Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 301127

Area Forecast Discussion
National Weather Service Great Falls MT
525 AM MDT Fri Sep 30 2016


Today through Sunday Evening...Region remains under southwest flow
pattern along eastern boundary of a broad Pacific upper-level trof.
Bands of moisture within the flow brought showers and even a few
thunderstorms to northwest MT and the Idaho Panhandle overnight.
Water vapor satellite imagery shows that area of moisture heading
into BC/Alberta, so expect our region will remain dry this morning,
except for a few spotty showers along the northern Rocky Mtn Front.
Secondary moisture plume over western Idaho is projected to lift
northeast and reach the Continental Divide area by early aftn, but
forecast models continue to show it moving more north-northeast
along the spine of the Northern Rockies, with almost all
precipitation confined to locations along/west of the Interstate 15
corridor. Though coverage will be spotty, there will be enough
instability and lift to generate a few thunderstorms that will
produce short periods of moderate rainfall and perhaps some small
hail/graupel. Convective activity diminishes overnight but
redevelops Saturday aftn as a surface cold front pushes west-to-east
through central MT. Model depictions of rainfall coverage remain
fairly broad, with most of the forecast area receiving 0.03-0.07
inch of rainfall with amounts of 0.1 to 0.15 inch possible in the
central counties through Sat eve.  A second round of rain is
possible for southwest MT on Sun aftn as a weak disturbance in the
flow tracks through northeast Idaho. After highs in the 70s today
and tomorrow, much cooler air arrives behind the front, with highs
on Sun only reaching the low-mid 60s.

Sunday night through Friday...Medium range models have a decent
handle on the overall timing of the movement of the upper level low
pressure trough through this period. However, differences in the
projected positioning of the low pressure center are causing
somewhat of a spread in forecasted precipitation amounts; the ECMWF
is a bit of an outlier by keeping the low pressure center closer to
the forecast area than the GFS/CMC. With the GFS having a better
track record with these systems recently, will lean more towards it
overall, but will increase the chance for precipitation in TFX`s
eastern zones due to better agreement among the models on placing
precipitation there. The trough will swing southeast from the
Pacific Northwest coast Sunday evening to the central Rockies Monday
afternoon, which will bring a good chance of showers to the area,
especially over the eastern TFX zones due to a diffluent flow
bringing enhanced lift there. A lingering frontal boundary over
Southwest Montana may also cause a few thunderstorms develop during
the afternoon and evening hours. As the trough then swings northeast
into the northern Great Plains through Tuesday, moisture wrapping
around the upper low pressure center will move the better chance for
showers into Eastern Montana. A weak ridge of high pressure will
then move into the area in the wake of this trough through Thursday,
decreasing the chance for showers and generally limiting them to the
mountains. A weaker trough will move into the Pacific Northwest on
Friday, bringing a slight chance of showers to much of the area once
again. With an upper level trough and its associated clouds/showers
generally over the area for much of the period, high temperatures
will remain 5 to 10 degrees below normal (mostly 50s expected)
during this period, while lows remain a bit above normal (mostly mid
30s to mid 40s).  Coulston


Updated 1125Z.

VFR conditions expected today. Only item of note for this morning is
chance of fog developing at KHVR, but any fog that does occur should
dissipate by ~15Z.  Clouds begin increasing around midday along the
Rocky Mtn Front and over southwest MT as narrow band of moisture
just moving into western MT at 11Z reaches the Continental Divide.
Lift and instability are sufficient for -SHRA and isolated -TSRA
along/west of a line from KCTB to KBZN this aftn/eve. Ceilings could
briefly drop to MVFR if locally moderate rainfall occurs near the
showers/thunderstorms.  Surface winds should remain at/below 15 kts
across the region today.


GTF  78  50  75  43 /  10  10  60  60
CTB  74  48  67  37 /  20  20  40  40
HLN  79  48  73  40 /  20  20  60  40
BZN  76  48  75  41 /  10  10  30  40
WEY  65  39  63  34 /  30  20  30  30
DLN  75  45  68  36 /  20  20  40  40
HVR  76  52  81  45 /   0  10  30  60
LWT  76  53  77  42 /   0  10  30  40



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