Area Forecast Discussion

KEY MESSAGES

- Widespread precipitation today, with accumulating snow most prevalent over the higher terrain of southwest MT tonight into Monday morning.

- Milder and drier conditions start by Tuesday and generally prevail through the rest of the upcoming workweek.

- With lighter winds and areas of high humidity levels, pockets of fog will be possible during late overnight and morning hours, mainly impacting central and north-central MT and the southwest valleys during the upcoming work week.

DISCUSSION

- Meteorological Overview:

A moist southwesterly flow aloft continues to overrun a cooler and nearly saturated air mass at the surface. The Pacific NW trough responsible for the moist southwesterly flow ejects eastward towards the Northern Rockies and sends waves of more widespread precipitation through the forecast area today through Monday morning. This system will be undergoing weakening and shearing as it approaches and its colder air aloft is not scheduled to arrive until tonight into Monday morning. So snow levels will be over 7000 ft for most locations, generally yielding liquid precipitation at and below pass level. The one exception to this will be along the Continental Divide where spotty light snow will fall before tonight.

H700 temperatures finally fall to around -5C late night into early Monday and this will bring more efficient accumulating snow, heaviest over the Madison/Gallatin ranges above 6000 ft. There will be a brief period with snow levels falling to the valley/plains floors Monday morning, although it does not appear that this will be long enough to support widespread accumulating snow impacts. Northwesterly winds may also get gusty later tonight, though gusts look to stay below 40 mph and the strongest winds should quickly move east of the forecast area Monday morning. Ensembles continue to favor ridging aloft to bring milder and drier conditions for much of the upcoming workweek; however, a weak system may move through around Wednesday and bring a few isolated showers to the southwest and along the Continental Divide. - RCG

As rain tapers off over Sunday night and Monday morning, lighter winds and higher overnight humidity levels will allow for areas of fog to potentially develop within central MT, north-central MT, and the southwest valleys. Due to limited winds during the rest of the Monday, there is the possibility that these areas of fog remain socked in as they have the past few days, resulting in daytime highs falling 5-10 degrees below what was expected in those areas.

Looking towards the rest of the week, an upper-level ridge begins to establish over the region beginning Monday evening. Accompanying this ridge will be mild and dry weather. Although the upper- level ridge is expected to weaken beginning Tuesday night, dry and mild conditions are currently expected to remain in place through at least Thursday afternoon. Although far out, a change in the weather is anticipated on Friday due to an upper-level trough beginning to move into the area, along with windier conditions. -ESH

- Forecast Confidence & Scenarios:

Light mountain snow and lower elevation rain and snow Sunday through Monday morning

The main trough swings through today through early Monday and brings a wave of widespread precipitation to much of the forecast area. Aside from some light snow along the Continental Divide and perhaps some spotty freezing drizzle over the plains west of I15, most of the precipitation will be falling in the form of rain until the colder air aloft moves in tonight. The overall pattern and NBM probabilities continue to favor elevations over 6000 ft in the Madison and Gallatin ranges for the best chance (60 to 80%) of snowfall amounts in excess of 3 inches. Lighter snows, generally up to a couple inches, look to fall along the Continental Divide and the central island ranges. The colder air is still not looking to be optimally aligned with the moisture for lower elevation snows, even in the Gallatin Valley where probabilities for 2 inches of snow fell from around 30% to around 10%. Decided to hold off on issuing any winter advisories with this forecast package given the brief window of opportunity for accumulating snow at and below pass level and H700 temperatures trending upward ever so slightly. - RCG

..Fog during the upcoming week

The main time period of concern will be Sunday night into Monday morning as precip moves through the region and tapers off. Lighter winds, along with residual moisture, will provide favorable conditions for fog development. The main impact associated with this would be a reduction in visibility and the potential for freezing fog due to overnight lows expected to be near or below freezing. If areas of denser fog develop, there is the potential for that fog to remain socked in and keep daytime high temperatures 5-10 degrees cooler than expected. Through the rest of the upcoming work week, pockets of fog may develop overnight, although these would likely be patchy and limited to areas near bodies of water and valleys. -ESH

AVIATION

01/18Z TAF Period

Dense fog is starting to lift over North Central MT as of 18z today, as light rain moves northward into North Central MT. Low clouds/ceilings will affect all sites through the period. There is the potential that some dense fog could redevelop once again tonight. Mountains/passes will be obscured across the CWA by precip/low clouds. Brusda

PRELIMINARY POINT TEMPS/POPS

TFX WATCHES / WARNINGS / ADVISORIES

Dense Fog Advisory until noon MST today for Bears Paw Mountains and Southern Blaine-Beaverhead and Western Madison below 6000ft- Canyon Ferry Area-Cascade County below 5000ft-Eastern Glacier, Western Toole, and Central Pondera-Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-Fergus County below 4500ft- Gallatin Valley-Helena Valley-Hill County-Judith Basin County and Judith Gap-Madison River Valley-Missouri Headwaters-Northern Blaine County-Northern High Plains-Southern High Plains-Western and Central Chouteau County.