textproduct: Great Falls

KEY MESSAGES

- Cumulative days of hot temperatures will maintain an elevated risk for heat related health affects today. - Increasing clouds and moisture this week will moderate temperatures some and bring a daily risk for showers and thunderstorms. - In addition to strong gusty winds and hail with some thunderstorms this week, locally intense precipitation becomes a concern with storms Tuesday night through Thursday.

DISCUSSION

/Issued Issued by National Weather Service Missoula MT/

The strong mid-level ridge of high pressure that brought record heat to much of the state over the weekend shifts slightly east and south this week. The Northern Rockies and MT remain on the northwest periphery of the ridge however for continued warmth with some moderation as temperatures aloft have peaked and moisture circulating around the ridge brings increased cloud-cover.

Afternoon temperatures today will still be hot, but around 5 degrees lower than yesterday and generally a couple of degrees below daily records. A heat advisory continues today considering the cumulative affect of several days of afternoon heat and mild overnight temperatures with more noticeable moderation of temperatures anticipated by Tuesday.

A plume of moisture circulating around the ridge with sub-tropical connections is already evident on satellite imagery streaming north across the Great Basin and Northern Rockies and this feed of moisture is expected to deepen and expand across the region by the middle of this week. Also of note is an unseasonably continuous upper level jet on the NW periphery of the ridge, enhanced at times by Pacific energy lifting across the Pacific NW into BC.

Shower and thunderstorm coverage today is likely to be limited by the still dry lower levels, but this will also create favorable conditions for stronger wind gusts with widely scattered convection lifting from SW MT late this afternoon through north- central MT this evening. Shower and thunderstorm coverage increases on Tuesday with lingering moisture likely to maintain daily shower/thunderstorm activity later this week into the weekend, though confidence in daily risk/coverage decreases beyond Wednesday. The deep moisture and presence of moderate flow aloft will support a risk for both organized storms (strong winds/hail) and storms with locally intense precipitation rates Tuesday and Wednesday. Hoenisch

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 95 63 90 61 / 20 40 30 80 CTB 89 63 83 59 / 10 20 50 60 HLN 95 66 91 65 / 20 30 30 70 BZN 96 60 93 61 / 20 20 20 50 WYS 95 51 94 53 / 0 10 10 10 DLN 93 58 90 58 / 10 30 40 60 HVR 96 66 94 62 / 10 30 40 80 LWT 92 59 90 57 / 0 10 20 70

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.