textproduct: Great Falls

KEY MESSAGES

- Cool and unsettled weather pattern continues with periods of showers and possible afternoon thunderstorms through Wednesday night and likely again Friday night into Saturday.

- Breezy to windy conditions through Wednesday with locally strong winds possible across portions of central Montana on Wednesday.

- Accumulating snow for elevations above 5500 feet, especially across the mountains of Glacier National Park.

UPDATE

Updated forecast is out. Only change this evening was to add some light showers for a few more hours in the Havre/Harlem areas. Overall, most of the thunderstorms have diminished, but an isolated lightning strike can't be ruled out overnight.

For the high wind watch, latest guidance is favoring a small area for high winds, mainly east of MacDonald Pass and the lower elevations of Judith Basin County. We will take a closer look when all of the 00z models come in this evening before making a final decision. Otherwise, the rest of the forecast is on track. Brusda

DISCUSSION

- Meteorological Overview:

A secondary push of colder air in the mid-upper levels surges east across the area this afternoon and evening supporting scattered to numerous showers and a few thunderstorms, favoring areas from southwest MT through eastern portions of central MT. The main upper level low/trough with this initial series of shortwaves shifts across the Northern Rockies Wednesday, bringing additional cooling along with windy conditions and some showers, mainly across north- central MT later Wednesday afternoon and overnight as moisture wraps around the back side of the upper trough. Snow levels will lower to around 7000 ft today and may fall to as low as 6000 ft Wednesday, though any mountain snowfall is likely to be brief with little accumulation. A drier period follows Thursday through at least early Friday before the next series of shortwaves begins to move in from the NW later Friday and Saturday.

Confidence in the upper level pattern decreases later this weekend into next week, but most longer range model ensembles favor upper level ridging briefly amplifying offshore of the Pacific NW/BC by early next week. This could bring some warming but also maintains a northwesterly flow aloft with potential for additional disturbances to move through and a risk for showers and thunderstorms. Hoenisch

- Forecast Confidence & Scenarios:

Accumulating Snow tonight through Wednesday evening...

Snow levels falling to between 5500ft to 6500ft throughout the timeframe will allow rain to mix in with or completely changeover to snow across the mountains of Southwest through North Central Montana, with accumulating snow expected above 6500ft. NBM5.0 probabilities for snowfall accumulations greater than 2" of snow for locations above 6500ft, most notably along the Continental Divide, Little Belts, Tobacco Roots, and Gallatin/Madison Ranges, are in excess of a 60% chance with 20-40% chance for accumulations to exceed 4" across most of these locations. The one exception is across the mountains of Glacier National Park where probabilities are significantly higher at around a 70% chance for 4" or more of snow, with even a 10% chance that amounts exceed 12" or more of snow.

Strong Winds on Wednesday...

Climate anomaly indicators such as the NAEFS and ECMWF EFIs continue to suggest the potential for a climatologically unusual to very unusual, with respect to June Climatology, wind event across portions of Southwest through North Central Montana. H700 wind speeds of 30-40kts, with speeds in excess of 50kts within the critical layer, are expected to overspread the Northern Rockies from late this evening through Wednesday afternoon. The core of these strong winds is still expected to move over Central Montana during this timeframe, Jenelle along and north of the US Hwy 12 and Montana Hwy 200 corridors but also including the Rocky Mountain Front south of US Hwy 2. Uncertainty begins to rise in how precipitation overspreading the aforementioned areas will affect mixing during the event, especially early on with respect to mountain wave activity. NBM5.0 probabilities for wind speeds of 34kts or greater range from a 30-40% chance across Judith Basin and much of the Rocky Mountain Front, with probabilities for wind gusts greater than 50kts varying from a 10-30% chance between the US Hwy 12 and MT Hwy 200 corridors to a 40-80% chance along the Rocky Mountain Front. At this time it is likely that isolated winds up to High Wind criteria will occur, but with coverage being so isolated I am hesitant to upgrade the High Wind Watch. At the same time this event has the potential to be a high impact/low magnitude event given that vegetation is leafed out and summer recreation is underway. For now we will maintain the High Wind Watch given the uncertainty. - Moldan

AVIATION

10/00Z TAF Period

Initial concern this TAF period will be for scattered showers and thunderstorms across the region through the mid-evening timeframe. Gusty and erratic winds will be the primary hazard with the stronger thunderstorms that do form.

Increasing cross barrier flow will result in at least breezy winds and gusts developing along the Rocky Mountain Front, which will spread elsewhere across the region through the day Wednesday. Mountain wave turbulence and a few instances of low-level wind shear will develop this evening and overnight across the region. All the while, high mountain snow and lower elevation rain near the Continental Divide this evening will result in increasing mountain obscuration through Wednesday. -AM/thor

PRELIMINARY POINT TEMPS/POPS

GTF 45 60 45 70 / 10 50 80 0 CTB 42 55 41 66 / 20 80 60 0 HLN 43 58 43 71 / 40 20 50 0 BZN 36 57 37 69 / 70 50 40 0 WYS 27 51 24 62 / 70 60 30 0 DLN 36 55 33 70 / 40 40 10 0 HVR 45 61 43 70 / 20 80 100 40 LWT 39 58 39 65 / 40 50 90 20

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Watch from 3 AM MDT Wednesday through late Wednesday night for Big Belt, Bridger and Castle Mountains-Fergus County below 4500ft-Gates of the Mountains-Helena Valley-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains-Meagher County Valleys-Upper Blackfoot and MacDonald Pass.