High ImpactSub-Advisory Winter Weather

John R. ScalaCenter for Disaster Research & EducationMillersville University of PennsylvaniaBroadcast Meteorologist, WGAL-TV

Gregory A. DeVoirNational 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 climatologyCookGarciaMagic Chart

Traditional Methods(empirical relationships based on observations):

Quasi-geostrophic forcing for ascentMoistureInstability (gravitiational, inertial, slantwisePrecipitation 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 deformationlayer frontogenesis

700 heights/layer frontogenesislayer temp advection

Winter storms which meet warningcriteria are well publicized; publiccan modify transportation needsaccordingly.

But, what if the expected eventis sub-advisory, intense, and ofshort duration?

Winterthur Insurance

Snow Advisory -

Weather conditions are expected to cause significantinconveniences and may be hazardous, especially tomotorists. If caution is exercised, however, thesesituations should not become life threatening.

Examples of conditions for which advisories are issuedinclude snow, blowing snow, frost, freezing drizzle, anddangerous 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 causeinconvenience or difficulty to travelers, or forpeople who must be outdoors, but conditionsgenerally 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 totraffic 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 todevelop a comprehensivenational program for roadweather research, development,and decision support to winterroad maintenance managers.

Maintenance Decision Support System (MDSS)

The MDSS is based on diagnostic andprognostic weather research capabilitiesand road condition algorithms, whichare being developed at national researchcenters.

http://www.rap.ucar.edu/projects/rdwx_mdss/index.htmlNCAR

• 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 specificconditions results in a HIGH IMPACTevent even though advisory criteriaare not met.

Washington State DOT

SPECIAL WEATHER STATEMENTNATIONAL WEATHER SERVICE STATE COLLEGE PA1036 AM EST TUE JAN 6 2004

PAZ005-006-011-012-017>019-025>028-034-035-037-041-045-046-061730-BEDFORD-BLAIR-CAMERON-CLEARFIELD-FULTON-HUNTINGDON-JUNIATA-MCKEAN-MIFFLIN-NORTHERN CENTRE-NORTHERN CLINTON-NORTHERN LYCOMING-POTTER-SOUTHERN CENTRE-SOUTHERN CLINTON-SOUTHERN LYCOMING-TIOGA-1036 AM EST TUE JAN 6 2004

...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 STATEMENTNATIONAL WEATHER SERVICE STATE COLLEGE PA1036 AM EST TUE JAN 6 2004

PAZ005-006-011-012-017>019-025>028-034-035-037-041-045-046-061730-BEDFORD-BLAIR-CAMERON-CLEARFIELD-FULTON-HUNTINGDON-JUNIATA-MCKEAN-MIFFLIN-NORTHERN CENTRE-NORTHERN CLINTON-NORTHERN LYCOMING-POTTER-SOUTHERN CENTRE-SOUTHERN CLINTON-SOUTHERN LYCOMING-TIOGA-1036 AM EST TUE JAN 6 2004

...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, 2004January 6, 2004

January 6, 2004January 6, 2004

January 6, 2004January 6, 2004

PENNDOT Roadway Weather Information

System (RWIS)

PENNDOT Roadway Weather Information

System (RWIS)

SPECIAL WEATHER STATEMENTNATIONAL WEATHER SERVICE STATE COLLEGE PA1122 AM EST TUE JAN 6 2004

PAZ012-019-026>028-035>037-041-042-045-046-049-050-056-063-061715-

CUMBERLAND-FRANKLIN-FULTON-HUNTINGDON-JUNIATA-MIFFLIN-NORTHERN

CLINTON-NORTHERN LYCOMING-PERRY-SNYDER-SOUTHERN CENTRE-SOUTHERN

CLINTON-SOUTHERN LYCOMING-SULLIVAN-TIOGA-UNION-

1122 AM EST TUE JAN 6 2004

...ARCTIC COLD FRONT TRIGGERING LOCALIZED HEAVY SNOW SQUALLS...

BITTER COLD ARCTIC AIR...SOME OF THE COLDEST OF THE SEASON...WILLSPILL INTO CENTRAL PENNSYLVANIA DURING THE DAY TODAY. ACCOMPANYINGTHE COLD FRONT WILL BE SNOW SHOWERS AND EVEN SOME LOCALLY HEAVYSQUALLS.

SOME OF THE HEAVIER SQUALLS COULD DROP A QUICK COATING TO AN INCH ORTWO OF SNOW. THIS WILL CAUSE ROAD CONDITIONS TO DETERIORATE RAPIDLYIN SOME LOCATIONS. GUSTY WINDS COULD ALSO CAUSE BLOWING AND

DRIFTINGOF ANY NEW SNOW THAT FALLS...REDUCING VISIBILITIES TO NEAR ZERO ATTIMES.

