Remainder of July hurricane outlook
Not much has changed in the Atlantic since my early July Atlantic hurricane outlook posted two weeks ago. Tropical cyclone activity typically picks up a bit during the last half of July, but we are still a month away from when hurricane season really gets going. Since the current active hurricane period began in 1995, nine of 14 years (63%) have had a named storm form during the last half of July. We had two last-half-of-July named storms last year--Christobal and Dolly. As seen in Figure 1, most of the late July activity occurs in the Gulf of Mexico, Western Caribbean, and Carolina waters. However, a few long-track "Cape Verdes" hurricanes begin to occur. These are spawned by tropical waves that come off the coast of Africa. Tropical waves serve as the instigators of about 85% of all major hurricanes.
Figure 1. Tracks of all tropical storms and hurricanes 1851 - 2006 that formed July 16 - 31.The Gulf of Mexico coast is the preferred strike location. There are still very few major Cape Verdes-type hurricanes forming in the last half of July.
Sea Surface Temperatures
Sea Surface Temperature (SST) anomalies have warmed slightly over the past two weeks, and are about 0.3°C (0.5°F) above average over the tropical Atlantic between Africa and Central America (Figure 2). These are some of the coolest SST anomalies for this time of year that we've seen since 1994. The strength of the Azores-Bermuda high has been near or slightly below average over the past two weeks, driving slightly below average trade winds. Weaker trade winds don't mix up as much cold water from the depths, and cause less evaporative cooling. The latest 2-week run of the GFS model predicts continued near-average or slightly below average-strength trade winds through the end of July, so SSTs should remain slightly above average during this period.
Figure 2. Sea Surface Temperature (SST) departure from average for July 13, 2009. SSTs were about 0.3°C (0.5°F) 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
El Niño conditions continue to amplify over the tropical Eastern Pacific. Ocean temperatures in the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region", are now 0.4°C above the threshold for a weak El Niño, according to the Australian Bureau of Meteorology (Figure 3). An increase of another 0.1°C will push the current El Niño into the "moderate" category. NOAA's Climate Prediction Center issued an El Niño Advisory earlier this month. The latest set of mid-June runs of the El Niño computer models are almost universally calling for El Niño conditions to become well-established for the peak months of hurricane season, August - October. It is likely that Atlantic hurricane activity will be suppressed in 2009 due to the strong upper-level winds and resulting wind shear an El Niño event usually brings to the tropical Atlantic.
Figure 3. 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 8, 2009, SSTs in the Niño 3.4 region had risen to 0.9°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: Australian Bureau of Meteorology.
Wind shear
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's band of strong high-altitude winds is the main source of wind shear in July over the Atlantic hurricane breeding grounds, since the jet is very active and located quite far south this time of year.
The jet stream over the past three months has been locked into a pattern where a southern branch (the subtropical jet stream) brings high wind shear over the Caribbean, and a northern branch (the polar jet stream) brings high wind shear offshore of New England.
The jet stream is forecast to maintain this two-branch pattern for the next week. However, during the final week of July, the subtropical jet is forecast to weaken. This will leave regions of low wind shear over the Caribbean and Gulf of Mexico for the final week of July (Figure 4), increasing the chances of hurricane development.
Figure 4. Wind shear in m/s between 200 mb and 850 mb on July 31, 2009, as forecast by the 00Z July 15, 2009 run of the GFS model. The subtropical jet is forecast to weaken by this time, leaving regions of low wind shear over the Caribbean and Gulf of Mexico for the final week of July. Wind speeds are given in m/s; multiply by two to get a rough conversion to knots. Thus, the red regions of low shear range from 0 - 16 knots.
Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa, and the Saharan dust storms have been quite active over the past month. Expect dust from Africa to be a major deterrent to any storms that try to form between Africa and the Lesser Antilles Islands in July. Several well-developed African waves have been done in by dry air from Africa over the past few weeks.
Steering currents
The steering current pattern over the past few weeks has not changed much. A persistent trough of low pressure has remained entrenched over the Eastern U.S. all summer, bringing cool and relatively moist weather to the eastern half of the country. This trough is 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. At present, it appears that the coming two weeks will maintain the strong trough over the Eastern U.S., which decreases the hurricane risk to the U.S. Gulf Coast. There is no telling what might happen to the steering current pattern during the peak months of August, September, and October, but it is often difficult to break a months-long steering current pattern like the current one.
