From WDSU News

NEW ORLEANS —We’re a ways off from being at the halfway point of the Atlantic Basin hurricane season, which began June 1 and ends November 30. The Atlantic Basin includes the Gulf of Mexico, Caribbean Sea, and the Atlantic Ocean. To date, the Atlantic Basin has produced three tropical depressions, two of which were able to intensify to hurricane status: Arthur and Bertha.

WHERE WE STAND, BY THE NUMBERS: As of Aug. 7, we are 68 days into the season. That leaves us with 114 days to go until the season officially ends. Our climb continues towards the official peak of the season, which is Sept. 10 (See Figure 1).

 

Figure 1. Hurricane season climatology.

 

FORECAST UPDATE: Evaluation is good where any data are concerned. Klotzbach & Gray, the team out of Colorado State University that issues seasonal hurricane forecasts, released a revised forecast for the season based on activity through July. The forecast continues to indicate a less active Atlantic hurricane season that normal (See Figure 2).

Figure 2. Atlantic Basin seasonal hurricane forecast, with revisions fom CSU.

THE LATEST ON EL NINO: Aside from wind shear, one of the contributing factors to the below-normal hurricane season forecast is the slightly cooler than normal sea surface temperatures in the tropical and sub-tropical Atlantic. An El Nino Watch continues, as ENSO neutral conditions remain. The chance for an late summer El Nino is 70 percent. The chance that El Nino will develop in the fall or winter is 65 percent.

Figure 3. Updated El Nino forecast from from National Centers for Environmental Prediction.

THE TAKEAWAY: Forecasts in no way indicate where a storm will make landfall. They only help to give an idea of season potential, based on the conditions of the ocean and the atmosphere. The reality remains that it only takes one system to ruin a day, a week, a month or beyond.

Here is a recap of the season so far:

ARTHUR: Arthur has been the strongest system of the season, so far, strengthening to category two status at its peak. Arthur made two landfalls: the initial in the United States over the Shackleford Banks in North Carolina on July 4 and the second in Canada near Port Maitland, Nova Scotia. Arthur’s development into a tropical depression on July 1 and its intensification to hurricane status on July 3 bucked the climatological odds (July 9 and Aug. 10, respectively). (See Table 1. NHC.NOAA.GOV.)

T.D. TWO: Tropical Depression Two formed without incident July 21 and dissipated July 23.

BERTHA: Invest 93L was upgraded to Tropical Storm Bertha late Thursday July 31 (CDT). The system made landfall in the Dominican Republic, before impacting the Bahamas August 3. Bertha strengthened to Category 1 hurricane status Aug. 4 and was declared post-tropical Aug. 6. This system had no direct impacts on the United States.

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For more on this story go to: http://www.wdsu.com/weather/hurricanes/2014-atlantic-hurricane-season-where-we-stand-by-the-numbers/27356910#ixzz39nwEwDCV

Invest 94L forms off African Coast, forecasters warn ‘one to watch’

From Caribbean360

BRIDGETOWN, Barbados, Sunday August 10, 2014 – The US-based National Hurricane Center (NHC) announced Sunday morning that a new tropical weather system, Invest 94L, had formed off the African coast.

According to the Sunday morning NHC bulletin, Invest 94L was located near 11.1N 20.5W and is moving westward at 15-20 MPH. It is early days yet but the current movement and conditions put the system reaching the Lesser Antilles by Saturday according to Dr Jeff Masters’ Weather Underground blog.

“This disturbance is producing a large area of cloudiness and disorganized showers and thunderstorms, some slow development of this system is possible over the next several days” said the NHC.

Dr Jeff Masters from Weather Underground indicated that “satellite loops show the wave has a modest amount of spin and respectable amount of heavy thunderstorms. Water vapor satellite images and the Saharan Air Layer analysis show that 94L is located in a fairly moist environment, with the dry air coming off of Africa located well to the north and west of the disturbance”.

For more on this story go to: http://www.caribbean360.com/news/invest-94l-forms-off-african-coast-forecasters-warn-one-to-watch?utm_source=Caribbean360%20Newsletters&utm_campaign=4c7a693bfa-Vol_9_Issue_024_Sunday_News8_10_2014&utm_medium=email&utm_term=0_350247989a-4c7a693bfa-39393477

NOAA Atlantic Hurricane Season Outlook

Issued: 7 August 2014

The following is an EXCERPT

The 2014 Atlantic hurricane season outlook is an official product of the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC). The outlook is produced in collaboration with hurricane experts from the National Hurricane Center (NHC) and the Hurricane Research Division (HRD). The Atlantic hurricane region includes the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.

