30 JULY 2200 EDT
It has taken a while but we are now seeing strong signs of life developing in the MDR — MAIN DEVELOPMENT REGION– all of the Atlantic tropical basin. Over the years there has been significant research that shows important connections between tropical activity and the MJO. (Madden Julian Oscillation) . For those who do NOT know what the MJO is here is a GREAT video that briefly explains the MJO featuring Dr. Phil Klotzbach of Colorado State .
http://www.youtube.com/watch?v=IzwfoZF3hlc… you may need to copy and paste this link
The MJO has 8 specific phases and each phase has a corresponding connection to the overall weather patterns over Australia South American and North America and each of the 8 phases favors or suppresses different types of tropical activity in the MDR of the Atlantic Basin.
This first image represents the current MJO path/ status as of JULY 29. Notice that we can see the track of this feature from back in the middle of JUNE when the MJO was in Phase 8. The circle in the middle of the diagram represents the “circle of death” or Neutral zone. Thus when the MJO is in the ” circle of death ” the MJO has no particular impact on weather patterns because it is too weak. As you can to see the MJO is it currently coming out of the circle of death or neutral zone and is now very weak just inside phase 6.
This table shows the significance of hurricane activity and the connection between the MJO . This table comes from some Colorado State university and Dr. Phil Klotzbach / Dr William Grey. As you can see there is a significant increase in activity in the MDR when the MJO is in Phase 1 and phase 2…. with significant decreases or below normal activity when the MJO is in Phase 6 and 7.
But wait… didnt I just say that the current MJO is in Phase 6… which corresponds to below normal activity in the tropics? Yes I did. Because the key is the MJO forecast track and position as remove into August and September.
This image shows the forecast from several different models.. the GFS ensemble… the European and the British MJO models. As you can see the vast majority of data shows that by Mid AUGUST the MJO will be in Phase 1 ….. which corresponds to above normal activity in the Atlantic Basin.
But besides the MJO phase… the 200 MB Vertical Velocities Plots also show a significant change coming. Remembering your basic physics strong vertical loss of these mean rising motion the atmosphere which supports thunderstorm and tropical development where as sinking air suppresses it. In this image… as I clearly state the areas of BROWN represent sinking air or negative vertical velocity which restricts or suppresses all thunderstorm development. The more of the brown areas or squiggly lines that we see through more suppressive or hostile the pattern is for any sort of storm development. Likewise the GREEN areas represent rising ocean which enhances thunderstorm development and Low pressure area in general.
As you can see from the image over the Atlantic Basin on July 21 not only was there an excessive amount of Sahara dust across the central and eastern Atlantic tropical regions. But there was also a huge area of sinking air across the entire Atlantic Basin… stretching from the West Coast all the way to Spain and Africa with a concentration of fixing air over the Atlantic tropical basin itself. However by July 29 we see that the Brown areas have moved east into Africa and a large an increasingly strong area of rising motion — the GREEN squigglly lines – is moving towards Central America from the eastern Pacific. Clearly over the next weaker so this much more favorable setup will move into the Gulf of Mexico the Caribbean and eventually the tropical Atlantic.
Thus by using these two unknown in overlooked aspects of hurricane forecasting we can see that as we move into AUGUST the MDR is clearly becoming much more favorable for tropical development . Now if we only had some tropical waves ( TW) there were coming off southwest Africa far not to the south so that the severe dust off of Northwest Africa out was not interfering with the TWs.
Well low and behold we have exactly that.. 99L and another feature over the northeast portion of the Caribbean basin. These satellite picture from earlier today –July 30 –shows the features quite nicely. If we take a look at the current areas of wind shear over the MDR… we can see a fairly strong wind shear over the tropical wave that is located over the Northeast Caribbean…. but over the southern portions of the tropical Atlantic the areas around 99L features remarkably low wind shear.
Taking a look at the trends in the Low level wind shear… We see that it’s increasing just to the north of Puerto Rico and the northeast portions the Caribbean but the low level wind shear appears to be neutral in and around the area of 99L.
If we look of the steering flow to figure out where these two systems might go over the next few days… The winds again at the lower level since we are dealing with weaker systems…. is essentially east to west with the 99L system which strongly implies that both the systems will continue to the west. However the other tropical wave over the Northeast Caribbean is seeing ESE – WNW winds which may take the system into the Bahamas over the next few days.
Lets take a look at some of the hurricane model guidance and see where this feature might be heading. Typically when you have a system of this latitude the hurricane model guidance is usually very good because is mostly based off the GFS and when you are at the low latitudes like this the problems and cold bias that the GFS has does a really come into play. However once you move north of 25° latitude the GFS cold bias and persistent over development of troughs on the East Coast usually plays havoc with th front again accurate forecast. Which is why one should use the European model since its vasa superior with regard to the depiction of trough amplitudes .
Here are the 12z hurricane model plots for 99L. As you can see most of the modesl seem to take it into the Northeast Caribbean and south of Puerto Rico at Day 6. The 0z JULY 31 hurricane models showed more of a band in some of the models… at day 5…. with one cluster or tracks passing north to Puerto Rico and another cluster of tracks over the Central Caribbean well to the south of Puerto Rico…..
In addition we also have the GFS ensemble spread…. from 18z and what is obvious to note is that most of the models take this system south the Puerto Rico or very close across the Hispano Island clipping Cuba and into the Bahamas by DAY 10 .
Again however I urge a lot of caution in using hurricane models past five days. It becomes much more important to be able to read the atmospheric overall pattern as tropical systems approach across 60° W longitude
The Canadian model for early this morning has been showing 99L developing into the fifth name tropical cyclone of the season by day 6 and a passing north of Puerto Rico and approaching the Bahamas by AUG 8-9. However as you can clearly see the Canadian model shows a fairly strong short wave trough tracking through the Great Lakes AUG 5-8 . This would imply that this system approaching the Bahamas would be forced to turn to the north then NNE. However this is somewhat speculative at this time since we did not know where this feature may be located next week with respect to the Bahamas nor do we know how deep for certain the trough is going to be cross into the Great Lakes and New England..
The European model at day 5 however also seized some sort of tropical feature in the North East Caribbean…which undoubtedly would be the TW that has been designated “99L.” Both the European model and the European ensemble show a gap in the central north Atlantic between the AZORES ridge and the western Atlantic or ” Bermuda High” ridge just off the East Coast. This gap if is big enough could force or turn 99L to the NW early…. IF 99L passes northeast the Puerto Rico. However if the European models correct this feature will be moving into the Caribbean which will slide under the Gap.
At day 8…. the 12z operational European and the European Ensemble again clearly show some sort of significant tropical feature to the northeast and Puerto Rico. Both models do show a weakness in the rage over the Central Atlantic Ocean. It’s possible that this weakness may turn 99L… by that time likely be a tropical storm or hurricane… out to sea. But again since we don’t know what the position of this features going to be ith relationship to the Caribbean islands .. this is just one possibility.
By day 10 the European model ensemble shows a distinct trough over the northeast U.S. and a break in the pattern . Note the western Atlantic Rudge / Bermuda High over the southwest Atlantic Ocean.. and the heat dome that has been oveer the Plains. This allows for a weakness was gap to appear on the East Coast so any tropical systems coming in from the Northwest Caribbean or the southwest Atlantic would be pulled towards the Southeast U.S. Coast. That being said indeed event that there is no break in the Track over the southeast then the pattern were clearly favor Central and western Caribbean track.