A multi-day, potentially historic heavy rainfall event may produce catastrophic and life-threatening flooding through Saturday from the Ozarks into the Ohio River Valley. Several clusters of strong to severe storms are expected from the Southern Plains, Mid Mississippi Valley to the Ohio/Tennessee Valleys and the Mid Atlantic. Read More >
Overview
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A slow-moving, anomalously deep upper level trough moved from the Missouri Valley towards the Tennessee Valley and Southeast US over the course of October 27 – October 29, 2021. Meanwhile, strong high pressure was present over northern New England and much of southeast Canada. As the trough slowly moved eastward, favorable jet dynamics and increasing baroclinic forcing allowed secondary low pressure began to develop close to the southern mid-Atlantic coastline on October 29. This coastal low would go on to affect the region for much of October 29 and 30 before slowly moving northward into New England. This system brought several impacts to the local area, but chief among them was tidal flooding. The storm’s slow movement caused a long duration period of onshore flow to occur, driving water towards the coast and into local bays and tidal rivers, with the persistence of the flow preventing water from draining back out to sea. Many local tidal gauges exceeded flood stage as a result, including some instances of moderate to major flooding on the Chesapeake Bay, Delaware Bay, and tidal Delaware River. As the storm made its closest approach, winds increased in strength over the area. The low pressure center was not particularly strong, with a central pressure of only around 995 to 1000 mb. However, it was large in size, and the presence of the strong high to the north created a tight pressure gradient. Onshore (easterly) winds became gusty as a result. Strong winds were mainly a concern only near the coast and over the waters, but in those areas, some gusts of near to above 60 mph were measured. A period of rainfall, which was locally heavy, also affected much of the region, but freshwater hydrologic impacts were very limited as rainfall amounts were insufficient to cause more than nuisance flooding. This page summarizes the storm’s effects on the area, particularly the significant and rather unusual tidal flooding event which occurred during its passage. |
 WPC Surface Analysis maps from 00z 29 Oct 2021 through 00z 31 Oct 2021, showing the evolution of the coastal storm which affected the area. Click to enlarge. |
Synoptic Overview at 00z 29 Oct 2021 (courtesy of SPC)
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| Figure 1: 250-mb analysis | Figure 2: 300-mb analysis | Figure 3: 500-mb analysis |
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| Figure 4: 700-mb analysis | Figure 5: 850-mb analysis | Figure 6: 925-mb analysis |
Synoptic Overview at 12z 29 Oct 2021 (courtesy of SPC)
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| Figure 7: 250-mb analysis | Figure 8: 300-mb analysis | Figure 9: 500-mb analysis |
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| Figure 10: 700-mb analysis | Figure 11: 850-mb analysis | Figure 12: 925-mb analysis |
Synoptic Overview at 00z 30 Oct 2021 (courtesy of SPC)
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| Figure 13: 250-mb analysis | Figure 14: 300-mb analysis | Figure 15: 500-mb analysis |
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| Figure 16: 700-mb analysis | Figure 17: 850-mb analysis | Figure 18: 925-mb analysis |
Synoptic Overview at 12z 30 Oct 2021 (courtesy of SPC)
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| Figure 19: 250-mb analysis | Figure 20: 300-mb analysis | Figure 21: 500-mb analysis |
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| Figure 22: 700-mb analysis | Figure 23: 850-mb analysis | Figure 24: 925-mb analysis |
NWUS51 KPHI 300822
LSRPHI
PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
422 AM EDT SAT OCT 30 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0500 AM COASTAL FLOOD 1 WNW GRASONVILLE 38.96N 76.22W
10/29/2021 QUEEN ANNE'S MD DEPT OF HIGHWAYS
MD 18 CLOSED DUE TO FLOODING BETWEEN MARSHY
CREEK ROAD AND WELLS COVE ROAD.
0730 AM COASTAL FLOOD 3 SE OXFORD 38.65N 76.13W
10/29/2021 TALBOT MD EMERGENCY MNGR
FLOODING REPORTED ALONG CHLORAS POINT ROAD.
