Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 181133

Area Forecast Discussion
National Weather Service Great Falls MT
533 AM MDT Sun Mar 18 2018

Aviation Section Updated


Areas of snow, along with some fog and patchy freezing drizzle,
will continue to cause minor impacts across Central Montana
through this morning, mainly from Helena and Great Falls north.
Snow showers will develop areawide this afternoon and again Monday
afternoon. Temperatures moderate early this week, before a cooler
and more unsettled pattern potentially returns late in the week.



Today through Monday night...Satellite imagery this morning reveals
a compact and somewhat disorganized upper level low centered very
close to Helena. Along and north of this low, bands of light to
moderate snow continue to impact portions of Central Montana.
Overall, the precipitation is more convective in nature (ie.
multiple bands of snow as opposed to one solid band of heavier
snow). Because of this, snowfall amounts will vary quite a bit as
this low continues to lift NE from Helena to Havre today. Along and
east of the low track (ie. Great Falls to Lewistown), the models
continue to differ on the depth of moisture, which continues to make
the precip-type less certain. In this area, the thinking is there
will be a mix of fog, very light freezing drizzle, and light snow.
Where moisture is deepest, the precip type will be snow and vice-
versa. The greatest impacts in this area may simply be from reduced
visibility as the lower precip rates and warm road temps look to
ultimately limit travel impacts. Any area of freezing drizzle should
be confined to this morning as the depth of moisture increases
areawide by this afternoon with a developing moist, NW flow. Ice
amounts look to only be a light glaze at worst. Given all this,
we`ll cancel the Winter Weather Advisory for Cascade County, but
leave the remaining advisories as-is and continue to re-evaluate the
situation through the morning. Overall, though, this does not
appear to be a higher-impact event. By this afternoon, the precip
will be more showery in nature and widespread impacts from
falling snow are not expected. Therefore, we`ll not extend the
Advisories into this afternoon.

Scattered snow showers will likely develop once again on Monday
afternoon as the upper level trough axis swings through. In its
wake, downsloping and subsidence aloft should help to scour out the
lower clouds that have plagued Central MT of late. Precip will wind
down as well, but may linger a bit longer in the mountains. MARTIN

Tuesday through Saturday...main forecast concerns are warming
temperatures, followed by a return to wet and unsettled weather.

H500 ridge over the Desert Southwest/Great Basin will amplify over
the Rockies Tuesday and into the day on Wednesday, all the while a
shortwave over the Gulf of Alaska digs towards the Pacific Northwest
and deepens. A weak shortwave traversing the amplifying ridge Tuesday
afternoon will bring some slight chance POPs to Southwest Montana
where better low- to mid-level moisture resides, but otherwise will
generally just bring some high clouds to the remainder of North
Central Montana for the afternoon/evening hours on Tuesday. During
the day on Wednesday the H500 ridge axis slides east across
Southwest and North Central Montana, with the flow becoming
increasingly southwesterly by the evening/overnight hours. 1000-500
mb thickness values on Wednesday climb to around 435-445dam and warm
air advection increases ahead of the approaching H500 trough over
the Pacific Northwest, which should support high temperatures in the
40s/50s south and west of a Browning, to Great Falls, to Lewistown
line, and the 30s north of this line (with the coolest reading over
Blaine and Hill counties where a substantial snowpack still
resides). Precipitation chances over the mountains of Southwest
Montana and along the Continental Divide on Wednesday will be on the
increase as the H700-H500 southwest flow begins to usher in
additional Pacific moisture, but will likely be very showery in
nature. High temperatures on Thursday will be similar to that of
Wednesday as 1000-500mb thickness values remain in the 435-445dam
range, however, increasing mid-level clouds and precipitation
coverage could hold temperatures down slightly. As just stated,
precipitation coverage is expected to be on the increase across most
of Southwest and North Central Montana during the day and night on
Thursday, as synoptic scale ascent associated with a shortwave
rotating through the main H500 trough moving from the Pacific
Northwest and towards the Northern Rockies overspreads the region.
Model solutions begin to diverge with the evolution of the upper
level pattern from Friday onwards, which will have an affect on the
timing of additional rounds of precipitation for Southwest and North
Central Montana. However, continued wet and unsettled weather is
expected to end the work week and persist into the first half of the
weekend. - Moldan


Updated 1133Z.

Low clouds and snow to continue impacting much of Central MT this
morning with widespread VLIFR/LIFR/IFR conditions.

Satellite imagery this morning shows a well defined upper level low
spinning over KGTF as of this writing. This low will continue to
lift NE through the KHVR area and eventually into southern Canada.
Low clouds, snow, and patchy BR/FZFG associated with the low will
continue to cause poor aviation conditions across Central Montana
this morning. By this afternoon, the snow will become more showery
in nature and will spread south into SW MT. By tonight, SHSN
activity will diminish some, but low clouds/fog/mtn obscurations
will remain a concern areawide with IFR/MVFR conditions likely.


Updated 18/400 AM.

Daytime high temperatures will be on the rise at both low and high
elevations into the middle of the upcoming work week, with
temperatures peaking on Wednesday and Thursday in the 40s and 50s
across most of Southwest and North Central Montana. Precipitation
coverage will be on the increase from Wednesday and into the first
half of the weekend, with some of the precipitation falling in the
form of rain, especially at lower elevations (i.e. valleys and over
the plains of North Central Montana). Increased snowmelt is likely
by the middle of the work week, as the combination of warming
temperatures and rain falling on the antecedent snowpack occurs.


GTF  32  20  40  23 /  60  30  20  10
CTB  32  18  34  21 /  90  60  30  10
HLN  36  25  41  24 /  60  40  30  10
BZN  37  21  39  21 /  40  30  30  30
WEY  30  13  31  12 /  40  50  40  40
DLN  34  16  36  17 /  50  30  30  20
HVR  31  18  31  17 /  80  30  30  10
LWT  32  20  37  22 /  30  40  20  20


Winter Weather Advisory until noon MDT today above 5000 feet for

Winter Weather Advisory until noon MDT today Eastern Glacier...
Eastern Pondera...Eastern Teton...Hill...Liberty...Toole.

Winter Weather Advisory until noon MDT today Central and
Southern Lewis and Clark...Northern Rocky Mountain Front...
Southern Rocky Mountain Front.

Winter Weather Advisory until noon MDT today Chouteau.

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


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