Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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665 FXUS65 KTFX 291149 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 549 AM MDT Wed May 29 2024 ...Updated Aviation Section... .SYNOPSIS... A cold front with scattered showers and thunderstorms will move east over the area throughout the morning, leaving gusty westerly winds in its wake. Its associated disturbance will continue showers and thunderstorms south of a line from Helena to Havre through early this evening. The main threats from the storms are strong erratic wind gusts and some heavy downpours. The gusty westerly winds with a chance of light showers will persist through the day on Thursday, as temperatures remain slightly below normal. After a relatively quiet day on Friday, mild and unsettled are forecast for the weekend into at least early next week. && .DISCUSSION... Today through Thursday... The main concern today into this evening will be the passage of a cold front across North Central, Central, and Southwest Montana and its associated disturbance. Along the cold front itself, scattered showers are moving across the area early this morning. Westerly winds will then increase and become gusty behind the front, and the resulting downsloping will help limit further activity in the afternoon on the plains adjacent to the Rocky Mountain Front. Have therefore increased National Blend of Model (NBM) mean forecast winds to between them and their 90th percentile to better represent widespread 20 to 30 mph winds with gusts to 40 mph at times over the plains. Overall, temperatures will struggle to warm back up to seasonal normals. However, increased moisture and lift on the leading edge of the main disturbance will combine with daytime heating to focus increased shower and thunderstorm activity along and east of a line from Dillon to White Sulphur Springs to Lewistown. The enhanced winds aloft also on the leading edge of the disturbance will increase the chance of strong winds translating down to the surface, so the Storm Prediction Center (SPC) has put a Marginal risk (5-15%) that a few severe thunderstorms may occur, mainly due the strong wind gust potential. Also, ensemble guidance indicates that the precipitable water over that area will be 1 to 2 standard deviations above normal, so the Weather Prediction Center (WPC) also has that area under a Marginal risk (5-15%) that localized minor flash flooding could occur (please see the HYDROLOGY section for more details). As the disturbance starts exiting the area overnight, precipitation will decrease from west to east, but a strong surface pressure gradient at the surface and continued moderate westerly winds aloft, breezy westerly surface winds should persist over the plains. Overall, though, ensemble guidance only gives a 50 percent chance that areas on the Rocky Mountain Front will exceed 55 mph wind gusts. Also, snow levels will mostly lower to between 4500 feet and 5500 feet overnight, but the tapering off of precipitation will limit snow accumulation to 2 to 4 inches above 7500 feet. On Thursday, the northwest flow aloft on the back side of the disturbance will bring a chance of additional rain and mountain snow showers, but any additional amounts will be minor. However, a persistently strong surface pressure gradient and winds aloft will cause breezy northwest winds to become gusty again by afternoon, as temperatures remain below normal. Friday through next Wednesday... A weak high pressure ridge is forecast to move into the area on Friday, bringing at least one day of drier conditions, lighter winds, and a return to near normal temperatures. Model clusters indicate that a more progressive westerly flow pattern will develop for the weekend into the middle of next week, but there is some disagreement on when the disturbances will move through the area. As a result, the NBM keeps the area slightly warmer than normal with a chance for showers and thunderstorms each day, especially over the mountain areas of Central and Southwest Montana. -Coulston && .AVIATION... 29/12Z TAF Period VFR conditions will prevail for the KHLN, KGTF, KCTB, and KHVR terminals for this TAF period. At the KBZN, KEKS, KWYS, and KLWT terminals thunderstorms are likely (60%) to occur and reduce visibility to MVFR levels Wednesday afternoon and evening. During thunderstorms there will be gusty and erratic winds and small hail. Wednesday afternoon at the KBZN, KEKS, and KLWT terminals ceilings will be reduced to MVFR levels. At the KBZN, KEKS, KWYS, and KLWT terminals there will be rain showers Wednesday afternoon and evening due to a front moving through the area. At the KLWT and KWYS terminals winds will gust up to 24 kts Wednesday afternoon and evening. At the KLWT terminal there is a 20% chance for showers and thunderstorms during the first 6 hours of this TAF period. At the KHVR, KHLN, and KGTF there will be rain showers Wednesday morning. At the KGTF and KHVR terminals winds will gust up to 28 kts from the middle of this TAF period through the end. At the KCTB terminal during this TAF period through the end winds will gust up to 40 kts. At the KHLN terminal from the middle of the TAF period through the end winds will gust up to 29 kts. There will be mountain obscuration during this TAF period. During this TAF period there will be mountain wave turbulence. Across North-central Montana at the end of this TAF period there is the potential for isolated occurrences of low-level wind shear. -IG Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .HYDROLOGY... A disturbance will move through the area today into this evening, with the focus for potentially heavy downpours from thunderstorms being over the area along and east of a line from Dillon to White Sulphur Springs to Lewistown. The main concern is that the rain will fall on warm mountain snowpack, causing rapid runoff. Most rivers should handle the extra water, as river levels are not exceptionally high, but some creeks and streams will run high and possibly out of their banks in some spots especially out of the Madison, Gallatin, Little Belt, and Big and Little Snowy mountain ranges. We are not anticipating widespread issues, but will monitor the situation closely for possible localized flooding. -Coulston && .PRELIMINARY POINT TEMPS/POPS... GTF 64 40 61 38 / 80 10 20 0 CTB 61 38 59 37 / 70 10 20 0 HLN 64 40 64 40 / 70 10 10 0 BZN 61 34 60 34 / 100 60 10 0 WYS 59 30 53 28 / 90 70 10 0 DLN 61 33 59 33 / 90 20 0 0 HVR 69 41 62 41 / 80 10 50 10 LWT 61 36 56 35 / 100 40 20 0 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls