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Area Forecast Discussion
Issued by NWS Glasgow, MT
Issued by NWS Glasgow, MT
652 FXUS65 KGGW 130852 AFDGGW Area Forecast Discussion National Weather Service Glasgow MT 252 AM MDT Sat Jul 13 2024 .DISCUSSION... KEY MESSAGES: 1) Thunderstorms are expected this afternoon and evening. Some will become severe with damaging winds and large hail. Timing will range around 2PM to 11PM over the bulk of northeast Montana. 2) Hot conditions will continue into the weekend with the heat advisory extended in the far south into Saturday. WEATHER PATTERN OVERVIEW: Upper ridge will remain in place over the western half of the CONUS for the next several days. A cold front will arrive Sunday evening and drop temps off about 5-10 degrees through Wednesday. Heat will be back in sway by late next week with 100s creeping back in as the ridge to the west restablishes itself. FORECAST CONFIDENCE & DEVIATIONS: The heat advisory currently for the bottom third of counties for Saturday has been left up due to momentum, but the cold air from the thunderstorms outflow yesterday has backed temps off for Saturday to a max of around 97 degrees which is probably too low for it. In addition, there is a chance that widespread thunderstorms in the early afternoon may produce enough cloud coverage to take the impact edge off the highs as well. Will be giving the day shift a chance to let it ride and the option to cancel it early if things do not look to be going that way. While Sunday does have temps reaching up to 100 degrees before the front arrives that evening, there is cooler temperatures Saturday night which will allow for some recovery in the heat risk. Therefore, will be delaying issuing an extension for Sunday for now. Keeping an eye to smoke models has allowed for some haze to sneak into the area over the next couple days. Air quality only remains moderate but will need to be monitored through the next several days. CAMs models for thunderstorms this afternoon have spread out the area being affected toward the south by about a county and half compared to 24 hours ago. They have also clumped the initiation of the storms around Noon to an hour or two after. However, there is an outlier in the HRRR which doesn`t bring storms through till nearly 6PM. It looks like mesoscale details such as lingering outflow boundaries from yesterday will be the deciding factor for this afternoon and this is adding quite a bit of uncertainty to timing and spatial placement of storms in addition to the strength of the heat. First 24 hours of PoPs were again updated to include a 50/50 blend of the SREF NPoP along with the NBM PPI01 in order to capture the afternoon thunderstorms better with the marginal threat for severe weather in the area. This bumped PoPs up from a meager 20%-30% to nearly 60% around the most certain areas. First 48 hours ended up getting a bump with the shrink stretch tool over the CWA for afternoon highs and overnight lows, as RTMA/URMA map passes are continuing to miss, skip, or average out ASOS/AWOS highs by roughly 2 to 4 degrees and this is clearly biasing baseline NBM downward compared to the reality of these 100 degree+ extremes which have occured over the last 2 days. GAH && .AVIATION... LAST UPDATE: 0830Z FLIGHT CAT RANGE: VFR - MVFR DISCUSSION: While the morning will start out near clear, afternoon and evening thunderstorms will move across the area today. Some of these storms will become severe with large hail and damaging winds. Brief MVFR visibility may be possible with some of these storms. Used Prob30s to handle the bulk of this from roughly 21 to 05Z. Will switch to tempos when T-storms start showing up on radar later today. Isolated HAZE is expected across the area through the cycle but should not drop conditions below VFR on it`s own. DENSITY ALTITUDE: Afternoon highs could break 100 degrees in some places through Sunday. This will make for less dense air and lead to a need for increased power and longer runway takeoffs. WIND: North to northeast at 5 to 15 kts through the morning. Veering to the east to southeast in the afternoon and evening hours. Outflow boundaries from any thunderstorms that occur will create gusty and erratic winds. Then becoming light and variable after midnight. Enriquez/GAH && .GGW WATCHES/WARNINGS/ADVISORIES... Heat Advisory until 9 PM MDT this evening for Garfield-Petroleum- Prairie. && $$ weather.gov/glasgow