textproduct: Great Falls

KEY MESSAGES

- Hot, dry, and breezy to windy condtions will bring elevated to critical fire weather conditions followed by scattered to numerous gusty thunderstorms with localized damaging winds.

- Strong westerly winds develop behind the cold front tonight into Thursday, with accumulating snow over the higher terrain of Glacier National Park.

- Unsettled and cooler conditions continue this weekend into early next week.

- Sunday looks to be coolest day and with the best opportunity for mountain and some lower elevation snow.

DISCUSSION

/Issued 542 AM MDT Wed May 13 2026/

- Meteorological Overview:

A tightly wound and sharply negatively tilted closed low is moving onto the Pac NW coast this morning, with an unstable southerly flow already developing over the Northern Rockies. Waves of lighter end showers, virga, and even a clap of thunder or two will move through central and eastern areas through the morning hours. As this trough and its attendant cold front approaches, the south to southwesterly flow aloft will strengthen with H700 flow up to around 50 kts and as high as 90 kts at the H500 level. This will bring breezy to windy conditions over much of the forecast area while afternoon highs rise to the middle 80s to lower 90s. Southwestern areas look to continue being the recipients of the strongest synoptic winds and most critical fire weather conditions where wind gusts will approach and even exceed 60 mph.

Once the trough's forcing begins to interact with the strong diurnal heating, clusters or broken lines of showers and thunderstorms will develop over the higher terrain of western and southwestern MT before racing northeastward onto the plains. While ML CAPE itself isn't overly impressive at around 200 to 500 J/kg. The wind shear and mid -level lapse rates are with bulk shear nearing 50 kts and mid level lapse rates in the 8.5 to 9C/km range. Forecast soundings show tall inverted V profiles up to around 600 mb and it won't take much for any shower or storm to transfer the stronger winds aloft to the surface, especially if they become outflow dominant. Reasonable peak wind gusts with this activity will generally fall in the 60 to 80 mph range, highest for both southwest and north-central locations east of I15.

Showers and thunderstorms look to quickly move east of the forecast area by around 8 or 9 pm this evening. Robust westerly flow aloft and a tightening surface pressure gradient will then bring gusty to strong westerly surface winds to much of central and north-central MT tonight into Thursday. H700 flow in excess of 70 kts will sweep through and push wind gusts up to 90 mph along the Rocky Mountain Front and up to 75 mph elsewhere. Even through this flow does weaken some during the day on Thursday, mixing aloft should make up the difference. There will be limited moisture on the backside of this system, but there will be sufficient forcing and orographic lift for a period of accumulating snow along the Northern Rocky Mountain Front, particularly for the GNP area. Then there will be some scattered showers over the lower elevations to the east, mostly along the Hi-Line.

A colder westerly flow aloft will become more established heading into the weekend for continued breezy to windy conditions with widely scattered shower activity. A more defined split trough looks to arrive late Saturday through early Monday and bring more widespread precipitation with sufficiently cold air for snow at all elevations for a period. Of course, split troughs lead to lower forecast confidence, so this system will be closely monitored over the next few days. - RCG

- Forecast Confidence & Scenarios:

Winds and potentially damaging thunderstorm wind gusts this afternoon and evening...

Just to show how dynamic this system is, there already have been a few gusts over 40 mph being reported with the low dbz showers heading towards the Continental Divide early this morning. In addition to that, convection has been ongoing all night along the Cascades directly in advance of the approaching trough. Synoptic winds look to begin increasing over the southwest by around noon with peak winds between 3 and 9 pm. The high wind watch for the southwest was upgraded to a high wind warning and the Big Belt zone was added. Probabilities for wind gusts over 55 mph are around 70 to 90% from central Beaverhead County eastward towards the higher terrain of Madison and Gallatin counties. The Bozeman area has been largely left out of this higher probability area, but the hires guidance brings in stronger winds here.

Clusters of showers and thunderstorms develop over the higher terrain of west/southwest MT by early afternoon before racing northeastward to the plains and valleys. This is slightly earlier than some of the previous model runs were showing. Theoretically, the convective gust threat should peak between 3 and 8 pm. Although any location may see severe convective wind gusts in excess of 60 mph, areas between Dillon and Bozeman and then between Great Falls and Harlem look to be most susceptible to the higher end damaging wind gusts as high as 80 mph. Slightly lower CAPE and cloud cover may result in slightly less lightning than one would expect with a dynamic system like this, but there will be at least some cloud to ground lightning and even some instances of severe hail with the stronger cores.