0834 AM COASTAL FLOOD FENWICK ISLAND 38.46N 75.05W
10/29/2021 SUSSEX DE TRAINED SPOTTER
MINOR STREET FLOODING FROM TIDAL INUNDATION
IN FENWICK ISLAND. REPORT VIA SOCIAL MEDIA.
1000 AM COASTAL FLOOD OXFORD 38.69N 76.17W
10/29/2021 TALBOT MD EMERGENCY MNGR
SEVERAL ROADS FLOODED IN OXFORD, INCLUDING
AT THE INTERSECTIONS OF STEWARD AND TILGHMAN
STREETS, WILSON AND BANK STREETS, AND MARKET
AND SOUTH STREETS. ADDITIONALLY, FLOODING
WAS REPORTED ALONG THE CAUSEWAY.
1141 AM NON-TSTM WND DMG MILLINGTON 39.26N 75.84W
10/29/2021 KENT MD TRAINED SPOTTER
REPORT OF A TREE DOWN AT THE SPOTTERS
RESIDENCE IN MILLINGTON.
1215 PM COASTAL FLOOD 1 E CHESTER 38.97N 76.26W
10/29/2021 QUEEN ANNE'S MD DEPT OF HIGHWAYS
MD 18 CLOSED BETWEEN DUNDEE AVE AND PINEY
NARROWS ROAD DUE TO FLOODING.
1225 PM COASTAL FLOOD TILGHMAN 38.72N 76.33W
10/29/2021 TALBOT MD EMERGENCY MNGR
FLOODING ALONG BLACK WALNUT POINT ROAD.
0106 PM COASTAL FLOOD 1 ENE PORT MONMOUTH 40.43N 74.09W
10/29/2021 MONMOUTH NJ PUBLIC
SOCIAL MEDIA REPORT OF BROADWAY BETWEEN
CHURCH AND MAIN IN PORT MONMOUTH
EXPERIENCING MINOR FLOODING.
0130 PM COASTAL FLOOD 1 SE PLEASANTVILLE 39.38N 74.50W
10/29/2021 ATLANTIC NJ DEPT OF HIGHWAYS
FLOODING REPORTED ON US 40.
0135 PM COASTAL FLOOD LEWES 38.77N 75.14W
10/29/2021 SUSSEX DE PUBLIC
FROM SPOTTER NETWORK: BROADKILL RIVER/ LEWES
REHOBOTH CANAL FLOODING ONSHORE AND
SURROUNDING AREA.
0139 PM COASTAL FLOOD 1 N TALBOT 38.84N 76.28W
10/29/2021 TALBOT MD EMERGENCY MNGR
CLAIBORNE LANDING ROAD AND BAYSIDE DR.
FLOODED.
0209 PM COASTAL FLOOD 3 WSW LONG NECK 38.60N 75.21W
10/29/2021 SUSSEX DE PUBLIC
FROM SPOTTER NETWORK: WARWICK COVE TIDAL
WATER MOVING ONSHORE RAPIDLY.
0300 PM COASTAL FLOOD 1 N NORTH WILDWOOD 39.01N 74.80W
10/29/2021 CAPE MAY NJ DEPT OF HIGHWAYS
NJ 147 CLOSED DUE TO FLOODING.
0300 PM COASTAL FLOOD 1 SW BOZMAN 38.76N 76.29W
10/29/2021 TALBOT MD EMERGENCY MNGR
RIVER RIDGE ROAD FLOODED.
0302 PM COASTAL FLOOD 1 E MILLSBORO 38.59N 75.29W
10/29/2021 SUSSEX DE PUBLIC
FROM SPOTTER NETWORK: INDIAN RIVER FLOODING
CUPOLA PARK IN MILLSBORO DELAWARE, RIVER
ESTIMATED AROUND 4 ABOVE AVERAGE WATER
ALMOST INTO HOMES IN HUNTERS POINTE
DEVELOPMENT. PLAYGROUND AND PARKING LOT
SUBMERGED.
