Characteristics of High Impact Sub-Advisory (HISA) Snow Events in Central Pennsylvania Gregory A. DeVoir NWS State College, PA Gregory A. DeVoir NWS State College, PA

Short duration, high intensity snow (falling in bursts or squalls) with 1-2” amounts in a 1 to 2 hour period, greatly magnifies impact. Instantaneous whiteouts and rapidly changing road conditions make chain reaction accidents more likely. The terms “high intensity”, “high impact” and “sub-advisory” (HISA) are used here to describe such events. Short duration, high intensity snow (falling in bursts or squalls) with 1-2” amounts in a 1 to 2 hour period, greatly magnifies impact. Instantaneous whiteouts and rapidly changing road conditions make chain reaction accidents more likely. The terms “high intensity”, “high impact” and “sub-advisory” (HISA) are used here to describe such events. High Impact Sub-Advisory Snowfall

January 6, 2004 CDT PHOTO/MICHELLE KLEIN CDT PHOTO/NIKI DESAUTELS CDT PHOTO/MICHELLE KLEIN Photo Credits: Centre Daily Times (CDT) and AP Photo Credits: Centre Daily Times (CDT) and AP CDT PHOTO/NIKI DESAUTELS AP PHOTO

Across central PA, the frequency of sub- advisory snow events increases as one moves north, and towards Lake Erie. The following slide contains indicates % of and # of days of: Snowfall > 1” Snowfall > 4” 1” < Snowfall < 4” Data extracted from Cember and Wilks (1993) Across central PA, the frequency of sub- advisory snow events increases as one moves north, and towards Lake Erie. The following slide contains indicates % of and # of days of: Snowfall > 1” Snowfall > 4” 1” < Snowfall < 4” Data extracted from Cember and Wilks (1993) High Impact Sub-Advisory Snowfall - Frequency

Nov 8-15 Avg. % of Days > 1” Avg. # of Days > 1” Avg. % of Days > 4” Avg. # of Days > 4” Avg. % of Days 1” < x < 4” Avg. # of Days 1” < x < 4” ~ ~ < < 1 ~ ~ < 1 < 0.2 ~ 3 < ~ < 1 < 0.1 ~ 1 < 0.1 BFD UNV MDT BFD UNV MDT BFD UNV MDT BFD UNV MDT BFD UNV MDT Dec 1-15 Jan 1-31 Feb Mar Cember and Wilks 1993

On average, throughout the winter months, sub-advisory snowfalls occur 3 to 4 times as often as snow Advisory and Warning events. Diagnosing and properly communicating the character (intensity, duration) of snowfall for HISA events is very important. Understanding what meteorological conditions distinguish HISA snow events from milder sub- advisory snow events is critical. On average, throughout the winter months, sub-advisory snowfalls occur 3 to 4 times as often as snow Advisory and Warning events. Diagnosing and properly communicating the character (intensity, duration) of snowfall for HISA events is very important. Understanding what meteorological conditions distinguish HISA snow events from milder sub- advisory snow events is critical. High Impact Sub-Advisory Snowfall – Cember and Wilks

Background and research… The timing, intensity and duration of snow in relation to traffic volume (time of day and day of week), greatly effect overall public impact (PENNDoT, 2001). Impacts tend to be greatly magnified during periods of higher traffic volume, especially weekday rush hours and weekend afternoons. Background and research… The timing, intensity and duration of snow in relation to traffic volume (time of day and day of week), greatly effect overall public impact (PENNDoT, 2001). Impacts tend to be greatly magnified during periods of higher traffic volume, especially weekday rush hours and weekend afternoons. Contributing Factors to HISA Events

The Effect of Precipitation on the Number and Severity of Crashes: Research has consistently found that precipitation increases the severity and frequency of crashes (in all three crash categories - fatal, injury, and property damage without injury). However, a recent study shows that wetter than normal months are associated with a statistically significant decrease * in the number of overall fatal traffic crashes in that month. * CONTRADICTION? How can these both be true? The Effect of Precipitation on the Number and Severity of Crashes: Research has consistently found that precipitation increases the severity and frequency of crashes (in all three crash categories - fatal, injury, and property damage without injury). However, a recent study shows that wetter than normal months are associated with a statistically significant decrease * in the number of overall fatal traffic crashes in that month. * CONTRADICTION? How can these both be true? Eisenberg, Daniel, 2004: The Mixed Effects of Precipitation on Traffic Crashes, University of California Berkeley. Contributing Factors to HISA Events

