textproduct: Great Falls

KEY MESSAGES

- Strong to severe thunderstorms possible this afternoon and evening, particularly across central and southwest MT with a risk for heavy rain, strong winds and large hail.

- Another round of strong to severe thunderstorms is possible Friday.

- Intense rainfall rates with storms today and tomorrow could produce localized flooding, particularly if occurring over burn scars or urban areas.

DISCUSSION

/Issued Issued by National Weather Service Missoula MT/

- Meteorological Overview:

MT remains on the fringe of an expansive mid-upper level ridge covering much of the CONUS from the Rockies eastward. An upper level low initially centered offshore of the OR coast will slowly lift around the NW periphery of the ridge into BC by Saturday then across western Canada this weekend, temporarily flattening the northern margins of the ridge before it builds back to the NW early next week.

The upper level system clipping the Pacific NW will maintain an enhanced southwesterly mid-upper level flow across the Northern Rockies today and tomorrow. This combined with still elevated lower to mid level moisture in place across the area and more efficient daytime heating with lower cloud-cover than recent days will support moderate instability for thunderstorm development today and again Friday with with sufficient wind shear for some organized/strong-severe storms.

Elevated moisture/instability will maintain a risk for afternoon/evening showers/thunderstorms on Saturday with mainly dry conditions expected Sunday as the upper level system passing north of the area turns winds aloft more westerly. Moisture looks to circulate back into the region early next week however as the ridge rebuilds and flow aloft turns back to the southwest. Temperatures gradually warm over he next several days with most areas peaking this weekend with afternoon temps in the mid 90s. Hoenisch

- Forecast Confidence & Scenarios:

Thunderstorm initiation is expected over SW MT this afternoon with hi-resolution model ensembles focusing the greatest coverage of thunderstorms from northern Beaverhead county NE through central MT with more isolated activity to the north through western portions of north-central MT. Late day thunderstorm development over Northern ID may clip across NW portions of the forecast area overnight tonight. Large hail is a hazard with severe storms this afternoon across central MT with the greatest overlap of CAPE>1500 J/kg and shear >35kts along a corridor from southern Lewis and Clark county east through southwest Judith Basin County. A similar setup exists on Friday with broader coverage of moderate CAPE and shear which may lead to a more organized wind hazard.

Anomalous moisture remains in place across the area (PWAT's > 1.00" with 50-60F dewpoints) for a risk of heavy downpours with any thunderstorms today and tomorrow. A Flash Flood Watch remains in effect through this evening for the particularly sensitive landscape near the Horse Gulch burn scar, but any areas that experience intense rainfall rates or repetitive storms today could see some quick runoff into low-lying or urbanized areas. Hoenisch

AVIATION

16/12Z TAF Period

Primary aviation concern will be thunderstorm development across southwest MT late this afternoon with storms generally tracking northeast/east across central MT this evening. There is a risk for large hail, gusty winds and brief heavy downpours with stronger storms across central MT including the KHLN terminal. Brief MVFR/IFR conditions may occur with storms, otherwise VFR conditions prevail with mainly clear skies through this morning before cumulus initiates this afternoon. Hoenisch

PRELIMINARY POINT TEMPS/POPS

GTF 89 61 94 62 / 10 20 30 60 CTB 85 59 89 60 / 20 30 40 50 HLN 91 63 94 63 / 50 40 50 70 BZN 91 59 93 58 / 30 30 40 60 WYS 87 50 87 50 / 30 40 50 60 DLN 88 57 89 56 / 20 30 50 60 HVR 91 62 96 63 / 0 20 10 50 LWT 84 55 89 57 / 30 20 20 60

TFX WATCHES/WARNINGS/ADVISORIES

Flash Flood Watch from 2 PM MDT this afternoon through this evening for Big Belt, Bridger and Castle Mountains.