Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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421 FXUS65 KTFX 091803 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1203 PM MDT Sun Jun 9 2024 ...Updated Aviation Section... .SYNOPSIS... Showers and thunderstorms increase over Southwest Montana this afternoon before spreading northward into central and north- central areas later in the evening and the overnight hours. The stronger storms may contain strong, gusty winds, hail, and heavy downpours, particularly for southwestern locations. Shower and thunderstorm activity diminishes Monday with breezy and much drier conditions expected heading into the mid-week period. && .Update... Morning update has been published, with only minor adjustment made to the on-going forecast. Main adjustment was to increase PoPs an additional 10-20% across the CWA through tonight as latest Hi-Res guidance continues to indicate numerous showers and thunderstorms lifting east and northeast across Southwest through North Central Montana through the overnight hours. Early morning convection across Southwest Montana will predominately remain near the Idaho border through this morning, with additional showers and thunderstorms developing between 10am-2pm over the higher terrain of Northeast Idaho/Southwest Montana and then lifting northeast over lower elevations of Southwest and into Central Montana through 6pm. Additional showers and thunderstorms will then overspread the remainder of Central and North Central from west to east from the late evening through morning hours on Monday as the shortwave moves overhead. A Marginal Risk for severe thunderstorms continues southeast of a general Dillon, to Three Forks, to WSS, to Lewistown line, with damaging wind gusts being the primary concern. Additionally, bursts of heavier rain over the higher terrain where an appreciable snowpack remains could lead to rises on creeks and streams emanating from the mountains. - Moldan && .AVIATION... 9/18Z TAF Period VFR conditions will prevail for the KGTF, KCTB, KHVR, and KLWT terminals during this TAF period. For those terminals tonight into tomorrow morning there is 40 - 70% percent chance for rain showers which could reduce ceilings to low VFR levels and so a PROB30 group was included in those TAFs for it. At the KGTF, KHVR, and KLWT terminals there is a 20% chance for thunder with the rain showers. Another aviation concern for the KGTF, KHVR, and KLWT terminals is wind gusts of 19 - 23 kts at the very end of the TAF period. For the KEKS and KWYS terminals showers and thunderstorms will reduce visibility and ceilings to MVFR levels for the duration of this TAF period. These showers and thunderstorms will move north this afternoon and evening. They will begin affecting the KBZN and KHLN terminals between 23Z and 00Z. These thunderstorms will have lightning, gusty to strong winds, small hail, and brief heavy rain. For the KHLN and KBZN terminals VFR conditions will prevail until the showers and thunderstorms move in between 23Z and 00Z. MVFR conditions will then prevail for the KBZN and KEKS terminals through the end of this TAF period. Due to the showers and thunderstorms there will be intermittent mountain obscuration primarily across Southwestern Montana through the duration of this TAF period. -IG Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PREV DISCUSSION... /ISSUED 527 AM MDT Sun Jun 9 2024/ Lower level north/easterly flow has persisted through the overnight hours for continued breezy conditions and scattered to broken clouds, mostly over central and north-central MT. Lower level cloud cover associated with this set up is expected to retreat to the northeast as a southwesterly flow aloft begins to develop ahead of a shortwave currently moving into the Pacific Northwest. An initial wave of weakening showers and isolated thunderstorms moving through Central Idaho is already being tracked on radar and should reach Beaverhead County later this morning in the form of isolated lighter end showers/virga and perhaps a lightning strike or two. Southwesterly flow aloft becomes more established ahead of the approaching shortwave later today, providing ascent, shear, and modest instability for afternoon and evening shower and thunderstorm development. The highest CAPE (500 to 1,000 J/kg) looks to be for areas south of I90, where gusty winds and hail may accompany the stronger cells. Precipitable water values will generally range in the 0.75 to 1 inch mark, so localized heavy downpours can also be expected. This activity will lift northward into Central/North- central MT during the evening and the overnight hours, but the loss of diurnal heating will result in a more benign brand of showers and general rains. While hi-res guidance does bring a secondary round of showers and storms to the southwest late tonight, a more stabilized atmosphere should preclude the potential for stronger development in a similar fashion as northern areas. Westerly winds increase and things dry out on Monday following the passage of the shortwave, but another weak perturbation does move across the region during the afternoon hours for additional isolated to scattered showers and thunderstorms, mostly along the Hi-Line and over and near areas of higher terrain elsewhere. Ensembles remain in fairly good agreement with a zonal flow aloft settling in for the mid-week period. H700 to H500 flow will be on the stronger side for this time of time of year, mostly between 40 and 50 kts or 2 to 3 standard deviations above climatology according to the NAEFS (North American Ensemble Forecast System). Deep layer mixing should encourage breezy to windy conditions, especially over the plains and the Rocky Mountain Front on Wednesday. Wind exceedence probabilities suggest that peak wind gusts will mostly fall between the 35 and 55 mph range except along the Rocky Mountain Front and adjacent high plains where gusts up to and exceeding 60 mph will be more common. The primary impact will be to summer outdoor recreation and travel; however, afternoon temperatures in the 70s and 80s combined with the said winds and minimum relative humidity values in the teens and 20s is expected to accelerate the curing/drying of grassland fuels. While no fire products are anticipated this time around, this may set up fire weather concerns in the future. Uncertainty remains in terms of forecast details for the upcoming weekend, but ensembles continue to highlight a Pacific trough exerting an influence over the Northern Rockies. At this time, there is an expectation for breezy conditions, isolate to scattered shower and thunderstorm activity, and a slight cool down for Saturday and Sunday. - RCG && .PRELIMINARY POINT TEMPS/POPS... GTF 74 56 80 50 / 10 70 30 10 CTB 69 51 77 49 / 10 60 30 0 HLN 79 59 84 51 / 30 80 30 0 BZN 77 55 78 45 / 60 80 50 0 WYS 74 47 71 39 / 70 80 60 0 DLN 76 51 76 45 / 90 90 20 0 HVR 73 53 77 49 / 10 70 50 10 LWT 73 54 76 46 / 10 70 40 10 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls