textproduct: Great Falls

KEY MESSAGES

- Scattered showers and thunderstorms develop this afternoon and evening, with isolated severe winds and hail possible.

- Dry and warm conditions settle in Wednesday and Thursday.

- Breezy winds and showers and thunderstorms move in through the weekend.

DISCUSSION

- Meteorological Overview:

For the rest of today, a small shortwave trough moving through the region will continue to develop scattered showers and thunderstorms across the region. Weak instability of 500-1,000 j/kg and mid level moisture will help aid in thunderstorm development. One thing that is lacking is shear, with ~20kts of shear (closer to 30kts in Southwest MT) will limit longevity of storms. However, deep mixing and a dry surface layer creates a conducive environment for gusty to isolated severe wind gusts for mature/collapsing storms. DCAPE reaching near 1,000 j/kg supports this environment. Decent mid level lapse rates also supports a localized hail threat with the strongest storms.

After this shortwave trough passes, upper-level ridging builds in for Wednesday and Thursday. The lack of forcing will keep the region dry. Upper-level ridging continues into Friday with a trough building in along the West Coast U.S. Though, embedded vorticity maximums and an increase in monsoonal moisture returns showers and thunderstorm chances.

The trough propagates east over the weekend and into early next week, keeping showers and thunderstorms in the forecast. With the upper-level ridge breakdown comes an increase in mid level winds Saturday into Sunday and therefore surface winds. There remains some uncertainty in how strong the winds gets but there is good confidence for breezy winds for the weekend.

- Forecast Confidence & Scenarios:

There is some uncertainty in how strong the winds get this weekend. However there is better agreement in the timing of the winds (late Saturday through Sunday morning. Right now, there's a good coverage of 30-70% chances for 30 mph gusts Saturday and Sunday, with 20-40% chances for 40 mph winds along the Hi-line Sunday. Seems like the main limiting factor for fire weather is low confidence for critical relative humidities, but can't rule out elevated fire weather concerns if winds keep trending this weekend. Another thing to watch for this weekend is high PWAT values of an inch bringing a localized flash flooding threat to burn scars. -Wilson

AVIATION

18/18Z TAF Period

SHRA and TSRA will be the main concern this afternoon and evening, with some of the stronger TSRA possibly producing some gusty winds. While most terminals have at least some risk of seeing some SHRA or TSRA within 20 miles, I kept mentions out of KHLN, KCTB, and KHVR for now, with all other TAF locales having either a tempo or PROB30 group. TSRA will wind down by 03z, with VFR conditions prevailing thereafter. Smoke will also be a minor concern, but should remain above VFR levels with slantwise visibility being the main impact.

For KGTF: Expect winds to gradually increase through the afternoon from the west, with gusts to around 15 kts. While this direction will keep crosswinds just below 15 kts, expect winds to shift to the northwest, which will bring the probability of seeing crosswinds over 15kts to around 80%, with TSRA in the vicinity that will be capable of producing additional gusts. Winds will weaken around 3z, at which point the risk for crosswinds will diminish. Ludwig

PRELIMINARY POINT TEMPS/POPS

GTF 54 86 55 85 / 20 0 0 0 CTB 52 82 52 78 / 20 0 0 0 HLN 52 86 53 89 / 30 0 0 0 BZN 49 83 50 90 / 30 0 0 0 WYS 37 79 39 84 / 30 0 0 0 DLN 48 83 49 86 / 20 0 0 0 HVR 55 87 55 87 / 20 0 0 0 LWT 52 82 52 85 / 20 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.