textproduct: Great Falls

KEY MESSAGES

- Daily chances for afternoon showers and thunderstorms through Friday.

- Scattered thunderstorms in Southwest Montana on Wednesday have the potential to produce heavy rain and localized flash flooding.

- Temperatures will warm up into the upper 90s to mid 100s through the weekend with the warmest temperatures expected on Friday and Saturday.

DISCUSSION

- Meteorological Overview:

A shortwave will move up from the southwest today bringing a chance for showers and thunderstorms primarily south of the I-90 corridor. Combined with the monsoonal moisture in the area and a lack of steering flow there is an increased potential for localized flash flooding in the higher terrain of Southwest Montana (see the HYDROLOGY section for more detail).

Another weak shortwave moves through the area on Thursday with another round of afternoon showers and thunderstorms possible, this time extending further north towards the Hi-Line but with a lower chance for heavy rain compared to Wednesday.

Heading towards the weekend the upper-level ridge starts to build over the Intermountain West leading to well above normal temperatures across much of the region, especially along the Northern Plains. Dry air will start advecting along an upper-level jet streak from the west which will both bring lower relative humidity values and the potential for gusty winds to the region. The potential combination of hot, dry, and windy conditions will lead to elevated fire weather concerns through at least the weekend (see the FIRE WEATHER section for more detail).

An fast moving upper-level trough will push through on Sunday which will help to moderate temperatures back down into the 90s for early next week but with the upper-level ridge potentially building back towards the middle of next week chances remain high that above normal temperatures will continue at least through the remainder of July.

- Forecast Confidence & Scenarios:

Thunderstorms Wednesday: Tall, skinny CAPE and PWATs exceeding 1.25" in the model soundings confirm the potential flash flood risk across Southwest Montana. When evaluating the current Flash Flood Watch, there still is not enough confidence to extend it to the lower elevations. There may be some localized heavy rainfall in the valleys of Southwest Montana but the higher confidence and concern is still in the mountainous areas of Beaverhead, Madison, and Gallatin counties.

As for the Horse Gulch burn scar, the models are still very uncertain on whether showers/thunderstorms will make it that far north so for now that area will be left out of the watch. But it will be something to keep an eye on as the day progresses.

Beyond the heavy rainfall there are some hints in the models that a couple storms may become strong to severe, particularly along the Idaho/Montana border. Hail would be the primary threat and if a heavy rain plus accumulating small hail scenario were to develop it could enhance the flash flood concerns already present in the mountains. Overall the severe threat remains marginal but it is something that folks should be aware of as the afternoon progresses, especially those who might be recreating in the backcountry.

Thunderstorms Thursday: Hi-res model guidance shows CAPE in the 1000- 2000 J/kg range across a good portion of the region with generally little cap and modest lapse rates which all suggests the potential for thunderstorm development in the afternoon. What will be noticeably lacking on Thursday is shear. So thunderstorms could potentially develop but it might be a lot more sporadic and popcorn in nature. With DCAPE values as high as they are there is naturally a chance for some gusty winds with any thunderstorm that develops. But otherwise Thursday afternoon feels pretty conditional and there is still a lot that needs to be resolved in the models to increase confidence in some of the higher-end threats.

Heat Friday thru Sunday: Across much of the US-87 corridor from Great Falls to Havre, the US-2 corridor east of Rudyard and along the Upper Missouri River Breaks there is a greater than 60% chance for 100+ degree temperatures on Friday. On Saturday, the risk for 100+ expands to include much of the the Northern Plains with a 60% chance for 100+ temperatures along the 287 corridor between Helena and Three Forks. 20-30% probabilities stick around on Sunday but beyond that temperatures start to moderate back down into the 90s early next week.

Heat-related impacts will be a big concern this weekend. Right now the highest concerns are in Hill, Blaine, Chouteau, and Fergus counties with surrounding areas having more moderate concerns as locations further west will at least be able to cool down a couple more degrees overnight compared to those locations further east. Heat-related illness will be a concern for those recreating and spending an extended amount of time outdoors and anyone without air conditioning. In particular, vulnerable populations and those most likely to experience heat-related illness will be at an even higher risk through the three day period.

While the worst of the heat is expected to be over after Sunday, it may be a while before a true cool down is expected as long-range models suggest the upper-level ridge could persist through next week. Therefore, those who are the most sensitive to the heat should be on the lookout for compounding heat stress, especially if they are without air conditioning or unable to find a cool location to recover. -thor

AVIATION

22/06Z TAF Period

Monsoon moisture will continue to stream in from the west/southwest and bring variably cloudy skies and periods of scattered showers and thunderstorms. Convective activity currently situated along the MT/ID/WY borders will slowly diminish and end during the overnight hours. Patchy fog will then develop and may bring compromised flight categories in the KWYS area between 22/09 and 22/15Z. A subtle shortwave moves in and initiates scattered to numerous showers and thunderstorms over southwest and portions of central MT between 22/18 and 23/06Z. The primary concern with this activity will be brief heavy downpours, lightning, and gusty/erratic winds. MVFR and even lower flight categories will be present beneath showers and storms. - RCG

FIRE WEATHER

Hot, dry, and windy conditions are expected through the weekend which will lead to elevated fire weather concerns across much of the region. The main limiting factor will be the potential for wetter showers and thunderstorms both Wednesday and Thursday.

There is a concern that most of the wetter showers will not make it north of a line between Helena and White Sulphur Springs which would put North-Central Montana at a much higher risk for elevated fire weather conditions through the weekend. Additionally, if the wetter storms on Wednesday and Thursday are unable to materialize, or they are a lot more isolated than what is currently expected, then that could also elevate fire weather concerns for Central and Southwest Montana.

As it stands there is a generalized concern for elevated fire weather conditions starting on Friday but much of the threat will not be determined until Thursday evening when it becomes clear which locations were able to pick up some wetting rain ahead of the incoming hot, dry, and windy conditions. -thor

HYDROLOGY

On Wednesday the Weather Prediction Center (WPC) has issued a Slight Risk (15% chance for flash flooding within 25 miles of a point) for flash flooding across most of Southwestern Montana. Between the I-90 corridor and a line between Augusta to Great Falls to Lewistown WPC has issued a Marginal Risk (5% chance for flash flooding within 25 miles of a point)for flash flooding on Wednesday. In addition across the mountains of Southwestern Montana there is a 50 - 70% chance for rainfall rates in excess of one half inch per hour on Wednesday. In addition there is around a 50% chance for rainfall rates in excess of three quarter of an inch per hour across the mountains of Southwestern Montana on Wednesday. There is a 20 - 40% chance for an inch of rain or greater across the mountains of Southwestern Montana on Wednesday. As a result a Flash Flood Watch was issued for the mountains south of I-90 from Noon to Midnight on Wednesday. -IG

PRELIMINARY POINT TEMPS/POPS

GTF 90 62 95 63 / 0 10 10 30 CTB 87 58 93 62 / 0 10 0 10 HLN 90 63 92 63 / 20 30 20 20 BZN 90 58 90 59 / 40 50 30 20 WYS 82 51 81 50 / 70 70 50 10 DLN 85 55 86 56 / 60 60 20 10 HVR 93 60 100 64 / 0 0 10 30 LWT 86 57 89 59 / 10 10 30 10

TFX WATCHES/WARNINGS/ADVISORIES

Flood Watch from noon MDT today through this evening for Gallatin and Madison County Mountains and Centennial Mountains- Northwest Beaverhead County-Ruby Mountains and Southern Beaverhead Mountains.