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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yup agreed, so I will go with him lol
either way we will see what happens, I would think the NHC thinks this has a shot or else it wouldnt be on the TWO
Link
slow develop is possible over the next few days, if they really felt it would get blasted by shear in a few days they would certainly not have written that
tells me their thinking is shear backs off around the islands too
lol... Ace is a Troll Tampa. The only reason I don't ignore most trolls is because I find them highly entertaining.
Tropical Update
I continue to watch several strong tropical waves but none is showing signs of development as yet. QuikSCAT last night and this morning continue to show one of these vigorous waves near 30W, possess a low level circulation and thus should be watch as it head west. This morning satellite imagery showed few shower activity along any of these features due to warmer sea surface temperatures and some dust. Most models expect this tropical wave to head west and should reach the islands by Sunday. Most of the computer models are now backing down shear, and along with dispersing dust and progressively warmer waters, these features now stand a chance even in the Caribbean. I will continue to monitor these features
Figure 1 - Wave Tracking 850MB Vorticity
(Source: CIMSS Tropical Cyclones)
Yeah, that's one of the things I've been noticing. The wave's strongest vorticity is at 850 mb, typical of healthy waves.
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
205 AM EDT FRI JUL 17 2009
...TROPICAL WAVES...
A TROPICAL WAVE EXTENDS FROM 18N29W TO 13N32W TO 7N32W MOVING W
NEAR 10 KT. THIS WAVE HAS BEEN MOVED E OF ITS 16/1800 UTC
SURFACE ANALYSIS POSITION TO COINCIDE WITH A LOW TO MIDDLE LEVEL
CYCLONIC CIRCULATION BASED ON RECENT SATELLITE IMAGERY. THE
QUIKSCAT PASS AROUND 16/2045 UTC INDICATED CYCLONIC CURVATURE IN
THE FLOW NEAR THE SURFACE IN THE VICINITY OF THIS WAVE.
SCATTERED MODERATE CONVECTION IS FROM 8N-15N BETWEEN 30W-33W
There's very few statements that I read on here that really blow my mind, but this one takes the cake so far. Forecasting is about trying to keep people safe by using scientific cooperation. Its NOT about bragging rights and scores. Thats just a completely irresponsible view towards forecasting and what we try to accomplish here on the blogs and to be quite frank, if you hold that attitude, I hope you never become a forecaster since that will cloud your judgment.
regarding wave at 32 n
A TROPICAL WAVE EXTENDS FROM 18N29W TO 13N32W TO 7N32W MOVING W
NEAR 10 KT. THIS WAVE HAS BEEN MOVED E OF ITS 16/1800 UTC
SURFACE ANALYSIS POSITION TO COINCIDE WITH A LOW TO MIDDLE LEVEL
CYCLONIC CIRCULATION BASED ON RECENT SATELLITE IMAGERY. THE
QUIKSCAT PASS AROUND 16/2045 UTC INDICATED CYCLONIC CURVATURE IN
THE FLOW NEAR THE SURFACE IN THE VICINITY OF THIS WAVE.
SCATTERED MODERATE CONVECTION IS FROM 8N-15N BETWEEN 30W-33W.
UPPER LEVEL DIFFLUENCE ASSOCIATED WITH THE UPPER LEVEL
HIGH AND THE UPPER LEVEL RIDGE IS SUPPORTING THE CONVECTION
ASSOCIATED NEAR THE ITCZ AND ALONG THE TROPICAL WAVE EXTENDING
FROM 18N29W TO 13N32W TO 7N32W.
He will blast every single person in here whether they have credentials or they dont
Atlantic Tropical Weather Outlook
000
ABNT20 KNHC 170540
TWOAT
TROPICAL WEATHER OUTLOOK
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
200 AM EDT FRI JUL 17 2009
FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...
DISORGANIZED SHOWER ACTIVITY CONTINUES IN ASSOCIATION WITH A
TROPICAL WAVE LOCATED SEVERAL HUNDRED MILES WEST-SOUTHWEST OF THE
CAPE VERDE ISLANDS. ADDITIONAL DEVELOPMENT OF THIS SYSTEM...IF
ANY...IS LIKELY TO BE SLOW TO OCCUR AS IT MOVES WESTWARD ABOUT
15 MPH. THERE IS A LOW CHANCE...LESS THAN 30 PERCENT...OF THIS
SYSTEM BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS.
ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.
$$
FORECASTER BLAKE
I am not staying up for that lol, just want to see what the NHC says at the TWO
TROPICAL WAVE LOCATED SEVERAL HUNDRED MILES WEST-SOUTHWEST OF THE
CAPE VERDE ISLANDS. ADDITIONAL DEVELOPMENT OF THIS SYSTEM...IF
ANY...IS LIKELY TO BE SLOW TO OCCUR AS IT MOVES WESTWARD ABOUT
15 MPH. THERE IS A LOW CHANCE...LESS THAN 30 PERCENT...OF THIS
SYSTEM BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS.
ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.
$$
FORECASTER BLAKE
chances stay low, ok well I am off to bed
night all
DMAX is the diurnal maximum, and occurs at nighttime over the ocean waters when ocean temperature is warmer than air temperature. Because warm air is less dense than cold air, it will rise until it encounters air that is as warm or warmer than itself, at which point it will begin to sink, creating subsidence. This is why disturbances over the tropics tend to gain an increase in convection at nighttime, and why developing disturbances (and sometimes, even newly-formed tropical cyclones) struggle to intensify and/or lose convection, because during the day the air temperature is warmer than the ocean temperature, which creates an overall lack of instability. This latter cycle is referred to as the diurnal convective minimum.
Both the DMIN and DMAX peak before sunrise and sunset, respectively.
The Caribbean may want to keep an eye on this too
Thanks, but I would not have learned it had it not been for this excellent community. :)
Good night all, I'm out. We'll see what our wave does tomorrow. Still not expecting any significant development prior to 75W, though. If I'm wrong, so be it.
Great explaination. Thanks. I learned something new. :)
also note the new cmc run drops the sw fla storm it showed earlier today not surpised there
No invest. Probably wont be one until Sunday.
Hmmmm? NWS Lake Charles
LONG TERM...WILL MAINTAIN CHANCE PRECIP INTO THE WEEKEND AS A
WEAK BACKDOOR COLD FRONT PROVIDES SOME FOCUS TO FIRE UP DAYTIME
SHOWERS AND THUNDERSTORMS. HEAT RETURNS SATURDAY IN ADVANCE OF THE FRONT.
WEAK BERMUDA HIGH PRESSURE WILL PROVIDE A LIGHT MOIST FLOW OFF THE
GULF WHICH SHOULD BE FAVORABLE FOR TYPICAL SEABREEZE TYPE DIURNAL
SHOWERS AND THUNDERSTORMS OVER THE SOUTHERN ZONES NEXT WEEK
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