textproduct: Great Falls

KEY MESSAGES

- A cold front will bring cooler conditions from Wednesday through the end of the work week, especially on Thursday and Friday..

- While most areas will see beneficial rainfall from Wednesday through Friday a few stronger storms on Wednesday and Thursday could lead to strong wind gusts, hail, and localized flooding of normally prone areas.

- Scattered shower and thunderstorm activity capable of strong wind gusts and cloud to ground lightning will continue into the weekend while temperatures trend above average again.

DISCUSSION

- Meteorological Overview:

After weeks of hot and overall dry conditions a shift to a "cooler" and wetter, albeit short-lived, pattern is expected across the Northern Rockies from Wednesday through the end of the work week. High temperatures over this timeframe will fall some 10 to nearly 20 degrees below normal as a cold front sags south from Canada, with multiple opportunities for rain, especially from Wednesday night through Thursday night. This Wednesday night through Thursday night timeframe remains the most favored for widespread rainfall, especially for areas along and north of the I-90 corridor.

While most areas will see beneficial rains and cooler temperatures, the convective nature of activity on Wednesday and Thursday (most notably) may also bring a few strong thunderstorms with localized strong gusty winds, hail, and heavy rainfall capable of flooding normally flood prone areas (i.e. burn scars and urban areas). Areas along and south of the US Hwy 12 corridor on Wednesday will be at greatest risk of seeing strong outflow winds from collapsing showers and thunderstorms given inverted-V soundings, which is especially concerning given on-going wildfire activity across Southwest Montana in Beaverhead County.

Ridging aloft begins to build back in heading into the weekend, but another trough approaching from the Pacific NW will bring southwesterly flow aloft back to the Northern Rockies. This will result in temperatures warming back up, along with daily scattered shower and thunderstorm activity. - Moldan/RCG

- Forecast Confidence & Scenarios:

Increasing Chances for Rainfall from Wednesday through Friday...

Ensemble model guidance continues support multiple opportunities for showers and thunderstorms from the middle through end of the work week; with climate anomaly indicators suggesting that some of the showers and thunderstorms could produce periods of heavy rainfall. NAEFS analysis suggest that PWATs will climb above 1 standard deviation above normal by Wednesday evening across the Northern Rockies and remain so through Friday evening, with PWATS peaking as high as 2 standard deviations above normal during the day on Thursday across all of Southwest through North Central Montana. ECMWF EFIs on Wednesday support the potential for unusual rainfall amounts along and north of a Lewistown, to Great Falls, to Browning line with showers and thunderstorms; with the potential for unusual rainfall amounts (with respect to climatology) shifting south of the Montana Hwy 200 corridor on the day on Thursday.

The tables below list the NBM10Pct, NBM50Pct, and NBM90Pct for rainfall for select locations across North Central through Southwest Montana; with the 10Pct equating to a 90% chance that rainfall amounts exceed the listed values while the 90Pct equates to 90% chance that rainfall amounts will be lower than what is shown.

6am Wed. thru 6am Thur. Rainfall Scenarios LOCATION 10 Pct | 50 Pct | 90 Pct Browning 0.10 | 0.35 | 1.15 Cut Bank 0.00 | 0.22 | 0.90 Havre 0.00 | 0.16 | 0.70 Great Falls 0.00 | 0.10 | 0.45 Lewistown 0.02 | 0.30 | 0.80 Helena 0.00 | 0.00 | 0.14 White Sulphur Springs 0.00 | 0.01 | 0.40 Bozeman 0.00 | 0.00 | 0.05 Dillon 0.00 | 0.00 | 0.05 Ennis 0.00 | 0.00 | 0.02 West Yellowstone 0.00 | 0.00 | 0.05

6am Thur. thru 6am Fri. Rainfall Scenarios LOCATION 10 Pct | 50 Pct | 90 Pct Browning 0.10 | 0.75 | 1.40 Cut Bank 0.16 | 0.60 | 1.35 Havre 0.30 | 0.85 | 1.70 Great Falls 0.10 | 0.45 | 0.95 Lewistown 0.16 | 0.70 | 1.50 Helena 0.05 | 0.35 | 1.50 White Sulphur Springs 0.14 | 0.01 | 0.95 Bozeman 0.00 | 0.30 | 1.05 Dillon 0.00 | 0.16 | 0.70 Ennis 0.03 | 0.55 | 1.05 West Yellowstone 0.03 | 0.40 | 0.90

