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.

Poundings for Florida ending; powerful shortwave next week

As many people have noted recently, the end of March has been unusually cool across much of the northern part of the country, as arctic air has settled in.  Here's this morning's approximate low temperatures across the country:

Fig 1 -- Surface temperature (colors) and pressure (contours) at 11Z, March 31, 2011. From the HOOT website.

Note that 40 degree Fahrenheit lows exteded all the way down to the Gulf coast, with temperatures in the teens and 20s across the upper midwest.  Very winter-like, even though by all measures we have definitely moved into spring.

Of course, as cold air moves south, so does the jet stream.  Remember that the jets above are directly tied to temperature gradients below through the thermal wind relation.  So, as the gradient of temperature between arctic air to the north and warm subtropical air to the south moves south, so do the jet streams above.  Here's this morning's 500mb chart (from College of DuPage--the HOOT site seems to be behind on its graphics).  The yellow contours are isotachs showing wind speed.

Fig 2 -- 500mb geopotential height (blue contours) and winds (yellow dashed contours) for 12Z, March 31, 2011.  From the College of DuPage.

Jet stream winds usually follow the contours of geopotential height since things are roughly (but not quite) in geostrophic balance up there.  We can see that there is a large-scale trough over the eastern half of the US, which corresponds well with the pool of colder air we're seeing at the surface.  One jet streak is analyzed over the central plains with another over the far southeastern US.  As the jet stream shifted south, the jet streaks shifted with it.  Since divergence associated with jet streaks helps drive surface cyclogenesis, the storm track also has shifted south.  As a result, places along the Gulf Coast, particularly Florida, have seen repeated rounds of strong storms as jet streak after jet streak and storm after storm move over them.  Today is no exception.

Fig 3 -- Base reflectivity composite over the southeastern US from 1808Z, March 31, 2011.  From weather.gov.

A tornado watch is in effect for central Florida at the moment.  The past several days have seen multiple mesoscale convective systems move through the state.  This is more than the typical diurnal cycle of convection over Florida...

But the end is in sight!  The European model shows that the broad trough over the eastern US is going to start moving eastward.  Here's the 500mb forecast for Friday morning:

Fig 4 -- ECMWF 24-hour forecast of 500mb winds (colors) and geopotential height (contours) for 12Z, Friday, April 1, 2011.  From the HOOT website.

The trough axis is really sharpening up and has moved further to the east.  There is a cyclonically-curved jet streak forecast in the base of the trough.  Since we expect to see divergence aloft in the exit region of a cyclonically-curved jet streak, we see that the best place for a surface cyclone to be developing is now well off-shore.  The convergent entrance region of the jet streak is forecast to move into the southeast by tomorrow morning, meaning widespread subsidence and generally clearing conditions.

The pattern is forecast to continue to be progressive, with the next shortwave moving through on Sunday into Monday.

Fig 5 -- ECMWF 96-hour forecast of 500mb winds (colors) and geopotential height (contours) for 00Z, Monday, April 4, 2011.  From the HOOT website.

The broad troughing in the eastern US has flattened out by this point and somewhat of a weak ridge has built up across the midwest.  However, there is a clear and rather sharp shortwave forecast to be digging through the four-corners region by this point.  The ECMWF sharpens this shortwave enen more by Monday night.

Fig 6 -- ECMWF 120-hour forecast of 500mb winds (colors) and geopotential height (contours) for 00Z, Tuesday, April 5, 2011.  From the HOOT website.

It will be very interesting to see what happens as this shortave comes through.  Based on this 500mb forecast and following jet streak divergence aloft, we'd expect a surface cyclone to be forming somewhere over, say, western New Mexico on Sunday night. However, the divergent jet streak region has moved all the way up to northern Michigan by Monday night (if we treat the jet streak forecast for Monday night as a straight jet streak).  That seems awfully fast motion to me--from New Mexico up to northern Michigan in 24 hours?  I suspect with this forecast we might end up seeing an occlusion of the first low followed by new cyclogenesis taking place further north. Just a thought.

