Irene update on 08/26/11

I don’t have any major changes to my track ideas as I’m now in complete agreement with NHC and the majority of the forecast models. I caution that the track of the eye doesn’t matter much anymore as everyone along the East Coast is going to be impacted by Irene. If the storm makes a second landfall in NJ, the Jersey Shore will be dealt a big impact in storm surge and high winds. Currently, I am liking the idea of keeping the storm just offshore, similar to Hurricane Gloria (1985).

Morning visible imagery of Irene courtesy of NRL
12z track guidance for Irene on 08/26/11

Satellite imagery shows the storm slowly stalking the NC coast. The storm motion is north at 14 mph with winds currently estimated to be 90 knots or 105 mph. The eastern shore of NC is in a slight risk for tornadoes later this afternoon and that threat will move north with time. 12z track guidance continues to be in excellent agreement (5 runs in a row now), so the question goes from where is Irene going to how strong will it be when it gets further north. NC should brace for winds sustained around 95-105 mph with gusts over 120 mph, mainly from Morehead City to Cape Hatteras on north. The storm will weaken some and should be an 85 knot (100 mph) storm when off the Virginia Beach coast late Saturday night. I now see Irene being a CAT 1 hurricane when it impacts the Delmarva and NJ coastlines, but that doesn’t mean it should be taken lightly. NJ should expect 60-70 mph winds sustained with gusts around 90-100 mph. The worst conditions (stronger winds) should remain offshore, but the rain will be excessive with amounts closing in on 15″. I have included the HPC Day 1-3 precipitation forecast and 11 a.m. NHC forecast track below.

HPC Day 1-3 precipitation accumulation forecast Fri morning.
11 a.m. NHC track forecast for Irene

For the DC/Baltimore area, the winds will be sustained from 35-40 mph with gusts around 50-60 mph quite possible. Rainfall amounts will be on the order of 4″-5″ for DC and closer to 8″-10″ for Baltimore and Aberdeen areas. I don’t see there being a tornado threat with this storm as the northern/western quadrants don’t usually have the dynamics necessary for this to happen.

As a side note: There has been question as to why the winds in Hurricane Irene are not as strong as the pressure would indicate. There is a good reason for this. The hurricane is embedded in a larger environment of lower pressure as indicated by the map below. When this occurs, the wind field expands and the pressure continues to drop, but you don’t build up the necessary gradients near the surface (and sometimes aloft) to increase the winds. A pressure of 947 mb as indicated by aircraft this morning would typically yield a CAT 3/4 hurricane with winds over 120 mph sustained. Irene does not have a well-organized eyewall (possibly due to continued dry air entrainment from an old SAL outbreak/environmental dry air and some light to moderate southwesterly shear), but the wind field has expanded to the point where hurricane force winds extend upwards to 90 miles from the center and tropical storm force winds extend much further than that. Dr. Pete Kozich also noted that the convection in Irene has not been as deep as one would expect for the sea surface temperatures and outflow that currently exist around the hurricane. This is most likely contributing to the weaker winds being observed by aircraft and buoys. The effects will be felt far and wide though with storm surge becoming a big issue. NC will experience a 10′-12′ surge and NJ will experience closer to 3′-6′ of surge. Waves will be 15′-20+’ on top of this.

Seal-level pressure map courtesy of RAMMB on 08/26/11

To emphasize. . .if you are reading this from coastal NC up through Long Island, I would be prepared for power outages (possibly for days), high winds causing some damage, very heavy rainfall from DC east, and possible tornadoes for Eastern NC and VA tonight through Saturday and Long Island into southeastern New England on Sunday.

I’ll update as needed. . .possibly later today

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