Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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929 FXUS65 KTFX 171828 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1228 PM MDT Tue Sep 17 2024 ...Updated Aviation Section... .SYNOPSIS... A fall storm system will spread an increasing coverage of showers and thunderstorms into the region this afternoon and evening with some strong to severe thunderstorms possible across eastern portions of north-central Montana. Widespread rain develops tonight and continues through WEdnesday across north-central MT with gusty winds also developing across most of north-central and southwest Montana. Drier weather develops to end the work week. && .UPDATE... No major changes made to this morning`s update other than to update POPs and to expand the gusty winds from thunderstorms this afternoon to line up with the SPC outlook. Ludwig && .AVIATION... 17/18Z TAF Period Showers and thunderstorms are moving north into Montana from Idaho and Northwestern Wyoming. These will slowly spread north through the first 12 hours of this TAF period. By 18/06Z rain showers will transition to all rain across all terminals. This rain will continue through the end of the TAF period. During heavier rain showers and any thunderstorm visibility will be reduced to MVFR levels. At the KHVR and KLWT terminals thunderstorms will produce strong to severe wind gusts between 18/00Z and 18/06Z. By the end of this TAF period at all terminals there will be gusty winds. At the KHVR, KLWT, and KGTF terminals winds will be strong by the end of this TAF period. During any heavier rain shower or thunderstorm there will be mountain obscuration and there will be mountain obscuration during the last half of this TAF period. At the end of this TAF period there will be mountain wave turbulence over North-central and Central Montana. -IG Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .HYDROLOGY... A flash flood watch remains in effect for the Horse Gulch fire burn scar area from this afternoon through Wednesday with initial concerns for brief higher intensity precipitation with thunderstorm activity later this afternoon and evening. While the potential for intense precipitation rates decreases tonight, steadier moderate precipitation will maintain some threat for flash flooding and debris flows in the burn scar area. Local minor flooding concerns may also arise on Wednesday with persistent moderate rainfall across portions of north-central MT, particularly where heaviest amounts are expected near the central MT mountain ranges, where some low lying areas and locations with poor drainage may begin to see standing water buildup. Hoenisch && .PREV DISCUSSION... /ISSUED 525 AM MDT Tue Sep 17 2024/ Key Points: -Scattered severe thunderstorms this afternoon/evening across eastern portions of N-central MT -Significant (once in 10-20yr) rainfall amounts for September likely across portions of N-central MT tonight through Wednesday -Strong winds developing Wednesday An early fall storm system is on track to bring several types of impactful weather to the region over the next several days as a closed mid-upper level low, currently centered over NE NV lifts out into WY by this afternoon, then deepens while tracking northward through eastern MT into SK by Wednesday night. Today...showers and thunderstorms will increase in coverage this afternoon as flow aloft turns southeasterly and becomes increasingly diffluent while importing deeper moisture into the region. This sets up a favorable environment for strong to severe thunderstorms this afternoon and evening across eastern portions of north-central MT, where the the greatest amount of buoyancy (500-1000 J/kg CAPE) is supported by hi-resolution model ensembles along with sufficient wind shear for organized thunderstorm development. Primary severe threat in this area (JB, Fergus Hill and Blaine counties) will be strong wind gusts and hail with any thunderstorms today also bringing a risk for locally heavy rainfall in addition to lightning. Convection this evening is likely to evolve into an area of widespread rain overnight as the mid level low and an associated surface low lift north into eastern MT by Wednesday morning. Unusually deep moisture is drawn around the the mid level low with moderate to locally heavy rainfall developing within a TROWAL/deformation feature wrapping around to the west of the low. The location of this feature and heaviest precipitation has shifted slightly east from previous forecasts (near I-15) to an axis now generally from Hill county south through Judith Basin and Fergus counties. Much of the Golden Triangle portion of north-central MT still has an 70% or higher probability for amounts in excess of one inch with 50% or better probabilities for amounts in excess of 2 inches now over much of Judith Basin, Fergus and Chouteau counties. (See hydrologic discussion for potential impacts from excessive rainfall). Snow levels only fall to as low 8500 ft, limiting any measurable snow to only the highest peaks. Strong low level (850-700MB) winds also wrap around the low as it slowly tracks north across eastern MT on Wednesday, resulting in very windy conditions for much of the area with strongest winds likely across Judith Basin and Fergus counties where probabilities for gusts in excess of 55 mph are above 60% and a High Wind Watch remains in effect. Another area of concern for stronger winds on Wednesday will be across Toole, Liberty and western Hill counties, where probabilities of 55+ mph winds are currently around 40%. The storm exits the area into SK Wednesday night with drier conditions and lighter winds across the area on Thursday. A more progressive west to northwest flow aloft pattern is generally supported by most medium range model ensembles, however considerable differences exist in the location and timing of troughing within this pattern, leading to lower than average confidence in forecast details for this weekend into early next week. Hoenisch && .PRELIMINARY POINT TEMPS/POPS... GTF 72 52 60 48 / 60 90 90 60 CTB 70 51 63 46 / 40 70 60 30 HLN 71 51 63 48 / 80 90 90 50 BZN 67 46 60 43 / 90 80 90 50 WYS 56 34 52 33 / 90 90 80 60 DLN 61 44 58 40 / 70 80 60 30 HVR 79 55 65 50 / 50 80 90 80 LWT 74 48 56 45 / 50 100 90 80 && .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 through Wednesday afternoon for Big Belt, Bridger and Castle Mountains. && $$ http://www.weather.gov/greatfalls