textproduct: Great Falls

KEY MESSAGES

- Cooler temperatures persist across the region through the end of the week with showers and thunderstorms around at times.

- A few thunderstorms will be capable of producing heavy rain, which would be impactful if they were to move over a city or burn scar.

- Temperatures trend closer to normal by the end of the weekend, with continued opportunities for afternoon and evening showers and thunderstorms.

DISCUSSION

Meteorological Overview:

A southwesterly flow aloft will more or less persist across the Northern Rockies through the weekend into early next week. This flow is initially more cyclonic in nature, which will foster plenty of showers and thunderstorms through Friday evening across the region. Persistent surface winds from the east and northeast will continue to usher in cooler temperatures. Precipitable water values will tick a touch higher today, resulting in increased precipitation efficiency with showers and thunderstorms that develop. The most robust thunderstorms that develop over the next few days will be capable of producing at least briefly heavy rainfall. Should one of these stronger storms move over urban areas or burn scars, some localized flash flooding cannot be ruled out. Lightning and gusty winds will also be a concern with stronger thunderstorms.

Heading into the weekend the nature of the southwesterly flow aloft becomes a bit more anti-cyclonic. That said, moisture working in from the southwest will combine with temperatures trending closer to normal to result in scattered afternoon showers and thunderstorms.

Greater uncertainty in the pattern develops toward the middle of next week. At this point the main takeaway will be for scattered showers and thunderstorms most days with temperatures near average. -AM

- Forecast Confidence & Scenarios:

Flash flooding concerns:

HREF 10km neighborhood probabilities for a quarter of an inch of rain over an hour is at or greater than 50% over the Pioneer Mountains today. Given the recent intense fire activity resulting in hydrophobic soils and steep terrain within the fire footprint, it will not take much precipitation over a short period to result in some flash flooding. As such, this area was added to a Flash Flood Watch valid for this afternoon and evening. Another Flash Flood Watch may be necessary for the same area for tomorrow as well. Ongoing Flash Flood Watch for the Horse Gulch Burn Scar remains in effect unchanged from the previous forecast update.

Risk for strong thunderstorms:

While inverted-V soundings will be around across SW MT the next two days, these inverted-Vs are rather shallow all things considered. This will help reduce the risk for a strong to severe gust. At this time the main concern will continue to be lightning, aside from briefly heavy rainfall. -AM

AVIATION

13/12Z TAF Period

Scattered showers/thunderstorms will affect the CWA through the period. The main impact from any storms will be lightning, brief heavy rainfall and gusty winds. The combination of thunderstorms/smoke will continue to obscure mountains across the CWA through the period.

For KGTF: Surface winds will generally remain out of the northeast, and generally less than 15 knots through the period. Some higher gusts are possible in/near thunderstorms. At this time, the forecast models all have different timing of when the storms will hit, thus confidence is low at this time exactly when a gusty wind is possible from a stronger storm. Brusda

HYDROLOGY

Precipitable water values will continue be around an inch or so across the region the next couple days. This will allow for the most robust thunderstorms to produce at least briefly heavy rainfall. Should these thunderstorms move over sensitive areas such as urban areas, burn scars, or active fires, excessive rainfall leading to flash flooding will be possible. A Flash Flood Watch is in effect for the Horse Gulch burn scar. The Flash Flood Watch was also expanded to include the Pioneer Mountains, where steep terrain and hydrophobic soils will need very low total rainfall amounts to result in flash flooding. -AM/Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 67 49 64 48 / 80 90 60 40 CTB 57 43 60 43 / 90 80 40 20 HLN 71 52 68 51 / 30 80 70 50 BZN 76 49 70 47 / 40 90 90 80 WYS 71 38 66 35 / 70 70 70 80 DLN 75 48 70 45 / 70 90 90 70 HVR 65 47 68 46 / 90 60 20 40 LWT 66 47 64 45 / 70 80 80 70

TFX WATCHES/WARNINGS/ADVISORIES

Flash Flood Watch this evening for the Horse Gulch burn scar.

Flash Flood Watch from 1 PM MDT this afternoon through this evening for the Pioneer Mountains burn scar in Beaverhead County.