Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 300457
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1057 PM MDT Sat Apr 29 2017

...UPDATED AVIATION SECTION...

.UPDATE...

Per the latest water vapor imagery, the shortwave that brought
scattered showers/isolated t-storms to part of the area today is now
moving into eastern Montana. In its wake, a stabilizing atmosphere
and subsidence aloft is causing any lingering shower activity to
quickly diminish. For the update, I lowered pops for the remainder
of the evening over the plains with showers on the decrease.
Overnight, a tightening pressure gradient along the Rocky Mtn Front
should allow an increase in the wind there, especially after
Midnight. The previous forecast seemed a bit low in that area and
I upped the wind some. Still should stay well below High Wind
criteria, though.

Looking ahead to tomorrow, another shortwave will swing through the
area, not unlike today`s. However, it may move through a bit
earlier in the day than today`s with the best chance of showers
being a bit further east of I-15 (ie. along/east of a Havre to
Bozeman line). I adjusted pops slightly for tomorrow afternoon to
account for this change. The previous forecast had a chance of
thunder in and around the Bozeman area and I expanded it north to
include the Havre and Lewistown areas as instability tomorrow
looks similar to what was realized today. Main threats from any
storm still looks to be gusty winds and small hail. Martin

&&

.AVIATION...
Updated 0457Z.

VFR conditions are expected at least through the day on Sunday, but
brief MVFR ceilings are possible with showers. Mid and high level
cloudiness will increase overnight from the west ahead of another
disturbance in the westerly flow aloft. Showers will also increase
over the Western and Southwest Montana mountains during this time
and continue past 00Z. Some showers will spread out over the plains
between 10Z and 16Z, but increasing westerly downslope winds will
limit showers along the east slopes of the Rockies (KCTB KGTF) after
that time. Better instability will likely bring another round of
showers and possibly a few thunderstorms east of the east slopes.
Coulston

&&

.PREV DISCUSSION...
/ISSUED 600 PM MDT Sat Apr 29 2017/

Tonight through Sunday night...Cumulus development continues this
afternoon across the forecast area with weak instability and
plenty of surface moisture available along with some forcing from
a weak shortwave aloft. Scattered to numerous showers have
developed over much of N-central MT with best, albeit weak,
instability focused over eastern portions of N-central MT, where
a few isolated thunderstorms cannot be ruled out through early
this evening. Shear is minimal however and short-lived/pulse type
convective cells will be the dominant theme. A weakening upper
level ridge progresses east across the forecast area tonight as
it is quickly flattened by an upstream Pacific upper level
trough/jet that moves east across the region on Sunday. Showers
and cumulus will diminish quickly this evening followed by
increasing cloudiness from the west later tonight. Deepening
surface low in southern AB Sunday will produce breezy west winds
across the forecast area with the upper level trough and embedded
shortwave energy in WNW flow aloft also triggering some showers.
Surface low in AB moves off to the east Sunday night with
additional shortwave energy dropping into N-central MT as flow
aloft turns more northwesterly. A surface frontal boundary may
accompany this energy as it moves southeast across N-central MT
Sunday night along with the potential for some showers with snow
levels lowering back down to around 4000 ft by early Monday
morning. Hoenisch

Monday through Saturday...Overall active weather to start the
upcoming work week...becoming warm and dry mid week...then some
uncertainty towards next weekend. Unsettled northwest flow will
be found Monday through Tuesday as a large upper low is found
across the Midwest...and a ridge of high pressure builds in the
Pacific Northwest. Monday`s activity looks to be scattered with
snow levels above 5000 feet. A weak disturbance in the northwest
flow may bring more widespread showers on Tuesday...with snow
levels possibly lowering to 4000 feet on Tuesday morning. More
instability and increased shear from a strong jet streak will
also be found Tuesday...creating perhaps some scattered
thunderstorms during the afternoon hours. We then see the long
awaited return of ridging on Wednesday...which now looks to
continue through the day Thursday. Warm temperatures and mainly
dry conditions are expected...with perhaps some breezy winds at
times. Most models indicate a cold front will move through at
some point on Friday...returning chances for showers to much of
the area. The weekend then becomes the most uncertain part of the
forecast. Most models develop some sort of large disturbance
across the western US coast. How it influences our weather
pattern by the weekend is still up for interpretation. For now it
is a monitor type situation...however...active weather may
continue through next weekend. Temperatures look to remain below
normal through Tuesday. Ridging and downsloping flow could then
return high temperatures in the 60s and perhaps 70s Wednesday and
Thursday. Friday into Saturday could even be near to above normal
temperatures. There is some concern warm temperatures combined
with melting of fresh mountain snow could bring some flooding
concerns by mid to late next week. Another item to keep a close
eye on in the long term. Anglin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  38  59  35  55 /  10  20  20  30
CTB  38  56  35  53 /  20  20  30  30
HLN  40  59  36  57 /  10  20  10  20
BZN  34  57  33  55 /  20  40  20  20
WEY  25  46  25  44 /  20  30  40  20
DLN  35  55  31  54 /  20  30  30  10
HVR  38  63  38  58 /  10  20  30  20
LWT  34  56  35  52 /  20  40  30  20

&&

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

$$

weather.gov/greatfalls



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