textproduct: Great Falls

KEY MESSAGES

- Showers and thunderstorms increase in coverage each afternoon through the remainder of the week.

- Trending cooler with more widespread precipitation this weekend into early next week.

- Impacts to recreation this weekend, including the risk for higher elevation snow.

DISCUSSION

- Meteorological Overview:

Upper level ridging will slowly drift east and over the Northern Rockies through the end of the work week, which will allow temperatures to warm near to slightly above normal. As the flow begins to back to more of the southwest later this afternoon and evening shower and thunderstorm chances will begin to increase, especially across Southwest and Central Montana. By Thursday and Friday shower and thunderstorm chances will increase further and across all areas, with gusty south to southwest winds developing along and south of the I-90 corridor in Southwest Montana on Friday. This weekend will then see a large upper level trough begin to move over the Northern Rockies, which will bring widespread precipitation and cooler temperatures across all of Southwest through North Central Montana through the day on Monday.

- Forecast Confidence & Scenarios:

Gusty Winds on Friday...

Anomalous, with respect to late June climatology, southerly H700 winds of 30-40kts will overspread Southwest Montana from the late morning through early evening hours. This timing combined with the flow orientation, with respect the north-south valleys and mountain passes along the Idaho border, will help to support a period of strong and gusty winds due to mixing and terrain funneling. Latest NBM5.0 probabilities support a 50-80% chance that gusts exceed 40 mph along and south of a Dillon, to Ennis, to Big Sky line, with even a 30-60% chance that gusts exceed 50 mph. Furthermore there is even a 20% chance that sustained winds within the Madison Valley reach 40 mph for a period of time from late Friday morning through the early evening hours. While these probabilities and deterministic guidance do not currently support high wind highlights it will be none-the-less windy on Friday, which will cause some difficulties for those operating high profile vehicles.

Showers and Thunderstorms from Wednesday through Friday...

Marginal instability and shear throughout the timeframe will help to limit most convection to the garden variety type, with gusty winds of 45-55 mph and hail up to the size of nickles; however, a few marginally severe thunderstorms producing wind gusts in excess of 60 mph and hail larger than quarters can't be ruled out, especially on Thursday across Southwest Montana. Additionally, with Precipitable Water (PWATs) climbing to between 0.5" to 1" by Thursday and Friday, which is 1 to nearly 2 standard deviations above normal per NAEFS analysis, localized heavy rainfall will become possible. The main concern from these heavier bursts of rain will be debris flows/flooding across recent burns scars like the Horse Gulch Area.

Widespread Precipitation this Weekend through Monday...

Climate anomaly indicators like the ECMWF EFIs continue to support the potential for a climatologically unusual precipitation event for much of Southwest Montana and portions of North Central Montana throughout the timeframe. This is significant given that fact that June is climatologically one of if not the wettest months of the year for these areas. Latest NBM5.0 probabilities for liquid equivalent amounts of 1" or greater from Saturday morning through Monday night range from a 40-80% chance across all of Southwest through North Central Montana, with a 20-50% chance that values exceed 2" across the Glacier National Park region and within an area from east of the I-15 corridor and west of the US Hwy 89 corridor across Southwest and Central Montana (i.e. Ennis, Bozeman, and White Sulphur Springs Areas). Lowering snow levels this weekend and early next week will also help to support rain changing over to snow in the mountains, with a 30-60% chance that snowfall amounts exceed 4" at elevations of 7000ft or higher across Southwest Montana. The combination of wet and raw conditions will lead to an increased risk of hypothermia for those recreating in the backcountry at any elevation, especially those recreating above 7000ft. - Moldan

AVIATION

24/06Z TAF Period

Weak ridging aloft will maintain general VFR conditions tonight. A shortwave moves in from the west and brings widely scattered shower and thunderstorm activity on Wednesday, mostly for southwestern areas after 24/18Z. VFR conditions will still generally prevail during this period as well, though low VFR clouds will be present near any showers or storms. - RCG

PRELIMINARY POINT TEMPS/POPS

GTF 75 49 76 52 / 10 10 80 10 CTB 73 47 75 50 / 0 0 70 20 HLN 78 52 77 54 / 10 50 70 20 BZN 79 47 76 48 / 10 30 70 20 WYS 77 39 72 40 / 40 40 80 10 DLN 79 46 77 49 / 30 40 40 10 HVR 75 48 78 50 / 0 0 30 30 LWT 69 45 70 46 / 30 0 70 40

TFX WATCHES/WARNINGS/ADVISORIES

None.