Event Summary
     National Weather Service, Raleigh NC

February 1, 2008 Low Level Wind Shear Event
Updated 2008/09/03





Event Headlines -

...The event featured a hybrid cold air damming pattern with a very strong southerly low level flow riding above a shallow cold air mass...
...Forecasters noted the potential for non convective low level wind shear several days before the event in the 100 AM EST Tuesday, January 29th Area Forecast Discussion (AFD)...
...Non convective low level wind shear values from the surface to just under 2000 feet exceeded 50 knots for a time on February 1, 2008...


Event Overview -
The weather pattern that developed on February 1st, 2008 was a typical hybrid cold air damming event. On the evening of January 31st, a strong, neutrally tilted mid level trough was located across the Southern Plains, encroaching upon the Lower Mississippi Valley with its attendant surface low positioned over central Mississippi. Further east, a strong 1039mb surface high was located over northern New England with strong ridging extending down into the Carolinas.

The air mass associated with this high pressure system was cold and dry. Temperatures across central North Carolina were in the mid to upper 40s as of 7pm January 31st, while dew points were in the upper teens to lower 20s.

The strong mid level trough acquired a negative tilt as it continued its eastward progression early in the morning on February 1st. Strong southwesterly winds above the surface served as a very efficient moisture transport mechanism, bringing moist Gulf of Mexico air into the Carolinas. This warm air advection regime resulted in strong lift above the stable surface cold dome that was in place due to the surface ridging.

Widespread precipitation developed across central North Carolina. As is the case with cold air damming events, evaporative cooling took place as the rain fell into the cold, dry air mass. This allowed surface ridging to maintain itself across the northern and northwest Piedmont, despite the fact that the parent surface high was retreating to the northeast. Winds at the surface responded by staying light out of the northeast at KRDU and KGSO during the morning hours. Meanwhile, the strong low level winds above the surface were out of the southwest at up to 50 knots. The 12Z February 1st KGSO sounding revealed the extent of the cold air damming. Such a significant change in wind direction and speed in the lowest 2000 ft produced very dangerous low level wind shear conditions across the Piedmont of North Carolina.



Aviation Forecast Program and Non-Convective Low Level Wind Shear

In addition to issuing forecasts and warnings for public interests, the National Weather Service (NWS) plays a critical role in aviation forecasting. Each NWS office has specific airports for which they issue Terminal Aerodrome Forecasts (TAF's). NWS Raleigh is responsible for issuing TAF's for 5 airports in central North Carolina:
  • INT (Smith Reynolds Airport – Winston-Salem, NC)
  • GSO (Piedmont Triad International Airport – Greensboro, NC)
  • RDU (Raleigh-Durham International Airport)
  • FAY (Fayetteville Regional Airport)
  • RWI (Rocky Mount-Wilson Airport)
In aviation forecasting, meteorologists are responsible for focusing on several key weather elements that directly affect the aviation community. Forecasters closely monitor the potential for low ceilings (cloud bases), low visibilities, thunderstorm potential, surface wind direction and speed, and non convective low level wind shear (LLWS) within 2,000 ft of ground level.

The National Weather Service describes wind shear as “...a change in horizontal wind speed and/or direction, and/or vertical speed with distance, measured in a horizontal and/or vertical direction. Wind shear is a vector difference, composed of wind direction and wind speed, between two wind velocities. A sufficient difference in wind speed, wind direction, or both, can severely impact airplanes, especially within 2,000 ft of ground level because of limited vertical airspace for recovery.” In simple terms, wind shear is essentially the change of wind speed and direction with height. There are two type of wind shear that can occur, convective and non convective. When thunderstorms are included in a Terminal Aerodrome Forecast, wind shear is inferred as any downburst or microburst may produce wind shear. Wind shear is only separately forecast in the TAF's if it is not associated with a thunderstorm. This type of wind shear can be even more dangerous as it can occur on an otherwise quiet weather day. Non convective wind shear can occur due to the following:
  • frontal passage and other boundaries (i.e. sea breeze)
  • low level temperature inversion
  • nocturnal low level jet
  • orographic effects
The NWS Aviation Directive requires a non-convective low level wind shear forecast is included in a TAF whenever:
  1. One or more pilot reports are received of non-convective LLWS within 2,000 feet of the surface, at or in the vicinity of the TAF airport, causing an indicated air speed loss or gain of 20 knots or more, and the forecaster determines the report(s) reflect a valid non-convective LLWS event rather than mechanical turbulence, or

