Great Falls, Montana

• Periods of dense fog continue through the morning hours, most widespread over central and north-central Montana and the southwest valleys.
• Precipitation becomes more widespread today, with accumulating snow most prevalent over the higher terrain of southwest Montana tonight into Monday morning.
• Milder and drier conditions then generally prevail through much of the workweek.

• Meteorological Overview:

A moist southwesterly flow aloft continues to overrun a cooler and nearly saturated air mass at the surface. This has maintained an abundance of low clouds, while waves of light rain showers move northward through the forecast area. Surface temperatures have risen well above freezing for most locations, so freezing drizzle or light freezing rain hasn`t been much of a problem tonight; however, there is some stubborn colder air at the surface north of the Hi-Line, mainly over northern Hill and Blaine counties, where there maybe some isolated instances of freezing drizzle through the morning hours. Also, some colder air is slowly seeping southeastward over the plains along the Rocky Mountain Front. Fortunately, these areas are mainly seeing fog rather than falling precipitation. Speaking of fog, the aforementioned light precipitation delayed the onset of fog development over much of central and north-central MT tonight, but the fog looks to begin expanding over the area heading towards the early dawn hours. Also, dense fog has developed over the Gallatin Valley and looks to expand to other southwest valleys as the night progresses.

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. Forecast uncertainty increases towards the end of the workweek, but most ensembles highlight zonal troughs bringing breezy to windy conditions and mostly mountain precipitation heading into next weekend. - RCG

• Forecast Confidence & Scenarios:

Areas of dense fog through the morning hours

The fog has been most widespread over the plains west of I15 so far tonight, but will begin to expand to other areas of central and north-central MT with light shower activity diminishing and ending. Also the southwestern valleys have also been observing patchy dense fog development, which looks to expand in coverage prior to the arrival of the more widespread precipitation later this morning. Impacts will mostly be directly related from the reduced visibility, but frost deposition or even some freezing drizzle may cause some slippery spots to develop on roadways, especially the bridges and elevated surfaces. These roadway impacts will generally be confined to plains locations along and west of I15. The dense fog advisory covering much of central/north-central MT was left as is and some southwest valleys were added to the advisory, including the Bozeman, Dillon, and Helena areas.

...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

01/06Z TAF Period

Initially, low level clouds, patchy dense fog, and drizzle will be the primary concern through the morning hours. Greatest risk of visibilities below 1/2 mile are across north-central MT where northeast flow is aiding surface moisture. While the threat is lower across southern terminals, isolated dense fog cannot be ruled out especially terminals KBZN, KWYS, and KEKS. Moving into Sunday afternoon, rain becomes increasingly widespread pushing in from the southwest. This will bring renewed concerns of decreased visibility with rainfall in combination of fog as well as low cloud bases through the later half of the TAF period. Mountain obscuration will continue through the entire TAF period. -JR

GTF  44  35  53  33 /  90 100   0   0
CTB  43  29  52  29 /  70  30   0   0
HLN  44  34  50  28 /  90  90  10   0
BZN  43  31  44  24 /  90 100  40   0
WYS  41  27  42  19 /  90  90   0   0
DLN  41  32  45  26 / 100  90  10   0
HVR  45  31  48  29 /  70  90   0   0
LWT  48  31  46  28 /  80 100  40   0

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-Missouri Headwaters-Northern Blaine County-Northern High Plains-Southern High Plains-Western and Central Chouteau County.