textproduct: Great Falls

KEY MESSAGES

- 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.

..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

AVIATION

01/12Z TAF Period

Widespread IFR/LIFR conditions and mountain obscuration from stratus and areas of dense fog will persist through the morning hours. Waves of precipitation will bring improved visibility in a south to north fashion by around 01/20Z, but MVFR/IFR/LIFR ceilings will more or less persist for much of the remaining TAF period, while the main precipitation shield traverses the forecast area. - RCG

PRELIMINARY POINT TEMPS/POPS

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

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