Tropical Storm Irene

Tropical Storm Irene Forecast/Discussion:

As of late Saturday evening, newly formed tropical storm Irene is producing a fairly large but disorganized area of deep convection, with a small but potent surface circulation located under the southwestern edge of this convection. Irene is exhibiting good outflow as well and is moving towards the WNW.

As Irene continues moving WNW over the coming couple of days, both the GFS/ECM showing the upper level anti-cyclone moving with the cyclone towards the WNW and continuing to provide low shear and favorable outflow for intensification of Irene over warm waters. Dry air also appears to be a minimal issue, especially given the broad nature of Irene’s moisture field. In addition, low level winds are rather light over the eastern Caribbean, for a change, and will be while Irene transverses the area:

This means that Irene will have until Monday morning to align its centers in a low shear environment. If that occurs Irene may well become a hurricane over warm waters with good outflow before interacting with land, and could end the Atlantic Basin futility streak at 8.

While Irene’s intensity starting Monday will be highly dependant on how much land the system runs into, the upper level anti-cyclone will remain over the cyclone as it continues trekking NWward in the general direction of the SE US. Combined with warm waters, the combination of rather weak shear and good outflow may provide for an intensifying Irene when she is over water.

There are some indications that Irene may miss the bulk of Hispaniola to the south (see track reasoning below) and will track over eastern Cuba. This will have an impact on intensity but likely won’t gut a large but well defined system, meaning Irene may still be potent north of Cuba.

In the near term, Irene will continue to follow the low level easterlies on the southern periphery of a subtropical ridge between Bermuda and the Azores. The cyclone will begin to approach a weakness in ridging located along the longitude of the eastern US seaboard. This will mean a generally WNW motion over the next couple of days, but a gradual right turn thereafter as Irene begins to deepen some and follow the deeper steering currents, and as Irene moves towards the weakness in the ridging.

There are two tough aspects to the track forecast which are dependant on the timing/strength of a shortwave moving over the eastern US around day 4-5.

  1. Does Irene completely miss Hispaniola to the south or move over part of the Island?
  2. Once Irene clears Cuba, will the trough to the north come in late enough that she doesn’t turn due north until she is east of Florida, over Florida, or west of Florida?

Given fairly strong ridging with no significant weaknesses north of Irene west towards the Bahamas and no troughs expected to move off the east coast until Irene is at the longitude of Hispaniola, I believe there will be no significant northward turn in Irene’s motion will occur until the system is past Hispaniola and that Irene will move over southern Hispaniola. Most global models agree with this scenario. Some models kept Irene just south of Hispaniola in their 12z runs, however an extrapolation of the current motion brings Irene over Hispaniola. And while I expect no major northern turn with Irene before Hispaniola, I expect no southern turn either.

Note how the 12z GFS (above) does not show a weakness opening up in ridging north of Irene until it is just passing the westernmost tip of Hispaniola.

Out of the 12z model suites, only the operational ECM was east or along the east coast of Florida. The GFS was along the western coast of the state, with strong signals from the ensembles of both models of Irene getting clearly into the eastern Gulf of Mexico by the end of the upcoming week. The CMC is also clearly into the Gulf of Mexico on its 12z run.

So the question becomes, how plausible is the idea of Irene getting west into the Gulf?

Over the next 5 days through midweek, the pattern will resemble a fast moving subtropical jet-stream over the northern CONUS with a large and persistent upper low over Alaska with shortwaves breaking off and zipping east in the flow. There will also be a weakly negative NAO in place with higher heights than normal over the Davis Strait. This will result in higher than normal heights over the western US with a large ridge of high pressure in place with below normal heights along the US east coast, with a slight weakness in ridging.

By late next week into next weekend, the ECM Ensemble means hint at the pattern changing, with the upper level low over Alaska weakening significantly, along with the negative NAO. This will mean the ridging may attempt to build back towards the eastern US (ignore the area of lower heights over the northern Gulf and SE due to the ECM ensemble’s interpretation of Irene), which would force a farther south/west track of Irene should one of the shortwaves that comes through before the end of the week not completely pick up Irene and recurve it.

Given Irene will still be in the Caribbean, the trough that moves off the east coast Tuesday will NOT be enough to completely pick up Irene. It will however open a weakness in the ridging north of Irene, causing the system to slow and gradually turn more to the NW.

The next shortwave in the flow will move north of Irene somewhere around Thursday. At this point Irene will likely be north of Cuba. Right now, given the rather weak nature of the –NAO (somewhat zonal flow) it does not appear this shortwave will carve out a particularly strong trough over the eastern US, meaning the will it may turn Irene slightly more to the right, a turn to the due north or NNE may not be likely. In addition, the westerlies will remain well north of the system as this shortwave passes, meaning wherever Irene goes near the end of the week it will do so slowly.

After the shortwave moves out, some models (most notably the ECM Ensemble mean) attempt to build ridging back in north of Irene which would most certainly force the cyclone into the eastern Gulf.

There is also the chance the shortwave slows down some due to the weakening –NAO and that this ridging does not rebuild north of Irene.

Given the uncertainty, will not try to be too fine and will forecast a track for days 6-7 over far western Florida. Intensity will be highly dependant upon whether or not the system makes it into the Gulf.

Track forecast (through day 7) and intensity forecast (through day 5):

12 hours/8am EDT Sunday: 50 knots (TS)

24 hours/8pm EDT Sunday: 60 knots (TS)

36 hours/8am EDT Monday: 65 knots (Cat 1)

48 hours/8pm EDT Monday: 70 knots (Cat 1)

72 hours/8pm EDT Tuesday: 55 knots (TS)

96 hours/8pm EDT Wednesday: 45 knots (TS)

120 hours/8pm EDT Thursday: 60 knots (TS)

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2 Responses to Tropical Storm Irene

  1. nice write-up/forecast as always…still alot of questions over the short-term…

  2. Thanks David. Questions in the short term and long term, as is usually the case with these tropical systems.

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