textproduct: Great Falls

KEY MESSAGES

- Shower and thunderstorm activity continues over central and north-central Montana today, with localized gusty winds, brief downpours, and occasional lightning being the primary hazards. - Drier and warmer conditions move in for Friday and most of Saturday, but showers and thunderstorms will increase from southwest to northeast by Saturday afternoon.

- There will be a slight cooldown Sunday and Monday before a more summer like pattern with daily shower and thunderstorm activity sets up during the middle and end of next week.

DISCUSSION

- Meteorological Overview:

A moist northwesterly flow aloft will maintain scattered to numerous showers and thunderstorms for another day. H500 temperatures dip to as low as -10C over the plains with a sharp transition to warmer air aloft south of boundary that roughly runs along the highway 12 corridor. Shower and thunderstorm activity will be most pronounced along and north of this boundary. While surface CAPE values will generally not exceed 500 J/kg, the cooler air aloft, its associated frontogenesis, along with added shear from the northwesterly flow aloft itself will be sufficient for instances of localized gusty winds, small hail, and brief downpours in addition to cloud to ground lightning.

This activity decreases in coverage later this evening and tonight with the loss of diurnal heating and the colder air aloft moving off to the east. Weak transient ridging develops Friday into Saturday for a temporary warming and drying trend. However, by Saturday morning, monsoon moisture and shortwave energy will already sending waves of showers and thunderstorms into southwest MT. This activity will move northeastward into central/north- central MT by late Saturday afternoon and evening. This all occurs while a Pacific trough and attendant cold front swing in from the northwest. Given these parameters, Saturday afternoon through Saturday night will offer the strongest and most widespread shower and thunderstorm activity. CAPE values don't look overly impressive at this time, but there will be pockets over 500 J/kg and PWAT values will creep up to around three quarters of an inch over the southwest thanks to the added monsoon moisture.

There will be a cooldown and additional opportunities for isolated showers and thunderstorms Sunday into Monday while the trough axis traverses the forecast area. Most ensembles highlight weak ridging aloft bringing a more summer like pattern for the middle and end of next week. Temperatures look to climb above average in response to the ridging, although weak shortwaves will bring daily opportunities for scattered showers and thunderstorms. - RCG

- Forecast Confidence & Scenarios:

Shower and thunderstorm activity today through Sunday...

The most concerning thunderstorm days will be today and Saturday. Even though the CAPE will be modest today, there will be sufficient shear and forcing for a few stronger storms. This activity will be most widespread over the plains east of I15, although any central/north-central location will be susceptible to at least isolated storms. Impacts will be greatest for outdoor recreation and will include localized wind gusts over 40 mph, small hail, and brief downpours in addition to instances of cloud to ground lightning.

Saturday's shower and thunderstorm activity will initially be driven by shortwave energy and monsoon moisture moving in from the southwest. The one factor that stands out will be PWATs creeping up to the three quarter to one inch range over southwest and portions of central MT. This is about one standard deviation above climatology per the NAEFS climate anomaly index. Should this occur, then localized heavy downpours capable of producing isolated instances of flooding impacts to normally flood prone areas such as burn scars will be plausible. CAPE values don't look very impressive for widespread strong to severe storms Saturday afternoon through Saturday night; however, this could change, particularly if the trough and cold front moving in from the northwest deepen and offer a more amplified arrival. - RCG

AVIATION

18/18Z TAF Period

Scattered showers and thunderstorms will mainly affect the central and northern portions of the CWA through 02z Fri. Expect clearing skies overnight. A few mountains over Central MT could be obscured at times this afternoon by passing showers. Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 69 44 73 48 / 50 20 0 0 CTB 64 40 70 46 / 40 20 0 0 HLN 77 46 79 50 / 10 20 0 0 BZN 79 42 79 47 / 0 0 0 10 WYS 75 36 80 43 / 0 0 0 10 DLN 80 43 81 49 / 0 0 0 10 HVR 69 41 72 48 / 80 20 0 0 LWT 66 39 70 45 / 60 20 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.