July Atlantic hurricane outlook
It's mid-July, and we have yet to see a named storm form in the Atlantic this month. The computer models are not predicting any development through at least July 20, and if we make it all the way to the end of the month without a named storm forming, it will be the first July since 2009 without a named storm. Since the current active hurricane period began in 1995, 13 of 17 years (76%) have had a named storm form during July. The busiest July occurred in 2005, when five named storms and two major hurricanes formed. These included Hurricane Dennis and Hurricane Emily--the strongest hurricanes ever observed so early in the season. Only eight major hurricanes have formed in July since record keeping began in 1851. As seen in Figure 1, most of the last half of July activity occurs in the Gulf of Mexico and waters off the Southeast U.S. coast. These type of storms form when a cold front moves off the U.S. coast and stalls out, with the old frontal boundary serving as a focal point for development of a tropical disturbance (as happened for Alberto, Beryl, Chris, and Debby in 2012.) There will be at least two cold fronts moving off the U.S. Mid-Atlantic coast over the next two weeks. The first of these fronts will push offshore around July 20, and we will need to watch the waters offshore of North Carolina for development then. Formation potential will be aided by ocean temperatures that are about 0.7°C (1°F) above average along the U.S. East Coast.
Figure 1. Tracks of all tropical storms and hurricanes 1851 - 2006 that formed July 16-31. The U.S. coast from North to Texas are the preferred strike locations. Only a few storms have formed in the tropical Atlantic and Caribbean in July. Wind shear is typically too high and SSTs too cool in July to allow African waves in the Caribbean and tropical Atlantic to develop into tropical storms. However, a few long-track "Cape Verdes" hurricanes have occurred in July, spawned by tropical waves that came off the coast of Africa. African tropical waves serve as the instigators of about 85% of all major hurricanes.
Figure 2. The seasonal distribution of Atlantic hurricane activity shows that July typically has low activity. Image credit: NHC.
Sea Surface Temperatures: slightly above average
The departure of Sea Surface Temperatures (SST) from average over the tropical Atlantic between Africa and Central America was about 0.3°C above average during June (Figure 3.) This figure has not changed much over the first two weeks of July. These temperatures are not warm enough to appreciably affect the odds of a July named storm or hurricane. The strength of the Azores-Bermuda high has been near average over the past two weeks, driving near-average trade winds. The latest 2-week run of the GFS model predicts continued average-strength trade winds through late-July, so SSTs should remain about 0.3°C above average during this period, due to average amounts of cold water mixing up from below due to the wind action on the water.
Figure 3. Sea Surface Temperature (SST) departure from average for July 12, 2012. SSTs were 0.3°C above average over the tropical Atlantic's Main Development region for hurricanes, from Africa to Central America between 10° and 20° North Latitude. Note the large region of above average SSTs along the Equatorial Pacific off the coast of South America, the hallmark of a developing El Niño episode. Image credit: NOAA/NESDIS
El Niño on the way?
For two consecutive weeks, ocean temperatures 0.5 - 0.6°C above average have been present in the tropical Eastern Pacific, which is right at the threshold for a weak El Niño episode. NOAA's Climate Prediction Center has issued an El Niño Watch, and gives a 61% chance that El Niño conditions will be present during the August - September - October peak of the Atlantic hurricane season. The likely development of a full-fledged El Niño episode means that Atlantic hurricane activity will probably be suppressed in 2012, due to the strong upper-level winds and high wind shear these events typically bring to the tropical Atlantic.
Figure 4. Sea Surface Temperature (SST) departure from average for the the equatorial Eastern Pacific (the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region"). El Niño conditions exist when the SST in this region rises 0.5°C above average. As of July 9, 2012, SSTs in the Niño 3.4 region had risen to 0.5°C above average. To be considered an "El Niño episode", El Niño conditions must occur for five consecutive months, using 3-month averages. Image credit: NOAA Climate Prediction Center.
