textproduct: Great Falls

KEY MESSAGES

- Transition to a cooler system begins this afternoon, with lower elevation rain and higher elevation snow becoming more widespread across Southwest Montana this evening.

- Better precipitation arrives across North-central Montana, particularly west of I-15, Sunday evening.

- Precipitation gradually moves out late Monday, though unsettled conditions remain through the remainder of the week.

DISCUSSION

Large scale upper level troughing will bring a couple waves of precipitation to the region over the next few days. The initial push of better precipitation arrives through the remainder of the day and tonight, primarily across Southwest Montana into portions of Central Montana. Snow levels fall with the arrival of the coolest air aloft, dropping down to as low as 6,000 ft or so at times, lowest south of I-90. Brief spurts of snow levels lower than 6,000ft will not be unreasonable given the precipitation rates that look to exist as convective elements slowly transition to be more stratiform in nature.

As the initial core of the upper level low fills across Southwest Montana into Sunday, deepening within the troughing looks to ensue further north across North-central Montana. A closed mid-level low looks to develop somewhere over northern Montana as a result, and gradually nudge back westward during the day Sunday. This will result in not only a favorably forced environment from a dynamical perspective over the plains, but there will be some upslope enhancement as northerly or northeasterly flow on the western side of the mid level flow interacts with terrain. All this to say there looks to be a favorable regime for widespread precipitation over North-central Montana beginning Sunday afternoon, but peaking Sunday night into early Monday before gradually diminishing. Peak precipitation amounts will follow where the best forcing ultimately settles, but also occur in upslope favored areas along the Rocky Mountain Front. The overall orientation of terrain favors the Northern Rocky Mountain Front over the southern ever so slightly for greater precipitation amounts. A flood watch remains in effect for areas in and near the eastern side of Glacier NP east of the Continental Divide.

A surface low looks to follow a similar track as the aforementioned mid level low Sunday into Monday. As this occurs, gusty winds look to develop over the plains. While the overall magnitude of these winds/gusts is not particularly notable for the region, the time of the year and direction of the stronger winds (Direction will vary quite a bit, dependent on where the surface low tracks with respect to any given area) does look to be a bit abnormal, which may pose greater impacts.

The upper level disturbance moves out into Tuesday, though a lingering southwesterly flow aloft and general troughing across the west will keep afternoon and evening showers and thunderstorms in the forecast for much of the remainder of the week. -AM

AVIATION

27/18Z TAF Period

Scattered showers and thunderstorms will affect the CWA throughout this TAF period. Thunderstorms will dissipate this evening, moving southwest to northeast out of the CWA by early tomorrow morning. More consistent precipitation will begin following the exiting of these thunderstorms. There will be MVFR conditions and mountain obscuration at times due to passing showers. -Dzomba

HYDROLOGY

Periods of rain will affect the CWA from today through Monday. Widespread rainfall amounts of 1 to 3 inches are expected across the CWA over the next 3 days.

In the Glacier Park region, rainfall could be a bit heavier, possibly up to 4 inches. Additionally, with some high elevation snow still left, the combination of rainfall/snowmelt could lead to some flooding on the east side of Glacier National Park. Thus a flood watch has been issued for this region starting on Sunday.

Additional flood statement might be needed if rainfall amounts start to exceed 3 inches elsewhere across the CWA. Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 49 58 46 59 / 90 90 90 100 CTB 49 63 48 60 / 60 90 100 100 HLN 46 54 45 57 / 100 100 100 100 BZN 41 52 39 55 / 100 100 90 100 WYS 32 48 29 48 / 90 90 90 100 DLN 38 50 37 56 / 100 90 90 90 HVR 50 67 47 66 / 80 100 90 90 LWT 44 55 41 57 / 90 90 80 90

TFX WATCHES/WARNINGS/ADVISORIES

Flood Watch from Sunday evening through Tuesday morning for East Glacier Park Region.

Winter Weather Advisory from 6 PM this evening to noon MDT Monday for Elkhorn and Boulder Mountains-Gallatin and Madison County Mountains and Centennial Mountains-Ruby Mountains and Southern Beaverhead Mountains.

Winter Storm Warning from 6 PM this evening to noon MDT Monday for Northwest Beaverhead County.