Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 210500

Area Forecast Discussion
National Weather Service Great Falls MT
1100 PM MDT Tue Mar 20 2018

...Aviation Section Updated..


Only minor updates to the going forecast this evening. Skies are
clearing out fairly quickly as the sun sets, which should allow
areas of freezing fog to form over the plains of north central
Montana northeast of a Cut Bank to Lewistown line. Decreasing
winds and still plenty of slowly melting snow cover around this
area will help develop more fog than previously expected, so have
adjusted the forecast to reflect this. Widespread dense fog
appears unlikely at this time, but it cannot be ruled out. The
rest of the forecast is on track.



Generally dry conditions will prevail across north-central and
southwest Montana through Wednesday with temperatures ranging
from near to slightly above seasonal averages. A large Pacific
storm system will gradually move inland late this week through
the upcoming weekend, bringing another period of unsettled
conditions with temperatures turning cooler.


Updated 0500Z.

VFR conditions are expected to persist at most terminals across
Southwest and North Central Montana through the afternoon hours on
Wednesday, with the only exception being the KHVR terminal where
fog/mist may reduce visibilities to MVFR/IFR conditions during the
early morning hours on Wednesday. Mid and high level cloudiness will
begin to increase across Southwest Montana during the afternoon
hours on Wednesday as bountiful Pacific moisture begins to stream
into the Northern Rockies. Showers and lower clouds may then begin
to obscure some mountain tops after 18Z, especially across Southwest
Montana and along the Continental Divide. Snowmelt across North
Central Montana during the day on Wednesday, combined with light
winds and limited cloud cover during the early evening hours, may
lead to the development of fog during the evening hours on
Wednesday. This fog, should it develop, would most likely impact the
KCTB, KHVR, and KGTF terminals. Given that this fog is at the end of
the TAF period and confidence is not high in its development, have
held off on placing any mention in the current TAF package. -



Overall, moderating temperatures this week will cause increased
snowmelt and subsequent runoff, which will increase the potential
for additional minor low land flooding and water approaching some
rural roadways. The threat for more widespread precipitation will
increase Wednesday night into Thursday and continue through
Thursday night before decreasing on Friday. The relatively warm
temperatures will keep the snow level relatively high, which will
increase the chance for rain to fall on mainly mountain snowpack
and cause enhanced runoff. The greatest potential for this to
happen will be in far southwest Montana, where 0.25 to 0.50
inches of precipitation (rain and melted snow combined) could
fall. Have therefore issued a Flood Watch for the southern halves
of Beaverhead, Madison, and Gallatin Counties for late Wednesday
night through Thursday night. Will continue to evaluate the
situation and model guidance to see if further flood statements
are needed. MARTIN/Hoenisch/Coulston


/ISSUED 555 PM MDT Tue Mar 20 2018/

Weak ridging across the region will amplify tonight through
Wednesday in response to a digging upper level trough off the
west coast. This will keep dry conditions in place across north-
central and southwest Montana tonight. Somewhat breezy west winds
area-wide will diminish this evening and this may allow some fog
to slide back SW into Hill and Blaine counties overnight.
Attention then turns to the first surge of moisture associated
with the incoming Pacific storm system, which lifts NE across the
region late Thursday and Thursday night with an associated mid
level shortwave, while a strong upper level jet moves around the
base of the large scale trough and lifts NE across WY by Friday
morning. Deep moisture from a decaying atmospheric river is
pulled NE across ID Thursday night, resulting in some significant
precipitation for areas along the ID border, primarily east of
Monida Pass. Snow levels also look to rise to around 8000 ft
Thursday night, raising hydrologic concerns for rain on snow,
accelerating the snow-melt across these areas. A flood watch will
be issued to highlight these concerns, primarily for far southern
portions of Beaverhead, Madison and Gallatin counties, near the
Idaho border. A secondary area of organized precipitation
develops and moves east along the Hi line Thursday night, with
enough warming aloft to present the possibility of freezing rain
where low level east winds and a cold snow-pack will keep surface
temperatures near freezing. Additional pieces of shortwave energy
move through the region Friday through Sunday as the large scale
trough moves inland and weakens. Showery and cool conditions are
likely to persist through the weekend with snow levels lowering
back to 3000-5000 ft by Friday and lowering further by the
weekend. Precipitation amounts and intensity look fairly light
this weekend however with little organized upper level support
but temperatures will trend back to below seasonal averages.
Drier and slightly warmer conditions look to return early next
week as the upper level trough exits into the central United
States. Hoenisch


GTF  30  49  28  48 /   0  10  10  20
CTB  24  41  23  38 /   0  10  10  40
HLN  30  55  31  51 /   0  10  10  30
BZN  28  48  30  53 /   0  10  10  30
WEY  21  38  27  43 /  10  50  40  80
DLN  26  47  29  50 /  10  20  20  40
HVR  21  34  20  37 /   0   0  10  10
LWT  27  47  29  51 /   0   0  10  10


Flood Watch from late Wednesday night through late Thursday
night Beaverhead...Gallatin...Madison.


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