textproduct: Great Falls

KEY MESSAGES

- Daily rounds of shower and thunderstorm activity continues through the weekend, although storm coverage drops off somewhat Friday and Saturday.

- Thunderstorm hazards will include strong, gusty winds, hail, and downpours in addition to cloud to ground lightning.

- Dry, hot, and breezy to windy conditions will raise elevated to critical fire weather concerns for the weekend, especially Friday and Saturday.

- The summer heat will continue through the weekend into next week, but will be most intense Friday and Saturday.

DISCUSSION

- Meteorological Overview:

Slightly drier air moves in today behind the exiting shortwave; however, another shortwave moves in from the southwest and brings more widely scattered showers and storms to much of the forecast area. Forecast soundings continue to show inverted V profiles, which will result in stronger thunderstorm outflow winds than the last couple of days. ML CAPE values generally running between 800 and 1200 J/kg will also be sufficient for hail, brief heavy downpours, and cloud to ground lightning. The combination of the thinning moisture and the building ridge will also warm afternoon temperatures well into the 90s and even 100s for most lower elevation locations.

A drier southwesterly flow aloft will continue to thin the moisture profile from west to east on Friday and Saturday for mostly dry conditions and intensifying heat, but there will be enough lingering instability and moisture for at least some showers and storms. Model guidance has steadily increased Saturday's thunderstorm activity over central and southwest MT with a more defined shortwave passage during peak diurnal heating. Otherwise, strengthening dry southwesterly flow will increase surface winds for at least the first half of the weekend and introduce elevated to critical fire weather concerns, especially for northern areas.

Southwesterly flow aloft continues over the Northern Rockies heading into next week. This will maintain the summer heat while passing shortwaves interact with diurnal instability for at least daily isolated shower and thunderstorm activity. There will also be periodic breeziness to go along with the hot and mostly dry conditions, but there's no clear signal for wind speeds strong enough to warrant fire weather concerns just quite yet. - RCG

- Forecast Confidence & Scenarios:

Shower and thunderstorm activity will be far more scattered in nature than yesterday, but the risk area will cover virtually the entire forecast area, not just central and southwest MT. Thinning moisture profiles will begin to reduce the threat for localized flooding while increasing instances of strong wind gusts near showers and storms. In addition to the strong wind gust threat storms will also be capable of hail, brief downpours, and cloud to ground lightning. Peak timing will generally run between 3 pm and midnight.

Even drier air moves in on Friday while the heat intensifies. Isolated showers and storms will mostly be confined to the central and southwest mountains. A more defined shortwave looks to bring a slight uptick in shower and thunderstorm activity on Saturday. Central and southwestern areas look to see the most widespread activity with storm hazards including localized strong wind gusts, hail, and downpours in addition to lightning. - RCG

AVIATION

23/06Z TAF Period

Lingering showers across Central and Southwest Montana will diminish by around 23/09Z. Impacts are generally not expected at any of the terminals outside of KWYS which will have continue to receive some light rain before tapering off.

There is a high chance for fog developing at KWYS overnight with slightly lesser confidence in KEKS that will depend on whether the clouds are able to fully clear out. The fog will start to lift after 23/15Z as the next shortwave trough starts to move into the region bringing another mid-level cloud deck and a chance for precipitation.

Another round of isolated to scattered thunderstorms are possible Thursday afternoon although confidence in impacts to any one terminal remains rather low. The highest confidence for impact will be across Southwest Montana including KBZN, KEKS, and KWYS which all have PROB30 groups for the convective timeframe. For all other terminals, confidence was less than 10% and thus too low to include in the TAF at this time. The main concerns for any thunderstorm that develops will be mountain obscuration, lightning, and gusty winds.

Wildfire smoke will continue to impact slantwise visibility through the period. There are some hints in the hi-res models that conditions might worsen Thursday afternoon/evening towards the end of the TAF period but there is still disagreement in the models as to how much surface visibility and ceilings will be impacted. -thor

FIRE WEATHER

Intensifying heat, critically low minimum humidity, and westerly winds routinely gusting in the 20 to 30 mph range will raise daily elevated to critical fire weather concerns this weekend into next week. In the near term, the worst fire weather conditions will be over the north-central plains Friday and Saturday. Additionally, isolated to widely scattered showers and thunderstorms may produce localized strong wind gusts and cloud to ground lightning, mostly for central and southwest Montana Saturday afternoon and evening. A fire weather watch has been issued for zones 112, 113, 114, 115, and 117 for Friday and Saturday to address the elevated to critical fire weather conditions that may develop in this setup. - RCG

PRELIMINARY POINT TEMPS/POPS

GTF 96 64 99 63 / 20 20 10 0 CTB 93 61 92 58 / 20 20 0 0 HLN 94 63 98 63 / 20 20 10 10 BZN 90 60 97 61 / 30 20 0 20 WYS 80 50 87 51 / 50 30 0 10 DLN 87 57 93 57 / 20 20 0 10 HVR 99 65 102 63 / 20 20 0 0 LWT 90 61 95 61 / 20 20 10 10

TFX WATCHES/WARNINGS/ADVISORIES

Fire Weather Watch from Friday morning through Saturday evening for Central and Eastern Lewis and Clark National Forest Areas- Chouteau and Fergus Counties-Eastern Glacier/Toole/Central/Eastern Pondera/Liberty-Hill and Blaine Counties-Lewis and Clark National Forest Rocky Mountain District- Rocky Mountain Front.

Extreme Heat Watch from Friday morning through Sunday evening for Bears Paw Mountains and Southern Blaine-Cascade County below 5000ft-Fergus County below 4500ft-Hill County-Judith Basin County and Judith Gap-Northern Blaine County-Western and Central Chouteau County.