textproduct: Great Falls

KEY MESSAGES

- Numerous showers and thunderstorms this afternoon, a few of which look to be on the strong side.

- Additional showers and thunderstorms Friday and Saturday ahead of a cooler day on Sunday with additional precipitation into Monday.

- Uncertainty remains in the exact track of the greatest precipitation amounts for this weekend into early next week.

DISCUSSION

Broad upper level troughing with embedded upper level low presently in the eastern Gulf of Alaska will continue to drift southeast toward the Pacific Northwest through the remainder of the day and through the end of the week. Flow aloft over the Northern Rockies becomes increasingly southwesterly as a result, with embedded waves ejecting from the approaching broader trough supporting several rounds of showers and thunderstorms through Saturday.

The first wave moves through today, though there looks to be a bit of a split regime from a parameter space perspective. Areas over the plains looks to have the greatest instability, while areas over Southwest Montana look to have the greatest shear. Where both of these better parameter spaces overlap this afternoon (Most likely between the MT-200 and I-90 corridors), there will be the risk for large hail in the strongest thunderstorms.

Forcing increases a bit heading into Friday as the core of the upper low approaches the Pacific NW. This forcing looks sufficient to result in plenty of cloud cover and showers around through the morning near the Continental Divide. This keeps confidence low with respect for any stronger thunderstorms developing in these areas by Friday afternoon. Areas further east have a better opportunity to receive some daytime heating, which will be the primary focus for afternoon thunderstorm development. While this occurs, stronger southwesterly flow aloft looks to nudge in from the south. This will result in gradually increasing shear from south to north through the afternoon. Should the instability develop in areas where the shear is able to move in quick enough, there will be a localized risk for a stronger thunderstorm, most favored across Central Montana east of I-15. Also worth noting will be the risk for brief heavy downpours with the strongest thunderstorms given anomalous moisture in place. Areas susceptible to impacts from briefly heavy rain should be aware of this risk, including areas around recent burn scars such as Horse Gulch.

The increasing southwesterly flow aloft mentioned prior will also result in a generally breezy day for most areas south of I-90 from late Friday morning through the afternoon. The risk for impactful gusts (58+ mph) appears low/localized at this time, however.

Additional showers and thunderstorms will be around again on Saturday as the core of the upper low begins to translate across the Northern Rockies. The risk for any stronger thunderstorm over the plains and adjacent areas of Central Montana will be conditional on how much surface moisture will be in place. Guidance that features higher Tds, closer to 55-60F, features ample instability and shear that would be capable of producing strong to severe thunderstorms. Guidance that keeps Tds closer to 50F keeps instability much lower and as a result would be result in a more run-of-the-mill type of thunderstorm day. All that said, there is a marginal risk for severe weather Saturday near and east of US-87.

There remains quite a bit of uncertainty with respect to the track that the most favorable forcing for stratiform type precipitation will take later weekend into next week. Most guidance keeps the most favorable forcing along the Rocky Mountain Front, including in Glacier NP. Latest trends in some guidance actually places the better forcing west of the Continental Divide by Sunday into Sunday night. The main takeaway will be for cooler conditions heading into Sunday, with snow levels falling furthest where the upper level low tracks. Cold/raw back country conditions will exist, particularly where the greatest precipitation falls.

General troughing holds through the middle of next week, which will keep things unsettled. Temperatures near to a touch below average with dally opportunity for showers and thunderstorms is a reasonable expectation at this point for this timeframe. -AM

AVIATION

25/18Z TAF Period

Primary concern this TAF period will be for numerous showers and thunderstorms developing across the region this afternoon and early evening. Gusty winds, hail, and lightning will be the primary concern from the strongest thunderstorms. Precipitation diminishes through the late evening and overnight before developing once again along the Continental Divide later tomorrow morning. Gusty southwest winds slowly develop late tomorrow morning across portions of Southwest Montana. -AM

PRELIMINARY POINT TEMPS/POPS

GTF 53 76 51 70 / 20 70 70 50 CTB 52 69 48 68 / 30 60 40 20 HLN 56 75 51 67 / 20 80 70 80 BZN 51 78 47 65 / 30 50 70 100 WYS 42 73 40 56 / 30 30 60 100 DLN 53 74 46 62 / 10 50 70 100 HVR 50 80 51 75 / 40 50 90 40 LWT 47 76 47 67 / 50 60 70 90

TFX WATCHES/WARNINGS/ADVISORIES

None.