Winter rains ease Southeast U.S. drought; Brazilian storm could go subtropical

By: Dr. Jeff Masters , 5:59 PM GMT on February 29, 2008

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The winter of 2007-2008 is in the books, as today marks the last day of meteorological winter (December, January, and February). Winter rains have eased the drought gripping the Southeast U.S., where the area covered by extreme to exceptional drought has shrunk by about 50% since the beginning of the year (Figure 1). Some regions of southern Georgia and southern Alabama, where winter rains have been more than six inches above average (Figure 2), are no longer suffering drought conditions at all.


Figure 1. Drought categories for the Southeast U.S. from December 25, 2007, and February 28, 2007. Image credit: U.S. Drought Monitor.

However, A large swath of the Southeast U.S., including Atlanta, Charlotte, and Huntsville, have received 1-4 inches of precipitation below usual for this time of year. The shortfall is particularly acute in northern Alabama, where Huntsville has received only 6.77" this year, compared to the normal 10.47". The below average rains during this winter rainy season bode ill for the summer, when drought conditions could easily return to last year's extreme levels. The Southeast badly needs one or two landfalling tropical storms or hurricanes in 2008 to help break the drought.

Central Florida surrounding Lake Okeechobee is also suffering from below average rains this winter. The lake, which reached its all-time low water mark of 8.82 feet on July 2, 2007, has risen to 10.02 feet, but this is still a record low for this time of year. The surface area of the lake has shrunk to about 2/3 of normal, and the water level is more than four feet below normal. Part of the reason for the record low lake levels is the fact that the lake was deliberately drawn down before the 2006 hurricane season, in anticipation of another very active hurricane season.


Figure 2. Departure of precipitation from average for January and February 2008. Image credit: NOAA Advanced Hydrologic Prediction Service.

The forecast
The short-range rainfall forecast is good for the Southeast, with significant rainstorms possible both Tuesday and Thursday. The longer range three-month forecast calls for a continuation of below average precipitation for the spring season, thanks to the continued presence of a strong La Niña event in the Equatorial East Pacific. La Niña events usually deflect the jet stream into a pattern that takes the Southeast U.S. out of the the usual storm track needed to bring typical spring rains. However, for the summer months of June, July, and August, NOAA's CFS Climate Forecast System Model is predicting a return to normal levels of rainfall over the Southeast U.S.

Severe weather outbreak coming on Monday
A strong low pressure system is forecast to develop over Texas on Sunday, bringing a slight chance of severe weather to eastern Texas Sunday afternoon. By Monday afternoon, the storm is expected to track northeastwards over the Ohio Valley, dragging a strong cold front across the south. A significant severe weather outbreak is possible Monday afternoon in advance of this cold front.

Interesting South Atlantic storm could become subtropical
An extratropical storm centered near 31S 30W, a few hundred miles east of the Brazil-Uruguay border, has begun to acquire subtropical characteristics and could become a subtropical storm this weekend. The storm is not expected to hit land. NASA/MSFC has a clickable satellite image of Southern Hemisphere one can use to zoom in on the storm. An ASCAT pass at 5:29am EST this morning showed winds of 50 mph near the center of the storm. Water temperatures are about 26°C, which is right at the boundary where tropical storm formation can occur. Subtropical and tropical storms are quite rare in the South Atlantic. I'll update this section of the blog through the weekend if the storm develops. There is no naming system in place to name any tropical or subtropical storm that may form in the South Atlantic. It would be up to the World Meteorological Organization (WMO) to institute such a scheme. The last time I checked into this, they had no plans to consider a naming system. Here's nice MODIS image of the storm from 15:30 GMT today.



Figure 3. Visible satellite image of extratropical low off the coast of Brazil that is beginning to acquire some subtropical characteristics. Image credit: NASA/MSFC.

Jeff Masters

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160. Weather456
8:17 AM AST on March 02, 2008
Good Morning, for the hurricane seasons of 2004, 2005, 2006 and 2007. I used these indicators to help put together a hurricane season forecast at the beginning of each month for June, July, August, September and October. I donot make one forecast in June for the whole season because some of these indicators are highly variable from June to September and September to December.

