textproduct: Great Falls

KEY MESSAGES

- Showers and thunderstorms overnight, some of which could produce gusty winds and brief downpours.

- Cool and breezy Sunday with highs in the 60s for most lower elevation locations, with 50s in the mountains.

- Drier Monday, but showers and thunderstorms return by Tuesday.

DISCUSSION

- Meteorological Overview:

A deep upper level low moving into the Pacific Northwest will continue to funnel moisture into North Central and Southwestern Montana today on southwesterly flow aloft. Early afternoon radar has shown numerous showers and thunderstorms developing across Southwestern Montana, with a few also developing over the Rocky Mountain Front and Little Belt and Snowy Mountains. While severe wind gusts are not expected today, I cant entirely rule out a stronger storm producing a rogue gust over 50 mph or so, along with some heavier downpours, especially if they can get a bit more organized. The most likely area for the winds to get gusty will be across the plains, where clouds will hold off just long enough to allow for some better thermal mixing and deepening of the mixed layer. While storms should weaken after sunset, showers with a few rumbles of thunder will remain possible through much of the overnight thanks to persistently strong upper level dynamics.

As we head into the day on Sunday, winds shift to the west as the upper level low moves overhead, which will bring a surface cold front along with it late tonight/early Sunday morning. Behind this front, expect temperatures to be notably cooler tomorrow, with most locations only rising into the 60s, along with a breezy westerly wind with most areas on the plains gusting to 25-40 mph and 15-30 mph across the southwest.

While it will remain breezy on the plains/Rocky Mountain Front overnight Sunday night into Monday morning, expect more pleasant conditions to arrive for the daytime on Monday as upper level ridging begins to build back across the Northern Rockies. While this ridge will keep us dry on Monday, it will quickly move east as a strong trough moves towards the West Coast. As this trough approaches, southwesterly flow aloft will bring a surge of moisture into the area, resulting in afternoon thunderstorm development Tuesday and Wednesday. A stronger upper level low will likely move towards the Pacific Northwest late next week, which would allow for additional showers and thunderstorms Thursday and Friday.

- Forecast Confidence & Scenarios:

Rain today: Some of the convection allowing models have wanted to have some pockets of heavier rainfall (up to an inch) from some of the showers and thunderstorms today. While our soils should be able to readily absorb these heavier rainfall amounts, repeated heavy rainfall may result in some ponding of water in poor drainage areas.

Winds Sunday: Behind the cold front mentioned above, expect gusty winds to affect most of North Central and Southwestern Montana on Sunday. While cooler temperatures, recent rainfall, and RH values generally staying above 25-30% will prevent critical fire weather conditions, wind gusts will still be notable for late August. Expect an 80-90% chance of seeing wind gusts over 40 mph along the Rocky Mountain Front, northern slopes of the Little Belt and Snowy Mountains, and around the Bears Paw Mountains, and a 30-60% chance of seeing gusts over 48 mph. While not impossible, the risk of seeing high wind gusts (over 58 mph) appears to be low, generally under 10-20%, I can not completely rule out an isolated instance of high winds tomorrow. Ludwig

AVIATION

30/00Z TAF Period

Showers and thunderstorms will continue to affect the CWA through the overnight hours. Some storms will produce gusty winds. Expect an area of steady rain to develop over Southwest MT after 8z Sun and then move northeastward up into Central MT. Overall, much of the precipitation will exit the CWA by 18z Sun. Windy conditions, with clearing skies move in from the west starting Sunday afternoon. Mountains/passes will be obscured through at least 18z Sun.

For KGTF: It will become windy on Sunday afternoon. Sustained winds over 20 knots with gusts in the mid 30 knots will be common through the afternoon hours. Winds should begin to diminish after 01z Monday.

PRELIMINARY POINT TEMPS/POPS

GTF 50 65 45 77 / 90 50 0 0 CTB 46 61 44 74 / 60 10 0 0 HLN 46 67 43 78 / 80 20 0 0 BZN 47 67 39 78 / 70 70 0 0 WYS 36 62 30 71 / 70 70 0 0 DLN 44 69 38 79 / 70 30 0 0 HVR 51 68 47 77 / 80 60 10 0 LWT 47 64 42 77 / 80 90 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.