Watches Issued In Caribbean For Future Ernesto

Hermine is now emerging off the East Coast, beginning a long saga of coastal flooding, high surf, and beach erosion along a swath of the Eastern Seaboard Labor Day weekend and into the coming work week.

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To be clear, this storm will likely not be nearly as large as Superstorm Sandy. However, you don't need a storm as large as Sandy to be destructive, and moderate to major coastal flooding is now expected along parts of the East Coast.

(MORE: Latest News | Interactive Hermine Tracker)

Despite being a post-tropical cyclone, tropical storm warnings have been extended eastward to include the Connecticut coast, including Long Island Sound, and tropical storm watches have been extended to include all of coastal Rhode Island and the Massachusetts coast up to Sagamore Beach, including Martha's Vineyard, Nantucket, and Cape Cod.

National Weather Service (NWS) policy allows the National Hurricane Center (NHC) to issue advisories on and issue tropical storm watches and warnings for post-tropical cyclones when the system continues to pose a significant threat to life and property. This policy change was implemented after Superstorm Sandy.

No matter what meteorologists call this storm off the East Coast in the days ahead, the impacts will be dangerous.

Here are details on the setup, the potential impacts and the remaining uncertainty.

(MORE: Send Your Hermine Photos)

When a Storm Gets "Trapped"

Hermine was steered northeast around the west side of the Bermuda-Azores high-pressure system. It was also pulled somewhat by a southward dip in the jet stream, or trough, moving through the Great Lakes and Northeast.

But now something strange is happening.

That Great Lakes/Northeast upper trough has bypassed Hermine into the North Atlantic without grabbing hold of the storm and taking it along, as more frequently happens in the western Atlantic.

Upper-level high pressure building to the north and east of Hermine will trap the system for several days, including the entire Labor Day holiday weekend, and several days after.

Impacts

There are still uncertainties regarding Hermine's "post-tropical" life off the East Coast that will have a major bearing on impacts:

• Where exactly the low-pressure center stalls or meanders
• How strong it remains, or how much it may intensify
• How long it lingers

Here are the potential impacts.

Coastal Flooding/Beach Erosion/High Surf

• Probability: High
• Where: From southeast New England to the Outer Banks of North Carolina, particularly Long Island, the Jersey shore, and Delmarva peninsula; Also, the west end of Long Island Sound, Chesapeake Bay, Delaware Bay, Raritan Bay, and other inlets will be susceptible.
• Threats: High, possibly damaging surf, potentially moderate to major coastal flooding, significant beach/dune erosion, rip currents
• Timing: Seas build from south to north Saturday, peak Sunday and Sunday night, remaining high Monday into Tuesday, before winding down gradually Wednesday.
• The water could reach 3-5 feet above ground level if the peak surge occurs at the time of high tide from on the Virginia Coast and 2-4 feet northward to Bridgeport, Connecticut, according to the National Hurricane Center (NHC). In the Outer Banks and Pamlico Sound, 1-3 feet of water above ground can be expected mainly this weekend.
• Maps: NHC Potential Storm Surge Inundation | NHC Prototype Storm Surge Alert Map

(FORECAST: Ocean City, Maryland | Atlantic City, New Jersey | Nantucket Island)

Here are some of the next high tides along the East Coast:

Norfolk, Virginia

Ocean City, Maryland

Atlantic City, New Jersey

Sandy Hook, New Jersey

High Winds

• Probability:
• • High (Outer Banks, Virginia Tidewater, southern Delmarva peninsula)
  • Moderate (rest of Northeast seaboard from the Delmarva peninsula and Jersey shore to southeast New England coast, including Nantucket, Martha's Vineyard)
  • Slight: I-95 corridor from Washington D.C. to Boston
• Threats: Wind gusts over 50 mph may lead to power outages, downed trees, and possibly some structural damage to homes and buildings.
• Timing: One strong wind core will pass through the "high risk" area Saturday; threat persists Sunday and Monday, possibly beyond depending on where the low lingers.
• Uncertainty: The exact path of this storm will dictate how far these winds spread inland, or whether the core of potentially damaging winds remain out to sea.

Heavy Rain

• Probability:
• • High (Outer Banks of North Carolina)
  • Moderate (Jersey shore, Virginia Tidewater, southern Delmarva peninsula)
  • Slight (Long Island, southeast New England coast, including Nantucket, Martha's Vineyard)
• Threats: Bands of locally heavy rain could add rainfall flooding component, particularly in urban areas and along smaller creeks.
• Timing: One push of heavy rain will pass through the "high" and "moderate" risk areas Saturday; Threat may persist for several days beyond depending on where the low lingers.
• Uncertainty: The exact path of this storm will dictate how far the rainbands spread inland, or whether the heaviest rain stays offshore.

(MORE: Why Long-Range Model Forecasts in the Tropics Can't Often Be Trusted)

Why It's Not Another Sandy

Hermine's post-tropical remnant may be an impactful coastal storm, but it will pale in comparison to Sandy in many ways.

Sandy's destruction was due to its giant wind field, churning up the ocean ahead of it, then piling it into the East Coast. The post-tropical low's wind field will be impressive, but Sandy was in another class.

Inundation is some areas may come close to that of Sandy. 

Sandy had a previous pedigree of a Category 3 hurricane coming north from the Caribbean Sea, spending days churning the western Atlantic Ocean water up.

(MORE: Sandy Track History from Weather Underground)

Hermine will have spent some time over land early Saturday before re-emerging over the ocean.

Occurring just before Halloween, Sandy also had a sharper air mass contrast to feed its intensification near landfall, with temperatures in the 30s and 40s over the Ohio Valley, Appalachians, and eastern Great Lakes. You may recall it even snowed heavily in the Appalachians during Superstorm Sandy.

(MORE: The 35 Strangest Weather Events I've Seen)

Post-tropical Hermine won't have nearly that sharp a temperature contrast in early September.

The upper-level pattern was also more extreme for Sandy, featuring a bizarre blocking upper high from the north Atlantic Ocean to the Canadian Maritimes, helping to shove Sandy westward into New Jersey, and a rather pronounced southward jet-stream dip from the Ohio Valley helping to provide atmospheric lift for its final explosive development moving ashore.

Post-tropical Hermine will be trapped by high pressure aloft, and a bit of a blocking high to its east, but that won't likely steer it abruptly ashore. It should just linger and loiter.

Also, the upper-level low poised to meet up with Hermine is much weaker than the bullish Sandy upper trough.

However, one thing "Hermine" may have that Sandy didn't is longevity, one important factor in wave generation by any storm.

With the potential to hover offshore for 3-4 days over multiple high tide cycles in some areas, this storm may be able to inflict more beach erosion and coastal flooding than a faster-moving storm of similar intensity.

For example, the legendary March 1962 "Ash Wednesday" storm hammered the coast from Long Island to North Carolina, damaging or destroying 45,000 homes, destroying the boardwalk in Ocean City, Maryland and cutting a new inlet through Hatteras Island.

While a storm of this magnitude is far from a certainty in this case, we're pointing out the potential of this "post-Hermine" storm.

Check back with us at weather.com for the latest on this situation.

MORE ON WEATHER.COM: Hermine Impacts (PHOTOS)