textproduct: Great Falls

KEY MESSAGES

- A few storms may produce localized strong, gusty winds and even some instances instances of brief downpours capable of localized flooding, mostly over central and southwest Montana.

- Some patchy fog will develop again by early Sunday morning, mostly for the southwest valleys and the river valleys over the plains.

- Daily rounds of showers and thunderstorms will continue through mid- week, most widespread Monday evening through early Tuesday.

- Seasonable temperatures will climb back above average towards the end of next week with shower and thunderstorm activity increasing once again next weekend.

DISCUSSION

- Meteorological Overview:

A moist southwesterly flow aloft will become more westerly and undergo some drying heading into early next week. This drying will bring increased breeziness and lowering minimum relative humidity values. Daily scattered shower and thunderstorm activity will continue through mid-week, most widespread during a shortwave passage late Monday into Tuesday.

Ridging aloft strengthens over the Great Basin with heights rising as high as the 588 to 594 dm range over the Northern Rockies by Friday. This will reduce shower and storm coverage and push afternoon temperatures back to the upper 80s to near 90 while seasonal breeziness continues.

A Pacific trough approaches the region sometime next weekend and brings increased shower and thunderstorm activity with the warmest temperatures of week looking to occur just ahead of this system. This will be followed by breezy to windy conditions and slightly cooler temperatures. - RCG

- Forecast Confidence & Scenarios:

Daily rounds of showers and thunderstorms through Tuesday...

Scattered showers and thunderstorms are already developing over S ID with W MT not far behind. This activity will move toward the northeast into central and southwest MT and interact with peak ML CAPE generally running between 500 and 1000 J/kg and effective shear above 20 kts. With surfaces beginning to dry out, localized strong, gusty winds, lightning, and perhaps some small hail will be the primary hazards, but PWATS as high as 0.80 inches may bring some heavier downpours capable of localized flooding, particularly for the more sensitive burn scars. No flood products are planned for today given the isolated nature of the heavier downpours, but conditions will be closely monitored through the evening hours. Showers and storms will weaken in coverage and intensity as they move northward into north-central MT later this evening into the overnight hours.

Westerly flow aloft will begin to thin the moisture profile Sunday and Monday, but will maintain at least similar shear and instability for continued scattered shower and thunderstorm activity. Dryer sub- cloud layers will increase instances of strong to severe wind gusts while decreasing the localized flooding threat. All locations will be subjective stronger storms, although southwest MT and areas along the Hi-line will be most susceptible. Showers and storms will be most widespread during a shortwave passage late Monday into Tuesday, although the timing of the trough and attendant cold front will determine whether this activity will be able to take advantage of the peak diurnal instability for stronger storm initiation. - RCG

AVIATION

15/18Z TAF Period

Low CIGS over the plains and valleys of Central and North Central Montana will continue to slowly lift through 21z this afternoon, with otherwise low-VFR/VFR CIGS expected through 03-09z Sunday before CIGS lower once again and patchy fog develops over portions of Southwest through North Central Montana. Showers and thunderstorms will develop through 21z this afternoon across Northeast Idaho and Southwest Montana before lifting northeast across the remainder of Southwest and portions of Central Montana. Main threat from these storms will be strong winds gusts and the potential for heavy rainfall.

For KGTF: Light surface winds through the duration of the TAF period will yield a low risk of crosswinds exceeding a 15 kt crosswind; however, thunderstorms lifting northeast into Central Montana from 02z-06z have the potential to produce outflow boundaries that could briefly exceed the 15kt threshold. The probability for these thunderstorms is around a 30% chance. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 77 51 83 52 / 0 20 30 30 CTB 71 46 77 47 / 0 10 20 20 HLN 78 50 82 53 / 20 80 20 30 BZN 77 47 79 48 / 20 60 30 40 WYS 72 34 75 35 / 50 80 20 20 DLN 74 44 77 46 / 70 70 30 30 HVR 76 49 84 50 / 0 30 20 30 LWT 73 48 79 49 / 30 50 30 30

TFX WATCHES/WARNINGS/ADVISORIES

None.