textproduct: Great Falls

KEY MESSAGES

- Strong to very strong winds are expected Sunday, with strong winds primarily along the Rocky Mountain Front and across the North Central Plains. - Multiple rounds of light, mainly mountain, snow through the week, including a round along a cold front Sunday night/Monday morning. - Yet another round of widespread strong winds looks likely on Thursday.

DISCUSSION

- Meteorological Overview:

Upper level ridging over the area today will break down tonight and tomorrow as an upper level trough moves through Central British Columbia and Alberta. As this occurs, a strong upper level jet streak will develop over the Northern US, which will result in the development of mountain waves in the lee of the Rocky Mountain Front. This will bring strong winds from aloft to the surface, resulting in strong to very strong winds to the Rocky Mountain Front and North Central Plains on Sunday, with gusts of 90-100 mph expected along the Front and gusts in excess of 65 to the Plains, with some gusts in excess of 70 in some of the wind prone areas on the Plains. Winds will taper off Sunday afternoon and evening from north to south as a cold front slides in from Alberta.

Overall zonal/westerly flow will set up across the area Monday and Tuesday, which will result in cooler but still breezy and unsettled weather with gusty winds on the plains and some light mountain snowfall across the area. Overall, Monday and Tuesday do not look overly impactful, though the continued winds may hamper any recovery work that needs to be done after Sunday.

Our next period of more impactful weather will be Wednesday and Thursday as yet another significant jet streak pushes into the Northern Rockies. While there is still naturally quite a bit of uncertainty, it is worth mentioning that there is some potential for Thursday to be a rather significant event for at least portions of the area and for some could produce the strongest wind gusts of the season, but I will talk about this more below.

While upper level and surface winds taper off Friday and Saturday, models seem to be in good agreement that a round of snowfall is likely as we close out the week.

- Forecast Confidence & Scenarios:

Winds Sunday: Confidence is fairly high (70-80%+) in a widespread strong to very strong wind event across the North Central Plains and Rocky Mountain Front tomorrow as a jet streak across the region. Most model solutions are showing a robust jet streak with 700 mb winds of 50-80kts (~60 to 90 mph) moving across the Divide tomorrow, which will result in widespread downsloping/mountain wave activity across the region. These winds will translate to wind gusts of up to 90-100 mph along the Rocky Mountain Front (where pressure gradient forces and mountain waves will cause some local accelerations of the winds as they move towards the surface), while further east across the plains wind gusts of 60-70 mph will be expected to be common for much of the day. There are a few ways for things to wind up not as intense (or at least not as long duration), however. The first will be the formation of a chinook arch, which may limit the thermal mixing component further east across the plains. This would reduce the opportunity for wind gusts to repeatedly reach the upper end of what is possible tomorrow, though mountain waves will still result in localized wind gusts to the upper end of the forecast envelope tomorrow. The other potential limiting factor will be the arrival of a cold front from Alberta, which will more or less undercut the winds as soon as or very shortly after it pushes through the area. Current model runs have tried to bring this cold front in during the mid to late afternoon, but models do sometimes tend to underestimate the speed at which cold air can propagate southward. If this front moves through earlier in the day, it would reduce just how long the winds will last tomorrow afternoon.

Thursday winds: Models are in unusually good agreement that yet another (and possibly stronger) upper level jet streak affecting the area on Thursday. While I hesitate to start sounding the alarm on an event before a separate event has even started, I will note that current model guidance for the Thursday event is concerning for a few reasons. First is that models are quite consistent in their handling of this event, though I will admit that this may be limiting my view of the possibilities of how this event will unfold. However, the solutions that are presented both show 700 mb winds of 60-90kts (70-100 mph), and 500 mb winds in the 90-110kt range (100-130 mph), which would make this event stronger than the one tomorrow, with the core of these winds primed to affect mainly Central and Southwestern Montana. At this point, it is a bit difficult to say just how efficiently these winds would mix to the surface. I will, however, note that some indices are eerily reminiscent of what we were seeing at similar lead times to a very impactful event earlier this season. While the wind event tomorrow will remain our focus for the next 24 to 36 hours, we will be keeping a very watchful eye on the forecast for Thursday as well. Ludwig

AVIATION

07/18Z TAF Period

Strong winds will affect portions of North Central MT this evening, while expanding to all of North Central MT and the northern portions of SW MT on Sun. Expect increasing clouds on Sun afternoon, especially over N Central MT, along with a cold front moving southward into N Central MT towards 00z Mon. Brusda

FIRE WEATHER

Strong winds on Sunday will result in increased fire danger across the plains of North Central Montana, even with RH values in the 20-30 percent range. Winds are expected to continue to be gusty through the early afternoon, but a cold front coming in from the north will bring both a wind shift to the north and a rapid decrease in wind speeds. Ludwig

PRELIMINARY POINT TEMPS/POPS

GTF 46 59 31 43 / 10 10 50 20 CTB 43 53 17 35 / 0 50 80 30 HLN 39 59 34 45 / 10 10 40 40 BZN 32 59 34 48 / 10 0 10 40 WYS 26 43 29 45 / 30 20 20 40 DLN 32 56 36 50 / 0 0 0 10 HVR 41 58 20 41 / 10 30 90 40 LWT 40 59 29 41 / 0 10 50 50

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Warning until midnight MST Sunday night for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Judith Basin County and Judith Gap-Northern High Plains- Southern High Plains-Southern Rocky Mountain Front.

High Wind Warning from 3 AM Sunday to midnight MST Sunday night for Bears Paw Mountains and Southern Blaine-Big Belt, Bridger and Castle Mountains-Canyon Ferry Area-Cascade County below 5000ft-Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-Elkhorn and Boulder Mountains-Fergus County below 4500ft- Gates of the Mountains-Helena Valley-Hill County-Little Belt and Highwood Mountains-Meagher County Valleys-Missouri Headwaters- Northern Blaine County-Snowy and Judith Mountains-Upper Blackfoot and MacDonald Pass-Western and Central Chouteau County.

High Wind Watch from 3 AM MST Sunday through Sunday evening for Gallatin Valley.