Great Falls, Montana

• Periods of dense fog and even some isolated instances of icing over elevated surfaces will continue through Sunday morning, most widespread over central and north-central Montana.
• Precipitation becomes more widespread by Sunday, with accumulating snow most prevalent over the higher terrain of southwest Montana Sunday night into Monday morning.
• Temperatures trend closer to average next week.

• Meteorological Overview:

Low level easterly flow continues to support low stratus and areas of dense fog development over central and north-central MT, while a moist southwesterly flow aloft overruns the cooler air mass and sends waves of light showers, sprinkles, and even some isolated instances of freezing drizzle. This general pattern will more or less persist through Sunday before a closed mid -level low over the Pacific NW heads towards the Northern Rockies and brings more widespread rain and snow Sunday through early Monday. The fog impacts will be most widespread over central and north-central areas during the overnight and early morning hours.

The closed low will begin migrating towards the Northern Rockies late Saturday night into Sunday while being weakened and elongated by synoptic shearing. With the colder air aloft not scheduled to arrive until Sunday night, snow levels will be over 7000 ft for most locations, except along the Continental Divide. This will bring mostly liquid precipitation for lower elevations with the exception of some isolated instances of light freezing drizzle over the plains west of I15. Some freezing precipitation may also occur for the colder southwest valleys along the MT/ID/WY borders.

H700 temperatures finally fall to around -5C by Sunday 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 get gusty Monday morning, though gusts look to stay below 40 mph. Ensembles continue to favor ridging aloft to bring drier conditions and near average temperatures for the remaining first half of next week, with unsettled conditions looking to return later in the week. - RCG

• Forecast Confidence & Scenarios:

Dense fog through Sunday

Hi-Res Ensemble Forecast (HREF) guidance supports the development of fog between 9pm this evening and 3am Sunday over the plains of Central and North Central Montana, with this area of fog persisting through 9am to 3pm Sunday before an area of precipitation lifting northeast from Southwest Montana brings an end to the reduced visibilities. The aforementioned HREF guidance largely support probabilities of between 40-60% with respect to visibilities falling below 1/4 mile during the overnight hours, and for this reason a Dense Fog Advisory has been issued yet again through 12 pm Sunday. Confidence is highest in visibilities falling to 1/4 mile or less along and west of the I-15 corridor, MT Hwy 200/US Hwy 87 corridor, and in Milk River Valley where persistent low level east to north flow will support upslope fog.

• Moldan

...Light mountain snow and lower elevation rain, snow, and some mixed precipitation Sunday through Monday morning...

The main trough moves through Sunday 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 and over the higher southwest valleys, most of the precipitation will be falling in the form of rain until Sunday night. The overall pattern and NBM probabilities favor the Madison and Gallatin ranges for the best chance (60 to 80%) for 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, but will monitor the Gallatin valley where there is around a 30 to 40% chance for a couple inches of snow on the south/eastern fringes of the valley Monday morning. - RCG

31/18Z TAF Period

KCTB is expected to have its dense fog and VLIFR conditions end by 20Z this afternoon.

Low level easterly flow combined with a moist southwesterly flow aloft will bring periods of IFR/LIFR conditions and mountain obscuration from low stratus, fog, and spotty light showers. Additionally, steadier rainfall will begin moving into the region out of the SW around 10Z. The worst and most widespread degraded conditions will be over central and north- central MT after 05Z. - RCG/ESH

GTF  35  39  36  47 /  20  90 100   0
CTB  29  41  29  50 /  10  60  50   0
HLN  35  43  36  49 /  50 100  90  10
BZN  34  41  34  41 /  20  90 100  30
WYS  32  39  26  39 /  80 100  90   0
DLN  37  40  33  43 /  70 100  90   0
HVR  30  42  33  46 /   0  70  90  10
LWT  34  45  33  42 /   0  60 100  40

Dense Fog Advisory from 9 PM this evening to noon MDT Sunday for Bears Paw Mountains and Southern Blaine-Cascade County below 5000ft-Eastern Glacier, Western Toole, and Central Pondera-Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-Fergus County below 4500ft-Hill County-Judith Basin County and Judith Gap-Northern Blaine County-Northern High Plains-Southern High Plains-Western and Central Chouteau County.