554 FXUS65 KTFX 191724 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1023 AM MST Sat Jan 19 2019 ...Updated Aviation Discussion... .UPDATE... Made minor tweaks to the grids and forecast through tonight. Trended more aggressive with PoPs for Sunday afternoon across central and parts of north-central Montana. Shear vorticity, sufficient moisture, and good overrunning should yield brief, localized moderate or heavy snow rates somewhere across the central and north-central counties. We will work to nail those details further as the event approaches. && .SYNOPSIS... A decaying Pacific storm system will sweep east across the region today bringing gusty winds and mild temperatures. Winds relax tonight as weak high pressure builds into the region before the next storm system impacts the region Sunday into Monday. Snow, possibly mixed with rain across the valleys of central and southwest Montana Sunday will transition to all snow Sunday night. Accumulating snow and falling temperatures late Sunday and Sunday night could create difficult driving conditions through Monday morning. && .AVIATION... Updated 1723Z. Main impact to aviation today will be increasing winds and associated mountain wave turbulence/low level wind shear in areas east of the Rockies. Strongest winds initially along the east slopes will spread east across the plains this afternoon and early evening. Winds relax tonight while clouds increase ahead of the next system spreading snow into SW MT by early Sunday morning. && .PREV DISCUSSION... /ISSUED 415 AM MST Sat Jan 19 2019/ Today and Tonight...Upper level ridge over the Northern Rockies this morning is rapidly flattened today as a deep upper low off the BC coast moves inland with trailing shortwave energy tracking east along and N of the Canadian border this evening. Plume of Pacific moisture associated with the system will shift east across the region this morning with some light precipitation possible over the mountains, but overall moisture with this system is decreasing as it moves across the region with only an inch or two of snowfall anticipated today across areas near the continental divide. Increasing westerly flow aloft and a deepening lee-side trough of low pressure will produce gusty winds today, especially across north- central MT, where winds will also scour out the lingering cold airmass at the surface. 700 MB flow increasing to 50-60kts this afternoon should translate to 40-50 mph wind gusts across western portions of north-central MT this afternoon with 60-70 mph wind gusts along the immediate east slopes of the Rockies. Still some marginal concern that high wind thresholds could be reached, particularly in the Cut Bank area this afternoon, but snow-cover across most of the area should limit mixing some, however this will need to be monitored closely today. Still some potential for localized blowing/drifting snow today in areas along the east slopes, primarily this morning before temps rise well above freezing this afternoon. Winds slowly diminish tonight as the lee-side trough weakens and winds aloft decrease. Hoenisch Sunday through Saturday... The week begins with a somewhat complex system affecting the region. As a low pressure system approaches from the west on Sun, a warm front may lift north at least through part of the CWA. Highs in the upper 30s to mid 40s are expected for much of the lower elevations, especially for the SW Valleys. As a result, whatever snow may fall at lower elevations Sun morning may mix with or change to rain during the afternoon hours. However, as the low passes through the CWA Sun evening, winds will shift and draw cooler air back south from Canada, allowing for a quick change back to snow for most places. The heaviest snow for north-central Montana is likely to be later Sun eve into Sun night, with the heavier (but likely moderate at most) snowfall shifting southward late Sun night into early Mon afternoon. At lower elevations, snow totals generally between 1 and 4 inches are expected. Mountain snow totals are currently forecast to be mostly in the 5 to 10 inch range, and advisories may be necessary for many of our passes. After discussing with neighbors, will hold that decision to the day shift, or next night shift. Drier air will work into the state Mon night into Tue, allowing for a period of drying and clearing skies as the eastern edge of a ridge builds in from the Pacific. However, the axis of this ridge will remain offshore, which will keep us in northwestern flow aloft, with the potential for an active pattern from Wed into the weekend. The next decent shot for precip appears to be Wed into Wed night. Beyond that, more systems are certainly possible, but model agreement is low, so POPs and QPF amounts were kept on the lower side for now. After the aforementioned warmth Sun, temps will drop back toward normal Mon, with highs upper 20s to mid 30s. A warming trend then commences Tue as the ridging builds in from the west. Highs Tue are currently forecast to be in the 30s for the north-central plains, but mid 20s to lower 30s for the SW valleys. However, temps over the Plains may need to be raised a bit depending on the amount of sunshine and strength of downsloping winds we expect. Through the latter half of the week, general W-SW'ly flow is expected much of the time, allowing for highs to remain near to a bit above normal for much of the CWA. && .PRELIMINARY POINT TEMPS/POPS... GTF 30 40 24 34 / 20 80 60 50 CTB 25 37 18 32 / 10 40 40 30 HLN 28 39 22 32 / 30 70 60 50 BZN 28 42 22 32 / 60 60 60 60 WEY 23 33 17 26 / 70 100 80 70 DLN 29 42 24 32 / 40 30 50 50 HVR 18 37 18 32 / 0 30 60 30 LWT 27 39 23 31 / 20 60 50 60 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls