With winter safely behind us, the next weather-related topic to look forward to is the first 90 degree day of the year. I did some research this afternoon and found that during the last 50 years for Bowling Green, the earliest 90 occurred on 4/10/1995 while the latest was 6/25/2003 with a median date of 6/2. For Louisville during the same time period, the earliest occurred on 4/23/1960 while the latest was 7/4/1982 with a median date of 6/7. So what can we expect this year?
Synoptically, the best way to get an early season reading of 90 is to have a 500 mb ridge develop over the Rockies/plains for about a week that produces near-record temperatures in that region. Next, a negatively-tilted trough should move into the western U.S. which will push the ridge eastward over our area and produce enough low-level warm-air advection to maintain or even increase heights over the central U.S. Oftentimes, the negatively-tilted trough will produce a negative tilt in the ridge axis, meaning the ridge will extend from the northern plains southeastward into the Southeast. This can allow for a cold high to drop out of SE Canada into the Great Lakes. It is often just south of this frontal boundary that early season temperatures can soar into the lower 90s in the mid-south. Here is a nice example of such an event.
Looking into next week, it appears that this type of scenario will play out, although it is uncertain at this time whether 90 will be reached in KY. By early next week the ridge will be situated over the Rockies. By mid-week, a large trough will push across the Pacific Northwest and the ridge will move eastward. By next weekend, the ridge axis will be centered right over the eastern third of the country, which suggests temperatures in the middle 80s with dewpoints in the middle 60s. The CPC is fairly confident in this scenario. My main concern is a weakening shortwave that tries to undercut the ridge Thursday-Friday that could produce clouds, showers and thunderstorms, which could keep temperatures in the upper 70s. There is quite a bit of uncertainty in the ensembles with this shortwave (the shortwave is the kink in the red 576 line). Stay tuned on that. The current high temperature of the season is 84, last reached 4/3.
