textproduct: Glasgow

KEY MESSAGES

- More active weather is expected through the weekend with heavy rain possible, damaging wind, and large hail. A Lake Wind Advisory is in effect from 4 PM today through 4 AM Saturday.

- A Slight Risk of Severe thunderstorms is outlooked for central Montana today, an Enhanced risk for eastern Montana on Saturday, and Marginal Risk in the east on Sunday.

- Continued unsettled conditions expected into next week as temperatures gradually rise.

DISCUSSION

WEATHER PATTERN OVERVIEW: Upper trough digging into the Pacific Northwest will push eastward through the weekend with a closed low expected over Montana by Monday. The trough pattern is expected to be strongest this weekend, which is part of why there is severe weather expected to occur for various parts of the area through Sunday evening. Lee trough and surface cyclogenesis is currently transporting warm, moist air into Montana in the southeast low level flow. Combined with southwest flow aloft creating vertical shear, forcing and destabilization along the front, there is support for organized severe thunderstorms after 4 PM this evening and going through about midnight. Round 2 of severe weather is expected in the eastern part of the area tomorrow, and then round 3 for Sunday in the same area. On Saturday, the uncertainty lies in how the low clouds evolve in the morning. If they burn off with surface heating early in the day, severe weather is more likely. If they stay into the afternoon, it is less likely for severe weather to occur. There is uncertainty on the shortwave trough passage for Sunday, so timing is also uncertain for severe storm activity.

High precipitable water and surface dewpoints suggest heavy rain potential in individual storms for Saturday. Although storms are expected to move somewhat fast, training of storms is possible, which means that storms may form over the same location more than once. Rain totals often accumulate enough to create rises in creeks and streams temporarily with this type of activity.

The upper trough will move over Montana on Sunday and Monday with temperatures cooling. Winds will increase on the back side of the system with cold air advection.

With the upper trough moving east by midweek, temperatures will be on the rise into the holiday weekend. Fairly persistent weak trough over the Pacific Northwest will support occasional chances for showers and storm action.

FORECAST CONFIDENCE & DEVIATIONS: Increased surface winds for this evening into tomorrow with the oncoming surface low. The models often struggle with recognizing the strength of the surface spin.

There is high confidence in the Lake Wind Advisory for tonight, and that another one will be needed for Sunday and Monday.

There is moderate confidence of severe weather occurring in the southwest this evening, in the eastern half of the area on Saturday, and low confidence on Sunday in the east. There is a low to moderate of damaging winds and hail occurring with these storms, and an isolated tornado.

There is moderate confidence in rainfall amounts through Monday and in temperatures climbing above average in early July.

AVIATION

UPDATED: 1830Z

FLIGHT CATEGORY: VFR through 00z, then scattered MVFR with thunderstorms, then areas of IFR after 06Z, and then areas of MVFR after 18Z.

DISCUSSION: An upper trough is positioning itself over the area through Monday, and strong southwest flow will allow off and on showers and thunderstorms to occur through 06Z Sunday. There is high confidence of showers and thunderstorms occurring after 02Z this evening for all sites. There is low confidence of severe weather occurring for KGGW from 03Z to 07Z Saturday. There is moderate to high confidence of IFR ceilings for KGGW, KOLF, and KSDY from 06Z to 20Z Saturday, and similar confidence of MVFR in this period for KGDV. There is moderate uncertainty on when the low clouds will lift on Saturday afternoon.

WINDS: Southeast at 10 to 20 knots, slowly switching to west after 18Z Saturday. There is a moderate (50 percent) chance of winds being quite different from these base directions after 18Z with a surface low expected to move through, especially for KGDV and KSDY.

GGW WATCHES/WARNINGS/ADVISORIES

Lake Wind Advisory from 4 PM this afternoon to 4 AM MDT Saturday for Central and Southeast Phillips-Central and Southern Valley- Garfield-McCone-Petroleum.