Weather: Tropical Storm Andrea a flooding threat to southern New England

How could a storm down in the Gulf of Mexico threaten New England with flooding?

Better water up there than some other things.  Source:    Woodley Wonderworks

Better water up there than some other things. Source: Woodley Wonderworks

A river in the sky

Forecasters at the National Weather Service in Taunton, Massachusetts, included a pretty complete description of how that could happen in their Boston forecast discussion (as of the time I am writing this post – this link will take you to the current Boston NWS forecast).

I bolded the highlights:

FRIDAY INTO SATURDAY…

CONFIDENCE CONTINUES CONCERNING A SIGNIFICANT FLOODING THREAT…NOW
SPECIFICALLY FOCUSED ACROSS SOUTHERN NEW ENGLAND.

THE KEY TO THE FORECAST IS THE STRENGTH OF THE OFFSHORE RIDGE PARENT
WITH THE BROAD TROUGHING OVER THE GREAT LAKES REGION. THIS GREATLY
IMPACTS THE TIMING AND PLACEMENT OF THE POST-TROPICAL LOW MOVING
NORTHEAST TOWARDS AND IN THE VICINITY OF NEW ENGLAND. THE EXACT
TRACK AND TIMING OF THREATS REMAINS UNCERTAIN. IT REMAINS QUITE
PLAUSIBLE THAT THE SYSTEM AND ATTENDANT HEAVY RAIN WILL BE
PROGRESSIVE AND/OR REMAIN OFFSHORE THEREBY REDUCING THE THREAT.

SUCCINCT WITH YESTERDAYS DISCUSSION…INITIAL BAND OF HEAVY RAIN IS
ANTICIPATED TO MATERIALIZE SOMEWHERE ACROSS PA INTO UPSTATE NY
THURSDAY INTO FRIDAY
PARENT WITH THE H85 FRONT WHERE THE
STRONGEST MOISTURE FLUX MAGNITUDE CONVERGES WITH +10C H85
DEWPOINTS AND PWATS AROUND 1.5 INCHES BENEATH THE RIGHT-REAR
QUADRANT OF THE UPPER-LEVEL JET.

THIS ATMOSPHERIC RIVER THEN SHIFTS AND FOCUSES AHEAD OF THE MAIN SUB-
TROPICAL LOW EXPECTED TO GLIDE NORTHEASTWARD ALONG A DEFINING QUASI-
STATIONARY BOUNDARY IN THE VICINITY OF SOUTHERN NEW ENGLAND FRIDAY
INTO SATURDAY. BECOMING DEEP AND RICH WITH TROPICAL MOISTURE /LATEST
GEFS MEANS SIGNAL +2 INCH PWATS INTO SOUTH-COASTAL NEW ENGLAND +3-4
STANDARD DEVIATIONS ABOVE AVERAGE/ IN AN AREA OF DEEP-LAYER ASCENT
PARENT WITH THE LEFT-EXIT REGION OF A SECONDARY JET AXIS AHEAD OF
THE BROADER TROUGH ALOFT…THE PLAUSIBLE OUTCOME IS STRONG LOW-MID
LEVEL MOISTURE CONVERGENCE WITHIN THE NORTHEASTERN QUADRANT OF THE
SURFACE LOW YIELDING A POTENTIAL OF AXIS OF 2-3 INCHES OF RAINFALL
WITHIN A 12 HOUR PERIOD ACROSS PORTIONS OF SOUTHERN AND SOUTHEAST
NEW ENGLAND…WITH LOCALLY HIGHER AMOUNTS.
GEFS MEANS HAVE UP TO A
60 PERCENT PROBABILITY OF SUCH A SCENARIO. MINOR MUCAPE ON THE
ORDER OF 100-200 J/KG IS APPARENT…SO WILL RAISE A ISOLATED
THUNDER CHANCE MAINLY CLOSE TO SOUTH AND SOUTHEAST COASTAL NEW
ENGLAND.

