Scientific Forecaster Discussion
000 FXUS65 KTFX 241142 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 442 AM MST Sat Feb 24 2018 ...Update to Aviation... .SYNOPSIS... Windy conditions will continue to cause areas of blowing snow with blizzard conditions expected along the Rocky Mountain Front and over the adjacent plains Saturday morning and again on Sunday. Pacific moisture in a northwest flow aloft will also bring areas of snow to the mountains along the Continental Divide and in southwest Montana today through Monday. Lower elevations will remain mostly dry through the weekend. && .DISCUSSION... Today through Monday...Montana continues to reside beneath upper level troughing with a complex surface pattern causing multiple impacts across the forecast area. Strong and gusty winds will continue along the Rocky Mountain Front and in eastern Glacier County this morning causing widespread blowing/drifting snow and closing multiple roads. Have gone ahead and extended the ending time for the existing Blizzard Warning through noon. Winds are expected to diminish for 12 to 18 hours or so beyond this time before increasing again. Ideally, the warning will be dropped today, but as surface winds begin to increase again overnight I anticipate that another Blizzard Warning could be required for Sunday. This next blast of strong winds could see gusts exceed 90 mph in favored canyons, valleys, and passes of the Rocky Mountain Front from early Sunday morning through late Sunday evening. These strong winds will likely spread out across the plains as well and there is some likelihood that conditions over the plains could require highlights for high winds as well as blowing snow. Meanwhile, the existing Winter Weather Advisory for higher elevations of southwest Montana will remain in effect through midnight tonight with about 2 to 4 inches of accumulation likely. Am anticipating that this area should see a lull in precipitation from midnight tonight through about noon on Sunday, then another surge of moisture will bring additional snowfall to this region from Sunday afternoon through late Monday afternoon. Once again, I anticipate that additional winter weather highlights will be required for my southwest Montana zones with this next surge. Biggest wild card in all of this activity will be high temperatures for Sunday. Some models suggest that portions of the forecast area could see temperatures actually rise back above freezing for the first time in awhile on Sunday afternoon. This could end up causing blowing snow to be less than anticipated. However, any warmup will be short-lived as models bring highs back below freezing again on Monday. mpj Monday Night through Saturday...Medium range forecast models are in much better agreement with the forecast solution during this period than they were last night. A broad upper level low pressure trough over the western and central portions of Canada and the United States is still forecast to be the dominant feature through this period. One embedded shortwave trough will exit the state Monday night, then another shortwave will swing through Montana Tuesday through Tuesday night, resulting in periods of light mountain snow. A shortwave high pressure ridge will then move into the state for Wednesday through Thursday ahead of a deep upper level low pressure area. This ridge will help decrease the chance of snow over the mountains, while breezy westerly downslope winds help keep the lower elevations dry. The low pressure area will be off the Pacific Northwest coast Thursday through Friday, but forecast models differ on what to do with it after then; the Canadian model keeps it off the coast, the ECMWF moves it slowly into the Pacific Northwest, and the GFS moves it quickly into Montana. Will go closer to the GFS/ECMWF solution of bringing the low towards the forecast area for an increasing chance of snow for Friday through Saturday. However, this solution is fairly different than the forecast depicted last night, so confidence in the details of this part of the forecast is only low to moderate. Regardless, the overall temperature trend keeps temperatures between 5 and 10 degrees below normal through Thursday, with the potential for at least a minor cooldown for Friday and Saturday. Coulston && .AVIATION... Updated 1142Z. Northwest flow aloft will continue to prevail over Montana through early Sunday morning. Strong and gusty surface winds causing blowing snow along the Rocky Mountain Front and over the adjacent plains should diminish somewhat by 18Z allowing conditions to improve. Disturbance aloft will continue to bring areas of light snow to the mountains of the Continental Divide and across southwest Montana through the forecast period. Dry conditions expected at lower elevations for prevailing VFR conditions at most terminals. mpj && .PRELIMINARY POINT TEMPS/POPS... GTF 28 15 36 13 / 10 10 20 20 CTB 27 15 33 11 / 10 0 20 10 HLN 28 12 35 12 / 20 20 50 40 BZN 26 7 32 12 / 30 30 30 50 WEY 17 -1 20 11 / 80 70 80 90 DLN 22 10 29 11 / 40 30 30 60 HVR 24 11 33 10 / 10 10 10 10 LWT 28 16 33 14 / 10 20 20 20 && .TFX WATCHES/WARNINGS/ADVISORIES... Winter Storm Watch from Sunday morning through Sunday evening Blaine...Cascade...Chouteau...Eastern Pondera...Eastern Teton... Fergus...Hill...Judith Basin...Liberty...Toole. Winter Weather Advisory until midnight MST tonight above 6500 feet for Beaverhead...Gallatin...Madison. Blizzard Warning until noon MST today Eastern Glacier...Northern Rocky Mountain Front...Southern Rocky Mountain Front.