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DISCUSSION

KEY MESSAGES:

- A quick arctic cold front will continue to push southward through the region and remain here over the next few days.

- Another strong winter storm with significant snow accumulation is expected overnight tonight through Saturday morning. WEATHER PATTERN OVERVIEW:

The wet mix of rain and heavy snow from earlier this morning has given way to more scattered snow showers this afternoon and a sudden push of arctic cold temperatures and gusty winds from Saskatchewan.

Another round of headline hazard products have been issued to cover multiple winter conditions.

The cold air mass has already plunged southward across the international border. Afternoon temperatures continue to drop quickly, pushed along by gusty winds up to 40 to 50 mph from the north. To cover the concerns for the resultant hazardous wind chills, a Cold Weather Advisory and an Extreme Cold Warning have been issued. Temperature model data for this cold time period are highly variable, so, sudden fluctuations in wind chills and temperatures are possible. Next few shifts will need to watch closely for the possible need to adjust these headlines.

The next organized snow storm comes our way from the Canadian Rockies along an embedded strong low pressure system combining with plentiful moisture riding up and over the US Rockies. Models are in fairly good agreement showing roughly a 30-hour period of snowfall for the region. Locally a wide range of snowfall totals can be expected, with just a trace of snow for locations like Westby and Plentywood, and around 12 inches of snow for locations like Zortman and Winnett. A fairly tight gradient of snowfall totals can be expected for locations in between, showing a possible 3 to 8 inches. To cover these snowfall concerns, a Winter Weather Advisory and a Winter Storm warning have been issued.

Most of these winter conditions will quickly subside on Sunday as the strong low pressure moves away toward the east and warmer high pressure pushes into our region from the west. Temperatures are expected to rebound to much warmer, being aided by a chinook event off the east side of the northern Rockies.

But, areas with more snowfall will obviously take longer to accept the suddenly warmer temperatures on Sunday and Monday, and the warm up for these locations will likely be more gradual than is shown the model data. FORECAST CONFIDENCE & DEVIATIONS:

Sudden drop temperatures to below zero levels - confidence high. Exact details of how cold it actually gets - confidence low.

General idea of the next snow storm with widespread accumulation - confidence high. Exact snowfall amounts - confidence average to below average.

Given the recent large divergence between NBM and actual conditions experienced here this morning, I have deviated from the given NBM for the first 2 1/2 days in the forecast, increasing precip amounts, chances for precip, and snow amounts. Also dropping cold temperatures more than NBM.

AVIATION

Updated: 2140Z Dec 11

PATTERN/OVERVIEW: A very strong winter storm system over multiple days and with multiple impact will persist for northeast Montana now through this weekend.

FLIGHT CONDITIONS: LIFR ceilings and visibilities in scattered light snow showers today through this afternoon, then partially pausing and clearing to IFR/MVFR ceilings. LIFR ceilings returning from the west toward the end of this TAF cycle in widespread accumulating snowfall.

WINDS: This afternoon, from the north around 30 kts gusting to 40 kts at times. Gradually later tonight from the northwest at 15 to 20 kts. Friday, from the northwest at 5 to 10 kts. Friday evening, near calm.

GGW WATCHES/WARNINGS/ADVISORIES

Winter Storm Warning from 10 PM this evening to 8 AM MST Saturday for Central and Southeast Phillips-Central and Southern Valley-Dawson-Garfield-McCone-Northern Phillips-Petroleum- Prairie-Southwest Phillips-Wibaux.

Cold Weather Advisory from 7 PM this evening to noon MST Saturday for Central and Southern Valley-Dawson-Eastern Roosevelt-McCone-Northern Phillips-Richland-Western Roosevelt- Wibaux.

Extreme Cold Warning until noon MST Friday for Daniels-Northern Valley-Sheridan.

Winter Weather Advisory from 2 PM Friday to 8 AM MST Saturday for Daniels-Eastern Roosevelt-Northern Valley-Richland-Western Roosevelt.