Last week the Climate Prediction Center was predicting that the last few days of August/first few days of September would be cooler than normal (normal is High 85/Low 63) for the Mid-South. As we now approach the time period that was predicted to be cooler than normal, it is clear that the models are consistently underestimating the scope of this historic hot/dry August, as predicted temperatures over the next few days have highs near 90 with lows in the upper 60s.
The underestimation of the heat wave by guidance has been consistent all month, as none of the MOS values from days 3-5 predicted temperatures above 99 degrees. On 8/15 and 8/16, when the temperature soared to a record-breaking 104 and 106 in Bowling Green respectively, MOS guidance for day 1 was upper 90s and the official NWS forecast for day 1 was for a high of 99 both days. That’s a 5-7 degree error on day 1!!
The point is that the hot (but not necessarily the dry – as you will see later) conditions are not going to be leaving anytime soon, regardless of what the models suggest. The primary synoptic reason is that with a developing La Nina, the storm track and the supporting troughs will be too far north to allow frontal convection to hold together as the cold fronts move away from the troughs and try to push into the ridge. Yes, heights over the Mid-South will lower as these fronts approach, but the feedback mechanism of the dry soil will quickly allow the ridge to build back in and allow temperatures to quickly warm up. This means that any cool highs that bleed down from Canada will have at most a one-day brief cooldown quickly followed by a warmup to well above normal.
There is some evidence, however, that along with a return of heat during the post Labor Day period, the Mid-South could be in a more favorable position with regards to precipitation. Note the 850 mb flow for Wednesday, Friday, and Monday. You’ll see that for the next week, low-level advection of moisture from the Gulf of Mexico is non-existent in our area as the flow is weak and disorganized. During the middle of next week however, guidance suggests a trough will dig over the northern Rockies, and that starting Thursday and continuing through the weekend, low-level moisture from the Gulf will stream into the Southeast and provide fuel for whatever frontal system is associated with the trough.
There is no certainty with this as shown by the comparison of the 8-10 days means of the ECMWF and GFS. The GFS is much stronger with the digging trough than the ECMWF but both show the possibility for increased low-level moisture for our area.
Continued hot with highs around 90 from now through Labor Day. Widely scattered showers/t-storms can be expected through Thursday (8/29) then dry thereafter.
After Labor Day: Hotter and more humid with more widespread showers/t-storms possible Thursday-Saturday. Highs could again reach the upper 90s prior to the frontal passage.