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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AOI
AOI
The blob off NOLA is getting a bit bigger with time
I just happen to be a Wendy's executive and operate the Wendy's in NW Florida and S Alabama. Those vouchers may only be redeemed after November 30th. If we have no storms through then, I'll personally buy everyone a Frosty!
Wouldn't trampling be sufficient?
Unless my Google is lying to me.. again.. the little blob of NOLA.. I have looked at it all day.. and slowly getting bigger
I hope you have some pull with the one in Colwood BC, Canada.. I want one to :)
I think.. and we all know how dangerous I can be when I do that... that 456 mentioned the possibility of something breaking off the tail of that trough.. I could be wrong.. god knows it would not be a first.
Evening everyone!
Not only did Weather 456 mention this, but so did Storm W in his synopsis. Definitely something to watch!!
Yeah is does. There are westerly winds south of 10n in that area but the dry air is limiting any convection from developing. It may acquire more convection once it gets to 50 west in 2 days as the SSTs will increase to 82F but we can rule out any chance of development now. Thumbs down for the GFS. Rapidly becoming the most unrealiable model. What a real shame.
Something's gotta give.
I am hoping Texas gets much needed rain. It would be nice if that blob in the Gulf would bring it on over to you!!
Link
"Monster Cell" 1049 lightening strikes within one hour in a relatively small area.
TORNADO WARNING
NATIONAL WEATHER SERVICE MELBOURNE FL
748 PM EDT WED JUL 15 2009
THE NATIONAL WEATHER SERVICE IN MELBOURNE HAS ISSUED A TORNADO WARNING FOR...CENTRAL ORANGE COUNTY IN FLORIDA...THIS INCLUDES THE CITIES OF...CHRISTMAS...BITHLO...AZALEA PARK... SOUTH CENTRAL SEMINOLE COUNTY IN FLORIDA... THIS INCLUDES THE CITIES OF...OVIEDO...GOLDENROD...UNTIL 830 PM EDT.
* AT 748 PM EDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A SEVERE THUNDERSTORM CAPABLE OF PRODUCING A TORNADO NEAR UNION PARK...OR NEAR AZALEA PARK...MOVING NORTHEAST AT 10 MPH.
* THE TORNADO WILL BE NEAR...BITHLO AND OVIEDO BY 825 PM EDT...7 MILES NORTHWEST OF CHRISTMAS BY 830 PM EDT...PRECAUTIONARY/PREPAREDNESS ACTIONS...IN ADDITION TO THE TORNADO...THIS STORM IS CAPABLE OF PRODUCING QUARTER SIZE HAIL AND DESTRUCTIVE STRAIGHT LINE WINDS.
&&
LAT...LON 2873 8122 2853 8095 2839 8121 2856 8135 TIME...MOT...LOC 2348Z 235DEG 8KT 2854 8121
$$
Actually, the blob is forecast to move in the opposite direction. Back over the big bend of florida over the weekend due to a "cold" front moving through the area.
Maximum Potential Hurricane Intensity
GOM 60 Hour Water Surface Temperature Forecast Model
GULF OF MEXICO...
A SURFACE TROUGH IS OVER THE NORTH CENTRAL GULF OF MEXICO ALONG
30N89W 27N92W 26N95W. WIDELY SCATTERED MODERATE CONVECTION IS
ALONG THE COAST OF LOUISIANA FROM 27N-32N BETWEEN 90W-94W. HIGH
PRESSURE RIDGING EXTENDS FROM THE ATLANTIC TO THE E GULF OF
MEXICO. PATCHES OF WIDELY SCATTERED MODERATE CONVECTION IS OVER
FLORIDA. SIMILAR CONVECTION IS OVER THE YUCATAN PENINSULA FROM
16N-21N BETWEEN 88W-91W. IN THE UPPER LEVELS...AN UPPER LEVEL
HIGH IS CENTERED OVER ALABAMA PRODUCING MOSTLY NE UPPER LEVEL
FLOW OVER THE GULF OF MEXICO. AREAS OF UPPER LEVEL MOISTURE ARE
NOTED OVER THE E GULF E OF 93W. STRONG SUBSIDENCE IS OVER THE W
GULF AND TEXAS. EXPECT MORE CONVECTION OVER THE N GULF N OF 25N
...AND OVER THE BAY OF CAMPECHE OVER THE NEXT 24 HOURS.
Tommorow.. the Launch of Apollo 11 will be done Live on my Blog as it Happened 40 years ago.
Starting at 7am EDT and 6am CST.
img src="
Link
Probably has something to do with being Cowboys fans.
Guess I am a rookie...I don't see any surface spin...Please correct me if I am wrong.
11NORTH 24WEST LOOK AT THE BLUE BARBS
I wouldnt put much stock in it just yet, we will see what the next QS shows
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