October is a month of building arctic cold across the prairies of Canada, so when zonal flow across the northern United States persists for the first 20 days of the month as we have seen thus far, eventually the dam breaks and the arctic air must be discharged south. When that happens, you typically see the type of wild weather headed for the Mid-South over the next two weeks.
Presently, conditions are quite calm with a mostly zonal flow across the United States. The minimal jet streak over the Midwest is associated with a weak cold front that will bring showers to the Great Lakes overnight. The key feature from Day 1 is the 150 kt jet streak off the coast of the Pacific Northwest that will bring the next major storm to the United States. By Wednesday, the diving jet streak will have spun off a powerful upper level and surface low that will quickly become closed off from the upper level flow in the lee of the Rockies. Severe thunderstorms are possible in the central Plains as the low deepens. Heavy snow is possible on the backside of this storm from northern KS to southwestern MN.
By late Thursday, warm advection rainfall will be entering western KY and TN. This rainfall may become heavy during the day Friday as tropical moisture from the low along the Gulf of Mexico streams northward and interacts with the Midwestern system. Widespread rains of 0.75″ to 1.5″ should be common across the Mid-South with higher amounts possible depending on how much tropical moisture gets involved. If the Gulf storm stays farther south than modeled, it is possible that rainfall amounts could be much less since tropical moisture will be shut off to the Midwestern storm. Temperatures will be quite cool with highs in the upper 50s Friday and Saturday.
The real exciting weather starts early next week as another strong jet max dives into the Midwest late Sunday. This will bring the first true arctic blast of the season into the Mid-South Tuesday and Wednesday. There is some question regarding how far south the core of the arctic air will reach next week. The GFS is obviously bullish by bringing 528 dm thickness and -7 degree C air at 850 mb into Bowling Green, but the European model has a more zonal track to the arctic air which will keep the truly frigid air over the Great Lakes. The GFS ensembles also show a 50-50 mix between the meridional (GFS) and zonal (European) solutions. The key to which solution is right lies in the interaction between the tropical and midwestern storms this weekend. The GFS merges the two storms over Quebec, which leads to a negatively tilted trough and a very deep surface storm, which in turn raises heights downstream over Greenland. This very amplified blocking pattern then allows the arctic front to dive due south towards the Gulf of Mexico by the middle of next week. If the tropical and Midwest systems don’t merge to create the deep Quebec storm, then the more zonal solution should be correct. Time will tell.
So what if the GFS is right and the core of the arctic air moves across the Mid-South next week? Expect highs across KY in the low to middle 40s with wind chills around freezing with light rain showers or sprinkles. Even though the 1000-500 mb thickness (528 dm) and 850 mb temperatures (-7 degrees C) are cold enough to support snow, keep in mind that those indicators are really only useful to determine the rain/snow line during the winter months of December – March. Boundary layer temperatures are still too warm for frozen precipitation even with this impressive air mass. This type of pattern will be cold enough to bring snow showers/flurries as far south as Indianapolis and perhaps all the way down to the Ohio River. Heavy lake effect snows can be expected in places like Michigan City, IN, Marquette, MI, and Cleveland, OH. Snow showers could also be found in some of the higher elevations of West Virginia with this type of fetch. If the European solution is correct, temperatures across KY will more likely be about 10 degrees warmer but still well below normal. The lake effect snow bands would shift to places like Buffalo, NY and western MI.
The models depict a reinforcing shot of arctic air for Halloween that will once again crank up the lake-effect snow machine, but this cold shot will almost certainly be more zonal in nature due to the polar vortex that will form from the first arctic blast. There is a good chance that the Mid-South will experience a quick warm-up following the arctic blast as the high pressure settles into the western Atlantic and the return flow of the high brings cT air back into our region for the first week of November. This is an important fact, since any subsequent storms will have a good moisture source to work with. Therefore, this arctic blast of very cold and dry air may be just the thing to make November a wetter than normal month.