0313 PM COASTAL FLOOD OCEAN CITY 39.28N 74.58W
10/29/2021 CAPE MAY NJ PUBLIC
SOCIAL MEDIA VIDEO SHOWING COASTAL FLOODING
OF STREETS IN OCEAN CITY ON THE BAY SIDE.
0330 PM COASTAL FLOOD SAINT MICHAELS 38.79N 76.22W
10/29/2021 TALBOT MD EMERGENCY MNGR
NUMEROUS ROADS FLOODED INCLUDING WILLOW ST,
W HARBOR ROAD, AND CHURCH ST.
0442 PM COASTAL FLOOD 1 E CHESTER 38.97N 76.26W
10/29/2021 QUEEN ANNE'S MD DEPT OF HIGHWAYS
MARYLAND ROUTE 18 CLOSED BETWEEN KENT
NARROWS WAY AND BLUE STAR MEMORIAL HIGHWAY.
0457 PM COASTAL FLOOD OXFORD 38.68N 76.17W
10/29/2021 TALBOT MD DEPT OF HIGHWAYS
MARYLAND 333 EAST CLOSED BETWEEN JACKS POINT
ROAD AND DIVISION STREET.
0500 PM COASTAL FLOOD STONE HARBOR 39.05N 74.76W
10/29/2021 CAPE MAY NJ PUBLIC
STREET FLOODING AT 92ND AND THIRD AVENUE;
WATER ESTIMATED TO BE AT LEAST A FOOT DEEP.
0515 PM NON-TSTM WND DMG 1 E OAKS 40.13N 75.43W
10/29/2021 MONTGOMERY PA TRAINED SPOTTER
TREE AND WIRES DOWN NEAR THE INTERSECTION OF
EAGLEVILLE ROAD AND SOUTH PARK AVENUE. FIRE
IN WOODS NEARBY.
0530 PM COASTAL FLOOD SALEM 39.58N 75.47W
10/29/2021 SALEM NJ DEPT OF HIGHWAYS
NJ 45 CLOSED DUE TO FLOODING.
0539 PM NON-TSTM WND DMG 4 S ELLENDALE 38.75N 75.43W
10/29/2021 SUSSEX DE LAW ENFORCEMENT
TREE DOWN ON OLD STATE ROAD AT DUPONT
PARKWAY. TIME ESTIMATED FROM RADAR.
0553 PM COASTAL FLOOD 1 SW TUCKERTON 39.60N 74.34W
10/29/2021 OCEAN NJ TRAINED SPOTTER
FLOODING AT LEAST A FOOT DEEP OVER ROADWAYS
IN PHOTO.
0600 PM COASTAL FLOOD BROOKLAWN 39.88N 75.12W
10/29/2021 CAMDEN NJ DEPT OF HIGHWAYS
FLOODING REPORTED ON US 130 AND NJ 47 NEAR
WESTVILLE AND BROOKLAWN.
0600 PM COASTAL FLOOD SALEM 39.57N 75.47W
10/29/2021 SALEM NJ PUBLIC
SEVERAL ROADS CLOSED AROUND SALEM COUNTY
INCLUDING THE MARKET STREET BRIDGE, TILBURY,
GRIEVES, WARD, HANCOCK, AND HOWELL. TIME
ESTIMATED.
0601 PM COASTAL FLOOD 1 WNW DELAWARE CITY 39.58N 75.60W
10/29/2021 NEW CASTLE DE PUBLIC
PHOTO OF TIDAL FLOODING AT LEAST ONE FOOT
DEEP ALONG ROUTE 9 NORTH OF DELAWARE CITY.
REPORT VIA SOCIAL MEDIA.
0624 PM NON-TSTM WND GST 2 NNE STRATHMERE 39.22N 74.64W
10/29/2021 M62 MPH CAPE MAY NJ MESONET
WEATHERFLOW MESONET STATION XOCS OCEAN CITY.
