Weather Service Report


030 
FXUS65 KTFX 270946
AFDTFX

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

.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


&&

.AVIATION...
Updated 0500Z.

VFR conditions are generally expected over the next 24 hours, unless 
otherwise mentioned. A moist southwest flow aloft ahead of a 
shortwave trough will spread mid and high clouds with scattered rain 
and mountain snow showers over the area through the period, which 
will mainly obscure mountain tops. At lower elevations, showers will 
be more hit-and-miss as a disturbance moves over the area through 
12Z, especially over the plains. A few showers moving over the 
southwest valley terminals may lower ceilings to high MVFR levels, 
but any lowering will likely be brief. The shortwave trough 
mentioned above will likely bring another round of scattered showers 
to the area after 18Z, with a better likelihood of occasional MVFR 
conditions. A band of heavier snow showers may develop over the 
southwest possibly reducing conditions to IFR at times after 00Z.
Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
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 

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

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Western Regional Climate Center, wrcc@dri.edu