Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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124 FXUS65 KTFX 060547 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1147 PM MDT Sun May 5 2024 ...AVIATION SECTION UPDATED... .SYNOPSIS... Below normal temperatures, widespread precipitation, and strong and gusty winds are expected through the first half of the upcoming work week as a slow moving Pacific storm moves over the Northern Rockies and High Plains. Significant impacts to travel are likely over the Central Montana mountains and along the Continental Divide as heavy snow falls, especially from Tuesday afternoon through Wednesday night. && .UPDATE... An upper level disturbance pivoting through the intermountain west is carving out a surface low across NE WY and E MT this evening. This surface low will drift northward and perhaps a bit northwestward this evening, allowing for a pressure gradient to slowly develop across the region overnight into Monday. Westerly winds look to develop late tonight and slowly increase in magnitude through the day Monday. In the meantime, southerly flow ahead of this system has resulted in showers and a few thunderstorms across the region this afternoon and evening. Given waning surface heating, showers and any isolated thunderstorms are forecast to diminish in coverage over the next few hours. That does not mean precipitation chances are over for the evening though, as a more stratiform precipitation increases in coverage from southwest to northeast tonight across the region. Greatest impacts from this precipitation overnight look to be in the mountains of Southwest Montana, where a Winter Weather Advisory remains in effect. This area of precipitation will slowly move toward eastern portions of North-central and Central Montana tomorrow. No significant changes were needed with the forecast this evening. Small tweaks were made in the near term using latest guidance to account for trends. -AM && .AVIATION... 1147 PM MDT Sun May 5 2024 (06/06Z TAF Period) Rain showers, mountain snow, and isolated thunderstorm activity will increase over the southwest tonight before lifting northeastward and primarily impacting the eastern portions Central/North-central MT on Monday. Periods of low VFR/MVFR and even IFR conditions (most notably for KWYS) and associated mountain obscuration can be expected, particularly during times of precipitation. Gusty west to northwesterly winds and mountain wave turbulence develop and increase late tonight/early Monday morning, peaking in strength later in the afternoon. Areas along the Continental Divide from MacDonald Pass northward and over the central ranges will see the strongest winds as will their neighboring plains/valleys. - RCG Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .HYDROLOGY... A slow moving Pacific weather system will bring an extended period of lower elevation rain and mountain snow next week, with the heaviest precipitation falling Tuesday through early Thursday. NBM probabilistic guidance highlights the eastern portions of Central/North-central MT with the heaviest rainfall amounts for this period, largely supporting 1 to 3 inches of rainfall/snow melt liquid equivalent over locations east of a Great Falls to Shelby line Tuesday through Thursday. While flooding is generally not expected, some rivers, creeks, and streams along the Canadian border in Hill and Blaine Counties are expected to approach or reach action stage Wednesday and Thursday, mainly impacting unpopulated rural locations. Anyone near waterways should remain alert for changing conditions and be prepared to seek higher ground should water rises occur. - RCG && .PREV DISCUSSION... /ISSUED 305 PM MDT Sun May 5 2024/ Key Points: - Significant spring storm will impact the Northern Rockies through the middle of the upcoming work week; bringing widespread rain, heavy mountain snow, and strong winds. - Heaviest rainfall amounts continue to be favored over the plains of Central and North Central Montana, with a 50% or greater chance than amounts exceed 1" northeast of a Judith Gap, to Great Falls, to Chester line from Monday morning through Thursday morning. - Heaviest mountain snow continues to be favored across the Central Montana mountains, particularly in the Little Belt and Highwood Mountains, and across the Glacier National Park Area. The Little Belt Mountains have an 80% or greater chance of seeing a foot or more of snow from Tuesday through Wednesday, with a 30-70% chance of exceeding two feet of snow. - Strongest winds are expected to occur along and north of the Montana Hwy 200 corridor, with NBM probabilities supporting a 70% or greater chance that sustained wind speeds reach or exceed 40 mph at