(Premium) Tropical Storm Hermine Increasingly Likely to Affect our Region

We continue to carefully monitor developments with tropical storm Hermine currently over the Gulf of Mexico. It is forecast to make landfall near around the Florida Big Bend area, as a tropical storm. But it could intensity more — into a Category 1 Hurricane — before making landfall. Therefore, a hurricane watch has also been issued for this region. The main threat will still be heavy rainfall and perhaps some coastal flooding and storm surge along the coast, due to the new moon cycle.

For our region, we watch for at least the remnants of this system to affect our area. Model guidance has been indicating that a northern stream shortwave will break off the upper-level trough that will passing through the Northeast tonight and tomorrow. This will trail the that trough and create a new, powerful shortwave trough in the Ohio Valley — similar to what we sometimes see during our strongest snowstorms. Then this shortwave trough and energy will amplify the downstream pattern, and thus interact with the shortwave energy from Tropical Storm Hermine, as it moves up somewhere near the Georgia and Carolina coasts on Friday.

There are a number of questions, in regards to timing, strength and placement of the northern stream energy. This will be the main mechanism that could actually pull Hermine further back west, before going out into the open Atlantic waters and make another landfall over the Northern Mid-Atlantic or New England region. Also more phasing, plus the fact that the storm will be sitting underneath a blocking high, may turn this into a closed ULL, which in turn would cause more strengthening as a hybrid Nor’Easter or baroclinic system. Somewhat similar to Sandy to what happened before reaching here — though it would be far less intensity-wise, given that this is a weaker tropical system, and we don’t have strong jetstream dynamics.

 

12z GEFS at 500mb showing northern stream shortwave wave phaising with upper-level energy from Hermine into a closed 500mb low.

12z GEFS at 500mb showing northern stream shortwave wave phasing and capturing with Hermine into a closed 500mb low lifting into Chesapeake Bay region..

There are also number of questions, in regards to the strength and placement of a blocking ridge and high pressure to the storm’s north. The faster flow across the northern tier of United States, may initially cause the aforementioned upper-level trough to move quickly enough for the ridge to be forced from the northwest to the north and east of the storm — into the North Atlantic. The movement of the ridge into the North Atlantic is critical, as this will give the storm less room to escape out to sea, which increases the chances for the storm to at least stall or meander back west towards the Mid-Atlantic or New England coasts.

12z GEFS showing blocking high pressure moving to northeast, not allow TS Hermine remants to escape out to sea.

12z GEFS showing blocking high pressure moving to our northeast, not allowing TS Hermine remnants to escape out to sea.

Then there are questions regarding how far off the Southeast US coast will Hermine go, after she makes landfall over Florida. If she remains inland or too close to the coast, the system will decay faster as tropical cyclone, before reaching the region, especially if there is also less phasing with the northern stream energy to strengthen it baroclinically.

The storm could become non-tropical and turn into a Nor’easter. Model guidance shows this system being inside a right-entrance region of a strong upper-level jetstreak running to north. This will help ventilate this system and with the right baroclinic dynamics, also deepen this storm as a non-tropical entity as it moves further north, while still being able to maintain some of its tropical core due to the well-above-average sea-surface temperatures.

The 12z GFS showing strong diverence over Southeast US with TS Hermine

The 12z GFS showing strong diverence over Southeast US with TS Hermine

Regardless, this system can cause heavy bands of rain in the local region. It will also cause rough surf, and coastal flooding due to astronomical high tides with the new moon phase, through the Labor Day weekend. But the impacts may not be as severe with a decaying system meandering in the Atlantic.

The other possibility for this storm is that it sits and decays well to our south, also sparing us of the biggest impacts. With the blocking high pressure to our north trending stronger, as well as shifting into the North Atlantic — a “blockier” position — as well as stronger backside energy in the Ohio Valley, a phase could occur pretty far south off the Carolinas. It will then not have much room to go northward, and instead sit and move slowly underneath the blocking high. This would give the worst impacts to the Mid-Atlantic, but not for our area.

However, the lack of strong initial blocking also makes the above idea very precarious as far as impacts here. Since the block is initially to our northwest, the storm will initially have some room to move northward and even northeastward for a bit before the phase. This may make it hard for the storm to truly miss us way to the south. What instead could happen is that after some northward progress, the storm hooks back towards our area just to our south, rather than directly hitting us. What this does, though, is shove the heaviest rain to our southwest, but puts us in the northeast quadrant — where the winds are the strongest and where there is the most fetch from the Atlantic. This could spell a lot of trouble for NJ and LI beaches. The 18z GFS shows this scenario well, which has piqued our level of concern somewhat.

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Today’s 18z GFS valid for 2am Sunday morning shows a tucked in storm just off the NJ coast, pummeling the beaches with very strong winds.

The storm may also go under a warm seclusion, where it has a warm core but baroclinic properties. This may allow some relatively warm air at the surface, instead of a large inversion. What this does is allow for a mixed layer up to around 900mb, where winds could be between 50 and 70 knots — the mixed layer could then help transfer those winds to the surface, at times. This means wind gusts could potentially be in that vicinity as well, and considering the wind direction is onshore, it could spell a lot of trouble for the beaches.

Fortunately, we are still several days out, and there are still a myriad of potential solutions. It is way too early to start hyping any impacts, as there are still a lot of features in this pattern that need to be ironed out. It could retrograde anywhere from Virginia to Boston, or still miss us entirely. However, we are starting to become more confident in some sort of impact from this storm, and that should not be taken lightly.

Stay tuned for more updates the rest of this week as we continue to monitor the latest model and ensemble guidance to better gauge the potential for Hermine to impact the weather in our local region this weekend.

 

This article was written and edited by both Miguel Pierre and Doug Simonian.