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Philadelphia/Mt Holly

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October 29-30: Coastal Storm Brings Major Tidal Flooding

Weather.gov > Philadelphia/Mt Holly > October 29-30: Coastal Storm Brings Major Tidal Flooding

Overview

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.

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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.

Meteorological Environment/Setup

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
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