Models begin to diverge in their long-range outlook past Monday night.  First, the forecast from the ECMWF for 500mb on Tuesday night (00Z, Wednesday):

Fig 7 -- ECMWF 144-hour forecast of 500mb winds (colors) and geopotential height (contours) for 00Z, Wednesday, April 6, 2011.  From the HOOT website.

The trough axis is very well-developed with a coherent jet around it and is beginning to take on a positive tilt across the central Ohio River valley.  The main divergent region is probably over eastern Quebec.  The strong winds in the jet streak on the eastern side of the trough point to what would probably be a rather strong cold front at the surface below.

Now let's take a look at the GFS forecast for the same time:

Fig 8 -- GFS 132-hour forecast of 500mb winds (colors) and geopotential height (contours) for 00Z, Wednesday, April 6, 2011.  From the HOOT website.

The GFS shortwave trough is lagging behind the ECMWF trough quite noticeably.  The trough is more neutrally tilted and is located over Missouri/Arkansas instead of out over the central Ohio River valley.  Also the jet streak pattern is not nearly as well-developed as was seen in the ECMWF model.  Here there is a much smaller jet streak in the lower Ohio River valley, with a divergent region probably over Indiana--vastly different from the eastern Quebec divergent region in the ECMWF.  There are noticeable differences elsewhere, too.  The GFS has a well-developed, cut-off low sitting off the west coast of British Columbia whereas the ECMWF has some kind of crazy combination of shortwaves that isn't as coherent.  Of course, we're looking at several days out here, so our models aren't that trustworthy.  Even in the upper-level features that we are the best at predicting (overall), there is often great disagreement at this range.

So what does this all mean?  We know that the rounds of storms in Florida look to be coming to a close very shortly.  The flow will begin to flatten out as troughing moves off the east coast this weekend.  Then, a new shortwave is forecast to dig in out of the desert southwest and onto the plains from Sunday into Monday.  From there on out--things get dicey.  Trying to time this shortwave for its impacts on the east coast is very difficult right now. 

Even trying to figure out what kind of precipitation type is going to fall is somewhat of a mystery.  The long-range forecast from the Climate Prediction Center is forecast a "heavy snow event" for the northern plains and upper midwest on Sunday into Monday.

Fig 9 -- CPC precipitation hazards forecast for Apr 2-13, 2011.  As of March 30, 2011.  From the CPC.

They are also calling for a widespread area of heavy precipitation and flooding on the east coast from April 4th-6th (Monday through Wednesday).  These forecasts make more sense given the more southerly track of the shortwave forecast by the GFS model, so I'd guess they're leaning toward that solution.  They're cutting it fine on the precipitation type in the northern plains, though.  The GFS forecast critical thickness lines for Monday evening, which help make a first-guess at precipitation type, are shown below:

Fig 10 -- GFS 4.5 day forecast of critical thickness values for 00Z Tuesday (Monday evening), Apr. 5, 2011.  From the College of DuPage.

In the plot above, I'm guessing based on the scale that the purple shadings are something like relative humidity at some level.  Which level?  I'm not sure.  But regardless, the colored lines are critical thickness forecasts.  Remember with critical thickness that you want to be north of the lines (in the colder air) for snow and south of the lines (in the warmer air) for rain.  Here we see that on Monday night (the end of the CPC's April 3-4th heavy snow swath), the critical thickness lines go right through the region they've outlined.  As such, I don't think we can really make much of a claim for an exclusively heavy snow event in the areas they've outlined.  There definitely is the possibility for snow in the far northern plains based on this forecast solution.  But we need more evidence from more model agreeement before saying that for certain.

And given the disagreements in the models after Monday, it doesn't seem very useful to try and look at any more diagnostics past this point.

So...this weekend will be the test.  Will the models come into better agreement?  Or will we still by guessing by the time Monday rolls around?