High Wind in Montana, January 3, 2021

Area Forecast Discussion
National Weather Service Glasgow MT
235 PM MST Sun Jan 3 2021

.DISCUSSION...

AFTERNOON DISCUSSION:
Most of the changes from the morning update have held for the
afternoon. The period that was given a little more focus was
Tuesday morning. A disturbance similar in feel and timing to the
last two pacific systems that contained freezing rain is again
expected. Have attempted to break the 6 hourly data into 3 hour
intervals and spent a little time re-mixing ensemble output for
PoPs and QPF. Based on past experience, while rain is forecasted
to fall by the basic ForecastBuilder/NBM output the consideration
of frozen ground will lead to freezing rain and slick surfaces
with black ice Tuesday morning. Therefore, have coped over
Pot_Rain grids to Pot_FreezingRain where MinTs dropped below
freezing to get the freezing rain into the forecast over a wider
area during this time period. GAH

900AM UPDATE:
Change to the forecast this morning was to focus on the wind gusts
across the area through this afternoon and early evening. 850 mb
wind spikes above 50 kts exist in almost every model along with
lapse rates that easily bring it to the surface. Petroleum,
southern Valley, central Phillips, and Garfield almost seem
certain to reach criteria. However, there is some question with
the remaining counties reaching criteria due to differing
placement in the ensembles, timing, and the usual lucky exposed
wind sensors. So, they were added on to the High Wind Warning
anyways.

Have also gone in and revamped hourly PoPs through midnight with
a blend of the ConsShort NPoP, HREF, and NBM PPI01. This ended up
raising chance for a weak disturbance to move across the northern
tier of counties late in the afternoon and early evening with the
passage of the 850 mb wind spike. GAH

MORNING DISCUSSION:
Another cold front will bring stronger winds through Central and
Eastern Montana this afternoon. Many models have wind speeds of 45
to 55 kts at 850mb in areas where lapse rates are around 7-8
C/km, particularly along areas of higher terrain such as the
Little Rockies and to a lesser extent through Petroleum and
Garfield counties. A High Wind Warning has been issued for
southwest Phillips county. While confidence is high for gusty
winds to mix down to surface throughout the area, the magnitude
continues to be too marginal to expand the warning to surrounding
areas east of the Little Rockies.

Although last night started off with boundary layer decoupling,
winds started ramping up after midnight to quickly mix out the
cooler air. Compressional heating from terrain downsloping out of
the west will help to bring warm and dry air aloft to the
surface. Highs are expected to be in the 40s to lower 50s across
the area and with temperatures expecting to be warmer than
yesterday, more widespread lower 50s is a possibility and Glasgow
Airport could break the previous record high of 50 degrees.

A progressive weather pattern is in store beginning with an
amplified ridge on Monday with some sun, however, temperatures
will not be enhanced by downsloping and will be end up being about
5 to 10 degrees cooler. A Pacific low replace the ridge by
Tuesday and bring the next chance for precipiation beginning
sometime in the early morning hours. It could be cold enough at
the surface to have freezing rain, however, QPF is very low and
POPs will increase as the sun rises and the system moves east.
Confidence at this time is low on the scenario and will depend on
the timing and placement of the system as we near the event, so
close monitoring and reevaluating impacts will be necessary as
models update.

After a midweek ridge, a swing back to another trough appears
probable, but model consensus begins to diverge substantially.
Seasonably warm and dry weather is expected to linger for the
next couple weeks. Roxy

&&

.AVIATION...
LAST UPDATED: 2030Z

FLIGHT CAT: VFR

DISCUSSION: Strong and gusty winds are expected this afternoon and
early evening as a front passes through the area. An isolated
shower cannot be ruled out at KGGW in the afternoon, otherwise
expect dry weather.

LLWS: Strong winds above the surface could create a speed shear
difference. This threat will last through 02-04Z. These winds may
periodically gust down to the surface.

WIND: Veering and increasing to 15 to 25 kts with gusts up to 45
kts this afternoon. Becoming west and decreasing to 10 to 15 kts
tonight through Monday morning. backing to the south Monday
afternoon.

GAH

&&

.GLASGOW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 11 PM MST this evening for Central and
Southeast Phillips...Central and Southern Valley...Dawson...
Garfield...McCone...Petroleum...Prairie...Richland...Southwest
Phillips...Western Roosevelt...Wibaux.

&&

weather.gov/glasgow