textproduct: Great Falls

KEY MESSAGES

- Smoky skies are possible through much of the week across the region with some impacts to air quality expected.

- Hot temperatures will persist through the week with the hottest day expected on Saturday.

- Chance for isolated to scattered thunderstorms across Southwest Montana Wednesday and Thursday afternoon.

- Strong winds are possible late Saturday through Sunday with the strongest winds expected along the Rocky Mountain Front.

DISCUSSION

- Meteorological Overview:

Strong upper-level ridging will continue through the rest of the week keeping temperatures hot and funneling more smoke from wildfires in the PacNW into Montana. Monsoonal moisture lifting up and around the ridge will provide daily chances for afternoon thunderstorms with the main threat being lightning and strong gusts. Otherwise dry conditions will persist for most areas with low minimum relative humidity through the afternoon.

The ridge starts to break down this weekend as a strong trough moves into the region bringing strong winds to the region, particularly along the Rocky Mountain Front. A brief cool down is expected for Monday before things slowly start to warm back up again heading towards the middle of next week as another ridge tries to build across the western CONUS. -thor

- Forecast Confidence & Scenarios:

Thunderstorm Chances This Week...

Isolated to scattered thunderstorms are possible Wednesday and Thursday afternoon across Southwest Montana with some isolated thunderstorms possible further north. The uncertainty in the forecast centers around how thick the smoke is over the region. It is possible the smoke could limit thunderstorm development either partially or entirely depending on how things play out. But the main concern is that any thunderstorm that develops will be on the drier side which will increase fire weather concerns, especially if there is frequent cloud to ground lightning. The potential for severe weather remains quite low at this time but a stray gust up to 70 mph cannot be ruled out as DCAPE is over 1500 J/kg for a good portion of the region.

Outside of this potential most areas will remain dry through the rest of the week. -thor

Return of Extreme Temperatures by Saturday...

While temperatures will remain hot through the remainder of the work week climate anomaly indicators have honed in on Saturday being the hottest day of the upcoming week, with values becoming unusual to very unusual with respect to late July/early August climatology. The fact that these climate anomaly indicators are suggesting another period of hot temperatures is remarkable considering that August 1st is (at most locations) only one day past the climatological peak for average high temperatures across Southwest through North Central Montana. The table below represents the potential high temperature scenarios for select cities across Southwest through North Central Montana, with the "Low End" scenario being the NBM10Pct (10% chance that temperatures are lower than the listed value), "Most Likely" scenario being the NBM50Pct, and the "High End" scenario being the NBM90Pct (10% chance that temperatures exceed the listed value).

Saturday's High Temperature Scenarios LOCATION Low End | Most Likely | High End Browning 88 | 93 | 98 Cut Bank 95 | 100 | 104 Havre 103 | 108 | 111 Great Falls 102 | 105 | 107 Lewistown 98 | 101 | 104 Helena 97 | 100 | 104 White Sulphur Springs 95 | 102 | 101 Bozeman 98 | 98 | 105 Dillon 95 | 98 | 100 Ennis 94 | 98 | 100 West Yellowstone 89 | 93 | 96

Given the presence of regional wildfires across the Northern Rockies the "High End" scenario will be tough to reach due to decreased solar insolation from the smoke, with dense smoke having the ability to hold temperatures back some 5 to 10 degrees.

Strong Winds from Saturday night through Sunday...

Confidence is increasing that a period of strong, nearing high, winds will impact the Rocky Mountain Front from Saturday night through the day on Sunday due to the combination of mountain wave activity (from the late evening hours on Saturday through Sunday morning) and deep mixing (during the daylight hours on Sunday). H700 winds per NAEFS analysis are prog'd to reach 40-50kts from Saturday evening through Sunday morning across the aforementioned area, which is 2 to nearly 3 standard deviations above normal. The table below are the NBM5.0 probabilities for wind gusts to exceed 45 mph, 55 mph, and 65 mph for select locations along the Rocky Mountain Front for the previously mentioned timeframe.

Probabilities for Wind Gusts to Exceed LOCATION >45 mph | >55 mph | >65 mph Many Glacier Valley 55% | 15% | <5% Logan Pass 50% | 10% | 0% St Mary 80% | 40% | 10% Two Medicine Valley 70% | 30% | 10% East Glacier 75% | 25% | 10% Browning 70% | 25% | 5% Cut Bank 75% | 20% | 5% Heart Butte 80% | 30% | 20% Choteau 5% | 0% | 0% Augusta 20% | <5% | 0%

Those looking to travel or recreate along the Rocky Mountain Front should be prepared for strong winds; which could make driving and hiking difficult, lead to falling trees in the backcountry, and/or lead to the spread of existing/new wildfires. - Moldan

AVIATION

28/18Z TAF Period

Wildfire smoke will continue to result in reduced surface and slant- range visibility through the TAF period. A few showers or thunderstorms across portions of Southwest and Central Montana this afternoon and evening will bring a gusty and erratic wind threat. Otherwise, minimal impacts are forecast.

For KGTF: Diurnal mixing will result in breezier westerly winds mixing to the surface late afternoon through early evening before becoming light and variable near and after sunset. The probability that the westerly wind that develops this afternoon has a cross-wind component greater than 15 kts has increased slightly to 30%. -AM

PRELIMINARY POINT TEMPS/POPS

GTF 59 93 59 95 / 0 0 0 10 CTB 54 89 54 90 / 0 0 0 0 HLN 59 93 59 93 / 0 10 10 10 BZN 57 92 56 93 / 20 20 20 10 WYS 48 84 46 86 / 20 40 30 10 DLN 54 87 53 89 / 10 20 10 20 HVR 59 95 59 98 / 0 0 0 10 LWT 57 90 56 91 / 20 0 20 20

TFX WATCHES/WARNINGS/ADVISORIES

None.