textproduct: Great Falls

KEY MESSAGES

- Patchy dense fog continues this afternoon and is likely to expand in area tonight; potentially another round Friday night.

- Seasonable afternoon temperatures the next few days, with the potential for warmer afternoon temperatures by later next week.

- A good chance for widespread rain and snow Saturday night into Sunday over much of the region.

DISCUSSION

Today...

Areas of fog and low clouds persist across much of the north- central MT plains, where a very moist low level airmass is in place on the southwestern fridge of surface high pressure centered across the Canadian Prairies. Light low level easterly upslope flow is supporting some of the most widespread/dense fog affecting areas adjacent to the Rocky Mountain Front and Cascade county. A Dense Fog Advisory continues through 21z adjacent to the Rocky Mtn Front.

Tonight through Friday night...

Surface high pressure currently centered across southern AB/SK slowly slides eastward through Friday night and is likely to hold the shallow/moist airmass in place across the the plains, maintaining at least some areas of low clouds and fog across portions of north-central MT. Dense fog advisories will likely be needed again tonight through Friday morning for many of the same locations experiencing fog today. Latest high resolution model ensembles give a 60% probability for visibility 1/2 mile or less for areas of north-central MT along and west of I-15. Additionally, overnight temperatures near or slightly below freezing will support freezing fog again tonight as well, and even the potential for some patchy freezing drizzle with the risk for localized icing, particularly affecting bridges and overpasses.

This weekend... An upper level low over the Pacific Northwest will dig southeast towards Utah into Saturday. This will result in a moist southerly flow aloft to developing over Central MT. As a result, rain/snow is forecat to develop Saturday night over Southwest MT and then move into Central MT by Sunday morning. Latest model guidance continues to support widespread precipitation development Saturday night into Sunday with precipitation exiting the area Sunday night or Monday morning. This storm system will bring rain/snow to much of the CWA. Snow levels will be high to start, generally above 7000 feet on Saturday, but then are forecast to fall down to about 4000 feet Monday. By Monday, most of the precipitation will have exited the region, but mountain snowfall is likely Saturday night into Sunday. Snow accumulations around the Pony/Big Sky area are starting to approach warning criteria. Thus a winter storm watch might be needed for Zone 330. Otherwise, a winter weather advisory is looking likely for mountain areas of Southwest and portions of Central MT from Saturday night into Sunday night.

As the next upper level ridge starts to build over the northern Rockies. The main question is how strong will it be. Overall, we expect drier and slightly warmer conditions by the later portions of next week as this upper level expands. Afternoon temperatures might be a bit warmer than currently forecast for the later half of next week. Hoenisch/Brusda/WMR

AVIATION

29/18Z TAF Period

Main concern remains the areas of dense freezing fog over portions of North Central MT. Visibility is down to a 1/4SM at times in the dense freezing fog. Overall, the fog will likely be stick around along the Rocky Mountain front (KGTF/KCTB) due to the light winds and high clouds. Light snow showers are expected to decrease this afternoon. The dense fog will expand in area across north-central Montana after 02z bringing widespread IFR conditions. There is a chance that KBZN may see patchy fog between 11-17z, but confidence remains low at this time. Mountains will be obscured through the entire period. WMR/Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 30 39 30 52 / 10 0 10 0 CTB 27 36 26 50 / 30 10 10 0 HLN 28 45 31 50 / 0 0 20 0 BZN 27 48 30 48 / 0 0 10 30 WYS 23 52 26 45 / 0 0 50 70 DLN 30 50 33 48 / 0 10 20 30 HVR 23 46 26 50 / 0 0 0 0 LWT 25 47 29 53 / 0 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

Dense Fog Advisory until 3 PM MDT this afternoon for Cascade County below 5000ft-Eastern Glacier, Western Toole, and Central Pondera-Eastern Pondera and Eastern Teton-Gates of the Mountains- Northern High Plains-Southern High Plains.