Near Record Warmth, But Rainfall & Much Cooler Weather Ahead

There is an interesting change in record highs after September 15 that signal seasonal change.....We have a noted seasonal change ahead for the weekend & especially late in the month.

Posted: Sep 18, 2018 9:07 PM
Updated: Sep 18, 2018 10:33 PM

It was another hot one today with 88-94 over the area.  The 91 at the Purdue Airport was two degrees shy of the record high of 93 set in 1891.  The records for Purdue go back to 1879.

Tomorrow's projected 90 will fall short of the 94 record set in 1891.  Thursday's forecast high of 92 will fall short of the 95 set in 1940.

It is interesting to note that after September 16, the record highs drop suddenly, signaling a seasonal change. 

After record lows in the upper 30s to lower 40s September 1-12, the record earliest occurrence of a temperature below 35 is 34 set September 13, 1964.  The earliest freeze on record occurred September 14, 1880 when the mercury dropped to 31 at Purdue.

What may tie a new record is the unusual warmth Thursday night-Friday morning.  With a fairly strong southwest wind all night, the temperature should not drop below 74.  The record warmest night-time low is 74 set in 1931. 

However, Thursday night will likely NOT go down as a tie record because I think we will drop into the 60s prior to the end of Friday.  Still it is noteworthy to have such a warm night so late in the season.

The low temperature of 71 forecast for Wednesday will be 5 degrees short of the record warm low of 76 set in 1925 for Wednesday night-Thursday morning.  However, it may end up as the third warmest.

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As of 9:45 p.m., any isolated showers/storms have faded away. 

However, it would not surprise me to see a a few more develop overnight as the very weak cold front (that popped the rainfall earlier) begins to lift back northward.  This will turn the wind from north to east, then south later tonight & into tomorrow.

Some patchy fog will develop, as well.

As the front slowly moves northward tomorrow & eventually out of our area, a couple isolated storms are possible.  The best potential will be in the northern parts of the area, it appears.

Short-term high-resolution model data hints at this (note some cells popping overnight with the front):

Notice a few cells tomorrow-tomorrow night.....best chance north half of the viewing area.

Thursday looks mostly sunny, hot, rather humid, breezy to windy with p.m. gusts to 30 mph from the southwest.  Highs will run 90-95 & heat indices 93-99.  Thursday night looks windy with lows only at a record warm 74-76.  Note the gusts +30 mph during the overnight.

Meanwhile, severe weather outbreak will take places from Texas to southwestern Ontario Thursday before the squall line gusts out to our west by late Thursday night-early Friday morning. 

Showers storms should re-fire Friday, beginning mid-morning.  These will increase & pass through right through afternoon & even evening.  Part of this moisture will be remnants of the Texas tropical low pass to our north.

The winds at 10,000' should be parallel to the surface front, so it appears that a good chunk of the showers/storms may actually occur just behind the front.  Indeed, there may be some on the actual front & a few ahead of it, but the bulk should be just behind it.  This means decreased severe risk (even isolated severe), at least for now.

The dynamics & wind fields for organized severe will be well over Ontario & Quebec, so before storms would really have the chance organize to severe, the front will tend to outrun them.  So, after highs well into the 80s to 90 in the far southeast, temperatures should fall into the 70s to 80 in the afternoon with northeast, then north to northwest wind.

In terms of rainfall totals, I think the potential there for 1-2" totals.

The weekend gets tricky.  At first I though the front would get far enough south & we would have a nice, cooler weekend.  It then looked like the front would stall just south of I-70 & we would have some periodic rain Saturday & Sunday.

Model data has flopped back the other way again to the forecast I had the first time.  HOWEVER, I am reluctant to remove rainfall from the weekend.  These paralleling winds from mid levels to surface should make it hard for the front to really clear Indiana.  Yes, there is a strong surface high over Iowa & Minnesota to Wisconsin, but I would prefer to keep 40% rainfall coverage for Saturday & Sunday with clouds/some sun at times.

I am not a model rider & detest flip-flopping in the forecast, so I still like going with this forecast for now.  Check out the GEFS model ensembles.  Note how of the 20 members, 10 keep rainfall in for the weekend in our area.  That is not enough consensus.  This, along with knowledge of hows front behave with the set-up, will prevent a switch to keep the weekend dry.

Highs in the upper 60s to lower 70s seem reasonable.

However, the front should move back northward Tuesday with increasing potential of rainfall.  It is behind this front that should briefly.....briefly warm us up Wednesday.  Highs well in 80s with high humidity & southwest to southwest should dominate, but rain & storms should occur at times, especially late.

Much cooler pattern should settle in for late September with lows in the 30s to 40s with with some highs in the 50s & 60s.  First patchy frost since late April is possible as we exit September & begin October with lows 34-37.  Freezing is likely to northern Illinois.

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