Summary
Recent history suggests a 63% chance of a named storm occurring in the last half of July. Given that none of the computer models are forecasting tropical storm formation in the coming seven days, and SST and wind shear patterns look pretty average, I'll go with a 30% chance of a named storm forming this month. Such a storm would most likely form near the end of the month, when wind shear is expected to decline due to a weakening of the subtropical jet stream. The last time we went this long in the season without a named storm forming was in 2004, when the first storm (Alex) formed on August 1.
I'll have a new post on Friday.
Jeff Masters
Reader Comments
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Yeah, but they said a rain cover on a thruster is miss so they don't want much rain because it could freeze during launch
They fixed the covers by replacing them yesterday.
"Based on satellite we had people here saying Dolores was a hurricane. NHC looks at the satellite and says its a 40mph TS lol"
errm...no on several counts.
1) What I said back in 2362 of the previous blog was based on watching changes in the latitudes&longitudes of both HurricaneCarlos and the trailing Invest96E -- thus deducing changes in relative vectors -- and thinking that I was seeing the FujiwharaEffect producing a rapid spinup of the invest.
2) By the time I got back to the blog, Invest96E had become TropicalDepressionFIVE-E (aka TD05E), and predicted an equally rapid spinup of TD05E into TropicalStormDolores.
(At the time NHC was predicting "in the next 24 hours")
3) Based on HurricaneCarlos' northeast quadrant leaving an antishear trail for TSDolores to draft into -- much like the leading race car cutting drag for its closely following competitor -- I predicted/asked "Two hurricanes heading toward Hawaii?" (Dolores followed by Carlos)
4) Beyond what I've seen in and learned from the WeatherUnderground blog, my expertise in tropical weather prediction is zero.
Which is why I further asked "Or is TD05E [now TSDolores] still too far away from HurricaneCarlos (guesstimating ~900NauticalMiles) for the FujiwharaEffect to come into play?"
I've yet to receive any response to either question.
Oh.
It looks like a Tropical Depression at this point.
My post had nothing to do with yours, it was in reaction to other comments. In fact I hadnt even seen your original comment.
To answer your questions, Dolores and Carlos appear to be too far apart for any sort of fujiwara effect to take place. Also Carlos will go south of Hawaii and do to its miniture size will have very little impact, if any. Dolores will have weakened to a remnant low well to the east of Hawaii as well.
Reparied when the RSS was retracted.
I'll join you in that dance. According to the KHOU calendar, besides 7/7 when we had 1.48 inches of rain, we have had 0.67 inches of rain from 4/29 to now.
Whats the diffrence between the positive and negative NAO?
"note the diurnal effect"
GOM 60 Hour Surface Current Forecast,Loop current
Maximum Potential Hurricane Intensity
extremely dry! I think H-town has been hit harder than us over here farther east.. I live in the Orange/Beaumont area and we have been lucky enough to have a couple small showers... May the rains come!
See, us West Coasters can wishcast and overreact as well as anybody can.
Could very well be true, but I'd like to borrow your crystal ball for the rest of the season.
Carlos Multiplatform Tropical Cyclone Kinetic Energy and Intensity Graph
No crystal ball needed, NHC has had that stance for several days. It is pretty obvious to see that Carlos will not come close to Hawaii. It is also pretty obvious that Dolores will have weakened to a remnant low within a few days from now, well away from Hawaii
TUESDAY...TIME TO LOOK TO THE EAST!!! GLOBAL MODELS (GFS, ECMWF)
HAVE BEEN PERSISTENT IN DEVELOPING A RATHER HEALTHY TROPICAL WAVE
OVER THE WESTERN ATLANTIC JUST OFF AFRICA. PREVIOUS RUN OF BOTH, GFS
AND ECMWF, DEVELOPED A CYCLONIC CIRCULATION WITH THIS SYSTEM BY THE
TIME IT REACHES THE SOUTHEASTERN BAHAMAS, THEN RECURVED IT NORTHWARD
AROUND THE PERIPHERY OF THE ATLANTIC RIDGE. HOWEVER, LATEST RUNS
OF THESE MODELS PUSH THE SYSTEM FURTHER WESTWARD AND BRING IT
RATHER CLOSE TO SOUTH FLORIDA (TOO CLOSE FOR COMFORT) BEFORE
RECURVING. WHILE THIS IS STILL TOO FAR IN THE FUTURE AND THE
MODELS ARE BOUND TO CHANGE MANY TIMES, IT IS NOT OUT OF THE REALM
OF POSSIBILITIES AS WE ENTER THE ACTIVE PORTION OF THE ATLANTIC
HURRICANE SEASON, AND IT BEARS WATCHING.
