Event Summary
     National Weather Service, Raleigh NC


January 18, 2007 Winter Storm
Updated 2007/06/18






Event Headlines

...An average of around one half to one inch of snow accumulated across interior portions of the North Carolina Piedmont...
...Low and mid level moisture quickly surged into central NC from the south and west quickly overcoming the a dry low level air mass preceding the storm...
...The evolution of the precipitation type at RDU from the initial onset of precipitation through the peak in snow intensity was a result of numerous mechanisms including diabatic processes, precipitation rate, and dynamic processes...
...The Micro Rain Radar (MRR) provided a new observational tool for monitoring the precipitation type and intensity during the event...
...The Micro Rain Radar (MRR) provided a tremendous amount of insight into the physical processes during the storm...
...AMDAR aircraft soundings provided observations of temperatures and winds at lower and mid levels of the atmosphere which were a great resource for forecasters...


Event Overview

A strong jet stream with a wind max of 160-180 kts between 200-300 MB evolved between a broad ridge over the Gulf of Mexico and a transitory large scale wave over the upper Midwest. Dynamics associated with this strong jet resulted in broad scale lift across the Southeast and lowering surface pressures along the southeastern U.S. coast. This drew moist air northward in the low to mid levels of the atmosphere. At the surface, a 1040 MB high was centered on the New England coast with a cold air damming signature extending southward into the Piedmont region of the Carolinas. The moist air from the south lifted up and over the cold dry air at the surface, expanding cloudiness and initiating precipitation across central NC in the predawn hours on the morning of Thursday, January 1, 2007.

Precipitation started as snow across the interior portions of the Piedmont with a mix of freezing rain and sleet across much of central North Carolina. One half to around one inch of snow accumulated in a swath from the Triangle area westward to the southern portions of the Triad area. Across much of the remainder of the northern and northeast Piedmont, snowfall amounts ranged from a trace to one-half of an inch. In locations where snow fell, the snow was generally followed by an hour or two of freezing rain with most locations warming up sufficiently to change the precipitation over to all rain. A map of the snow and sleet accumulations is shown below.


Event Details

For much of the first half of January, the Southeastern U.S. experienced above normal temperatures as the eastern U.S. was dominated by an upper level ridge of high pressure with a trough setup across the Western U.S. The upper air pattern changed dramatically during the third week of January with an active southern jet stream and a trough developing over the eastern U.S.

On Wednesday evening, at 00Z/18 January (700 PM EST), a strong, cold, 1042 MB surface high pressure system was centered over New England. A ridge associated with this high pressure system extended across the Mid Atlantic and into the Upstate region of South Carolina and Georgia. The location and intensity of the surface high was in a favorable location for Cold Air Damming in North Carolina.

Between 00Z and 06Z, a low level southeasterly flow developed, transporting moisture into the region. The 0350Z AMDAR sounding from an aircraft landing at RDU shows an entirely sub freezing sounding with a hint of a warm nose developing between 900 and 850 MB.

The upper level pattern favored a progressive high pressure system. By 06Z, the high pressure system over New England was beginning to move offshore. The transport of cold, dry air into North Carolina began to wane after 06Z but a cold and dry air mass would remain in place at the surface, setting the stage for in-situ cold air damming (CAD) if enough precipitation would fall into the potential CAD region. Precipitation generated by the intense subtropical jet moved quickly across Alabama, Mississippi, and Georgia during the early evening hours. By 06Z, the precipitation had spread across South Carolina and was beginning its advance northward.

Between 06Z and 09Z, the precipitation rapidly developed and pushed northward into southern and central North Carolina. During this period, isentropic lift of warm, moist air was intensifying across Georgia and South Carolina. By 09Z, an inverted trough of low pressure had developed along the Southeast U.S. coast and a weak area of low pressure was developing along the east coast of Florida.

