Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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110 FXUS65 KTFX 220025 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 625 PM MDT Tue May 21 2024 UPDATED AVIATION SECTION .SYNOPSIS... After a few showers and thunderstorms this afternoon and evening, our next impactful spring snowfall is expected to arrive on Thursday. Expect heavy, wet snow across the higher terrain, with snow possibly even reaching the valleys of Southwestern Montana. Heavy wet snow that falls may result in some damage to trees and power lines, and will be difficult to move. This snow will begin to taper off Thursday night, though wet weather will continue through the weekend. && .DISCUSSION... Key Points: -A few gusty showers and thunderstorms are expected across the plains today -Another heavy wet snowfall is expected in the higher terrain Wednesday and Thursday that may lead to travel delays and damage to trees and power lines -Wet weather continues through the weekend, possibly resulting in stream and river rises Today and Tonight Widely scattered showers and thunderstorms have begun to pop up across North Central and Southwestern Montana this afternoon as moisture pushes in from the North thanks to a low pressure system in Southern Alberta and Saskatchewan. This moisture is interacting with a surface trough that is currently positioned across the area, which is creating just enough atmospheric lift to help these showers get going. For the most part, these showers will not be much of an issue today and just bring beneficial rainfall, though a few rumbles of thunder and isolated wind gusts will be possible, especially across the plains where the sun has been able to help create just a bit of instability. Across the Southwest, expect clouds to be a bit thicker, helping to limit surface heating and thus limiting how intense the showers are able to become. Ludwig Wednesday through Thursday night... The next round of impactful precipitation will begin to move into the area tomorrow. A closed upper level low will dive southward from BC into the Pacific NW tonight into Tuesday, slowly turning eastward Wednesday evening, moving across southern Idaho and vicinity before exiting eastward Thursday night. Ensembles continue to favor Southwest through Central Montana as the areas with the best chance for an inch of liquid precipitation, though a bit of uncertainty is creeping in. The first complicating factor to these chances is that the regime appears to be convective in nature through the day Wednesday, before transitioning to a more stratiform event Wednesday night. This would lead to decreased confidence in impacts at any one location through the day Wednesday. Additionally, there is a subset of guidance that forms a H7 low much further north than the H5 low that will be largely over southern ID. This would favor higher precipitation amounts further north, further adding to a complicated forecast. Regardless, mountain snow will be around in the afternoon Wednesday, with snow levels falling through the overnight into Thursday morning. Current guidance suggests that most major mountain ranges south of a line from Great Falls to Lewistown have at least a 50% chance of seeing at least 6 of snow, with the potential for some of the higher passes to see over a foot of heavy, wet snowfall. There is some concern that this heavy wet snow will result in some tree and power line damage, especially now that trees have begun to come out of dormancy and some trees may have experienced some stress from the heavy snowfall event earlier this month. This concern will extend into the far Southwestern Valleys, where snow levels will likely fall to the valley floors early Thursday morning, resulting in a coating of heavy wet snow at locations like Bozeman, Dillon, Ennis, and West Yellowstone. For now, I have included Bozeman, Dillon, and Ennis in a Winter Weather Advisory with a forecast of 1-3 inches of snow, though there is some concern that a period of heavier precipitation could result in snow lasting longer than currently forecast, which could result in notably higher snowfall totals in the lower elevations. (Note that West Yellowstone was not mentioned in the Advisory as they are currently under a Winter Storm Warning.) With all of this said, this system should not be nearly as impactful as our storm earlier in the month, though that is not to say that this will be a minor event. This system will likely result in scattered power outages, along with very difficult travel thanks to the heavy and wet nature of the snow, as heavy wet snow is very energy intensive to move. AM/Ludwig Friday through Sunday... The region looks to get a brief break Friday, as one system departs and another begins its approach. Showers will still be around given cool air aloft, but not expecting many impacts from these showers. Another system looks to work in quite similarly to the Wednesday and Thursday system for this weekend. An upper level disturbance will drop southward from BC into the Pacific NW and then turn eastward. There is a bit more uncertainty with respect to track and timing with this system however, which will play a role in impacts. The chance for a half an inch of precipitation with this system is largely lower than 50%, owing to the aforementioned uncertainty. Sunday night into early next week... Ensembles favor upper level ridging building in behind the departing system Sunday night into early next week, which will allow temperatures to trend back closer to normal Monday and then above normal for Tuesday. This period looks largely dry, but there will be a non-zero chance for a shower or thunderstorm. -AM && .AVIATION... 22/00Z TAF Period VFR conditions are expected to continue across North Central (KCTB, KHVR), Central (KLWT, KGTF, KHLN), and Southwest (KBZN, KEKS, KWYS) Montana through at least 22/15Z. Through 22/15Z: Weak high pressure aloft will cause lingering clouds, showers, and breezy westerly winds to decrease across the region through 03Z. However, a low pressure area will move into the Pacific Northwest after 09Z, bringing increasing mid-level cloudiness. Increasing southwest winds on the leading edge of this low will also cause a period of mountain wave turbulence and low level wind shear over far Southwest Montana, mainly south of a KEKS- KBZN line. After 22/15Z: Increasing moisture and instability the southwesterly flow aloft will spread mountain obscuring low VFR/MVFR conditions in showers over the region. Southerly winds will also become quite gusty south of Interstate 90, while scattered thunderstorms could cause erratic wind gusts up to 45 kt at times across much of Southwest Montana. -Coulston Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .HYDROLOGY... A Flood Watch remains in effect for the Little Belts Wednesday night into Friday. The chances for an inch and two inches of precipitation have increased slightly (Largely around 70% and 30% respectively) for the Little Belts over this period. There is some uncertainty as to how low snow levels fall with this system. Nonetheless, if most precipitation across the Little Belts Falls as rain, there are concerns for flooding on creeks and streams in the area given recent precipitation/saturated soils. -AM && .PRELIMINARY POINT TEMPS/POPS... GTF 43 57 37 48 / 20 80 100 80 CTB 40 53 34 51 / 20 90 80 40 HLN 45 61 37 48 / 20 90 100 90 BZN 37 60 32 45 / 10 80 100 100 WYS 31 48 25 41 / 20 90 90 90 DLN 40 57 30 44 / 10 90 90 90 HVR 40 57 37 53 / 40 70 70 40 LWT 37 56 34 43 / 20 70 90 90 && .TFX WATCHES/WARNINGS/ADVISORIES... Winter Storm Warning from 6 PM Wednesday to noon MDT Thursday for Elkhorn and Boulder Mountains-Northwest Beaverhead County- Ruby Mountains and Southern Beaverhead Mountains-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass. Winter Weather Advisory from midnight Wednesday night to 6 PM MDT Thursday for Gates of the Mountains. Flood Watch from Wednesday evening through Friday morning for Little Belt and Highwood Mountains. Winter Storm Warning from 6 PM Wednesday to 6 PM MDT Thursday for Big Belt, Bridger and Castle Mountains-Gallatin and Madison County Mountains and Centennial Mountains-Little Belt and Highwood Mountains. Winter Weather Advisory from 6 PM Wednesday to 6 PM MDT Thursday for Beaverhead and Western Madison below 6000ft-Gallatin Valley- Madison River Valley-Missouri Headwaters. && $$ http://www.weather.gov/greatfalls