textproduct: Great Falls

KEY MESSAGES

- Scattered to widespread showers and thunderstorms develop this evening along and north of Highway 200, with isolated large hail possible.

- Periods of widespread rainfall Monday and Tuesday, with periods of moderate to heavy rainfall across Rocky Mountain Front and Sweet Grass Hills.

- Drier conditions Wednesday and Thursday.

- Another round of precipitation for next weekend.

DISCUSSION

- Meteorological Overview:

A passing shortwave will develop scattered to widespread showers and thunderstorms mainly north of the MT Highway 200 corridor late this afternoon through the evening. The incoming trough brings a good synoptic environment for the threat of a few severe thunderstorms across North-Central MT, with hail as the primary threat. High PWATs nearing an inch this afternoon will bring an isolated flash flood threat to where soils have been saturated from recent precipitation. Behind this system, a vertically stacked low pressure system will move into the area Monday morning through Tuesday morning, bringing periods of widespread rain for mainly North-Central MT. Based on the current low track, periods of moderate to heavy rainfall will be possible for the Rocky Mountain Front and US Highway 2 corridor. This system will also bring in cooler fall-like temperatures Monday and Tuesday. Monday night into Tuesday morning, low to mid level winds increases around the closed low. This results in breezy to windy conditions across North-Central MT.

After the system exits during the day Tuesday, drier conditions move in for Wednesday and most of Thursday. 700mb winds increasing along the Rocky Mountain Front Thursday will also increase surface winds, though confidence is low for high winds.

Heading into late Thursday night into Friday, a cold front will re-introduce low end chances for precipitation. Though, better chances for precipitation arrived with the next incoming trough Saturday and Sunday. Ensembles hint at a cooler system incoming next weekend with snow levels falling to at least the higher mountain peaks. Though details and confidence in this next system is low.

- Forecast Confidence & Scenarios:

The biggest question mark of the day today was cloud cover and instability limiting severe thunderstorm potential today. The synoptics providing both directional and speed shear (40-50kts) bring a favorable environment for thunderstorms to become severe across North-Central MT. The morning cloud cover and fog could've limited instability development, but there looks to be a few breaks in the clouds early this afternoon to keep an eye on isolated severe storms. Since all the instability will be elevated, the storms will be elevated and therefore a primary hail threat, with a low end severe wind gust threat. The best window for severe thunderstorms to develop will be between 6-10pm with the best spatial corridor along and north from the Helena to Lewistown line. A limiting factor late tonight as storms get closer to the Hi-line is the drop off of instability and increase in saturated profiles. Therefore, it seems like those initial storms in the early to mid evening will be the storms to watch.

Models have been consistently showing additional 1-3" rain accumulations along the Rocky Mountain Front and the Sweet Grass Hills. How far east along US Highway 2 does this extend remains uncertain, but this northwest shift in the low track favors the Rocky Mountain Front for the corridor of heaviest precipitation. With thunderstorms today and tomorrow, some high-res guidance show up to isolated 4" if that area gets good convective rain rates. Rainfall amounts drop off significantly south of the MT Highway 200 corridor to less than a quarter of an inch. -Wilson

AVIATION

06/18Z TAF Period

Low clouds/light precip will affect portions of North Central MT early this afternoon. Then a wave of strong storms will move from southwest to Northeast through the Central and North Central MT this evening and overnight. Expect strong winds and some hail with this wave. The stronger storms transition to more of a stratiform rain by Monday morning over North Central MT. Some rain likely in Southwest MT by Monday morning, but precip will not be as heavy over the Southwest. Expect mountains/passes to be obscured at times across the CWA by low clouds/precip.

For KGTF: Main concern will be thunderstorm potential between 00z and 03z this evening. Some storms could have strong erratic wind gusts or hail. Given current conditions, kept hail out of the forecast for now. Expect low clouds/light rain Monday morning, with some improving conditions towards mid day Monday. Brusda

HYDROLOGY

Areas along the Rocky Mountain Front have received 0.75" to 1.5" of rainfall over the past day. There hasn't been much of a response in creeks/streams yet due to the drier grounding soaking all the rain up. The main concern is another additional widespread 1 to 3 inches and isolated 4 inches through Tuesday morning on a more saturated ground. Areas that get the higher end 3 to 4 inch totals may see minor overflow in creeks/streams along with muddied rural roads. The wet conditions can also cause rock/debris slides in the higher terrain. Therefore, a flood watch was issued from this afternoon through Tuesday. Farther east across North-Central MT will see wetting rain, but confidence for those higher end totals remain low. One thing to watch for is any eastern shift of these amounts farther east along US Highway 2 towards Hill County. -Wilson

PRELIMINARY POINT TEMPS/POPS

GTF 51 64 47 66 / 90 100 40 50 CTB 46 56 44 60 / 90 100 100 80 HLN 51 66 44 70 / 80 100 20 10 BZN 48 69 40 71 / 30 80 10 0 WYS 41 60 30 64 / 90 40 10 0 DLN 46 66 36 71 / 90 50 0 0 HVR 51 68 45 64 / 90 100 90 90 LWT 50 68 44 64 / 90 90 20 30

TFX WATCHES/WARNINGS/ADVISORIES

Flood Watch through Tuesday morning for East Glacier Park Region- Eastern Glacier, Western Toole, and Central Pondera-Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-Northern High Plains-Southern High Plains-Southern Rocky Mountain Front.