10.2 High Impact SubAdvisory Snow Events: The Need to Effectively Communicate the Threat of Short Du

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10.2 High Impact Sub-Advisory Snow Events The
Need to Effectively Communicate the Threat of
Short Duration High Intensity Snowfall
20th Conference on Weather Analysis and
Forecasting SESSION 10 PUBLIC FORECAST AND
WARNING ISSUES

• Gregory A. DeVoir
• NOAA/NWS State College, PA
• January 13, 2004

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NWS Tiered Forecasting Approach

• The National Weather Service (NWS) utilizes a
  multi-tiered approach to inform customers of the
  likelihood, potential severity and impact of
  impending weather events.
• When potentially hazardous weather conditions are
  expected, routine forecast products are headlined
  and complimented with specific event-driven
  Watches, Warnings, Advisories or Special Weather
  Statements (SPSs).

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NWS Tiered Forecasting Approach

• Warnings are ideally preceded by Watches,
  alerting to the potential threat to life and
  property well in advance of impending weather
  events.

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NWS Tiered Forecasting Approach

• Our Goal To provide sufficient lead time so that
  preventative actions can be taken to mitigate the
  impacts of adverse weather conditions. Save
  lives and property.
• In most instances of hazardous weather, the
  tiered forecast product approach works well. User
  groups are informed well ahead of time in the
  majority of significant events.

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NWS Tiered Forecasting Approach

• However, a small number of events occur annually
  which fail to meet Advisory (and/or) Warning
  criteria, yet still pose a significant hazard.

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Sub-Advisory Events

• Not confined to cold-season phenomena.
• Patchy Dense Fog.
• Approaching-severe Thunderstorms.
• Non-severe wind gusts.
• Many occur during the winter season, where a
  combination of precipitation intensity, duration
  and type, along with fluctuations in wind and
  temperature present a multitude of hazards.

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Sub-Advisory Snowfall

• Occurring in short-lived bursts or squalls of
  snow, total snow accumulation is a secondary
  concern.
• Severe impact is created by blinding snowfall,
  rapidly deteriorating road conditions, and
  increasing driver anxiety and confusion.
• The terms high intensity, high impact and
  sub-advisory are used here to describe such
  events.

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Central PA SnowAdvisory/Warning Criteria

• In central Pennsylvania, Winter Weather Advisory
  snow criteria is met when an average of at least
  2 to 4 inches of snow accumulates in a 12-hour
  period
• A midpoint value of 3 inches or more is
  required.
• Warning criteria is reached when 6 inches of snow
  or more falls within a 12-hour period (again, a
  midpoint value).

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High Impact Sub-Advisory Snowfall

• In areas which receive substantial annual
  snowfall, forecasts calling for one to two inches
  of snow do not typically arouse substantial
  public concern.
• For this reason, the NWS does not headline its
  forecasts for such amounts in these areas.

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High Impact Sub-Advisory Snowfall

• However, snow falling at a high intensity
  (bursts or squalls), with 1-2 amounts in a 1
  to 2 hour period, greatly magnifies the public
  impact.
• When snowfall rates are high, yet advisory
  criteria are not met, advisories typically are
  not issued and the public is not aware of the
  potential impact of the snowfall.

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An Ongoing Forecast Problem

• The NWS predicts weather. But clearly, there are
  numerous other factors that affect the outcome of
  a short burst of snow in a few hours.
• Road conditions, air and road surface
  temperatures, as well as how drivers act and
  respond to available information and road
  conditions are factors the NWS cannot control.

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An Ongoing Forecast Problem

• NOAA strategic plan, mission goal 4
• "Support the Nation's commerce with information
  for safe, efficient, and environmentally sound
  transportation.
• To fulfill its mission, the NWS must do all it
  can to ensure its users are informed of all
  impending weather hazards. Critical short-term
  weather information must reach our customers.

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High Impact Sub-Advisory Snowfall - Frequency

• Across the U.S., the frequency of sub-advisory
  snow events increases as one moves north, and
  towards areas downwind of the Great Lakes.
• The following slide contains data extracted from
  Cember and Wilks (1993), indicates of and of
  days of
• Snowfall 1
• Snowfall 4
• 1

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Nov 8-15
Cember and Wilks 1993
Dec 1-15
Jan 1-31
Feb 15-28
Mar 24-31
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High Impact Sub-Advisory Snowfall Cember and
Wilks

• 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.

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High Impact Sub-Advisory Snowfall

• 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.

