Brief Record-Setting Heat; Chilly Fall Weather to Follow

As many of you probably have heard from your local meteorologists, a brief period of hot weather is on the way this week. In fact, temperatures may even spike as high as 96 across the area on Tuesday. Thankfully, a series of strong cold fronts will follow this brief surge of hot air, causing temperatures to drop nearly 25 degrees over a 48 hour period. On the cusp of the cold front will come scattered showers and thunderstorms Wednesday and Wednesday night. The coverage of thunderstorm activity will be dependent upon how strong differential vorticity advection becomes over the area ahead of the digging trough, as well as the depth of low level moisture and ultimately instability.

So, let's get into the details.

Here are the current analysis charts, for the surface and 500 MB respectively:

On the surface chart, you can clearly see the larger than normal temperature differences between the cloudy, cool and muggy lower Great Lakes and the warm, dry Great Plains. Temperatures across Nebraska, Kansas, parts of Missouri and Oklahoma are soaring past 95 degrees, with rather dry conditions, while the humidity remains across parts of Missouri and east of the Mississippi River, with rather cooler air in place.

But as you can see from the 500 MB chart, the heat ridge will be shifting east ahead of a weaker shortwave [the same shortwave that will ultimately usher in the development of a new Eastern Canadian trough later in the week], causing a dramatic rise in temperatures across the Ohio Valley and East by later this week. In fact, temperatures could be so high as to set record-high temperatures in places like Chicago, Indianapolis, Detroit, etc. Around here, temperatures are expected to range in between 93-96 degrees.

What are the main factors that are allowing this burst of heat?

• Ultimately, the strong EML advection from the Desert Southwest, aided by the strengthening gradient around the surface high in the plains. Because of how warm the West has been lately, this dry hot air will eventually mix to the surface, where the cooler, moist air resides and cause extremely warm temperatures. 
• Dry soil conditions- For the past three to four weeks, the area has yet to see little IF ANY rain. Grounds are becoming cracked and dry, making it increasingly difficult to harvest produce from fields and gardens. Even so, clear sunny conditions will contribute to "baking the soil" and allowing temperatures to soar far passed what they would normally reach under similar conditions.
• Clear skies and mid-level subsidence will contribute as well, especially before the ridge begins to be "squelched" by the developing upper level trough that will descend from Canada by Wednesday.
• Strong boundary layer wind and mixing will transport hot, dry air from immediately above the PBL and into the surface layer, producing rather hot conditions. This will also allow temperatures to increase quickly Tuesday morning.

12z GFS forecast at hour 120 at 500 MB

Ultimately, these factors will allow temperatures to soar well-above normal, and possibly break records in many different areas. In fact, I would not doubt that most of our current computer guidance is UNDERESTIMATING this period of heat. From my own experience, usually brief-excursions of heat usually lead to rather unusual temperature anomalies, so it would not surprise me if MOST areas reach the mid-90's and possibly the UPPER-90's. However, given this summer's rather cool 'record', I don't think it would be wise to go with temperatures above 95/96 degrees this time around. Even so, the possibility is still on the table and needs to be monitored.

However, because of hot, dry air mixing down from the mid-levels, we do not need to prepare for "unbearable" humidity levels. In fact, I think its rather likely that dew points will only approach slightly uncomfortable levels Tuesday afternoon, possibly in the mid-upper 60's [which is rather high for this time of year, but not unbearable]. Dew points will then increase into the lower 70's Wednesday afternoon as moisture pools along the approaching Canadian front.

After the cold front passes through, the weather will turn rather chilly across the area, leaving temperatures in the LOW-70's and possibly upper 60's for a few days. In fact, the above GFS forecast clearly shows that the first strong longwave trough will set up across the East and Maritime Canada later this week, keeping an attendant high pressure system across the area and the associated chilly Canadian air mass.

As for the longer range forecast, details are rather lacking, but more and more signs are coming up that are pointing to a chilly middle part and end to September but a possibly warmer October. However, don't let this fool you about winter; signs are showing that November could be our transition month, much like November 2010 [Thanksgiving week went from being in the upper 60's with heavy rain and thunderstorms (we ended up with about four inches of rain that week) to the upper 20's and lake effect snow showers by the end of it], which eventually lead to a shockingly cold and snowy winter for most of the United States. More details on what I am expecting for the long-range outlook will likely come by the end of September either on my blog or in the newspaper.