Tropical Forecasting

a. Surface Pressure and SST Anomalies

b. Upper Wind Anomalies (Westerly, Increasing)

c. Location and intensity of the monsoon trough, ITCZ, subtropical high-pressure ridge*, African Easterly Jet.

d. The Southern Oscillation

e. The Madden-Julian Oscillation (MJO)

f. Quasi-biennial Oscillation (QBO)

g. The North Atlantic Oscillation (NAO)

h. The Saharan Air Layer (SAL)

i. The Tropical Upper Tropospheric Trough/Lows (TUTT)


Medium-Long Range Forecasts 30-60 days

These are just brief descriptions, details will be posted as the hurricane season nears

During the QBO, Atlantic tropical cyclones are more frequent when 30 mb winds are westerly and increasing, rather than easterly and increasing.

During El Nino, high SST over the eastern Pacific causes more deep convection there. The resultant outflow aloft enhances upper tropospheric westerlies over the Caribbean and western equatorial Atlantic. Consequently, the 200 mb anticyclonic flow necessary for tropical cyclones to develop is reduced. During Neutrals and weak to moderate La Ninas, low SSTs over the eastern Pacific supresses deep convection there. The resultant subidence enhances lift and weak to moderate upper level easterlies over the Tropical Atlantic Summer, which favors tropical cyclone development.

During the hurricane season in the Caribbean basin, below normal monthly mean sea level pressure is associated with increased hurricane activity. Pressure anomalies tend to persist from spring through summer.

During times of wind surges into the Monsoon trough, NECZ or ITCZ during late July, August and September, expect increase tropical convection and activity.

During the MJO Active Phase, low level rising and upper level drivergence, thus enhanced convection is favored.

Increase velocities of the African Easterly Jet was found to increase in the intensity and frequency of African Tropical Waves. Examples include Ivan (2004), Florence and Helene (2006) and Dean (2007).

SAL can affect tropical cyclones is several ways but during the period from 2004-2007 I observed only two. Increase African Dust actually lowers SSTs not the entire tropical Atlantic but the area just west of the Cape Verde. Increase African Dust creates a dry enviroment within the mid-levels which enhances subsidence and downdraft and suppresses tropical convection.

The Positive NAO index phase shows a stronger than usual subtropical high pressure center and a deeper than normal Icelandic low. The negative NAO index phase shows a weak subtropical high and a weak Icelandic low. African easterly waves are impelled along the southern periphery of the Azores High away from coastal West Africa towards North America and the Caribbean, sometimes triggering tropical cyclogenesis. The position and intensity of high also affects upwelling along the African Coast, how arid West African gets during the summer months (SAL), surges into the ITCZ, Monsoon Trough, etc., the pressure gradient between a TC low pressure and the high pressure cell. All these factors are compiled and made into a forecast. A stronger High also increases SSTs in the Caribbean and Gulf of Mexico due to downwelling (the opposite of upwelling). A stronger high also indicates that tropical cyclones move faster and make it more westward than usually (like Frances, Ivan, Dean). It must be remembered that the Bermuda High is just a westward extentsion of the Azores High and only becomes seperated during the passage of frontal troughs and in rare cases, tropical cyclones (Maria 2005).

The Famous TUTT, my second most favorite tropical feature of the summer months (Tropical Cyclone 1st). The TUTT should not be confused with troughs in the mid-laltitude westerlies because unlike those troughs, the TUTT is tropical and mainatined by subidence. The TUTT extends from the North central or Northeastern Atlantic roughly 35-40N/40W all the way to Central America in some cases. The TUTT is more of a short term forecasting tools as it is so variable during the hurricane season. TUTTs can either increase wind shear over tropical system or enhanced outflow through outflow channels. Also TUTTS are respobsible for the sudden "blow-up" in convection of inverted V tropical waves in the Eastern Caribbean which can lead to genesis (Ernesto 2006).

Well most of this information came from those four hurricane seasons. But Climatology is an important tool and the Atlantic Ocean is the most well studied basin in the world. Another useful tool is by looking at past tropical cyclone season and see how each phenomena mentioned above affected that season. Like Neutral-Enso in 1995.

I hope this help those who want to try a hand at forecasting.


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159. KoritheMan
8:12 AM GMT on March 02, 2008
Member Since: March 7, 2007 Posts: 521 Comments: 19128
158. OUFan919
4:59 AM GMT on March 02, 2008
The snowstorm back home in St. Louis looks pretty bad from what I have seen too.

