Potential Tropical Cyclone Eight—50mph/1006mb

[WxWatcher007]

Although we do not have an established low pressure yet, the NHC is designating the area off the southeast coast an orange with 40% odds of development. Given how quickly the timeline for development could be, I think it’s worth separating out the discussion.

 

1. Offshore the Southeastern U.S.:
A non-tropical area of low pressure could form along a frontal 
boundary a few hundred miles off the southeastern U.S. coastline 
this weekend. Thereafter, the low may develop some subtropical or 
tropical characteristics, and a subtropical or tropical depression 
could form early next week while the system moves generally 
northwestward toward the coast. Additional information on this 
system can be found in products issued by your local National 
Weather Service Forecast Office and High Seas Forecasts issued by 
the National Weather Service.
* Formation chance through 48 hours...low...10 percent. 
* Formation chance through 7 days...medium...40 percent.

Those following know that I’ve been talking about the southeast coast since late August. I view this latest homebrew development as part of a broader window that has been open since early September—a window open due to repeated troughing in the east that has left stalled and quasi-stalled frontal boundaries off the coast.

We saw it with 99L, which did not develop but brought rain and wind to Nova Scotia as an unusually strong late summer extratropical (read: baroclinic) low.

We then had a follow up wave after 99L that quickly moved OTS.

Now we’re looking at another stalled boundary off the coast and a non-tropical low (or two) that looks increasingly likely to develop along the boundary. 

 

 

The one we care about looks to develop off the Carolina coast. Again it’s non-tropical at the start but as the boundary decays it’s possible the low acquires tropical characteristics or becomes subtropical. The model signal has ebbed and flowed, but has come back today—and unlike 99L I think this has a better chance.

1) Unlike 99L, this is likely forming further south and west than 99L, in a more tropical environment.

 

2) With ridging likely to build over the top and some troughing to the west, this is likely to be steered into the US. 

 

 

Now, we’re not talking about a major hurricane or anything like that, especially with shear possible nearby, but strong winds and coastal flooding seem likely. We’ll have to watch for intensity trends this weekend.

[AccuChris]

Long range 0z HRRR indicating a Cat 1 hurricane Sunday


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[calm_days]

Yes , here it is in terms of radar.  18z was of course somewhat different in terms of timing and area. 

[WxWatcher007]

Even though we’re relatively close in, I’d still use the ensembles to a degree. There’s still a lot of spread along the coast, even up to the Mid-Atlantic. Not any real strong members at this time. 

It will be interesting though to see if the GFS is a little stronger at 00z. I think the Euro is playing catch up again. 

[Kevin Reilly]

1 hour ago, calm_days said:

Yes , here it is in terms of radar.  18z was of course somewhat different in terms of timing and area. 

Looking at overall wind shear vectors dry air around and strong high pressure to the NNE of this system it appears more of a subtropical storm then purely tropical but still would have potential impacts for sure.  Not what the vulnerable Carolina Coast needs. 

[yoda]

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
200 AM EDT Sat Sep 14 2024

For the North Atlantic...Caribbean Sea and the Gulf of Mexico:

Active Systems:
The National Hurricane Center is issuing advisories on Tropical 
Storm Gordon, located over the central tropical Atlantic Ocean. The 
Weather Prediction Center has issued the final advisory on 
Post-Tropical Cyclone Francine, located inland over Arkansas.

1. Offshore the Southeastern U.S.:
A non-tropical area of low pressure is expected to form this 
weekend along a frontal boundary a few hundred miles off the 
southeastern U.S. coastline. The system could gradually acquire 
subtropical or tropical characteristics thereafter over the warm 
waters of the Gulf Stream, and a subtropical or tropical depression 
or storm could form early next week while the system moves 
generally northwestward toward the coast. Additional information on 
this system can be found in products issued by your local National 
Weather Service Forecast Office and High Seas Forecasts issued by 
the National Weather Service.
* Formation chance through 48 hours...low...20 percent. 
* Formation chance through 7 days...medium...50 percent.


High Seas Forecasts issued by the National Weather Service
can be found under AWIPS header NFDHSFAT1, WMO header FZNT01 
KWBC, and online at ocean.weather.gov/shtml/NFDHSFAT1.php

Forecaster Reinhart

[SouthWake33]

We are down at bald head island through Monday. Keeping a close eye on this.


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[StantonParkHoya]

Please tell me we’re not trying to extrapolate the HRRR out to 84 hours on here

[calm_days]

i was surprised that long range HRRR was uniquely accurate for both Ian (2022) and Debby this year and i have seen people saying over the years that it often can have some functional purpose alongside the ensembles.  i will take your advice nonetheless!!   

 

 

 

[AccuChris]

The 12z long range HRRR is running now, so lets see if it is consistent with itself from its 0z bullish run last night


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[calm_days]

[calm_days]

.SHORT TERM /SUNDAY THROUGH TUESDAY/...
Key messages:
*Confidence in a low pressure system developing (potentially
 with a band of heavy rain and wind around at least part of the
 center) has increased somewhat.
*Questions remain as to where this system may cross the coast.
*Wherever this system tracks, heavy rainfall is likely mainly
 near and north through east of the center (with peak amounts in
 the 2-5 inch range possible) and much lighter rain expected on
 its western and southern sides.

Discussion:
[...]

Meanwhile, surface low pressure is expected to develop and
gradually deepen along a stalled front well offshore on Sunday
and Sunday night while being steered by a combination of high
pressure to its north, a coastal trough extending north of the
surface low, and a broad and weak mid-level low over the Deep
South. These steering factors continue to yield considerable
uncertainty in the forecast timing and track of this low, which
results in a range of differences in the location of impacts
between Monday and Tuesday. Although the latest guidance suite
is coming into somewhat better agreement than we have seen over
the last few days that a low pressure system will develop and
track towards the coast, considerable differences remain in
where the rain associated with this system will fall.

