Winter Weather and Aviation
Mike BardouMeteorologist
National Weather Service Romeoville, IL
Jonathan LefflerMeteorologist
Center Weather Service Unit Aurora, IL
Summary
• NWS Background
• Weather Basics
• Hazards
• Planning
Weather Forecast Offices (WFO)
National Weather Service WFORomeoville, IL
• Located about 30 miles southwest of Downtown Chicago
• 27 full-time staff members
• At least 2-3 per shift
WFO Aviation Weather Services
• TAFs (every 2 hrs on weekdays/3 hrs on weekends)
- KORD - KMDW - KRFD - KDPA - KGYY
• Aviation Forecast Discussion• Soaring Forecasts
TAF Sites
Center Weather Service Units (CWSU)
Center Weather Service UnitAurora, IL
Located within the City of Aurora, IL about 40 miles west of Downtown Chicago
Staffed by 4 NWS meteorologists from 5:00 am – 9:00 pm, 7 days a week.
CWSU Aviation Weather Services• Provide Professional Meteorological Services to FAA
Air Traffic System
Accomplished with:
– Liaison to NWS Forecast Offices
– In-person and telephone briefings to local FAA facilities
– Center Weather Advisories (short-fuse “warnings”)
– Collaborative Convective Forecast Products (CCFP)
– Web-based products/decision aids
Weather Phenomena
• High Pressure• Low Pressure• Cold Front• Warm Front• Occluded Front
High Pressure
• Overall sinking motion, stable conditions• Calm or near calm surface winds, except at far
perimeter of high• Can have Cu growth, but normally little vertical
extent to clouds• Favorable for fog development, esp. if ground is
wet and skies are clear overnight• Snow covered ground can keep persistent IFR as
airmass changes very little
Low Pressure
• Overall rising motion, unstable conditions• Moderate to very strong surface winds in
cyclonic (counter-clockwise) motion• Great vertical extend of clouds (TSRA, SHRA)• Fog development typically not favorable due
to stronger winds & mixing of drier air• Can help “scrub” atmosphere from stagnant
high pressure
Warm Front
• Shallow sloping surface = less vertical extent• Winds usually SE shifting to SW after passage• Thickening & lowering of clouds ahead of
front with widespread, layered precipitation• Warm and stable once warm front passes
Cold Front
• Steep sloping surface = greater vertical extent• Winds usually SW shifting NW after passage• Lines of TSRA/SHRA ahead/along front with
residual -RA/DZ in some cases• Skies clearing and becoming stable
Occluded Front
• Acts similar to cold front, but normally not as strong
• Can maintain SHRA and possible TSRA if atmosphere remains unstable (upper level low)
• Wind direction varies based on position of low
Low Pressure Lifecycle
Weather Map Features
Tools
Average Snowfall
Weather Hazards
• Winds• Ceilings• Visibility• Precipitation• Icing• Turbulence
Wind Speed and Direction
Ceilings
• Clouds typically develop as saturation occurs at a particular level
• Cloud base is dependent on the level that saturation occurs
• Thickness of cloud cover is dependent on depth of the saturated layer
Ceilings
Visibility
Radiation Fog• Develops overnight with
weak flow• Common under high
pressure
Advection Fog• Typical ahead of warm fronts• Can occur with weak
movement of warmer air over snow cover…can be prolonged
• Strong low level inversion develops (warm air moving over cold air)
• South or east component to the wind is favorable
Precipitation
• Need upward motion and moisture to develop precipitation
• Precipitation type (RA, SN, FZRA, PL…) is dependent on the vertical temperature structure through the atmosphere
• Depth of cold/warm layers determines if freezing/melting occur as precipitation falls to the ground
Freezing Precipitation Frequency
Precipitation Intensity
• Determines visibility reductions in falling precip
• Strength of upward motion and amount of moisture in the atmosphere helps determine intensity
• 2 to 3 miles with light snow is common• 1 mile or less is common in heavy bands of
snow
Biggest TAF Challenges
• Stratus or Fog?? Both?? Slight improvements/deterioration (eg. LIFR to IFR etc.)
• Precipitation Type• Thunderstorm coverage• Thunderstorm timing
Icing
Areas Where Icing Occurs• Most non-convective icing occurs in late fall through early
spring• Significant icing potential increased in areas of:
– Cold air advection – Relative humidity greater than 85% (the higher, the more
potential)– Enhanced upward vertical motion (trofs, fronts, orographic
lift) • Icing can occur anywhere super cooled water droplets come in
contact with an aircraft surface temperature below freezing• 85% of all icing conditions occur in vicinity of frontal systems
Type is largely temperature dependent
Turbulence
Turbulence
• Most common in cold season (fall-winter) as jet speeds increase
• Associated with speed & directional shears (horizontal & vertical) - not just jet strength
• Satellite clues and soundings can help
Satellite Clues
Sounding Example 1
Sounding Example 2
Sounding Example 3
Planning Ahead
• TAFs– TAF Trends
• Forecast Discussion• ADDS Tools– Icing/Turbulence– AIRMETs/SIGMETs– PIREPS
• Winter Weather Headlines
Aviation Forecast Discussion
• Issued by most local offices that issue TAFs• Explains the reasoning behind the TAF• May also indicate confidence in the forecast
and potential deviations that may occur• Can also help provide info for conditions
between the TAF sites
Winter Weather Headlines
Aviation Weather Resources
• ADDS
• Forecast Maps
• NWS Chicago
• CWSU (ZAU)
How You Can Help…PIREPs!! -Limited observational data above the surface -They help us with the forecast -They also help other pilots
http://adds.aviationweather.gov/pireps/java/
Questions??
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