A high-amplitude, high-energy, highly interesting pattern
[Have added an update at the end with info on Tropical Storm Sandy.]
There’s not been anything (at least not yet!) of the extremity of what I wrote about in a recent blog entry describing October’s meteorological split personality, but in 2012 the month is again showing its wild side!
This past week, a deep, large, strong trough over the central-eastern U.S. was reminiscent of the one which begat the April 2011 tornado superoutbreak.
The maps below show 500 millibar heights and wind speeds on April 27, 2011 (top) and October 18, 2012 (bottom):
Image credit: Plymouth State University.
Fortunately this time the atmosphere wasn’t nearly as hot, moist and unstable – on 4/27/2011 temperatures in Alabama were as high as 91 with a dewpoint of 70 – but even so, tornadoes of rare intensity for Mississippi in October were spawned.
Base reflectivity and velocity radar images of the Conehatta, Mississippi EF3 tornado early in the morning of Thursday, October 18, 2012. Image credit: Gibson Ridge
And here on a satellite image from Friday is something you don’t see every day: a cloud of dust which got kicked up by powerful winds in the Plains blowing over extremely dry soil and carried all the way almost to Atlanta!
Image credit: NASA/GSFC MODIS Rapid Response
Now, this coming week, a vivid signal is showing up in model forecasts for the pattern aloft: strong ridging (indicated by the band of bright red colors on the map below) all the way from west of Alaska to east of Greenland.
ECMWF model ensemble mean 500 millibar heights and departures from average for this coming Wednesday. Image credit: Allan Huffman's Model and Weather Data Page
South of that, the remnant of Rafael, which combined with a system from the Labrador Sea, is being forced south back toward the Azores, and it looks like what’s left of it will head into Portugal and then maybe bring stormy weather eastward across southern Europe.
In the U.S., a deep trough is setting up over the West with inclement weather there, and models indicating it will progress to the central and eastern U.S. by next weekend, with a big temperature swing from balmy to chilly.
And over the Caribbean is Invest 99L, almost certain to become a tropical depression and likely then Sandy.
Exactly where it ultimately goes as it heads north into the Atlantic later this week will depend on how it interacts with that trough expected to be over the Southeast and the ridge out ahead of it. Most model forecasts have portrayed just a brush of Florida and then staying out at sea, though recent runs of one model (the GFS) have shown it eventually hooking back to New England or the Mid-Atlantic. While that looks goofy, patterns such as this can sometimes lead to goofy outcomes. [Update mid-afternoon Sunday: Now the ECMWF model shows it doing that even more sharply and quickly.]
Regardless of that or how strong 99L gets wind-wise, it poses a serious threat of heavy rain, flash flooding and mudslides in the Greater Antilles.
GFS model forecast of precipitable water amounts for this coming Wednesday, showing deep tropical moisture flowing squarely into the Greater Antilles. Image credit: Wright-Weather
UPDATE MONDAY EVENING OCTOBER 22
The low pressure system in the Caribbean formed on schedule and is just insulated enough from atmospheric conditions not far to the northwest which are hostile to it (wind shear, dry air), that it quickly organized and was upgraded from Invest 99L to Tropical Depression Eighteen and then Tropical Storm Sandy, and it is expected to further strengthen. No matter how much the winds are able to increase, the serious rainfall threat noted in the original entry above continues to be present.
As the storm heads north, models are consistent in predicting it to get energy from the jet stream and remain solid rather than getting sheared by those strong upper-level winds and falling apart, suggesting that it becomes somewhat of a hybrid with some characteristics of a non-tropical cyclone while retaining at least some of its tropical nature.
Models have shown critical and at times extreme differences in their track forecasts, however. Some runs have depicted a wild scenario with the storm getting sucked up into the mid-Atlantic and northeast U.S. and producing everything from torrential rain, strong winds, and coastal flooding/erosion to heavy interior high-elevation snow. That has created a buzz; it's important to both take such a threat seriously, while not overreacting to particular model runs and prematurely making pronouncements. Other model runs have swerved it far out to sea. Most recently, there is less model guidance that portrays the worst-case scenario; it's still too early to declare a verdict, as subtle differences in the atmospheric jigsaw puzzle upstream/downstream can mean the difference in outcomes between a historic weather event and a near-miss. There's plenty of time for advance warning and preparation if necessary.
Tropical Storm Sandy. Image credit: UW-Madison SSEC/CIMSS