Wind shear: above average
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart. The jet stream has two bands of strong high-altitude winds that are currently bringing high wind shear to the Atlantic. The southern branch (subtropical jet stream) is bringing high wind shear to the Caribbean, and the northern branch (polar jet stream) is bringing high wind shear to the waters offshore of New England. This configuration often leaves a "hole" of low shear between the two branches, off the Southeast U.S. coast and over the Gulf of Mexico. The jet stream is forecast to maintain this two-branch pattern over the coming two weeks. Wind shear has been about 10 - 20% higher than average over the first two weeks of July, and is predicted to be mostly above average for the coming two weeks. This will cut down on the odds of a July storm.
Figure 5. Vertical instability over the Caribbean Sea in 2012 (blue line) compared to average (black line.) The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Instability has been lower than average, due to an unusual amount of dry air in the atmosphere, reducing the potential for tropical storm formation. Image credit: NOAA/NESDIS/CIRA.
Dry air: above average
As seen in Figure 5, there has been an unusual amount of dry, stable air in the Caribbean this year creating low levels of vertical instability. This has occurred due to a combination of dry air from Africa, and upper-atmosphere dynamics creating large areas of sinking air that dry as they warm and approach the surface. The Gulf of Mexico and tropical Atlantic between the coast of Africa and the Lesser Antilles have also seen low vertical instability this summer. June and July are the peak months for dry air and dust coming off the coast of Africa, and the Saharan dust storms have been quite active over the past two weeks. Expect dry air to be a major deterrent to any storms that try to form in the tropical Atlantic during July.
Steering currents: average
The predicted steering current pattern for the next two weeks is a typical one for July. We have an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. These troughs are frequent enough and strong enough to recurve any tropical storms or hurricanes that might penetrate north of the Caribbean Sea. Steering current patterns are predictable only about 3 - 5 days in the future, although we can make very general forecasts about the pattern as much as two weeks in advance. There is no telling what might happen during the peak months of August, September, and October--we might be in for a repeat of the favorable 2010 and 2011 steering current pattern, which recurved most storms out to sea--or the unfavorable 2008 pattern, which steered Ike and Gustav into the Gulf of Mexico.
Summary: a below average chance of a July tropical storm
Given that none of the computer models are forecasting tropical storm formation in the coming seven days, SSTs are only slightly above average, and wind shear and vertical stability are above average, I'll go with a 30% chance of a named storm forming in the Atlantic during the remainder of July.
Figure 6. Hurricane Emilia over the Eastern Pacific at 20:35 UTC July 10, 2012. At the time, Emilia was a Category 3 hurricane with 125 mph winds. Emilia peaked earlier in the day as a Category 4 storm with 140 mph winds--the strongest hurricane in the East Pacific so far in 2012. Image credit: NASA.
An active Eastern Pacific hurricane season
It's been a very active start to the Eastern Pacific hurricane season, where we've already had six named storms, four hurricanes, and three intense hurricanes. A typical season has 4 named storms, 2 hurricanes, and 0 intense hurricanes by July 14. The formation of Tropical Storm Fabio on July 12 marks the 4th earliest formation of the Eastern Pacific's season's sixth storm. The record is held by the year 1985, when the season's sixth storm formed on July 2. Record keeping began in 1949.
Have a great weekend, everyone, and I'll be back Monday with a new post.
Jeff Masters
Reader Comments
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Interestingly, the GFS 06Z run latches on to the low exiting the W African coast and brings it to a position of about 50W closed off. The run also shows a significant moisture increase along and on either side of 10 degrees N. The next few runs will be interesting but if this holds it could signal a shift to more active conditions and the start of the CV season
El Niño on the way?
For two consecutive weeks, ocean temperatures 0.5 - 0.6°C above average have been present in the tropical Eastern Pacific, which is right at the threshold for a weak El Niño episode. NOAA's Climate Prediction Center has issued an El Niño Watch, and gives a 61% chance that El Niño conditions will be present during the August - September - October peak of the Atlantic hurricane season. The likely development of a full-fledged El Niño episode means that Atlantic hurricane activity will probably be suppressed in 2012, due to the strong upper-level winds and high wind shear these events typically bring to the tropical Atlantic.
