Thursday, July 28, 2011

Heavy rain in the midwest, tropical storm down south

Repeated rounds of storms around the periphery of the still-persisting ridge in the southern US have contributed to rainfall totals that are well above climatology for the last few weeks in some locations.  In Chicago this is the wettest July on record with 9.75 inches so far this month.  Furthermore, this month is now the 9th wettest month of all months in Chicago since records began--pretty impressive.

There's a lot of water vapor to work with in the atmosphere over that region at the moment.  Here's the satellite water vapor image today:
GOES-E water vapor image from 1615Z, July 28, 2011.
The upper-level ridge across the south causes air to advect clockwise around its periphery.  As such, you can see a big plume of moisture stretching from Arizona and New Mexico through the central plains, into the midwest and out through the mid-Atlantic states.  This plume of moisture has been in place over the same area for several days now, and any storms that form in that area follow the same path.  Repeated rounds of storm after storm (MCC after MCC...) following the same path have contributed to the high rainfall totals.

The more moisture there is in the atmosphere, the greater the potential for heavy rainfall.  This seems like a very obvious statement.  To that end, one parameter that meteorologists use to evaluate the potential for heavy rainfall is called the precipitable water value (often abbreviated as PWAT or PWTR).  It's a pretty simple index to derive--basically, all precipitable water does is look at the profile of moisture content over a given location.  It calculates how deep the water on the ground would be if all of the water vapor over a given location were to immediately condense out and fall as rain.  It's saying, if all the water in the air over your head immediately condensed into rain and fell out, how much rain would fall.  Clearly this is never fully realized, as the atmosphere never dumps out all of its water vapor content at once.  But, in general, the higher the precipitable water values, the higher the likelihood for heavy rain.

Most soundings that you see will compute the precipitable water value for that sounding location.  The HOOT website soundings are no different.  Here's this morning's 12Z sounding from Davenport, Iowa, which is in the middle of that moisture plume.
KDVN sounding from 12Z, July 28, 2011.
First, let's look at this sounding for a moment.  Notice how the dewpoint trace (the green line) is rather close to the temperature trace (the red line) from the surface up to somewhere around 430mb.  That's a very deep layer of very moist air.  We'd therefore expect the precipitable water values to be rather high.  You can see the calculated preciptable water value under PWTR above the upper right corner of the sounding.  The black value is the value for the current sounding, and it shows a PWTR value of 2.29 inches.  So that's saying that, based on this sounding, if all the water vapor in the atmosphere over Davenport were to immediately condense out and fall as rain, 2.29 inches of rain would fall.  That's a lot.

But we've already established that this is a record-breaking month for rainfall in this region.  Just how anomalously moist is the atmosphere within this plume of moisture?  The Rapid City, SD forecast office of the National Weather Service has put together a really nice webpage with climatological values of precipitable water for each sounding site.  Here's their yearly graph of climatological precipitable water values for Davenport:
Monthly climatological precipitable water values for KDVN.
There are several curves on this graph, which are explained by a legend in the upper left corner.  Each curve represents a different percentile of the normal precipitable water values based on the averages from 1995 to 2010.  The mean value (the 50th percentile) is given by the red curve.  You can see that for the month of July, Davenport's mean PWTR value is about 1.2 inches.  This morning's value of 2.29 inches is well above that.  In fact, if you look at the range of values for July, you'll see that 2.29 inches isn't quite up to the maximum curve (the light green curve) but it is above the 99th percentile curve (the dark green curve).  That means this precipitable water is in the 99th percentile of all PWTR values seen during July over the past 15 years or so.  That's very high.

Furthermore, the dashed green line shows a value that is two standard deviations above normal.  This value is often used by forecasters to try and separate extreme values from more climatologically normal (but still high) values.  Any time you're more than two standard deviations above normal, it's considered to be a more "extreme" case with a high potential for heavy rainfall.  2.29 inches is definitely above the two-standard-deviations line.  All of this points to the potential for very heavy rain.

Unisys publishes an interpolated precipitable water map for the US after every sounding release time.  So, here's their map of precipitable water across the US this morning at 12Z:
Interpolated precipitable water at 12Z, July 28, 2011.
There's a swath of elevated precipitable water values from eastern Nebraska east through southern Michigan.  This is the corridor where rounds of storms are able to produce very heavy rainfall and bring us these extreme events.

Also notable is the large area of very high precipitable water values right on the Gulf Coast.  They cite 2.4 inches at a point near Lake Charles, Louisiana.  There, too, precipitable water values are in the 99th percentile and thunderstorms with heavy rain are forecast for the next few days.

Of course, a very potent rainmaker looks to be bearing down on the Texas gulf coast in the next day or two.  Tropical Storm Don has formed in the Gulf of Mexico and is moving northwestward.

NHC predictied 5-day path cone of TS Don as of 10AM CDT, July 28, 2011.
It's not forecast to strengthen beyond tropical storm strength, which is good news for the gulf coast in terms of lessening the potential for damaging winds.  What's even better news is the amount of rain this storm could bring to central Texas, which is currently suffering from "exceptional" drought, as seen on the map below.  In contrast, with the extreme rainfall over Chicago recently, I'm pretty sure the little "A" for "abnormally dry" over northern Illinois is going to be disappearing soon...
US Drought Monitor from the Climate Prediction Center/UNL, as of July 19, 2011.

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