2014 Atlantic Hurricane Season Outlook: Summary

NOAA’s updated 2014 Atlantic Hurricane Season Outlook indicates that a below-normal season is likely. The outlook calls for a 70% chance of a below-normal season, a 25% chance of a near-normal season, and only a 5% chance of an above-normal season. See NOAA definitions of above-, near-, and below-normal seasons. The Atlantic hurricane region includes the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.

The main difference between this updated outlook and the pre-season outlook issued on 22 May is that the updated outlook indicates a higher chance of a below-normal season (70% compared to 50%), along with lower chances of a near-normal (25% compared to 40%) or an above-normal (5% compared to 10%) season.

Overall, atmospheric and oceanic conditions across the tropical Atlantic Ocean and Caribbean Sea (called the Main Development Region, MDR; Goldenberg et al. 2001) are expected to suppress tropical storm and hurricane formation, intensity, and duration throughout the peak months (August-October, ASO) of the hurricane season.

The suppressive atmospheric conditions include strong vertical wind shear, increased atmospheric stability, drier air, enhanced sinking motion, stronger trade winds, a southward shift of the African easterly jet, and a suppressed west African monsoon system.

The suppressive oceanic conditions include near- or below average sea surface temperatures (SSTs) in the MDR, which are already anomalously cool relative to the remainder of the global tropics. This relative cooling is even larger than the models had predicted in May, and is expected to persist throughout ASO.

In addition, the possibility remains for El Niño to develop during the late summer or early fall. El Niño typically suppresses hurricane activity by increasing the vertical wind shear, atmospheric stability, and sinking motion in the western and central MDR. If El Niño develops, it could make the already suppressive atmospheric conditions even less conducive to hurricane formation.

Based on the current and expected conditions, combined with model forecasts, we estimate a 70% probability for each of the following ranges of activity during 2014:

7-12 Named Storms

3-6 Hurricanes

0-2 Major Hurricanes

Accumulated Cyclone Energy (ACE) range of 40%-90% of the median.

The seasonal activity is expected to fall within these ranges in 70% of seasons with similar climate conditions and uncertainties to those expected this year. These ranges do not represent the total possible ranges of activity seen in past similar years.

These expected ranges are centered below the official NHC 1981-2010 seasonal averages of 12 named storms, 6 hurricanes, and 3 major hurricanes.

Compared to the pre-season May outlook, the predicted range of named storms has decreased to 7-12 (from 8-13 in May), and the predicted range of major hurricanes has lowered to 0-2 (from 1-2 in May). For the remainder of the season, the outlook is predicting 5-10 named storms, of which 1-4 are expected to become hurricanes and 0-2 are expected to become major hurricanes.

Activity to Date:

Two named storms (Arthur and Bertha), both of which reached hurricane strength, have formed in the Atlantic basin to date. Arthur made landfall in North Carolina as a Category-2 hurricane. On average 1-2 Atlantic named storms form during June-July.

Hurricane Landfalls:

It only takes one storm hitting an area to cause a disaster, regardless of the overall activity predicted in the seasonal outlook. Therefore, residents, businesses, and government agencies of coastal and near-coastal regions are urged to prepare every hurricane season regardless of this, or any other, seasonal outlook.

Predicting where and when hurricanes will strike is related to daily weather patterns, which are not reliably predictable weeks or months in advance. Therefore, it is currently not possible to accurately predict the number or intensity of landfalling hurricanes at these extended ranges, or whether a particular locality will be impacted by a hurricane this season.

DISCUSSION

1. Expected 2014 activity

Climate signals and evolving oceanic and atmospheric conditions, combined with model forecasts, indicate that a below-normal Atlantic hurricane season is likely. This outlook calls for a 70% chance of a below-normal season, a 25% chance of a near-normal season, and only a 5% chance of an above-normal season. See NOAA definitions of above-, near-, and below-normal seasons.

