textproduct: Great Falls

KEY MESSAGES

- Another warm and dry day today across the region.

- Hot, breezy to gusty winds, and showers and thunderstorms return Wednesday.

- Daily chances for precipitation and breezy to windy conditions continue through the weekend.

DISCUSSION

- Meteorological Overview:

Brief upper-level ridging continues today, which will bring another day of dry conditions. Temperatures will also be well above normal today, with upper 70s to mid 80s forecasted across lower elevations. There is a low end chance for a few showers in Beaverhead County this afternoon.

Southwest flow aloft increases late Tuesday into Wednesday as a closed low moves onshore across the Western U.S. This southwest flow advects in warm, moist air across the region. Temperatures will be the hottest on Wednesday, where current afternoon temperatures are forecasted to reach the upper 80s to low 90s for most of the region. Increasing southwest winds ahead of the main trough will bring breezy to gusty winds, especially across Southwest MT. Strong vertical daytime mixing will help lower relative humidities, bringing a concern for fire weather in Southwest MT and the Hi-line.

Decent mid to high level moisture helps develop showers and thunderstorms across the state of Montana Wednesday afternoon and evening. Forecast soundings show deep inverted V soundings due to the very dry surface layer. This combined with decent mid level winds results in strong to severe wind gusts being the main hazard with showers and thunderstorms. The best potential for strong to severe storms will be Southwest MT through North-Central MT south of the Highway 2 corridor.

Models start to diverge slightly heading into Thursday on the track and speed of this upper-close low/trough moving through Montana. This will mainly impact timing of gusty winds moving into the region, as well as precipitation track. Both the Euro and GFS models hint at a strong 700mb jet forming across North-Central MT as this trough axis moves through, which will bring breezy to gusty winds across the area throughout the day Thursday.

Friday through the weekend, westerly flow aloft looks to be persistent. A few shortwaves move through during this time frame, which will continue daily chances for precipitation. Decent cross- barrier flow also helps keep breezy to gusty conditions mainly across North-Central MT during this time frame. Temperatures cool back to near to slightly below averages. Snow levels falling Thursday through the weekend indicate light snow may reach the mountains/passes, but recent warm temperatures will make any snow struggle to accumulate.

- Forecast Confidence & Scenarios:

Thunderstorm Risk Wednesday:

Models are showing modest CAPE of up to 1,000 j/kg across Southwest and North-Central MT (with the highest corridor being in North- Central MT). Strong daytime vertical mixing will create a very dry surface layer, resulting in deep inverted V forecast sounding profiles. Strong to severe wind gusts will be the primary concern with showers and thunderstorms. One thing to watch for, is how much cloud cover there is in the morning. This will affect how much daytime heating occurs, and can limit instability development and strong daytime vertical mixing. However, with strong synoptic winds already in the area, this might be able to overcome this limitation. Southwest winds aloft increases throughout the day across Southwest. Any stronger shower/thunderstorm will help mix these winds down to the surface. Although the 700mb jet increases throughout the day Wednesday across Southwest MT, most of the high wind gusts look to be mainly convective driven. Therefore, high wind watches have been held off for now.

Winds late Wednesday Night Into Thursday:

As this main shortwave moves through the region, stronger 700mb winds move in behind across North-Central MT. Both the GFS and European models hint at peak winds happening very early Thursday through the morning hours. Although this isn't the best timing for mixing winds down to the surface, both of these models to try to mix down some of these winds down. By the afternoon the mid level jet still peaks to 40-50kts, especially closer to Blaine/Fergus Counties. Therefore, strong winds look to continue through the day Thursday. Timing of these winds are still a little uncertain, but the signal has been more consistent the past few models runs to give enough confidence for issuing High Wind Watches along the Rocky Mountain Front and North-Central MT. -Wilson

AVIATION

12/06Z TAF Period

Ridging aloft moves in and brings widespread VFR conditions for the duration of the TAF period; however, high level clouds will be increasing after 12/18Z and there may be a few cloud builds or even an isolated shower or storm over the higher terrain of far SW MT after 13/00Z. Winds will become easterly over the plains and light and variable over the southwest. - RCG

FIRE WEATHER

Elevated to near-critical fire weather conditions are expected on Wednesday as temperatures soar well above normal and into the mid- 80s to low 90s, with the potential for numerous locations to set new daily record highs for May 13th across Southwest through North Central Montana. The warm temperatures will be accompanied by strong and gusty south to west winds; most notably along and south of the I- 90 corridor within the north-south orientated valleys of Southwest Montana, west of the I-15 corridor over the plains of Central and North Central Montana, and across Hill and Blaine Counties. Further enhancement of winds are likely beneath and/or near shower and thunderstorm activity, especially across Southwest and portions of Central Montana or generally along and south of the US Hwy 12 corridor. Dry conditions, with relative humidity values in the teens will be common across much of the plains and valleys. Probabilities for the Hot-Dry-Windy Index (HDWI) rising to within the 95Pct of Climatology has increased to a 60% or greater chance across most lower elevations across Southwest Montana south of the I-90 and across Hill and Blaine Counties in North Central Montana, with Fire Weather partners indicating that fuels would be receptive to fire, especially across Southwest Montana. -Moldan

The current fire weather watches for FWZ113 and Southwest MT remains on track for Wednesday. -Wilson

..RECORD OR NEAR RECORD MAXIMUM TEMPERATURES FORECAST FOR MAY 13

LOCATION FORECAST HIGH RECORD HIGH Boulder 82 83 Bozeman MSU 87 86 Chester 86 86 Choteau 86 85 Conrad 85 87 Cut Bank 82 84 Dillon 85 87 Fort Benton 92 88 Great Falls 88 89 Helena 88 89 Havre 92 92 Lewistown 87 88 Shelby 83 84 Stanford 87 86 West Yellowstone 80 81

PRELIMINARY POINT TEMPS/POPS

GTF 75 43 79 50 / 0 0 0 0 CTB 69 38 76 46 / 20 0 0 0 HLN 77 42 83 49 / 0 0 0 0 BZN 78 39 81 45 / 0 0 0 0 WYS 76 36 80 42 / 20 20 0 0 DLN 80 42 84 49 / 0 0 0 0 HVR 72 38 78 47 / 20 0 0 0 LWT 72 39 74 46 / 0 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Watch from late Wednesday night through Thursday evening for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Northern High Plains-Southern High Plains-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

Fire Weather Watch from Wednesday afternoon through Wednesday evening for Hill and Blaine Counties.

High Wind Watch from late Wednesday night through Thursday evening for Bears Paw Mountains and Southern Blaine-Fergus County below 4500ft-Hill County-Northern Blaine County-Snowy and Judith Mountains-Western and Central Chouteau County.

High Wind Watch from late Wednesday night through Thursday afternoon for Cascade County below 5000ft-Eastern Toole and Liberty-Helena Valley-Judith Basin County and Judith Gap.