0656 PM COASTAL FLOOD 3 S KITTS HUMMOCK 39.06N 75.40W
10/29/2021 KENT DE LAW ENFORCEMENT
MAIN STREET CLOSED DUE TO FLOODING AT
HUBBARD AVENUE IN BOWERS BEACH.
0656 PM COASTAL FLOOD LITTLE CREEK 39.17N 75.45W
10/29/2021 KENT DE LAW ENFORCEMENT
MAIN STREET CLOSED DUE TO FLOODING AT LITTLE
CREEK ROAD.
0656 PM COASTAL FLOOD 2 NW WEST DOVER 39.19N 75.62W
10/29/2021 KENT DE LAW ENFORCEMENT
FLOODING HAS CLOSED PEARSONS CORNER ROAD AT
CENTRAL BRANCH ROAD.
0656 PM COASTAL FLOOD 2 NNW MAGNOLIA 39.09N 75.49W
10/29/2021 KENT DE LAW ENFORCEMENT
FLOODING HAS CLOSED PONDEROSA ROAD AT
CYPRESS BRANCH ROAD.
0656 PM COASTAL FLOOD 2 NNW MAGNOLIA 39.10N 75.49W
10/29/2021 KENT DE LAW ENFORCEMENT
FLOODING HAS CLOSED CYPRESS BRANCH ROAD AT
EVERGREEN ROAD.
0656 PM COASTAL FLOOD 1 SE ODESSA 39.45N 75.65W
10/29/2021 NEW CASTLE DE LAW ENFORCEMENT
OLD CORBIT ROAD CLOSED DUE TO FLOODING AT
MAIN STREET.
0656 PM COASTAL FLOOD 2 WNW ODESSA 39.47N 75.69W
10/29/2021 NEW CASTLE DE LAW ENFORCEMENT
MARL PIT ROAD CLOSED AT BRICK MILL ROAD DUE
TO FLOODING.
0656 PM NON-TSTM WND DMG ODESSA 39.46N 75.67W
10/29/2021 NEW CASTLE DE LAW ENFORCEMENT
FLOODING HAS CLOSED MARL PIT ROAD.
0656 PM COASTAL FLOOD 3 ENE MILFORD 38.92N 75.38W
10/29/2021 SUSSEX DE LAW ENFORCEMENT
CEDAR BRANCH ROAD CLOSED BETWEEN MILLS ROAD
AND DRAWBRIDGE ROAD.
0656 PM COASTAL FLOOD 2 SSW NASSAU 38.69N 75.17W
10/29/2021 SUSSEX DE LAW ENFORCEMENT
CAMP ARROWHEAD ROAD CLOSED DUE TO FLOODING.
0656 PM COASTAL FLOOD 1 ENE LONG NECK 38.62N 75.13W
10/29/2021 SUSSEX DE LAW ENFORCEMENT
LONG NECK ROAD CLOSED SOUTH OF COVE ROAD DUE
TO FLOODING.
0700 PM NON-TSTM WND DMG 1 SE TROOPER 40.14N 75.39W
10/29/2021 MONTGOMERY PA TRAINED SPOTTER
TREE DOWN ON WIRES WITH POLE DAMAGED.
0701 PM COASTAL FLOOD NEW CASTLE 39.66N 75.56W
10/29/2021 NEW CASTLE DE TRAINED SPOTTER
PHOTO OF AT LEAST A FOOT OF COASTAL FLOODING
ALONG DELAWARE STREET.
0701 PM NON-TSTM WND GST 4 SE MYSTIC ISLAND 39.52N 74.32W
10/29/2021 M69 MPH OCEAN NJ MESONET
WEATHERFLOW MESONET STATION XTUK IN
TUCKERTON.
0709 PM NON-TSTM WND GST ATLANTIC CITY 39.38N 74.42W
10/29/2021 M60 MPH ATLANTIC NJ OTHER FEDERAL
USGS SITE AT ATLANTIC CITY MARINA.
0714 PM NON-TSTM WND GST SEASIDE PARK 39.94N 74.07W
10/29/2021 M66 MPH OCEAN NJ MESONET
WEATHERFLOW MESONET STATION XSEA AT SEASIDE
HEIGHTS.
