High Wind in Montana, January 29 to 30, 2017

Area Forecast Discussion
National Weather Service Great Falls MT
416 AM MST Sun Jan 29 2017

UPDATED AVIATION

.DISCUSSION...

Today through Tuesday...The high pressure ridge over the forecast
area will be flattened by a shortwave trough today and tonight,
bringing strong westerly winds at times to much of North Central
and Central Montana. The surface pressure gradient and mid level
winds will be strongest along the Rocky Mountain Front. The strong
gradient will bring sustained winds of 30 to 50 mph to the Front
this afternoon and evening, while the 65 to 75 kt 700 mb winds
may get mixed down to the surface at times. Have therefore
upgraded the High Wind Watch to a Warning for the afternoon and
overnight hours. However, forecast models indicate that the peak
wind period will be from about 4 pm until 10 pm, then the winds
should gradually decrease after midnight. The pressure gradient
does not spread out onto the rest of the plains until the evening
and overnight period, but mid level winds will not be nearly as
strong. Regardless, the winds will keep temperatures quite mild,
with highs mostly in the mid 40s to lower 50s and lows in the mid
20s to lower 30s. Winds will mostly not be mixed down into the
Southwest Montana valleys due to lingering temperature inversions,
which will keep high temperatures in the 20s to lower 30s and lows
in the single digits and teens. The area will remain dry during
this time as well, with only areas of mid and high cloudiness
passing overhead.

The passage of the aforementioned shortwave trough and the
approach of another shortwave trough from Canada will start to
change the weather pattern across the area Monday into Tuesday.
The westerly flow aloft on Monday will become increasingly moist
and northwesterly on Tuesday. Breezy westerly winds will linger
into the day on Monday with only a chance of snow in the western
mountains. A Canadian cold front will also move into northern
Montana on Monday, cooling highs mostly back into the 40s in the
north, while the southwest valleys remain in the 20s to lower 30s.
A strong jet ahead of the approaching shortwave will help bring
extra lift over the forecast area Monday night into Tuesday, which
will combine with the increased moisture to produce an increasing
chance of precipitation. It will start out as mostly rain over the
plains Monday evening, but the Canadian cold front will push south
over the plains during the late evening hours, causing the
precipitation to become mostly snow. This front will likely linger
over a line from west central Montana to south central Montana
into the day on Tuesday, focusing the best chance of snow over
central Montana (including the Helena, Great Falls, and Lewistown
areas). Cooler air with the front will keep lows Monday night
mostly in the teens, with highs only warming into the 20s for
Tuesday. At least minor snow accumulations are expected over most
of the forecast area on Tuesday, so will continue to monitor the
situation for possible winter weather highlights.
Coulston

Tuesday Night through Friday...Fairly confident the stationary
front will remain draped across Southwest Montana Tuesday night
into Wednesday morning as zonal flow aloft persists. Isentropic
lift along and near the front, enhanced by the right entrance
region of a mid- to upper-level jet streak, and plentiful
moisture should result in periods of light to moderate snow. By
Wednesday afternoon, forecast confidence decreases. The 00Z/Sun
GFS suggests the stationary front will remain across Southwest MT
Wednesday afternoon through Friday morning before diminishing
thereafter as zonal flow aloft persists. This pattern would allow
periods of light to moderate snow to continue over much of the CWA
into Friday morning. However, the 00Z/Sun ECMWF suggests a sharp
shortwave trough will move southeastward over the CWA later
Wednesday, allowing the front to exit the CWA to the south as colder
air overspreads the area. Then, Thursday into Friday, the ECMWF
suggests a mid to upper-level high pressure ridge will build
eastward from the Pac NW. This scenario would result in drier
conditions later Wednesday through Friday. As a result, adjusted
POPs downward, but only slightly, for later Wednesday through Friday
due to this new discrepancy in model guidance. Temperatures are
expected to be colder than normal during the period, especially in
North-Central MT.

Friday Night through Sunday...Both the latest ECMWF and GFS are
in general agreement that zonal flow aloft will occur during this
period. Accordingly, moist upslope flow should result in
additional periods of snow along western aspects of mountains.
Dry conditions should occur elsewhere, including the plains and
valleys, due to downslope flow. This pattern should allow
temperatures to moderate somewhat, but at this time, readings
look to be at least several degrees below normal.
Jaszka

&&

.AVIATION...
Updated 1116Z.

VFR expected next 24-hours. However, abundant mid- to upper-level
cloud cover will continue to spill-over a mid- to upper-level high
pressure ridge building into our area. This ridge is expected to be
broken-down by a weak weather disturbance after 00Z/Mon, causing
flow aloft to back from northwesterly to west-northwesterly. Breezy
to gusty winds are expected in North-Central MT and at KEKS, while
LLWS is possible at times in North-Central MT. Lastly, mountain
obscuration from lower clouds and snow showers is possible along the
Rocky Mountain Front after 00Z/Mon.
Jaszka

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  51  32  46  20 /   0   0   0  50
CTB  48  30  42  16 /   0   0   0  40
HLN  33  17  33  16 /   0   0   0  60
BZN  33  11  33  17 /   0  10   0  40
WEY  20   3  22  13 /   0   0  10  40
DLN  36  17  37  22 /  10   0   0  20
HVR  50  29  43  17 /  10  10   0  20
LWT  52  31  45  16 /   0  10   0  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from 2 PM this afternoon to 5 AM MST Monday
Northern Rocky Mountain Front...Southern Rocky Mountain Front.

&&

weather.gov/greatfalls