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Critical Fire Weather for Southern California; Snow for the Great Lakes Region

Critical fire weather conditions will continue for southern California through early this week. A clipper system to bring snow showers to the Great Lakes along with renewed surge of Arctic air to the Upper Midwest and Ohio Valley. Rain increases for the central Gulf Coast today and Georgia into Florida on Monday. For Alaska, a powerful storm will impact most of the state through Monday. Read More >

Philadelphia/Mt Holly

Weather Forecast Office

July 21: Severe Weather with Hail and Multiple Areas of Significant Wind Damage

Weather.gov > Philadelphia/Mt Holly > July 21: Severe Weather with Hail and Multiple Areas of Significant Wind Damage

Overview

A cold front approached the mid-Atlantic from the north-northwest on July 21. The front was also associated with an unseasonably potent mid-level trough, which helped increase shear and lift over the region. With a seasonably warm air mass in place ahead of the front, widespread thunderstorms developed during the afternoon. An initial cluster of thunderstorms developed as early as late morning along the sea breeze in eastern New Jersey. These storms quickly became severe, producing both damaging winds and hail as well as locally heavy rainfall. As the front itself approached, additional storms developed, taking the form of mainly linear and multi-cellular clusters. Several of these storms also became severe. The most noteworthy event of the day was a very strong straight line wind event over Montgomery County, PA. Numerous trees and utilities were downed in that area by winds that likely reached 80 mph. Another damaging wind event occurred in Queen Anne’s County, MD. That event prompted some speculation of a tornado touchdown, but radar data and the nature of the damage suggested straight line winds were responsible there as well. As the cold front moved through, drier air moved into the region, and convection had largely dissipated or moved offshore by mid-evening.
 

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TPHL base velocity loop from about 2:50PM to 3:30PM on July 21, 2021. Intense winds, radar estimates at times to 80 mph, are seen over Montgomery County, which caused extensive damage.

Meteorological Environment/Setup

Synoptic Overview at 12z 21 Jul 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 00z 22 Jul 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
 

Surface Maps (courtesy of WPC)

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Figure 13: 09z surface map Figure 14: 12z surface map Figure 15: 15z surface map
 
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Figure 16: 18z surface map Figure 17: 21z surface map Figure 18: 00z surface map

Observed Soundings
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Figure 19: 12z Long Island, NY sounding Figure 20: 12z Sterling, VA sounding Figure 21: 12z Wallops Island, VA sounding
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Figure 22: 00z Long Island, NY sounding Figure 23: 00z Sterling, VA sounding Figure 24: 00z Wallops Island, VA sounding
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