textproduct: Great Falls

KEY MESSAGES

- The shallow Arctic airmass will continue to slowly retreat northeastward tonight, with mild temperatures returning to all areas by Sunday.

- Winds increase and become strong along the Rocky Mountain Front on Sunday and spread over portions of the central and north- central Montana on Monday.

- A strong Pacific front looks to bring widespread strong, gusty winds and locally intense snow showers on Wednesday.

- The Arctic air begins to sag southward heading into next weekend, although it's southern extent remains uncertain at this time.

DISCUSSION

- Meteorological Overview:

Ridging aloft gradually replaces the current cold northwesterly flow with the shallow Arctic air continuing to thin and retreat northeastward through Sunday morning. Despite the ridging aloft and well above normal temperatures during the first half of the week, westerly flow aloft will remain strong and offer multiple opportunities for strong, gusty surface winds, most widespread Monday and again on Wednesday.

Monday's winds will be driven by 55 to 75 kt H700 flow ahead of an approaching shortwave in addition to a tightening surface pressure gradient and mountain wave activity. On Wednesday, a potent Pacific shortwave and attendant cold front look to bring cold air advection and deep layer mixing while H700 flow exceeds 75 kts. The sharp frontogenesis with this system quickly drops H700/H500 temperatures to around -15C/-30C behind the front, which may also encourage locally intense snow shower/squall development in addition to adding support for strong winds. This proposed event is still four days away and things could change, but it bears watching.

The Arctic air begins to sag southward during the second half of the week with the return of colder westerly flow aloft; however, the southward trajectory looks to be weak, resulting in northern areas getting clipped with the colder air at times while milder air remains in place in the southwest. The passage of shortwaves will maintain periods of mostly mountain snow during this time. - RCG

- Forecast Confidence & Scenarios:

Winds Sunday through Wednesday...

Periods of strong, gusty winds will accompany the return of milder conditions through mid- week. Winds initially increase and become strong along the Rocky Mountain Front as early as Sunday morning. H700 flow exceeding 70 kts combined with localized mountain wave activity looks to bring at least a brief period of winds approaching 80 mph in the East Glacier/Browning areas where probabilities for the high wind criteria gusts are now topping around 70%. Given this probability is for the higher 75 mph criteria, a high wind watch has been posted. Other north-central MT locations look to see winds mostly gusting in the 25 to 45 mph range on Sunday.

There is some potential for blowing/drifting of snow with the onset of the gusty west/southwest winds on Sunday. This would initially affect areas along the Rocky Mountain Front and areas northeast of the Little Belts and Highwoods by Sunday afternoon. Fortunately, above freezing temperatures are likely to settle the snowpack across most areas before strongest winds develop Sunday night and Monday.

An approaching shortwave will bring stronger and more widespread winds Monday through early Tuesday. While most central and north- central locations will see widespread gusts in the 35 to 55 mph range, the Rocky Mountain Front and high plains west of I15 currently have up to a 90% chance for gusts exceeding 55 mph and a 70 to 80% chance for 75 mph gusts or greater. Another area to closely watch will be the central MT highway 87/200 corridor where the combination of mountain wave activity and strong flow aloft push the chances for gusts exceeding 55 mph to the 40 to 60% mark. High wind watches were sent out for these normally higher wind prone areas, including the Rocky Mountain Front mountain/plains zones, eastern Toole/Liberty zone, and the lower elevations of Cascade and Judith Basin counties. Other areas may be added by incoming shifts as confidence increases.

On Wednesday, a much stronger and dynamic Pacific trough and attendant cold front will bring an opportunity for widespread strong winds and scattered locally intense snow showers/squalls. Deterministic models seem to become more and more aggressive with this system as time goes by. As of this writing, it checks nearly all the boxes for a high impact wind event across north-central and southwest MT. It looks to have a vertically stacked wind profile greater than 70 kts, strong cold air advection, and timing near the time of peak diurnal mixing. widespread strong to damaging winds can be expected should this event unfold as currently advertised. It's important to note that this event is four days away and before getting overly excited I'd like to see more run to run consistency among ensemble members. Some caveats that could weaken or preclude the winds include a stronger/faster Canadian front undercutting the strong winds aloft, models over forecasting upper level flow, and frontal passage timing shifts. This one will be closely monitored over the next few days. - RCG

AVIATION

13/18Z TAF Period

VFR conditions are expected to persist through the period. Low clouds will gradually lift across the region by 13/22Z with SCT to BKN high level clouds continuing through the rest of the period. Southwest winds start to pick up after 14/06Z across the northern plains (KCTB, KHVR, KGTF) with instances of LLWS possible at KCTB and KHVR. -thor

PRELIMINARY POINT TEMPS/POPS

GTF 16 49 39 55 / 0 0 0 0 CTB 15 46 33 51 / 0 0 0 10 HLN 27 49 31 54 / 0 0 0 0 BZN 30 52 28 53 / 0 0 0 0 WYS 14 44 14 39 / 0 0 0 0 DLN 31 53 29 53 / 0 0 0 0 HVR 0 44 30 51 / 0 0 0 0 LWT 22 49 34 50 / 0 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Watch from Sunday morning through Sunday evening for East Glacier Park Region-Northern High Plains.

High Wind Watch from Monday morning through Tuesday morning for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Eastern Toole and Liberty-Northern High Plains- Southern High Plains-Southern Rocky Mountain Front.

High Wind Watch from Monday afternoon through Tuesday morning for Cascade County below 5000ft-Judith Basin County and Judith Gap.