textproduct: Great Falls

KEY MESSAGES

- Afternoon and evening showers and thunderstorms many days this week, a few of which look to be on the stronger side.

- Very warm temperatures most days this week.

DISCUSSION

- Meteorological Overview:

Upper level troughing with embedded closed low is shifting southeastward off the BC coast toward the Pacific NW this afternoon, eventually becoming cutoff over CA/NV mid-week. Multiple waves ejecting out of this troughing within a southerly flow aloft through the week will result in several rounds of primarily afternoon and evening showers and thunderstorms. At this point Tuesday, Thursday, and Friday are most favored for at least isolated strong to severe thunderstorms. The probability for stronger thunderstorms this afternoon/evening and Wednesday are only slightly lower.

The other primary concern through the week will be warm temperatures. Daytime highs Tuesday across the plains east of I-15 look to peak in the upper 80s to mid-90s. While this is certainly on the hot side for this time of the year, these temperatures only look to be around for a day before slight cooling into Wednesday. Given this is the case, we are holding off on a Heat Advisory for Hill/Blaine counties at this time (Where temperatures look warmest).

The cutoff low eventually gets re-absorbed in the jet stream late week, lifting north across the region. How quick this occurs will dictate how long precipitation will linger into the weekend across the Northern Rockies. -AM

- Forecast Confidence & Scenarios:

Strong to severe thunderstorms this week:

The main limiting factor to organized severe weather this week looks to be shear. Ample moisture working in from the east at the surface and aloft from the southerly flow ahead of the upper low will result in a favorable thermodynamic environment most days. The issue is that flow aloft does not appear to be overly strong most days. This will limit shear that develops, keeping the risk for a more organized persistent severe threat lower (Think supercell thunderstorms). That said, marginal shear, ample instability and sufficient DCAPE will result in at least a gusty wind threat most days, in addition to an occasional hail threat as well.

As better moisture works in through the week, rainfall rates will begin to climb in the strongest thunderstorms. Areas with concerns for higher rainfall rates - such as burn scars - should be aware of this increased risk of excessive rainfall. -AM

AVIATION

25/18Z TAF Period

Main concern will be increasing chances for showers/thunderstorms this afternoon. Expect the storms to develop in Southwest MT by mid afternoon and slowly move northward into North Central MT by evening. Gusty surface winds will be the main impact today. Mountains, passes will be obscured at times by passing showers/thunderstorms.

Looking ahead...showers/thunderstorms are possible everyday this week, with some storms be severe later this week. Brusda.

PRELIMINARY POINT TEMPS/POPS

GTF 54 88 55 82 / 20 40 60 30 CTB 51 88 53 79 / 30 30 60 60 HLN 50 83 52 80 / 30 40 50 60 BZN 47 80 46 81 / 20 60 50 60 WYS 39 70 38 77 / 20 70 30 20 DLN 46 76 44 77 / 40 50 10 60 HVR 57 95 59 93 / 0 10 40 30 LWT 52 87 52 83 / 20 20 30 50

TFX WATCHES/WARNINGS/ADVISORIES

None.