MULTIPLE VEHICLE ACCIDENTS HAVE ALREADY BEEN REPORTED IN SOME OF THESQUALLS OVER PARTS OF WESTERN AND CENTRAL PENNSYLVANIA.

$$

SPECIAL WEATHER STATEMENTNATIONAL WEATHER SERVICE STATE COLLEGE PA1122 AM EST TUE JAN 6 2004

PAZ012-019-026>028-035>037-041-042-045-046-049-050-056-063-061715-

CUMBERLAND-FRANKLIN-FULTON-HUNTINGDON-JUNIATA-MIFFLIN-NORTHERN

CLINTON-NORTHERN LYCOMING-PERRY-SNYDER-SOUTHERN CENTRE-SOUTHERN

CLINTON-SOUTHERN LYCOMING-SULLIVAN-TIOGA-UNION-

1122 AM EST TUE JAN 6 2004

...ARCTIC COLD FRONT TRIGGERING LOCALIZED HEAVY SNOW SQUALLS...

BITTER COLD ARCTIC AIR...SOME OF THE COLDEST OF THE SEASON...WILLSPILL INTO CENTRAL PENNSYLVANIA DURING THE DAY TODAY. ACCOMPANYINGTHE COLD FRONT WILL BE SNOW SHOWERS AND EVEN SOME LOCALLY HEAVYSQUALLS.

SOME OF THE HEAVIER SQUALLS COULD DROP A QUICK COATING TO AN INCH ORTWO OF SNOW. THIS WILL CAUSE ROAD CONDITIONS TO DETERIORATE RAPIDLYIN SOME LOCATIONS. GUSTY WINDS COULD ALSO CAUSE BLOWING AND

DRIFTINGOF ANY NEW SNOW THAT FALLS...REDUCING VISIBILITIES TO NEAR ZERO ATTIMES.

MULTIPLE VEHICLE ACCIDENTS HAVE ALREADY BEEN REPORTED IN SOME OF THESQUALLS 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, 2004January 6, 2004

January 6, 2004January 6, 2004

CDT PHOTO/MICHELLE KLEIN CDT PHOTO/MICHELLE KLEIN

CDT PHOTO/NIKI

DESAUTELS CDT PHOTO/NIKI

DESAUTELS

CDT PHOTO/MICHELLE KLEIN CDT PHOTO/MICHELLE KLEIN

Photo Credits:

Centre Daily Times (CDT) and AP

Photo Credits:

Centre Daily Times (CDT) and AP

CDT PHOTO/NIKI

DESAUTELS CDT PHOTO/NIKI

DESAUTELS

AP PHOTOAP PHOTO

AP PHOTOAP PHOTO

(3 separate multi-vehicle accidents)

PA I-80

December 28, 2001 Exit 185 Logantown 63 vehicles, 45 injuries

Mile Marker 202 20 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

High Impact Sub-Advisory Snowfall - Frequency

Nov 8-15Nov 8-15

Avg.% of Days > 1”

Avg.# ofDays> 1”

Avg.% of Days > 4”

 

Avg.# of Days> 4”

Avg. % of Days

1” < x < 4” 

Avg. # of Days

1” < x < 4”

 10

 0.8

 ~2

 0.2

 8

 .6

 3

 0.2

 ~1

 0.1

 2

 .2

 1

 0.1

 < 1

  0.1

 < 1

 ~ 0

 24

 3.8

6

 ~1

 18

 2.9

 8

 1.3

 1

 0.2

 7

 1.1

 4

 0.6

 < 1

 < 0.2

 ~ 3

 < 0.5

 25

 7.8

 5

 1.6

 20

 6.2

 13

 4

 2.5

 0.8

 10.5

 3.3

 9

 2.8

 1.5

 0.5

 7.5

 2.3

 21

 2.9

 4

 0.6

 17

 2.4

 10

 1.4

 2

 0.3

 8

 1.1

 6

 0.8

 1.5

 0.2

 4.5

 0.6

 10

 0.8

 ~2

 0.2

 8

 0.6

 5

 0.4

 1.5

 0.1

 3

 0.2

 2

 0.2

 < 1

 < 0.1

 ~ 1

 < 0.1

 BFD

 UNV

 MDT

 BFD

 UNV

 MDT

 BFD

 UNV

 MDT

 BFD

 UNV

 MDT

 BFD

 UNV

 MDT

Dec 1-15Dec 1-15

Jan 1-31Jan 1-31

Feb 15-28Feb 15-28

Mar 24-31Mar 24-31

Cember and Wilks 1993Cember 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)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 SnowfallHISA 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 IMPACTstatement

Communicate the severity (impact) of event in conciseeasily understood terminology

Partner with NWS to deliver an effective message to allpotential 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…A winter storm’s impact is not just how much snow…but how and when that snow is delivered…and thisbut how and when that snow is delivered…and thisinformation needs to be communicated effectively toinformation needs to be communicated effectively tothe public. the public.

Junker (2000)

Wetzel and Martin (2001)

Summary of ingredients and diagnostics for forecastingWinter season precipitation.

Technique for forecasting snowfall using mixing ratioson 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) and850mb 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"