Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 211812
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1150 AM MDT Tue Mar 21 2017

Aviation Section Updated

.UPDATE...

Areas of fog and drizzle continue to linger across southwestern
Montana this morning. Some areas...mainly at pass level...are
seeing temperatures right around or below freezing...causing some
freezing fog and freezing drizzle at times. Made changes to
include this in the forecast through the morning hours. Daytime
heating and mixing should dissipate any lingering fog and drizzle
this afternoon. Otherwise little changes made to the forecast.
There was some consideration given to adding in thunderstorms
across the southwest...mainly for Madison and Gallatin Counties.
New NAM model indicating a weak boundary may lift the moist air
with MU Capes approaching 500 J/KG and modest shear. Something to
keep any eye on for later today...and any storms that can form
will be isolated and sub severe.


&&

.AVIATION...
Updated 1750z.

IFR conditions will continue across most terminals through the
remainder of the afternoon, with the exception of Southwest Montana
where MVFR conditions may linger for a couple more hours due to
lower ceilings. A weak boundary currently across Southwest/Central
Montana, will lift northeastward as a warm front during the late
afternoon and into the evening hours. This front will be the focus
for rain showers, and possible even light freezing rain near the
KCTB and KHVR terminals in North Central Montana during the
nighttime hours. The models have been persistent in keeping a
shallow layer of cold air (right around freezing) over North Central
Montana for the past few days for tonight/early tomorrow morning as
the front lifts north, so decided it was best to mention the
possibility of freezing rain at this time.

&&

.HYDROLOGY...
400 AM Update.

We will continue with the point flood warnings across Beaverhead,
Hill and Blaine counties this morning. The point forecast
warnings in Blaine county should be able to cover the flooding
impacts, as most problems are occurring on the Lodge and Battle
Creeks, along with the Milk River. River levels in Blaine County
have all shown a slow decrease over the past 24 hours, but it will
take a little more time to get all the water through the system.

In Beaverhead County, an ice jam is causing flooding problems on the
Big Hole River between Dewey and Wise River. This is a very
unpredictable problem/impact, and additional highlights might be
needed for this river later today.

A side from the potential of additional ice jams on rivers, no other
flooding impacts are expected at this time. Brusda

&&

.PREV DISCUSSION...
/ISSUED 554 AM MDT Tue Mar 21 2017/

Today through Thursday...The main challenges over the next few days
are the potential for a light wintry mix tonight along the Hiline,
the potential of accumulating snow along the MT/ID border, and
flooding concerns. For the latest flooding information, see the
Hydrology section below.

Showers this morning associated with the stationary front draped
across SW MT will gradually diminish as the best lift/moisture
shifts east of the local area. In the wake of the showers, patchy
fog (dense in a few spots) is likely this morning across parts of
SW MT where most of the precip fell yesterday and where low level
moisture lingers. The front will begin to move north as a warm
front later today as another shortwave approaches from the SW.
Lift with the wave isn`t overly impressive, but should be enough
to allow areas of light precip to develop from SW to NE later
today through tonight. Along the Hiline, temperatures may fall
near/just below freezing late tonight allowing a brief window of
opportunity for a wintry mix. The amount of warm air aloft moving
in makes the temp forecast tonight tricky and the models vary
quite a bit on just how cold it will get. Overall, confidence is
low to moderate on wintry precip from Cut Bank to Havre tonight.
Regardless, impacts look very minimal, if any at all, given the
short duration and ground/road temps only hovering around 32
degrees.

A dry and breezy westerly flow develops in the wake of tonight`s
system with precip decreasing at lower elevations (but still
lingering in the mountains). Late Wednesday night into Thursday,
there is the potential that parts of far SW MT (ie. in and around
the W. Yellowstone area) will get clipped by a strong storm system
moving through the central Rockies. I upped pops during this time
given the better model agreement. However, the highest
precip/snow amounts looks to stay east of the local area, but it
bears watching. Right now, only thinking light accumulations of
snow in that area with only minor impacts. Martin

Thursday night through Tuesday...An upper level ridge of high
pressure will build over the region for Thursday night and Friday.
Thus expect mostly dry conditions away from the Rocky Mountain
Front, along with comfortable spring time temperatures on Friday.
For the rest of the extended, the GFS/EC models differ on
timing/location of precipitation events. The GFS is wetter Saturday
afternoon through Sunday, while the EC is wetter Monday
afternoon/night. Thus confidence is low for now in terms of how wet
this weekend will be, especially across Southwest MT. By early next
week, both models bring in enough cold air, that snow could mix in
at all elevations from time to time, but especially at night/early
in the morning. Overall temperatures will be the warmest/well above
normal on Friday, then gradually cool to near/slightly above
seasonal normals over the weekend and into early next week. Brusda

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  54  38  64  36 /  20  50  30  10
CTB  47  32  56  32 /  40  70  30   0
HLN  55  37  62  36 /  30  40  30  10
BZN  52  37  61  34 /  30  40  30  10
WEY  43  32  45  28 /  30  70  60  40
DLN  55  35  57  33 /  30  50  30  10
HVR  47  32  61  34 /  20  40  30  10
LWT  46  36  61  35 /  20  40  20  10

&&

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

$$

weather.gov/greatfalls



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