  2. When, in the forecaster’s judgment, non-convective vertical wind shear of 10 knots or more per 100 feet in a layer more than 200 feet thick are expected or reliably reported within 2,000 feet of the surface at, or in the vicinity of, the airport.

Local users have expressed that the criteria for vertical wind shear of 10 knots or more per 100 feet in a layer more than 200 feet thick within 2,000 feet of the surface is too restrictive and would result in very few if any forecasts of non-convective low level wind shear. Local guidance based largely on user needs suggests that non-convective low level wind shear should be included in the TAF when 25 knots or more of shear is expected in the 2000 foot layer near the surface.

Meteorologists use several tools in order to forecast and detect low level wind shear. Some of the tools include:
  • model forecasts
  • RAOBS (weather balloon observations)
  • radar Velocity Azimuth Display (VAD) Wind Profiles (VWP)
  • other wind profilers
  • aircraft soundings
  • pilot reports
  • wind shear table/spreadsheet technique


Forecasting Non-Convective Low Level Wind Shear

Forecasters began to mention possible low level wind shear conditions in the outlook portion of the aviation forecast discussion as early as the early morning hours of January 29th. By the morning of January 30th, confidence increased that a hybrid damming event was likely and non convective low level wind shear would be a concern on the morning of February 1st. LLWS was first introduced into the RDU and GSO Terminal Aerodrome Forecasts at 12z on January 31st, providing approximately a 20 hour lead time. Model forecasts were essential in helping meteorologists determine the severity of the anticipated LLWS.

One tool forecaster's use is forecast soundings. These soundings show a vertical profile of temperature, dew point, and winds that a particular model is forecasting at a specific time. Taking a look at the 3 hourly NAM forecast soundings at RDU from the 00Z Thursday, January 31 run, valid from 00Z through 21Z Friday, February 1, 2008, it is readily apparent that the model was indicating a strong cold air damming event developing during the early morning hours of February 1st. Note the strong low level temperature inversion that develops by 09Z as the atmospheric column cools and becomes saturated. While this is occurring, very strong winds (50kt and greater) develop near the top of the inversion. This represents an ideal case for LLWS.

Loop of the NAM forecast soundings at RDU from 00Z January 31, 2008 run - click to enlarge

Determining the magnitude of low level wind shear is not typically as simple as using scalar subtraction from one wind speed near 2,000 ft and the wind speed near the surface. Scalar subtraction can only be used in a unidirectional wind profile when there is no variation of wind direction from one height to the other. When there is a difference in vectors from one height to the next, vector subtraction must be used. NOAA Technical Memorandum NWS FCST-23 introduced a wind shear computation table to easily and quickly determine wind shear values. For today’s use, the chart has been converted from meters per second to knots, so some of the values have been rounded. Essentially, anything that yields a value greater than 20 knots is considered significant enough to mention in the Terminal Aerodrome Forecasts.

NOAA Technical Memorandum NWS FCST-23 wind shear computation table  - click to enlarge

In the February 1st case, the table below which was created based on the RDU AMDAR data, was calculated using the wind shear spreadsheet. The surface wind would be the w1 in the table and the 2000 ft wind would be w2. The angular difference is simply the difference between the wind vector at w1 and w2.