The Lagged Effects of Precipitation Across Days: While more precipitation on a given day is linked to an increase in fatal crashes on that day… More precipitation on a given day also correlates to/predicts a decrease in fatal crashes on the following day. * In addition, crash rates peak around moderate/ medium levels of snow, then continue to decrease as snow depth increases. The Lagged Effects of Precipitation Across Days: While more precipitation on a given day is linked to an increase in fatal crashes on that day… More precipitation on a given day also correlates to/predicts a decrease in fatal crashes on the following day. * In addition, crash rates peak around moderate/ medium levels of snow, then continue to decrease as snow depth increases. * Eisenberg, Daniel, 2004: The Mixed Effects of Precipitation on Traffic Crashes, University of California Berkeley.

How might these studies relate to HISA events? Crashes are more likely on wet days which are preceded by prolonged dry periods due to oil buildup on roads. Additionally, road pretreatment (salting) is only effective for 5 to 7 days after application. A soaking rain or a dry stretch of several days will remove existing road treatments. People are not conditioned to drive in wet conditions after long dry spells.  Fridstrom et al. (1995) found that the first snowfall of the winter causes more crashes than would be expected simply by the amount of snowfall. Crashes are more likely on wet days which are preceded by prolonged dry periods due to oil buildup on roads. Additionally, road pretreatment (salting) is only effective for 5 to 7 days after application. A soaking rain or a dry stretch of several days will remove existing road treatments. People are not conditioned to drive in wet conditions after long dry spells.  Fridstrom et al. (1995) found that the first snowfall of the winter causes more crashes than would be expected simply by the amount of snowfall.

Contributing Factors to HISA Events How might these studies relate to HISA events? As snowfall events increase beyond medium sized events, fatal crashes decrease. Traffic density decreases during extended periods of moderate to heavy snowfall – people adjust to the conditions. Not so for HISA events! How might these studies relate to HISA events? As snowfall events increase beyond medium sized events, fatal crashes decrease. Traffic density decreases during extended periods of moderate to heavy snowfall – people adjust to the conditions. Not so for HISA events!

Contributing Factors to HISA Events How might these studies relate to HISA events? Crashes are more likely on days preceded by long dry periods. * Road pretreatment (salting) is only effective for up to 7 days after application. A dry stretch of several days, or a soaking rain, will remove road treatment, making the next snowfall potentially more hazardous for drivers. How might these studies relate to HISA events? Crashes are more likely on days preceded by long dry periods. * Road pretreatment (salting) is only effective for up to 7 days after application. A dry stretch of several days, or a soaking rain, will remove road treatment, making the next snowfall potentially more hazardous for drivers. * Eisenberg, Daniel, 2004: The Mixed Effects of Precipitation on Traffic Crashes, University of California Berkeley.

What do HISA Events LOOK like? January 6, 2004

December 2, 2003

High Inversion Heights

Can You Find the Cold Front?

10 to 15 degree temperature drops in one hour behind the front

What do HISA Events LOOK like? Events occurring below 25 degrees or with rapid temperature drops (as with a passing arctic front) can present the most problems. Tires can partially melt the snow, which ends up freezing on roadways, creating a solid ice. Pretreatment is essential to preserving safe travel conditions in these situations – and requires advance notification from the NWS. This is one of the main purposes of our calls to the PSP and PENNDOT for these events. Events occurring below 25 degrees or with rapid temperature drops (as with a passing arctic front) can present the most problems. Tires can partially melt the snow, which ends up freezing on roadways, creating a solid ice. Pretreatment is essential to preserving safe travel conditions in these situations – and requires advance notification from the NWS. This is one of the main purposes of our calls to the PSP and PENNDOT for these events.