This follow up Nina, if it come to pass, looks like it will be a weak one with the ensemble forecast hovering around -.5 to -.6 in the winter months. Last year was a moderate Nina and there's never been a moderate or strong Nina that followed a moderate Nina. Only one moderate ever produced a follow up at all. So we may yet land in neutral territory as 80 percent of moderate events produce a La Nada the following season.
With weak being the most likely scenario if we get the Nina, the follow up weak years temps were BN across the entire SE including the Tennessee Valley in the Sept-Nov timeframe. The west coast was particularly cool for those falls as well. So it might be a big pattern shift for them as we hit fall. When you factor in every follow up Nina year the Southeast still had BN temperatures in fall. There was a stark difference out west though, it was quite warm for most areas west of the Mississippi, especially the upper Midwest to the Pac NW. Which would strongly indicate the stronger the Nina the warmer the West in Autumn.
The winter seasons for the weak Nina follow up years have a similar pattern to what has already been discussed here. Very cold upper midwest, slightly AN southeast with the Plateau being the approximate dividing line between normal temps and AN temps. We really saw that play out last year in the Nina. The February 10th-20th period saw several days with a massive difference from Tri-Cities to the Plateau and points westward with the classic SE ridge battle vs super cold air sliding down from the Upper Midwest. There were 20 degree differences in temperatures across each of the Grand Divisions with Tri in the mid 50s, the Plateau in the lower 30s, upper 10s in Clarksville/Nashville and lower 10s in NW Tn/NE Arkansas.
The two anomalies in the follow up weak Ninas are torchy 2018 and 1985 with it's legendary winter. Oddly if you remove them there's almost no change to the map for most of the region. Take away 2018 and northern Arkansas and NW Tennessee are slightly BN instead of near normal with nothing much changing for anywhere else. Take away 1985 and the only area that warms much is a small corridor from Chattanooga, to Knoxville to Tri-Cities with everyone else remaining roughly the same as far as departure from normal.
If we do indeed avoid the Nina and go La Nada, things looks slightly better for the valley. The AN area slips over the Smokies and lines up from Cherokee to Boone and points East. The Upper Midwest remains frigid those years, which is where we get our cold from 90 percent of the time. As long as Eastern Montana to Wisconsin gets really cold we have a chance of getting a good cold outbreak, especially the central and western Tennessee valley regions.
All the Nina talk is just one factor of many though. We sometimes see the atmosphere defy ENSO and act in opposite fashion to what ENSO would normally suggest.