textproduct: Great Falls

KEY MESSAGES

- Light mountain snow and lower elevation rain will continue through the day.

- Cooler temperatures will persist through Tuesday morning before more seasonal temperatures return mid-week.

- Another system arrives on Wednesday bringing another round of light mountain snow and lower elevation rain as well as a chance for afternoon thunderstorms.

DISCUSSION

- Meteorological Overview:

An upper level trough will continue its way across the region bringing a chance for light mountain snow and lower elevation rain throughout the day. As the trough starts to depart, northerly flow behind it will keep temperatures well below normal through Monday. More zonal flow returns on Tuesday, allowing for seasonal temperatures to return. Just as temperatures start to gradually warm up, a shortwave trough is expected to move into the region on Wednesday bringing another chance for showers and thunderstorms. Mountain snow will generally stay above pass level for this storm with only slight probabilities of snow levels dropping low enough to provide impacts at pass level.

Towards the end of the week cluster guidance points towards upper- level ridging building along the west coast bringing a return to northwesterly flow over Montana. The result will be more seasonal to above-normal temperatures with generally dry conditions persisting across the region.

- Forecast Confidence & Scenarios:

Sunday Snow:

Overall, snowfall amounts have decreased compared to previous model runs with the mountains and valleys of Southwest Montana expected to see an additional 1 to 3 inches with isolated higher amounts possible along the peaks. For the Northern Rocky Mountains and the Little Belts, 2 to 4 inches are possible at pass level with higher amounts expected along the peaks. But generally, the models have moved away from the previously forecasted 6 to 12 inches along the peaks. This is not to say it is completely out of the range of possibilities. But the probability of occurrence now sits at around 10-20%.

While there is still some potential for enhanced snowfall along north-facing slopes, the models have backed off significantly compared to previous runs. In particular, the probabilities for snowfall amounts greater than 6 inches have decreased significantly across the region. That being said, there is still enough snowfall expected to produce impacts that are worthy of the current suite of Winter Weather Advisories. So no changes additions or subtractions were made to the advisories with this forecast package.

Wednesday Precipitation:

Given that this shortwave trough is not expected to be as cold as the current system moving through, light snow will be primarily confined to the higher peaks of the mountains. Locations at and below pass level may pick up a few flakes but nothing looks particularly impactful at this time. Lower elevations will see a chance for rain throughout the day with a chance for thunderstorms in the afternoon/evening hours. Currently, the Euro focuses more of the instability in Central Montana whereas the GFS pushes it towards Southwest Montana and the eastern Hi-Line. So the generalized 15-20% chance for a thunderstorm across a large portion of the region matches the current confidence of suggesting there could be something but there is still a bit of uncertainty as to where it will happen. -thor

AVIATION

17/06Z TAF Period

A large upper level disturbance moving over the Western US will continue to spread low to mid-level cloud cover across the Northern Rockies through the 1706/1806 TAF period, with the lowest CIGS expected to shift from Southwest Montana to over the valleys and plains of Central and North Central Montana through 09-15z Sunday. Precipitation coverage and intensity will begin to shift northward over this same timeframe as a cold front drops south from Canada and across the Northern Rockies, with the most persistent precipitation occurring in easterly and northerly upslope areas of Central and North Central Montana. While MVFR/low-VFR will generally dominate over the next 24 hours there will be times of LIFR/IFR conditions, most notably at the KCTB, KGTF, and KLWT terminals. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 40 27 52 29 / 80 40 10 0 CTB 38 25 55 29 / 70 10 0 0 HLN 43 30 54 32 / 70 30 10 0 BZN 45 28 49 26 / 30 30 20 10 WYS 37 21 43 19 / 20 20 10 0 DLN 45 23 50 26 / 40 20 0 0 HVR 44 29 53 28 / 80 50 10 0 LWT 39 27 45 25 / 70 60 40 10

TFX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory until midnight MDT tonight for Big Belt, Bridger and Castle Mountains-East Glacier Park Region-Elkhorn and Boulder Mountains-Little Belt and Highwood Mountains- Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

Winter Weather Advisory until 6 AM MDT early this morning for Gallatin Valley-Gallatin and Madison County Mountains and Centennial Mountains-Madison River Valley.