A few buddy's and I r going to North Texas/Southern Oklahoma tomorrow to chase. Hopefully we get a few isolated storms out in front of the main line that will form tomorrow afternoon. If the cap can get broken, things could get very interesting tomorrow.
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155. BahaHurican
11:20 PM EST on March 01, 2008
Hey, 23. I'm not thinking so much about forecasting when it will happen as I am about WHY it happens. After looking at all the different years, I have been wondering what causes such a drastic, and seemingly arbitrary, shift. I find it hard to believe it "just happens".
Member Since: October 25, 2005 Posts: 19 Comments: 20734
154. BahaHurican
11:12 PM EST on March 01, 2008
143. hurricane23 10:53 PM EST on March 01, 2008
Personally ive given up on forcasting el nino and nina.There are just to many things that are involved.For all we know a nino will come out of no were in september.Not likely though.


The data I've been working with suggests that if even a moderate La Nina is still around in April, we're very unlikely to get into an El Nino before the season is over. There was only one year, 1968, when an El Nino started before the year was out. Frankly, I don't see us getting back to ENSO-neutral before June. Even if we do get out of the La Nina, neutral years tend to yield slightly higher numbers formation-wise and an increased chance of US landfalls.

All this makes for a potentially interesting hurricane season. I may even try a long-range forecast this year LOL.
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152. hurricane23
11:09 PM EST on March 01, 2008
150. BahaHurican 11:08 PM EST on March 01, 2008
I wonder if anybody out there is speculating on what causes ENSO to cycle. I. E. why did we have that "sudden" flip-flop from the expected La Nina to El Nino in 2006? Why the flop back to La Nina in 2007? I've seen a fair amount of observation of the mechanism, but not much on what is driving the machine.

Baha its extremely difficult to forcast and to unpredictable for me atleast.
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151. hurricane23
4:09 AM GMT on March 02, 2008
Thankfully the severe weather threat for the southeast florida is not there next week as most of the energy will remain to the north of the area.Showers/thunderstorms will be on the increase though.
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150. BahaHurican
4:08 AM GMT on March 02, 2008
I wonder if anybody out there is speculating on what causes ENSO to cycle. I. E. why did we have that "sudden" flip-flop from the expected La Nina to El Nino in 2006? Why the flop back to La Nina in 2007? I've seen a fair amount of observation of the mechanism, but not much on what is driving the machine.

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148. BahaHurican
4:03 AM GMT on March 02, 2008
141. hurricane23 10:46 PM EST on March 01, 2008
Also el nino/nina are only a small portion of tropical cyclone formation.There are many factors that go into a developing tropical cyclone


Very true. I think it's interesting, though, that so many of the "smaller" facters, like some of those you mentioned, are related to larger synoptic events like MJO, PDO, ENSO, et al. I'm becoming more convinced over time that a better understanding of global weather patterns will lead to improved forecasting on a storm by storm basis.
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147. hurricane23
4:01 AM GMT on March 02, 2008
Nope....5-6 months for the real season.

Hope everyone has a great sunday!

www.AdriansWeather.com
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146. hahaguy
3:58 AM GMT on March 02, 2008
i see everyone is getting ansy about hurricane season. just playing
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145. hurricane23
3:55 AM GMT on March 02, 2008
I think i'll stick to forcasting a 2 week steering pattern rather then forcasting el nino/nina.
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143. hurricane23
3:53 AM GMT on March 02, 2008
Personally ive given up on forcasting el nino and nina.There are just to many things that are involved.For all we know a nino will come out of no were in september.Not likely though.
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141. hurricane23
3:46 AM GMT on March 02, 2008
Also el nino/nina are only a small portion of tropical cyclone formation.There are many factors that go into a developing tropical cyclone such as sst's , wind shear and the amount of SAL that comes off Africa. The water temp could be 85 degrees but if theres dry air in the vicinity convection will have a very tough time getting going.
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139. hurricane23
3:40 AM GMT on March 02, 2008
Feel free to read up on a good read from a friend up in melbourne his numbers and forcast for the 2008 season.He is usually on the money.