0730 PM NON-TSTM WND DMG 1 S SOUTH AMBOY 40.47N 74.28W
10/29/2021 MIDDLESEX NJ PUBLIC
CORRECTS PREVIOUS NON-TSTM WND DMG REPORT
FROM 1 S SOUTH AMBOY. TREE FELL ONTO A HOME
AND BROUGHT WIRES DOWN ON SCHUSSLER STREET.
0800 PM NON-TSTM WND DMG 1 W WANAMASSA 40.23N 74.05W
10/29/2021 MONMOUTH NJ PUBLIC
RAMP CLOSED OFF OF SOUTH ROUTE 18 DUE TO A
DOWNED TREE. TIME ESTIMATED FROM NEARBY
OBSERVATIONS.
0800 PM NON-TSTM WND DMG 2 W RICHLAND TWP 40.48N 75.37W
10/29/2021 BUCKS PA PUBLIC
WIRES BLOWN DOWN ON EAST CHERRY ROAD. TIME
ESTIMATED.
0805 PM NON-TSTM WND GST HARVEY CEDARS 39.71N 74.14W
10/29/2021 M62 MPH ANZ451 NJ MESONET
0806 PM COASTAL FLOOD 2 S INDIAN BEACH 38.66N 75.07W
10/29/2021 SUSSEX DE LAW ENFORCEMENT
DE ROUTE 1 CLOSED BETWEEN BETHANY AND
REHOBOTH DUE TO COASTAL FLOODING.
0820 PM NON-TSTM WND DMG 2 N RED BANK 40.37N 74.07W
10/29/2021 MONMOUTH NJ NEWSPAPER
*** 4 INJ ***
TREE FELL ON A LARGE PARTY TENT, INJURING
FOUR PEOPLE. TIME ESTIMATED.
0924 PM NON-TSTM WND GST 3 NNW BARNEGAT LIGHT 39.80N 74.12W
10/29/2021 M60 MPH OCEAN NJ MESONET
0930 PM COASTAL FLOOD 1 ESE PHILADELPHIA 39.95N 75.14W
10/29/2021 PHILADELPHIA PA EMERGENCY MNGR
STREET FLOODING REPORTED IN THE AREA OF
PENNS LANDING INCLUDING COLUMBUS BLVD AND
DELAWARE AVE. TIME ESTIMATED.
1030 PM COASTAL FLOOD 2 NE PENNSVILLE 39.68N 75.50W
10/29/2021 SALEM NJ PUBLIC
SIGNIFICANT ROADWAY FLOODING REPORTED
BETWEEN PENNSVILLE AND PENNS GROVE,
INCLUDING ALONG HOOK ROAD IN PENNSVILLE.
TIME ESTIMATED.
