PowellVolz

This could possibly be one of the best discos I’ve ever read… Main concern for the long term is the looming winter weather event for Friday night into Saturday. As such the discussion will focus primarily on that event. Sunday and beyond is rather uneventful, with mostly dry conditions and possibly some rain on Tue (mainly south of I-40) as a closed upper low slides by us to the south. Now, on to the winter weather event. Friday evening into Friday night a northern stream trough/upper low will shift east from the far northern plains and into the western Great Lakes region, while a southern stream wave moves east from Texas along the I-20 corridor across the deep south. Strong surface high pressure will dive south out of Canada behind the northern stream disturbance, pushing very cold air into the TN valley after midnight Friday night. Meanwhile, the phasing of these two disturbances coupled with the strong thermal gradient across the Ohio and Tennessee river valley will lead to a very strong jet streak developing over the eastern CONUS, stretching from the deep south up through the mid-Atlantic seaboard after midnight Fri night. Models forecast upwards of 160k jet streak to spread north from the deep south through the southern Appalachians and mid-Atlantic seaboard Friday night into Saturday. This will induce strong forcing for ascent, with deep layer upward omega and saturated air extending into and well beyond the dendritic growth zone for several hours. In other words, this is setting the stage for widespread snowfall, with precipitation rates that will likely overcome the warm ground and lead to significant accumulating snow across much of eastern Tennessee. Precipitation begins to move into the region from the southwest during the evening hours tomorrow, but really ramps up in intensity and coverage between 06z and 12z Sat. Expect some areas in the Cumberland plateau to begin seeing at least a rain/snow mix, if not all snow, by 2-3 AM EST Sat morning, with most all of TN valley seeing mostly snow by 5 AM EST or thereabouts. While some of the potential snowfall during this period will be lost to melting or mixed ptypes, I believe the strong CAA (and resulting increase in snow ratios) and rapid cooling of the atmosphere due to dynamic cooling will support a bulk of the QPF falling as snow. Further lending confidence to higher snowfall amounts is that the strong forcing will also be coincident with some negative EPV aloft, which would favor some convectively enhanced snowfall rates. The strongest signal in this regard will be mostly over the northern plateau and further north into our western SW Virginia counties, which is where the higher storm total snowfall amounts are. However, it also extends across the central and northeastern TN valley as well which is why snow amounts have been increased there. It would be a lie to say there's not some uncertainty remaining. Trends have been colder and more snowy over the last 24hrs, but there's also less QPF to work with in most guidance as well. Will that trend continue? Will the air temperatures drop quick enough, and/or precip rates be high enough, to overcome warm ground temperatures? These are questions that we won't know the answer to until Saturday morning. But based on the guidance we have available, it seems a significant late-season winter storm is in the offing. Let's talk about storm total amounts real quick. In general, accumulations of 2" or more should be mostly limited to areas from Bledsoe over to Monroe county and points northward. Areas that get 3" or more snowfall should generally speaking be along or north of the I-40 corridor, and amounts upwards of 4" should be limited to the northern plateau and up into Lee and Wise county in Virginia, and also in the Smokies. While the higher terrain of the southern plateau (e.g. Bledsoe down to Marion county and also Signal Mountain for instance) could see 2-3" as well, southern areas in general should be more in the 1-2" range. For a more user friendly graphical output, be sure to check our webpage and social media accounts. Lastly, have gone ahead and hoisted up a Winter Storm Watch for the entire CWA. A good argument could be made to have held off for the far south. But given the official forecast, issuing a watch seems like the best decision to make and leaving some southern counties out is probably splitting hairs at this point. Will let the upcoming night shift make a determination on what areas to upgrade to a Warning and what areas to upgrade to an Advisory .

Just go ahead and cancel it now .

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I think theses a WSW for everyone .

Literally rinse and repeat 90% of the time and I can’t blame them. .

WSW for the plateau .

Yesterday it looked good. Has anyone seen it today? .

Going to be interesting to see what MRX does. They are running out of low end mods to roll with. .

Moving faster? .

I just hope this doesn’t follow the trend of the year and goes NW in the 11th hour .

I think this also could be a deal where odd locations go snow faster than climo locations based on snow rates. .

No one gets mad at the weather guys when they predicted 2 inches and we get 6. .

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What are those rates along the northern plateau? .

Hope I’m not overreacting but that looks similar to the 3k NAM .

Maybe I’m wrong but it seems like the NAM always leads the way with the first big adjustment. Maybe it’s too amped which would pull it NW but if it doesn’t start to move back on the next run or two, the valley is in trouble IMO. Hopefully it’s bias is showing up .

Doesn’t make sense that SE trends benefited W/M Tennessee but hurt ETn. .

Just thinking out loud… less GOM, less warm nose = higher ratios? .

If flatter means less amped, would that suggest less chances of warm nose issues? .

This is correct. In this storm the western side of the apps had major downsloping issues but for whatever reason, they didn’t make it west of the mountain counties. The valley NW of 75/40 and west of 81 had a CAD type situation. I’m just north of Knoxville and I stayed all snow and never made it out of the 20’s. At the same time Gatlinburg was in the mid 40’s with rain. I’ve actually wondered if there could have been a weak inverted trough up the valley which negated the downsloping and also added more lift in the valley to help keep the column cold. .

As @Carvers Gap mentioned before… the closer you are to the rain, the heavier your rates will be. In this case Knoxville is closer to the heavier rates than the plateau. We won’t see much dynamic cooling in this event but with heavy rates comes some atmospheric cooling from the top down and it’s likely this is beating back the warm layer (which is probably very shallow). Another possibility is we have seen before with a NW wind, the valley can “trap” cold air all the way up to the lower mid levels in a perfect setup. .

I don’t believe I’ve seen a mod suggest that either .

What could go wrong they said…. .

I’m trying not to get excited but here we are. .

Interesting .