rocky Mountain
Front. It also spread some smoke south from wildfires in British
Columbia, which is reducing visibility in some areas to less than
5 miles. Am expecting both the fog and smoke to dissipate as the
morning inversion is broken. However, smoke from fires in west
central Montana may still move over Central Montana this
afternoon, but this should remain aloft. Otherwise, a weak
disturbance in the west-southwest flow aloft will bring isolated
to scattered thunderstorms to Southwest Montana this afternoon and
evening. Gusty winds will likely be the main threat. The high
pressure ridge will then begin to build back into the forecast
area Tuesday into Wednesday, which will keep the area generally
dry with only passing clouds. However, another disturbance is
forecast to move through the ridge Tuesday night, which will bring
a slight chance of thunderstorms to Central Montana. Again, these
storms should remain weak. Near-normal temperatures today will
warm to between 5 and 10 degrees above normal by Wednesday.

Wednesday Night through Monday...Broad ridging aloft will
maintain dry conditions over the forecast area Wednesday night.
The ridge begins to break down Thursday as a closed upper low
advances through southern British Columbia. This shortwave trough
crosses southern Alberta on Friday, resulting in a westerly flow
across the region. Although an isolated shower can`t be ruled
out, weather remains dry through the end of the week. The most
notable concern is the potential for rather breezy west winds
developing behind the passing shortwave Friday afternoon. The
combination of higher wind speeds with low afternoon humidity
will once again produce at least elevated fire weather concerns.
Temperatures return closer to seasonal averages Friday and
Saturday, though remain rather warm with highs in the mid- to
upper-80s. Model solutions vary over the weekend under the weak
zonal flow but overall maintain dry conditions with high
temperatures remaining in the upper 80s. Temperatures trend
warmer to end the period on Monday as large scale ridging
amplifies over the interior southwest.


Updated 0430Z.

VFR conditions are expected through the 1706/1806 TAF period across
all terminals in North Central and most in Southwest Montana (with
the exception of the KEKS terminal where afternoon showers and
thunderstorms may lower ceiling to near MVFR). Confidence in MVFR
conditions being realized at KEKS is low this far out, so have kept
this TAF package at VFR conditions. Otherwise, breezy winds are
slowly diminishing across the region, with generally light winds
expected for the remainder of the night and into the day on Monday.
A wind shift line was drifting south out of Alberta Canada, and will
primarily affect the KCTB and KHVR terminals with a slight northerly
component to the wind direction. In addition, smoke in the lower
levels was evident behind this wind shift line. Was not confident
enough at this time to introduce smoke for KCTB and KHVR, but did
introduce some lower ceilings to around 6000ft agl at KCTB where
smoke may pool up against the Rocky Mountain Front due to the
nocturnal temperature inversion. Additional smoke is possible across
the region from distant Western CONUS fires, however decreased
visibilities and/or low ceilings are not anticipated. - Moldan


GTF  83  49  87  54 /   0   0   0  20
CTB  77  45  82  48 /   0   0   0  10
HLN  83  53  89  56 /  10  10   0  10
BZN  83  49  87  51 /  10  10   0  10
WEY  78  40  80  42 /  30  30  10  10
DLN  83  50  86  50 /  10  10   0   0
HVR  82  50  88  54 /   0   0   0  10
LWT  80  50  85  54 /   0  10  10  20



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