A Wet...Wet...Wet July

A Wet…Wet…Wet July
Despite a dry start to the month, July 2011 has moved into the record books as the wettest July on record and one of the wettest months ever on record for Chicago. The official climate sites for Chicago (O’Hare International Airport) and Rockford reported no measurable rainfall for the first 10 days of the month, and for the first 21 days, only reported 0.45 and 0.19 inches, respectively. Since then monthly totals as of July 29th have risen to 11.15 inches and 4.60 inches, respectively. For Chicago, this is now the 7th all-time wettest month on record.
The so-called “Ring of Fire” pattern (figure 1), which has been responsible for the recent heat and rain, occurs when an extension of the Bermuda High, a semi-permanent region of high pressure over the Atlantic Ocean, becomes anchored across the southern or southeastern United  States.  
 

300MB Geopotential Height

Figure 1: 300mb Geopotential Height Composite for July 1st to July 24th 2011 showing upper level high anchored across the southern United States.
 
This high pressure is also reflected at the surface (Figure 2) across the southeastern United States with a quasi-stationary frontal boundary lingering along the northern periphery of the high. While areas south of this frontal boundary baked under intense heat, moisture from the Gulf of Mexico wrapped around the high, pooling along the frontal boundary, setting the stage for periods of record setting rainfall. July 22nd and 23rd alone saw 8.41 inches of rainfall in Chicago, which includes a single day record for July 24th of 6.86 inches.

 

Mean Sea Level Pressure

Figure 2: Composite Mean Sea Level Pressure (MSLP) for July 1st to July 24th 2011 showing a ridge of high pressure extending from the Southwestern United States.

 

Over the past week, as the quasi-stationary frontal boundary pushes up against the ridge of high pressure and is held up, it takes on a west to east orientation. Steering winds become aligned parallel to the front, meaning that thunderstorms that develop on the boundary then move east (slowly at times due to the weak winds aloft) along the boundary. The sounding from Davenport, IA during the evening of July 23rd in figure 3 shows this setup. Notice the weak winds in the column, only about 50kts at 200mb. The steering winds, which can be approximated by the 850-300mb mean wind, are nearly westerly, parallel to the frontal boundary as analyzed in Figure 5. Also notice the very high precipitable water values of nearly 52mm, or just over 2 inches, indicative of the high amount of moisture in the atmosphere.

Davenport Sounding

Figure 3: 00Z Sounding from July 24th at Davenport, IA.

As additional thunderstorms continue to develop, they will travel a similar route along the front. This process, called thunderstorm “training” leads to prolonged periods of heavy rainfall over a given area, and are often responsible for flash flooding. Comparing the northern periphery of the high (look at the white area on Figure 2) with the precipitation totals on Figure 4, it’s easy to see where this frontal boundary has been positioned on average over the past couple weeks.
 

Monthly Precipitation

 Figure 4: Rainfall totals from July 1st to July 28th 2011 (From Midwest Regional Climate Center)
 

 Figure 5: 00Z Surface Analysis from July 24th.

So how long will the pattern persist? As of July 29th, long-range model guidance shows minimal variation in the overall synoptic pattern. Outlooks from the Climate Prediction Center show elevated probabilities for above normal precipitation and temperatures in both the 6 to 10 day outlook and the 8 to 14 day outlook suggesting we may be in this wet pattern for at least a couple more weeks. The one-month outlook, however, shows equal chances for above or below normal precipitation signaling a possible end to the very wet weather. 

 

7/29/11 BMD



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