textproduct: Great Falls

KEY MESSAGES

- It will be dry and warm this afternoon.

- It will be windy on Tuesday across North-central, Central, and Southwestern Montana.

- Light mountain snowfall will return for Tuesday afternoon through Thursday with cooler temperatures.

DISCUSSION

- Meteorological Overview:

This afternoon there is an upper-level ridge over North-central, Central, and Southwestern Montana. This will bring warm temperatures with dry weather to the area this afternoon. On Tuesday an upper- level low associated with a surface cold front tracks along the Canadian/Montana Border. This will briefly bring a strong surface pressure gradient to North-central Montana. Low-level and mid-level winds increase during the day Tuesday. This will lead to windy conditions across North-central, Central, and Southwestern Montana on Tuesday. See the Forecast Confidence & Scenarios section for more details on the winds. The cold front/upper-level low will bring isolated light rain/snow showers to North-central Montana Tuesday afternoon and evening. Tuesday afternoon across Southwestern Montana there will be isolated rain/snow showers ahead of the surface cold front. With these showers there is the possibility of a few rumbles of thunder. See the Forecast Confidence & Scenarios Section for details.

On Wednesday there will be an upper-level trough over North-central, Central, and Southwestern Montana. On Wednesday due to the cold frontal passage and upper-level trough it will be colder across the area with light mountain snow. Wednesday morning strong winds will linger across Blaine and Fergus Counties. On Thursday there will be upper-level zonal flow with a brief upper-level shortwave over North- central, Central, and Southwestern Montana. This will bring another day with cool temperatures and mostly dry weather. On Friday there will be upper-level zonal flow over North-central, Central, and Southwestern Montana. This will allow temperatures to warm up slightly with mostly dry weather. On Saturday and Sunday an upper- level ridge will move over North-central, Central, and Southwestern Montana. This will allow temperatures to warm up with mostly dry weather. -IG

- Forecast Confidence & Scenarios:

Across Gallatin and Madison Counties Tuesday afternoon there is a 10% chance for thunder. On Tuesday there will be a strong pressure gradient for a few hours across North-central Montana and winds aloft will increase. The most impactful winds will occur, in the areas where winds at low and mid levels peak in the mid to late afternoon. Another area of uncertainty is that cold air from Canada could move in faster than currently the models are projecting and undercut the strong winds along the Hi-Line. However in Glacier County there will be several hours of strong wind gusts in excess of 75 mph before the cold front moves in and shuts off the winds.

On Tuesday there is greater than a 50% chance for wind gusts of 55 mph or greater across most of North-central, Central, and Southwestern Montana. Across the Rocky Mountain Front down to Macdonald Pass and the plains west of I-15 there is greater than an 95% chance for wind gusts in excess of 55 mph. Along the Rocky Mountain Front there is greater than a 80% chance for wind gusts in excess of 75 mph on Tuesday. As a result across most of North- central, Central, and Southwestern Montana High Wind Warnings were issued for Tuesday. For the I-90 Corridor from Whitehall to Bozeman there is very brief period late afternoon/early Tuesday where there could be strong wind gusts. Since this period is brief and wind gusts don't look to be much more than 60 mph in those areas, they were excluded from the High Wind Warnings at this time. Isolated locations in the higher terrain of Southwestern Montana that aren't in the High Wind Warning have a chance to exceed 60 mph wind gusts. Given the low confidence in multiple areas hitting and low confidence in it being more than a one and done event, they were left out of the warnings at this time. On Wednesday there is greater than a 70% chance for wind gusts in excess of 55 mph across portions of Blaine and Fergus County. As a result for the Bears Paw area and the lower-elevations of Fergus County a High Wind Warning was issued from Tuesday afternoon through Wednesday morning. Northern Blaine County and the Snowies and Judiths were left out of the warning due to low confidence in any wind gusts causing impacts. See the TFX WATCHES/WARNINGS/ADVISORIES Section for details. -IG

AVIATION

06/18Z TAF Period

VFR conditions will largely prevail throughout the duration of the 0618/0718 TAF period; however, temporary IFR/MVFR CIGS are possible at the KGTF and KCTB terminals through 18-20z this afternoon thanks to low level easterly winds, which have been supporting low level clouds along and eastern slopes of the Rocky Mountain Front. Outside of these low clouds the primary concern over the next 24 hours will be increasing winds between 09-15z Tuesday, with winds becoming strong beyond 15z across all terminals. Mountain wave turbulence and instance of low level wind shear will increase beyond 09z Tuesday. - Moldan

FIRE WEATHER

Tuesday afternoon relative humidity values will drop near critical levels across Central and Southwestern Montana. This combined with gusty to strong winds will lead to elevated fire weather conditions across Central and Southwestern Montana. Additionally the Hot, Dry, and Windy index exceeding the 95th Percentile will support the elevated fire weather conditons. The greatest risk for any fire spread will be in those areas where there hasn't been any greenup. -IG

PRELIMINARY POINT TEMPS/POPS

GTF 41 63 26 49 / 0 20 40 10 CTB 37 56 21 44 / 0 60 30 0 HLN 40 65 30 56 / 0 10 10 0 BZN 35 68 27 57 / 0 20 10 0 WYS 27 58 25 52 / 10 20 10 0 DLN 38 65 27 58 / 0 10 0 0 HVR 32 63 26 43 / 10 20 20 0 LWT 33 65 24 45 / 0 30 40 10

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Warning from 3 AM to 9 PM MDT Tuesday for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera- Northern High Plains-Southern High Plains-Southern Rocky Mountain Front.

High Wind Warning from 9 AM to 9 PM MDT Tuesday for Beaverhead and Western Madison below 6000ft-Big Belt, Bridger and Castle Mountains-Cascade County below 5000ft-Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-Elkhorn and Boulder Mountains-Gates of the Mountains-Helena Valley-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains-Madison River Valley-Meagher County Valleys-Northwest Beaverhead County- Upper Blackfoot and MacDonald Pass-Western and Central Chouteau County.

High Wind Warning from 3 PM Tuesday to noon MDT Wednesday for Bears Paw Mountains and Southern Blaine-Fergus County below 4500ft.