Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 011514
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
913 AM MDT Sat Oct 1 2016

.UPDATE...
The forecast is on track today with only a few minor adjustments to
the PoPs. Short range models are showing multiple rounds of showers
and a few thunderstorms moving though the area today. The area of
best instability and shear still appears to be along and east of a
Havre-Great Falls-Helena line where a few storms could be strong to
marginally severe. The main threat from these storms will be strong
winds. Storms will also be capable of producing brief heavy rain and
small hail. The activity will quickly shift east out of the area
this evening with mainly dry conditions for the overnight hours.
Langlieb

&&

.AVIATION...
Updated 0550Z.
A weak upper level disturbance will move northeastward through the
region through 12z Sat producing a few light showers/thunderstorms.
A stronger wave will move through the region on Sat
afternoon/evening producing more widespread chances for
showers/storms. Overall VFR conditions will prevail but some MVFR
conditions are possible in/near showers. Brusda

&&

.PREV DISCUSSION...
/ISSUED 438 AM MDT Sat Oct 1 2016/
Today through Monday Evening...Our run of warm and mostly dry
conditions the past few days will come to an end later today as the
broad upper-level trof that has been hanging off the Pacific coast
begins to bring more concentrated energy and moisture inland.
Initial cooling and precipitation will start with the shortwave trof
and associated surface cold front making their way through the Idaho
Panhandle this morning. Scattered showers have already developed
well ahead of the front over southwest MT this morning and
additional, widespread precip is expected east the Divide this
aftn/eve as the cold front crosses the Rocky Mtn Front. Also,
increasing instability and lift (CAPE values of 500-900 J/kg and
lifted indices of -2 to -4) ahead of the front will generate
isolated to scattered thunderstorms mainly along and southeast of a
Havre-Great Falls-Helena line from late aftn through this eve. Small
hail and moderate rainfall will be possible with some storms. The
front and convective activity heads into eastern MT overnight, with
slightly drier conditions expected through midday Sun.  At that
time, a second shortwave lifts northeast out of Idaho, bringing
another round of showers starting over southwest MT and then
spreading northeastward by aftn/eve.

After struggling for several runs to discern a consistent evolution
of the main trof, latest models seemingly have settled on taking the
trof`s low pressure center along a track from Boise to near
Billings. This more northerly track will push more extensive
moisture over our region, resulting in widespread, all-day rainfall
on Mon. Initial rainfall estimates range from 0.25 to 0.35 inch for
the Rocky Mtn Front and far southwest MT to 0.5 to 1.0 inch for
locations in the central and Hiline counties through late Mon night.
However, given the inconsistent model solutions, forecast confidence
is marginal at best right now, and further adjustments to the
storm`s track and resulting precip totals are likely. About the only
sure bet is that we are done with 70-degree temperatures for a while
after today, as highs Sun only reach the 60s and drop into the upper
40s and low 50s on Mon.
Waranauskas

Monday night through Saturday...There is better agreement now among
the forecast models regarding the passage of the Pacific storm
system during the early/middle portion of next week. The GFS/CMC
(Canadian) models are more in line with what the ECMWF was showing
last night. In other words, the Pacific system is now forecasted to
stay fairly close to the forecast area and bring more widespread
precipitation. The low pressure center within the upper level trough
is now forecasted to move slowly northeast from the Montana/Wyoming
border Monday evening across Eastern Montana on Tuesday then into
southern Saskatchewan/Manitoba through Wednesday. As a result,
moisture wrapping around the upper low will combine with a deep
northerly upslope flow to bring widespread showers to the TFX
forecast area into Tuesday night, mainly east of Interstate 15.
Precipitation will generally range from around 0.25 inches west of
Interstate 15 to almost 1.5 inches across the eastern TFX plains
(including the Havre and Lewistown areas).

This system will also drag much cooler Canadian air south into
the area behind a cold front, lowering snow levels to around 5000
feet along the Hi-Line, to around 6000 feet across Central
Montana, and to around 7000 feet in the southwest. As a result,
accumulating snow is expected in the mountains, with 4 to 8 inches
possible above 7000 feet. There may be periods Monday night and
Tuesday night when heavier precipitation may even bring wet,
slushy accumulation to some favored upslope areas down to around
4500 feet. Exact amounts are difficult to pin down at this time,
so we will monitor this for potential winter weather highlights.
Precipitation will decrease from the west Tuesday night into
Wednesday as the system exits the area. A few showers may linger
through Thursday before weak high pressure moves in on Friday to
help dry out the area. However, another weaker system is
forecasted to move into the area by Saturday, to bring another
chance of showers. Below normal temperatures with lows in the mid
20s to mid 30s and highs in the 40s to lower 50s are expected with
the Pacific system, then temperatures will warm about 5 to 10
degrees (closer to normal) through the end of the week. Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  73  42  62  43 /  50  60  10  40
CTB  65  36  59  40 /  30  30  10  40
HLN  71  39  62  42 /  50  40  20  30
BZN  75  40  63  43 /  20  20  30  40
WEY  62  32  60  37 /  30  20  40  40
DLN  68  35  61  40 /  40  30  30  40
HVR  79  43  65  44 /  50  70   0  50
LWT  78  41  62  43 /  40  50  10  60

&&

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

$$

weather.gov/greatfalls



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