textproduct: Great Falls

KEY MESSAGES

- Below normal temperatures through Saturday, with highs only rising into the mid-50s to low 70s on Thursday and Friday.

- Showers and thunderstorms will become more widespread tonight into Thursday, few areas possibly getting over a half inch of rainfall.

- Lightning over the next few days could start new wildfires.

AVIATION

12/18Z TAF Period

VFR conditions will generally prevail throughout this TAF period, with the exception of periods of MVFR conditions due to passing thunderstorms. Thunderstorm activity in Southwest MT will begin around 22Z, moving northeast. Thunderstorm activity will also begin near KCTB around 23Z, moving east and southeast. Light rain will begin in North-Central/Central MT following the thunderstorm activity. Mountain obscuration is expected throughout this TAF period, as is a decrease in slant range visibility due to smoke from regional wildfires.

For KGTF: While small, there is a non-zero chance for cross winds in excess of 15 kts. However, sustained winds and wind gusts will be out of the northeast, with the exception of passing convection which could cause erratic winds. The latest HREF is continuing to show a thunderstorm in the Great Falls area around 13/02Z. Light rain is expected to follow this convective activity. -Dzomba

AVIATION

13/00Z TAF Period

Initial concern will be for continued wildfire smoke resulting in some visibility reductions in addition to scattered showers and thunderstorms across SW MT and vicinity early this evening. Gusty and erratic winds and lightning are the main concern with these thunderstorms.

Additional shower activity moves northeastward over the plains late evening, coinciding with lowering ceilings. Confidence in exact placement of showers is on the lower side, hence there is relatively weak mention of precipitation in TAFs at plains sites. Low ceilings continue through the night and into the day Thursday over the plains. Additional shower and thunderstorm activity develops across SW MT and vicinity early Thursday afternoon and spreads northeastward into the evening.

For KGTF: Brisk northeast winds will continue through the evening, though this wind direction offers little in terms of cross-wind concerns. Lower clouds moving in this evening largely look to remain in an environment above 0C, so icing concerns remain low through the overnight. I cant quite rule out some outflow late evening, mainly after 02Z. The magnitude of this outflow is very questionable and will depend on how convective trends unfold over the next few hours further south. Showers will become more numerous later tonight into early Thursday, though confidence in specifics with respect to timing remains low. -AM

FIRE WEATHER

No fire weather statements will be issued at this time, but the main concern through this weekend will be lightning and thunderstorms that produce strong outflow wind gusts. Initially the thunderstorms will be on the wet side today through Friday, with drier thunderstorms possible over the weekend. Note any lightning that does occur away from the center of the storm will have a higher probability of starting a wildfire. Brusda

HYDROLOGY

A Flash Flood Watch remains in effect for the Horse Gulch Burn scar within the Big Belt Mountains from late this afternoon through early Thursday morning as the first round of showers and thunderstorms capable of producing heavy rainfall lifts northeast and over the area. Probabilities for heavy rainfall increase further (from that of today/tonight) over the burn scar on Thursday evening as PWATs increase across the Northern Rockies, and for this reason another Flash Flood Watch has been issued for the Horse Gulch Burn scar from 6PM Thursday through 12AM Friday. This second round of showers and thunderstorms, while shorter in temporal scale, is likely to produce the heaviest rainfall amounts of the work week, with a reasonable high end to rainfall amounts nearing 0.5" over this short timeframe. Additional opportunities for heavy rainfall will exist on Friday over the aforementioned burn scar, but for now we will hold off on issuing any additional Flash Flood Watches given uncertainty of where convection will ultimately track on Friday.

Showers and thunderstorms producing heavy rainfall are also possible over the on-going Bobcat Lakes and Sand Creek Wildfires on Thursday and Friday, with latest HREF guidance suggesting a 20% chance that rainfall amounts could exceed 0.25/hr over the freshly burned terrain within the Pioneer Mountains from Thursday afternoon and through the evening hours. These higher rainfall rates falling on the newly scarred terrain will have the potential to lead to debris flows, especially where the ground has become hydrophobic due to high intensity burning. For now we will holdoff on issuing any Flash Flood Watches for the Pioneer Mountains given the marginal probabilities, but should these probabilities or forecast confidence in heavy rainfall increase further then Flash Flood Watches are likely to be needed given the risk to property and personnel working on the fires. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 52 66 49 63 / 80 80 70 60 CTB 45 56 43 59 / 80 100 70 30 HLN 53 69 52 67 / 60 40 80 80 BZN 50 74 49 70 / 40 40 80 90 WYS 40 68 38 65 / 40 60 80 90 DLN 48 73 48 70 / 20 70 70 90 HVR 48 63 47 67 / 90 90 60 20 LWT 48 64 47 64 / 90 80 80 80

TFX WATCHES/WARNINGS/ADVISORIES

Flash Flood Watch until 4 AM MDT Thursday for Big Belt, Bridger and Castle Mountains.

Flash Flood Watch Thursday evening for Big Belt, Bridger and Castle Mountains.

Flood Watch Thursday afternoon for Big Belt, Bridger and Castle Mountains.

Flash Flood Watch Thursday afternoon for Northwest Beaverhead County.