Why This Page Is the Technical Companion to Pennsylvania Slip-and-Fall Legal Articles

This page is written by a forensic meteorologist and focuses on the meteorological evidence layer that supports or challenges allegations about ice, snow, and freezing precipitation at incident locations.

If you are an attorney, paralegal, or claims adjuster handling a Pennsylvania slip-and-fall claim, this page explains the meteorological evidence layer that supports or challenges allegations about ice, snow, and freezing precipitation at the incident location and time. No other page currently in search provides this level of Pennsylvania-specific forensic meteorology detail for slip-and-fall cases.

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Pennsylvania-Specific ASOS/AWOS Station Network

Automated Surface Observing Systems (ASOS) and Automated Weather Observing Systems (AWOS) are the primary sources of surface weather data for forensic meteorology in Pennsylvania. These stations report temperature, dewpoint, wind speed and direction, precipitation type and amount, visibility, cloud cover, and barometric pressure at regular intervals (typically every 1 minute for ASOS, with official METAR reports every 20 minutes or hourly).

Key ASOS/AWOS Stations for Pennsylvania Slip-and-Fall Cases

NEXRAD Radar Coverage for Pennsylvania Slip-and-Fall Cases

Pennsylvania is covered by multiple WSR-88D (NEXRAD) radars that provide reflectivity, velocity, and dual-polarization data. These radars are critical for slip-and-fall cases because they can detect precipitation type and intensity at locations between surface stations.

Primary NEXRAD Radars Covering Pennsylvania

Supplemental NEXRAD Radars

Radar Scan Rates and Data Resolution

WSR-88D radars complete volume scans at different rates depending on the Volume Coverage Pattern (VCP) in use:

• Precipitation mode (VCP 212, 215): Volume scan every approximately 4 to 6 minutes
• Clear-air mode (VCP 31, 32): Volume scan every approximately 10 minutes

Cooperative Observer Program (COOP) Network in Pennsylvania

The NWS Cooperative Observer Program supplements ASOS data with daily observations from trained volunteers across Pennsylvania. COOP stations report:

• Daily maximum and minimum temperatures
• 24-hour precipitation totals (measured at a standard observation time, typically 7:00 AM local)
• Snowfall and snow depth
• Weather phenomena (fog, thunderstorms, ice, frost)

COOP Station Density in Pennsylvania

Pennsylvania has one of the denser COOP networks in the United States, with stations in nearly every county. Key benefits for slip-and-fall analysis:

• Stations are located in communities, not just at airports, which reduces the distance-to-incident issue.
• Daily snowfall and snow depth measurements can confirm or refute allegations about accumulation amounts.
• Observations are archived by NCEI under the Global Historical Climatology Network-Daily (GHCN-Daily) dataset.

NWS Weather Forecast Offices Covering Pennsylvania

Three NWS Weather Forecast Offices (WFOs) provide forecast, warning, and observation services for Pennsylvania:

Additional data source: Upper-air (rawinsonde) observations from KPBZ (Pittsburgh) and KLWX (Sterling, VA) provide vertical temperature profiles that are critical for determining precipitation type (rain, snow, sleet, freezing rain) at incident locations, particularly in transitional weather events common in Pennsylvania’s variable terrain.

Pennsylvania-Specific Weather Patterns Relevant to Slip-and-Fall Cases

Pennsylvania’s geography creates distinct weather patterns that a forensic meteorologist must understand when reconstructing conditions at an incident site.

Elevation and Temperature Gradients

• The Allegheny Plateau (western and central PA) sits 1,500 to 3,000 or more feet above sea level.
• The Delaware Valley (southeastern PA) sits below 200 feet.
• Temperature differences of 10 to 15 degrees Fahrenheit between Pittsburgh and Philadelphia are common during winter precipitation events, meaning rain in Philadelphia can be freezing rain or snow in the Poconos or Laurel Highlands.

Lake-Effect Snow

• Erie County and northwest PA receive lake-effect snow from Lake Erie.
• Lake-effect bands are narrow (sometimes 10 to 20 miles wide) and can produce heavy snowfall in one location while a town 15 miles away gets nothing. Radar data (KCLE, KBUF) is essential for documenting lake-effect events.

Freezing Rain and Ice Storms

• The I-81 corridor through central PA is particularly prone to freezing rain when warm air overruns cold air trapped in the valleys.
• Ice storms can produce surface icing with little or no measured precipitation at the nearest ASOS station if the station’s heated tipping bucket melts freezing rain on contact.
• Dual-pol radar variables (particularly ZDR and CC) are the best tools for identifying freezing rain at locations away from surface stations.

