textproduct: Great Falls

KEY MESSAGES

- Daily chances for isolated to scattered showers and thunderstorms through Friday.

- Hotter temperatures continue through Friday.

- Wetter widespread precipitation and cooler temperatures arrives Saturday through Monday

DISCUSSION

- Meteorological Overview:

The closed low over California and Nevada continues to linger through the weekend. This low will slowly phase towards the Intermountain West through the rest of the week. This will give a dominant southwest flow aloft pattern through Saturday, which will keep advecting in warm moist air for daily convection through Friday. Steep lapse rates and a dry surface layer will keep the primary threat for strong to severe wind gusts and smaller hail this afternoon and Thursday. Friday looks to have better instability and increasing shear during the day, which results in a slightly higher chance for severe wind and hail. Southerly flow will also keep the well above normal temperatures through Friday, with the hottest temps near Hill and Blaine Counties.

Saturday through the beginning of next week, another upper-level trough translate east across the Western U.S and interacts with the remnants of that weakened closed low. A low pressure system/cold front brings widespread precipitation to the region and the cold front also cools down temperatures Saturday. This trough lingers into the beginning of next week, which keep scattered to widespread precipitation and cooler temperatures through Monday. There are differences in the models on the interaction of the new trough and how much precipitation/where the corridor of best precipitation sets up. However, the main message is confidence is increasing for cooler and wetter conditions this weekend and Monday.

- Forecast Confidence & Scenarios:

Heat Today Through Friday:

Well above normal temperatures continue through Friday, especially across the Central Hi-line. The current Heat Advisory for Hill and Blaine Counties remain in effect through 9pm tonight. Although mid to upper 90s temperatures are forecasted for that area again Thursday and Friday, overnight lows will be slightly cooler. Therefore, I did not make any extensions to the current Heat Advisory.

Thunderstorm Risk Through the Weekend:

Current mesoanalysis shows instability has increased to 500 to 1,000 j/kg across the region with downdraft CAPE at 600 to 1,000 j/kg. Shear is low today, with up to 25 to 30kts effective. Therefore, thunderstorms are less likely to be well organized/sustaining, meaning chances for severe thunderstorms is low. However, with deep inverted V profiles from a dry surface layer, we can't rule out a few strong to an isolated severe wind gust. Thursday's thunderstorms look similar to today in hazards, but storm motions may be a little bit slower towards Central MT which can increase the risk slightly for localized flash flooding over burn scars.

Better instability and shear increases Friday, which will increase chances for marginal severe wind gusts and large hail. PWATs nearing an inch will bring a minor threat for localized flash flooding particular in burn scar areas/urban areas.

Saturday's thunderstorm threat will mostly be focused to the Hill/Blaine/Fergus County areas.

Weekend System Bringing Better Chances for Precipitation:

There are still some differences for how this closed low phases with the new incoming trough, therefore, there remains a good spread in precipitation amounts. Saturday through Sunday, there is an 80 to 90% chance for 0.25" of precipitation, 60 to 80% chance of 0.5" and a 40 to 70% chance for an 1", and an up to 15% to 30% chance for 2" inches. Snow levels look to drop to above pass level, which can bring light, slushy snow accumulations to the higher peaks. -Wilson

AVIATION

27/12Z TAF Period

VFR conditions will prevail across all terminals during this TAF Period. Showers and thunderstorms will begin to affect Southwestern Montana by around 27/18Z. Showers and thunderstorms will then move northward through the afternoon. By around 27/21Z showers and thunderstorms will begin to make it up to North-central Montana. All thunderstorms will produce frequent lightning and gusty, erratic winds. -IG

PRELIMINARY POINT TEMPS/POPS

GTF 85 55 91 57 / 20 20 20 30 CTB 83 52 88 53 / 50 30 20 40 HLN 85 53 90 54 / 20 20 20 20 BZN 84 47 89 48 / 20 20 20 20 WYS 79 39 82 40 / 20 20 10 0 DLN 79 45 84 46 / 30 20 10 0 HVR 95 55 97 57 / 20 30 20 20 LWT 85 52 89 53 / 20 20 20 20

TFX WATCHES/WARNINGS/ADVISORIES

Heat Advisory until 9 PM MDT this evening for Hill County- Northern Blaine County.