Tropical Storm Hermine Taking Aim on Tri-State

My expectation was the upcoming holiday weekend would be mainly dry. Maybe some clouds and rough seas at worst, but as of tonight models suggest it is going to be much more than that. Latest models are pointing to a significant rain and wind event as Tropical Storm Hermine tracks up the east coast. Here is the latest probable path from the National Hurricane Center:

Before I talk about the potential impacts this storm may bring, I figured I will detail why the models made a dramatic shift west with the track. Ultimately, it comes down to the strength of Hermine which dictates its speed. A weaker storm typically moves faster while a stronger storm moves slower. 


Let's compare 500mb height anomalies between Monday's GFS against today's. The timestamp is valid for Friday evening. 

Monday 12z GFS (top) Vs. Wednesday 18z GFS (bottom) 

The glaring difference is Hermine is much stronger. In Monday's run, she was barely a depression over North Carolina whereas in today's run she is a Tropical Storm over South Carolina. The Atlantic Ridge is further west on the latest GFS, and the trough over the northwest Atlantic is weaker / faster. These realities are part of the reason why Hermine will track along or just off the coast rather than drift out to sea. 

The other reason - most important - why Hermine could track close to the coast is because the trough at the 250mb level captures Hermine and retrogrades her back west. The strong jet streak will bring about a dynamical aspect of the system that would allow strong winds to mix down to the lower levels, or surface, of the atmosphere. 

Some say when Hermine gets here she may not be a Tropical Storm. Maybe a sub-tropical or extratropical system. Honestly, who cares. Call it a Nor'easter if you want. But if the afternoon model runs come to fruition then expect lots of rain, high winds, and surge at the beaches. In my eyes this will be a Tropical Storm when it gets here due to the dynamics that will be at play. 


Check out this animation:

http://tropic.ssec.wisc.edu/real-time/mimic-tpw/natl/anim/latest72hrs.gif

This loop shows atmospheric moisture, or precipitable water. In the Gulf of Mexico we have Hermine and off the coast of North Carolina there is Tropical Depression 8 (look for the circular motions). The orange and red colors indicate a moist atmosphere. As Hermine tracks north, there is a good chance it is going to take this moist atmosphere and move it into our area. This would enhance rainfall amounts and cause added headache to those along the coast. This too will play part of the systems dynamics. 

A closer look at the 500mb level shows Hermine getting 'stuck' under a strong block to its north. The ridge you see in central Canada on the 500mb maps I posted above tracks east-southeast and traps Hermine under it. So between the 250mb trough and 500mb ridge, Hermine's effects will be felt for almost 24 hours. More in some places. 

The EURO is especially impressive with the wind and rain. 850mb winds on the EURO are near 80kts!! 

The GFS has 925mb winds (closer to the surface) near 70kts. Just further proof that the system will be fairly potent when it gets here. 

The Canadian models suggests rainfall could be anywhere from 3 to 6 inches all within a 24-hour period. That would cause extensive flooding, especially in flood prone zones. 

IMPACTS

1. There is still 3 important model suites to run before Hermine gets here. There is a chance models intensify Hermine more in the event they are downplaying the dynamics. I am impressed with the jet stream at the 250mb level. I think the GFS is too weak with the storm at the moment, and a blend between it and the EURO is not a bad forecast at this time. 

2. Rainfall is likely to be in the 3" range. The difference between 2-3" and 5-6" will depend on the exact track of the storm. Since we've been mainly dry the last 10 days getting a heavy dose of rain in a short period of time could bring flooding concerns to much of the state. 

3. Since there will be a retrograde back west, this is likely to enhance storm surge along the immediate coast. The NJ shore and southern shores of LI could ill-afford their beaches being destroyed or homes receiving property damage. Once the exact path of Hermine is nailed down, we'll have a better idea of the surge and how bad it may be. 

4. Wind is expected to be at least Tropical Storm strength for pretty much everyone in the Tri-State. Gusts could be as high as 65-70mph with sustained winds in the 50mph range. The combination of high winds and heavy rain could lead to power outages across the area. 

I'll have another update on Friday! 

Have a good night.

Best,

Francesco Paparatto