SO CONCERNING MODEL PREFERENCE….NOTEWORTHY FROM WPC/HPC…A
COASTAL ROUTE IS BEST RATHER THAN OCCLUSION AGAINST THE OFFSHORE
WEAK RIDGE OF HIGH PRESSURE. GFS/GEFS CAN TRACK UNORGANIZED
TROPICAL/SUBTROPICAL CYCLONES TOO QUICKLY…WHILE ECMWF GUIDANCE
ACCELERATES SYSTEMS INTO THE WESTERLIES TOO SLOWLY…SO WILL BLEND
THE 05.0Z ECMWF AND 05.12Z GFS WITH THE FORECAST DISCOUNTING ALL
OTHER GUIDANCE. MEANWHILE THE 05.12Z ECMWF HAS WOBBLED SLIGHTLY
INLAND…AND WILL DISCOUNT IT FOR THIS FORECAST…BUT MUST EMPHASIZE
THAT THIS WILL LIKELY BE THE MAIN CONTENTION THE NEXT 1-2 DAYS…
THE EXACT COASTAL TRACK AND COLLOCATED PRECIPITATION MAXIMUM. THERE
IS STILL UNCERTAINTY WITH THE FORECAST 2 DAYS OUT IN REGARDS TO
TIMING AND PLACEMENT.

HAZARDOUS WEATHER OUTLOOK WILL ECHO THE THREATS ACCORDINGLY WITH
HEAVY RAIN AND POTENTIAL FLOODING THE BIGGEST CONSIDERATION. WHILE
THE SYSTEM MAY BE PROGRESSIVE AND THE FLOODING THREAT MAY BE
LESSENED…IT STILL SHOULD BE GREATLY EMPHASIZED THAT THERE REMAINS
A RISK FOR URBAN…LOW-LYING…AND SMALL STREAM FLOODING. FLOOD
PRONE AREAS SHOULD BE ON ALERT FRIDAY INTO SATURDAY.

In relatively plain English, and after checking out the National Hurricane Center Discussion #1, Andrea is likely going to land in Florida, probably as a tropical storm, and then kind of merge with a system that is currently heading east across the central US. This will set up that “atmospheric river,” although its exact location is still unclear this far out. Computer models are most reliable in the short term, roughly 24 to 48 or 36 hours.

Right now the GFS model shows it happening like this (click to enlarge):

Andrea and its remnant can shift that Gulf water vapor all the way up the coast, and possibly dump it over New England. Of course, if it ends up tracking further east, out to sea, then flooding threats will be less. Hope that happens.

Dams

New England always gets floods, and this process also can happen in colder weather. A few years ago, a tremendous ice storm up here in the Northeast happened this way.

Flooding is just something people live with, but New Englanders have also been living with dams for a long time, in many cases, literally centuries, and some of those dams are at risk.

Now might be a good time to take a closer look at that, when it’s not about politics but about the real world case of holding back flood waters. I checked out the US Army Corps of Engineers online dam inventory and got some graphics that show the problem in New England.

Basically, there are a lot of old, hazardous dams, and many of them do not have an emergency action plan (EAP) …

… in Massachusetts

US Army Corps of Engineers National Dam Inventory website

US Army Corps of Engineers national dam inventory website (hazardous only – EAP graphic didn’t load)

… in Connecticut

US Army Corps of Engineers National Dam Inventory website

US Army Corps of Engineers national dam inventory website (hazardous only – EAP graphic didn’t load)

… and in Rhode Island.

US Army Corps of Engineers National Dam Inventory website

US Army Corps of Engineers national dam inventory website

Downstate New York also has a lot of dams, but I couldn’t separate out the data from the overall state listing and so didn’t include it here. Of note, the NID does show, that in New York, 389 of the 688 dams that represent a significant hazard of potential damage and require an EAP do not have one.

These dams have held for many, many years. They all withstood Superstorm Sandy. Each will continue standing as long as it can, and nobody can predict when it will fail – only that failure will happen eventually, if something isn’t done. People are trying – it just takes a lot of time, and unfortunately, money.

It’s too late to do anything now about the hazards, but knowledge is always power in a risky situation. If you live near a dam or just want to learn more about this, FloodSmart.gov has a good page on that.

Be safe – I value each and every one of you readers.


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