Shear strength based on Amdar aircraft sounding text data


Observing Non-Convective Low Level Wind Shear

By January 31st, it was apparent to forecasters at the Raleigh forecast office that a hybrid damming event was likely and non convective low level wind shear would be a concern. Model forecasts were essential in helping meteorologists determine the severity of the anticipated LLWS.

Although model forecasts are critical in forecasting LLWS in advance, it is just as important to have tools able to detect the onset and evolution of LLWS. On the morning of February 1st, forecasters had several methods of observing LLWS conditions including upper air RAOB soundings, AMDAR Aircraft Soundings, WSR-88D VAD Wind Profiles, Other Wind Profilers, Pilot Reports (PIREPS), and the Graphical Airman’s Meteorological Advisory (G-AIRMET).



Upper Air RAOB Soundings

RAOB is an acronym for "rawinsonde observation" which are observations made by a rawinsonde unit attached to a weather balloon. RAOB Soundings are typically conducted every 12 hours at 00Z and 12Z although they may be done more frequently for significant events. The observations provide details on the thermal, moisture, pressure and wind fields in the vertical atmosphere.

The Skew-t plot below from Greensboro depicts the 12Z, February 1, 2008 RAOB which shows a significant near surface based inversion with a tremendous amount of shear, both from a veering wind (directional shear) and increasing velocities with height (speed shear).

KGSO Skew-T diagram from the 12Z, February 1, 2008 - click to enlarge


The wind, temperature, moisture, and pressure data can be interpolated to provide a detailed estimate of the data on small vertical intervals. The chart below displays the wind speed and direction in the fourth column with the height in feet above ground level (AGL) at 100 foot intervals.

Tabular data from the KGSO RAOB at 12Z, February 1, 2008



AMDAR Aircraft Soundings

AMDAR is an acronym for Aircraft Meteorological DAat and Reporting (AMDAR) which is an international effort within the World Meteorological Organization to coordinate the collection of environmental observations from commercial aircraft. In the United States, we often refer to the Meteorological Data Collection and Reporting System (MDCRS) which is a private/public partnership facilitating the collection of atmospheric measurements from commercial aircraft to improve aviation safety.

AMDAR is very useful for short term forecasting situations where conditions are changing rapidly and in particular for aviation forecasting. Regarding winter weather events, AMDAR data can provide forecasters with the height of the freezing level, the presence of elevated warm layers, indications of thermal advection and dry layers. All of these are necessary for accurate precipitation type forecasts. The availability of this upper air data at times and locations where RAOB data may be lacking is invaluable.

During the 30 hour period from 12Z on 01/31 through 22Z on 02/01 there were over 25 AMDAR soundings available at RDU.

The image below contains a loop of AMDAR soundings at RDU during the event. There are 19 soundings in the loop that run from 1301Z on Thursday, January 31 through 2130Z on Friday, February 1, 2008. A Java Loop of AMDAR soundings from 1301Z on 01/31 through 2130Z on 02/01 that can be stopped, controlled and zoomed is also available.

The aircraft soundings in this case show a dramatic increase in southwest winds just above the surface starting with the 09Z sounding. As precipitation fell through the surface cold dome, a very pronounced temperature inversion became established from the surface to near 900 mb. This inversion separated the light northeast surface winds with the strong southwest winds at the top of the inversion, resulting in a classic low level wind shear event.

Amdar aircraft sounding loop


The table below was created based on the text output of selected AMDAR soundings. The aircraft sensor reports multiple observations during the course of its ascent or descent and each observation is included in the text output. In the table, one can see how the wind shear dramatically increased by 12Z.

Data from the selected AMDAR soundings used in the table below...
0231Z Graphic or Text  |   0918Z Graphic or Text  |   1142Z Graphic or Text  |   1357Z Graphic or Text  |   1450Z Graphic or Text

Shear strength based on Amdar aircraft sounding text data

It should be noted that the aircraft soundings report heights based on a standard atmosphere and are not adjusted to local barometric pressure like a cockpit altimeter would be. Therefore, there is some variability in the heights reported by the AMDAR observation and have to be converted based on local barometric pressure readings.



