Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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364 FXUS65 KTFX 291800 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1155 AM MDT Mon Apr 29 2024 .SYNOPSIS... A slow moving and strong upper level disturbance will bring unsettled conditions to the Northern Rockies through the middle of the upcoming work week, with accumulating snow becoming more likely along the Rocky Mountain Front and across the Island Ranges of Central Montana from Tuesday evening through Thursday morning. && .UPDATE... The morning update has been published. Large scale ascent and an eastward advancing Pacific cold front will increase shower and isolated thunderstorm activity this afternoon and evening, mostly for Southwest MT and central/north-central locations south and east of a Helena to Havre line. Primary concern will be the potential for isolated instances of stronger wind gusts up to 50 mph mixing down to the surface during any shower or storm activity, especially for the north to south oriented southwest valleys. This in addition to a few lightning strikes, brief downpours, and perhaps some small hail or graupel given mid level lapse rates around 7 to 7.5C/km. H700 temperatures fall to -10C behind the front, so a period of wet snow can also be expected this evening over and near the higher terrain of Central and Southwest MT. Although this may be a quick burst of heavier snow, impacts should remain brief and minor. This situation will be monitored as the day progresses. The main update this morning was to adjust pops, temperatures, and winds to current trends. - 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. && .PREV DISCUSSION... /ISSUED 433 AM MDT Mon Apr 29 2024/ Today through Tuesday morning... A shortwave trough ahead of the main event will bring rain to lower elevations and snow to the mountains. Precipitation amounts over a quarter of an inch are most likely to the southeast of a line from Chinook to Great Falls with amounts up to a third of an inch possible around Lewistown and the Judith mountains. There is some convective potential this afternoon in central and southwestern Montana which may produce locally higher amounts. Otherwise, the other main threat this afternoon into Tuesday morning will be gusty winds up to 50 mph along the Rocky Mountain front and across southwestern Montana. The timing of the highest wind gusts in southwestern Montana will be this afternoon while the highest wind gusts are expected Tuesday morning for the Rocky Mountain front. Tuesday afternoon through Thursday morning... The timing of the snow has shifted as the trough is now expected to move slower through the region, delaying the onset of the heaviest snowfall by about 6-8 hours compared to the previous day. This means that the highest impacts will be generally confined to all day Wednesday. When considering highlights, the start time on the winter storm watch along the East Glacier Park Region was kept the same but extended to Thursday morning to account for the increased time window. Additionally, a watch was added for the Little Belt and Highwood mountains. Snow totals increased a bit from the previous forecast and there is now high enough confidence in at least 10 inches of snow across a wider portion of the zone to warrant adding it to the watch. This segment of the watch has a 12 hour delay to the start time compared to the East Glacier watch to account for the delay in when snow is expected to start further east but both watches end Thursday morning. When considering other zones, there was not enough confidence or widespread impact to add additional zones to the watch. There are a couple zones, particularly along the Rocky Mountain front and the Big Belts, where limited areas might be near criteria. But when looking for more widespread impacts and higher snow amounts over a larger portion of the zone, there was not enough confidence to warrant adding them at this time. Thursday afternoon through next weekend... Ensemble guidance currently points towards a shortwave moving through the area between Friday morning and Saturday afternoon which will keep precipitation chances up. Beyond that, ensembles point towards a trough coming onshore along the western CONUS early next week which could continue a wetter pattern across the state. -thor && .PRELIMINARY POINT TEMPS/POPS... GTF 32 52 32 46 / 60 20 30 80 CTB 31 50 32 42 / 10 40 50 70 HLN 32 52 30 50 / 60 30 10 60 BZN 24 49 24 49 / 80 40 40 50 WYS 20 40 19 39 / 70 50 50 60 DLN 24 47 23 46 / 60 30 20 30 HVR 35 56 33 48 / 40 40 50 70 LWT 28 49 27 45 / 90 20 10 70 && .TFX WATCHES/WARNINGS/ADVISORIES... Winter Storm Watch from Tuesday evening through Thursday morning for East Glacier Park Region. Winter Storm Watch from Wednesday morning through Thursday morning for Little Belt and Highwood Mountains. && $$ http://www.weather.gov/greatfalls