textproduct: Great Falls

KEY MESSAGES

- Well below normal temperatures linger through Tuesday, with a moderating temperatures thereafter.

- Dense fog is possible from the evening hours tonight through Tuesday morning across most lower elevations.

- Overall dry conditions are expected through next Monday.

DISCUSSION

- Meteorological Overview:

Some light precipitation is expected over SW MT south of I-90 this afternoon as a weak embedded upper disturbance pivots in on the northeastern side of the upper low centered over Eastern Oregon. - Moldan/AM

Subtle ridging begins to move in late tonight, which will help clear skies a bit. Given recent precipitation the concern will be for both cold overnight lows as well as for fog by Tuesday morning in many areas. This concern more or less exists Wednesday morning as well, though there will be uncertainty with how much snow pack remains by then.

After upper troughing moves across AB/SK Wednesday night into Thursday an upper level ridge begins to move in. Dry conditions remain in place, with highs trending back above average by the weekend. -AM

- Forecast Confidence & Scenarios:

Dense Fog through Tuesday morning...

Recent snowfall and subsequent melt, light winds, and clearing skies will set the stage for the potential of dense developing from the evening hours tonight and persisting through the morning hours on Tuesday. Latest Hi-Res Ensemble Forecast (HREF) guidance supports the development of fog over the plains of Central and North Central Montana and in the valleys of Southwest and Central Montana. Additionally, BUFKIT soundings show saturated low levels beneath mid- level dry air associated with a region of subsidence moving over the Northern Rockies.

One limiting factor to the development of dense fog, most notably along the immediate Rocky Mountain Front and Island Ranges of Central Montana will be the direction of surface winds, which will gradually turn to the southwest or west. These southwest to west surface winds, even while light in nature, tend to have a "drying" effect and subsequently limit the development of fog. Given an overall favorable setup and support from ensemble guidance thought it was best to issue a Dense Fog Advisory from 9PM this evening through 12PM Tuesday. - Moldan

AVIATION

12/18Z TAF Period

Low ceilings will continue to be a concern for all terminals for the first part of the TAF period. Across Southwest Montana, scattered rain/snow showers will bring occasionaly drops in visibility but overall reductions should limited. Ceilings gradually rise as our storm system exits east, but the moisture behind will promote patchy areas of fog or even drizzle late tonight into the morning hours. -JR

PRELIMINARY POINT TEMPS/POPS

GTF 22 42 21 49 / 10 0 0 0 CTB 16 45 19 53 / 10 0 0 0 HLN 19 41 20 49 / 20 0 0 0 BZN 20 39 18 46 / 20 0 0 0 WYS 20 44 14 47 / 20 10 0 0 DLN 21 39 17 46 / 20 0 0 0 HVR 18 44 16 54 / 10 0 0 0 LWT 18 40 21 48 / 10 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

Dense Fog Advisory from 9 PM this evening to noon MDT Tuesday for Bears Paw Mountains and Southern Blaine-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-Meagher County Valleys-Missouri Headwaters-Northern Blaine County-Western and Central Chouteau County.