textproduct: Great Falls

KEY MESSAGES

- Hot temperatures persist through the rest of the day with a chance for isolated showers and thunderstorms this evening.

- Monsoonal moisture arrives Tuesday and Wednesday with more showers and thunderstorms expected; a few may produce heavy rainfall, gusty winds, and hail.

DISCUSSION

Today will be the last day of excessive heat for now. Temperatures will cool off Tuesday through Thursday as southwesterly flow starts to take over and bring more clouds and rain into the region. Towards the weekend, another upper-level ridge will start to build bringing the potential for more 90+ degree weather through next weekend.

Shower/Thunderstorm potential: There is a chance for isolated thunderstorms this afternoon and evening across portions of Southwest and Central Montana. These thunderstorms will be on the drier side towards Southwest Montana and trend wetter as they head north. The main concerns will be strong, gusty winds with small hail possible.

Things will trend wetter heading into Tuesday and Wednesday with the additional threat of heavy rain entering the picture with any shower or thunderstorm that develops. Hi-res guidance highlights the potential for heavy total rainfall and heavy rainfall rates across the region. With widespread precipitable water values in the 1-1.5" range, there is a concern for heavy rain and flash flooding with any thunderstorm that develops. Thus, a Flash Flood Watch has been issued for portions of North-Central, Central, and Southwest Montana. Poor drainage and urban areas as well as burn scars will be the most at risk for flooding. Those recreating outdoors should be prepared to seek higher ground if needed to avoid flood-prone areas.

Wednesday will have a similar set up but with less confidence on which areas will be at greatest risk, the decision was made to start with issuing the watch for Tuesday and seeing how future model runs play out to determine the threat level and spread for Wednesday. -thor

AVIATION

13/12Z TAF Period

A southwesterly flow aloft streams variable mid to high level clouds and moisture across the region with VFR conditions prevailing. Widely scattered high-based showers and thunderstorms develop across southwest MT in the afternoon with gusty showers/thunderstorms tracking northeast into central/north- central MT this evening. Generally light surface winds this morning become east to southeasterly this afternoon. Hoenisch

FIRE WEATHER

Initially high-based shower and thunderstorm development this afternoon will bring a risk for gusty downdraft/outflow winds as well as isolated lightning. High resolution model ensembles focus the greatest risk for lightning and isolated strong wind gusts along an axis from southwest through central MT late this afternoon and evening. Wetting precipitation chances with convection initially today remains low and recent heat/dryness has likely made vegetation in some areas more receptive to lightning ignition, especially across southwest MT. Hoenisch

HYDROLOGY

The influx of Monsoon-related moisture into the region by Tuesday will increase the risk for locally intense precipitation/heavy downpours with thunderstorm activity through Thursday. Precipitable water amounts of 1.50" or more are forecast, which is at or above the 90th percentile of climatology. There is a marginal (5-15%) risk for excessive rainfall across the area Tuesday and Wednesday with potential for precipitation rates that could cause localized flash flooding, particularly in sensitive/steeper terrain areas or burn scars. Hoenisch

PRELIMINARY POINT TEMPS/POPS

GTF 63 91 60 84 / 40 20 70 60 CTB 61 82 58 80 / 20 30 70 50 HLN 65 91 64 87 / 30 10 70 60 BZN 59 94 60 87 / 20 10 50 70 WYS 50 94 53 85 / 10 10 10 30 DLN 57 90 57 86 / 30 10 40 40 HVR 65 95 61 85 / 30 20 70 60 LWT 58 91 56 82 / 10 10 70 60

TFX WATCHES/WARNINGS/ADVISORIES

Heat Advisory until 9 PM MDT this evening for Bears Paw Mountains and Southern Blaine-Canyon Ferry Area-Cascade County below 5000ft-Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-Fergus County below 4500ft-Gallatin Valley-Gates of the Mountains-Helena Valley-Hill County-Judith Basin County and Judith Gap-Madison River Valley-Missouri Headwaters-Northern Blaine County-Southern High Plains-Western and Central Chouteau County.

Flood Watch from Tuesday afternoon through Tuesday evening for Bears Paw Mountains and Southern Blaine-Big Belt, Bridger and Castle Mountains-Canyon Ferry Area-Cascade County below 5000ft- Elkhorn and Boulder Mountains-Fergus County below 4500ft- Gallatin Valley-Gallatin and Madison County Mountains and Centennial Mountains-Gates of the Mountains-Helena Valley-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains- Meagher County Valleys-Missouri Headwaters-Snowy and Judith Mountains-Southern High Plains-Southern Rocky Mountain Front- Upper Blackfoot and MacDonald Pass-Western and Central Chouteau County.