High Wind in Montana, November 21 to 22, 2017

Area Forecast Discussion
National Weather Service Great Falls MT
1015 PM MST Tue Nov 21 2017

UPDATED AVIATION

.SYNOPSIS...

A warm front will gradually lift northeast across the area through
tonight, possibly bringing a brief period of very light
precipitation to the plains and valleys. Steadier precipitation will
be found in the mountains, especially northern sections of the
Continental Divide. In the wake of the front, breezy conditions will
develop over the plains, with strong winds expected along the Rocky
Mountain Front.

&&

.UPDATE...Surface warm front continues to push eastward through
central Montana this evening. However, radar imagery and latest
short term models indicate that the chance for light precipitation
is a bit less than earlier forecasts suggested. Have updated to
lower forecast PoPs over the plains this evening and overnight.
Have also made updates to overnight low temperatures since some
locations, especially in my northeast zones, have fallen to near
or below previous forecast lows. In addition, made some tweaks to
winds using latest observations and short term model consensus
tools. Finally, kept a mention for a slight chance of freezing
rain in locations where temperatures remain below freezing this
evening and added some patchy fog overnight. Concerns for strong
winds continue for the Rocky Mountain Front and Eastern Glacier
County so have left the High Wind Warning in place. Have also
issued a partner email to address concerns for increased runoff
over the next two days (see Hydrology section below). mpj

&&

.AVIATION...
Updated 0515Z.

Westerly flow aloft and primarily VFR expected next 24-hours as high
pressure aloft builds over the area. Also, a subtle, E'ward-moving
disturbance will exit the region by 18Z/Wed. At the surface, a warm
front extends SE'ward from just north and east of KCTB to just north
and east of KLWT. This front will continue moving NE'ward and should
exit North-Central MT between 06Z and 09Z/Wed. Behind this front,
gusty southwesterly winds are expected over North-Central MT,
especially along/near the Rocky Mountain Front. This includes KCTB.
Except for the Madison Valley, including KEKS, lighter surface winds
are expected over SW MT. Lastly, patchy BR may develop in the
sheltered valleys of SW MT, such as the Helena Valley, around
12Z/Wed.
Jaszka

&&

.HYDROLOGY...
Updated 830 PM MST Tuesday

Snow levels will rise above 8000 feet tonight and remain elevated
through late Thanksgiving Day. Rainfall amounts of 1 to 2 inches
are expected in the mountains along the Continental Divide from
the Canadian border southward along the Rocky Mountain Front. This
moisture will fall on a mountain snowpack with snow water
equivalent moisture ranging from 1.5 inches to over 7 inches. This
combination will cause increased runoff in small streams and
creeks along the Rocky Mountain Front and especially in Glacier
National Park. As a result, there is a potential danger for
localized flash flooding and mud slides. mpj

&&

.PREV DISCUSSION...
/ISSUED 453 PM MST Tue Nov 21 2017/

Tonight through Thanksgiving Day...Warm front is starting to lift
across the region bringing increased cloud cover and warming
temperatures. Models indicate precipitation is still possible
along and ahead of this front this evening into tomorrow morning.
Areas along the Hi-Line that are slow to warm may see freezing
rain at times with precipitation from this warm front, while most
other areas will see rain or higher elevation snow. Meanwhile an
strengthening jet and weak disturbance in a flattening ridge will
increase southwesterly winds this evening into Wednesday morning.
Low end High Wind Warning criteria may still be met across the
Rocky Mountain Front and adjacent high plains. Thus have left the
High Wind Warning as is. Warm frontal precip will generally
diminish overnight tonight. Wednesday will see chances for rain
and snow linger across the western mountains and portions of the
southwest, while breezy and mainly dry conditions will be found
elsewhere. Thanksgiving Day will also see mainly dry conditions to
start, although rain and high elevation snow may linger across the
Rocky Mountain Front perhaps impacting those traveling across Marias
Pass. An approaching trough and cold front will also sharpen the
pressure gradient on Thursday, bringing the potential for very
strong winds. With 700 mb winds approaching 70 knots, most areas
across the central and northern portion of the CWA may see High Wind
Criteria. A watch may be needed after the current warning expires
tomorrow morning. With all this wind will come warming temperatures.
Wednesday will see highs in the 50s to near 60 while Thursday sees
potentially record setting highs in the 60s. Anglin

Thursday night through Tuesday...An amplified but progressive flow
pattern persists across the region late this week into early next
week with general upper level ridging over the western US,
occasionally broken down by passing upper troughs Thursday night and
again Mon/Tues of next week. Upper trough passing through the region
Thursday night will be accompanied by strong mid level flow aloft
along with a NW to SE Pacific frontal passage. Strong wind potential
will exist across much of the forecast area Thursday night as a
fairly large area of 60-80kt winds at 700MB spreads east across
north-central and SW MT with some recent model data suggesting a
belt of 80kt+ winds possible within this layer over north-central
MT. This combined with the shortwave and frontal passage creates a
favorable setup for strong winds to impact a fairly large portion of
the forecast area Thursday night, including southwest MT (vs
strongest winds typically confined to areas only along the east
slopes of the Rockies). Precipitation will also increase Thursday
night along the continental divide with a period of precipitation
likely to accompany the frontal passage through central and SW MT
Thursday night. Snow levels initially above pass levels will lower
with the frontal passage Thursday night, but may not lower all the
way to valley bottoms. Cooler but still above average temperatures
follow on Friday with upper level ridging building over the region
through the weekend for temperatures warming back to well above
seasonal averages again by Sunday. The next Pacific trough and
associated cold front are expected to sweep east across the area on
Monday with some model spread yet with regard to timing and depth of
this wave. In any case, a period of stronger winds and at least
scattered precipitation will accompany the passage of this
system on Monday followed by cooler conditions.
Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  39  59  48  64 /  20  10  10  10
CTB  23  54  43  61 /  10   0  10  20
HLN  33  53  36  59 /  20  10  10  10
BZN  31  52  35  57 /  20  20  10  10
WEY  31  41  32  43 /  60  50  20  20
DLN  37  56  37  57 /  30  20  10  10
HVR  22  56  41  66 /  20  10  10   0
LWT  37  59  45  65 /  20  10  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 11 AM MST Wednesday Eastern Glacier...
Northern Rocky Mountain Front...Southern Rocky Mountain Front.

&&

http://www.weather.gov/greatfalls

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
516 PM MST TUE NOV 21 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0600 AM     SNOW             11 ESE LEWISTOWN        47.00N 109.21W
11/21/2017  M2.0 INCH        FERGUS             MT   CO-OP OBSERVER

            24 HOUR TOTAL

&&

MARTIN