Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 171014

Area Forecast Discussion
National Weather Service Great Falls MT
414 AM MDT Sat Mar 17 2018


A complex storm system will move through the area this weekend
bringing areas of snow and freezing drizzle. In the mountains,
the precipitation will mainly fall as snow. At lower elevations,
the precipitation type will fluctuate between snow and freezing
drizzle. Precipitation gradually winds down from west to east by
Sunday night.



Today through Sunday night...The forecast over the remainder of the
weekend is a complicated one with only moderate confidence, at best.
A slow-moving upper level low continues to edge east through the
Pacific NW early this morning. Several shortwaves have been
pinwheeling around this low over the past several days and this will
continue through the weekend.

The next wave to impact our forecast area is moving north through
Nevada at this time. This wave will continue north, eventually
merging with the low over the Pac NW, before pushing NE through
Montana. The models have continued the northward trend (seen over
the past few days) with the track of this low, which now places the
axis of heaviest snow further north as well. Further complicating
the forecast, the areas along/south of the track of this low may see
more dry air than originally forecast, making snow/rain totals
lower/less certain. This also makes precip-type more questionable,
especially over the plains of Central Montana.

So, here is the way things look to shake out. An area of
steadier/heavier precip moving north out of Idaho will lead to
impactful snow at higher elevations of SW MT, primarily above 6500
feet. The Winter Weather Advisory in this area has been adjusted and
now is only in effect for mountain passes and does NOT include
valleys such as Dillon, Ennis, or Bozeman. This area of snow should
weaken with time as it pushes north. By this afternoon/evening,
another band of steadier precip looks to setup from near Helena NE
through parts of Central Montana. Again, the models have trended
further NW with this band which puts more emphasis on the Rocky Mtn
Front and areas west of I-15. Because of this trend, the Winter
Weather Advisory was expanded north. Along and east of I-15, the
precip-type will fluctuate between snow and freezing drizzle.
Ultimately, this will lead to lower snowfall amounts than originally
forecast, but the combination of snow and freezing drizzle will lead
to travel concerns and the Winter Weather Advisory will stand for
now. Precip should begin to wind down from west to east on Sunday,
lingering longest in the mountains. Of note, across SW MT, enough
instability is expected for a few lightning strikes this afternoon
and again Sunday afternoon. MARTIN

Monday through Friday...main forecast concerns over this portions of
the forecast period are chances for precipitation and temperatures
peaking around the Wednesday/Thursday timeframe.

As was the case last evening, a very messy upper level pattern to
start the forecast period, as the main H500 trough/closed low over
the past week fragments into numerous shortwaves strewn across the
Western CONUS/Canada. A few of these disjointed shortwaves will
cross both North Central and Southwest Montana during the day on
Monday, which will allow for continued chances for mostly light
snow, however, some light rain may mix in at lower elevations across
North Central Montana. Following the passage of the aforementioned
shortwaves, northwest flow sets up over the Northern Rockies for
Monday night and most of the day on Tuesday before another fast
moving shortwave dives southeast across the CWA Tuesday
afternoon/evening. This shortwave will be moisture starved save for
across extreme Southwestern Montana, where slight chance POPs exist
for the afternoon and evening hours on Tuesday. Transitory ridging
then quickly moves over the Northern Rockies behind the departing
shortwave during the day on Wednesday, and amplifies over the
Northern High Plains for the day on Thursday. Beneath this ridge
high temperatures are expected to climb into the 40s to mid-50s
(with the exception of Liberty, Hill, and Blaine Counties where an
expansive snow pack still resides). As the H500 ridge amplifies over
the Northern High Plains on Thursday, a longwave trough will begin
to slide southeast from the Gulf of Alaska and into the Pacific
Northwest. This will in turn cause the upper level flow to back to
the southwest across the Northern Rockies, which will allow for
increasing Pacific moisture to work in across Southwest and portions
of North Central Montana. Precipitation chances will begin to
increase throughout the day on Thursday from southwest to northeast
across the CWA. Precipitation chances continue then into the day on
Friday, as the main H500 shortwave begins to work east across the
Intermountain West and into the Northern Rockies. - Moldan


Updated 0600Z.

FZFG will continue to cause significant aviation impacts across
Central MT through SAT AM.

The main concern overnight will be continued LIFR/VLIFR conditions
across Central MT due to very low clouds and FZFG. Seeing as there
is no real mechanism to mix the fog out, conditions will remain
poor through SAT AM, possibly even lasting into early SAT
afternoon. Attention then turns to the next wave moving north from
the Great Basin. This system will bring rain and snow showers to
SW MT, possibly even accompanied by some lightning and gusty winds,
with areas of light snow expected over portions of Central MT.
The worst conditions, aviation-wise, look to be over Central
Montana where low clouds and snow will lead to continued IFR/LIFR
conditions. MARTIN


Updated 17/400 AM.

General lowland flooding from snowmelt will continue to be a
concern over the weekend, especially across SW MT where temps
won`t be as cool as further north. We will continue to cover these
hazards with Hydrologic Outlooks and Flood Advisories for now.

Temperatures will warm each consecutive day to start the upcoming
work week, with highs peaking on Wednesday and Thursday in the mid-
40s to mid-50s across Southwest and portions of North Central
Montana, or generally along and southwest of a Lewistown, to Great
Falls, to Browning line. This will lead to continued snowmelt for
snow remaining at both lower and even high elevations during the
daytime hours. A large upper level disturbance is then expected to
move across the Northern Rockies during the second half of the work
week, which could bring accumulating snow to the mountains of
Southwest and North Central Montana and rain/snow to lower
elevations. Any liquid precipitation on top of the remaining and
warmed snowpack, will only lead to additional flooding concerns
where poor drainage exists. MARTIN/Moldan


GTF  35  24  36  20 /  60  70  50  30
CTB  32  22  32  17 /  30  70  70  30
HLN  37  28  38  22 /  60  60  70  50
BZN  40  22  39  19 /  40  30  40  30
WEY  35  14  29  12 /  70  30  50  50
DLN  38  20  36  17 /  60  40  60  50
HVR  33  19  34  16 /  10  60  70  40
LWT  37  22  35  20 /  20  30  30  30



Flood Advisory continues until further notice for snowmelt in
Jefferson...Broadwater...and Northern Gallatin Counties until Friday

Winter Weather Advisory from 3 PM this afternoon to noon MDT
Sunday above 5000 feet for Central and Southern Lewis and

Freezing Fog Advisory until 9 AM MDT this morning Blaine...
Cascade...Chouteau...Eastern Glacier...Eastern Pondera...Eastern
Teton...Hill...Liberty...Northern Rocky Mountain Front...
Southern Rocky Mountain Front...Toole.

Winter Weather Advisory from midnight tonight to noon MDT Sunday
Eastern Glacier...Eastern Pondera...Eastern Teton...Hill...

Winter Weather Advisory until 6 AM MDT Sunday above 6500 feet
for Beaverhead...Gallatin...Madison.

Winter Weather Advisory from 6 PM this evening to noon MDT
Sunday Cascade...Chouteau.

Winter Weather Advisory from 6 PM this evening to noon MDT
Sunday Northern Rocky Mountain Front...Southern Rocky Mountain


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