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Severe Weather of April 27th and 28th, 2011
Information Updated: 17 June 2011
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OVERVIEW:
Unseasonably warm and moist air over the region on Wednesday April 27th fueled severe thunderstorms.
The thunderstorms occurred ahead of a cold front which would move through on Thursday morning. This was the same storm system responsible for the deadly outbreak of tornadoes across the Deep South that same evening and overnight. One tornado occurred in the evening of the 27th, and five more tornadoes occurred in Central PA after midnight in the early morning hours of April 28th, 2011. The strength of these tornadoes ranged from EF0 to EF2 on the Enhanced Fujita Scale.
The thunderstorms developed in an environment that was conducive to producing Supercell Thunderstorms. Supercells have the structure to produce the most-violent tornadoes.
Initially, on the afternoon of the 27th, thunderstorms did develop over Central Pennsylvania. The paths of the storms took them over the same places as heavy rain that occurred the past two days (Monday and Tuesday). This produced flooding in many locations, and some of those storms produced wind damage as well.
Most of the storms that produced tornadoes did not form until after midnight, that is, very early Thursday morning. One tornado did occur in the early evening, in Potter County, Northern PA.
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The Ayers Hill Tornado
Path Map and highlight summary of the Ayers Hill Tornado of April 27th, 2011
Text Information of the Ayers Hill Tornado of April 27th, 2011
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The Broad Top City Tornado
Path Map and highlight summary of the Broad Top City Tornado of April 28th, 2011
Text Information of the Broad Top City Tornado of April 28th, 2011
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The East Waterford Tornado
Path Map and highlight summary of the East Waterford Tornado of April 28th, 2011
Text Information of the East Waterford Tornado of April 28th, 2011
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The Round Top Tornado
Path Map and highlight summary of the Round Top Tornado of April 28th, 2011
Text Information of the Round Top Tornado of April 28th, 2011
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The Palmyra Tornado
Path Map and highlight summary of the Palmyra Tornado of April 28th, 2011
Text Information of the Palmyra Tornado of April 28th, 2011
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The Ono Tornado
Path Map and highlight summary of the Ono Tornado of April 28th, 2011
Text Information of the Ono Tornado of April 28th, 2011
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How common are Tornadoes in Pennsylvania?
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Local Tornado Historical Climatology
PA Map of Tornadoes (by County through 2009)
PA Tornado Magnitude and Frequency (by Month)
Map of PA Tornado Occurrance Times (by Hour of Day)
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What is the EF Scale?
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The Enhanced Fujita Scale
(EF Scale) |
The EF Scale, which became operational on February 1, 2007, is used to assign a tornado a 'rating' based on estimated wind speeds and related damage.
When tornado-related damage is surveyed, it is compared to a list of Damage Indicators (DIs) and Degrees of Damage (DoD) which help estimate better the range of wind speeds the tornado likely produced. From that, a rating (from EF0 to EF5) is assigned.
The EF Scale was revised from the original Fujita Scale to reflect better examinations of tornado damage surveys so as to align wind speeds more closely with associated storm damage.
The new scale has to do with how most structures are designed.
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EF SCALE
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EF Rating
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3 Second Gust (mph)
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0
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65-85
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1
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86-110
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2
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111-135
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3
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136-165
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4
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166-200
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5
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Over 200
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*** IMPORTANT NOTE ABOUT EF SCALE WINDS: The EF scale still is a set of wind estimates (not measurements) based on damage. Its uses three-second gusts estimated at the point of damage based on a judgment of 8 levels of damage to the 28 indicators listed below. These estimates vary with height and exposure. Important: The 3 second gust is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in open exposures, using a directly measured, "one minute mile" speed.
Assigning a Tornado Rating Using the EF Scale
The NWS is the only federal agency with authority to provide 'official' tornado EF Scale ratings. The goal is assign an EF Scale category based on the highest wind speed that occurred within the damage path. First, trained NWS personnel will identify the appropriate damage indicator (DI) [see list below] from more than one of the 28 used in rating the damage. The construction or description of a building should match the DI being considered, and the observed damage should match one of the 8 degrees of damage (DOD) used by the scale. The tornado evaluator will then make a judgment within the range of upper and lower bound wind speeds, as to whether the wind speed to cause the damage is higher or lower than the expected value for the particular DOD. This is done for several structures not just one, before a final EF rating is determined.
Enhanced F Scale Damage Indicators
NUMBER
(Details Linked) |
DAMAGE INDICATOR |
ABBREVIATION |
| 1 |
Small barns, farm outbuildings |
SBO |
| 2 |
One- or two-family residences |
FR12 |
| 3 |
Single-wide mobile home (MHSW) |
MHSW |
| 4 |
Double-wide mobile home |
MHDW |
| 5 |
Apt, condo, townhouse (3 stories or less) |
ACT |
| 6 |
Motel |
M |
| 7 |
Masonry apt. or motel |
MAM |
| 8 |
Small retail bldg. (fast food) |
SRB |
| 9 |
Small professional (doctor office, branch bank) |
SPB |
| 10 |
Strip mall |
SM |
| 11 |
Large shopping mall |
LSM |
| 12 |
Large, isolated ("big box") retail bldg. |
LIRB |
| 13 |
Automobile showroom |
ASR |
| 14 |
Automotive service building |
ASB |
| 15 |
School - 1-story elementary (interior or exterior halls) |
ES |
| 16 |
School - jr. or sr. high school |
JHSH |
| 17 |
Low-rise (1-4 story) bldg. |
LRB |
| 18 |
Mid-rise (5-20 story) bldg. |
MRB |
| 19 |
High-rise (over 20 stories) |
HRB |
| 20 |
Institutional bldg. (hospital, govt. or university) |
IB |
| 21 |
Metal building system |
MBS |
| 22 |
Service station canopy |
SSC |
| 23 |
Warehouse (tilt-up walls or heavy timber) |
WHB |
| 24 |
Transmission line tower |
TLT |
| 25 |
Free-standing tower |
FST |
| 26 |
Free standing pole (light, flag, luminary) |
FSP |
| 27 |
Tree - hardwood |
TH |
| 28 |
Tree - softwood |
TS |
Other background information:
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Map of rainfall amounts for April 26th
Map of rainfall amounts for April 27th
Map of rainfall amounts for April 28th
2011 Weather Features:
- October 29th (Record) Snowfall
- Williamsport Sets Annual Precipitation Record
- Flood Inundation Maps for Jersey Shore, PA begins on Wednesday, Oct. 26th
- NWS State College Remembers the 100 Year Anniversary of the Austin Dam Break and Flood
- Harrisburg Sets September and Annual Rainfall Records
- New River Forecast Points on Daily River Forecast
- Unofficial (subject to review) river crests for rain due to remnants of Lee (7 to 10 Sep 2011)
- Storm Total Rainfall Reports Sept 4-8, 2011
- Latest Aug 23rd Earthquake information.
- Historical Central PA Temperature Extremes for July 4th
- 2011 Tornado Summary (through June 14th)
- Two Tornadoes in York County on June 12th
- 2011 Central PA Tornadoes (so far)
- Tioga County Tornado of May 30th, 2011
- Huntingdon County Tornadoes of May 27th, 2011
- Tornadoes of May 26th, 2011
- Tornadoes of May 23rd, 2011
- Dauphin County Tornado of May 18th, 2011
- Tornadoes and Flooding of April 27th and 28th, 2011
- Flooding and Tornado of April 16th, 2011
- Flooding of March 10-13
- Feb 20-21 Snow Event
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