WKU Meteorology

Yesterday’s squall line dropped 0.11 inches of rain at the WKU Farm with a maximum wind gust of  27.5 miles per hour. While certainly not the most impressive total, this was the first time in nearly two weeks that a single event brought Bowling Green more than a tenth of an inch of rain. While this helped a little bit, Bowling Green is still substantially below the average rainfall for the month of September.
Typically, Bowling Green receives 4.13 inches of rain in an average September. This September, however, we have only received 1.41 inches. This lines up with the moderate La Nina that is currently occuring, which typically brings warmer and drier conditions to the southeastern U.S. That said, now that the blocking pattern has  broken down, what is the likelihood of some of this deficit being made-up?

While neither the GFS or the NAM explicitly calls for much precipitation over the course of the next few days, the National Weather Service in Louisville has included a 50% chance of rain for Monday, with accumulations between a tenth and a quarter of an inch. These chances hinge on the development of a high amplitude trough over the Eastern U.S. The forecast precipitation areas in the models have been trending west, making it more and more likely that we should see some rain.

At long last, the summer heat has broken! The dominant ridge that has been sitting in place over Kentucky for the past several days has been replaced by a trough, bringing in substantially cooler temperatures that will finally make it feel like summer has ended. While we cannot rule out the possibility of a fall heat wave, it would be rather unusual for that to occur.

The cold front associated with this transition brought our area a somewhat impressive squall line. While the SPC did not receive any reports of severe weather, a series of thunderstorms dropped plenty of rain across the state as Bowling Green temperatures dropped from the upper 80s to the mid 70s over the course of an hour. A more detailed analysis of this system will come tomorrow, as the Kentucky Mesonet releases precipitation totals for today across the state.

For the past several days, we Kentuckians have been stuck with the same aggravatingly hot conditions generated by this more or less stationary ridge over the southeast. Bowling Green started off fall with the mercury tying a record high temperature of 95°F today. Forunately, these hot conditions may only have another day left, as a long-anticipated cold front associated with a mid-latitude cyclone moving across the northern part of the country will bring cooler conditions to the area on Saturday, along with a chance of thunderstorms as the front moves through Friday afternoon.. Forecast highs over the next week should drop into the low 70s by Monday, with a slight rebound towards the upper 70s by the end of next week. This should also bring a fair amount of precipitation into the region, with the National Weather Service calling for a 40% chance of rain on Monday.

Of particular interest is the way that the GFS and the NAM are handling the increased odds of a precipitation event on Monday. The NAM, shown below on top, does not call for any rain immediately over Bowling Green, instead favoring precipitation falling more or less everywhere around it. On the contrary, the GFS, on the bottom, is calling for upwards of 2 inches in the Warren County area. While chances are that the GFS is overestimating the amount of precipitation associated with this airmass, it is indicative that our dry spell may finally be coming to an end.

The National Weather Service has issued a Red Flag Warning for Warren County for more click here.

Hot conditions will continue for the next few days, and Bowling Green even has a shot at breaking its record high of 98 today. On Friday, the cold front should finally approach the area, bringing a limited shot at some rainfall. Highs could still be in the low 90s on Friday, depending on how much cloudiness and rainfall we get, but by Saturday the front will be through the area and a much more comfortable air mass will be in place.

For the last several runs, both the GFS and the ECMWF have been calling for a lobe of energy to break off from the jet stream and drop south somewhere over the central Plains or Mississippi Valley, possibly forming an upper level cutoff low. The models differ on the timing, location, and magnitude of this feature, but they agree that some sort of cutoff low should form somewhere over the central U.S. beginning Sunday. This is something to watch for over the weekend as it could bring cooler temperatures and some good rain chances for the Bowling Green area.

12Z GFS forecast of 250 mb heights and wind speed at 7 AM Monday, showing the lobe of cutoff jet stream energy over the Plains.

High pressure will continue to dominate the area over the next few days, keeping the hot and dry weather around. Temperatures are forecast to top out in the mid 90s on both Wednesday and Thursday. There is a slight chance of a shower or thunderstorm tonight, as both the RUC and the GFS paint a few hundredths of an inch of precip over the area. However, the low levels of the atmosphere are very dry due to our continuing lack of rainfall, and this rain chance will probably not amount to anything.

Over the weekend, the persistent ridge should break down enough to allow a cold front to pass through the area and bring cooler conditions. Moisture will be limited with this boundary and the surface cyclone will be well to our north, but there still might be enough convergence to fire some scattered thunderstorms on Friday and Friday night ahead of the front.

Forecast surface map at 7 AM Saturday morning from the HPC, showing cold front past Bowling Green.

With a broad ridge of high pressure still parked over the mid-South and the jet stream well to the north of the area, conditions are forecast to stay dry and warmer than normal for the next several days. Average highs for Bowling Green this time of year are around 80, but high temperatures are expected to be in the low 90s until at least Friday with nighttime lows in the low to mid 60s. Temperatures will peak around 95 on Tuesday as the center of the ridge moves over the area. On Wednesday a frontal boundary may move close enough to the area to provide some cloud cover and slightly cooler temperatures, but the front will retreat on Thursday. Over the weekend, the front is forecast to finally push through the area and bring relief from the heat, but this is a long way out and subject to change.

Bowling Green has only received 0.93 inches of rainfall in September, which is 1.74 inches below average. We are still 2.05 inches above average for the year to date, but with the continued dry forecast, there is concern that drought conditions may develop in the area. Here’s a quick glance at NOAA’s latest drought monitor and seasonal drought outlook.

More complete drought reports can be found at http://www.drought.unl.edu/dm/monitor.html.

The passage of the cold front early this morning has left the area with much cooler and drier air in place, but the heat shall return for the weekend and well into next week.  The main reason for this is due to the overall lack of soil moisture.  This lack of soil moisture is leaving the boundary layer much drier, and the jet stream pattern has gone to a more summer-like pattern as opposed to something that would be expected in mid-September, with the main core of jet energy well to the north over southern Canada, leaving much of the United States high and dry.  Lack of moisture is going to allow for wide temperature fluctuations between day and night, and indications show that next week daily maximum temperatures could reach into the mid 90s—rather unseasonable warmth for this time of year.  This pattern is somewhat reminiscent of what was seen during the summer and early fall of 2007.  All indications are that the next week will be dry and hot, only exacerbating the drought conditions.

The 17Z GOES Skew-T sounding shows much greater CAPE than earlier, just shy of 4000, as well as a lifted index of -7.  The SWEAT index was also close to 300 as well in this sounding.  At the same time, the latest (18Z) SPC Mesoanalysis does not concur with that very well at all, showing CAPE of 1000 in the area.  Much of the dynamics needed to fuel severe thunderstorms are concentrated farther north in Illinois, Indiana and Ohio, and storms are already firing up in central and northern Indiana and into northern Ohio.  All of that being said, severe storms cannot be ruled out here in the area, but conditions just are not that conducive here for widespread severe weather.  Severe weather, if any, will likely be very isolated.

A low centered about over Lake Michigan is bringing Bowling Green area much of the Mid-South some rain and thundershowers this morning.  Indications from the SPC, yesterday, showed a Slight Risk for severe weather, but in accordance with some of the latest model runs, that threat has been shifted more toward the east, with Bowling Green and places to the west and immediately to the east of I-65 not really expecting to see the greatest amount of convection, as these areas are on the far western extent of the risk area.

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