textproduct: Great Falls

KEY MESSAGES

- Scattered showers and thunderstorms this afternoon capable of producing strong wind gusts and large hail.

- Isolated showers and thunderstorms across Southwest and Central Montana Thursday and Friday afternoon.

- Trending hotter toward the weekend, with Saturday and Sunday looking hottest.

DISCUSSION

- Meteorological Overview:

Upper level troughing well off to the northwest over northern BC will progress eastward through the day today, eventually emerging into northwestern AB by the evening. Flow aloft across the region will largely be zonal, but be ever so slightly cyclonic in nature given the disturbance off to the northwest. This weakly cyclonic nature of the flow will combine with sufficient daytime heating to result in at least widely scattered showers and thunderstorms across the region by mid-afternoon. Deep inverted-V soundings suggest wind will be the primary concern with the strongest thunderstorms, but increasing shear through the afternoon combine with respectable instability will yield a large hail threat as well. This activity exits eastward late evening or early overnight.

The main upper level disturbance passes eastward into Thursday, though some lingering instability across SW MT will yield a few isolated showers or thunderstorms Thursday and Friday afternoon. Thereafter an upper level ridge begins to build in across the Central CONUS and interior west, which will allow for temperatures to climb heading into the weekend. While the hottest conditions look to remain off to the east and southeast, afternoon temperatures rising well into the 90s in many areas this weekend will pose impacts to those with outdoor plans. Plan for plenty of hydration and rest breaks if planning to be outside this weekend.

Heading into next week the ridge looks to slide eastward ever so slightly, which looks to open the door for some afternoon and evening showers or thunderstorms. -AM

- Forecast Confidence & Scenarios:

Severe Thunderstorms and Heavy Rainfall through this Evening...

Confidence has increased through the morning that an isolated severe thunderstorm or two will occur through the timeframe that the Storm Prediction Center (SPC) has introduced a Marginal Risk for severe thunderstorms across portions of Southwest through North Central Montana, generally along and east of a Havre, to Great Falls, to Helena, to Butte, to Big Hole Pass line. BUFKIT soundings across these areas show deep inverted-V's with CIGS of 9-12kft, which would suggest that damaging winds would be the primary hazard for these areas. With that being said BUFKIT soundings over the plains of Central and North Central Montana, especially across Fergus and Blaine Counties, do support sufficient instability within the hail growth zone for the potential of hailstones to approach 1" but less than ideal shear will likely be the limiting factor for hail to exceed severe criteria.

Hot Temperatures this Weekend...

Climate anomaly indicators continue to support the potential for an unusual to very unusual period of hot temperatures, with respect to early to mid-July climatology, across the Northern Rockies; especially along and east of a Lewistown, to Helena, to Chief Joseph Pass line where EFIs are in excess of 0.6. This is not to say that areas north and west of this line will not also see hot temperatures, but with respect to July climatology readings will not be as anomalous.

The tables below depict Low End (NBM5.0 10th Percentiles), Most Likely (50th Percentiles), and High End (NBM5.0 90th Percentiles) scenarios with respect to high and low temperatures over the weekend; with larger spreads between the three scenarios suggesting greater uncertainty due to a combination of cloud cover/wind speeds and directions/latent heat release while smaller spreads suggest greater certainty.

Saturday's High Temperature Scenarios LOCATION Low End | Most Likely | High End Browning 83 | 87 | 91 Cut Bank 89 | 93 | 97 Havre 93 | 100 | 104 Great Falls 94 | 99 | 103 Lewistown 91 | 96 | 100 Helena 93 | 98 | 101 White Sulphur Springs 91 | 95 | 97 Bozeman 95 | 98 | 100 Dillon 91 | 96 | 99 Ennis 92 | 96 | 98 West Yellowstone 86 | 90 | 91

Sunday's High Temperature Scenarios LOCATION Low End | Most Likely | High End Browning 78 | 87 | 94 Cut Bank 84 | 93 | 99 Havre 92 | 101 | 107 Great Falls 93 | 100 | 106 Lewistown 92 | 99 | 106 Helena 92 | 99 | 106 White Sulphur Springs 92 | 99 | 103 Bozeman 97 | 101 | 105 Dillon 92 | 98 | 103 Ennis 96 | 100 | 103 West Yellowstone 90 | 94 | 96

Sat. Night's Low Temperature Scenarios LOCATION Low End | Most Likely | High End Browning 55 | 58 | 63 Cut Bank 54 | 59 | 66 Havre 59 | 62 | 68 Great Falls 57 | 62 | 67 Lewistown 55 | 59 | 64 Helena 61 | 64 | 68 White Sulphur Springs 53 | 57 | 60 Bozeman 57 | 61 | 65 Dillon 55 | 58 | 62 Ennis 57 | 60 | 63 West Yellowstone 47 | 52 | 57

Sun. Night's Low Temperature Scenarios LOCATION Low End | Most Likely | High End Browning 49 | 58 | 65 Cut Bank 50 | 60 | 68 Havre 56 | 63 | 71 Great Falls 57 | 63 | 70 Lewistown 54 | 59 | 66 Helena 61 | 67 | 71 White Sulphur Springs 54 | 58 | 63 Bozeman 57 | 62 | 66 Dillon 56 | 61 | 65 Ennis 57 | 60 | 65 West Yellowstone 49 | 54 | 59

At this time these scenarios would tend to suggest that while daytime temperatures will be hot across most areas, especially on Sunday, temperatures will cool sufficiently enough during the overnight hours to provide some relief from the heat. This relief offsets the need for Extreme Heat highlights at this time, but at the event draws closer Heat Advisories may be need for some locations. - Moldan

AVIATION

08/18Z TAF Period

Isolated to scattered showers and thunderstorms will move from west to east across Southwest through North Central Montana between 18z this afternoon and 00-06z this evening, with the greatest concentration of showers and storms occurring along and north of the US Hwy 12 corridor. VFR conditions will generally prevail throughout the duration of the 0818/0918 TAF period with the exception of beneath showers/storms when brief MVFR/low-VFR conditions will occur. Additionally, patchy fog may develop near the KHVR terminal during between 10-14z Thursday, but confidence in this occurring is low given that only a few Hi-Res members support this scenario. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 55 88 55 93 / 20 0 0 0 CTB 54 82 54 86 / 0 0 0 0 HLN 56 88 56 91 / 20 0 0 0 BZN 52 88 54 90 / 20 10 10 0 WYS 45 83 45 84 / 20 20 30 0 DLN 50 86 52 89 / 20 0 0 0 HVR 56 88 56 92 / 60 0 0 0 LWT 52 83 53 89 / 40 10 10 10

TFX WATCHES/WARNINGS/ADVISORIES

None.