850 mb sweet spot?

[Warren Eckels]

Is there a sweet spot regarding dewpoint depression at 850 mb that promotes convection, or is the rule simply "the more moisture, the better"?

I', asking because a thin line of garden-variety thunderstorms approached Lake County, Indiana around 1:00 pm; they were entering a regime of surface temperatures and surface dewpoints that was slightly lower than the Illinois regime which maintained them well below severe guidelines.

At about 1:40, a storm W of Crown Point blew up as it moved northeastward. Winds approached severe levels in Merrillville as the line broadened and backfilled. Around 2:15 pm on an errand, I saw a cell behind the line started to develop near the I-65/US 30 interchange. Maybe I'm imagining things, but I thought I saw a wall cloud develop. (The winds immediately behind the squall line were from the ESE, upper level winds were from the southwest, but weak).

It seemes strange that a squall line would wait until it reached a region of lower CAPE to intensify. One somewhat unusual factor was a pool of relatively dry air at 850 mb behind the remnants of Tropical Storm Arlene; the intensification coincided rather nicely with that increase in T-Td at 850 mb.

Have any of you observed a similar situation, that is, an increase in storm intensity coincident with a decrease in moisture at 850 mb? (Around here, 850 mb was about 1300 m overhead yesterday).

 

[Mike Johnston]

Well, somewhat speculation on my part, but if there were higher temps at 850mb to the east in Indiana, there may have been some local capping inversion in place - allowing moisture and heat to build as the day progressed - and then, when sufficient lift approached coincident with the line of t-storms from IL, the cap was broken releasing this energy. The stacked low system yesterday seemed to be occluded, and there were evidently some temperature inversions in place over various areas downstream - such as Iowa.

Besides the thermodynamics, another observation re: yesterday was that the surface low seemed to trough out a little to the east and north. Therefore, winds at the surface in the prime target areas were not backing very well, and in fact veering in many places. I wasn't closely watching Indiana, but perhaps there were some backing surface winds that far east? If so, maybe low level shear was enhanced which also contributed to intensification there.

 

[Warren Eckels]

Arlene winds

Let's see... Data from Weather Underground

All times are CDT

Winds in Valparaiso (KVPZ, about 14 mi east) on June 13

10:53 WSW 7
11:53 SSW 6
12:53 W 6
13:53 SSE 7 *** squall line approaching from southwest
14:17 SSE 6 *** squall line almost overhead
14:44 WSW 14 *** rain, thunderstorm

Winds in Gary (KGYY, about 10 mi NW) on June 13. Note that KGYY is about 2 mi SW of Lake Michigan.

10:50 WNW 8
11:50 WNW 8
12:50 Var 6
13:50 Var 6 ***squall line passed through 10-15 min earlier
14:50 SE 8

I do not remember winds backing before the line passed through Merrillville, but I was observing the storm through a window...

Now, the batch of cells that developed in back of the squall definitely had directional shear to work with. Where I was, the wind switched to the ESE at the surface while the cells moved from the WSW. Fortunately for the populated areas of NE Lake and N Porter counties, no severe weather developed.