Ghost of Karen: What to expect from this transformed, tropical remnant and why

October 9, 2013

Rejuvenated coastal storm will linger for days with rain and wind for Mid-Atlantic

The eleventh named tropical cyclone in the Atlantic fell short of becoming a hurricane, then wallowed in the Gulf while weakening.  Now, Karen has taken on a new identity, transformed into a coastal low currently visiting the Southeast and Mid-Atlantic.

Before we delve into the specifics of ex-Karen’s interesting impacts on the Mid-Atlantic, let’s discuss the various fates of a hurricane or tropical storm once it moves out of the tropics and into mid-latitudes.  We will also briefly define what is meant by a coastal low.

Satellite view (enhanced infrared) of the coastal storm just off the Eastern Seaboard (weathertap.com).
Satellite view (enhanced infrared) of the coastal storm just off the Eastern Seaboard (weathertap.com).
Surface weather map (8 AM Oct 9) showing the coastal low, associated fronts and area of rain. Note the large high pressure cell over New England impeding northward progress of the storm.
Surface weather map (8 AM Oct 9) showing the coastal low, associated fronts and area of rain. Note the large high pressure cell over New England impeding northward progress of the storm.

The post-tropical phase of hurricanes: Storms with an identity crisis

The National Hurricane Center defines a hurricane as post-tropical when its tropical characteristics have been lost; namely, the eye has vanished, the eyewall fragments or disintegrates, the wind field expands, and there is no longer a well-defined radius of maximum winds.

Post-tropical systems can dissipate further, or they can combine with pre-existing weather systems in the mid-latitudes, such as a frontal boundary or an upper level trough in the jet stream.

If the remnant vortex begins assuming the characteristics of an extratropical cyclone i.e. it acquires a frontal wave-like structure (warm front, cold front), an asymmetric rain shield, and a central core of cold (as opposed to warm) air, we describe this evolution as an extratropical transition.

Some infamous examples of extratropical transition in the Mid-Atlantic include Hazel (1954), Camille (1969), Agnes (1972) and Floyd (1999).  Each of these storms, incidentally, were highly destructive, producing heavy rainfall and/or sustained high winds, sometimes well inland and days after landfall.

The inland phase sometimes reaches destructive proportions because extratropical transition provides the remnant tropical system with additional sources of energy. These sources include vigorous, sustained uplift ahead of a jet stream trough, or strong temperature contrasts along a frontal zone.  Inland, post-tropical systems can even re-intensify, a process called rejuvenation.

So how would we describe Tropical Storm Karen’s transition to an extratropical, coastal low, now slowly crawling northward along the Delmarva?    Karen’s remnant vortex and tropical moisture combined with a strong cold front.  The cold front was part of the same extratropical cyclone that created the blizzard over South Dakota, the violent tornado outbreak across Nebraska, and the extensive squall line along the East Coast.

The coastal low is now part Karen and part Atlas (Atlas is the name bestowed upon the extratropical cyclone by The Weather Channel).

What is a coastal low?

Coastal lows are a flavor of extratropical cyclone, in which part of most of the circulation resides over Atlantic or Gulf waters.   There are several facets that distinguish coastal storms from their inland cousins.

First, the nature of the hazards is such that high winds blowing over a long fetch of water raise potentially enormous waves, creating a flood risk and high storm tide.   Beach erosion and damage to beach structures can be as widespread and costly as that wrought by a hurricane.

Related: A “nor’easter,” a “northeaster,” a northeast storm, or does it even matter?

Second, in addition to the energy generated by strong temperature (frontal) contrasts, coastal lows derive a significant fraction of their energy by extracting heat and moisture from the Atlantic’s warm Gulf Stream current.

Third, coastal lows are primarily creatures of the cool (winter) months.  During  this time, temperature contrasts are maximized, the jet stream aloft is intense and dynamic, and the oceanic Gulf Stream remains quite warm in spite of the cooling sea surface.  Intense coastal lows are termed Nor’easters.  These powerful storms produce sustained northeasterly gales (up to hurricane force), a prolonged and damaging storm tide, and (sometimes) heavy snow.

Impacts of this week’s coastal low on the Mid-Atlantic

1. Lots of cloud cover, cool and humid conditions will prevail.    A combination of the coastal low to our east and high pressure parked over New England will usher in a moist, cool onshore flow, at least through Friday morning.   Thanks to high pressure nosing down the Piedmont from New England, there is a hint of cool air damming this morning – building a shallow wedge of cool air buttressed against the Appalachians.  This will only enhance the thick overcast and may contribute to our area’s rain totals.

2. A gradient of significant rainfall will span the region.  The nature of the rainfall will be stratiform – that is, light to moderate, widespread, falling from thick layers of cloud, rather than deep convective cloud (think heavy showers, thunderstorms) in nature.   But the rain is expected to be heavier closer to the low, meaning coastal regions and the Eastern Shore will likely see greater rain totals.  Periods of rain will chill the air further;  temperatures may not climb out of the mid-upper 50s for some portions of the area.

NOAA Weather Prediction Center total rainfall accumulation expected for the 5-day period, Oct. 9 through Oct. 14. (NOAA).
NOAA Weather Prediction Center total rainfall accumulation expected for the 5-day period, Oct. 9 through Oct. 14. (NOAA).

Because the system will meander off the Delmarva for several days, rain totals could add up along the Eastern Shore – providing the proverbial “soaking rain” that so much of our region needs.  This is why the official NWS forecast reflects periods of rain all the way through Saturday.

3.  This will be a stubborn pattern taking a few days to clear out.   The storm is cut off from the polar jet stream, adrift within a sprawling ridge of upper atmospheric high pressure.  Such orphaned or cutoff lows can frequently stall, and even retrograde (move toward the west).  Strong high pressure over the western North Atlantic will retard northward progress.

Upper atmospheric forecast chart for tomorrow (Oct. 10) showing the very isolated nature of the vortex attending the surface coastal low. Yellow shades denote regions of maximum spin. (Unisys Corp.)
Upper atmospheric forecast chart for tomorrow (Oct. 10) showing the very isolated nature of the vortex attending the surface coastal low. Yellow shades denote regions of maximum spin. (Unisys Corp.)

4.  This will not be a potent Nor’easter.   The storm, while stubborn, does not have much jet stream support – in other words, it does not sit beneath a trough that can ventilate the system, add impetus to rising air and rapidly “bomb out” the system.

Yes, the rain could be heavy at times;  the rain will add up to a few inches along the coast;  the winds from the northeast will be stiff;  high waves will lash at beaches, leading to some erosion and potential flooding.  Flooding is probable since this storm will persist across multiple high tide cycles.  But this storm won’t likely become a monster.


(National Weather Service)

What could go wrong with this forecast?   If the storm sits for long enough time over the warm Gulf Stream, it could tap enough energy to deepen more significantly, even lacking jet stream support.  The models generally do not portray this scenario.

How about “Karen Reincarnated”,  becoming…once again…an intensifying tropical system?  Not likely;  thunderstorms would really have to begin firing near the core…and even if this were the case, the resulting, more intense storm would be a “hybrid” rather than a pure tropical system.

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Don Lipman · October 9, 2013