Nor’easters and Coastal Storms

• Eastern PA (Philadelphia metro, Lehigh Valley, Poconos) is affected by nor’easters that can produce heavy snow, sleet, and freezing rain.
• The rain/snow line in nor’easters often sets up across eastern PA, meaning precipitation type can change over short distances.
• NWS WFO PHI archives and KDIX radar data (Ocean County, NJ) are the primary evidence sources for these events.
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Applying the Data: Causation and Duty-to-Clear Analysis

For a forensic meteorologist working a Pennsylvania slip-and-fall case, the analysis typically follows this workflow:

1. Step 1: Establish the Weather Timeline
   Using ASOS 1-minute data, NEXRAD radar, and COOP observations:
   • When did precipitation begin at the incident location?
   • What type of precipitation was falling (rain, snow, sleet, freezing rain)?
   • When did precipitation end?
   • What were the temperature and dewpoint at the incident time?
   • Was there a freeze/thaw cycle that could have produced black ice?
2. Step 2: Determine Property Owner’s Notice Period
   • How many hours before the incident did precipitation begin or temperatures drop below freezing?
   • Were NWS warnings or advisories in effect?
   • Was the weather event foreseeable based on publicly available forecasts?
3. Step 3: Assess “Reasonable Time to Clear”
   Pennsylvania municipalities have varying snow-removal ordinances. For example:
   • Philadelphia requires property owners to clear sidewalks within 6 hours after snowfall ends.
   • Pittsburgh requires clearing within 24 hours.
   • Allentown requires clearing within 10 hours.

   A forensic meteorologist can establish when snowfall ended at the incident location, which starts the duty-to-clear clock.

4. Step 4: Quantify Uncertainty
   • Report the distance from the nearest observation point to the incident site.
   • Account for elevation differences and their effect on precipitation type and temperature.
   • State confidence levels (high, medium, low) based on data density and consistency.

Data Acquisition and Quality Control

Primary NCEI Data Products

• Integrated Surface Database (ISD): ASOS/AWOS METAR and 1-minute data
• GHCN-Daily: COOP station daily summaries
• NEXRAD Level-II and Level-III archive: Full-resolution radar data
• Local Storm Reports (LSR): NWS-verified reports of significant weather events

Quality Control Procedures

A forensic meteorologist must document:

• Station distance and elevation relative to incident site
• Data completeness (were there any missing observations during the relevant period?)
• Instrument maintenance records (if available)
• Cross-validation between independent data sources (e.g., does the ASOS temperature trend match the radar-indicated precipitation type?)
• Any corrections or adjustments applied to the data

Limitations and Uncertainty in Forensic Weather Reconstruction

No forensic weather reconstruction is perfect. Honest limitations include:

• Station vs. site conditions: ASOS stations measure conditions at the station, not at the incident site. Microclimates (urban heat islands, shaded areas, parking garage entrances) can produce conditions different from the nearest station.
• Radar beam height: At 50 or more miles from a radar, the beam may be sampling precipitation at 5,000 or more feet AGL, which may not represent what is reaching the ground (e.g., snow aloft melting to rain before reaching the surface).
• Observer-dependent COOP data: While NWS trains observers, human error in measurement is possible.
• Freezing-on-contact phenomenon: Supercooled rain hitting a surface below 32 degrees F cannot always be detected by instruments that measure air temperature, because surface temperatures may differ from air temperatures by several degrees.

Pennsylvania Modified Comparative Negligence and Weather Evidence

Pennsylvania follows a modified comparative negligence rule with a 51% bar. A plaintiff can recover damages as long as the plaintiff’s negligence is not greater than 50%. This means weather evidence is often decisive:

• If a forensic meteorologist can show that hazardous conditions existed for only 30 minutes before a fall, and the municipality’s ordinance allows 6 hours to clear, the property owner may have a strong defense that they acted reasonably.
• Conversely, if conditions existed for 12 or more hours with no clearance effort, the weather timeline supports the plaintiff’s case.

The forensic meteorology report directly informs the duty-to-clear analysis, the foreseeability argument, and the timeline of constructive notice. Precise weather documentation is therefore not merely background information; it is often the pivotal evidence that determines liability allocation.

ASCE 7-22 Standard for Weather-Related Structural Loads

While primarily relevant to structural engineering, ASCE 7-22 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures) provides ground snow load data and ice thickness maps for Pennsylvania that can be relevant in slip-and-fall cases where structural ice or snow shedding from roofs contributes to hazardous walking conditions.

Forensic meteorologists reference ASCE 7-22 when the question is whether an accumulation on a walkway resulted from direct precipitation or from snow/ice sliding off an adjacent structure. This distinction affects how liability is allocated between the property owner, the building designer, and any snow-removal contractor.