times from Monday afternoon/evening through Wednesday. A strong and slow moving spring storm will bring widespread impacts to the Northern Rockies through the middle of the work, with the highest impacts to travel, agriculture, and outdoor recreation expected along and north of the I-90 corridor across portions of Southwest, Central, and North Central Montana. High temperatures through the middle of the upcoming work week will be below to well below normal, with Tuesday and Wednesday being the "coldest" days thanks to widespread cloud cover and periods of moderate to heavy precipitation. A gradually moderation in temperatures is then expected for the second half of the work week as the aforementioned storm pulls away from the region, with temperatures rising above normal for the upcoming weekend. A majority of ensembles continue to highlight a closed Pacific low lifting northeastward into eastern MT while merging with another shortwave streaming eastward out of the Pacific NW. There will be a bit a lull in activity on Monday as steadier stratiform rains remain to our east/northeast and mountain snows temporarily diminish. H700 northwesterly flow begins to increase by Monday afternoon, ushering cold air advection and increasing surface winds along the Rocky Mountain Front and much of Central/North-central MT. By Tuesday, periods of moderate, to at times heavy, rainfall begin to settle in and more or less remain in place through Thursday. Keep in mind that there will be some areas situated southeast of higher terrain that see reduced rainfall amounts due to downsloping winds. This widespread rainfall will generally be beneficial for most, but some minor hydrologic impacts are expected for those who see the heaviest storm totals, please view the hydrology section for more information. Strong and cold northwesterly flow aloft (H700 winds around 40 to 60 kts, temperatures near -10C) will be occurring during this timeframe as well, resulting in windy conditions for all areas and heavy wet mountain snow over the central ranges. Monday night through Tuesday looks to be the overall windiest period, with most locations in both North-central and Southwest MT seeing 60 to 90% chances for wind gusts in excess of 45 mph, while the 55 mph exceedance higher probabilities (70% +) remain confined along the Continental Divide, the Central MT highway 200 corridor, and the eastern portions of the north-central plains. As mentioned earlier, H700 temperatures will be cold enough to support mountain snow, heaviest over the central ranges where a Winter Storm Watch remains in effect Tuesday through Thursday and the probability for snowfall amounts in excess of 2 feet continues to run 70% and higher, mostly for the mid- slopes and higher peaks of the Little Belt Mountains. The combination of strong winds and heavy wet snow is expected to cause dangerous conditions for outback recreation over higher terrain and there is also the expectations for at least some isolated impacts to the power infrastructure in areas that receive the heaviest snow and stronger winds. Additional winter weather highlights may be needed over the higher terrain of the southwest, the Rocky Mountain Front, and possibly for the Bear`s Paw Mountains. Lower elevations will mostly see cold, wet, and raw conditions for outdoor recreation and agriculture activities with daytime temperatures mostly in the 40s Tuesday and Wednesday. - RCG/Moldan && .PRELIMINARY POINT TEMPS/POPS... GTF 44 56 40 47 / 30 30 20 70 CTB 43 54 38 46 / 20 10 30 80 HLN 43 56 38 49 / 50 20 10 40 BZN 39 53 30 47 / 60 60 20 50 WYS 30 45 25 39 / 70 80 70 80 DLN 38 51 28 43 / 60 40 30 40 HVR 46 58 41 48 / 20 70 70 90 LWT 42 51 36 42 / 50 80 30 80 && .TFX WATCHES/WARNINGS/ADVISORIES... High Wind Warning from noon Monday to 6 PM MDT Tuesday for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Northern High Plains-Southern High Plains-Southern Rocky Mountain Front. High Wind Watch from Monday evening through late Tuesday night for Bears Paw Mountains and Southern Blaine-Eastern Toole and Liberty-Fergus County below 4500ft-Hill County-Judith Basin County and Judith Gap-Northern Blaine County-Snowy and Judith Mountains-Western and Central Chouteau County. Winter Storm Watch from Tuesday afternoon through Wednesday evening for Big Belt, Bridger and Castle Mountains-East Glacier Park Region-Snowy and Judith Mountains. Winter Storm Warning from noon Tuesday to midnight MDT Wednesday night for Little Belt and Highwood Mountains. Winter Weather Advisory until noon MDT Monday for Gallatin and Madison County Mountains and Centennial Mountains-Northwest Beaverhead County-Ruby Mountains and Southern Beaverhead Mountains. && $$ http://www.weather.gov/greatfalls