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Area Forecast Discussion...CORRECTED TIME National Weather Service Great Falls MT 520 AM MDT Fri May 3 2024

UPDATED AVIATION SECTION

SYNOPSIS

One more cool and showery day is expected today before milder temperatures move in for the weekend. Another cool and wet Pacific weather system impacts North-central and Southwest Montana Sunday through at least mid next week, with the most significant precipitation and coolest temperatures expected Monday through Wednesday.

DISCUSSION

The latest satellite and meso-analysis shows abundant cloud cover within a northwesterly flow aloft. Light snow showers have been ongoing all night, mostly over the northerly upslope areas adjacent to the Little Belt, Highwood, and Snowy Mountains. This shower activity will more or less persist through this afternoon with the continued northwesterly flow, though accumulation amounts will generally be negligible. Transient ridging builds in tonight for clearing skies with temperatures bottoming out in the 20s for most locations. Patchy fog development can be expected in spots, but shouldn't be a widespread problem. With one more day of ridging aloft, Saturday should be the driest day over next week while temperatures climb to near of slightly above average.

Deep southerly flow aloft develops over Southwest MT Saturday night ahead of a closed low moving onto the Pacific NW coast. This system will trek eastward into S ID on Sunday, placing the Northern Rockies on the ascent side of the system for northward expansion of shower and isolated thunderstorm activity, though temperatures will remain near average for one more day. H700 flow slows down by Sunday afternoon and MU CAPE looks to be around 100 to 300 J/kg (highest southwest MT), so not expecting anything beyond general thunderstorm risks at this time.

Ensembles remain in decent agreement with the aforementioned closed low swinging NE into E MT and slowing down some Monday through early Thursday. Although, there are differences on the precise path of the low, most favor cool, wet, and windy periods during this timeframe. According to the NAEFS Standardized Anomaly, H700 temperatures are forecast to be around -8C for much of this event, which would support mountain accumulating snow, but mostly rain at lower elevations. Current probabilistic forecasts for rainfall highlight the eastern portions of North-central MT with the highest chances (50 to 70%) for 1 inch of rainfall during the 72 hr period ending next Thursday at 6 am, with nearly all locations in the CWA standing at least a 50% for a quarter inch of rainfall for the same time period. The one drier exception looks to be southwestern valleys from Dillon to the Three Forks area.

As mentioned earlier, thermodynamic profiles should confine accumulating snow to mountain areas, with the Little Belts and the Madison/Gallatin ranges seeing 60% + chances for 8 inches of snow or more Monday through Thursday, with most of it falling Tuesday night into Wednesday. H700 flow on the backside of this low will be at or above the 30 to 40 kt range for an extended period, resulting in gusty to at times strong northwesterly surface winds.
While all days Monday through Wednesday look to be on the windier side, Tuesday is being singled out by the EC Extreme Forecast Index and NBM probabilistic forecasts as the windiest day overall, with many locations seeing 70% + chances for wind gusts above 40 mph.

Drier and milder conditions move in for Thursday and Friday of next week, though around 50% of the ensembles support weaker troughing remaining in place for non-zero chances for precipitation and near average temperatures heading into next weekend. - RCG

AVIATION
520 AM MDT Fri May 3 2024 (03/12Z TAF Period)

North Central (KCTB KHVR) and the plains of Central Montana (KGTF KLWT): Northerly upslope flow will continue widespread mountain-top obscuring low VFR/high MVFR ceilings through at least 15Z. However, lingering light snow along north-facing slopes (mainly east of KGTF into the KLWT area) will keep conditions there as IFR, obscuring those mountains. High pressure will start moving into the area from the west after 15Z, decreasing the snow and allowing broken ceilings to lift to at least low VFR levels with partial clearing starting after 18Z. However, weak instability during the afternoon may cause isolated to scattered light snow showers to form again between 19Z and 00Z, possibly causing periods of MVFR conditions.

Southwest Montana (KBZN KEKS KWYS): VFR conditions are expected to continue through at least 04/12Z. Broken ceilings will continue through at least 18Z, then partial clearing is expected after 18Z.
-Coulston

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

PRELIMINARY POINT TEMPS/POPS
GTF 50 26 63 42 / 30 0 0 10 CTB 49 26 59 37 / 20 0 0 10 HLN 54 31 68 44 / 10 0 0 10 BZN 51 28 66 41 / 10 0 0 10 WYS 46 20 60 36 / 0 0 0 30 DLN 52 30 65 44 / 10 0 0 40 HVR 50 27 63 39 / 40 0 0 0 LWT 42 23 59 37 / 40 0 0 10

TFX WATCHES/WARNINGS/ADVISORIES
None.

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