Saturday, July 23, 2011

Mesoscale Discussion #1715

Discussion... window of opportunity exists for organized severe storms to erupt in area outlined above. From 9:00 PM CDT to 10 PM, it is possible organized severe convection will occur in that area. Problem with that assumption is how it is possible CINH is increasing. CINH is a warm layer in the atmosphere that suppresses stronger storms. Current radar indicates a small line of strong storms has already develop, but it remains to be seen if it will be the high CAPE or CINH that will win out for the storms.
Should the CAPE win out and storms develop, they will work their way SE, entering the heavily-hit Chicagoland area overnight yet again, with several more inches of rain possible. Radar indicates that other than congealing more into a small line, no strengthening has been observed in the storms themselves, which would indicate there is some resistance in the atmosphere to letting the CAPE win out.
I predict that the CAPE will probably come through and ignite a more broad expanse of storms. If the SPC believed the CINH would dominate, the slight risk of severe weather would have been removed by now.

July 23, 2011: Images of Thunderstorm



Radar during July 23 Chicago storms.

We were able to get a couple of images of the radar during the storm's evolution that started the flooding in Chicago.
In this image we see how the line of storms developed in Cook County and appeared to start moving north. Out west, we see the bowing line structure approaching North Illinois.
In this image we see how storms had regenerated and kept popping up in the same area over and over again, non-stop.

The Weather Centre Image Library (TWCIL)


Copyright The Weather Centre

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Severe Weather Forecast Discussion- July 23 Afternoon

Mesoscale Discussion #1708 has been issued for North IA and Minnesota.

Discussion... Thunderstorms have initiated in Minnesota and are congealing into a line moving SE. This line will strengthen into a full blown squall line, which may weaken as the line moves SE, according to short range models.
Because it appears the SPC is seeing something in the models that i'm not, I don't feel it is proper to give out a discussion at this time. However, when it becomes clear, I will provide one.

Mesoscale Discussion #1708

Terms to know:
*Synoptic Front: A front. Example: cold/warm front.
*Deep Layer Shear: Winds moving in different directions and at different speeds.
*Forcing: A mechanism that usually enables warm air to rise, which creates thunderstorms.
*


MESOSCALE DISCUSSION 1708
   NWS STORM PREDICTION CENTER NORMAN OK
   0152 PM CDT SAT JUL 23 2011
   
   AREAS AFFECTED...FAR ERN SD...MUCH OF CENTRAL AND SRN MN AND FAR NRN
   IA.
   
   CONCERNING...SEVERE POTENTIAL...WATCH LIKELY 
   
   VALID 231852Z - 232015Z
   
   STORMS ARE EXPECTED TO INCREASE IN COVERAGE AND INTENSITY ACROSS
   PORTIONS OF CENTRAL/SRN MN THROUGH THE AFTERNOON...AND ALSO POSSIBLY
   DEVELOPING WWD INTO ERN SD AND SWD INTO NRN IA. A WW LIKELY WILL BE
   NEEDED IN THE NEXT HOUR OR SO FOR PORTIONS OF THIS AREA.
   
   ELEVATED STORMS HAD RECENTLY DEVELOPED IN AN E-W BAND ACROSS WEST
   CENTRAL MN...NORTH OF A STC-VVV LINE. THESE STORMS WERE LOCATED
   AHEAD OF THE SEWD MOVING SYNOPTIC FRONT...THAT EXTENDED FROM NWRN MN
   SEWD INTO ERN SD. SUFFICIENT DEEP LAYER SHEAR AND INSTABILITY EXISTS
   FOR MARGINALLY SEVERE HAIL WITH THESE STORMS THE NEXT HOUR OR TWO. 
   
   HOWEVER...WITH ADDITIONAL HEATING/BOUNDARY LAYER DESTABILIZATION AND
   STRONGER LARGE SCALE FORCING SPREADING IN FROM THE WEST... MORE
   STORMS ARE FORECAST TO DEVELOP AND BECOME SURFACE BASED. ONCE THIS
   HAPPENS...EFFECTIVE SHEAR AT 40-50 KT AND MODERATE INSTABILITY WILL
   BE FAVORABLE FOR SUPERCELLS. WHILE SEVERE HAIL/WINDS SHOULD BE THE
   PRIMARY THREATS...THE VERY MOIST BOUNDARY LAYER AND THE FORECAST OF
   STRONGLY VEERING LOW LEVEL WIND PROFILES WOULD SUPPORT A TORNADO OR
   TWO. BY LATE AFTERNOON...HIGH RESOLUTION AND OPERATIONAL MODELS ARE
   IN AGREEMENT OF STORMS CONGEALING INTO A SEWD MOVING LINEAR MCS...
   WITH WIND DAMAGE BECOMING THE PRIMARY THREAT AT THAT TIME.

Chicago Past 24 Hours Rainfall- July 23 Flooding

This is an estimated 24 hour rainfall total. Notice the 3-6 inch totals in North Cook County into Lake County, IL.

July 23, 2011 Chicago Flooding Pictures

Copyright The Weather Centre
Retention Pond Flooded


Gazebo underwater

Full view. Est. depth: 10 feet+

Lake unusually full

Boat dock ravaged by flooding

Children's play place under water

Beach sign underwater

RECORD REPORT

Chicago, IL has recorded its wettest day ever today, July 23, 2011.
So far, 6.91 inches of rain has occurred, and it may increase.
We will watch this situation.

July 22, 2011 Bismarck Storms Q & A

Q: Was the line of storms a derecho?
A: Looking at storm reports, it looks like there were violent wind gusts of 60 MPH, so it can be defined in at least some ways as a derecho, or at least a bow echo.

Q: Was there a tornado in Bismarck?
A: I had issued a tornado warning for Bismarck, but so far, there is no evidence of a tornado.

Early July 23, 2011 Chicago Storms Q & A (Historic Rainfall)

Radar as of 12:30 AM CDT
Q: Around 11 PM, I started hearing all this thunder and lightning, but it kept going for hours. Why?
A: We tracked this cluster on radar. A few pop-up storms developed around 11 PM in southern Cook County. It started out as only a few cells. Those cells intensified and congealed into a line. Behind that line, another line of elevated storms developed. Then another line. It just wouldn't stop.

Q: Why was the thunder so loud?!?
A: Some storms just happen to have very loud thunder. These storms were in that category. Also, CAPE values were considerably high, so that likely helped the storms become very active.

Q: I heard a LOT of rainfall. How much?
A: From 1 AM to 7 AM, officially 6.91 inches of rain was measured at O'Hare Airport in Chicago. That make it the wettest day in history in Chicago.

Q: Is that rainfall total historic?
A: Yes.