textproduct: Great Falls

KEY MESSAGES

- Hot and dry conditions will prevail over the next few days, along with periods of scattered showers and thunderstorms.

- Some of these storms may produce strong, gusty winds and dangerous cloud to ground lightning.

- The combination of the heat and breeziness, along with the scattered thunderstorm activity will result in elevated fire weather conditions over southwest and portions of central Montana.

- A Pacific weather system brings cooler temperatures, additional showers and thunderstorms, and periods of gusty winds this weekend into early next week.

DISCUSSION

- Meteorological Overview:

Ridging aloft continues to build over the interior west, while a trough approaching the Pacific NW gradually brings a more unstable southwesterly flow aloft over the Northern Rockies. Widely scattered showers and thunderstorms have expanded northward over the last couple of days and will continue to do so through Thursday. Additionally, thunderstorm coverage and intensity will also be on the increase as shortwaves move through the southwesterly flow and interact with steep lapse rates above 8C/km and ML CAPE in the 300 to 900 J/kg range. Very warm and dry sub- cloud layers will make localized strong, gusty winds and lightning the primary concerns with this activity. While most storms will occur in the afternoon and evening, there will also be at least some overnight convection. The combination of the very warm and dry conditions with scattered thunderstorm activity will also bring elevated fire weather conditions to southwest and portions of central MT, see the Fire Weather section for more information.

Ensembles are coming into agreement with the trough ejecting eastward into the Northern Rockies this weekend into early next week. Friday looks to be a transition day, with showers and storms beginning to retreat back to southwest MT and temperatures beginning to trend cooler. The main trough then moves through the Northern Rockies and brings more areawide scattered showers and some thunderstorms, gusty winds, and cooler temperatures. CAPE will be highest on Saturday, but still not as high as earlier in the week. The combination of afternoon temperatures cooling below average with more productive showers should limit severe weather and fire weather concerns. In fact, some of the latest most aggressive deterministics bring well below average temperatures on Sunday, and even a bit of high mountain snow. The evolution of this system will be closely monitored over the next few days. Southwesterly flow aloft quickly returns to the Northern Rockies early next week as another trough moves onto the Pacific NW coast. This trough looks to swing eastward heading towards the middle and end of next week, although its depth and overall impact are uncertain at this time. - RCG

- Forecast Confidence & Scenarios:

Scattered showers and thunderstorms today through Friday

Showers and thunderstorms will initiate over west and southwest MT by mid- afternoon before moving towards the plains and valleys to the east later in the afternoon and evening. This activity will expand farther north and increase in coverage and intensity compared to previous days. Forecast soundings show tall inverted V profiles, with DCAPE values in the 1100 to 1400 J/kg range. This indicates that strong, gusty winds will be the primary hazard with this activity. This is corroborated by the latest suite of hires guidance, which highlight wind gusts in between 40 and 60 mph with this broken line of storms, strongest for locations near and west of I15. Hail shouldnt be a significant problem in this environment, but with effective shear values approaching and exceeding 25 kts, some hail cant be ruled out with the strongest cores. Similar conditions unfold on Thursday, though the forcing may be a bit stronger and the storm coverage maybe a little more widespread. By Friday shower and thunderstorm activity begins to retreat to the southwest and ML CAPE values look to fall below 500 J/kg in most cases for weaker storm intensity. - RCG

AVIATION

26/06Z TAF Period

Wildfire smoke will lead to periods of mountain obscuration along the Continental Divide and portions of Southwest Montana over the next 24 hours. Showers and thunderstorms are expected to develop in Southwest Montana after 26/18Z and become more scattered into Central Montana after 26/21Z. Thunderstorms will linger across the region through at least 27/03Z and start tapering off further into the evening with the loss of daytime heating. The main concerns will be lightning and gusty/erratic winds. Mountain obscuration will be a concern wherever showers/thunderstorms develop.

For KGTF: Wildfire smoke is generally expected to remain aloft and impact slantwise visibility through the period with no major impacts to surface visibility expected. Winds will remain light through the night into the mid-morning with a slight pick up to 6-8kts out of the east expected after 26/20Z.

The main concern for Wednesday will be how the thunderstorms develop and progress across the region. The latest hi-res models show thunderstorms splitting to the north and south of Great Falls with around a 30% probability of a direct impact to KGTF. The main concern would be if a thunderstorm starts to follow the I-15 corridor from the south off the mountains which would greatly increase the chances that it impacts the terminal. Even without a direct impact, there is a concern that any thunderstorm in the general area could produce an outflow boundary that results in a strong crosswind at the airport. In terms of timing, thunderstorms are expected to start moving off the Continental Divide between 26/22Z and 27/01Z and reach the KGTF vicinity by around 27/00Z to 27/03Z. -thor

FIRE WEATHER

Very warm, dry, and breezy conditions will combine with thunderstorm activity for elevated fire weather conditions for the next couple of days, especially for southwest and portions of central Montana. The primary concern will be passing waves of showers and thunderstorms, some with little rainfall and cloud to ground lightning. The strongest storms may produce erratic gusty winds over 50 mph,which may help spread new fires. Thursday and Friday look to be the breeziest and driest days for central and southwestern locations north of I90, with up to a 20% chance for the Hot Dry Windy Index to exceed the 90th percentile. Fire weather products are not planned to be sent out at this time given the marginal conditions, but the situation will continue to be monitored. A Pacific trough brings additional showers and thunderstorms and periods of gusty winds this weekend; however, cooler temperatures and more productive storms looks to reduce the fire risk. - RCG

PRELIMINARY POINT TEMPS/POPS

GTF 90 58 92 56 / 20 30 20 30 CTB 87 53 87 53 / 20 30 20 20 HLN 90 55 88 56 / 30 30 40 30 BZN 88 52 88 51 / 20 30 30 30 WYS 81 41 81 42 / 30 30 30 40 DLN 86 50 84 51 / 30 30 20 20 HVR 93 60 96 60 / 0 10 20 30 LWT 88 56 92 56 / 20 30 30 30

TFX WATCHES/WARNINGS/ADVISORIES

None.