Area Forecast Discussion

KEY MESSAGES

- A few strong thunderstorms are expected over southeast Montana through this evening. Main threat is heavy rain and strong winds. - Flood Watch in effect through Sunday for much of the area. Widespread precipitation through the weekend. - Widespread 50-95% chance of an inch of rainfall. 50-80% chance of two inches of rainfall for portions of south central Montana.

DISCUSSION

Rest of Today through Friday...

Widespread rain will continue over the next 24 hours. An estimated 0.75-1.25" of rain has fallen in a swath extending from Sweet Grass County and northeast through northern Custer County. Localized heavier amounts up to 3 inches has occurred within this watch as well. Expect more rain to occur over the next 24 to 30 hours.

The root cause of all this reain is an upper level low that is positions over southwest Idaho as of noon. Moisture brought in with it the Pacific, as well as a sharp easterly surge of low- level moisture has PWATs approaching 1.3" at and east of Yellowstone County. While widespread rain will persist, the larger issue is keeping an eye out for pockets of localized, heavier rainfall.

Analysis shows CAPE up to 500 J/kg across southeast MT this afternoon, with modest shear (especially for mid September). These ingredients combined with the ample ascent provided by the upper low will bring about storm potential through the evening. Soundings show long, skinny CAPEs alongside a deep, saturated layer, further indicating the heavy rain potential of these storms. Due to all of this, a rare Moderate risk of Excessive Rainfall exists from Yellowstone County through Custer and Powder River counties, meaning that any storm has at least a 40% chance of inducing flash flooding. These rainfall rates could be as high as 3" of rain per hour, and the most likely timeframe of these storms occurring is now through 9pm. Do not wait for a storm to occur, have a plan ready now in case of possible flash flooding. Flood Watch for all of southeastern MT and north-central WY continues to remain in effect.

Relatively low amount of cooler air is brought in with this system, only dropping snow levels at around 10,000 ft. So elevations above this will see 1-4 inches of snow.

As the upper low shifts through Wyoming on Sunday, we will still receive additional rain from the wrap-around. However, little instability exists during this period, so the chance for localized areas of heavy rain with rain rates of 2-3" per hour is much lower. The only exception (while still being a lower chance than today) is areas at and south of US-212 from Crow Agency through Alzada, where CAPE up to 200 J/kg is forecasted to occur. All rain is expected to shift east of Montana by midnight Sunday night.

As we enter the work week, expect a quick warm-up to slightly above normal temperatures and mainly dry conditions throughout the week. A week shortwave is currently progged to cross southern MT on Wednesday, bringing with it a small (less than 20%) chance of showers and storms to the area. Vertz

AVIATION

19z Discussion.

Widespread showers and some embedded thunderstorms continues through tonight, with patchy nighttime fog as well. MVFR/IFR flying conditions will be common, and all TAF sites will be impacted by this system (though KLVM will see low amounts of rain compared to all other terminals). Mountains will be mostly obscured through the period. Vertz

PRELIMINARY POINT TEMP/POPS

Sun Mon Tue Wed Thu Fri Sat ----------------------------------------------------------- BIL 051/062 050/070 049/077 054/079 054/081 054/080 053/072 +8/W 20/B 00/U 01/B 00/U 00/B 11/B LVM 043/066 042/074 045/078 049/078 048/081 049/081 048/072 62/W 00/U 00/U 10/B 10/U 00/B 13/W HDN 049/063 047/072 045/079 049/080 048/083 049/083 049/075 +8/T 30/B 00/U 11/B 00/U 00/U 11/B MLS 053/061 049/069 050/077 053/078 053/082 056/083 054/075 ++/W 20/B 00/U 01/B 00/U 00/U 01/B 4BQ 053/061 049/067 049/076 054/079 053/082 057/083 055/076 99/T 61/E 00/U 12/T 20/U 00/U 02/W BHK 048/061 044/067 044/073 048/079 050/083 052/084 052/078 99/W 40/E 00/N 03/T 30/U 00/N 11/B SHR 046/062 044/070 042/077 048/078 046/082 048/083 049/077 88/T 61/B 00/U 12/W 00/U 00/U 01/B

BYZ WATCHES / WARNINGS / ADVISORIES

MT...Flood Watch in effect through Sunday afternoon FOR ZONES 29>34-36-37-40-42-56>58-63>68-138-139-141-169>173-228-235. WY...Flood Watch in effect through Sunday afternoon FOR ZONES 198-199.