textproduct: Great Falls

KEY MESSAGES

- Showers and thunderstorms increase in coverage the next few afternoons.

- Cooler this weekend into next week, particularly on Sunday.

- Still some uncertainty with exactly how far snow levels fall, but the risk for mountain snow remains for Saturday night through Monday morning.

DISCUSSION

- Meteorological Overview:

Transient upper level ridging between broad troughing to the east and stronger troughing well off to the northwest off the far northern BC coastline will be the initial feature dictating weather across the region. Although this ridging is in place, there still looks to be sufficient afternoon heating today to result in showers and thunderstorms, mainly near Central and Southwest Montana terrain. The overall combination of the kinematics and thermodynamics does not offer much confidence in a stronger thunderstorm today, though one cannot be completely ruled out.

The upper level troughing well off to the northwest gradually drifts southeast through the remainder of the week, which will slowly introduce a more southwesterly flow aloft. Showers and thunderstorms increase in coverage across the region as a result of this more favorably forcing. The environment is split for Thursday, with better instability further north and more favorable shear further south. At this time organized strong to severe thunderstorms look to be isolated at best. Should there be any trend in a more favorable direction in parameter space, the risk for organized strong to severe thunderstorms would increase.

Upper troughing continues its approach into Friday, bringing potential for a couple impacts. The first will be for another round of widespread showers and thunderstorms, primarily in the afternoon and evening. Ample forcing from the approaching troughing will have no issue supporting shower and thunderstorm activity, but thick cloud cover early in the day and into the afternoon raises questions with respect to just how much destabilization will occur. At this time the risk for stronger thunderstorms appears low, but should instability trend higher, there would be plenty of shear in place to support stronger thunderstorms. Trends will continue to be monitored.

The other concern for Friday will be for breezy southwest winds near the Idaho border between Monida Pass and West Yellowstone. A narrow mid-level jet looks to moves across this portion of Southwest Montana Friday morning into the afternoon. While these stronger winds aloft to look to at least briefly mix to the surface, the overall magnitude of winds at the surface as well as the duration of these winds is questionable - again related to forecast cloud cover across the region.

The core of the upper level disturbance looks to be off the WA/OR coastline by Saturday, keeping the Northern Rockies squarely within a persistent southwesterly flow aloft. Given the ample forcing associated with the nearby troughing, showers and thunderstorms will be fair game beginning quite early in the day across Southwest Montana, spreading northeastward with time into Saturday afternoon.

By Saturday evening the core of the upper disturbance begins tracking eastward, moving somewhere over the Northern Rockies into Sunday. A more stratiform type of precipitation ensues thereafter, with greatest amounts generally on the northwest side of the core of the disturbance. Snow levels look to drop lowest where the low tracks, which is still uncertain. Those with recreation plans this weekend in the terrain should be aware of the risk for the colder conditions and risk for snow.

By early next week the disturbance moves northeast, but lingering troughing hangs around. This looks to keep temperatures on the cooler side of average with continued opportunities for precipitation into mid-week. -AM

- 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

24/18Z TAF Period

The primary concern this TAF period will be for afternoon showers and thunderstorms, mainly near terrain in Central and Southwest Montana. Prob30s were added where confidence was greatest in showers or thunderstorms being around this afternoon. Otherwise attention turns to tomorrow where showers will be around by early morning, mainly across Central and Southwest Montana. Additional showers and thunderstorms develop by late morning into early afternoon across the entire region. -AM

PRELIMINARY POINT TEMPS/POPS

GTF 49 75 52 74 / 10 70 20 80 CTB 47 73 50 68 / 0 60 30 80 HLN 53 77 55 73 / 40 70 40 90 BZN 47 76 50 76 / 20 70 30 70 WYS 40 70 42 70 / 40 80 30 50 DLN 47 77 51 71 / 30 50 20 60 HVR 49 78 50 78 / 0 30 30 70 LWT 44 70 46 74 / 10 70 40 80

TFX WATCHES/WARNINGS/ADVISORIES

None.