Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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088 FXUS65 KTFX 261732 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1132 AM MDT Wed Jun 26 2024 ...Updated Aviation Section... .SYNOPSIS... An exiting high pressure ridge will help warm temperatures up to 15 degrees above normal today, but an increasingly moist and unstable southwesterly flow aloft will bring scattered showers and thunderstorms for this afternoon and evening. A Pacific weather system will then bring a chance of isolated to scattered strong to possibly severe thunderstorms for Thursday. Cooler temperatures with more showers than thunderstorms are expected in the wake of the system on Friday, leading into a drier and warmer Saturday. && .UPDATE... Today there will be showers and thunderstorms primarily across Southwestern Montana this afternoon and evening. Isolated thunderstorms will produce strong to severe wind gusts primarily south of I-90. The Storm Prediction Center issued a Marginal Risk (5% chance within 25 miles of point) for severe wind gusts for that area. For the update I increased PoPs across Southwestern Montana and Southern Central Montana based on current radar imagery and hi-res model guidance. I made a minor update to QPF and Sky grids based on current observations and trends. The rest of the forecast is on track. -IG && .AVIATION... 26/18Z TAF Period VFR conditions will prevail across all terminals during this TAF period. Wednesday across all terminals it will be warm and so there will be density altitude issues from the beginning of the TAF period through Wednesday evening. Across the Southwestern Montana terminals (KBZN, KWYS, and KEKS) there will be isolated showers and thunderstorms Wednesday afternoon through Wednesday evening. Due to the isolated nature of these showers and thunderstorms only VCSH was used in the TAFs. Some of the thunderstorms will produce strong to severe wind gusts. For the KHLN terminal Wednesday evening there is a 40% chance for a rain shower or two and so a PROB30 group was included for it. There is a 20% chance for these showers to affect the North-central Montana Terminals (KGTF, KHVR, KCTB, and KLWT) Wednesday evening through early Thursday morning. Thursday morning across all terminals there will be isolated showers and thunderstorms. For the KHVR, KLWT, and KGTF terminals these thunderstorms will be strong to severe with hail and wind gusts as the primary threats. Some of the thunderstorms Thursday morning will produce heavy rainfall and there is a low chance of intermittent reductions in visibility to MVFR levels because of it. There will be isolated instances of low-level wind shear across the Southwestern Montana terminals (KBZN, KWYS, and KEKS) Thursday morning. -IG Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PREV DISCUSSION... /ISSUED 540 AM MDT Wed Jun 26 2024/ Today... A low pressure trough off the Pacific Northwest coast will put a warm and increasingly moist southwesterly flow aloft over Southwest, Central, and North Central Montana today. This will warm temperatures up to 15 degrees above normal and put much of the area under a Minor heat risk with a few areas of Moderate; this will cause difficulty for those sensitive to the heat, especially for those outdoors for a long period of time or for those without air conditioning. This warmup will also help increase afternoon and evening instability, resulting in scattered showers and thunderstorms spreading northeast over the area with the passage of a disturbance in the flow aloft. While severe storms are not generally expected today, weak directional wind shear ahead of a cold front over the Northern Rockies and warm/dry low levels of the atmosphere may combine to produce a strong to weakly severe thunderstorm with strong wind gusts. Showers and storms should decrease overnight. Thursday... The trough is forecast to move into the Pacific Northwest on Thursday, which will tap into a deeper feed of moisture for the further increasing southwest flow aloft over the area. Although instability will decrease somewhat overnight, this increase in moisture will combine with the passage of a Pacific cold front to increase the instability enough late in the morning to increase coverage of showers and thunderstorms before noon across the area. The 0-3km wind shear associated with the flow aloft mostly increases to mostly between 30 and 40 kt for the area east of a line from Bozeman to Helena to Havre, which puts that area under a Marginal risk for isolated severe thunderstorms (gusts of 58+ mph and/or hail 1+ inch in diameter). However there is a core of 40 to 60 kt wind shear over portions of Meagher, Judith Basin, and Fergus County, where the Storm Prediction Center (SPC) has issued a Slight risk for isolated to scattered severe thunderstorms. In fact, it has a portion of Fergus County under a 10 percent probability of damaging winds (75+ mph gusts) and very large hail (2+ inches in diameter), and a less than 5 percent probability of a tornado. The favored period for the strong storms overall seems to be between 10 am and 4 pm. Broad scale westerly winds will also increase behind the cold front, bringing widespread 15 to 25 mph winds with gusts up to 40 mph; there is also a 60+ percent probability of 50+ mph gusts along the Rocky Mountain Front, but High Wind criteria is unlikely to be reached. Thunderstorm activity should decrease during the late afternoon and evening hours, as the trough starts to move through the area. Overall, a brief cooling trend begins on Thursday, as temperatures should struggle to reach seasonal averages. Friday... The cooling continues as the low pressure trough forms more of a closed off low pressure area and slows down its progress over Northeast Montana. Moisture wrapping around this system will combine with the passage of a Canadian cold front to shift the breezy winds more northerly to focus another round of showers (with fewer and weaker thunderstorms) more so over North Central and Central Montana. Temperatures will mostly remain up to 10 degrees below normal, which will lower snow levels to mostly between 7800 and 8500 feet, limiting light measurable snowfall to the mountain peaks well above pass level. The low pressure area is then forecast to exit the area Friday night, taking the showers with it, as overnight lows fall into the lower to mid 40s at lower elevations and into the 30s in the mountains. Saturday through next Wednesday... A broad high pressure ridge between systems will bring dry conditions for Saturday and near normal temperatures for Saturday into Sunday. However, a broad low pressure trough will bring an increasing chance for showers and thunderstorms again for Sunday through Monday, as temperatures cool back to slightly below normal. As of now, the long-term model guidance is forecast another broad high pressure ridge to dry out the area and warm temperatures back slightly above normal for Tuesday into Wednesday. -Coulston && .PRELIMINARY POINT TEMPS/POPS... GTF 86 57 72 50 / 10 40 80 20 CTB 81 55 69 46 / 20 30 50 40 HLN 89 60 75 52 / 20 40 80 10 BZN 86 55 77 44 / 40 40 80 20 WYS 78 48 72 37 / 50 30 50 10 DLN 83 52 74 40 / 40 40 50 0 HVR 84 58 74 51 / 0 30 90 30 LWT 81 54 72 44 / 10 30 90 20 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls