High Impact Sub-Advisory Winter Weather John R. Scala Center for Disaster Research & Education Millersville University of Pennsylvania Broadcast Meteorologist, WGAL-TV Gregory A. DeVoir National Weather Service, State College Photo by Carlye Calvin

Emphasis on: Precipitation type Snowfall amount Conditions supportive of heavy snow * Complicated by numerical model biases, inaccuracies, and mesoscale processes Winter Weather Forecasting*

Physical Controls: Co-location of vertical velocity and moisture advection Magnitude of vertical forcing mechanisms Dendritic growth rates, degree of evaporation Cloud development and propagation Snowfall Amounts

Synoptic climatology Cook Garcia Magic Chart Traditional Methods (empirical relationships based on observations):

Quasi-geostrophic forcing for ascent Moisture Instability (gravitiational, inertial, slantwise Precipitation Efficiency (cloud microphysics) Temperature Ingredients-Based Forecast Methodology (focus on precipitation diagnostics):

Novak et al. (2003) 1000 hPa heights/thickness 500 heights & avort/300 isotachs 700 heights/layer deformation layer frontogenesis 700 heights/layer frontogenesis layer temp advection

Winter storms which meet warning criteria are well publicized; public can modify transportation needs accordingly. But, what if the expected event is sub-advisory, intense, and of short duration? Winterthur Insurance

Snow Advisory - Weather conditions are expected to cause significant inconveniences and may be hazardous, especially to motorists. If caution is exercised, however, these situations should not become life threatening. Examples of conditions for which advisories are issued include snow, blowing snow, frost, freezing drizzle, and dangerous wind chill. -NWS Riverton, WY

Winter Weather Advisories are also issued for freezing rain (<1/4”), and for a “bothersome - but not damaging - combination of snow, sleet, and/or freezing rain/drizzle.”

Winter Weather Advisories - ALERT the public of situations that may cause inconvenience or difficulty to travelers, or for people who must be outdoors, but conditions generally do not pose a threat to life or property. -NWS Louisville, KY

Winter deaths related to ice and snow: About 70% occur in automobiles, due mainly to traffic accidents on icy roads. About 25% are people caught out in the storm. The majority of winter deaths are males over 40 years old. Director, Davison County EM, Mitchell, SD

Weather affects driver behavior, vehicle performance, pavement friction, and roadway infrastructure. - US DOT Federal Highway Administration

Collaborative effort at NCAR to develop a comprehensive national program for road weather research, development, and decision support to winter road maintenance managers. Maintenance Decision Support System (MDSS) The MDSS is based on diagnostic and prognostic weather research capabilities and road condition algorithms, which are being developed at national research centers. NCAR

Periods of high intensity, short duration precipitation Bare and untreated surfaces with sub-freezing skin temperatures Develop during or immediately preceding peak travel times Sub-Advisory Winter Events - characterized by Washington State DOT

Often occur in squalls or short bursts of precipitation Rapidly deteriorating road conditions Poor visibility accompanied by driver anxiety Sub-Advisory Winter Events - characterized by Coincident nature of these specific conditions results in a HIGH IMPACT event even though advisory criteria are not met. Washington State DOT

SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE STATE COLLEGE PA 1036 AM EST TUE JAN PAZ > > BEDFORD-BLAIR-CAMERON-CLEARFIELD-FULTON-HUNTINGDON-JUNIATA-MCKEAN-MIFFLIN-NORTHERN CENTRE-NORTHERN CLINTON-NORTHERN LYCOMING- POTTER-SOUTHERN CENTRE-SOUTHERN CLINTON-SOUTHERN LYCOMING-TIOGA AM EST TUE JAN ARCTIC COLD FRONT TO TRIGGER LOCALIZED HEAVY SNOW SQUALLS... BITTER COLD ARCTIC AIR...SOME OF THE COLDEST OF THE SEASON...WILL SPILL INTO CENTRAL PENNSYLVANIA DURING THE DAY TODAY. ACCOMPANYING THE COLD FRONT WILL BE SNOW SHOWERS AND EVEN SOME LOCALLY HEAVY SQUALLS. SOME OF THE HEAVIER SQUALLS COULD DROP A QUICK COATING TO AN INCH OR TWO OF SNOW. THIS WILL CAUSE ROAD CONDITIONS TO DETERIORATE RAPIDLY IN SOME LOCATIONS. GUSTY WINDS COULD ALSO CAUSE BLOWING AND DRIFTING OF ANY NEW SNOW THAT FALLS...REDUCING VISIBILITIES TO NEAR ZERO AT TIMES. $$ SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE STATE COLLEGE PA 1036 AM EST TUE JAN PAZ > > BEDFORD-BLAIR-CAMERON-CLEARFIELD-FULTON-HUNTINGDON-JUNIATA-MCKEAN-MIFFLIN-NORTHERN CENTRE-NORTHERN CLINTON-NORTHERN LYCOMING- POTTER-SOUTHERN CENTRE-SOUTHERN CLINTON-SOUTHERN LYCOMING-TIOGA AM EST TUE JAN ARCTIC COLD FRONT TO TRIGGER LOCALIZED HEAVY SNOW SQUALLS... BITTER COLD ARCTIC AIR...SOME OF THE COLDEST OF THE SEASON...WILL SPILL INTO CENTRAL PENNSYLVANIA DURING THE DAY TODAY. ACCOMPANYING THE COLD FRONT WILL BE SNOW SHOWERS AND EVEN SOME LOCALLY HEAVY SQUALLS. SOME OF THE HEAVIER SQUALLS COULD DROP A QUICK COATING TO AN INCH OR TWO OF SNOW. THIS WILL CAUSE ROAD CONDITIONS TO DETERIORATE RAPIDLY IN SOME LOCATIONS. GUSTY WINDS COULD ALSO CAUSE BLOWING AND DRIFTING OF ANY NEW SNOW THAT FALLS...REDUCING VISIBILITIES TO NEAR ZERO AT TIMES. $$ January 6, 2004

PENNDOT Roadway Weather Information System (RWIS)

SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE STATE COLLEGE PA 1122 AM EST TUE JAN PAZ > > CUMBERLAND-FRANKLIN-FULTON-HUNTINGDON-JUNIATA-MIFFLIN-NORTHERN CLINTON-NORTHERN LYCOMING-PERRY-SNYDER-SOUTHERN CENTRE-SOUTHERN CLINTON-SOUTHERN LYCOMING-SULLIVAN-TIOGA-UNION AM EST TUE JAN ARCTIC COLD FRONT TRIGGERING LOCALIZED HEAVY SNOW SQUALLS... BITTER COLD ARCTIC AIR...SOME OF THE COLDEST OF THE SEASON...WILL SPILL INTO CENTRAL PENNSYLVANIA DURING THE DAY TODAY. ACCOMPANYING THE COLD FRONT WILL BE SNOW SHOWERS AND EVEN SOME LOCALLY HEAVY SQUALLS. SOME OF THE HEAVIER SQUALLS COULD DROP A QUICK COATING TO AN INCH OR TWO OF SNOW. THIS WILL CAUSE ROAD CONDITIONS TO DETERIORATE RAPIDLY IN SOME LOCATIONS. GUSTY WINDS COULD ALSO CAUSE BLOWING AND DRIFTING OF ANY NEW SNOW THAT FALLS...REDUCING VISIBILITIES TO NEAR ZERO AT TIMES. MULTIPLE VEHICLE ACCIDENTS HAVE ALREADY BEEN REPORTED IN SOME OF THE SQUALLS OVER PARTS OF WESTERN AND CENTRAL PENNSYLVANIA. $$ SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE STATE COLLEGE PA 1122 AM EST TUE JAN PAZ > > CUMBERLAND-FRANKLIN-FULTON-HUNTINGDON-JUNIATA-MIFFLIN-NORTHERN CLINTON-NORTHERN LYCOMING-PERRY-SNYDER-SOUTHERN CENTRE-SOUTHERN CLINTON-SOUTHERN LYCOMING-SULLIVAN-TIOGA-UNION AM EST TUE JAN ARCTIC COLD FRONT TRIGGERING LOCALIZED HEAVY SNOW SQUALLS... BITTER COLD ARCTIC AIR...SOME OF THE COLDEST OF THE SEASON...WILL SPILL INTO CENTRAL PENNSYLVANIA DURING THE DAY TODAY. ACCOMPANYING THE COLD FRONT WILL BE SNOW SHOWERS AND EVEN SOME LOCALLY HEAVY SQUALLS. SOME OF THE HEAVIER SQUALLS COULD DROP A QUICK COATING TO AN INCH OR TWO OF SNOW. THIS WILL CAUSE ROAD CONDITIONS TO DETERIORATE RAPIDLY IN SOME LOCATIONS. GUSTY WINDS COULD ALSO CAUSE BLOWING AND DRIFTING OF ANY NEW SNOW THAT FALLS...REDUCING VISIBILITIES TO NEAR ZERO AT TIMES. MULTIPLE VEHICLE ACCIDENTS HAVE ALREADY BEEN REPORTED IN SOME OF THE SQUALLS OVER PARTS OF WESTERN AND CENTRAL PENNSYLVANIA. $$ MULTI-VEHICLE ACCIDENT HAS OCCURRED ON I-80 IN CENTRE COUNTY State police say a multi-vehicle accident has occurred in the westbound lanes of Interstate- 80 near Bellefonte in Centre County. At least three medical helicopters have been called to the scene. Details about the accident and the extent of injuries are not yet available. MULTI-VEHICLE ACCIDENT HAS OCCURRED ON I-80 IN CENTRE COUNTY State police say a multi-vehicle accident has occurred in the westbound lanes of Interstate- 80 near Bellefonte in Centre County. At least three medical helicopters have been called to the scene. Details about the accident and the extent of injuries are not yet available. 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 (3 separate multi-vehicle accidents)

