textproduct: Great Falls

KEY MESSAGES

- Flash Flood Watch remains in effect for the Pioneer Mountains through this evening.

- Temperatures moderate through the weekend and early next week, but remain below to near normal.

- Shower and thunderstorms chances will persist through the middle of the upcoming work week, most notably through tonight and then again Monday afternoon through Tuesday afternoon.

DISCUSSION

- Meteorological Overview:

Moist, southwest flow aloft and overall north-to-east low level flow will help to maintain precipitation chances and below-to-near normal temperatures through the middle of the upcoming work. Over this stretch Sunday and Monday will be the closest to normal, with respect to high temperatures, as surface winds briefly become southwesterly and supporting some "warming" as compared to the Saturday, Tuesday, and Wednesday. Monsoonal moisture lifting north and east within the southwest flow aloft will maintain daily chances for showers and thunderstorms, most notably from Monday afternoon/evening through the Tuesday afternoon. While thunderstorms from this weekend through the middle of the work week will generally be of the garden variety a few stronger storms producing strong wind gusts can't be ruled out as low level moisture gradually erodes leading to inverted-V soundings, especially across Southwest Montana. - Moldan

- Forecast Confidence & Scenarios:

Flooding risk through this evening:

12z Hi-Res Ensemble Forecast (HREF) guidance supports a 20-30% chance that hourly rainfall rates could exceed 0.25"/hr from the late afternoon through evening hours tonight over the burn scars across the Pioneer Mountains, with these probabilities appearing reasonable given morning observations of between 0.1"-0.3" across Southwest through North Central Montana. Given that the threshold for flash flooding is so low over the active fires in the Pioneers, and that they have already received a wetting rain over the previous 24 hours, a Flash Flood Watch remains in effect through 9 pm this evening.

Concerns for flash flooding on the the Horse Gulch Burn Scar remain lower than that of the Pioneer Mountain Burn Scars given a lower probability for hourly rainfall rates to exceed 0.5"/hr here, with these probabilities being around a 10% chance. While flash flooding can not be ruled across the Horse Gulch Burn Scar through the evening hours tonight confidence in any flooding remains too low to issue a Flash Food Watch. -Moldan/AM

AVIATION

14/18Z TAF Period

Scattered showers and thunderstorms will continue through this evening, resulting in lower ceilings and visibilities throughout the beginning of this TAF period. Ceilings will begin to rise for KBZN, KEKS, KWYS and KHLN around 14/05Z, while the other terminals will continue to see MVFR ceilings throughout the rest of this TAF period. KGTF and KLWT will have VLIFR conditions due to fog starting around 15/07Z. The fog is expected to lift around 15/11Z for KGTF, while it will continue in KLWT until around 15/15Z. Mountain obscuration will occur across North-Central, Central and Southwestern Montana throughout this TAF period.

For KGTF: Low ceilings and visibilities will dominate the beginning of this TAF period due to passing showers through the evening. The showers are expected to dissipate by 15/02Z, and while unlikely, we cannot rule out an isolated thunderstorm this evening. While visibilities will increase later this evening, MVFR ceilings will remain throughout the TAF period, with the exception of a period of IFR conditions 15/07Z - 15/11Z due to fog. Light winds will create a near zero probability of a 15 kt cross-wind. Icing remains a concern for the afternoon, as forecast soundings continue to feature at least a shallow layer of clouds in the 0C to -15C range until the early evening. -Dzomba

HYDROLOGY

A Flash Flood Watch remains in effect for the Bobcat Lakes and Sand Creek Burn Scars in the Pioneer Mountains through the evening hours tonight. Given the intensity of burning that occurred through much of the past week soils are likely to be hydrophobic, which combined with the steep and complex terrain of the Pioneer could lead to flash flooding and debris flows. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 51 77 51 84 / 50 10 10 10 CTB 44 73 46 78 / 20 10 10 10 HLN 50 77 51 82 / 40 30 30 10 BZN 46 76 46 79 / 60 30 50 0 WYS 34 71 34 74 / 70 40 60 10 DLN 46 73 45 77 / 40 70 50 10 HVR 49 77 49 85 / 50 10 20 0 LWT 49 72 50 78 / 60 30 50 20

TFX WATCHES/WARNINGS/ADVISORIES

Flash Flood Watch until 9 PM MDT this evening for the burn scars in the Pioneer Mountains.