Major Hurricane Isaias

[vortex95]

9 hours ago, GaWx said:

I’m glad the expected last minute weakening verified well saving from a MH landfall, but realize getting cat 1-2 H effects is of course still a big impactful event (I know first hand).
 

 Interestingly per Dr. Knabb, that same strong shear that caused the dramatic weakening just before landfall actually caused the worst of the thunderstorms/wind gusts (solid cat 2 gusts) to be in your area in the NW eyewall (where Stephanie Abrams great coverage was). It’s almost as if the NW eyewall weakening lagged behind. She said she had never seen so much lightning in an eyewall, which typically has limited lightning due to the upper atmosphere being too warm for as much ice in the clouds vs typically much more lightning in outer bands as Dr. Knabb explained.

 I thought TWC coverage was excellent between her, Cantore showing storm surge and sudden shift to warming S winds off Gulf in E eyewall, and Dr. Knabb.

Regarding the LTG frequency,  Isaias was rapidly going PT/ET, and strong baroclinic influences were present before landfall.  Dry air intrusion and wind shear are going to lead to stronger updrafts b/c more CAPE present.  Asymmetries in the circulation are going to result in frontal-type convergence zones, further promoting stronger updrafts

Also, chasers reported it was a "cold" hurricane. Temps along the FL coast were largely below 70 prior to landfall.  That's not your typical tropical environment.

LTG can be frequent in eyewall in an intense HU in the deep tropics.  Recons have reported this many times in strong Cat 4s and Cat 5s.  What happens is that when a TC gets this intense, updrafts in the eyewall, which normally max out just below typical tstm updraft strength, cross the threshold to make it favorable for LTG. 

 

[vortex95]

This SAR image clearly shows a sting jet, which is a characteristic of a PT/ET low pressure, not a TC.  Also, prior to landfall, there was no coherent, focused wind center.  It was quite elongated.  So despite the satellite appearance w/ all the deep convection, structurally one could argue it had transitioned to more PT/ET than tropical prior to landfall.  That sudden jump of pressure up 12 mb to 970 on recon when it has been holding steady 958-960 mb also indicates a rapid change structurally to away from tropical status.  That suggested a distinct cool/dry air penetration into the core (front).  Once that happens, a system is PT/ET.

Note: SAR shows sustained winds max 110 kt.  The SAR tends to have high bias.
https://www.star.nesdis.noaa.gov/socd/mecb/sar/sarwinds_tropical.php?year=2026&storm=AL092026_ISAIAS

I have always said when a system is transitioning like this, there should be a ramp down to subtropical status before PT/ET status.  Why not?   If a system can go from ET to ST to tropical, there is absolutely no reason to think it can not work the other way.  This is what should have been done for Sandy for its landfall.  A TC rarely goes instant PT/ET in just 6 hr.  It is *process* that takes time.

And I don't want to hear about the messaging issue.  ST storms get names just like tropical storms, and the watches and warnings are the same ST or tropical.  In fact, a few years ago, JTWC started assigning operationally ST status as systems transitioned from TC.
 

[vortex95]

One historical item concerning sting jets.  It likely that HU Edna in Sep 1954 when crossing Cape Cod in a very strong PT/ET transition (110 mph at landfall) had a sting jet.  Why?  BOS gusted to 101 mph out of the NW -- its highest gust on record!  10 days before HU Carol (115 mph at landfall) produced a gust to 100 mph at BOS out of the SE as the storm passed to the W, which is what you would expect for very high winds.

When HUs get to New England, the PT/ET transition is almost always ongoing and the typical cooler/more stable air NW of the center along w/ rapid forward motion that will reduce winds W of the track.  I always wondered how such a wind gust occurred in BOS from Edna from that direction.  Once the concept of the sting jet became front and center about 20 years ago, that explained it!

In the larger TC picture, Isaias in some ways is a watershed event.  From strictly a conventional satellite perspective (EIR and Dvorak), that much deep convection not all sheared N and NE of the center near/at landfall, that says it was still more tropical than not, and can be called a HU at landfall.  However, the SAR imagery tells a much different story!  It's overall elongated wind structure, a string jet present, and the lack of a coherent inner core says it was more PT/ET.

Other factors stating it was more PT/ET?  That abrupt rise in center pressure in a short time before landfall (something obviously happened at the core here) and the rather cool surface conditions directly ahead of the storm.  This shows it was not just a race for cooler/drier air trying to wrap in from the W and NW.  This less than favorable TC atmosphere was N and NE as well, and got ingested into the core before landfall.

This is an excellent example how new technology reveals things we could never quite detect, or detected/quantify at all prior.  SAR gives a very detailed wind profile of a TC when it is over water.  And as we know, the wind structure is paramount as to if it is a TC or not (tight or spread out?, symmetrical or asymmetrical?)  And it also makes us rethink what we thought we knew well, and adjust accordingly.  This is good b/c the science is always evolving. 

Imagine when SAR-type imagery is available on geostationary satellites?  That will revolutionize analysis and modelling for the current data sparse oceans.

[vortex95]

I was looking back at SAR imagery for Isaias.  Look at the difference in structure only 12 hr apart!
  

[GaWx]

Meanwhile, the remnants of Isaias are directly responsible for tornadoes today in NC, including a damaging one in Fayetteville and several others in the Triangle region! This is serious stuff and is the 2nd day in a row of this.

[WxWatcher007]

47 minutes ago, GaWx said:

Meanwhile, the remnants of Isaias are directly responsible for tornadoes today in NC, including a damaging one in Fayetteville and several others in the Triangle region! This is serious stuff and is the 2nd day in a row of this.

Yeah so much for the severe risk busting.