WSR-88D VAD Wind Profile

The National Weather Service Doppler radar has the capability of detecting both precipitation and wind speeds, based on how fast the precipitation picked up on radar is moving toward or away from the radar beam. The software on the WSR-88D can calculate wind direction and speed at various levels in the atmosphere and produce a Velocity Azimuth Display (VAD) Wind Profile (VWP) product. The radar produces both a text product and a graphical product which helps forecasters determine the estimated winds speeds in the lowest 2,000 feet.

The image below (click to enlarge) is from the KRAX WSR-88D near Raleigh, NC at 1058Z on Friday, February 1, 2008. The increasing amounts of low level wind shear can be seen at 1058Z with a northeast wind at around 15 knots observed at 1,000 feet, a southerly wind at 25 knots at 2,000 feet and a south-southwesterly wind at 40 knots at 3,000 feet. While the temporal resolution of the VWP product is great (it is available with each volume scan, typically around every 5 minutes in precipitation mode), the vertical resolution is limited to every 1,000 feet.

A Java Loop of Velocity Azimuth Display (VAD) Wind Profile (VWP) products from 0303Z through 1502Z on Friday, February 1, 2008 is also available.

KRAX vad wind profile image - click to enlarge



Other Wind Profiler

A wind profiler is a type of weather observing equipment that uses radar or sound waves (SODAR) to detect the wind speed and direction at various elevations above the ground.

The North Carolina Department of Environment and Natural Resources (DENR), Division of Air Quality (DAQ) has a wind profiler located near Raleigh, NC. The purpose of this equipment is to measure winds from the surface up to approximately 16,000 ft MSL. The output is very much like the WSR-88D VAD Wind Profile. This tool can be extremely useful to forecasters as it provides very good vertical resolution. In addition, on clear weather days when the WSR-88d radar wind profile will not be as accurate due to lack of radar returns needed to estimate the winds, the profiler is sensitive enough to provide good data. Finally, the wind profilers can provide detailed data in locations that are far away from the radar.

The two images below (click to enlarge) are from the DAQ wind profiler data near Raleigh on February 1, 2008. The data is from around 06Z to around 15Z with the vertical height up to around 10,000 feet in the first image and up to around 3,000 feet in the second image. The rapid increase in wind shear can be seen from around 07Z to 11Z as the near surface flow remains east to northeasterly at 10 to 15 knots while the flow at around 2,000 feet rapidly increases and veers to south-southwesterly at around 45 knots.

NCDAQ Ralegh wind profiler data from 06Z to 15Z on 2007/02/01 up to 10,000 feet - click to enlarge

NCDAQ Ralegh wind profiler data from 06Z to 15Z on 2007/02/01 up to 3,000 feet - click to enlarge

Data from various wind profilers across the country can be accessed via the Cooperative Agency Profilers (CAP) project web site produced by GSD (formerly FSL). The web site collects wind profiler data from various participating agencies, apply GSD's data quality control algorithms, and make these value-added data available on the web and to the National Weather Service.

More information on the CAP project is available at http://madis-fsl.org/cap/index.jsp. Real time profiler data is available via the Graphical Data Display page which contains time-series displays of Wind Speed and Direction, Spectral Moment Displays (Signal Power, Signal to Noise Ratio, Radial Velocity, and Velocity Variance), and RASS Temperature Displays. Real time data from the Raleigh profiler is available via this link.



Pilot Reports (PIREPS)

A pilot report or PIREP is a report of actual weather conditions encountered by an aircraft in flight. This information is usually relayed by radio to the nearest ground station. The message is then encoded and relayed to weather offices and air traffic service units.