6am Fri. thru 6am Sat. Rainfall Scenarios LOCATION 10 Pct | 50 Pct | 90 Pct Browning 0.00 | 0.00 | 0.16 Cut Bank 0.00 | 0.00 | 0.13 Havre 0.00 | 0.00 | 0.30 Great Falls 0.00 | 0.04 | 0.35 Lewistown 0.03 | 0.30 | 0.90 Helena 0.00 | 0.19 | 0.65 White Sulphur Springs 0.00 | 0.35 | 0.85 Bozeman 0.00 | 0.30 | 0.95 Dillon 0.04 | 0.25 | 0.65 Ennis 0.07 | 0.45 | 0.95 West Yellowstone 0.00 | 0.35 | 0.90

While multi-day (Wednesday morning through Friday night) rainfall amounts of 2+" can not be ruled of for isolated locations, most notably north of the I-90 corridor, these higher amounts would be heavily dependent on thunderstorm activity; with reasonable multi- day rainfall amounts of between 0.25" to 1" for areas along and south of the I-90 corridor and 0.5" to 1.5" for areas along and north of the I-90 corridor. This beneficial rainfall is not expected to pose any flooding risks outside of burn scar areas, with the most significant impact being an increased risk for rockfall in prone areas.

Below Normal Temperatures on Thursday and Friday...

ECMWF EFIs, with respect to high temperatures, support unusually "cool" temperatures for the end of the work week with respect to mid- August climatology as surface high pressure and a cold front slowly noses south from Alberta; initially over the plains of Central and North Central Montana on Thursday and then across all of Southwest through North Central Montana on Friday. The tables below list the "Low End" (NBM10Pct), "Most Likely" (NBM50Pct), and "High End" (NBM90Pct) temperature forecast scenarios for select sites across North Central through Southwest Montana on Thursday and Friday.

Thursday's High Temperature Scenarios LOCATION Low End | Most Likely | High End Browning 54 | 59 | 63 Cut Bank 57 | 63 | 67 Havre 64 | 70 | 76 Great Falls 66 | 72 | 76 Lewistown 62 | 68 | 73 Helena 69 | 74 | 79 White Sulphur Springs 65 | 72 | 77 Bozeman 71 | 78 | 82 Dillon 70 | 77 | 81 Ennis 71 | 76 | 80 West Yellowstone 66 | 72 | 77

Friday's High Temperature Scenarios LOCATION Low End | Most Likely | High End Browning 57 | 62 | 68 Cut Bank 60 | 64 | 68 Havre 65 | 70 | 75 Great Falls 64 | 69 | 75 Lewistown 59 | 64 | 70 Helena 66 | 71 | 78 White Sulphur Springs 62 | 67 | 73 Bozeman 66 | 71 | 77 Dillon 67 | 71 | 76 Ennis 65 | 70 | 74 West Yellowstone 63 | 66 | 70

The presence of regional wildfire smoke and/or increasing cloud cover from monsoonal moisture will favor temperatures falling somewhere between the "Low End" and "Most Likely" scenarios as solar insolation is significantly limited. - Moldan

AVIATION

11/12Z TAF Period

During this TAF period, VFR conditions will prevail for all terminals except KBZN, KEKS and KHLN, which will see periods of MVFR conditions due to reduced visibilities caused by wildfire smoke from wildfires in the Pioneer Mountains. All terminals will experience reduction in slant range visibility due to wildfire smoke. While clouds will be minimal, SCT030 was put into the TAF for KBZN due to thick smoke plumes. Expect periods of mountain obscuration in Southwest MT due to thick smoke.

For KGTF: Between 11/23Z and 12/04Z, there will be a 15-30% chance of easterly cross winds in excess of 15 knots. From 12/05Z to 12/07Z this chance drops below 10%. However, it increases again starting at 12/08Z, where there is a 30-50% chance of north- westerly cross winds in excess of 15 knots until 12/14Z. After this period, chances for 15 knot cross winds decrease to less than 5%. Slant range visibility will be reduced due to wildfire smoke. -Dzomba

PRELIMINARY POINT TEMPS/POPS

GTF 54 77 53 69 / 0 10 80 70 CTB 49 66 47 60 / 20 40 90 100 HLN 54 79 53 71 / 0 10 50 70 BZN 53 86 51 75 / 0 10 50 50 WYS 47 80 41 68 / 0 30 30 70 DLN 52 84 49 73 / 0 30 30 60 HVR 52 76 50 66 / 10 10 90 100 LWT 51 77 49 66 / 0 10 90 80

TFX WATCHES/WARNINGS/ADVISORIES