My only concern is the strength of this high. As speed shear may come into play with this system.
Thank You and Have a Great Weekend.
Oh No,,, "The Chart" lol
Please see corrected time for the GFS run to 06Z. The strength of the high is the reason the GFS run is basically a straight line to 50W. Easterly shear from the high could be a problem.
Post# 6 shows tracks right into C America but as we all know these patterns change from one week to the next. Also I don't think there is a set position of the high during El-nino as I have seen many different patterns from El-nino to the next.
My granddaughter just asked a question that has me stumped (though I should probably know the answer)
Can there be lightning without rain (meaning rain clouds, not that it is actually raining)?
TIA
This phenomenon is most common in dry, arid parts of the world, but yes, it can happen. In what is called a "dry thunderstorm," all or almost all rain from the thunderhead converts into water vapor due to the tendency of dry air to absorb water. Actually, the dry lightning produced in these storms is especially dangerous when vegetation is present because a vicious wildfire can be ignited.
1:26 PM GMT on July 13, 2012
I've learned not to make forecasts beyond 10 days! The odds of being wrong are too high.
Jeff Masters
Thanks! Just read you reply to my granddaughter, she thanks you as well
yes...
.DAY ONE...TODAY AND TONIGHT.
.THUNDERSTORM IMPACT...
ADEQUATE MOISTURE...DAYTIME HEATING AND A SOUTHEAST WIND FLOW
AROUND THE ATLANTIC RIDGE WILL ALLOW ISOLATED SHOWERS AND
THUNDERSTORMS TO DEVELOP ACROSS THE COASTAL COUNTIES BY EARLY
AFTERNOON AND PUSH INTO THE INTERIOR THROUGH THE LATE AFTERNOON
AND EARLY EVENING HOURS. THE PRIMARY HAZARDS WILL BE CLOUD TO
GROUND LIGHTNING STRIKES AND GUSTY WINDS WITH STORMS TODAY. SEEK
SAFE SHELTER IF STORMS DEVELOP IN YOUR AREA.
.FLOOD IMPACT...
LOCALLY HEAVY RAINFALL AMOUNTS OF UP TO TWO INCHES MAY OCCUR WITH
A FEW STORMS CAUSING TEMPORARY FLOODING OF LOW-LYING AND POOR
DRAINAGE AREAS.
.MARINE THUNDERSTORM GUST IMPACT...
GUSTY WINDS WILL BE POSSIBLE WITH STORMS TODAY ACROSS THE INLAND
LAKES. BOATERS SHOULD BE PREPARED TO SEEK SAFE HARBOR.
.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY.
SCATTERED LIGHTNING STORMS ARE EXPECTED EACH AFTERNOON FOCUSED
MAINLY OVER THE INTERIOR SECTIONS. FREQUENT LIGHTNING...LOCALLY
HEAVY RAIN AND GUSTY WINDS SHOULD BE THE PRIMARY THREATS.
.SPOTTER INFORMATION STATEMENT...
SPOTTER ACTIVATION IS NOT ANTICIPATED TODAY.
Thanks Dr. That is good enough for me.....
I'd imagine it must be pretty impressive...
It was my pleasure to educate you this fine morning :)
They made for a beautiful drive into work this morning.
You can also have spectacular lightning with volcanic eruptions, but those clouds of ash (that generate the lightning) are still clouds.
I'm not aware of any phenomena that can produce (real) lightning without having a cloud first.
it is raining it just evaporates before hitting the ground
Thank you for the comprehensive post!
GULF OF MEXICO...CENTRAL LOUISIANA...SOUTH CENTRAL LOUISIANA...
SOUTHWEST LOUISIANA...WEST CENTRAL LOUISIANA AND SOUTHEAST TEXAS.