Strong winds and Rocky Mountain Front snow tonight into Thursday...

The robust H700 flow will be as much as 4 standard deviations above climatology tonight through Thursday morning according the the NAEFS Standard Anomaly Index. This combined with the tightening surface pressure gradient and mountain wave activity will bring periods of widespread strong surface winds to central and north-central MT despite the poor timing for diurnal mixing. Wind gusts will approach 90 mph along the Rocky Mountain Front and up to 75 mph for other locations. One uncertainty with this system is how far south the stronger winds will go. I considered adding the Big Belt zone for this second round of winds, but held off due a more isolated and shorter period of winds. Another uncertainty is whether the Helena Valley will be able to mix higher end gusts to the surface at night, so I maintained the high wind watch for now.

Colder air air and orographic lifting will bring snow along the Rocky Mountain Front tonight into Thursday morning. A quick wind whipped 3 to 6 inches of snow at Logan Pass seems plausible. Since we are approaching the recreation summer season with an increasing number of visitors and road maintenance workers in the park, I matched up with the MSO office and posted a winter weather advisory. The primary impact will be slushy road accumulations, tree damage/power outages, and dangerous conditions for outdoor recreation. - RCG

AVIATION

13/18Z TAF Period

VFR conditions will prevail at all terminals during this TAF Period. This afternoon and early evening there will be scattered thunderstorms across North-central, Central, and Southwestern Montana. The primary risk with any thunderstorm will be strong and erratic wind gusts and frequent lightning. The strongest thundestorms will produce wind gusts in excess of 60 kts. Additionally outside of any thunderstorms it will be windy across Southwestern Montana this afternoon. Then after 14/00Z winds will increase at the KCTB, KHVR, KLWT, and KGTF terminals and remain strong through the rest of the TAF Period. There will be instances of low-level wind shear and mountain wave turbulence during the duration of this TAF Period. There will also be brief periods of mountain obscuration across North-central Montana during this TAF Period. -IG

FIRE WEATHER

Hot, dry, and breezy to windy condtions will bring elevated to critical fire weather conditions followed by scattered to numerous gusty thunderstorms with localized 60 to 80 mph wind gusts. Probabilities for the Hot-Dry-Windy Index (HDWI) rising to within the 95Pct of Climatology has increased to a 80% or greater 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. Red Flag Warnings remain in effect for these aforementioned areas. Other areas across the region will see localized elevated fire weather conditions, with slightly greener and wetter fuels, so no additional fire weather products are anticipated at this time. Shower and thunderstorm activity will be most numerous over north-central and southwestern areas east of I15. - RCG

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

LOCATION FORECAST HIGH RECORD HIGH Boulder 83 83 Bozeman MSU 88 86 Chester 88 86 Choteau 86 85 Conrad 86 87 Cut Bank 82 84 Dillon 84 87 Fort Benton 93 88 Great Falls 89 89 Helena 88 89 Havre 95 92 Lewistown 90 88 Shelby 84 84 Stanford 88 86 West Yellowstone 81 81

PRELIMINARY POINT TEMPS/POPS

GTF 89 47 64 44 / 70 70 0 0 CTB 82 42 59 39 / 60 20 0 0 HLN 88 42 65 41 / 70 50 0 0 BZN 89 38 68 36 / 20 50 0 0 WYS 81 31 65 32 / 40 20 0 0 DLN 84 38 68 36 / 50 10 0 0 HVR 95 43 68 42 / 50 90 30 0 LWT 90 43 64 41 / 20 90 0 0

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Warning from 6 PM this evening to midnight MDT Thursday night 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.

Winter Weather Advisory from midnight tonight to noon MDT Thursday for East Glacier Park Region.

Red Flag Warning until 9 PM MDT this evening for Hill and Blaine Counties.

High Wind Warning from 3 AM Thursday to midnight MDT Thursday night 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 Warning until 9 PM MDT this evening for Beaverhead and Western Madison below 6000ft-Big Belt, Bridger and Castle Mountains-Canyon Ferry Area-Gallatin Valley-Gallatin and Madison County Mountains and Centennial Mountains-Madison River Valley- Missouri Headwaters-Northwest Beaverhead County-Ruby Mountains and Southern Beaverhead Mountains.

High Wind Watch from late tonight through Thursday afternoon for Helena Valley.

High Wind Warning from midnight tonight to 6 PM MDT Thursday for Cascade County below 5000ft-Eastern Pondera and Eastern Teton- Eastern Toole and Liberty-Judith Basin County and Judith Gap.