RWS Tropical Season Forecast 2010 (Forecaster Carlos)

RWS Tropical Season Forecast 2010
Complementary discussion to RWS’ Matt (AtownWxWatcher) :
This discussion completes and complements the forecast thoughts on the 2010 Tropical cyclone season provided by AtownWxWatcher. It will touch on the points talked about by him and also include a few extra points.
This discussion will touch on and include the following global parameters and conditions relevant to all tropical cyclone season:

• North Atlantic Basin SSTs

1. North Atlantic SST Tripole
2. North Atlantic SST Tripole: Upward and Downward motions

• ENSO Change

1. El Nino --> Neutrality --> Weak La Nina
2. Atlantic Basin Shear
3. Convergency

• MJO (Madden Julian Oscillation)
• 2010 Tropical Cyclone Season Main Tracking
• AMO Phase (Atlantic Multidecadal Oscillation)
• ITCZ
• QBO Phase (Quasi-biennial Oscillation)
• ACE (Accumulated Cyclone Energy)
• Analog Seasons

• Season Tropical Cyclone Forecast Numbers
• Low end Numbers (Tropical Cyclone Formation Hindering Parameters)
• Medium end Numbers (Tropical Cyclone Formation Encouraging Parameters Outweighing Hindering Ones)
• High End Numbers (Tropical Cyclone Formation Encouraging parameters)

North Atlantic Basin SSTs:

This hurricane season will feature much above average Sea Surface Temperatures (SSTs) across much if not all of the tropical Atlantic basin waters which include the tropical eastern Atlantic waters off the coast of Northern Africa, the Atlantic ocean Hurricane Alley, Caribbean sea and the Bahamas Atlantic waters as well as the waters off the Southeast CONUS coast courtesy of the Gulf stream current. Additionally, the Gulf of Mexico (GOM or GOMEX) waters has been experiencing a rapid warm of its waters from the much colder aspect it has had during the early spring.

The result of such warmer SSTs though the vast majority of the tropical Atlantic ocean is that SSTs around 26 °C (~80 °F), are conducive for the formation and sustainability of tropical cyclones , as long as other important factors related to tropical cyclone formation are contributing. Therefore, the higher the SSTs, the stronger the tropical cyclones as the feed off the latent heat released by the enormous amounts of evaporation and condensation those much warmer SSTs would allow. This tropical cyclone parameter alone, is one of the main forces behind the formation of a tropical system, and given how warm they are for this season, this is undoutly one of the main positives for an at least average season.

The North Atlantic SST Tripole:

The Atlantic basin SSTs are responsible for the formation of the North Atlantic Tripole, which basically is 3 different areas of SSTs throughout the Atlantic Basin; either warm cold warm or cold warm cold. For this tropical Atlantic season, the tripole has set itself up to the warm cold warm configuration, which is best for tropical formation enhancement as it leads to much more upward than downward motion.

North Atlantic SST Tripole Upward and Downward Motions:

By having the North Atlantic Tripole configured to its warm-cold-warm pattern, the SSTs distribution then allows for much more of the vital upward motion over the basin as opposed to the downward motion which hinders tropical cyclone formation and deepening, as it allows for sinking air to be dominant instead of rising air, which is seen with upward motion. The formation of tropical cyclones are dependent on upward motion because rising air kick starts the building of cumulu-nimbus clouds which then result in the beginning of the convective processes that lead to thunderstorm formation and eventually the falling of the air/surface pressure and then surface low pressure system formation. Additionally the North Atlantic Tripole, influences the paths tropical systems take as the warm SSTs areas encourage upward motion and thus an overall area of lower pressures over the warm SSTs, encouraging the tropical cyclones to follow the upward motion areas.

ENSO Change:

The next major player for any hurricane season is the ENSO status. For this season, we are seeing the collapse of the positive ENSO (El Nino) to be replaced by ENSO neutral and even possibly a weak negative ENSO (La Nina). This change greatly influences many aspects related to the formation of tropical cyclones, and is therefore very important if not most chief when making a tropical season forecast, as it affects the overall global pattern and hence the more localized Atlantic ocean basin pattern.

