Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 200740

Area Forecast Discussion
National Weather Service Great Falls MT
1239 AM MST Tue Feb 20 2018

...Update to Aviation...


A few flurries or brief light snow showers could linger across
northern and central portions through this evening...while the
remainder of the areas sees partly cloudy...cold...and dry
conditions. Bitterly cold temperatures and dangerous wind chills
are then expected for tonight for much of the area. See the Wind
Chill Advisories for the latest details. Southwest winds will
bring a slight warming trend for Tuesday...with below normal
temperatures overall still expected. Mainly dry conditions are
also expected...with some light snow possible across the southwest
and portions of the Hi-Line.



Satellite and radar imagery, as well as the latest short term
models, suggest that cloud cover and the risk for light snow
were somewhat overstated in the afternoon forecast package. As
such, I have updated to reduce cloud cover and PoPs tonight
through Tuesday. Have also made some minor changes to overnight
low temperatures and hourly diurnal temperature trends using
observations and short term model consensus tools. Main risk for
tonight continues to be bitterly cold wind chills across north-
central and central Montana. Current batch of advisories appear to
be on-track and do not anticipate any further updates this
evening. mpj


Update 0739Z.

An update this morning was required for EKS as low clouds have
dropped CIGs to IFR level. Latest obs indicate no apparent impact to
sfc visibility, with CIGs ranging between 002 and 004. Expect IFR
conditions to persist for the next several hours. Will continue to
monitor the future extent of IFR conditions with the next TAF
update. KLG

Previous Update 0517Z.

Areas of low clouds continue to spread slowly to the southeast
across south-central MT impacting HLN, EKS and BZN terminals with
relatively low CIG`s in place. Another area of patchy low to mid-
level clouds continue across GTF to HVR but should not last too long
after daybreak. Otherwise, no major or impactful weather conditions
are expected throughout the period. KLG


/ISSUED 420 PM MST Mon Feb 19 2018/

Tonight through Tuesday....Large mid level low continues to
progress across the WY area while a broad tough remains over the
region. Stuck in this trough is a decent amount of moisture
bringing areas of clouds and flurries/light snow to the northern
half of the CWA...with cold and partly clear conditions elsewhere.
Expect these clouds and flurry chances to linger though the
evening hours until the trough moves out and a sfc high moves
south out of Canada. With this high will come very cold air. As
this high stall across the south...west to southwest flow will
increase just enough to cause dangerous wind chills for central
and northern areas. There remains some uncertainty on the strength
of the winds and tonight`s low temperatures. The lows tonight
will depend greatly on if this remnant moisture exits tonight and
can bring clear skies. Temperatures will plummet in this
scenario. Even if the skies don`t clear...temperatures will still
drop to below zero readings. Decided to issue a Wind Chill
Advisory for those areas with the best wind and cold potential.
Some of these areas are close to Wind Chill Warning
criteria...however...confidence was not high enough to issue.
Southwest MT will also be bitterly cold tonight...however...the
stalled high pressure will limit winds needed for any highlights.
Still can`t rule out very cold wind chills and it will need to be
monitored. Tuesday then sees a slight warming trend with the
mentioned southwest winds. Temperatures will still be below normal
with highs only in the single digits to teens above zero. Overall
dry conditions are expected for Tuesday...although some lingering
light snow is possible in the southwest...and some scattered
light snow showers are possible in the north from a weak clipper
system in northwest flow. Anglin

Tuesday Night through Sunday...A deep, positively-tilted low
pressure trough will remain over the western and central United
States through this period, keeping the forecast area generally
under a northwesterly flow aloft. A series of disturbances will
move through this flow, bringing a chance of snow mainly to the
western and southwest Montana mountains...with only slight chances
across the valleys and plains as the disturbances move through.
Some breezy winds may also accompany these disturbances...perhaps
bringing some areas of blowing snow. These winds should help
bring a gradual warming to the area, but temperatures will only
warm from around 20 degrees below normal for Tuesday
night/Wednesday to around 5 to 10 degrees below normal by Saturday
night/Sunday. The strongest disturbance of the long term is
forecast to move south along the Pacific coast, which will focus
the best precipitation there. However, the greatest chance for
(what is currently forecast to be) light snow over the entire
forecast area will be during the upcoming weekend, as a
broad disturbance and cold front moves northwest to southeast
over Montana. Widespread light to perhaps moderate snow is
possible from Saturday into Sunday. Accumulations and impacts are
hard to nail down at this point as a westerly component to the
disturbance could bring hindering downslope flow to some areas.
Stay tuned to future updates for the latest details on this
developing system. Coulston/Anglin


GTF -18  13 -12  16 /  10  10  10  20
CTB -19  15 -15  14 /  10  10  20  10
HLN -14  13  -8  18 /  10  10  10  10
BZN -18  10  -9  16 /  20  10  10  20
WEY -27   8 -17  13 /  20  10  20  20
DLN -20  12  -8  13 /  10  10  20  20
HVR -20   8 -16   8 /  10  10  20  20
LWT -18  11 -11  16 /  20   0  10  10


Wind Chill Advisory until noon MST Tuesday Blaine...Cascade...
Chouteau...Eastern Glacier...Eastern Pondera...Eastern Teton...

Wind Chill Advisory until noon MST Tuesday Fergus...Judith

Wind Chill Advisory until noon MST Tuesday Central and Southern
Lewis and Clark...Northern Rocky Mountain Front...Southern Rocky
Mountain Front.


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