&&
$$
NOUS41 KPHI 300515 PNSPHI DEZ001>004-MDZ012-015-019-020-NJZ001-007>010-012>027-PAZ054-055-060>062-070- 071-101>106-301715- Public Information Statement National Weather Service Mount Holly NJ 115 AM EDT Sat Oct 30 2021 ...HIGHEST WIND REPORTS... Location Speed Time/Date Provider ...Delaware... ...Kent County... Dover Air Force Base 46 MPH 0225 PM 10/29 AWOS ...New Castle County... New Castle County Airport 42 MPH 0315 PM 10/29 ASOS ...Sussex County... 5 E Long Neck 55 MPH 0314 PM 10/29 Mesonet Rehoboth Beach 53 MPH 0224 PM 10/29 Public Delaware Coastal Airport 47 MPH 0343 PM 10/29 ASOS ...New Jersey... ...Atlantic County... Atlantic City 60 MPH 0709 PM 10/29 Other Federal Atlantic City International 52 MPH 0717 PM 10/29 ASOS ...Cape May County... Ocean City 62 MPH 0624 PM 10/29 WXFLOW 2 NW Cape May 56 MPH 0448 PM 10/29 CO-OP Observer North Cape May 44 MPH 0536 PM 10/29 Broadcast Media ...Cumberland County... Millville Municipal Airport 44 MPH 0702 PM 10/29 ASOS ...Mercer County... Trenton Mercer Airport 40 MPH 0816 PM 10/29 ASOS ...Monmouth County... 1 ENE Manasquan 56 MPH 0810 PM 10/29 Mesonet Avon-by-the-Sea 50 MPH 0542 PM 10/29 Trained Spotter ...Ocean County... Tuckerton 69 MPH 0701 PM 10/29 WXFLOW Seaside Heights 66 MPH 0714 PM 10/29 WXFLOW 3 NNW Barnegat Light 60 MPH 0924 PM 10/29 Mesonet 1 S Island Beach State Park 57 MPH 0714 PM 10/29 Mesonet 2 SSW Barnegat Light 54 MPH 0924 PM 10/29 Mesonet 1 SW Beach Haven 53 MPH 0734 PM 10/29 Mesonet Harvey Cedars 52 MPH 0634 PM 10/29 Law Enforcement ...Pennsylvania... ...Delaware County... 1 E Tinicum Twp 44 MPH 0121 PM 10/29 Other Federal ...Lehigh County... Lehigh Valley International 43 MPH 0742 PM 10/29 ASOS ...Philadelphia County... Northeast Philadelphia Airpo 43 MPH 0703 PM 10/29 ASOS null Philadelphia Internati 41 MPH 0447 PM 10/29 ASOS ...New Jersey... ...Maritime Stations... Harvey Cedars 62 MPH 0805 PM 10/29 Mesonet &&
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| Figure 1: Hydrograph for the New Jersey Atlantic Coast at Sandy Hook tidal gauge (SDHN4) during the late October 2021 coastal flooding event. The solid blue line represents observed values which have already occurred. The purple line with small square boxes along it represents forecast values. | Figure 2: Same as Fig. 1, for Watson Creek at Manasquan (MSNN4). | Figure 3: Same as Fig. 1, for Barnegat Bay at Barnegat Light (BGLN4). |
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| Figure 4: Same as Fig. 1, for the New Jersey Atlantic Coast at Atlantic City (ACYN4). | Figure 5: Same as Fig. 1, for the tidal Delaware River at Burlington (BDRN4). | Figure 6: Same as Fig. 1, for the tidal Delaware River at USCG Station Washington Street, Philadelphia (PHBP1). |
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| Figure 7: Same as Fig. 1, for Delaware Bay at Reedy Point (RDYD1). | Figure 8: Same as Fig. 1, for Chesapeake Bay at Tolchester Beach (TCBM2). | Figure 9: Same as Fig. 1, for Eastern Bay at Claiborne (CLNM2). |
Coastal flooding is a common occurrence in the local area. The presence of the Atlantic Ocean and several local bays, inlets, and tidal rivers and tributaries present a constant and complex challenge to forecasters. In some cases, it can take only small deviations, on the order of a few tenths of one foot, from the normal astronomical tide to cause tidal flooding. Tidal flooding is so common that NWS Mount Holly will not issue a Coastal Flood Advisory until forecast water levels exceed three tenths (0.3) of one foot above a tidal gauge’s documented minor flood stage. Coastal Flood Watches and Warnings are reserved for the less common instances of moderate and major tidal flooding.
The late October, 2021 tidal flooding event was notable for a couple of reasons. The synoptic pattern which led to the flooding event was one that is commonly associated with tidal flooding: coastal low pressure leading to a strong and long duration onshore wind event, which pushes large amounts of water towards the coast and local tidal areas, and prevents that water from escaping back out to sea as it does in a normal tidal cycle. However, a couple of abnormalities were present in this case. First, the event occurred during a period of astronomically low tides. Tidal magnitude is primarily governed by the lunar cycle, with astronomical tides reaching their maxima at the times of a full or new moon. In this case, however, the moon was squarely in between a full moon on October 20 and a new moon on October 29. While tidal flooding can and does occur during periods of low astronomical tides, it requires a significantly higher meteorological surge component to compensate. Such a surge, of 3 to 5 feet above astronomical tidal levels, was present in this event. Because of this unusually high surge, it is very fortunate that astronomical tides were not higher, or flooding would have been signifcantly worse.
