Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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814 FXUS65 KTFX 300239 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 839 PM MDT Mon Apr 29 2024 .SYNOPSIS... A slow moving Pacific system will bring below average temperatures, periods of rain and snow, and even a few thunderstorms through mid- week. Breezy to windy conditions will accompany the unsettled conditions. Temperatures moderate some heading towards the weekend, but the typically active spring weather pattern will persist for the foreseeable future. && .Update... Evening update has been published, with the primary adjustments being to raise overnight temperatures across much of Central and North Central Montana and (raise) PoPs along and east of a Bozeman, to Great Falls, to Havre line. Showers, with embedded thunderstorms, continue to lift east and northeast across portions of Southwest through North Central Montana this evening, predominately east of the I-15 corridor. Rain has switched over to snow across some valleys of Southwest Montana this evening as intense precipitation rates have cooled the lower levels efficiently enough to allow for a changeover to all snow. Snow accumulations across most lower elevations of Southwest Montana will generally remain below 2 inches where precipitation changes over to all snow, with 2-6" being possible in the mountains. Further north over the plains of Central and North Central Montana, precipitation will predominately remain as all rain; however, a little wet snow may mix in near the end of the precip shield exiting the area near Tuesday morning. With widespread cloud cover and continued breezy and gusty southwest to west surface winds expected through the overnight hours tonight, I have raised low temperatures by approximately 2-4 degrees. While temperatures aloft will be cooling throughout the night in wake of the Pacific cold front, expect the aforementioned winds and cloud cover to inhibit radiational cooling and keep temperatures a little "warmer". - Moldan && .AVIATION... 645 PM MDT Mon Apr 29 2024 (30/00Z TAF Period) VFR conditions are mostly expected to persist across North Central, Central, and Southwest Montana through at least 01/00Z. However, ceilings will likely be low enough for most of the period that mountain top obscuration can be expected at times. Scattered showers with isolated thunderstorms over much of the area will decrease from the west through around 08Z as a disturbance exits the area. Some storms may produce brief heavy showers that reduce conditions to MVFR/IFR with erratic wind gusts up to 40 kt. After 18Z, another disturbance will then likely bring areas of rain to North Central Montana and more scattered showers to Southwest Montana. Moderate mid level westerly winds will keep some mountain wave turbulence over Southwest Montana through the period, while temporarily decreasing winds on the plains will allow periods of low level wind shear between 03Z and 15Z. -Coulston Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PREV DISCUSSION... /ISSUED 645 PM MDT Mon Apr 29 2024/ Shower and isolated thunderstorm activity associated with an approaching Pacific trough continues to increase from the west /southwest. Although current mesoanalysis data only show around 100 to 250 J/kg of CAPE, cooler air aloft is keeping mid-level lapse rate around the -8C/km mark. Some of the stronger cells may contain gusty winds over 40 mph, brief downpours, and a few lightning strikes. This is mostly likely to occur for central/southwest areas along and south of a Helena to Lewistown line. A period of wet snow is also expected this evening, mostly over and near the central ranges and the Madison/Gallatin ranges. Although some of these areas may pick up a quick 1 to 3 inches of snow on grassy surfaces, no winter weather highlights are being considered at this time given the short period of occurrence and the warmer antecedent ground conditions; however, the situation will continue to be monitored and the appropriate actions will be taken should things take an unexpected turn. Precipitation diminishes and ends in a west to east fashion later this evening into tonight. The aforementioned broad Pacific trough will slowly traverse the region Tuesday through early Thursday, bringing periods of rain/snow, gusty winds, and well below average temperatures. Guidance has been fairly consistent with the positioning of the system, with one closed low moving along the Canadian border and a secondary circulation splitting off farther south along the MT/ID border. However, timing and borderline temperatures have added forecast challenges in regards to overall snow impacts. H700 temperatures in the -10 to -15C degree range are definitely supportive to dendritic snow growth, but the latest trends are delaying the optimal northerly flow aloft to the Wednesday/Wednesday night timeframe for some areas. Given that the we are at the end of April, the higher sun angle increases loss of snow from insolation and lowers roadway impacts, even in the mountains. This leaves a brief opportunity for higher impacts Wednesday morning and then again Wednesday night. The inherited Winter Storm Watches do highlight the locations where the highest impacts are expected and snow exceedance probabilities are running above 50% for 6 inches of snow or more Tuesday through Thursday. The East Glacier zone was upgraded to a Winter Storm Warning and Winter Weather advisories were posted for the adjacent Northern Plains and Southern Rocky Mountain Front zones given higher confidence of impacts late Tuesday night into Wednesday morning. The Little Belt Mountain Winter Storm Watch was left as is for now, since the timing of impacts looks to have shifted later on Wednesday/Wednesday night. Winter Weather Advisories will be considered for other locations over the next 24 hours, with the most likely candidates being the Madison/Gallatin mountains and perhaps the Big Belts and the northerly upslope areas along the highway 200 corridor between Great Falls and Lewistown. Regardless of snowfall amounts, anyone with outdoor plans will encounter cold and wet conditions, especially Tuesday night through Thursday. In addition to that, low temperatures will be near or below freezing each morning for the remainder of the work week, with the highest probabilities (70% +) for 32F or lower readings along the Rocky Mountain Front and central/southwestern areas along and south of the highway 200 corridor. Ensembles maintain general troughing and below average temperatures over the Northern Rockies for the remainder of the work week, with another passing shortwave to bring another round of lower elevation rain showers and mountain snow Friday through early Saturday. This system doesn`t appear to be quite as wet and cold as the previous one and this is reflected in the NBM probabilities of a quarter inch of rainfall/liquid equivalent precipitation for the 24 hour period ending 6 pm Friday currently running less than 40% for all areas. Transient ridging dries things out Saturday through early Sunday before another Pacific system moves in for the first part of next week. Despite the overall unsettled conditions, Temperatures slowly creep toward seasonal averages this weekend into early next week. - RCG && .PRELIMINARY POINT TEMPS/POPS... GTF 37 52 32 43 / 80 20 30 90 CTB 34 50 32 38 / 10 40 50 90 HLN 36 52 30 47 / 60 30 10 70 BZN 27 49 24 46 / 80 40 40 50 WYS 21 40 19 38 / 50 50 50 60 DLN 25 47 23 45 / 50 30 20 30 HVR 39 56 33 50 / 50 40 50 80 LWT 32 49 27 45 / 100 20 10 80 && .TFX WATCHES/WARNINGS/ADVISORIES... Winter Storm Warning from midnight Tuesday night to noon MDT Thursday for East Glacier Park Region. Winter Weather Advisory from midnight Tuesday night to noon MDT Thursday for Northern High Plains-Southern Rocky Mountain Front. Winter Storm Watch from Wednesday morning through Thursday morning for Little Belt and Highwood Mountains. && $$ http://www.weather.gov/greatfalls