Tropical Storm Karen

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Original story HERE (http://www.accuweather.com/en/weather-news/tropical-development-possible-1/18390176) at accuweather.com




Tropical Storm Karen Forms in Gulf of Mexico
By Michael Doll, Meteorologist

October 03, 2013; 9:16 AM

Tropical Storm Karen has formed in the Gulf of Mexico and will move into the southern United States this weekend with heavy rain, gusty winds and rough seas.

Karen will continue to move along a curved northward path over the central Gulf of Mexico through Friday and will most likely make landfall along the upper Gulf Coast Saturday night.

There is a chance Karen becomes a hurricane before making landfall.

Near and just east of where Karen makes landfall, minor coastal flooding is possible. Wind gusts in the neighborhood of 60 mph can cause minor property damage, downed trees and power outages.

Showers and thunderstorms will become more frequent from the Florida Panhandle to southeastern Louisiana Saturday.

As Karen strengthens, seas will gradually build over the central and eastern Gulf of Mexico.

As seas build, the frequency and strength of rip currents will increase along with the possibility of beach erosion.

Rainfall can be heavy enough to alleviate recent dry conditions in some locations of the South and may raise the risk of flooding for parts of the region this weekend.

A pocket of 3- to 6-inch rainfall can occur close to the center of the storm track.

Sometimes as tropical systems make landfall, tornadoes can be produced. This is also a possibility at this point, near and east of the landfall.

Showers and thunderstorms with this system have been barely spinning across the southern and central Caribbean since early in the week.

Through Wednesday, disruptive winds had been an inhibiting factor in development.

This weekend over the mainland United States, a strong cold front will move across the Mississippi Valley then toward the Appalachians.

This will help pull the moisture from this Gulf of Mexico system northward and enhance rainfall from the Gulf coast to part of the interior South.

The aforementioned cold front will eventually bring showers and some thunderstorms to the mid-Atlantic and Northeast late on Sunday into Monday.

There is a chance that moisture from Karen enhances the rainfall across parts of the mid-Atlantic and New England early next week, where the rain is most needed and the weather of late has been more like summer.

Content contributed by Alex Sosnowski, expert senior meteorologist.

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[The Stig]

Original story HERE (http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2540) at Weatherunderground

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Hurricane Watches are flying along the U.S. Gulf Coast as Tropical Storm Karen heads north-northwest into the Gulf of Mexico. Karen, the eleventh named storm of the 2013 Atlantic hurricane season, formed about 8 am EDT Thursday in the Southeast Gulf of Mexico. It's not often that one sees a new storm start out with 60 mph sustained winds, but that's what an Air Force hurricane hunter plane found this morning near 7:30 am EDT, when they sampled the northern portion of the storm. A ship located about 50 miles northeast of the northeast tip of the Yucatan Peninsula measured sustained winds of 51 mph near the same time. Satellite loops show that Karen is a medium-sized storm with an area of very intense thunderstorms along its northern and eastern flanks. Wind shear has risen since Wednesday, and is now a moderately high 20 knots, thanks to strong upper-level winds out of the west-southwest. These strong winds are keeping any heavy thunderstorms from developing on the west side of Karen's center of circulation, by driving dry air that is over the Yucatan Peninsula and Western Gulf of Mexico into Karen's core. As a result, Karen has a lopsided comma-shape on satellite imagery. Karen has a strong upper-level outflow channel to its north that is helping ventilate the storm, though, and ocean temperatures are a very warm 29°C (84°F). Between 7 am and 9:30 am EDT the Hurricane Hunters made three passes though the center of Karen, and the central pressure stayed roughly constant at 1004 mb, so Karen is not undergoing much change.


Forecast for Karen

Wind shear will steadily increase as the storm heads north-northwest, and shear will reach a high 25 knots by Saturday morning as Karen closes in on the U.S. Gulf Coast, according to the latest SHIPS model forecast. The atmosphere will grow drier as Karen moves into the Northern Gulf of Mexico, and the drier air combined with increasing wind shear will retard development, making only slow intensification likely through Friday. A trough of low pressure and an associated cold front will be moving through Louisiana on Saturday, and the associated upper-level westerly winds will be able to turn Karen more to the northeast as it approaches the coast on Friday evening and Saturday morning. The higher shear at that time should be able to induce weakening, and the 8 am EDT Thursday wind probability forecast from NHC gave a 28% chance Karen will be a hurricane at 2 am EDT Saturday, down from 44% on Friday afternoon. Most of the models predict landfall will occur along the western Florida Panhandle Saturday afternoon or evening.

The usually reliable European model has Karen making landfall over Eastern Louisiana, though. If Karen does follow this more westerly path, the storm will be weaker, since there is more dry air and higher wind shear to the west.

