textproduct: Great Falls

KEY MESSAGES

- There will be isolated showers and a thunderstorm across mainly Southwest MT this afternoon and evening.

- Warm temperatures with scattered showers and isolated thunderstorms across the region Saturday.

- Sunday and Monday turn slightly cooler and wetter.

- A mid week system brings another round of cooler, wetter, and breezy conditions.

DISCUSSION

- Meteorological Overview:

Southwest flow aloft from the incoming upper-level trough will lift small 500mb vorticity maximums north across Southwest MT this afternoon and evening. This has begun to develop isolated showers across Southwest MT. These showers will lift north throughout the afternoon and evening. As they track to Central MT, showers will likely fall apart. Weak instability along the MT/ID border this afternoon will bring a low end threat for a thunderstorm or two.

Saturday, a stronger upper-level shortwave moves through. Winds aloft increase slightly Saturday afternoon, which will bring breezy to gusty winds to Southwest MT. Moisture and warm air advection will bring scattered showers and enough buoyancy for thunderstorms to develop in the afternoon and evening.

Better chances for wetter precipitation will be Sunday into Monday as a closed 500mb wave moves across the Great Basin Region. Temperatures cool slightly then, but will relatively stay mild. Most impacts from snow looks to be at and above pass level, with There is a low end chance Monday for an isolated thunderstorm or two. Once this system exits, there will be a brief lull in active weather before another trough moves through Wednesday and Thursday. Wind aloft increases, which will help mix down breezy to gusty winds at the surface. This system looks to bring cooler temperatures and another round of wetter precipitation.

- Forecast Confidence & Scenarios:

Thunderstorm risk This Afternoon and Saturday:

Overall thunderstorm risk today is low, and mostly confined to the MT/ID border. Dry air at the surface and strong mid level lapse rates may try to produce one or two 40-50 mph wind gusts with a storm.

Better instability builds in tomorrow to around 500 j/kg. Though, overall thunderstorm risk remains isolated. Good lapse rates and inverted V soundings show that a few strong sub severe storms will be capable of producing gusty winds and small hail. I can't rule out one marginal severe gust of 60 mph though.

Precipitation Sunday and Monday:

Probabilities for moisture amounts haven't change much since the last forecast. There's still a 30-60% chance for 0.5" of moisture between the Boulder Hill to Great Falls to Lewistown areas. Probabilities are less across the Southwest MT Valleys and Hi- line. Snow levels do cool down Sunday night to Monday morning to ~6,000ft. There doesn't look to be strong cold air advection aloft or frontogenesis to cause snow levels to crash much further. Therefore, impacts from snow look to stay at above pass level, but even impacts at pass level look to be light. -Wilson

AVIATION

10/18Z TAF Period

Main concern this taf period will be increasing showers/thunderstorms this afternoon/evening over Southwest MT. The storms will move into Central MT early this evening before diminishing. Another round of showers/thunderstorms is expected on Sat afternoon/evening. The main impact from any thunderstorms will be gusty winds/lightning. Some mountains/passes will be obscured at times by clouds/precipitation. Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 40 72 41 59 / 10 40 60 40 CTB 33 66 36 53 / 0 20 20 20 HLN 42 71 38 62 / 20 70 60 70 BZN 39 70 36 63 / 20 30 50 60 WYS 33 58 31 54 / 20 60 60 70 DLN 39 66 35 61 / 20 50 30 50 HVR 35 74 40 60 / 0 10 50 20 LWT 40 73 40 60 / 10 30 60 50

TFX WATCHES/WARNINGS/ADVISORIES

None.