Hurricane Irene Forecast #2

As of mid-Monday afternoon—Irene is developing an inner core as it transverses the small gap between the Puerto Rico and northern Hispaniola coastlines. Irene is also exhibiting excellent outflow, especially on the northern half of the storm which is likely contributing—along with rather warm sea surface temperatures—to the continual firing of deep convection.

Radar out of Puerto Rico indicates that Irene at the very least has an open eyewall, with deep convection around mainly the western side of the circulation. An eye has occasionally tried popping out on visible imagery but currently there are no signs of an eye on IR imagery and no microwave images to clarify the structure of Irene’s inner core. However, over the past few hours the half eyewall visible on radar has tightened some which may be indicative of some strengthening of Irene.

Irene has been and is expected to remain under an upper level anti-cyclone. This is in tandem with an upper low well east of Irene providing for the excellent outflow noted on satellite imagery. There is some dry air in the vicinity of the hurricane, with arc clouds once in a while emitting out of the core, however this does not appear to be having a major impact on the inner core of Irene which is continuing to fire consistent deep convection.

Over the coming days global models generally agree on Irene remaining right under or very near the upper level anti-cyclone that is above it now. This will continue to provide for outstanding outflow/ventilation, which will promote continued deep convection within the inner core of the hurricane whenever the system is over water, which should be a good part of its remaining life. Water temperatures are very warm in the path of Irene and the warmth is somewhat deep over the far southwestern Atlantic, which along with good upper level divergence and low vertical wind shear may result in significant intensification:

The question becomes at what point does the intensification stop, and how strong is Irene when this occurs?

The global models (GFS used as an example) generally show Irene remaining under the upper level anti-cyclone until it gets to near the latitude of the Carolinas. North of here, Irene will begin to interact with the westerlies (which may briefly provide for a burst of intensification as they vastly improve upper level lift over the cyclone as it approaches them). However, up until the latitude of the Carolinas upper level wind shear should remain fairly light and Irene should be able to maintain good outflow, with the westerlies beginning to add to the upper level divergence in the vicinity of Irene as it approaches the Carolinas.

As Irene nears the US coastline continental dry air will become a possible issue. Shear may remain low enough from the Carolinas points south that significant dry air entrainment does not occur; however the 12z GFS (shown above) along with the 0z ECM indicate continental dry air edging right up to Irene’s moisture field, which may cause some disruptions within the inner core as Irene gets near the US mainland.

While Irene will move over very warm sea surface temperatures even up to the Carolina coast, the depth of the warm water becomes much shallower as one moves north of the Bahamas (shown above) which may also result in some decrease in instability available for Irene if her forward motion is slow enough off the southeast US coast.

So essentially, will forecast a steady state or slight weakening days 4-5 as Irene nears the coast, however based on outstanding upper level conditions and only minor issues otherwise I do not believe Irene will significantly fall apart upon landfall (assuming the system does make a landfall).

Irene will interact with Hispaniola over the next 24 or so hours, but then will have at least 3-4 days before possible interacting with the southeastern US. Irene as mentioned will be in a very favorable environment for intensification above sufficiently high TCHP waters to support a very strong cyclone. So at the very least will need to ramp up Irene to minimal category 3 status within 72 hours, with some rapid intensification being possible between 24-96 hours out given the very warm sea surface temperatures the hurricane will be transversing  and the outstanding upper level pattern the hurricane should be under. Most models peak Irene somewhere between 72-96 hours out as a strong Cat 2 or low end Cat 3. Will peak at around 110 knots between hours 72-96, and will have to watch carefully for periods of rapid intensification. Will bring down to 105 knots by day 5 to reflect the slightly more hostile (but still relatively good) environment around Irene as discussed above.

A weakness in ridging has opened up north of Irene due to a trough moving off the US east coast. Initially this was thought to possibly not have a significant impact on Irene’s motion due to Irene being a good bit farther south, however center relocations and a deeper cyclone have allowed Irene to end up farther north than thought just 24-48 hours ago. Irene should continue to move WNW in the near term on the southwestern periphery of subtropical ridging centered to the east of the cyclone.

As the trough currently along the east coast lifts to the northeast, it will leave a small weakness in the ridging over the southeastern US. With good ridging still extending just about to the Carolina coastline and Irene being on the southwest side of it, a general WNW motion should continue for the next couple of days, taking it right into the Bahamas.

The next trough will move through on the rather zonal flow Thursday-Friday. How fast this trough moves through and how much it digs will likely determine if Irene makes a direct impact on the US southeast coast or not.

There are two main factors that will determine how much this trough digs:

  1. –NAO ridging over Greenland and the Davis strait.
  2. The +PNA troughing over the Aluetians/Alaska.

The NAO and PNA have generally been modeled to weaken some by the end of the week into the weekend. Since both the –NAO and +PNA support troughs along the east coast, the stronger these teleconnections are the more the trough digs along the east coast.

As you can see above, the 12z ECM does show the –NAO weakening noticeably and the +PNA remaining about the same. The ensemble NAO forecasts have come into slightly better consensus on the NAO bouncing back towards neutral by mid-week, which is crucial to where Irene tracks:

The GFS ensembles are in fairly good agreement in moving the first trough out relatively quickly and clearly leave Irene behind off the southeast coast:

The next trough comes through the flow roughly 36 hours later (Saturday night) and will most likely be the trough that accelerates Irene to the N and then NE.

At this point most global models have Irene just off the southeast coast, which would make escaping without landfall difficult. Out of the 12z model suite the Euro remained most consistent and indicated a landfall near the SC/NC boarder. The CMC/GFS indicated a brush/possible direct hit on the outer banks. The NOGAPS indicated a likely NC landfall and the UKMET remains well left of all guidance and brings Irene north along the Florida coast and into either Georgia or SC. The 12z GFS ensemble mean appeared to be in agreement with the op GFS/CMC. The 0z Ensemble mean appeared to indicate a landfall somewhere near the NC/SC boarder, but the spread out pressure fields indicated that some members had landfall in Georgia and some missed the outer banks to the east.

Given the above information and likelihood  of the trough on Friday leaving Irene behind off the southeast coast, will forecast a landfall near the NC/SC boarder and will continue moving Irene NE just off the Mid Atlantic coastline.

Track forecast (black dots are on a 24 hour interval from 18z Monday):

Intensity forecast:

Initial (18z Monday): 70 knots/Cat 1

12 hours (6z Tuesday): 75 knots/Cat 1

24 hours (18z Tuesday): 80 knots/Cat 1

36 hours (6z Wednesday): 85 knots/Cat 2

48 hours (18z Wednesday): 95 knots/Cat 2

72 hours (18z Thursday): 105 knots/Cat 3

96 hours (18z Friday): 110 knots/Cat 3

120 hours (18z Saturday): 105 knots/Cat 3

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