textproduct: Great Falls

KEY MESSAGES

- Scattered thunderstorms this afternoon capable of strong wind gusts up to 80 mph and marginally large hail, mainly east of I-15 ahead of a cold front.

- Gusty winds behind a cold front this afternoon, with elevated fire weather conditions.

- Generally near or above average temperatures this week with a few days of scattered showers and thunderstorms.

DISCUSSION

- Meteorological Overview:

Potent upper level troughing moving across the region today will bring an array of impacts, including strong to severe thunderstorms and elevated fire weather conditions. A cold front will be moving through today, which will delineate the transition between a more thunderstorm focused risk and fire weather risk.

Ahead of the cold front ample moisture looks to coincide with daytime heating and forcing from the passing upper level disturbance to result in scattered showers and thunderstorms, mainly east of I- 15. Deep mixing and shear on the order of 30-40kts will support the risk for large hail with initial storms before the risk transitions to be more wind focused. Latest mesoanalysis features DCAPE in excess of 1200 J/Kg over the plains, which would support the risk for wind gusts approaching 80 mph in the strongest thunderstorms that form. While PWATs are still elevated, the deep inverted-V profiles will support a mix of wet and dry thunderstorms.

Behind the cold front a period of breezy west winds is forecast to develop in most areas. The lowest portion of the atmosphere dries out quite quickly behind the front, which will allow for relative humidity to continue falling behind the front, even as temperatures cool. The combination of the breezy and dry conditions, as well as the potential for a few dry thunderstorms brings about an elevated fire weather risk across most of the region.

Heading into Monday the upper system responsible for active weather today will exit eastward. Weak ridging builds in across the west in wake of this system, though with some lingering monsoonal moisture near the Idaho border between Monida and West Yellowstone, a few thunderstorms cannot be ruled out by late afternoon. The rest of the region looks dry Monday.

The upper level ridging more or less persists much of the upcoming week, but as it shifts slowly eastward an upper level trough approaching the Pacific NW will slowly allow for showers and thunderstorms to spread further and further northward across the region each day. -AM

- Forecast Confidence & Scenarios:

The main source of uncertainty today is with respect to how ongoing convection over the plains north of Great Falls will modify the convective environment this afternoon. On one hand this activity may limit destabilization in some areas where forcing is greatest. Conversely this activity and associated outflow may serve as a focus for additional convective initiation mid-afternoon, resulting in perhaps a greater areal extent of convective activity. ADditionally, if this activity persists, it could eventually break the cap and intensify and become the main area of concern across the Hi-Line. Nonetheless, any thunderstorm that forms this afternoon will be capable of strong wind gusts, and at least briefly large hail. -AM

AVIATION

23/18Z TAF Period

The primary concern during the TAF period is isolated showers and thunderstorms developing this afternoon and early evening along and east of I-15. Severe wind gusts and large hail will be possible with thunderstorms, with best chances for isolated 50-65kts gusts at KHVR and KLWT. Breezy west to southwest synoptic winds settle in this afternoon as well. Storms clear during the evening, which will help clear out clouds.

For KGTF: West winds of 10 to 20 kts with gusts up to 35 kts will continue through 24/04z. Thunderstorm chances remain low at KGTF this afternoon. However, there's a 30% chance an isolated storm initiates over the terminal with little to no warning. Strong winds and hail are the primary hazards with thunderstorms. -Wilson

FIRE WEATHER

A surface front will bring gusty westerly winds through the afternoon hours across the region. Widespread gusts over 35 mph are possible and will be enhanced by any thunderstorm activity that develops. Minimum RH values will be most critical in Southwest Montana where the RFW is currently in effect for Sunday afternoon and evening. Further north, conditions will be windy but not quite as dry. Additionally there is some disagreement in the latest models as to how the thunderstorms will develop and how widespread they will be. With this uncertainty in place the decision was made to hold off on issuing a RFW for North-Central Montana for now. But elevated to near-critical fire weather conditions will be a concern across the entire region. Where thunderstorms do develop, the concerns will be frequent lightning and gusty/erratic winds which may lead to new fire starts and make new and/or existing fires difficult to contain. -thor

PRELIMINARY POINT TEMPS/POPS

GTF 54 81 53 83 / 0 0 0 0 CTB 50 77 48 79 / 0 0 0 0 HLN 50 84 52 85 / 0 0 0 0 BZN 48 82 49 84 / 10 0 0 20 WYS 39 79 41 79 / 20 0 20 20 DLN 47 84 50 85 / 0 0 0 20 HVR 55 82 51 84 / 10 0 0 0 LWT 52 79 48 82 / 30 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES