August 25, 3 PM Weather Forecast Update

Dangerous heat, some storms & heat relief ahead...

Posted: Aug 25, 2021 2:02 PM
Updated: Aug 25, 2021 4:31 PM

Much of the area did not pick up a lot of rainfall lastnight.  Some places saw only a trace to few hundreths of an inch.  The highest totals were centered around Roselawn with 1.81" measured at CoCoRaHs site there.  Around 1.50" was measured from Francesville to near Monon.

Wind gusts from the gusting out & weakening line of storms were 30-45 mph.  No measured gust exceeded 45 mph.  It is possible there were some +50 mph gusts in northern Newton & northern Jasper County & also far western Warren County, based on radar analysis.  The only gusts that looked severe on radar (+58 mph) were in northeastern Newton & northern Jasper counties.

Currently, "Ring of Fire" line of storms is riding south-southwestward over southern Indiana & that complex's outflow boundary run from Terre Haute to east of Champaign. 

It also appears to have produced a differential heating zone in our southern counties.  It is cooler in our southern areas underneath the clouds from the storm complex's tops than farther north.  This is creating a boundary along the Wabash River from Covington to Lafayette to west of Kokomo.  It is currently 80-82 in Montgomery & southern Clinton counties, but 88-95 north of that zone.  Heat indices north of that zone are trending anywhere from 91-110 with dew points 73-81.

Heat Advisory is up for all counties except Montgomery & Clinton.

Both boundaries have developing surface cumulus towers that are working to overcome capping in our viewing area.

Meanwhile, an MCV (old vortex of low surface pressure) is centered over eastern Iowa & is approaching northern Illinois.  It's outflow boundary is producing cluster of storms in northern Missouri.

The boundary in our area & the MCV all spell storm potential later today as surface heating & lowering surface pressue erodes the cap.

Microburst Composite parameters are high in eastern Illinois & expanding eastward:

Precipitable water values (PWAT) are high for waterloading in storms (heavy, heavy rainfall to pull down strong wind cores):

Downdraft CAPE numbers continue to increase.

These three parameters support risk of a couple to few wet microbursts from any more intense storm later today-evening.  Wet microbursts are small, localized wind cores that suddenly drop out of storms like a bowling ball, accompanied by torrential rainfall.

These wind cores can produce wind damage over a small area less than 2.5 miles in diameter.

So........

Even though the wind fields & shear will increase slightly with the MCV, wind fields are so incredibly weak overall in our area, modest uptick in them from that MCV won't necessarily cause great storm organization.

However, pulsey nature & some organization of a cluster or band of storms results in some severe risk.

Pretty slow moving nature of the storms & high rainfall rates will result in locally-heavy rainfall.  It will be variable though.  One spot may see +2", then another spot 8 miles away may only get 0.30".

Timing is generally 4 pm-3 am for storms.

Storms lastnight tended to cleanse the area of surface smoke, so our air quality has gone back to "Good" levels today after being "Moderate" & locally "Unhealthy for Sensitive Groups" yesterday.

Small, compact MCV may pop a couple isolated showers & storms tomorrow morning, followed by multi-cell storms tomorrow midday through afternoon with risk of a couple isolated wet microbursts.

Meanwhile, two new complexes/lines of severe storms will pivot around the periphery of the heat wave in the "Ring of Fire from the Dakotas to Minnesota & Iowa, then southern Wisconsin to northern Illinois & southwestern Lower Michigan tomorrow to tomorrow evening.

The two complexes/lines will likely merge into one & move our way.

They will be exiting stronger wind fields aloft & may encounter an environment less unstable from being worked a bit by earlier storms in our area as they approach.

So, this line should begin to gust out & weaken.

However, line lastnight, it will likely have gustiness to it with gust front surging out ahead of it.  This gust front may produce 30-50 mph gusts.  A couple isolated gusts to severe strength (+58 mph) cannot be ruled out, but it should gradually weaken as it progresses southeastward through the viewing area.

This would tend to pass in the 11 p.m.-4 a.m. time frame.

I am keeping storm coverage low to "isolated" status Friday as upper ridge flexes northward & the storm corridor shifts to Kansas to western Ontario to northern Michigan.

A couple isolated storms are possible in the strong southwest flow.

Highs of 90-96 are expected with heat indices near 100 to 112.

Storm risk looks isolated Saturday & Sunday right now with 91-97 for highs & heat indices near 100 to 113.

It appears there will be a southwest breeze 10-15 mph daily to help with the intense heat a little bit.

Storms are possible Monday with some rain lingering to early Tuesday morning.

Wind fields are weak for severe.  Any severe risk would be based on robust ML CAPE & Downdraft CAPE & it would tend to be "isolated severe".

Highs Monday should run 88-95 with heat indices near 100 to 113 as dew points to near 80 pool ahead of the front.

Only reason I went slightly cooler was the fact that there will be a lot of cumulus clouds turning the sky mostly cloudy at times.

Tuesday looks cooler & less humid with nice northeast wind 10-20 mph & highs 78-83.

Lows of 57-62 are likely Tuesday night.

Highs Wednesday, with northeast winds & lots of sunshine, should run 77-83 with dew points in the comfortable 50s.

Lows of 55-60 are likely Wednesday night.

It will turn warmer & more humid at the end of next week to the start of next weekend.

After early Tuesday morning, the next potential of rainfall is Saturday, September 4.  However, that is subject to change as it is so far out.

Also, the tropics will become increasingly active.  One wildcard are two clouds of Saharan dust in the Caribbean that will tend to inhibit development.

Thus, it would appear that the main corridors of tropical development appear to be shifting a bit for late August to early September from eastern Gulf to Atlantic to more like the Yucatan to western Gulf with potential impacts Louisiana westward. 

I think AFTER September 5, the focus will then return BACK to the eastern Gulf to East Coast for impacts as these two clouds of Saharan dust become increasingly diffuse & much less impactful.

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Decreasing clouds this afternoon with slightly warmer temperatures
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