textproduct: Great Falls

KEY MESSAGES

- Strong winds develop and become widespread across much of north-central and southwest Montana tonight through Tuesday morning.

- Widespread above normal temperatures continue through Wednesday.

- Precipitation is mainly focused along the continental divide through Tuesday where snow levels fluctuate at or above 6000 ft.

- Precipitation and the potential for accumulating snowfall becomes more widespread across north-central Montana later this week as a colder airmass also moves back south into the region.

DISCUSSION

- Meteorological Overview:

A very active weather pattern continues through the week as a strong westerly flow and plume of deep Pacific moisture remains oriented across the Northern Rockies and MT with embedded disturbances bringing multiple rounds of precipitation, initially focused along the continental divide, but expanding further east across central and north-central MT later this week. Meanwhile, mild Pacific air will maintain above average temperatures and relatively high snow-levels through the next few days before a much colder airmass surges back south into north-central MT later this week.

Mid-level westerly flow increases further tonight as a shortwave disturbance rapidly translates east from BC to AB with surface low pressure emerging in southern AB and tracking across southern SK Tuesday morning. Strong winds re-intensify this evening along the Rocky Mtn Front with the arrival of 70-80kt 700mb winds and spread east across north-central and central MT overnight tonight through Tuesday morning. Temperatures will remain mild tonight with many lower elevation areas seeing steady temperatures in the 40s as winds increase and thick cloud-cover persists. Precipitation remains mainly confined to areas along the continental divide through Tuesday morning where snow levels currently around 6000 ft rise to 7000 ft or more overnight, limiting any impactful snowfall to the highest terrain above passes.

Winds shift more northwesterly on Tuesday behind the surface low across eastern portions portions of north-central MT with enough cooling aloft and weak instability present to support some convective shower development, mainly across Hill/Blaine/Fergus counties. Temperatures cool just enough in these areas Tuesday afternoon to support snow or a mix of rain/snow with main impact being brief visibility reduction in these showers.

Another embedded shortwave moves across the Northern Rockies Wednesday with snow levels remaining high along the continental divide where the bulk of the precipitation occurs, while a period of lighter rain/snow shifts across the plains early Wednesday before westerly flow spreads mild but windy conditions back east across the area.

A colder airmass begins to spread back south into north-central MT Wednesday night through Thursday with the colder air lingering through at least Saturday morning before retreating back north. Meanwhile, additional shortwave disturbance move across the region with an east west frontal zone aloft becoming a focus for precipitation Thursday and Friday. Latest model guidance trends are supporting the potential for several inches of snow accumulation across portions of north-central and central MT Thursday and Friday with still a fair amount of uncertainty in specific amounts. Hoenisch

- Forecast Confidence & Scenarios:

Ingredients for mountain wave enhancement are focused mainly along the Rocky Mountain Front this evening, shifting to areas adjacent to the Little Belts across Judith Basin county late tonight through Tuesday morning. Wind gusts in excess of 75 mph are likely along the Rocky Mtn Front with a 30-40% chance of wind gusts reaching this magnitude across Judith Basin County, where winds of this magnitude are not as frequent and may be more impactful than those along the Rocky Mtn Front.

The arrival of colder air later this week has seen some fluctuations in timing with still considerable uncertainty in how far south the cold air will penetrate beyond central MT. Most model guidance keeps the colder airmass limited to the plains. By Friday it is possible that locations on the plains see afternoon temperatures in the single digits while southwest MT valley locations remain above average with afternoon temperatures in the 40s. Hoenisch

AVIATION

08/18Z TAF Period

VFR conditions will prevail across the KBZN, KEKS, and KHLN terminals during this TAF Period. During the majority of this TAF Period it will be very windy at all but the KHLN and KBZN terminals. There will be several hours of wind gusts in excess of 50 kts at the KGTF, KEKS, and KCTB terminals after 09/06Z. There will be instances of low-level wind shear and mountain wave turbulence across North-central, Central, and Southwestern Montana during the duration of this TAF Period. After 09/12Z across all terminals there is greater than a 30% chance for rain and/or a rain/snow mix. At the KHVR and KLWT terminals the rain and/or rain/snow mix will lower visibility to at least MVFR-levels with a 10% chance for visibility to lower to IFR-levels or lower. During the majority of this TAF Period there will be periods of mountain obscuration across North-central, Central, and Southwestern Montana. -IG

PRELIMINARY POINT TEMPS/POPS

GTF 42 50 33 54 / 20 50 60 70 CTB 38 43 25 44 / 30 40 30 40 HLN 40 48 33 52 / 50 70 80 80 BZN 36 47 30 54 / 20 50 70 80 WYS 25 39 26 41 / 80 90 90 90 DLN 38 48 33 53 / 30 50 70 50 HVR 31 45 20 41 / 40 70 20 80 LWT 34 46 24 48 / 30 70 40 90

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Warning until 10 AM MST Tuesday for Cascade County below 5000ft-East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Eastern Pondera and Eastern Teton- Eastern Toole and Liberty-Gates of the Mountains-Madison River Valley-Northern High Plains-Southern High Plains-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

High Wind Warning from 6 PM this evening to 3 PM MST Tuesday for Bears Paw Mountains and Southern Blaine-Big Belt, Bridger and Castle Mountains-Fergus County below 4500ft-Hill County-Little Belt and Highwood Mountains-Meagher County Valleys-Northern Blaine County-Snowy and Judith Mountains-Western and Central Chouteau County.

High Wind Warning until 3 PM MST Tuesday for Judith Basin County and Judith Gap.