An important measure of the total seasonal activity is NOAA’s Accumulated Cyclone Energy (ACE) index, which accounts for the combined intensity and duration of named storms and hurricanes during the season. This updated outlook indicates a 70% chance that the 2014 seasonal ACE range will be 40%-90% of the median. This range is centered at 65%. In May, the predicted ACE range was 40% to 100%, centered at 70% of the median. According to NOAA’s hurricane season classifications, an ACE value below 71.4% of the 1981-2010 median reflects a below-normal season.

The 2014 Atlantic hurricane season is expected to produce (with 70% probability for each range) 7-12 named storms, of which 3-6 are expected to become hurricanes and 0-2 are expected to become major hurricanes. These ranges are centered below the 1981-2010 period averages of 12 named storms, 6 hurricanes and 3 major hurricanes. For the remainder of the season, the outlook is predicting 5-10 named storms, of which 1-4 are expected to become hurricanes and 0-2 are expected to become major hurricanes.

For the U.S. and the region around the Caribbean Sea, the historical probability of multiple hurricane strikes decreases with decreasing seasonal activity. Nonetheless, for each region, there are many instances in the historical record of hurricane strikes during below-normal seasons (include this season), and even more instances of hurricane strikes during near-normal seasons. Also, the likelihood of at least one U.S. hurricane landfall is the same during El Niño as it is during La Niña and ENSO-Neutral. Already this year, the first hurricane of the season (Arthur) made landfall in North Carolina as a Category-2 hurricane.

Predicting the location, number, timing, and strength of hurricanes landfalls is ultimately related to the daily weather patterns, which are not predictable weeks or months in advance. As a result, it is currently not possible to reliably predict the number or intensity of landfalling hurricanes at these extended ranges, or whether a given locality will be impacted by a hurricane this season. Therefore, NOAA does not make an official seasonal hurricane landfall outlook.

2. Science behind the updated 2014 Outlook

The 2014 seasonal hurricane outlook reflects the likely continuation of exceptionally suppressive atmospheric and oceanic conditions across the MDR. In addition, the possibility remains for El Niño to develop during the late summer or early fall, coinciding with the peak of the hurricane season. If El Niño develops, it could make the atmospheric conditions even more suppressive to hurricane formation.

The outlook takes into account dynamical model predictions from the NOAA Climate Forecast System (CFS), the European Centre for Medium Range Weather Forecasting (ECMWF), the EUROpean Seasonal to Inter-annual Prediction (EUROSIP) ensemble, along with ENSO (El Niño/ Southern Oscillation) forecasts from models contained in the suite of Niño 3.4 SST forecasts compiled by the IRI (International Research Institute for Climate and Society) and the NOAA Climate Prediction Center.

a. Atmospheric conditions across the North Atlantic and Africa

Many non-conducive atmospheric conditions are expected to persist within the MDR throughout ASO. These include: 1) enhanced vertical wind shear and strong upper-level winds, 2) anomalously dry, stable, and sinking air, 3) near-or above average northeasterly trade winds, 4) a configuration of the African easterly jet (i.e. decreased cyclonic shear) that is less conducive to hurricane development from tropical cloud systems moving off the African coast (aka easterly waves), and 5) a suppressed west African monsoon system (Bell and Chelliah 2006).

The vertical wind shear across the MDR is currently much stronger than average. During June and July, the strength of the vertical shear was the highest in the 1970-2014 record. The CFS high-resolution (T-382) and lower resolution (T-126) forecasts both predict a continuation of anomalously strong vertical wind shear across the western and central MDR during ASO, along with near-average shear in the northeastern MDR.

The enhanced vertical wind shear is caused primarily by strong upper-level westerly winds resulting from 1) an amplified Tropical Upper Tropospheric Trough (TUTT) over the western and central MDR, and 2) a suppressed upper-level subtropical ridge that is shifted south of normal over the eastern MDR.

Sinking motion in the mid-and upper-atmosphere also currently stronger than average across the central MDR, resulting in drier and more stable air that suppresses thunderstorm growth necessary for hurricane formation. Anomalous sinking motion has been present in this region since August 2013, and also helped to suppress the 2013 Atlantic hurricane season (Bell et al. 2014).

