July 18, 11 PM Weather Forecast Update

New data is coming in regarding temperatures for the rest of July to August & t'storm risk this weekend & next week.

Posted: Jul 17, 2020 10:03 PM
Updated: Jul 18, 2020 5:57 PM

Growing degree days or GDDs are a measure of how much heat we have accumulated in a given growing season. 

With 50 degrees being the baseline & 86 being the zenith (nothing accumulates below 50 or above 86) for the mean temperature.

So, if your high was 78 & your low was 45, your mean temperature would be 61.5.  61.5 - your 50 base would = 11.5 GDDs.

If your high was 67 & your low 34 your mean would be 50.  50-50 = 0 GDDs.

Normally, the viewing area sees 2900-3500 GDDs per season from north to southwest.

GDDs are instrumental in certain floral & faunal life cycles.

For example, the "old-fashioned" lilacs usually begin flowering when 100 GDDs are reached.  Northern Catalpa flowers at around 320 GDDs.  Sugar Maple begins to flower at 50 GDDs.  Flowering Crabapple begins flowering at 55-60 GDDs. 

Butterfly bush will normally flower at 600 GDDs.

However, Silver Maple may begin to flower at only around 15.

Wheat needs about 1400 to complete maturity.

Corn needs 100-120 GDDs to germinate, but these are NOT based on soil temperatures.

Our GDDs show the pattern of warmth since April 1 in our viewing area after calculating & averaging all observations sites from Mesonet to ASOS/AWOS to COOP in our area.

The strip of 1700-1800 extends from our SE Covington site to near Chrisman & Paris, Illinois then to Terre Haute, Indiana.

Who has the greatest number of GDDs for 2020 so far?

It is in the southwestern tip of Indiana, west of Evansville near Mt. Vernon with +1900.

These current tallies are near normal (though it has been a hot June & July) due to such cold weather in part of May with some all-time May record low temperatures broken (as low as 23 degrees in May).

The southwestern Fountain County GDDs are a bit above normal, however.

Dr. Walt Beineke of Purdue University Forestry Genetics brought up a great point in that photo period is also a determinant of leaf, flower emergence & processes in plants.  Day length triggers processes in plants that trigger flowering, etc. 

One great example are Asters & Goldenrod species in the Fall.  They are not triggered by increasing daylength, but decreasing daylength triggering them to bloom!

Trees will also bud much faster in May when it is 65 than in March or April when it is 65, too (due to increasing sunlight hours).

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Nasty widespread damaging wind event (potential Serial Derecho) in the "Ring of Fire" will track from North Dakota & Manitoba to Minnesota, then northern Michigan & western Ontario.

Here, looks like nothing more than a small isolated shower/storm that can penetrate the cap.  The cap will be very strong, so the only reason I mention this is the subtle MCV tracking through Illinois to our area with some high/mid clouds & some cumulus.

Highs of 92-97 are expected with heat indices 100-107 with south-southwest wind at 10-25 mph.

MCS or bowing complex of severe storms to our northwest looks to completely collapse early Sunday morning, though we will pick up the convective debri (cloudiness) in the morning hours.

However, thinning of those clouds, increasing boiling heat & the MCV & outflow boundary look to be enough to overcome the strong capping in place.

A line of scattered storms is likely after 5 p.m. & before 11 p.m.

Given the boiling instability, a couple/few severe storms are possible with wind/hail.  Storms will be a bit pulsey in nature due to lack of good bulk shear & wind fields aloft.  However, the MCV will help to add a bit of bulk shear to the mix.

Highs of 92-99 with heat indices of 102-111 have been expected.  If clouds stay here a bit too long, then those numbers may be trimmed a hair.  I will keep them for now.

There is not much going on Monday with lack of a decent trigger.  Partly cloudy to mostly sunny skies & 90-95 with 96-107 heat indices should suffice.  I kept a 25% POP in the forecast for consistency & persistence forecast purposes.

Better storm risk comes in Tuesday & Wednesday with highs 88-93 & heat indices 90s to lower 100s possible.

Looks like a round Tuesday & Wednesday from a couple of MCVs & an eventual surface cold front & upper trough that contracts the hot upper ridge some.

It looks dry Thursday with 86-91 with heat indices 90-96 with sunshine.

88-93 are likely Friday, followed by 90s next weekend with scattered storms possible.  Heat indices at & exceeding 100 are likely given tropical flow from the south with south-southwest winds at 10-25 mph.  The nights look warm & muggy, too (in the 72-77 range).

There just isn't sustained heat & humidity relief currently seen until early August.

Hot, humid weather with above normal temperatures dominates now to July 31.

At least now-July 31 shows promise for welcome rainfall for the actively-growing & maturing crops.  We just don't need the wind & hail from any severe t'storms.

August does look overall warmer than normal here & cooler than normal in the Northern Plains & Rockies.

Looks drier than normal unless we see a tropical system make landfall on the Gulf Coast & move north to our area.  That is possible.  Otherwise, wet, stormy pattern dominates the Northern Rockies & Plains through Great Lake to New England with hot, dry weather Southern Plains, Midwest & Southeast (unless tropical systems impact that area).

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Some snow Wednesday to Wednesday night.....
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