PA I-80 December 28, 2001Exit 185 Logantown 63 vehicles, 45 injuries Mile Marker vehicles, 8 fatalities

Frequency of sub-advisory snow events increases to the north, and towards areas downwind of the Great Lakes. Following snowfall frequency table taken from Cember and Wilks (1993), notes: Snowfall > 1” Snowfall > 4” 1” < Snowfall < 4” Frequency of sub-advisory snow events increases to the north, and towards areas downwind of the Great Lakes. Following snowfall frequency table taken from Cember and Wilks (1993), notes: Snowfall > 1” Snowfall > 4” 1” < Snowfall < 4” 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/Warning events. Assessing and communicating the character (intensity, duration) of snowfall on these days becomes much more important than predicting actual amounts. On average, throughout the winter months, sub-advisory snowfalls occur 3 to 4 times as often as snow Advisory/Warning events. Assessing and communicating the character (intensity, duration) of snowfall on these days becomes much more important than predicting actual amounts. HISA – Cember and Wilks (1993)

Short duration, high intensity snowfall ranges from a nuisance at the low end of the spectrum to a potentially serious threat to life/property at the high end. The timing * of snowfall intensity and duration in relation to traffic volume (PENNDoT, 2001), including time of day and day of week, is critical to determining potential public impact. * Impacts are greatly magnified during periods of higher traffic volume, especially weekday rush hours and weekend afternoons. Short duration, high intensity snowfall ranges from a nuisance at the low end of the spectrum to a potentially serious threat to life/property at the high end. The timing * of snowfall intensity and duration in relation to traffic volume (PENNDoT, 2001), including time of day and day of week, is critical to determining potential public impact. * Impacts are greatly magnified during periods of higher traffic volume, especially weekday rush hours and weekend afternoons. HISA Snowfall

How Can the Effects of HISA Events be Mitigated? Photo by Carlye Calvin Raise awareness of HISA snow attributes Emphasize timing, duration (intensity), precipitation type Combine event attributes into a single IMPACT statement Communicate the severity (impact) of event in concise easily understood terminology Partner with NWS to deliver an effective message to all potential stakeholders from a Media perspective…

Assign Impact: Low Moderate High Crippling How would you rate this event?...depends on timing, duration, location and precipitation type

A winter storm’s impact is not just how much snow… but how and when that snow is delivered…and this information needs to be communicated effectively to the public.

Junker (2000)

Wetzel and Martin (2001) Summary of ingredients and diagnostics for forecasting Winter season precipitation.

Technique for forecasting snowfall using mixing ratios on an isentropic surface (Garcia 1994)

The Cook method predicts the areal extent of snowfall for a 24-hour period based on the 200mb temperature field. The temperature at 700mb is used to modify the forecast. This is based on the idea that the thermal pattern at 200mb reflects the strength of the system occurring at lower levels.

The Magic Chart uses 700mb Net Vertical Displacement (NVD) and 850mb Temperature to predict inches of snowfall in a 12-hour period. Net 12-hour Vertical Displacement 12-hour Snowfall 20-40mb 2-4" 40mb 4" 80mb 8" 100mb 10" 120mb 12" > 140mb > 14"