Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 281751
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1150 AM MDT Tue Mar 28 2017

Aviation Section Updated

.UPDATE...

Latest analysis shows an inverted trough from the front range of
the central Rockies north through eastern Montana. A band of
heavier precip associated with this trough has been sitting just
east of our forecast area this morning. West of the trough, drier
air resides over central and SW MT with only some high clouds
moving through. The main adjustment for the update was to slow
down the arrival of clouds this afternoon and, consequently, bump
highs up a bit, especially across the eastern half of our CWA (ie.
Havre to Lewistown to Bozeman), where clouds will arrive the
latest. Lastly, I lowered pops some tonight across central
Montana. Despite a shortwave passing through, it appears
downsloping will limit any shower activity there. Martin

&&

.AVIATION...
Updated 1750Z.
VFR conditions will prevail over the region through the period.
Expect some passing mid/high level clouds through 18z Wed along with
breezy west winds at times. Some mountains could become obscured
after 12z Wed with a few showers developing over the western
Rockies. Brusda

&&

.PREV DISCUSSION...
/ISSUED 500 AM MDT Tue Mar 28 2017/

Today through Wednesday night...Mid-level trough over the High
Plains will move east this morning, to be replaced by gradually
increasing westerly flow through Wednesday as weak upper ridging
slides by to the south. At the surface, broad lee-side troughing
will persist over the High Plains into Wednesday night. A sharp
upper trough moving into Pacific Northwest with a leading
shortwave disturbance will begin to increase synoptic-scale lift
across the region during the Wednesday overnight period.

As for resulting weather, skies will clear from west to east this
morning, but will replaced by increasing high clouds by this
evening as the westerly jet stream begins to push high-level
moisture over the region. This increasing westerly flow will also
help induce a west to southwest breeze across the region this
afternoon. Wednesday will see a bit stronger breezes out of the
southwest and continued mid- to high-level cloudiness, but
temperatures will also be a bit warmer, resulting in what is
likely to be the warmest day of the workweek for most parts of
north-central and southwest Montana. Most shower activity
Wednesday will be confined along and west of the Continental
Divide. However, it is interesting to note that some models
develop a few rain showers with a quick-moving upper disturbance
that traverses the north-central High Plains Wednesday morning
through early afternoon.

By Wednesday night, broad synoptic scale lift begins to develop
over the region, with showers beginning to develop east of the
Continental Divide, especially for central and southwest Montana.
Thickening clouds will likely result in warmer low temperatures
for most areas Wednesday night. Cassell

Thursday through Monday night...an unsettled pattern is still
expected, with multiple upper level disturbances bringing the
chance for precipitation to Southwest, Central, and portions of
North Central Montana. Primary focus will revolve around the
Thursday through Friday system, which will bring the most
widespread accumulating precipitation and potential for impacts to
the region. 00z models this evening are coming in a bit more
interesting, with the GFS developing a northern H700 low over
Southern British Columbia/Northern Idaho/Northwestern Montana by
Thursday morning, propagating east southeast across Central
Montana to East Central Montana by Friday morning. On the other
hand, the 00z NAM/Canadian/ECMWF favor a solution in which the
main H700 low associated with the shortwave diving from the
Pacific Northwest moves well south of the region. Despite these
tracks in 00z NAM/Canadian/ECMWF, persistent northerly upslope
flow and weak divergence aloft would still support precipitation
chances along and south of a KGTF to KHVR line, with the bulk of
the precipitation from these three solutions (00z
NAM/Canadian/ECMWF) falling over Southwest Montana where the
greatest upper level support resides.

The 00Z GFS scenario is the most interesting, as a combination of
strong H300 divergence, the track of the H700 low, and ideal
northerly flow (with Froude numbers around 0.4 to 0.6) would lead to
an enhanced area of precipitation across portions of Central and
North Central Montana Thursday afternoon and into the morning hours
on Friday. This would be especially true for any north facing slopes
and their adjacent foothills. Given this new wrinkle of the GFS,
increased POPs across Central and portions of North Central
Montana.

Questions that still remain with this system are 1) how much dynamic
cooling results, especially across Central Montana (which would
allow for some rain to change over to snow onto the Plains), and 2)
given the warm antecedent conditions as of late, will snow be able
to accumulate on road surfaces? While higher elevations across the
Little Belts, Highwoods, Continental Divide, and Southwest Montana
Mountains should see accumulating snow just about everywhere, am
only expecting minor accumulations on grassy surfaces on the Central
Montana Plains and Southwest Montana Valleys at this time.

Temperatures will remain mild through the weekend and into early
next week, with a return of intermittent chances for moisture
Saturday night into the first half of the upcoming work week. -
Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  55  38  61  42 /   0  10  20  10
CTB  51  34  55  36 /   0  10  20  10
HLN  54  35  60  41 /   0  10  10  20
BZN  55  29  57  38 /  10  10  10  30
WEY  40  16  41  29 /  20  10  10  40
DLN  52  27  56  37 /   0  10  10  40
HVR  61  33  63  39 /   0  10   0  10
LWT  55  33  59  40 /  10  10   0  20

&&

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

&&

$$

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