textproduct: Great Falls

KEY MESSAGES

- Slight risk for severe thunderstorms across portions of Central and North Central Montana, with the primary severe weather threats being hail larger than quarter size and damaging wind in excess of 60 mph.

- Marginal risk for flash flooding across portions of Southwest through North Central, most notably along and near the the Montana Hwy 200 corridor.

- Moisture circulating across the Northern Rockies and MT will maintain daily chances for showers and thunderstorms through the upcoming weekend.

DISCUSSION

The Northern Rockies and MT remain on the NW periphery of an expansive mid-upper level ridge centered on the N-central CONUS. A plume of subtropical sourced mid-upper level moisture circulating around the back side of the ridge will continue to deepen today with lower level easterly flow bringing dewpoints in the 50s to around 60 to central/north-central MT overnight. An embedded mid- level shortwave and upper level jet disturbance lifting out of northern CA will help to trigger and organize thunderstorm development across SW MT this afternoon with stronger cells tracking N/NE through central MT and eastern portions of n-central MT this evening. The presence of deep moisture and moderate shear will support some strong to severe thunderstorms with strong winds and hail possible in addition to localized heavy rainfall as storms peak in intensity early this evening.

The flow of anomalous moisture across the area looks to remain in place through the upcoming weekend, maintaining a daily chance for showers and thunderstorms. Cloud-cover on Wednesday will result in temperatures closer to seasonal averages and may inhibit instability and result in a lower coverage of showers/storms. Differences in the timing of embedded disturbances and the extent cloud-cover and instability leads to lower confidence in specific details beyond Wednesday. Hoenisch

AVIATION

14/18Z TAF Period

Showers and thunderstorms will develop through the afternoon hours across the higher terrain of Central Idaho and Southwest Montana before lifting north to northeast in Central and portions of North Central Montana. Thunderstorms moving into Central Montana late this afternoon and early this evening will be capable of producing large hail, damaging winds, and heavy rainfall that will lead to reduced VIS. Those terminals most at risk of these strong storms will be the KHLN, KGTF, KLWT and KHVR. While VFR conditions will largely prevail there will be period of MVFR/low- VFR conditions this afternoon and evening beneath showers and storms, with brief periods of IFR possible. Mountain obscuration will increase through the remainder of the day. - Moldan

HYDROLOGY

A Flash Flood Watch remains in place for much of central MT and adjoining areas for this afternoon and evening. High-resolution model ensembles are in fairly good agreement focusing stronger convection and potential for intense rainfall rates along an axis from southern Lewis and Clark County east across the Big/Little Belts, Judith Basin and Fergus counties where there is 50-80% probability of 1hr rainfall rates exceeding .50" and 30-50% probability for localized hourly rates exceeding 1". This puts the Horse Gulch burn scar at risk for flash flooding as well as other areas that may see these types of intense rainfall rates. Storms are expected to have reasonably quick motion, which may help to keep intense rainfall rates relatively brief but still impactful. Hoenisch

PRELIMINARY POINT TEMPS/POPS

GTF 59 82 59 88 / 90 50 20 20 CTB 56 79 57 83 / 50 30 30 30 HLN 62 86 62 90 / 60 60 30 30 BZN 58 86 57 91 / 40 50 40 30 WYS 52 84 49 87 / 10 30 30 10 DLN 56 85 55 89 / 20 50 30 10 HVR 60 82 58 89 / 80 50 30 0 LWT 55 79 54 85 / 90 70 40 40

TFX WATCHES/WARNINGS/ADVISORIES

Flood Watch until midnight MDT tonight for Bears Paw Mountains and Southern Blaine-Big Belt, Bridger and Castle Mountains- Canyon Ferry Area-Cascade County below 5000ft-Elkhorn and Boulder Mountains-Fergus County below 4500ft-Gallatin Valley- Gates of the Mountains-Helena Valley-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains-Meagher County Valleys-Missouri Headwaters-Snowy and Judith Mountains-Southern High Plains-Upper Blackfoot and MacDonald Pass-Western and Central Chouteau County.