When the M/J/J average SST anomaly is +0.6C or greater, 92% of the time the season storm total will be 8 named storms or less. When that average is -0.6C or greater, 92% of the time the season storm total will be 13 named storms or less (and 75% of the time the season storm total will be 9 to 13 named storms, i.e., a normal season. Note that a normal season, because of the increased activity since 1995, is now 11 named storms. Added: Since the start of the satellite era, the past 43 seasons in the Atlantic basin have averaged 11 named storms of which 6 became hurricanes and 2 of those became major hurricanes with sustained winds of at least 111mph.) A strong pre-season La Nina has never resulted in a high activity storm season. Ninety percent of high activity seasons occur under ENSO neutral pre-season conditions."

More Here
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138. sullivanweather
3:40 AM GMT on March 02, 2008
Adrian said the following:

I think its important to remember that a strong pre-season La Nina has never resulted in a high activity storm season.

I don't know what years he's looking at, but this is completely false.

One also cannot ignore the propensity for East Coast tropical cyclone strikes during these La Nina years.
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136. KoritheMan
3:18 AM GMT on March 02, 2008
Adrian,

I don't know why you keep on saying a strong pre-season La Nina has never resulted in an active hurricane season.


La Nina tends to enhance the strength of Atlantic hurricanes, and also the track (as was already stated above by Baha) of the storms (they tend to form further south and strike further south and west, like Dean, Felix, Gilbert, and Allen did, although Allen did not hit in a La Nina year). Numbers seem to be only affected a minimal amount.

But I have noticed, and this is probably what Adrian is referring to, is that La Nina tends to cool the Atlantic, and El Nino tends to warm it. This is, of course, speaking only from my observations, which have only been over the last few years.
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135. hydrus
3:14 AM GMT on March 02, 2008
BAHA-Dr.Gray was saying that during el Nino years the upper level winds associated with the Q.B.O would usually reduce hurricane formation in the atlantic basin by about 40%.Do you agree with his findings?
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134. stormdude77
2:29 AM GMT on March 02, 2008
Great information there, Baha. I can't wait until you're done with it.
133. BahaHurican
2:20 AM GMT on March 02, 2008
Thanks, guy. No one around here (I mean in the Bahamas) seems to be compiling this relatively simple to access data, so I needed to satisfy my own curiosity.

When I'm finished, I'll post the whole thing on my blog, hopefully with some graphics courtesy of Levi32.
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132. hahaguy
2:07 AM GMT on March 02, 2008
that is some nice info baha. i am glad you posted that i found it very interesting.
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131. BahaHurican
2:05 AM GMT on March 02, 2008
Sullivan, I have seen studies suggesting less active ATL seasons are correlated with El Nino rather than La Nina. La Nina seems to have a greater effect on WHERE the storms form and track than on how many form.
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130. BahaHurican
2:03 AM GMT on March 02, 2008
Evening all.

Here are my preliminary findings for

Bahamian Tropical Cyclone Strikes 1950 - 2007.

During this period, a total of 80 tropical systems, ranging from tropical depression to Category 4, passed within a 75-mile radius of the Bahamas, which works out to about 1.38 systems per year. Of these, 25 occurred in ENSO-positive periods, 28 in ENSO-negative periods, and 27 in ENSO-neutral periods. Thus it seems that ENSO does not have a markedly serious effect on the number of hurricanes that affect the Bahamas in a particular season. However, tropical cyclones were seen in only 12 ENSO-neutral seasons, as compared with 15 El Nino and 15 La Nina seasons. There may therefore be a slightly smaller chance that a TC will affect the Bahamas during ENSO-neutral years.

The vast majority of TC strikes in the Bahamas during the 1950 - 2007 period occurred in the peak months of August, September, and October, with 21, 23, and 22 occurrences respectively. By comparison, the TOTAL number of strikes for the other three months of the season was only 18, and this included one pre-season strike in May. A preliminary look suggests that August storms are considerably less likely to occur in El Nino years; only 4 were found. The converse was seen in September, when only 6 of the 23 systems found formed in La Nina years. In October, a whopping 13 of 22 systems passed through in ENSO-neutral years, compared with only 5 and 4 systems in El Nino and La Nina years respectively.

I need to do some further analysis to determine whether stronger systems are more likely to affect the Bahamas during a particular part of the ENSO cycle.

