After nearly a month of below normal temperatures across the Mid-South we are finally looking at the first truly big winter storm of the season. Unfortunately for those who love snow, most of Kentucky will be on wrong side of the rain-snow line for next weeks storm.
The synoptic pattern that has led to the recent turn to colder has been a pair of ridges located over the West Coast of the United States and the western Atlantic Ocean. This pattern is effectively shown by the current conditions and the primary result for the central United States is a deep trough with a northwest flow. This fast NW flow is not conducive to storms in our region since the pattern is very progressive due to the relatively long wavelength, which keeps storms from intensifying. What also hurts us is since the mean trough axis is located to our east, there is no way for any tropical moisture to get pulled into any storms until they are well to our east. Hence, most of the big storms that have developed out of this pattern have done so off the East Coast, well out to sea.
The trough axis is beginning to move westward however, which means the next two weeks will be very stormy over the Mid-South. By Monday, shortwave energy diving over the ridge and into the Southern Plains will help deepen a surface low in West Texas. Note the location of the arctic high just off the East Coast. Since there is nothing downstream to keep this high in place, the arctic high will be pushed farther offshore by the next arctic high that on the Monday map is located over southern Saskatchewan – note the cold front on the 850 mb map that separates the two air masses (look for the kink in the height field that connects the Texas low with a low over James Bay). The location of this departing offshore high and the timing of the fresh arctic air mass will almost certainly keep Kentucky warm enough for all rain, even as the Texas low cuts south of our region. By Tuesday, overrunning rain will overspread the region as Gulf air brings 50+ warmth into the Mid-South. Farther north across southern IL, IN and OH, it will probably be cold enough for snow, although the location of the rain/snow line is still uncertain. What is certain is that a departing high will always bring a warm, southerly return flow that will replace our arctic air from the upcoming weekend with mT air by the time the precipitation begins Monday night into Tuesday. By the time the arctic air finally does arrive on Wednesday, most of the moisture will be gone, although I do expect a changeover to light snow at the end. Note that by Wednesday, the storm has deepened into a blizzard for the Appalachians and a heavy rainstorm for the East Coast.
The lesson here is that for Kentucky to get a big snowstorm from this type of synoptic pattern, an arctic high needs to be anchored over the Ohio Valley by a downstream storm in the Maritimes. This slows down the storm and allows for greater deepening, greater moisture convergence, and a nice cold airmass for overrunning snow to fall into. In this case however, with no downstreams storms there is nothing to keep the arctic high from departing eastward into the Atlantic. The good news is that as the main trough axis continues its westward march over the next two weeks there will be many more storms forming in our region. This is backed up by the CPC longrange guidance for the 6-10 day and 8-14 day periods. With so much cold air nearby and an active storm track, there should be some snow chances in our future before Christmas.