El Nino --> Neutrality --> La Nina:

El Niño-Southern Oscillation, , is a global climate pattern that forms across the tropical Pacific ocean and can be also know as a quasi-periodic pattern and is most characterized by the warming of the waters off the coast of northern South America.. Over the Atlantic ocean basin, especially the tropical Atlantic, El Nino increases vertical wind shear, which greatly hinders tropical cyclone formation as the vertical wind shear rips apart the intense thunderstorms near the center of the tropical cyclones, which are responsible for the formation of lower pressure and eventually the surface low pressure center. Thus, increased vertical shear over the tropical Atlantic basin, diminishes tropical cyclone activity by hindering their formation and deepening.

As the El Nino collapses for the 2010 tropical cyclone season and gives way to a neutral ENSO, and possibly to a weak La Nina (Negative ENSO or Cold phase ENSO), the issue related to the vertical wind shear becomes much more tame and less of a problem, as the vertical wind shear would be less intense leading to shear values which tropical cyclones can withstand and therefore thrive . The more the ENSO swifts toward its cold phase, the more favorable the pattern of the Atlantic basin becomes for tropical cyclone formation and thus, lead to a more active season.

Convergence:

With the changing of the ENSO from its Warm phase to neutrality and perhaps towards its cold phase, the waters off the coast of northern South America become colder, and therefore create a good contrast between the waters there and those over the Caribbean and Gulf Of Mexico, as a result increased convergence becomes present , as the difference of the SSTs off the coast of northern South America (ENSO waters) and the much warmer ones over the Gulf of Mexico and especially Caribbean sea induce the blowing of winds towards the Caribbean sea and therefore the convergence of surface winds and air over that area, which then leads to the formation of rain, then thunderstorms and then consequently, surface low pressure formation as the force of the collision forces air to rise, initiating thunderstorms.

MJO (Madden Julian Oscilation):

The Madden–Julian oscillation is an equatorial region propagating pattern of anomalous precipitation which of global scale As a result, it is distinguished by the progression eastward of hefty areas of enhanced and suppressed tropical precipitation. Since it has both enhance and suppressed rainfall patterns, the MJO has wetter and drier phases. The drier phase results in a lesser moisture content in the troposphere and more downward motion, and therefore the drier phase usually hinders tropical cyclone formation. On the other hand, the wetter phases are characterized by much more and even abundant moisture in the troposphere, leading to much more upward motion, then increased thunderstorm formation, which is key to kick start the cyclogenesis necessary for tropical cyclone formation.

For this season, it is believed, that the MJO phase pulses will be much more predictable and will spend more time over the areas which are more conducive for tropical cyclone formation due to the collapse of the warm ENSO phase as the MJO is greatly more active during the cold ENSO phase. For this eason, taking into consideration that the ENSO is basically in a neutral state right now, an increased, more regular and predictable MJO behavior seems to be on the table for this tropical season over the Atlantic basin, as the MJO wet phase pulses, traverse eastward from the Pacific ocean into the Caribbean sea/ GOM and into the greater tropical Atlantic basin.

2010 Tropical Cyclone Season Main Tracking:

We believe that due to the aforementioned and discussed convergence and the configuration of the North Atlantic SSTs Tripole for this season, which its configuration indicates much more upward motion throughout the tropical Atlantic ocean, the main tracking for the tropical cyclones this season will be, to the north, the Atlantic Hurricane Alley on towards the Bahamas and SE CONUS, and then the climatological tracks due to the Coriolis Effect force, recurvature towards the Northeastern US, Southeast Canada and the northern areas of the North Atlantic ocean and to the south, the Caribbean sea, which is looking to be much more active this season due to much better convergence, upward motion and ENSO neutral resulting in a much friendly vertical wind shear profile, and then into the Gulf Of Mexico.

AMO Phase (Atlantic Multidecadal Oscillation):

Signal defined from the changes in the SST patterns of the North Atlantic Ocean and has a warm and cold phase. Currently, the AMO, which is a quasi-periodic cycle, is going through its warm phase. As a result of past observations, when the AMO is in its warm phase, the Atlantic Ocean sees a modest increase in number of category 1 and 2 hurricanes, while there is still no solid evidence that the warm AMO phase increases the number of major hurricanes over the Atlantic basin. Additionally, the warm phase of the AMO focus most of the Atlantic tropical cyclone tracks towards the east coast of the CONUS, which falls in line with our 2010 tracking resultant from the increased convergence and upward motion seen with the warm-cold-warm North Atlantic SST Tripole and the overall areas of above normal SSTs across the tropical Atlantic basin.