Since almost all of Karen's heavy thunderstorms will be displaced to the east by high wind shear, there will be relatively low rainfall totals of 1 - 3" to the immediate west of where the center makes landfall. Much higher rainfall totals of 4 - 8" can be expected to the east. To judge the possibilities of receiving tropical storm-force winds at your location, I recommend using the NHC wind probability forecast.

The highest odds of tropical storm-force winds (45 - 55%) are along the coast from Buras, Louisiana, to Pensacola, Florida.

I'll have a new post this afternoon.

Jeff Masters

[The Stig]

Original story HERE (http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2541) at Weatherunderground



Posted by: Dr. Jeff Masters, 8:25 PM GMT on October 03, 2013

Tropical Storm Karen is having trouble with dry air and high wind shear as the storm heads north-northwest at 12 mph into the Gulf of Mexico. An Air Force hurricane hunter plane is in the storm, and found top surface winds near 65 mph between 3:30 - 4:30 pm EDT Thursday, and a central pressure of 999 mb, 5 mb lower than this morning's. Satellite loops show that Karen is a medium-sized storm whose heavy thunderstorms have declined in intensity and areal coverage since this morning. The heavy thunderstorms are all on the northern and eastern flanks of the storm, and the low-level center is exposed to view. High wind shear of 20 knots, due to strong upper-level winds out of the west-southwest, is driving dry air from the Western Gulf of Mexico into Karen's core. Heavy thunderstorms are having difficulty building on the west and south sides of Karen's center of circulation because of the shear, resulting in a lopsided comma-shape on satellite imagery. Karen is attempting to build an eyewall, and has managed to wrap a band of heavy thunderstorm about half way around its center. If this band wraps all the way around, Karen will likely be able to intensify into a Category 1 hurricane. Karen has a strong upper-level outflow channel to its north that is helping ventilate the storm, and ocean temperatures are a very warm 29°C (84°F). Ocean heat content is about 30 kJ per square centimeter, which is fairly typical for this time of year, and does not increase the odds of rapid intensification. Strong southeasterly winds ahead of Karen are already pushing tides 1 - 1.5' above normal along the coast from Eastern Louisiana to Alabama, as seen on our wundermap with the storm surge layer turned on.

Forecast for Karen

Wind shear is expected to increase as the storm heads north-northwest, and shear will be quite high, 25 knots, on Saturday, as Karen closes in on the U.S. Gulf Coast, according to the 2 pm EDT SHIPS model forecast. The atmosphere will grow drier as Karen moves into the Northern Gulf of Mexico, and the drier air combined with increasing wind shear will retard development, making only slow intensification likely through Friday. A trough of low pressure and an associated cold front will be moving through Louisiana on Saturday, and the associated upper-level westerly winds will be able to turn Karen more to the northeast as it approaches the coast on Saturday. The higher shear, combined with ocean temperatures that will drop to 28°C, should be able to induce weakening, and the 5 pm EDT Thursday wind probability forecast from NHC gave a 23% chance Karen will be a hurricane at 2 pm EDT Saturday, down from 41% odds at 2 am EDT Saturday.

The models are split into two camps for Karen's track. The European, UKMET, and GFDL models have Karen making landfall over Central or Eastern Louisiana. These models keep Karen relatively weak, resulting in a path that follows the low-level winds more to the west, where there is more dry air and higher wind shear. The GFS model and HWRF model keep Karen stronger, and predict a landfall in the Western Florida Panhandle. NHC is splitting the difference between these two solutions, which is a reasonable compromise. Most of Karen's heavy thunderstorms will be displaced to the east by high wind shear when the storm makes landfall, and there will likely be relatively low rainfall totals of 1 - 3" to the immediate west of where the center. Much higher rainfall totals of 4 - 8" can be expected to the east. To judge the possibilities of receiving tropical storm-force winds at your location, I recommend using the NHC wind probability forecast. The highest odds of tropical storm-force winds (44 - 66%), according to NHC's 5 pm EDT Thursday forecast, are along the coast from Buras, Louisiana, to Pensacola, Florida.


A possible analogue for Karen: Hurricane Ida of 2009

We have little skill forecasting intensity, and I expect that at landfall, Karen has a 20% probability of being a Category 1 hurricane with 75 - 85 mph winds, and a 20% chance of being a minimal tropical storm with 40 - 45 mph winds. One possible scenario is a repeat of what happened with Hurricane Ida of 2009. Ida took a track very similar to Karen's, and was a hurricane with 75 mph winds just south of the mouth of the Mississippi River in Louisiana. High wind shear from an approaching trough of low pressure, combined with cooler ocean temperatures near shore, combined to cause a sudden weakening of the storm. Ida became extratropical a few hours before making landfall on the Alabama coast, and brought top sustained winds of 40 - 50 mph to the coast from Shell Beach, Louisiana to Waveland, Mississippi.

Read more at http://www.wunderground.com/blog/JeffMasters/karen-having-trouble-with-dry-air-and-high-wind-shear#rGZmrO53qrpavLKC.99