Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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384 FXUS65 KTFX 281205 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 600 AM MDT Tue May 28 2024 UPDATED AVIATION SECTION .SYNOPSIS... Today there will be showers and thunderstorms primarily across the eastern portion of North-central, Central, and Southwestern Montana. Some of these thunderstorms could have severe wind gusts. On Wednesday during the day the showers and thunderstorms spread eastward to the eastern portion of Southwestern and North-central Montana by Wednesday afternoon. On Thursday the weather remains unsettled with precipitation chances across North- central Montana and the mountains of Southwestern Montana. Friday begins a warming trend through the middle of next week with precipitation chances for the weekend. && .DISCUSSION... Today... Today there is an upper-level ridge over North- central, Central, and Southwestern Montana and so temperatures will be warmer today than yesterday. Today across North-central Montana there is a 90 - 100% chance for temperatures 80 or greater across the lower-elevations of Central and North-central Montana and in the Helena and Gallatin Valleys. From Fort Benton to Great Falls there is 50 - 85% chance for temperatures of 85 or higher. Late afternoon/early evening through Wednesday morning a cold front begins to move east across North-central, Central, and Southwestern Montana. This combined with southwest flow aloft and the warm temperatures will cause the atmosphere to become unstable. This will result in thunderstorms some of which could produce severe wind gusts. As result the SPC has issued a marginal risk (5%) for severe wind gusts this afternoon through the evening from Inverness south to Bozeman and west. These thunderstorms have the potential to occur beginning between 2 and 4 pm and continuing through rest of the evening. Due to anomalously high precipitable water values some of these thunderstorms will produce heavy rain. As a result and due to mountain snow pack being warm for the extreme Western portion of Southwestern Montana and just east of the divide the WPC has issued a Marginal Risk (5 - 15%) for flash flooding through 6 AM Wednesday. Today there is a 15 - 40% chance for locations across the western portion of North-central, Central, and Southwestern Montana receiving a tenth of inch of rain or greater. See the HYRDOLOGY section below for more information. Wednesday... On Wednesday the upper-level trough associated with the cold front begins to move over north-central, Central, and Southwestern Montana. This will bring cooler temperatures and rain to most of the area on Wednesday. As the cold front moves eastward Wednesday morning there will be instability generated across the eastern portion of Southwestern Montana and the extreme eastern portion of South-central Montana. This will create the potential for thunderstorms in those areas Wednesday afternoon and early evening. These thunderstorms are unlikely to be severe with the main threat being heavy rainfall. As a result and due to warm mountain snow pack the WPC has issued a Marginal Risk (5 - 15%) for flash flooding from 6 AM Wednesday to 6 AM Thursday for the eastern portion of Southwestern Montana and the extreme eastern portion of South- central Montana. Across the eastern portion of Southwestern Montana there is 50 - 80% chance for locations to receive a quarter inch of rain or greater. Across the rest of Southwestern Montana there is 40 - 80% chance for receiving a tenth of an inch of rain or greater. North of the Highway 200 corridor there is a 15 - 30% chance for locations receiving a tenth of an inch of rain or greater. South of the Highway 200 Corridor and east of Great Falls in Central Montana there is a 40 - 60 % chance for receiving a quarter inch of rain or greater on Wednesday. See the HYDROLOGY section below for more details. On Wednesday due to strong flow aloft and a strong surface pressure gradient there is a 60 - 90% chance for wind gusts exceeding 55 mph along the Rocky Mountain Front. Along the plains adjacent to the Rocky Mountian Front there is a 85% chance for wind gusts exceeding 47 mph on Wednesday. Due to only isolated locations of the plains adjacent to the Northern Rocky Mountain Front having a 50 - 60% chance for reaching 58 mph wind gusts or 40 mph sustained winds (High Wind Warning Criteria) no High Wind Products were issued. This will need to continue to be monitored. Late Wednesday evening the rain across the Rocky Mountain Front and the mountains of Southwestern Montana will transition to a rain/snow mix. Thursday through Friday... On Thursday the upper-level trough remains in place over North-central, Central, and Southwestern Montana. This will keep temperatures cool. On Thursday there is a 20 - 40% chance for locations along the central and eastern portion of the Hi-Line south to Fort Benton receiving a tenth of an inch of rain (total liquid amount for mountain locations) or more. On Thursday for the mountains of North-Central, Central and Southwestern Montana there is 15 - 40% chance for receiving a tenth of an inch of snow or more. On Friday the upper-level trough slowly begins to move to the east away from North-central, Central, and Southwestern Montana. This will allow temperatures to warm up to near seasonal averages. On Friday it will be dry across the area. Saturday through next Tuesday... On Saturday a weak upper-level ridge moves over North-central, Central, and Southwestern Montana. This will warm up temperatures and keep most of North-central and Central Montana dry with some precipitation chances for Southwestern Montana. On Sunday cluster analysis shows that an upper-level ridge remains over North-central, Central, and Southwestern Montana which will allow temperatures to warm up on Sunday. On Monday cluster analysis keeps the upper-level ridge over the area. Two of the clusters have a very strong ridge over North-central, Central, and Southwestern Montana. This could cause hot temperatures for the area. This will need to continue to be monitored. Next Tuesday three clusters (81% of ensemble members) keep the upper-level ridge over North-central, Central, and Southwestern Montana. The other cluster (19% of ensemble members) shows an upper-level trough beginning to move over Montana. This indicates that the weather pattern has the potential to change for the middle of next week. -IG && .AVIATION... 28/12Z TAF Period VFR conditions are expected to continue through at least 29/12Z across North Central, Central, and Southwest Montana, unless otherwise mentioned. The high pressure ridge over the area will gradually move east out of the area through the period. Mostly clear skies will become mostly cloudy from west to east after 18Z and southerly winds will become gusty ahead of an approaching cold front. The flow aloft will also shift more southwesterly and bring in increased moisture and instability, along with some mountain wave turbulence. Scattered showers and thunderstorms are then forecast to spread east of the Continental Divide after 22Z and over much of the area through 06Z. Winds behind the frontal passage will shift more westerly after 02Z along and just east of the Continental Divide, while increasing southerly winds will cause low level wind shear at KHVR. Thunderstorm activity should decrease somewhat after 06Z, but scattered showers will likely linger. -Coulston Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .HYDROLOGY... A Pacific weather system will bring a threat of heavy downpours with associated showers and thunderstorms, first to the Continental Divide and adjacent areas this afternoon and evening, then to the remainder of North Central, Central, and Southwest Montana tonight through tomorrow. The concern is that this precipitation will fall on warming high mountain snowpack, which could increase the amount and speed of runoff out of the mountains. While most rivers should be able to handle this runoff with a low risk of minor flooding, smaller creeks and streams may run out of their banks. This situation will continue to be monitored for possible flood highlights. -Coulston && .PRELIMINARY POINT TEMPS/POPS... GTF 84 52 64 42 / 10 40 50 10 CTB 81 48 60 38 / 20 50 20 10 HLN 85 54 64 42 / 20 50 60 20 BZN 82 47 64 35 / 10 40 90 50 WYS 72 42 60 29 / 0 40 90 50 DLN 80 47 64 33 / 20 50 80 20 HVR 84 54 69 42 / 0 30 50 10 LWT 78 50 63 37 / 0 20 90 40 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls