Bowling Green and clipper systems don’t mix

Any student who has taken my synoptic meteorology course knows that there is a very good reason snowfall forecasts tend to bust for Bowling Green. Synoptically speaking, every good snowfall storm track tends to miss this region. The three primary snowfall tracks are 1) Nor’easters, which typically only bring snow to far eastern KY (Dec 19-21), 2) Plains blizzards, which put us in the warm sector as all rain (Dec 24-25), and 3) Alberta Clippers (or Manitoba Maulers, if you will), which are most likely to break our hearts with a busted forecast (Today).

As I tell my students, clipper systems that drop down from the central plains of Canada are typically only good for snow across southern Indiana and the Bluegrass. The main reason why is because the moisture is so limited with these storms and forecast models (most notably the NAM) ALWAYS overestimate the southern extent of the precipitation shield. Time after time, models will suggest that several inches of snow are possible in Bowling Green from a clipper system only to have less than an inch verify.

The key to forecasting these systems is to follow the track of the 500 mb low. The best snowfall will occur in the difluent region downstream of the 500 mb vorticity maxima. As in this case, the vort max is over central IL, which means the heaviest snow at this time (Thursday afternoon) is over central IN. What the models never pick up on with their QPF is the very sharp snowfall gradient just south of the track of this difluent zone. As the radar shows, the precipitation shield becomes very raggedy from the Ohio river south. Since these clipper systems do not tap into Gulf moisture, the only moisture available for snowfall is from the dynamics associated with the vorticity maxima. So if you are located south of this vort max, you will always receive less snowfall than forecast due to lack of moisture. Hence, the snowfall forecasts for Bowling Green went from 4+ inches a few days ago, to 2-4 inches yesterday, to 2 inches last night, and now stand at less than an inch this morning.

The other problem with these clipper systems is that they almost always verify farther north than the models suggest, especially at the 48-72 hr forecast window. Hence, earlier models had the vort max digging much farther south over central KY, which also contributed to the busted forecast for south-central KY. Climatologically, the preferred storm track for clipper systems is from the Dakotas to central IA to central IN, which means climatologically, the 500 mb vort max and its associated snowfall shield will only clip northern Kentucky and cause lower than expected amounts in Bowling Green.

Finally, in less arctic conditions, the brief shift to a southerly flow ahead of the attendant surface low is often enough to cause these clippers to bring rain to Bowing Green, while the Bluegrass remains cold enough for snow. In this case, the boundary layer was cold enough everywhere for snow, but in past years when temperatures were close to freezing, a snowfall forecast for a clipper in Bowling Green ended up verifying as light rain and drizzle with a few flurries.

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