textproduct: Great Falls

KEY MESSAGES

- It will be hot and dry today, with the warmer temperatures extending through Sunday.

- There is a chance for thunderstorms in Southwest MT Friday, with chances for showers and thunderstorms in Southwest and Central MT Saturday and Sunday.

- A cold front will bring gusty winds across the CWA Sunday, and will cause below seasonal average temperatures Monday through the middle of next week.

DISCUSSION

- Meteorological Overview:

Montana continues to be under the influence of an upper level ridge today, creating another day of warm temperatures and dry conditions. While these warm temperatures will last through Sunday, shortwaves embedded in the upper level ridge and an increase in monsoonal moisture will bring chances for showers and thunderstorms tomorrow through the weekend. Thunderstorms will mainly impact Southwest MT tomorrow night, with a transition to more stratiform precipitation Saturday morning. Saturday evening will see chances for thunderstorms in Southwest and Central MT, although isolated convection may make its way up to the Hi-Line. Sunday afternoon into the evening, thunderstorm chances increase to include Southwest, Central and North-Central MT. The main threat with these thunderstorms will be gusty winds, small hail, frequent lightning and brief, heavy downpours. While conditions are currently not favorable for strong to severe storms this weekend, we cannot rule out the possibility of an isolated one forming.

The upper level ridge will begin to break down on Sunday as an upper level trough moves over the Northern Rockies from the west. This will bring breezy conditions across the CWA on Sunday. A cold front moving through Sunday night will decrease temperatures to below seasonal averages Monday and Tuesday. However, another upper level ridge building over Montana will cause temperatures to rise again on Wednesday. This rise in temperatures will be short lived as an upper level trough moves west once again, lowering temperatures and bringing back chances for showers and thunderstorms. -Dzomba

- Forecast Confidence & Scenarios:

The main source of uncertainty in this forecast is the timing of the ridge break down and subsequent cold front. This timing will determine when we see the highest non-convective winds. Currently, models are showing the ridge beginning to break down Sunday afternoon, which coincide with the peak of the breezy conditions. If this ridge break down and cold front arrive earlier or later, the timing of the gusty winds will follow suit.

Thunderstorms: There is a 15-20% chance of thunderstorms in Southwest MT tomorrow night into early Saturday morning. On Saturday, there is a 25-30% chance of thunderstorms in Southwest MT, and a 15-20% chance of thunderstorms in Central MT. North-Central MT will see low, but non-zero, chances of thunderstorms Saturday, generally below 15%. On Sunday, there is a 25-40% chance of thunderstorms in Southwest and Central MT, and a 15-20% chance of thunderstorms in North-Central MT. Over the weekend. The inflow of monsoonal moisture will increase precipitable water to over an inch, creating the risk of localized flash flooding over burn scars with a stronger thunderstorm. The potential for flash flooding will depend on how fast the thunderstorm is moving, and what the rainfall rates associated with it are.

Precipitation: Saturday through Sunday, there is a 50-70% chance of at least a wetting rain (0.1 inches) in Southwest MT. This chance decreases to a 15-30% chance in Central MT, and a 5-15% chance in North-Central MT. For a quarter of an inch during this same time period, there is a 15-25% chance in the valleys and a 35-50% chance in the mountains of Southwest MT. This chance decreases to less than 5% across Central and North-Central MT.

Wind: On Saturday, there is a greater than 50% chance for wind gusts greater than 25 mph throughout the CWA. For wind gusts greater than 30 mph, this chance remains greater than 50% east of a line between Havre and Lewistown, and south of Dillon. For the rest of the CWA, this chance decreases to 20-45%. For wind gusts greater than 40 mph, the only considerable chance will be in northern Blaine County, with a 50-75% chance.

On Sunday, there is a greater than 75% chance of wind gusts greater than 25 mph across the entire CWA. For wind gusts greater than 30 mph, there is a 65-85% chance in Southwest MT, a 50-80% chance in Central MT, and a 45-65% chance in North-Central MT, with the exception of northern Blaine County and along the Rocky Mountain front, in which chances are 80-100%. For wind gusts greater than 40 mph, these chances decrease to 25-50% in Southwest MT, 15-30% in Central MT, 15-30% along the Hi-Line east of Chester, 20-50% along the Hi-Line west of Chester to Cut Bank, and 50-70% along the Rocky Mountain front. There is a 25-50% chance of wind gusts greater than 50 mph along the Rocky Mountain front. -Dzomba

AVIATION

20/18Z TAF Period

VFR conditions prevail this TAF period. Regional wildfire smoke will reduce slant-range visibility at times, but is not forecast to impact surface visibility. Outflow from thunderstorms north of the International Border look to result in at least a brief wind shift over much of the plains tonight, but confidence in timing and magnitude remains low at this time.

For KGTF: Initial concern will be for winds to shift, at least briefly, to a more northwesterly direction this afternoon. Most guidance keeps winds through the remainder of the afternoon less than 10 kts, with gusts largely around 15 kts or less. One high- res model is more aggressive with mixing and develops a slightly stronger westerly to northwesterly wind, around 15 kts with gusts to 20-25 kts, near 23Z. Overall this more breezy scenario with stronger cross-winds has around a 20% chance to unfold. As the surface decouples around 02Z, winds look to decrease. -AM

PRELIMINARY POINT TEMPS/POPS

GTF 56 87 59 90 / 0 0 0 10 CTB 52 82 54 85 / 0 0 0 10 HLN 58 91 60 89 / 0 0 10 30 BZN 54 91 56 88 / 0 0 10 30 WYS 46 89 49 81 / 0 0 10 40 DLN 54 89 57 86 / 0 0 20 40 HVR 55 89 59 93 / 0 0 0 0 LWT 55 85 57 88 / 0 0 0 10

TFX WATCHES/WARNINGS/ADVISORIES

None.