Thursday, March 31, 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
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?

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