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FXUS65 KTFX 290542 AFDTFX

Area Forecast Discussion National Weather Service Great Falls MT 1141 PM MDT Sun Apr 28 2024

Updated Aviation Discussion

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 Wednesday.

UPDATE

Latest satellite imagery continues to show high level clouds along the Northern Rocky Mountian Front and across North Central Montana this evening. Current temperatures and winds are doing well in the forecast. No changes were needed this evening to the current forecast. Gusty westerly winds continue to diminish this evening with the exception of the Rocky Mountain Front, but returns for tomorrow afternoon and includes locations across Southwest Montana. Webb

AVIATION
1141 PM MDT Sun Apr 28 2024 (29/06Z TAF Period)

Westerly flow aloft with embedded moisture increases through Monday morning ahead of an incoming Pacific weather system with VFR conditions prevailing at most terminals while some showers track across SW MT overnight with local mountain obscuration and brief MVFR possible at KEKS and KBZN through 10z. A developing cold front across central and southwest MT will be the focus for increasing showers by Monday afternoon with a few thunderstorms possible with the frontal passage across eastern terminals (KLWT and KBZN) late Monday afternoon. In addition to more widespread mountain obscuration and brief MVFR with showers, a few stronger wind gusts may accompany thunderstorms across eastern portions of central and southwest MT Monday afternoon. A brief mix or change to snow showers is also possible at KEKS, KLWT and KBZN Monday evening before precipitation ends there. West to southwest surface winds increase Monday morning and shift to northwesterly briefly with the frontal passage before turning back to the west Monday evening. Hoenisch

Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information.

PREV DISCUSSION
/ISSUED 540 PM MDT Sun Apr 28 2024/

Rest of today through Tuesday...satellite imagery shows an unseasonably deep trough in the Gulf of AK with upper level jet to its south approaching the west coast. Flow splits downstream with a weaker shortwave lifting out across MT while the rest of the energy tracks across the SW US into an upper level system lifting out into the central US plains. Enough residual moisture in the flow and weak instability will help to maintain some isolated to widely scattered showers across the mountains of western and southwest MT through this evening.

The incoming upper jet and initial shortwave moving out of the Gulf of AK trough reach the Northern Rockies Monday with a cold front shifting east across most of the area by Monday evening. Larger scale ascent and some modest instability should support a fairly widespread coverage of showers and a few thunderstorms Monday afternoon across areas roughly along and southeast of a line from Lincoln to Great Falls and Havre. Precipitation amounts of 0.25" or more are most likely (50-70% probability) across eastern portions of central/SW MT (Judith Basin, Fergus, Meagher and Gallatin counties)
where probabilities for thunderstorms are also around 50-70%. These areas could also see some stronger wind gusts (35-45 mph) with any storms that develop due to the more favorably timed late afternoon frontal passage. Gusty west to northwest winds spread across most of the area behind the front Monday night and continue through Tuesday as surface low pressure develops in southern Saskatchewan to the north of a mid level circulation tracking east along the international border. - Hoenisch

Tuesday night through Wednesday night...is shaping up to be the most impactful stretch across Southwest through North Central Montana, most notably along the eastern slopes of the Rocky Mountain Front and Island Ranges of Central Montana, as a strong shortwave trough and associated closed low slowly pivots east over Northern Montana and Southern Saskatchewan. This upper level disturbance will help to pivot precipitation westward towards the Continental Divide through the evening hours on Tuesday, with precipitation then spreading south towards Central Montana through Wednesday morning and persisting here through Wednesday evening. NBM probabilities for 6" of snow throughout the period across the East Glacier Park Region generally ranges from 40-60%, with areas along and north of a Two Medicine to Hudson Bay Divide line having the highest probabilities.
With increasing recreation and subsequent travel on routes adjacent to Glacier National Park, most notably US Hwy 89 north of Browning, have decided to issue a Winter Storm Watch for the East Glacier Park Region given the potential impacts to those unprepared for a return to winter like conditions. Additionally, as the precipitation shifts southeastward towards the Montana Hwy 200 corridor through Wednesday morning, latest NBM probabilities support a 40-60% chance for snowfall accumulations to reach 8" across the Little Belt Mountains.
Have held off for now on a Winter Storm Watch, but should trends continue then one will need to be considered given the impacts to travel on US Hwy 89.

Thursday through next weekend...Ensemble cluster largely favor unsettled northwest to northerly flow over the Northern Rockies, which will help to maintain chances for precipitation and temperatures near or slightly below normal. - Moldan

PRELIMINARY POINT TEMPS/POPS
GTF 40 55 37 53 / 20 70 50 20 CTB 38 58 33 47 / 20 10 20 40 HLN 42 52 35 54 / 30 90 40 30 BZN 37 57 24 47 / 40 90 80 30 WYS 28 48 19 40 / 40 90 80 50 DLN 34 55 24 47 / 10 80 50 30 HVR 38 64 38 54 / 0 30 40 40 LWT 39 55 31 49 / 20 90 100 20

TFX WATCHES/WARNINGS/ADVISORIES
Winter Storm Watch from Tuesday evening through Wednesday morning for East Glacier Park Region.

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