textproduct: Great Falls

KEY MESSAGES

- Very warm and dry conditions prevail again today with widely scattered gusty showers and thunderstorms moving in from the southwest this evening into tonight.

- After another very warm day on Saturday, scattered afternoon and evening showers and thunderstorms will develop in a southwest to northeast fashion, some storms will contain strong, gusty winds, heavy downpours capable of localized flooding over burn scars, and dangerous cloud to ground lightning.

- A surface front will bring gusty winds and elevated fire weather concerns on Sunday, there will also be a few lingering showers and storms east of I15.

- There will be a modest cooldown early next week with some widely scattered showers and storms, mostly over southwest Montana.

DISCUSSION

- Meteorological Overview:

Strong ridging aloft will remain in place today before a trough moving onto the Pacific NW coast brings a more unstable southwesterly flow to the Northern Rockies tonight into Saturday. Very warm and dry conditions generally prevail through the weekend even while the ridge breakdown process takes place. Most lower elevation locations will see afternoon highs well into the 80s to the lower 90s. Subtle shortwaves will begin moving through and interacting with increasing instability this evening into tonight. This will send a wave of widely scattered high based showers and thunderstorms moving in a southwest to northeast fashion this evening into tonight with the primary impact being localized gusty winds and lightning, especially for central and southwest MT.

Stronger shortwaves look to send a more impactful wave showers and thunderstorms through the forecast area late Saturday afternoon into Saturday night. ML CAPE doesn't look exceptionally high in the 400 to 900 J/kg range, but forcing from the shortwave will make up some of the void and very warm and dry sub cloud layers will increase instances of strong gusty winds up to and exceeding 60 mph, especially if linear upscale growth takes place. Hail and brief downpours downpours capable of flash flooding of normally flood prone areas will also be present with bulk shear generally above 25 kts and PWATS approaching and even surpassing the one inch mark in some cases. The focus of this activity will be over southwest MT and and along the Continental Divide, though at least some of it will move northeastward onto the plains during the overnight hours.

The Pacific NW trough will swing northeastward through W MT and into Canada on Sunday while its attendant surface front traverses the CWA and brings gusty westerly winds and elevated fire weather conditions. There will also be lingering instability for widely scattered showers and thunderstorms, some on the strong side, mainly for locations east of I15. Temperatures modestly cooldown Monday and Tuesday with some shower and thunderstorm activity in southwest MT. This looks to be followed by ridging aloft briefly rebuilding before another Pacific trough approaches during the mid to latter part of next week. - RCG

- Forecast Confidence & Scenarios:

Scattered shower and thunderstorm activity this evening through Sunday, strongest and most widespread Saturday...

An unstable southwesterly flow aloft will send multiple waves of showers and thunderstorms through the forecast areas this evening through Sunday. The initial wave will move into southwest MT by this evening before moving northeastward during the overnight hours. This activity will be isolated in nature and mostly high based with little precipitation. The primary concern will be localized gusty winds over 30 mph and lightning, mostly for southwestern locations.

The stronger shortwave energy arrives Saturday afternoon into Saturday night. Strong gusty winds in excess of 60 mph and lightning will be the primary hazards on Saturday, especially if the storms become linear and undergo upscale growth. This threat will be highest over southwest MT and along the Continental Divide. Some weakening is expected with this activity as it moves northeastward, though these same hazards apply to plains locations as well. In addition to the winds, there will be localized heavy downpours capable of flash flooding of burn scars and other flood prone locations, mainly in southwest MT where there is around a 10% chance for excessive rainfall and localized flooding. The focus of this activity shifts eastward on Sunday with a few stronger storms for locations east of I15. Central/north-central locations east and southeast of the highway 87 corridor will see the strongest storm ceiling with strong, gusty winds, hail, and downpours all on the table. - RCG

AVIATION

21/06Z TAF Period

VFR conditions will prevail across the region through the period however there is a 30% chance of slight visibility reduction due to mist for the KCTB area. An outflow boundary is currently moving southward across KCTB, KHVR, and KGTF briefly increasing winds up to 15 - 25 KT until 07Z. Some smoke will also obscure distant mountain ranges across the region through the period. Scattered dry thunderstorms over Southwest MT are expected to develop after 22/05Z Friday night primarily impacting KEKS and KBZN during the period. Primary threats for these storms will be strong gusts and cloud-to-ground lightning.

For KGTF: VFR conditions for the period. Winds will be mostly light and variable tonight with a brief possible northerly gust of up to 18 KT between 06Z and 07Z. Then winds will shift northeasterly tomorrow afternoon. Currently, next chance for thunderstorms is predicted to be Saturday morning after 22/16Z. - Akins

FIRE WEATHER

The fire weather concerns begin when isolated showers and storms with cloud to ground lightning and little precipitation pass through this evening into tonight. Even though this activity will be isolated, some new fire starts can't be ruled out. More productive storms move in on Saturday, but again the presence of cloud to ground lightning may ignite new fire starts. The most cornering day will be on Sunday when a surface front brings gusty westerly winds amid continued very warm and dry conditions. The timing of this front will ultimately determine how long the dry windy conditions persist. The latest NBM probabilistic guidance highlights large areas of north-central and portions of southwest Montana with over a 60% chance for wind gusts over 30 mph. Although the fuels aren't quite as dry as they were in earlier in the month, elevated fire conditions will be present and the wind may be enough to ignite any lightning caused hold over fires from Friday and Saturday's storms. - RCG

PRELIMINARY POINT TEMPS/POPS

GTF 87 59 91 60 / 0 20 20 30 CTB 82 54 84 56 / 0 10 20 10 HLN 92 60 90 58 / 0 30 30 50 BZN 91 57 88 53 / 10 20 40 60 WYS 89 50 83 45 / 10 20 40 50 DLN 90 57 86 51 / 10 30 50 70 HVR 89 59 93 60 / 0 10 20 10 LWT 85 57 88 58 / 0 10 20 30

TFX WATCHES/WARNINGS/ADVISORIES

None.