In the lower atmosphere, strong northeasterly trade winds have been associated with a more southward location of the Atlantic Inter-Tropical Convergence Zone (ITCZ). These conditions are typically associated with a less active Atlantic hurricane season. During June 2014, the northeasterly surface wind speeds in the central MDR were comparable to those observed last year. The trade winds are expected to remain near- or- above average during ASO 2014.

Another prominent feature of the atmospheric circulation is that the mid-level African Easterly Jet (AEJ) is currently in its near-normal position near 12.5oN. This location is 2.5o-5o equatorward of its above-normal season average. As a result, much of the MDR is located in anticyclonic shear north of the jet axis, rather than in cyclonic shear south of the jet axis. This AEJ pattern favors weaker easterly waves that are more confined to the southern MDR.

All of the above conditions are associated in part with a suppressed west African monsoon system, which is expected to remain weaker than average during ASO. The west African monsoon is a major factor controlling the wind and air pressure patterns across the MDR. A weaker monsoon, as seen this year, is historically associated with a below-normal season because it produces all of the above anomalies that are now present.

b. Sea surface temperatures across the Main Development Region (MDR)

SSTs are currently below-average in the central and eastern MDR, and slightly above average in the Caribbean Sea. SSTs averaged across the MDR are -0.11oC below average, which is -0.37oC cooler than the remainder of the global tropics. This relative cooling is another indicator of below-normal activity during the hurricane season.

Many dynamical models are predicting near- or below-average SSTs to persist in the MDR throughout the hurricane season. The CFS high-resolution (T-382) and lower resolution (T-126) models are both predicting below-average SSTs in large parts of the MDR during ASO.

c. El Niño

El Niño typically suppresses Atlantic hurricane activity (Gray 1984) by producing non-conducive conditions within the MDR, including enhanced vertical wind shear, increased atmospheric stability, and sinking motion. If El Niño develops, it could make the already suppressive atmospheric conditions even less conducive to hurricane formation.

The average forecasts of the dynamical models (thick orange line) and statistical models (thick green line), which are contained in the suite of IRI/CPC Niño 3.4 SST forecasts, predict El Niño to be present during ASO. The official CPC/ IRI probabilistic ENSO forecast (updated August 7th) indicates a 55% chance of El Niño during ASO.

In that updated CPC/ IRI forecast, forecasters note that although sea surface temperatures across the central equatorial Pacific have recently returned to near average, this cooling is expected to be temporary. El Niño is now favored to emerge during August-October, and to peak at weak strength during the late fall and early winter.

The cooling of the surface and sub-surface temperature anomalies across the central and east-central equatorial Pacific during June and July produced a sharp drop in the oceanic heat content. A time series of the upper-ocean heat content shows that this cooling is linked to an upwelling equatorial oceanic Kelvin wave. Subsequently, a downwelling wave and its associated warming has moved into the central equatorial Pacific, which is one of the reasons why the recent cooling in that region is expected to be temporary.

3. Multi-decadal fluctuations in Atlantic hurricane activity

The current high-activity era for Atlantic hurricanes began in 1995 (Goldenberg et al. 2001). Hurricane seasons during 1995-2013 have averaged about 15 named storms, 8 hurricanes, and 4 major hurricanes, with an ACE index of 151% of the median. NOAA classifies 13 of the 20 seasons since 1995 as above normal, with eight being very active (i.e., hyperactive defined by ACE > 165% of median). Only three seasons since 1995 were below normal (1997, 2009, and 2013). If the current outlook verifies, 2014 will become the fourth below-normal season since 1995. It would mark the first time in two decades (since 1993-94) that two consecutive seasons were below-normal.

Two key aspects of the multi-decadal signal responsible for the current high-activity era are the warm phase of the Atlantic Multi-Decadal Oscillation (AMO) and its associated enhanced west African monsoon. Neither of these features is present this summer. It is too soon to know, however, if this change is a short-term deviation from the active Atlantic phase of the multi-decadal signal, or if it is signaling the onset of a transition to a low-activity era for Atlantic hurricanes.

4. Uncertainties in the Outlook

Uncertainties in this outlook are related to the persistence and strength of the exceptionally non-conducive atmospheric and oceanic conditions now present. Another uncertainty is related to the possible development of El Niño, including its timing and strength.

To read the whole NOAA Press Release go to: http://www.cpc.ncep.noaa.gov/products/outlooks/hurricane.shtml