Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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077 FXUS65 KTFX 220514 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1114 PM MDT Tue May 21 2024 ...Aviation Section Updated... .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. && .Update... Evening update has been published, with mainly minor adjustments made to PoPs and low temperatures. Bumped PoPs up slightly through the remainder of the evening hours, especially over the plains of Central Montana, to account for recent radar trends which continued to show scattered showers drifting east. These and other showers will become more isolated in nature with the loss of diurnal heating; however, a few elevated showers may persist into the early morning hours on Wednesday over the mountains and eastern zones. Rain and mountain snow will begin to increase in coverage beyond 3AM MDT west of the I-15 corridor with the arrival of the next upper level disturbance, especially along Continental Divide north of the Montana Hwy 200 corridor. Low temperatures were also nudged up by several degrees across most locations, with the anticipation that increasing cloud cover will limit radiational cooling throughout the nighttime hours. Finally, patchy fog was introduced across eastern portions of the Hi-Line where skies will remain partly clear through the overnight hours and where recent rain has fallen from today`s showers. - Moldan && .AVIATION... 22/06Z TAF Period VFR conditions prevail through the overnight under increasing mid level cloudiness. The next system arrives through the day Wednesday. The initial concern will be for lower elevation rain showers and scattered thunderstorms with higher elevation snow showers through daylight hours. Gusty winds and small hail will be the concern with thunderstorms that form. Then, precipitation transitions to be more stratiform in nature (Ceilings lowering) with snow levels falling down as low as valley floors at times Wednesday night. Expect mountain obscuration from Wednesday afternoon onward into Wednesday night. -AM 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 && .PREV DISCUSSION... /ISSUED 625 PM MDT Tue May 21 2024/ 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 && .PRELIMINARY POINT TEMPS/POPS... GTF 45 57 37 48 / 20 80 100 80 CTB 42 53 34 51 / 20 90 80 40 HLN 47 61 37 48 / 20 90 100 90 BZN 38 60 32 45 / 10 80 100 100 WYS 30 48 25 41 / 20 90 90 90 DLN 42 57 30 44 / 20 90 90 90 HVR 40 57 37 53 / 40 70 70 40 LWT 37 56 34 43 / 30 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