Marine Weather and Tide Forecast for South Hills, MT

DISCUSSION
/Issued 226 PM MDT Sun Apr 20 2025/

- Meteorological Overview:

Moist, unstable northwest flow over the Northern Rockies through Tuesday afternoon will help to maintain daily chance for rain/snow showers (and even a few thunderstorms) across all of Southwest through North Central Montana, with temperatures hovering near to slightly below normal across these regions. By Tuesday afternoon/evening the upper level flow will begin to back to west and eventually southwest as a compact shortwave digs from the Pacific Northwest to over the Great Basin through Wednesday morning. This shortwave will help to re-focus isolated to scattered rain/snow across Southwest and portions of Central (i.e. south of the Montana Hwy 200 corridor) Montana throughout the timeframe. By Wednesday afternoon/evening another shortwave digging southwest from British Columbia/Alberta will begin to phase with the shortwave over the Great Basin, which will help to support some (albeit limited) chance for showers further north into the remainder of Central and portions of North Central Montana. Transient ridging will then begin to slide east from the western seaboard from Thursday through Saturday, with this ridge helping temperatures to moderate on Thursday before climbing above normal for Friday and Saturday. - Moldan

- Forecast Confidence & Scenarios:

Thunderstorms thru Monday :

General thunderstorms producing gusty and erratic winds and small hail are possible through the timeframe, most notably from the late morning through early evening hours each day when solar insolation will be the strongest. While an isolated stronger thunderstorm producing marginal hail (i.e. 1/2" to 3/4") is possible given low freezing levels and a deep hail growth zone the overall lack of CAPE and shear will keep this shallow topped convection disorganized. None-the-less, accumulating small hail notorious beneath this springtime pattern, along with brief bursts of moderate to heavy rain given the moderately tall and skinny CAPE profile. - Moldan

Snowfall through Tuesday:

Overall, compared to previous forecasts there were no significant changes in the expected snowfall amounts. Most places along the Continental Divide, island ranges of central Montana and the mountains of southwest Montana will see 1 to 3 inches of new snow.
Confidence for amounts above 3 inches drops off pretty quickly with the probability of 4 inches or more sitting at less than 60% and the probability for 6 inches or more at less than 30%. When evaluating the potential for headlines, the heaviest snow is expected to be confined to the highest elevations and far above pass level. Thus there is no need for an advisory at this time but those in the backcountry should be prepared for difficult conditions over the next couple of days.

Rain through Tuesday:

Lower elevations across much of the region have a chance at seeing rain through Tuesday afternoon. The heaviest amounts are expected from this evening through Monday afternoon, with the highest storm total amounts expected in southwest Montana. Most locations are likely to see at least some precipitation, however the Rocky Mountain Front and portions of north-central Montana are expected to remain largely dry through this event with precipitation amounts above a tenth of an inch more likely south and east of a line from Havre to WSS to Helena. This same region also has around a 50% chance of a quarter inch or more of precipitation through Tuesday afternoon. Isolated higher amounts are also possible under heavier showers and thunderstorms. -thor

AVIATION
21/00Z TAF Period

A weakly unstable westerly flow aloft will continue scattered showers with a few lingering thunderstorms over North Central, Central, and Southwest Montana through at least 09Z, possibly with some periods of MVFR conditions. While activity will mostly decrease across the north, scattered rain and snow showers will persist across the south through around 12Z. The next push of moisture and instability will bring another round of scattered showers and isolated thunderstorms again after 18Z. Have not yet included mention of thunderstorms in this period of the TAFs, as there remains uncertainty as to whether these isolated storms will directly impact the terminals. The westerly flow aloft will also increase through 22/00Z, which will continue the threat for mountain wave turbulence, as well as gusty erratic winds with showers and thunderstorms. -Coulston

Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information.

PRELIMINARY POINT TEMPS/POPS
GTF 38 54 32 53 / 50 60 20 30 CTB 33 50 29 50 / 30 50 40 30 HLN 38 53 29 55 / 50 60 10 10 BZN 33 49 22 51 / 90 90 20 10 WYS 27 46 16 48 / 70 80 20 10 DLN 33 51 23 52 / 60 50 10 0 HVR 34 56 32 52 / 50 70 60 30 LWT 35 50 29 48 / 80 90 50 30

TFX WATCHES/WARNINGS/ADVISORIES
None.