Because reports are made by pilots, they may occur at any time and may contain a variety of information. Common to all reports should be location, time, altitude (MSL), type of aircraft, and at least one weather element:
  • Sky cover (MSL)
  • Flight visibility and weather (Statute miles)
  • Temperature (C)
  • Wind (knots)
  • Turbulence
  • Icing
  • Remarks
An example pilot report from the event is shown below.

Example pilot report - click to enlarge

If wind shear is reported in the PIREP that indicates a gain or loss of 20 knots, then LLWS must be included in the terminal forecast.

PIREPS can be accessed on the internet via the Aviation Weather Center (AWC) web page at http://adds.aviationweather.gov/pireps/. Real time PIREPS during the most recent 12 hours and within 200 miles of KRDU are available via this link.



Graphical Airman’s Meteorological Advisory (G-AIRMET)

The Aviation Weather Center, based out of Kansas City, MO, now offers a new suite of graphical guidance. The Graphical Airmet (G-Airmet) produces a 12 hour outlook highlighting the potential for numerous aviation hazards, including the possibility of non convective low level wind shear. The G-Airmet can be accessed on the internet by going to http://aviationweather.gov/testbed/gfa/

Graphical Airmet guidance product - click to enlarge



Surface Analysis

The surface analysis from 12Z Friday, February 1, 2008 shown below depicts a fairly strong (1042 MB) surface high pressure system located just northeast of Maine. The high pressure center was transitory and was no longer supporting the Hybrid cold air damming event that was ongoing across the Carolinas and Virginia. The inland advancement of the coastal front was indicative of the low level shear and warm advection across the Carolinas.

A Java Loop of surface analysis imagery from 12Z Thursday, January 31, 2008 through 00Z Saturday, February 2, 2008 shows the evolution of event.

Surface analysis from 12Z Friday, February 1, 2008



Regional Radar Loop

A Java Loop of regional reflectivity imagery from 1758Z on Thursday, January 31 through 1958Z on Friday, February 1, 2008 is available here. Note - this loop includes 51 frames.

The regional reflectivity image below is from 0658Z (158 AM EST on February 1, 2008) shows a large area of precipitation across much of central and western North Carolina. The precipitation was falling as snow across the North Carolina mountains with moderate rain and temperatures in the mid 30s reported across the Western Piedmont and Foothills.


Regional reflectivity image - click to load loop



Mesoscale Data

Analyzed surface pressure and wind barbs from SPC at 08 UTC on Friday, February 1, 2008
A high pressure center located over New England extended southwestward across the Mid Atlantic coast and into central Virginia and North Carolina. A light northeast to easterly wind was present across central North Carolina. A coastal front extended along the North Carolina coast southward into southern South Carolina and Georgia. While a surface ridge was in place, the pressure field was becoming baggy as the high pressure center began to move offshore.

SPC Analysis at 08 UTC on Friday, February 1, 2008



500 MB heights, temperatures (red), dew points (green), and wind barbs (black) from SPC at 08 UTC on Friday, February 1, 2008
A strong southwesterly flow was present across the Carolinas and the Tennessee Valley with between 50 and 70 knots of southwesterly flow over central North Carolina and up to 100 knots of flow across Tennessee.

SPC Analysis at 08 UTC on Friday, February 1, 2008.



850 MB heights, temperatures (red/blue), dew points (green), and wind barbs (black) from SPC at 08 UTC on Friday, February 1, 2008
A strong southerly flow up to 50 knots was located across the Carolinas. The temperatures shown in red indicate significant warm advection across North Carolina and especially Virginia.

SPC Analysis at 08 UTC on Friday, February 1, 2008.



Analyzed surface frontogenesis (red), temperatures (blue), pressure (black), and wind barbs (brown) from SPC at 08 UTC on Friday, February 1, 2008
Note the dramatic values of surface frontogenesis along the Carolina coastline where surface temperatures ranged from the lower 40s just inland from the coast to the lower 60s just offshore.