ITCZ (Intertropical Convergence Zone):

Area around the Equator parallel where the southern and northern hemispheres winds join together. It plays an important role on the tropical season because it can aid in the formation of tropical cyclones by allowing for low-level vorticity due to being an area where wind speed and change occurs, or if you will an area where horizontal wind shear takes place. So, keeping that in mind, once the ITCZ migrates to the north of its climatological area during the northern hemisphere summer, which coincides with a good part of the tropical cyclone season over the Atlantic basin, increased Coriolis force occurs and therefore, thunderstorm formation becomes more likely within African tropical waves AKA Eastern tropical waves. As the thunderstorms continue to grow taller, surface pressures fall and eventually a surface pressure system forms and cyclogenesis gets under way.

QBO (Quasi-biennial Oscillation):

Just like the ENSO, the QBO is also a quasi-periodic pattern or oscillation. It also has three phases: an easterly or negative phase, a neutral phase and a westerly or positive phase.
It has importance on the tropical Atlantic basin and on the forecast, because it influences the precipitation pattern and amounts on Africa, which then directly influences the African tropical wave train, either making them wetter and more robust, or drier, weaker and less frequent. Currently the QBO is going through its east phase, which is a less conducive phase for tropical cyclone formation over the Atlantic Ocean since it can change monsoonal precipitation and due to the facts aforementioned on this same paragraph.

So in the case of the QBO because of its –QBO phase, tropical cyclone activity over the Atlantic ocean would be weaker as long as the –QBO were to be the dominant parameter for this season.

ACE (Accumulated Cyclone Energy):

For general and learning purposes, ACE is an evaluation used to express the energy used by a single tropical cyclone or that used by all tropical cyclones during a season. Moreover, the ACE for a tropical cyclone season is used to categorize the level of activity seem during the season. Below is the equation:

As per NOAA:
• Above-average season: ACE value above 103, which is 117% of the 1951–2000 median, as long as at least two of the following three parameters go beyond the long-term average: number of tropical storms (10), hurricanes (6), and major hurricanes (2).
• Near-normal season: not above-normal and not below normal.
• Below-normal season: ACE value below 66, which is75% of the 1951–2000 median.

Additionally, another important point to make is that the Southern Hemisphere ACE has been seen during the last decade or so, to correlate quite nicely with the Northern Hemisphere ACE and the hurricane season. Therefore, since the ACE has been on a 30-year low, if ACE alone were the ruling factor, the tropical cyclone activity over the Atlantic basin would see lower activity this season.

Analog Seasons: (2005 and 2008):

By looking back over past tropical cyclone seasons for the Atlantic basin, 2005 and 2008 stand out, mainly because of the many similarities they share with the 2010 season. The most important ones among them, include the ENSO which was a collapsing El Nino into neutrality and then turned into an La Nina, the SSTs over the tropical Atlantic and the North Atlantic Tripole, which directly leads to more upward motion along with more frequent wet MJO phases, and more convergence directly related to the contrast in SSTs and wind surface patterns over the waters of northern South America and the Caribbean sea due to a neutral or cold ENSO status and the much warmer Caribbean SSTs.

Season Tropical Cyclone Forecast Numbers:

• Low end Numbers (Tropical Cyclone Formation Hindering Parameters):

Would occur in case the current –QBO, low ACE and the ENSO phase warm, were to become dominant. If so, them up to 8-9 named tropical cyclones would form.

• Medium end Numbers (Tropical Cyclone Formation Encouraging Parameters Outweighing Hindering Ones): [ Main RWS activity forecast number for the 2010 tropical cyclone season]

Will occur as long as the current parameters already favorable stay that way and the atmospheric pattern of the Atlantic basin remain as they are as well as the oceanic patterns. Therefore, 14-18 named tropical cyclones may form.

• High End Numbers (Tropical Cyclone Formation Encouraging parameters):

Would occur in case the ENSO switched to a weak to moderate La Nina, the –QBO turned to a +QBO and the ACE reversed from its 30 year low to its above average value of 103. If so, than as many as 19 to 21 tropical cyclones would form.

Forecast Graphic: