High Wind in Montana, February 9 to 11, 2017

Area Forecast Discussion
National Weather Service Great Falls MT
1010 AM MST Thu Feb 9 2017

.UPDATE...

Complex weather scenario continues to evolve over the region.
Observations are showing rapid temperature rises of more than 20
degrees in less than one-half hour as warm air aloft erodes the
cold air in place across the north-central plains. An earlier
update was needed to freshen weather grids for freezing rain
reported over Hill and Blaine counties. Short-term models continue
to advertise the development of high winds this afternoon, mainly
along and west of Interstate 15 and into portions of Judith Basin
county. Winds may taper somewhat overnight, then return Friday, so
will adjust wording in the high wind warning. The temperature
inversion may hold longest in the Helena valley, and made a minor
adjustment to remove Lewis and Clark county from the high wind
warning.

Winter weather advisories remain in effect across North-central MT
for blowing snow and light freezing rain. The freezing rain hazard
appears to have ended, but with extensive moisture showing on
satellite imagery cannot rule out the possibility of additional
light mixed precipitation over areas where colder air remains. An
interesting point is that temperature rises are occurring without
a simultaneous increase in winds. This may help mitigate the
drifting and blowing snow potential as snow begins to melt. Will
presently hold with current advisories, but likely expire them
early this afternoon.

The blizzard warning for the Rocky Mtn Front will go into effect
at 11am. This is the time when wind speeds are expected to
increase and continue near high wind criteria through the
afternoon. There is a deep, cold snow pack in place over this
region and would provide a source for extensive drifting and
blowing snow. However, temperatures west of Highway 89 now range
from 35 to 45 degrees with rain. Reports from the area this
morning indicate the top layers of the snow is thickening and
preventing drifting/blowing snow. Further, snow is sliding off
roof tops and blocking exits to structures. Highway 2 is closed
again due to avalanche danger. Minor street flooding is being
reported in East Glacier. The blizzard warning will be allowed to
continue given uncertainty of pending high winds, but may be
cancelled this afternoon in the absence of any blowing/drifting
snow reports. PN

&&

.AVIATION...
Updated 0535Z.

Predominant VFR cigs expected through the period as strong downslope
winds become established overnight just above the surface. This will
present wind shear issues especially at southwest TAF locations such
as KBZN and KEKS. With passage of the primary warm front from south
to north Thursday morning through early afternoon, any light snow or
brief period of freezing rain will give way to increasing
southwesterly winds, with the potential for significant visibility
reductions due to blowing snow especially toward the Rocky Mountain
Front. In sheltered areas, the rapid warming over the cold surfaces
may result in haze or fog at KHLN and other similar locations in the
2 or 3 hours after warm frontal passage.

Cassell

&&

.PREV DISCUSSION...
/ISSUED 810 AM MST Thu Feb 9 2017/

810 update: Quick addition to this morning's weather forecast. Have
received a report of light freezing rain already in the Fort Belknap
area, and KHVR (Havre) ASOS has reported periods of unknown
precipitation (sometimes associated with freezing rain). Have
therefore added a mention of freezing rain for the northeast portion
of our forecast area starting immediately. Significant ice
accumulations are unlikely at this time and the area is under a
Winter Weather Advisory for the freezing rain potential, but will
continue to monitor the situation.
Coulston

Today through Saturday...Short term models and guidance continue
to bring a significant change to the weather pattern today. Latest
observations from Deep Creek which is a few miles south-southeast
from Marias pass support this change as they currently show
southwest winds gusting into the 50s with temperatures above
freezing. Often times, models tend to push colder air away from
the front range faster than happens in real life, so the delay
starting the High Wind Warning until later this morning appears to
be appropriate at this time. We have been highlighting this
abrupt change in the pattern for several days now so I see little
need to rehash the intricate details of a very complicated
forecast, but will hit the main highlights for brevity's sake.
First, a warm airmass with abundant Pacific is currently
overriding the frigid airmass in place across the forecast area.
This will bring a risk for mixed precipitation at lower elevations
today with heavy snow and ice expected in the western mountains.
Next, the warm air will mix to the surface and push out the cold
air with strong and gusty west winds prevailing in the wake of the
exiting cold air. Existing snow will be picked up by these strong
winds causing blowing and drifting with many roads likely to
experience closures, especially in the area of Glacier Park where
very heavy snow fell earlier this week. An incredible plethora of
highlights have been issued for this pattern change and, as of
now, I am seeing no reason to make any changes to the long list of
existing warnings and advisories. By Saturday it is expected that
gusty west winds will continue but will have also fallen back
below high wind criteria. Also, with a couple of days of above
freezing temperatures it is likely that the risk for blowing and
drifting snow will have died off. Additional precipitation can be
expected, especially over the western mountains but significant
snow accumulations are not expected. mpj

Saturday Night through Thursday...Medium range models are in good
agreement late this weekend through much of next week, depicting a
longwave upper level ridge becoming established along or just inland
of the west coast with dry conditions prevailing through at least
Thursday under a northwest flow aloft. Weak upper level disturbances
occasionally move over the top of the ridge, bringing periods of
cloudiness and enhanced breezy winds over portions of north central
MT at times Sunday and Monday with generally sunny conditions
prevailing Tuesday through Thursday. Temperatures will start near
seasonal averages on Sunday and gradually warm to above seasonal
averages by the middle of next week. Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  47  37  47  29 /  20  10  10  10
CTB  43  37  44  28 /  20  10  10  10
HLN  29  27  38  19 /  30  30  20  20
BZN  40  27  38  21 /  40  20  30  30
WEY  35  34  34  17 /  90  90  60  40
DLN  43  35  43  23 /  30  20  30  20
HVR  41  35  44  29 /  60  20   0   0
LWT  47  36  46  25 /  30  10  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 PM MST this afternoon Cascade...
Central and Southern Lewis and Clark...Eastern Pondera...Eastern
Teton...Judith Basin...Toole.

Winter Weather Advisory until 5 PM MST this afternoon Blaine...
Chouteau...Fergus...Hill...Liberty.

Winter Weather Advisory until 5 PM MST this afternoon
Beaverhead...Gallatin...Madison.

High Wind Warning until midnight MST Friday night Cascade...
Eastern Pondera...Eastern Teton...Judith Basin...Toole.

Winter Weather Advisory from 9 PM this evening to midnight MST
Friday night Eastern Glacier...Northern Rocky Mountain Front...
Southern Rocky Mountain Front.

Blizzard Warning until 9 PM MST this evening Eastern Glacier...
Northern Rocky Mountain Front...Southern Rocky Mountain Front.

High Wind Warning from 9 PM this evening to midnight MST Friday
night Eastern Glacier...Northern Rocky Mountain Front...Southern
Rocky Mountain Front.

Winter Weather Advisory until 11 AM MST this morning Eastern
Glacier...Northern Rocky Mountain Front...Southern Rocky
Mountain Front.

&&

weather.gov/greatfalls