SPC Analysis at 08 UTC on Friday, February 1, 2008



0-3 Km Storm Relative Helicity (blue) and storm motion (brown) from SPC at 08 UTC on Friday, February 1, 2008
Note the 0-3 Km SRH values were extremely large averaging from 500 to 1000 units across central North Carolina.

SPC Analysis at 08 UTC on Friday, February 1, 2008



NWS Composite Reflectivity Imagery from 0832 UTC on Friday, February 1, 2008
The composite reflectivity imagery is from the approximate time in which the analysis imagery above is valid.

Composite Reflectivity Imagery from 0832 UTC on Friday, February 1, 2008



Archived Text Data from the Weather Event

Select the desired product along with the date and click "Get Archive Data."
Date and time should be selected based on issuance time in GMT (Greenwich Mean Time which equals EST time + 5 hours).


Product ID information for the most frequently used products...

RDUAFDRAH - Area Forecast Discussion
RDUZFPRAH - Zone Forecast Products
RDUAFMRAH - Area Forecast Matrices
RDUPFMRAH - Point Forecast Matrices
RDUHWORAH - Hazardous Weather Outlook
RDUNOWRAH - Short Term Forecast
RDUSPSRAH - Special Weather Statement
RDUTAFFAY - Terminal Aerodrome Forecast for FAY
RDUTAFGSO - Terminal Aerodrome Forecast for GSO
RDUTAFINT - Terminal Aerodrome Forecast for INT
RDUTAFRDU - Terminal Aerodrome Forecast for RDU
RDUTAFRWI - Terminal Aerodrome Forecast for RWI


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Lessons Learned

  • The LLWS computation spreadsheet is a quick and easy tool to help forecasters gage the magnitude of the wind shear based off differences in both speed and direction at two different heights.
  • Cold air damming events are typically conducive for non convective low level wind shear due to the sharp low level temperature inversion that is created.
  • AMDAR soundings have proven to be a reliable means to monitor the development and evolution of LLWS. However, availability of AMDAR soundings can vary greatly from day to day. In addition, one must be cognizant of the fact that the AMDAR observations reports heights based on a standard atmosphere with no adjustment based on local barometric pressure.
  • Model forecast soundings can be very beneficial in determining low level wind shear potential prior to the onset of conditions.
  • The Graphical Airmet produced by the Aviation Weather Center is a new tool forecasters can use to maintain situational awareness of possible non convective low level wind shear conditions.
  • In discussions with the general aviation community following this event, it was noted that low level wind shear of 15-20 knots can begin to cause significant issues for smaller aircraft. Although such a threshold cannot be relayed through the TAF, the aviation forecast discussion is the perfect medium in which to relay the threat of low level wind shear to both general aviation and commercial aviation.
  • Local users have expressed that the criteria for vertical wind shear of 10 knots or more per 100 feet in a layer more than 200 feet thick within 2,000 feet of the surface is too restrictive and would result in very few if any forecasts of non-convective low level wind shear. Local guidance based largely on user needs suggests that non-convective low level wind shear should be included in the TAF when 25 knots or more of shear is expected in the 2000 foot layer near the surface.


Acknowledgements

Many of the images and graphics used in this review were provided by parties outside of WFO RAH. The surface analysis graphic was obtained from the Hydrometeorological Prediction Center. The skew-T images were obtained from the University of Wyoming. Upper air analysis images were provided by the Penn State NARR display page. Surface temperature and dew point analysis images were obtained from Plymouth State University. AMDAR aircraft sounding data was obtained from the Earth System Research Laboratory - Global Systems Division (GSD). North Carolina Department of Environment and Natural Resources (DENR), Division of Air Quality (DAQ) profiler data was provided by the Cooperative Agency Profilers (CAP) project web page produced by Global Systems Division (GSD). Some radar imagery was obtained from various National Weather Service web sites.



References

Badner, Julius: Low-Level Wind Shear: A Critical Review. NOAA Technical Memorandum NWS FCST-23, 1979.



Case study team -
Jason Beaman
Jonathan Blaes

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