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Predictability of Events

• Increased computational resources and better
  tools (computer models, satellite and radar
  imagery) enable forecasters to diagnose more
  meso- and smaller-scale weather phenomena and
  instabilities than ever before.
• Operational application of snow microphysics
  concepts - Waldstreicher (2001) showed how BUFKIT
  and snow microphysics concepts can be applied
  operationally to better identify and more
  accurately predict the timing of heavier snow
  bursts.

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Cross-Hair Signature Waldstreicher (2001)
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Recent Examples in/near PA

• 22 February 2001 2 to 3 inches of snow in less
  than 2 hours throughout mid-Atlantic states,
  including Washington, D.C. area.
• 28 December 2001 - Lake Effect Snow Squall
  (Loganton, PA).
• 5 January 2003 1 to 3 inches of snow in less
  than 3 hours across south central PA (midday
  Sunday following church services).

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Recent Examples in/near PA

• In each of these examples, snow amounts were
  accurately forecast well ahead of time.
• Yet, for each example, the impact was out of
  proportion with the volume of the snowfall.
• INTENSITY was the biggest factor, along with
  traffic volume and subsequent road conditions,
  leading to multiple fatal chain reaction
  accidents.

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Recent Examples in/near PA

• NOWCASTs were in effect for all of these events,
  but
• NOWCASTs have a limited audience (NWS websites,
  NOAA Weather Radio, sporadic AM/FM radio
  stations).
• Many did not receive critical short term forecast
  information.

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Last week - January 6, 2004
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January 6, 2004
SPECIAL WEATHER STATEMENT NATIONAL WEATHER
SERVICE STATE COLLEGE PA 1036 AM EST TUE JAN 6
2004 PAZ005-006-011-012-017019-025028-034-035-0
37-041-045-046-061730- BEDFORD-BLAIR-CAMERON-CLEAR
FIELD-FULTON-HUNTINGDON-JUNIATA-MCKEAN-MIFFLIN-NOR
THERN 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. LA CORTE
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January 6, 2004
PENNDOT Roadway Weather Information System (RWIS)
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January 6, 2004

• SPECIAL WEATHER STATEMENT
• NATIONAL WEATHER SERVICE STATE COLLEGE PA
• 1122 AM EST TUE JAN 6 2004
• PAZ012-019-026028-035037-041-042-045-046-049-050
  -056-063-061715-
• CUMBERLAND-FRANKLIN-FULTON-HUNTINGDON-JUNIATA-MIFF
  LIN-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...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

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.
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January 6, 2004
CDT PHOTO/MICHELLE KLEIN
AP PHOTO
CDT PHOTO/MICHELLE KLEIN
CDT PHOTO/NIKI DESAUTELS
AP PHOTO
Photo Credits Centre Daily Times
(CDT) and AP
CDT PHOTO/NIKI DESAUTELS
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How Can We Better Communicate the Threat?

• High Intensity, Sub-Advisory snow events should
  be headlined or covered by Special Weather
  Statements (SPSs). NWS forecasters can exercise
  subjective judgment in the issuance of Advisories
  and Warnings (NWS Directives).
• Consistently appropriate forecast terminology
  needed to ensure the responsiveness of DOT
  agencies and road crews (i.e. snow bursts/squalls
  vs. generic snow shower wording).

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How Can We Better Communicate the Threat?

• Education and Outreach Partnerships with state
  police, DOT and other local agencies is
  essential.
• Intelligent Transportation Systems (ITS) such as
  Highway Advisory Radio systems and remotely
  queued variable text roadside signs can directly
  reach highway travelers with critical weather
  information.

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How Can We Better Communicate the Threat?

• NWS State College is actively pursuing additional
  partnerships with PENNDOT and law enforcement
  agencies regarding the recent and previous I-80
  incidents.
• Hope to expand existing RWIS infrastructure
  through existing partnerships with Penn State and
  PENNDOT, and make this data available to
  travelers in real time.

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Conclusion

• High Intensity, Sub-Advisory snowfall can be a
  significant threat to life and property. The
  timing of such events is critical to their
  resulting impact.
• More accurate assessment of snowfall character
  (intensity and duration) along with heightened
  efforts to partner with state and local agencies
  to more efficiently deliver this information will
  better serve our users.

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Conclusion

• To support the NOAA Strategic Mission, the NWS
  must do all it can to ensure its users are
  informed of all impending weather hazards.
• Progress in this area will decrease loss of life
  and property on our nations highways, and move
  the NWS closer to our organizational goal of
  becoming Americas No Surprise Weather Service.
• Questions?