textproduct: Great Falls

KEY MESSAGES

- Scattered to numerous showers and thunderstorms will move in a southwest to northeast fashion later this afternoon through the early overnight hours.

- Thunderstorm hazards will include strong, gusty winds, hail, and torrential downpours capable of localized flooding.

- Warmer temperatures and slightly drier conditions move in for the weekend, although there will still be widely scattered shower and thunderstorm activity.

DISCUSSION

A shortwave embedded within a southwesterly flow aloft will interact with monsoon moisture and an unstable profile for the development of clusters to broken lines of thunderstorms moving in a southwest to northeast fashion this afternoon into tonight. The most robust CAPE over 1,500 J/kg will be slightly farther east of the 30 +kt bulk shear area, but there will certainly enough overlap for at least scattered strong to severe thunderstorms. Additionally, anomalously high PWATS, generally between an inch and an inch and a half, will be capable of producing brief periods with rainfall rates in the half to one inch range and even higher in the stronger cores. A flash flood watch was sent out again for the Horse Gulch Burn scar for this afternoon and evening. Urban areas that receive these heavier rainfall rates will also be susceptible to localized flooding, especially over central and north-central MT, though confidence was not quite high enough for additional flood products at this time.

Slightly drier air moves in for the weekend as the flow aloft becomes more westerly and pushes the richest moisture off to the east. There will be another shortwave that passes through again on Saturday, but it will have less CAPE and moisture to interact with. The widely scattered showers and storms that do develop will contain gusty, to locally strong winds with RH's dropping back down to the teens Saturday afternoon. Similar conditions prevail for Sunday, though storm coverage may be even less than Saturday.

Aside from a brief cooldown on Monday thanks to a passing cold front, temperatures will be on the rise thanks to high pressure aloft building over the Great Basin area. The Northern Rockies will still be far enough north for shortwaves and moisture to compromise the high and bring at least widely scattered shower and thunderstorm activity. - RCG

AVIATION

17/18Z TAF Period

Another wave of showers and thunderstorms is expected to lift north and east across Southwest through North Central Montana this afternoon and evening, with the areal coverage being much higher across lower elevations than that of Thursday. Thunderstorms will develop initially over the higher terrain of Southwest and West Montana between 18-21z this afternoon before moving north and east over the adjacent plains and valleys through 03-06z Saturday. Primary threat with the strongest storms will be gusty and erratic winds; however, hail can't be ruled out, especially at those terminals nearest to surrounding terrain. VFR conditions will largely prevail, but MVFR/low-VFR conditions are possible near/beneath precipitation. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 61 95 61 95 / 90 10 20 0 CTB 60 91 57 87 / 70 10 0 0 HLN 62 93 61 94 / 70 30 30 10 BZN 57 92 57 93 / 70 20 20 10 WYS 48 87 48 86 / 80 20 10 0 DLN 55 89 55 90 / 70 20 10 0 HVR 64 97 63 95 / 70 20 20 0 LWT 56 89 57 90 / 60 20 20 20

TFX WATCHES/WARNINGS/ADVISORIES

Flash Flood Watch until midnight MDT tonight for the Horse Gulch Burn Scar.