Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 271121

Area Forecast Discussion
National Weather Service Great Falls MT
521 AM MDT Mon Mar 27 2017

Updated Aviation Discussion.


Today and Tonight...A fairly sharp upper level shortwave
disturbance located over north-central Montana will continue to
slowly exit east through the north-central and central Plains this
morning with minimal impacts outside of a stray shower or two.
The main system to watch moves through the area this afternoon and
evening, and will need to be watched carefully especially toward
our southern counties. An upper level trough splits over the
Pacific Northwest today through tonight. The southern piece of the
trough split will form an elongated closed low situated from
west-central Wyoming south through the 4-Corners region by Tuesday
morning. With the trough split occurring to our west, this will
leave much of Southwest Montana in a prolonged period of decent
synoptic-scale lift through this evening. Ahead of the northern
piece of energy moving slowly across our CWA through tonight, a
low-level front will strengthen this afternoon and evening as it
drops through Southwest Montana this afternoon. This will set up
a fairly impressive yet narrow deformation zone across our
southern counties of Southwest Montana this afternoon and into the
evening, with the primary impact being potentially moderate to
heavy precipitation for a period of time late this afternoon into
the evening. The big question then becomes how much dynamic
cooling occurs under the moderate to heavy precipitation band, as
well as where exactly this band sets up. This will result in a
huge spread in scenarios with regards to rain and snow amounts, as
well as snow levels across Southwest Montana today, particularly
south of US Highway 12. As an example, it is possible Bozeman
could see convective rain showers this afternoon change to a band
of moderate rain or snow this evening as temperatures drop through
the 30s, with subsequent snow accumulations anywhere from just a
trace to as much as 6 inches through the overnight as snow levels
drop. Given the warm temperatures of late, snow amounts below
5000-5500 feet will be driven by the intensity of snow rates,
with 1 inch/hour rates likely needed this evening to overcome
melting on surfaces. Given the large uncertainty currently
regarding temperatures, as well as intensity and location of the
narrow deformation zone, we will not issue any winter weather
hazards with the morning forecast package.

Tuesday through Wednesday...Upper troughing across the region
Tuesday morning will be replaced by upper transitory ridging
Tuesday night into Wednesday. Weak lee-side troughing at the
surface will allow for breezy conditions to develop, but without
strong subsidence in the lower levels stronger winds aloft will
have a hard time reaching the surface. Wednesday`s temperatures
are likely to be the warmest highs of the workweek with most of
the area between weather systems. Cassell

Wednesday night through Tuesday...An active and unsettled pattern
is in store for much of the long term, with a very progressive
upper level flow pattern. With this being said, primary focus and
concerns center around the late Wednesday night through Friday
timeframe, as a strong shortwave dives southeast from the Pacific
Northwest into the Great Basin. While much of the upper level
support and forcing from this shortwave will remain across far
southern portions of the CWA over this time frame, weak upper
level divergence and strong northerly flow should support upslope
conditions as far north as the Little Belts and near KGTF. This
flow would support north facing slopes for the heaviest
precipitation accumulation. Internally calculated Froude numbers
from this evenings 00z runs are giving values of 0.25 to 1.0 for
KGTF and the Little Belts, which would support ideal conditions
for uspslope flow across these areas. The question that remains is
how much cool air will be available to change precipitation over
to snow. Current model consensus is to keep precipitation in the
form of rain across Central Montana, save for the higher
elevations with snow levels lowering to around 5,000 to 5,500 feet
by Friday morning. Given that the upper level support is not all
that impressive over this time frame and location (Central
Montana), don`t believe that dynamic cooling processes will be
able to change precipitation over to snow onto the plains. Future
temperature forecasts will be closely monitored however, as any
cooling could lead to snow falling onto the Plains. With this
being said, given warm antecedent conditions, any snow
accumulations would be light on the Plains. Further to the south
across Southwest Montana, likely precipitation chances will exist
as upper level support will be strongest. Snow levels Wednesday
night of 6,500 to 7,500 feet, will lower to around 5,500 feet by
Friday morning. This will set the stage for accumulating snow for
elevations at or above 5,500 feet across Southwest Montana, with
several inches of snow being possible.

Beyond Friday...Temperatures will warm, with both high and low
temperatures remaining well above normal. Precipitation chances will
return to the region Saturday night and continue into early next
week as another system moves into the Pacific Northwest and
southwest flow aloft develops. - Moldan


Updated 1121Z.

VFR conditions are generally expected over the next 24 hours, unless
otherwise mentioned. Low-level flow increases out of the northwest
through the day as a weather front moves into Southwest MT this
afternoon/evening. Rain and higher elevation snow showers will move
south and east through the day from near KHLN to KBZN and KEKS. A
band of moderate to heavy rain/snow may develop over the southwest
possibly reducing conditions to IFR at times after 00Z and
persisting until 06-09z tonight. Cassell


GTF  57  31  54  34 /  20  30  10  10
CTB  54  33  53  32 /  20  10   0  10
HLN  54  33  53  32 /  40  40  10  10
BZN  54  27  50  22 /  50  70  20  10
WEY  37  19  37   8 /  50  50  20   0
DLN  48  24  49  26 /  60  60  10  10
HVR  59  32  59  31 /  20  20  10   0
LWT  55  31  51  29 /  40  50  20   0



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