In short, wherever this low happens to cross the coast, heavy
rainfall peaking in the 2-5 inch range will be possible
generally on the northeastern side of the low with lesser
amounts to its west and south. Therefore, if the low tracks
more north towards Cape Lookout or Cape Hatteras, then most or
all of the area should see little rainfall. NHC has raised
the probability of this system acquiring tropical
characteristics to 50% within the next few days before it
potentially reaches the coast. Thus, while gusty winds are
expected generally to the north of the low due to high pressure
over New England causing a tightened pressure gradient, a band
of enhanced wind and rain near the center of the low may develop
as well.

[WxWatcher007]

High resolution models like the NAM and HRRR aren’t designed for potential tropical entities. I’d lean on global models and ensembles. 

[weatherCCB]

Looking at Wilmington radar almost looks like a little circulation . Definitely trying.

[Windspeed]

High resolution models like the NAM and HRRR aren’t designed for potential tropical entities. I’d lean on global models and ensembles. 

It does not mean they won't simulate them for tracking purposes, but they don't simulate ocean coupling the same way as TC models and do not handle intensification realistically. The NAM, HRRR and RAP code simulate them as assymetric or baroclinic warm core surface lows. They do not simulate intensification accurately, which is why their handling of intensity is usually overdone.

I'm going to add to this a lot more. The NAM, HRRR and RAP are designed to simulate development and intensification of convection (precipitation, thunderstorms, etc.) within the confines of diabetic processes (heat exchange) between vapor and air parcel associated with diffluence (spreading wind vectors), frontal boundaries, surface troughs versus ridges, upper lows, surface lows, etc., for atmospheric lift/instability. Tropical lows are surface lows just like baroclinic surface warm core, but they are powered by latent heat release off of SST evaporation, which is not the same as latent heat release from air parcel and dew point derrived instability over land. Why am I explaining this? Simply put, the convection models mentioned above, by their design, cannot handle the latent heat release of SSTs correctly, so they overdo convection, which starts a feedbak loop. This inflates convection, driving down simulated pressures, which in turn over-intensifies the tropical surface low. This feedbak loop is kept in check over land where, again, dew points and diabetic processes drive convection. The feedbak loop is why the NAM, for example, will show hurricane over water bottoming out way lower in pressure than what is realistic. Convection over land is, of course, what those models were designed to correctly simulate in mesoscale and baroclinic processes. That is their purpose and also why we may switch to them after a hurricane makes landfall, as they then become excellent for simulating what the surface low will do over land for land-derrived convection.

Hope this helps. Year after year we always scream not to use the high resolution convection models in the tropical threads, but perhaps an explanation as to why we don't use them will deter people from posting them? Meh, who am I kidding?

Long story short, do not use the NAM, HRRR or RAP products for tropical lows over water. After they make landfall? Sure...

[purduewx80]

There's hints of another binary interaction on some of the guidance, both CAMs and globals, not unlike what occurred with Francine. The MCV off the coast and another broad wave on the east side of the loop are trackable now and could be the two players these models are seeing. The difference this time is that they're both frontal waves initially. Hopefully, if this occurs and a consolidated low develops, it happens close enough in that it runs out of time to intensify significantly due to the improved organization and improving jet dynamics. This seems to be the consensus, but there's also indications a weird hybrid mess with multiple centers could head up the coast, in which case a named system would be less likely, despite similar impacts. 

[Normandy]

The MCV of the coast to me looks to be the catalyst.  The further it slides east the more time over water this would get 

[StantonParkHoya]

12z GFS has this inland by Monday morning. Sped the time up.

[purduewx80]

36 minutes ago, Normandy said:

The MCV of the coast to me looks to be the catalyst. 

Definitely what the 12z GFS shows and why it’s so much earlier with landfall. 

[StantonParkHoya]

ICON looks like a nor’easter lol. DMV folks would be hyped if it was January.

[NorthHillsWx]

One word for this: Meh

[StantonParkHoya]

12z Euro is… not much

[jbenedet]

All the NAM caveats apply but this is a synoptic gradient event if it materializes. 

Currently, highest prob is a sub tropical system, 

More persistent easterly gales, higher surge and higher surface winds than what the min surface pressure implies is what to expect.

[StantonParkHoya]

18z GFS says it may not rain in Raleigh

[yoda]

13 minutes ago, StantonParkHoya said:

18z GFS says it may not rain in Raleigh

Huh?  It does, nearly 3" worth

 

[StantonParkHoya]

30 minutes ago, yoda said:

Huh?  It does, nearly 3" worth

 

‘Twas a joke. Just not much of a storm.

[calm_days]

mid morning through to now !!

[GaWx]

 The extratropical LLC is near 31.5N, 76.5W. Below is the latest SST map (couldn’t save as high res). The LLC is over 84F water in the Gulf Stream. On the progged path, it will cross water as warm as 85. But then they fall sharply to 77-8 at the coast of the Carolinas thanks to recent BN temps/clouds. I’d think that would help keep the storm from being too strong at landfall should it become tropical:

 

[WxWatcher007]

4 minutes ago, GaWx said:

 The extratropical LLC is near 31.5N, 76.5W. Below is the latest SST map (couldn’t save as high res). The LLC is over 84F water in the Gulf Stream. On the progged path, it will cross water as warm as 85. But then they fall sharply to 78 at the coast of the Carolinas thanks to recent BN temps/clouds. I’d think that would help keep the storm from being too strong at landfall should it become tropical:

 

Here you go:

 

Looking at 925mb vorticity you can see the boundary clearly and the quickly increasing vorticity along it when compared to earlier today. 

[GaWx]

 This is now Invest 95L.