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??