.DAY ONE...TODAY AND TONIGHT
DUE TO THE COMBINATION OF COPIOUS GULF MOISTURE...A NEARLY
STATIONARY TROUGH OF LOW PRESSURE ALOFT...AND ALREADY SATURATED
GROUND...LOCALIZED FLOODING MAY BE POSSIBLE TODAY. THUS...A FLASH
FLOOD WATCH CONTINUES TODAY FOR ALL OF SOUTHEAST TEXAS...CENTRAL
AND SOUTHERN LOUISIANA...AS ANY ADDITIONAL HEAVY RAINFALL MAY
CAUSE LOCALIZED FLOODING OF URBAN AREAS...LOW ELEVATION AREAS
PRONGED TO FLOODING...ESPECIALLY NEAR RIVERS AND STREAMS.
.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY
THE WEATHER PATTERN WILL BE SLOW TO CHANGE OVER THE WEEKEND AS
SHOWER AND THUNDERSTORM CHANCES REMAIN ABOVE NORMAL. BY EARLY NEXT
WEEK...THE UPPER LEVEL TROUGH OF LOW PRESSURE WILL SLOWLY WEAKEN
AND BE REPLACED WITH BUILDING HIGH PRESSURE. WITH LINGERING
MOISTURE AND DAYTIME HEATING...EXPECT AFTERNOON SHOWERS AND
THUNDERSTORMS.
.SPOTTER INFORMATION STATEMENT...
SPOTTERS ARE ENCOURAGED TO REPORT ANY FLOODING THAT MAY OCCUR
TODAY.
Heat lightning?
Edit: Nope, that comes from clouds too...
Virga, is that the blue pill????
GULF OF MEXICO...
A NARROW UPPER TROUGH DIPS S ACROSS E TEXAS AND THE FAR NW
CARIBBEAN STILL GENERATING SCATTERED SHOWERS/ISOLATED
THUNDERSTORMS INLAND AND IN THE GULF W OF A LINE FROM TERREBONNE
BAY LOUISIANA ALONG 27N92W TO BROWNSVILLE TEXAS. AN UPPER RIDGE
ANCHORED OVER SOUTH CAROLINA EXTENDS A RIDGE AXIS TO THE
MISSISSIPPI DELTA OF LOUISIANA COVERING THE FAR NE GULF. A WEAK
MESOSCALE LOW HAS MOVED OFF THE FLORIDA COAST AND IS NOW 1014 MB
CENTERED AT 13/0900 UTC NEAR 29N85W WITH A SURFACE TROUGH
EXTENDING FROM E OF TALLAHASSEE FLORIDA THROUGH THE LOW TO
26N84W GENERATING SCATTERED SHOWERS/ISOLATED THUNDERSTORMS
WITHIN 60 NM OF THE SURFACE TROUGH. THE REMAINDER OF THE GULF IS
DOMINATED BY AN UPPER LOW CENTERED NEAR 24N88W AND SUPPORTING A
SURFACE TROUGH THAT AT 13/0900 UTC EXTENDS FROM 26N87W INLAND
OVER THE YUCATAN PENINSULA NEAR 21N90W CONTINUING TO THE NW
CORNER OF GUATEMALA. ISOLATED SHOWERS/THUNDERSTORMS ARE WITHIN
120/150 NM OF THE TROUGH N OF 20N. CLUSTERS OF SCATTERED
SHOWERS/ISOLATED THUNDERSTORMS ARE OVER THE SE GULF S OF 26N TO
THE COAST OF CUBA BETWEEN 80W-84W INCLUDING THE STRAITS OF
FLORIDA...ALONG THE COAST OF MEXICO FROM 22N-25N W OF 95W...AND
WITHIN 60/75 NM ALONG THE COAST OF MEXICO IN THE W BAY OF
CAMPECHE S OF TUXPAN AND W OF THE ISTHMUS OF TEHUANTEPEC.
SURFACE TROUGH OVER THE NW YUCATAN PENINSULA WILL MOVE NW
THROUGH SUN AS A SURFACE RIDGE N OF THE AREA SLIDES S TO THE FAR
NE GULF BY MON. THE MESOSCALE LOW AND TROUGH WILL DISSIPATE
LATER TODAY.
I hope this is true for North Texas as well... all the precipitation is tracking just east and west of Dallas. We could sure use about a weeks worth of rain.
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