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

/Issued Issued by National Weather Service Missoula MT/

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

18/00Z TAF Period

Showers remain in the area with thunderstorms lingering over the Lewis Range and pushing eastward. Easterly winds at the surface from KGTF eastward this evening will shift to west southwest as the frontal boundary moves through. Expect showers and thunderstorms to push to the north northeast. Showers and thunderstorms will be the most widespread until around 06Z Sun when high resolution models show them becoming more scattered and dissipating to the north. Radar shows some gusty outflow boundaries stemming from the bulk of the storms which may impact areas around KGTF and KHLN through 02Z Sat. Temporary strong northwest winds possible with these storms. Overnight, skies will clear slowly which will increase radiational cooling. Areas that received rain and then clear will see some patchy fog, but widespread dense fog is less of a concern this evening. There is around a 10-15% chance of IFR CIGs around KCTB from 08Z-14Z Sat, which is one area that models are highlighting. A return of showery conditions is possible after 20Z Sat, but not expected to be as widespread as this evening. -Muessle

PRELIMINARY POINT TEMPS/POPS

GTF 95 61 95 61 / 20 90 10 20 CTB 90 60 91 57 / 30 70 10 0 HLN 94 62 93 61 / 50 70 30 30 BZN 93 57 92 57 / 30 70 20 20 WYS 85 48 87 48 / 50 80 20 10 DLN 88 55 89 55 / 70 70 20 10 HVR 98 64 97 63 / 0 70 20 20 LWT 90 56 89 57 / 10 60 20 20

TFX WATCHES/WARNINGS/ADVISORIES

Flash Flood Watch until midnight MDT tonight for Big Belt, Bridger and Castle Mountains.