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KEY MESSAGES

- Dangerous heat and humidity are expected to cause impacts to northeast Montana for today into tomorrow night. Highs will be in the 90s and 100s with lows in the mid 60s to mid 70s. An Extreme Heat Warning is in place from now through Sunday evening.

- There is a chance of dry thunderstorms in the west tomorrow afternoon. A severe gust is possible. - Temperatures will moderate early next week, but are expected to remain above normal. Precipitation chances begin to return Tuesday night.

DISCUSSION

WEATHER PATTERN OVERVIEW:

An upper ridge is over the area, and the peak heat is expected to build in tomorrow afternoon before the ridge breaks down slightly for Monday. The base NBM indicates lows staying above 65 degrees for tonight, Sunday night, and Monday night, so it will be difficult to stay cool enough to sleep comfortably. There are small wiggles in the upper level flow for various CAMs tomorrow afternoon, especially in the west half of the area. Any forcing over the west would be enough to generate an intense dry thunderstorm since cloud bases will be very high based on hot surface temperatures. We will keep an eye on this potential, but storm coverage is expected to be low at the very least. There is an extreme heat warning in place for the heat from now through Sunday evening.

There is a low (10 percent) chance of a shortwave moving through and using the sfc boundaries near or after peak heating to generate a strong thunderstorm or two today in the far north. There is a similar event expected on Sunday, with this low chance starting mostly in central Montana and moving into our CWA. One CAM suggests a severe gust or two with the passage. With near inverted Vs on Fcst soundings can not rule this out due to high DCAPE (1500 to 2600 J/kg).

The latest GFS and ECMWF ensemble suggests a toppling of the ridge crest to the east by late Monday evening, so central Montana is expected to start to see precipitation chances again by the time we get to Tuesday afternoon. The base NBM suggests these shower and storm chances will continue through late in the upcoming work week.

FORECAST CONFIDENCE & DEVIATIONS: The main deviation was to winds for tomorrow by using the HRWFV3.

There is HIGH confidence on temperatures exceeding 95 degrees for high temperatures and low temperatures struggling to drop below 65 degrees this weekend, and for highs exceeding 90 on Monday.

There is MODERATE confidence that fire weather will hold off for at least the next week. Recent rains are keeping the vegetation pretty green, so fire concerns are low.

There is moderate confidence of an isolated dry thunderstorm producing a 60 mph wind gust on Sunday from 2 PM to 8 PM in the western part of the area.

There is HIGH confidence of precipitation chances returning by Tuesday evening.

AVIATION

UPDATED: 1830Z

FLIGHT CATEGORY: VFR

DISCUSSION: Mainly dry weather and mostly clear skies are expected over the TAF cycle. A few model runs suggest isolated dry thunderstorms may impact KGGW tomorrow afternoon though. There is low confidence in isolated gusty winds as high as 60 mph from 21Z Sunday to 02Z Monday. There are almost no radar echoes showing up in the models, so a gust of wind may simply come out of an innocent looking cloud.

DENSITY ALTITUDE: Hot temperatures near and above 100 degrees will lead to needs of increased length of runway and increased power to take off. Worst times will be in the afternoon and evening from 18Z to 03Z today, and the same time frame for Sunday.

WIND: South southeast at 7 to 15 knots.

GGW WATCHES/WARNINGS/ADVISORIES

Extreme Heat Warning until 6 AM MDT Monday for Central and Southeast Phillips-Central and Southern Valley-Daniels-Dawson- Eastern Roosevelt-Garfield-McCone-Northern Phillips-Northern Valley-Petroleum-Prairie-Richland-Sheridan-Southwest Phillips- Western Roosevelt-Wibaux.