Great Falls, MT 59405 Weather

Key Messages

- Significant shift to cooler and wetter conditions will occur on Friday.

- Rain develops tonight west of I-15 with rain and mountian snow early Friday and a transition to lower elevation snow by Friday evening before precipitation ends.

- Gusty to locally strong northwest winds develop across north-central Montana on Friday with much cooler temperatures spreading across the area by Friday afternoon.

- Drier but continued cooler than average conditions are expected for the weekend into early next week.

Discussion

- Meteorological Overview:

A significant shift from the ongoing mild/benign weather is on track for Friday as an upper level trough moving onshore into CA/NV today lifts north across MT ahead of an eastward moving trough from northern Pacific.

The trough lifting out of CA/NV will spread unusually deep moisture for this time of year (PWATs>200% of normal) north into the area tonight through Friday morning with precipitation developing across western portions of the forecast area (mainly W of I-15) tonight as this moisture is drawn north along a deformation axis across the Northern Rockies. This precipitation will be a mix of decaying convection that develops over the Great Basin this afternoon with additional lift provided by upper jet energy lifting north from the Great Basin as well as in advance of the trough arriving from the NW.

The upper trough approaching from the NW crosses the Northern Rockies Friday afternoon, shifting the axis of precipitation east across the area while also bringing cooler air aloft and lowering snow levels before precipitation decreases from west to east Friday evening. Cooling aloft will support an initial drop in snow levels from above 9000 ft to around 6000-7000 ft across most of the area by Friday afternoon, resulting in a change to wet snow over the mountains. A secondary push of lower level cooling surges southeast out of AB as the northern upper trough axis emerges east of the Rockies and with surface low pressure deepening across eastern MT. Snow levels quickly lower to 3000-4000 ft across N-central MT as early as late Friday morning across Glacier county and by late Friday afternoon across eastern portions of north-central/central MT and Friday evening across SW MT valleys. Gusty to locally strong northwest winds develop with the surge of cooler air across N- central MT Friday, driven mostly by the surface pressure gradient behind the intensifying low to the east.

Mainly dry conditions follow for the weekend into at least early next week with temperatures remaining somewhat cooler than average for late October with afternoon temps in the mid-upper 40s and overnight lows in the 20s. A more unsettled period may develop by mid to late next week as the next trough digging southward along the west coast begins to shift inland, though there there is still considerable uncertainty with timing and details at this point. Hoenisch

- Forecast Confidence & Scenarios:

Aviation

22/18z TAF Period

Overall, VFR conditions are expected to continue through 06Z Friday. From then on, conditions begin decreasing as the system moves in. Southwest MT will be impacted first as rain showers move into the area after 08Z Friday. Steadier rainfall is expected to develop by 12Z in SW MT. Visibility and ceilings will gradually decrease as the morning progresses, resulting in areas of MVFR and IFR conditions. With lower ceilings and visibility, mountains and hills may become obscured. Conditons will begin to deteriorate after 12Z at the other terminals as rain spreads northward into the rest of the region. Impacts will be limited at KHVR and KLWT due to precip being limited to light rain/showers after 14Z. For KCTB, strong northerly winds will develop after 13Z with gusts up to 45KT possible. Looking beyond the TAF period, rain has the potential to transition to snow over Friday afternoon/evening. Additionally, areas of strong winds will continue to develop across North and Central MT.

Preliminary Point Temps

GTF 42 52 27 45 / 10 90 70 0 CTB 36 48 22 44 / 10 100 0 0 HLN 43 53 23 46 / 20 100 70 0 BZN 40 55 22 43 / 20 100 100 0 WYS 32 45 17 46 / 60 100 90 0 DLN 42 49 26 45 / 80 100 90 0 HVR 42 57 25 46 / 0 90 80 0 LWT 45 62 23 42 / 0 100 100 0