Also of note, in most of our region’s tidal flooding events, the worst flooding (of the moderate and major variety) is typically found near oceanfront locations, whether it be the New Jersey or Delaware oceanfront itself, or adjacent bays such as Barnegat Bay in New Jersey and Indian River Inlet in Delaware. However, in this event, the worst flooding occurred across upper Delaware Bay, the tidal Delaware River, and Chesapeake Bay, though significant flooding also occurred in some of the back bays of eastern New Jersey. Most tidal gauges near the oceanfront (as seen in the Hydrographs tab on this page) reached only minor flood stage, and in some cases failed to reach flood stage at all. On the other hand, moderate to major flooding occurred in those other aforementioned locations. While areas like the tidal Delaware River and Chesapeake Bay are no strangers to flooding, it is unusual to see this disparity from the oceanfront locations; conversely, it is relatively common for minor to moderate flooding to occur at oceanfront locations while little to no flooding occurs in other tidal areas.
There were a couple of reasons flooding was worse at some of the typically less prone locations. The greater impacts on Chesapeake Bay can partially be explained by the fact that Chesapeake Bay has a naturally lower tidal range than other tidal portions of the area. Thus, the astronomically low tides are less of a factor for determining flooding potential on the Chesapeake. On Delaware Bay and the tidal Delaware River, much of the disparity can be explained by the timing of the highest storm surge versus the time of high tides. Elevated tidal levels were present for several days leading into and during the coastal storm on account of the long duration onshore flow. However, the greatest tidal departures occurred during a concentrated surge in water levels on the evening and overnight of the 29th. This surge coincided with the time of low tide at the oceanfront, sparing those areas from worse flooding. However, the tidal Delaware River and most of Delaware Bay experiences high tide 3 to 6 hours later than local oceanfront locations. Thus, in those areas, the highest surge occurred closer to the time of high tide. In addition, in those locations, the duration of higher surge values lasted longer than at the oceanfront, due to the difficulty in water draining back out to sea from areas like the Delaware River thanks to the continued onshore flow pushing water in despite the falling tides attempting to drag water out.
A pair of images below illustrate this point. These annotated graphs show water levels at the Sandy Hook (Figure 1) and Reedy Point (Figure 2) tidal gauges during the event. In the graph, the blue line “Predictions” simply marks the expected astronomical tide, all other factors aside. The “Predictions” label is somewhat misleading; it is not a forecast made by NWS, it is simply the expected astronomical tide. The green line (labeled as “Verified”) shows the actual water levels which occurred. The purple line represents the difference between the green and blue lines. It can be thought of as the storm surge. Positive values represent times when the actual water levels were higher than the astronomical tide. In this case all the values are positive since water levels were elevated. (Note: there is no data for the red line marked “Preliminary” and users should disregard this.) These images help to show why flooding in locations such as Delaware Bay and the tidal Delaware Bay, areas which Reedy Point is a good representation of, was worse than in oceanfront locations such as Sandy Hook.
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| Figure 1: Annotated graph of water levels at the Sandy Hook gauge during the late October 2021 tidal flooding event. |
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| Figure 2: Same as Figure 1, for the Reedy Point gauge. |
This effect also helps to explain why flooding in eastern New Jersey back bay locations was worse than would normally be expected with only minor flooding at the oceanfront tidal gauges. Back bay high tides occur a little later than oceanfront high tides. But more importantly, the especially long duration of onshore flow was a major factor for the back bays in preventing water from draining. While other meteorological and hydrologic factors are involved, these facts explain a large part of the distribution of tidal flooding during the late October 2021 event.
A list of coastal flooding reports and some associated impacts can be found in the Summary LSR tab on this page.
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