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

17/12Z TAF Period

Scattered to numerous showers and thunderstorms will develop over west and southwest MT by around 17/19Z before moving east and northeast through central and north-central MT during the evening and early overnight hours. Thunderstorm hazards will include strong, gusty winds, hail, and torrential downpours. General VFR conditions will prevail, but there will be MVFR and even brief IFR conditions beneath the heavier showers and thunderstorms. There will also be some patchy fog development towards the end of the TAF period. - RCG

PRELIMINARY POINT TEMPS/POPS

GTF 95 61 95 61 / 30 50 10 40 CTB 89 59 91 58 / 40 60 10 0 HLN 93 62 93 62 / 50 50 20 30 BZN 93 57 92 58 / 40 40 10 40 WYS 85 49 85 48 / 50 60 20 30 DLN 88 55 89 55 / 50 60 10 20 HVR 97 62 97 62 / 10 50 10 20 LWT 89 56 90 57 / 10 70 10 30

TFX WATCHES/WARNINGS/ADVISORIES

Flash Flood Watch through this evening for Big Belt, Bridger and Castle Mountains.