FXUS65 KTFX 170650 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1249 AM MDT Fri May 17 2024 ...Aviation Section Update... .SYNOPSIS... Gusty west winds will increase and become more widespread through Friday as a Pacific weather system moves into the region with its associated cold front crossing the region by Friday afternoon. Scattered showers and isolated thunderstorms are possible in some areas through Friday evening, with cooler temperatures following through the weekend while an active weather pattern brings additional chances for precipitation late this weekend into next week. && .UPDATE... No significant changes needed to ongoing forecast for the overnight period. However, winds have already started to decrease on the Rocky Mountain Front and adjacent plains after widespread gusts in the 45 to 55 mph range. winds area a little slower to decrease across the northeast TFX plains, including Hill, Blaine, and Chouteau Counties, and there are a few lingering showers and weak thunderstorms. This is occurring on the trailing edge of an exiting disturbance, so winds and shower/storm activity should continue to diminish through midnight. Otherwise, winds and at least scattered showers will return tomorrow morning as a passing cold front cools temperatures back closer to seasonal averages. -Coulston && .AVIATION... 18/06Z TAF Period Note: The NWS Great Falls Office manages the following TAFs: KCTB, KHVR, KGTF, KLWT, KHLN, KBZN, KEKS and KWYS. All airfields begin this TAF period in VFR conditions. Beginning at, 17/19Z, KCTB, KHVR and KLWT fall into predominant MVFR conditions due to light rain showers and low ceiling (KCTB). The TAFs for all of our airfields include PROB30 groups that bring each airfield into MVFR conditions in rain or rain/snow (KLWT) and, in some cases, with low ceiling. These PROB30 groups begin as early as 17/15Z. Mountain obscuration is forecast. Generally, widespread strong gusty westerly winds are forecast throughout this period. Mountain wave is forecast. Areas of virga, and all of the associated hazards to aviation flight ops, should be expected. Some isolated thunderstorm activity is forecast to occur across portions of north central, central and southwest Montana, Friday afternoon/evening. As of this time, thunderstorm is not forecast to occur within the ten mile aerodrome of our airfields. - Fogleman Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PREV DISCUSSION... /ISSUED 720 PM MDT Thu May 16 2024/ An upper level trough digging into BC today will progress east across western Canada tonight through Friday. Meanwhile, A fairly vigorous shortwave and strong upper level jet (for May) move onshore in the Pacific NW tonight and across the Northern Rockies Friday with an associated Pacific cold front at the surface. Surface low pressure emerging ahead of these features in southern AB will move off to the east into western SK by Friday morning with the Pacific cold front trailing SW across central and western MT. Breezy to windy conditions persist through this evening across much of the area with 30-40 mph gusts over the mountains and across western portions of north-central MT. These areas will see winds persist through the overnight period while areas further east across north-central MT and in the SW MT valleys should see winds diminish some overnight. Showers and a few thunderstorms are possible near the Canadian border (mainly northern Hill/Blaine counties) through this evening with showers increasing later tonight along northern portions of the Continental divide near Glacier NP. Better support for organized thunderstorms will exist north of the border through this evening, but there may be some potential for wind gusts from storms in far SE AB and SK to reach into portions of Hill and Blaine counties. Wind continues to be the primary concern for tomorrow across much of the area with most areas likely (>60% probability) to see gusts in excess of 40 mph with a reasonable chance (30-50%) for localized gusts in excess of 50 mph, especially across areas adjacent to the Rocky Mtn Front and mountains of central and southwest MT. Strong winds (gusts in excess of 50kts) are not expected to be widespread but a few isolated higher gusts are possible, piratically with the passage of the cold front across SW MT Friday afternoon. Westerly/downslope flow will be a limiting factor for precipitation coverage/amounts on Friday, however there will be ample mid to upper level support from the incoming shortwave, upper jet energy and modest instability for showers, particularly across southwest MT and increasing in the afternoon and evening as moisture and another disturbance move southeast from AB. Hoenisch Saturday through Sunday...ensemble clusters strongly favor longwave troughing over the Northern Rockies throughout the weekend, with the leading mode of uncertainty being the overall amplitude of the longwave trough, specifically on Sunday. A deeper longwave trough on Sunday, which is overall favored in 3 of the 4 clusters, would result in cooler and "wetter" conditions through the weekend across much of Southwest through North Central Montana. Latest NBM probabilities for a 0.25" of precipitation from 12z Sunday to 12z Monday generally range from 20-40% south of the US Hwy 2 corridor and north of the US Hwy 12 corridor. Additionally, the probability for snowfall of 2" or more along the Continental Divide and across the mountains of Central Montana over this timeframe is approximately 30%, with 48hr probabilities (including Saturday and Saturday night) ranging from 30-50% for these same areas. High temperatures on Saturday in wake of Friday's Pacific front will fall into the 60s, with Sunday only seeing highs rise into the 50s. Monday through next Thursday...an unsettled and "cooler" pattern is favored through the upcoming work week, with the multi-model ensemble cluster suggesting overall longwave troughing over the Western CONUS and Western Canada and inconsequential periods of transient shortwave ridging moving over the Northern Rockies. Some uncertainty exists with the timing, position, and amplitude of individual shortwaves troughs rotating through the longwave trough, but the main message to put forward will be that their exists multiple chances for precipitation (some of which may fall as mountain snow) and below normal temperatures through the upcoming work week. - Moldan && .PRELIMINARY POINT TEMPS/POPS... GTF 50 61 40 64 / 10 40 30 10 CTB 45 55 35 60 / 10 70 40 10 HLN 51 62 39 67 / 0 40 10 0 BZN 43 65 30 66 / 0 50 10 0 WYS 37 61 24 57 / 10 30 10 0 DLN 43 63 29 66 / 0 20 0 0 HVR 50 64 39 65 / 20 60 80 10 LWT 45 61 36 61 / 10 60 40 10 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls