Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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818 FXUS65 KTFX 180026 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 625 PM MDT Tue Sep 17 2024 UPDATED AVIATION SECTION .SYNOPSIS... A weather system will bring widespread rainfall to North Central and Southwestern Montana over the next 48 hours, with some areas seeing in excess of 2 inches, mainly in an area bounded by Great Falls, Lewistown, and Havre. Rain will let up late Wednesday afternoon and evening, with mostly clear skies expected on Thursday. && .DISCUSSION... Key Points: -Scattered severe thunderstorms this afternoon/evening in North Central Montana, particularly east of a line from Havre to Great Falls to Lewistown -Significant (once in 10-20yr) rainfall amounts for September likely across portions of N-central MT tonight through Wednesday -In addition, gusty winds are expected Wednesday, especially in Judith Basin and Fergus Counties Short term (through Thursday)... A notable weather system will continue to push into the area this afternoon and evening, which will bring a fairly widespread soaking rain to the area over the next 48 hours. For the rest of the day today, rain showers will continue to gradually work their way further north from their current location in Southwestern Montana, Wyoming, and Idaho, and they will strengthen as they do so. Some of these showers may eventually grow into strong to severe thunderstorms, particularly east of a line from Havre to Great Falls to Lewistown, where a few instances of gusty winds and large hail will be possible later this evening and into the overnight hours. Elsewhere, expect rain showers and a few thunderstorms through the remainder of the afternoon, evening, and overnight hours, with some localized heavy rainfall possible. Wednesday will see rain continue and become mostly a widespread stratiform/steady rain, with a few areas of embedded moderate to heavy rainfall. Within this area of widespread rainfall, models have been fairly insistent of a band of moderate to heavy rainfall setting up across the plains of North Central Montana, and ensembles show some areas seeing at least a 50% chance of seeing 2 of rain, mainly in the area from Havre to Great Falls, then north of the Little Belt and Snowy Mountains to Lewistown in association with this persistent band of more intense rainfall. While soils are fairly dry given that we are not anticipating widespread river flooding, but a Flood Watch has been issued for the potential for flooding of low lying areas. In addition to the rainfall, well have to watch for a brief period of gusty winds as drier air works in behind this system Wednesday afternoon and evening, particularly in Judith Basin and Fergus Counties, where ensembles show at least a 60% chance of seeing wind gusts over 60 mph, which is backed up by a strong low level jet on the back side of this system that will likely mix to the surface as the rain comes to an end tomorrow evening. Behind this system, a surface high pressure system will push in from the Southwest, allowing for a dry day on Thursday. Ludwig Extended (Friday through next Tuesday)... 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 && .AVIATION... 18/00Z TAF Period Widespread showers with mainly VFR conditions over Southwest (KWYS, KEKS, KBZN) and Central (KHLN, KGTF, KHLN) Montana will continue to spread north into North Central (KHVR, KCTB) Montana through 06Z ahead of a low pressure system. However, scattered embedded thunderstorms moving into North Central Montana may produce periods of mountain-obscuring MVFR conditions in heavier rain, along with erratic wind gusts. After 06Z, precipitation will remain widespread, but it will become more stratiform as conditions lower to mountain- obscuring MVFR/IFR levels with some high mountain snow. The low pressure system will move east of the area, and winds will become quite gusty out of the west to northwest across the area, especially over the plains east of a KCTB-KGTF line. This overall increase in wind will cause mountain wave turbulence and low level wind shear. Conditions will very gradually improve after 18Z west of that line and into Southwest Montana, but conditions will remain quite low east of it. -Coulston 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 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. This is particularly true where heaviest amounts are expected near the central MT mountain ranges, such as the area around the Little Belt, Snowy, and Bears Paw Mountains, where some low lying areas and locations with poor drainage may begin to see standing water buildup. With this in mind, I have issued a Flood Watch for these areas. Hoenisch/Ludwig && .PRELIMINARY POINT TEMPS/POPS... GTF 52 62 48 73 / 100 100 40 10 CTB 51 65 45 71 / 60 30 10 0 HLN 51 64 48 74 / 100 100 40 10 BZN 46 61 43 71 / 100 100 50 10 WYS 34 52 33 62 / 100 80 60 20 DLN 44 59 40 67 / 50 50 20 10 HVR 56 66 49 73 / 100 100 100 10 LWT 49 56 46 68 / 100 100 100 10 && .TFX WATCHES/WARNINGS/ADVISORIES... Flood Watch from 6 PM MDT this evening through late Wednesday night for Bears Paw Mountains and Southern Blaine-Fergus County below 4500ft-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains-Snowy and Judith Mountains. High Wind Warning from 9 AM Wednesday to midnight MDT Wednesday night 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