Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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037 FXUS65 KTFX 161953 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 153 PM MDT Mon Sep 16 2024 .SYNOPSIS... An impactful period of weather will overspread the region through Thursday morning including burn scar flooding concerns, strong to severe thunderstorms at times, widespread rain with areal flooding concerns, and gusty winds. Concerns diminish heading into Thursday, though unsettled conditions remain heading into the weekend.&& && .DISCUSSION... Through Tuesday evening... A closed low across CA/NV is within a broad upper trough across the western CONUS drifting slowly eastward this afternoon. The initial upper level low will then lift northeastward tonight into Tuesday toward the northern high plains as another upper level low dives southward along the Pacific coast toward CA. The initial closed upper low will dig out a surface low across Wyoming and eventually eastern Montana that eventually meanders west (Becomes stacked with the upper low) toward eastern portions of the plains beginning late Tuesday (But mostly Wednesday). The result of all of this will be for multiple rounds of showers and thunderstorms through the evening Tuesday. The initial push of showers and thunderstorms arrives this afternoon across Southwest Montana initially, with activity moving northward through the evening into Central Montana. Forecast soundings do feature inverted- v profiles, but aren`t overly deep. Hence gusty winds will be a concern, but the threat for severe gusts (58+ mph) is low today. These showers and thunderstorms diminish somewhat late evening before another round of showers and a perhaps few thunderstorms moves in from the south as better forcing arrives from the upper level low late tonight. Showers will be ongoing across southern portions of the region heading into Tuesday as the upper level disturbance continues its approach. This should help to keep instability on the low side across Southwest Montana Tuesday. Relatively warm, relatively moist air being advected in form the east across the eastern plains Tuesday will lead to a complicated severe thunderstorm setup. Guidance ranges from dew points in the lower 50`s all the way up into the lower 60`s by late Tuesday afternoon across eastern areas, with members that advect in better moisture also featuring better instability. To further complicate things, there looks to be some degree of a capping inversion across these areas, bringing into question how many thunderstorms will even be able to form in a theoretically more favorable environment otherwise. All that said, if thunderstorms do manage to form in a favorable environment, hail and gusty winds will both be a concern late tomorrow across eastern areas. The Storm Prediction Center highlights this risk with a slight risk for severe weather along and east of a line from Havre to Lewistown. Localized impacts to burn scars will be possible with thunderstorms. See the hydrology section for more information. Wednesday into early Thursday... The closed upper low becomes stacked with the surface low Wednesday (Indicative of a mature storm) as the two stall briefly across the region. Strong forcing around the western and southwestern side of this system will result in a period of heavy rain, across the majority of Central and North- central Montana Wednesday into Wednesday night. The chance for 2 inches of rain east of I-15 is at or above 50% with this system. See the hydrology section for flooding concerns. Another concern Wednesday will be a risk for strong winds across the plains on the west and southwest side of the stacked system. Forecast soundings show winds aloft (2,000ft to 5,000ft AGL on the plains) on the order of 45-60kts at times. There is concern that these winds will get mixed to the surface across the plains where precipitation is heaviest as well as due to terrain influence. At this point there is a greater than 50% chance for sustained winds of 40 mph Wednesday across the lower elevations of Judith Basin county and eastern Fergus county. The chance for a 58 mph gust is largely the same chance over these same areas. With this in mind, a High Wind Watch was issued where confidence was highest in stronger winds. Additional watches may be needed as confidence in the situation grows. Snow levels look to drop to around 8,000 ft, perhaps briefly lower, in portions of Southwest Montana Tuesday night through Wednesday evening, though overall amounts look minimal outside of the highest peaks. -AM Thursday afternoon into early next week...As this trough/closed low system move out of the region Thursday, northwest flow aloft moves in. Embedded upper air disturbances along this northwest flow aloft will keep low chances for precipitation through the weekend. However, there is uncertainty regarding to the timing of this trough exiting, which can affect precipitation chances for the later half of the week. However, the northwest flow aloft will keep temperatures below average through the forecast period. -Wilson && .AVIATION... 16/18Z TAF Period A Pacific trough in the Great Basin area will maintain a difluent southerly flow over the Northern Rockies for variable cloudiness and periods of widely scattered showers and thunderstorms. Primarily VFR conditions are expected, though low VFR clouds will become more common, especially during times of precipitation. Shower and thunderstorm activity will become more widespread heading towards the end of the TAF period. - RCG Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .HYDROLOGY... Several rounds of precipitation will move across the region over the next few days. Rounds of showers and thunderstorms will be the initial concern this afternoon and tomorrow afternoon, with primary concern for recent burn scars. The 25km neighborhood probability for a half inch of precipitation over an hour in the vicinity of the Horse Gulch burn scar Tuesday afternoon is 75%. A Flood Watch has been issued for this burn scar related to potential for flash flooding. Precipitation becomes more widespread going into Wednesday and Thursday, with the focus shifting northward onto the plains. In the Golden Triangle area there is a greater than 50% chance for 2 inches of rain (Peaks around 70-80% in and around Judith Basin and Chouteau counties). In fact there is nearly a 50% chance in these two counties for 3 inches of rain at lower elevations. Local flooding concerns may arise in flood prone areas and areas near terrain where heavier rain is most persistent. -AM && .PRELIMINARY POINT TEMPS/POPS... GTF 47 71 52 59 / 20 50 90 90 CTB 45 71 52 60 / 20 50 90 90 HLN 50 72 51 60 / 40 80 90 80 BZN 45 68 45 56 / 40 90 90 80 WYS 35 55 34 50 / 50 90 100 80 DLN 45 62 44 59 / 40 70 80 40 HVR 50 80 55 66 / 20 50 90 100 LWT 47 75 47 56 / 30 50 90 90 && .TFX WATCHES/WARNINGS/ADVISORIES... High Wind Watch from Wednesday morning through Wednesday evening for Fergus County below 4500ft-Judith Basin County and Judith Gap-Snowy and Judith Mountains. Flash Flood Watch from Tuesday afternoon through Wednesday afternoon for Big Belt, Bridger and Castle Mountains. && $$ http://www.weather.gov/greatfalls