Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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153 FXUS65 KTFX 291956 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 156 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. && .DISCUSSION... 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 && .AVIATION... 1155 AM MDT Sun Apr 28 2024 (29/18Z TAF Period) Expect VFR conditions to give way to periods of MVFR to IFR conditions today as showers and isolated thunderstorms move in and develop across portions of Central and Southwestern Montana today. These showers and storms will bring periods of IFR to even LIFR conditions in the heaviest storms, particularly near and south of a line from KHLN to KLWT. Periods of reduced sigs and vis will continue into the overnight hours as rain changes to snow across SW MT (including KBZN and KEKS), with periods of visibility less than what the TAFs show. Ludwig Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PRELIMINARY POINT TEMPS/POPS... GTF 32 52 32 43 / 60 20 30 90 CTB 31 50 32 38 / 10 40 50 90 HLN 32 52 30 47 / 60 30 10 70 BZN 24 49 24 46 / 80 40 40 50 WYS 20 40 19 38 / 70 50 50 60 DLN 24 47 23 45 / 60 30 20 30 HVR 35 56 33 50 / 40 40 50 80 LWT 28 49 27 45 / 90 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