On the contrary, NHC admitted to an extremely late labeling of Vince
1) Vince was sitting on "[paraphrasing] water too cold to support tropical cyclone development." Should have rapidly become extra-tropical.
2) Vince was heading directly toward Europe, which was even weirder.
3) The NHC had no problem calling Catarina a hurricane, even though it was in the SouthAtlantic. Which was such an unusual first that the Brazilian meterology service denied its hurricane status even after landfall.
Wow. Somebody finally said it.
just like TD 1 did except w/ the cold frontal boundary...ok...I understand
AT THE SAME TIME...A TROPICAL WAVE OVER THE FAR EASTERN ATLANTIC
WATERS WILL MOVE WEST TO NORTHWEST TOWARDS SOUTH FLORIDA THROUGH
THE WEEKEND...BRINGING IN SOME DRIER AIR TO THE AREA FOR THIS
WEEKEND. THE TROPICAL WAVE COULD THEN MOVE INTO SOUTH FLORIDA
EARLY NEXT WEEK ALLOWING FOR DEEPER TROPICAL MOISTURE TO WORK INTO
THE AREA. SO HAVE KEPT THE POPS IN THE SLIGHT CHANCE TO LOW END
CHANCE CAT THIS WEEKEND FOR SOUTH FLORIDA, THEN THE HIGH IN CHANCE
CAT FOR EARLY NEXT WEEK.
TUESDAY...TIME TO LOOK TO THE EAST!!! GLOBAL MODELS (GFS, ECMWF)
HAVE BEEN PERSISTENT IN DEVELOPING A RATHER HEALTHY TROPICAL WAVE
OVER THE WESTERN ATLANTIC JUST OFF AFRICA. PREVIOUS RUN OF BOTH, GFS
AND ECMWF, DEVELOPED A CYCLONIC CIRCULATION WITH THIS SYSTEM BY THE
TIME IT REACHES THE SOUTHEASTERN BAHAMAS, THEN RECURVED IT NORTHWARD
AROUND THE PERIPHERY OF THE ATLANTIC RIDGE. HOWEVER, LATEST RUNS
OF THESE MODELS PUSH THE SYSTEM FURTHER WESTWARD AND BRING IT
RATHER CLOSE TO SOUTH FLORIDA (TOO CLOSE FOR COMFORT) BEFORE
RECURVING. WHILE THIS IS STILL TOO FAR IN THE FUTURE AND THE
MODELS ARE BOUND TO CHANGE MANY TIMES, IT IS NOT OUT OF THE REALM
OF POSSIBILITIES AS WE ENTER THE ACTIVE PORTION OF THE ATLANTIC
HURRICANE SEASON, AND IT BEARS WATCHING.
Do they say where it would go after hitting south Florida? They are expecting the wave to approach from the SE. Will the trough still take it out to sea? And what does "CAT" mean in this context? Sorry for all the questions. Still learning. :)
Broke records for Jan- Jun~ was the warmest for land & ocean in N Hemisphere (2nd globally) warmest land for Northern hemisphere & globally yet.
The upper atmo was mostly 4-7th warmest.
You might want to reign in your emotions, you do realize you are talking about professionals right?
CAT is short for Category I think
Thanks Sefhurricane. And the question about the NWS and NHC. I think they are two seperate agencies. The NHC deals with tropical weather and the NWS is more general about any kind of weather but only covers a small area. I think. Lol
Why because they mention the wave? So what?
Latest National Hurricane Center Tropical Weather Discussion
000
AXNT20 KNHC 151724
TWDAT
TROPICAL WEATHER DISCUSSION
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
205 PM EDT WED JUL 15 2009
Oh ok oops. :) That is an interesting question though. Ive noticed in the past our local NWS will not mention the tropics when the NHC does. It does get confusing.
Reflector site for those at work, which includes Weather456, daily updates
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