Only about 20 of the 80 systems surveyed affected the Bahamas as hurricanes, and more than half were depressions for their entire passage through Bahamian waters. Notably, 2 of 20 systems between 1970 and 1990 were hurricanes in the vicinity of the Bahamas. One other thing I noticed was the large number of systems which had the Bahamas as either a beginning or ending point. I never thought of my homeland as the hurricane nursery or the hurricane graveyard . . .
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129. sullivanweather
1:21 AM GMT on March 02, 2008
Adrian,

I don't know why you keep on saying a strong pre-season La Nina has never resulted in an active hurricane season.

1950 ONI peaks @ -1.7
1950 Atlantic ACE - 243!!

1955 maintained a moderate La Nina throughout the season (~-1.0)
1955 Atlantic ACE - 199!!

1999 ONI peaks @ -1.5
1999 Atlantic ACE - 177!!

2000 ONI peaks @ -1.7
2000 Atlantic ACE - 116!!

These are all above normal seasons, in some cases 'hyperactive'

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127. 1900hurricane
12:42 AM GMT on March 02, 2008
123. StormW 6:21 PM CST on March 01, 2008
121. Patrap 6:11 PM EST on March 01, 2008
3day GFSx 500 mb Plot, UNYSIS
Link

WOW!

Goes pretty quick from a positive tilt to a negative tilt trof. Good vort max.


I've got a question. What does that mean for the severe threat for SE TX/Upper TX Coast? Does that enhance severe activity?
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126. surfmom
12:40 AM GMT on March 02, 2008
Dog is taking me out for a walk - tomorrow
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124. surfmom
12:24 AM GMT on March 02, 2008
WFL surfers: looks like the anticipated active weather will also be bring waves to WFL/GOMEX The next front may deliver the biggest waves thus far this winter (of course that is relative in the GOMEX Looking to be chest high plus? On Tuesday rising seas out of the southwest at your winter spots. Wednesday the swell will be out of the northwest with an easy breeze. Presently the Gulf is 64 degrees. Start planning your "i can't get to work" excuses now!

East of I75 SRQ it was hot as blazes, worried we are going to look like a dust bowl by the end of the summer. Hope this front brings some quality rain. I'd skip catching waves in order to get rain.
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122. hurricane23
11:14 PM GMT on March 01, 2008
John Gerard is a great guy ive meet him a few times watch him all the time on CW during the week.
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121. Patrap
11:11 PM GMT on March 01, 2008
3day GFSx 500 mb Plot, UNYSIS
Link
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119. hydrus
11:04 PM GMT on March 01, 2008
CCHSWEATHERMAN-Could you please give me some info about this storm that the computers are predicting?
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118. KoritheMan
8:55 PM GMT on March 01, 2008
My numbers are 13/7/3.

Sounds reasonable.

Only takes one so preps should always be completed come june1.

Well said.
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117. cchsweatherman
8:04 PM GMT on March 01, 2008
Michael,
Just was wondering what you expect for Florida with this impending storm and the next one depicted in the models?

Also, it seems like you have done a great amount of research on La Nina and El Nino. Keep it up since I do read your info and learn from it. I find it quite useful in trying to predict what could happen in hurricane season.
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114. cchsweatherman
7:26 PM GMT on March 01, 2008
To be honest, I don't know GBlet. Maybe Adrian or Storm could help you with that.
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113. GBlet
7:19 PM GMT on March 01, 2008
I happen to feel like a sitting duck this year, especially after Greensburg. I live in Great Bend,Ks.
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112. GBlet
7:17 PM GMT on March 01, 2008
CC, does anybody have any info yet on upcoming tornado season in relation to La Nina?
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111. cchsweatherman
7:04 PM GMT on March 01, 2008
Where did everyone go? Seems like everyone just disappeared at the same time.
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110. cchsweatherman
6:15 PM GMT on March 01, 2008
109. hurricane23 12:53 PM EST on March 01, 2008
108. weatherbro 12:51 PM EST on March 01, 2008
Any chance this possible historic superstorm could surpass '93?

Dont think so my friend.The threat for any real severe weather atleast for southeast florida is not there next week.Those type of events are rare.


Like Storm and I have repeatedly said, it cannot be ruled out since it depends upon how far south the low develops. If it develops over the SE, then South Florida may get some significant weather coming to them by late in the week into the weekend. Just saying that it can't be ruled out since even the SPC and NWS have very little, if any handle, on this future storm.
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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.