Model forecasts and analysis showed significant isentropic lift at the 285-295K degree theta levels with a confluent flow and saturation advancing northeastward across the southern tier of central North Carolina at around 12Z on January 18. The Triad area and northern tier of North Carolina remained on the edge of the large scale lift with the flow becoming more parallel to the pressure contours and drier air. In fact, low level moisture rapidly moved into the Raleigh area as noted by the RUC dev2 analysis at 08Z, 09Z and 10Z. Despite the very dry air in place across central N.C. at 09Z the precipitation moved into the RDU area (METAR's | radar loop) from the west right around 1000Z or 500 AM EST. The area of precipitation was structured in northwest to southeast oriented bands that moved eastward across the state.

The precipitation initially fell as sleet in Raleigh (15 hours of observations at RDU) and then a mix of sleet and freezing rain. A zoomed in skew-T display of the RUC dev2 analysis at 10Z for RDU shows a warm nose or a near freezing isothermal layer centered around 900 MB with the temperature warming above freezing at 875 MB or around 4,000 feet. During the next hour, the precipitation fell as a mix of freezing rain and sleet. Surface temperatures were in the mid 20s.

As the precipitation intensified between 1030Z and 1100Z, the warm nose indicated by the dev2 analysis at 11Z for RDU eroded. Initially the cooling produced by evaporation and then the cooling produced by melting worked to trim back the warm nose. In addition, dynamic cooling played a role in cooling the layer and mitigating the initial impact of warm air advection. An AMDAR aircraft sounding from a plane landing at RDU at 1120Z shows that the warm nose had cooled sufficiently to a near freezing isothermal layer. The mixed precipitation reported across the Raleigh area changed to all snow at around 1100Z.

With a near freezing isothermal layer established aloft and an area of somewhat heavier precipitation moving over RDU, a change over to accumulating snow arrived between 1100Z and 1300Z. The intensity of the snow then began to relax after 1239Z. By 1302Z the precipitation had changed to a mix of sleet and snow.

It should be obvious that the evolution of the precipitation type during the event was a result of numerous mechanisms including diabatic processes, precipitation rate, and dynamic processes. Many more details on the evolution of the precipitation type in Raleigh is contained in the Analysis of the Event with MRR Data section below.

Low level warm air advection continued from just above the surface to 850 MB during the mid morning hours. By the 14Z, the warm nose was even more prominent and it was centered at around 925 MB. The RUC dev2 analysis at 14Z for RDU and a 14Z AMDAR sounding from an aircraft departing RDU both show a profile that would no longer support frozen precipitation. The observations at RDU only contain rain or freezing rain after 14Z and no snow or sleet. As the surface temperatures gradually warmed, forecasters used the surface wet bulb temperatures to track the freezing line as it moved northwest from near Fayetteville-Rocky Mount at around 12Z to near Southern Pines-Clayton-Roanoke Rapids at around 16Z and to near Wadesboro-Pittsboro-Oxford at 18Z.

The greatest isentropic lift and moisture convergence shifted east to eastern North Carolina after 15Z. By 16Z the precipitation had begun to diminish in coverage and intensity with only scattered light precipitation remaining across central North Carolina at 18Z. By 400 PM, only a few isolated pockets of freezing precipitation were reported, primarily across the Triad. Most locations across North Carolina had warmed above freezing by 400 PM with most of the significant precipitation confined to eastern portions of the state.


Snow and Sleet Accumulation Map

Snow and sleet accumulation map from the January 18, 2007 Winter Storm




Surface Analysis

The surface analysis from 12Z Thursday, January 18, 2007 shown below, depicts the weather pattern just after daybreak. An inverted trough of low pressure is shown along the Southeast U.S. coast with a weak area of low pressure developing off the east coast of Florida. A cold area of high pressure (1040 MB) centered off the Northeast U.S. coast is moving slowly eastward. This area of high pressure is no longer able to supply cold air to support the cold air damming across the Carolinas and Virginia.

A Java Loop of surface analysis imagery from 00Z January 17 (700 PM Tuesday) through 12Z January 19, 2007 (700 AM Friday) shows the evolution of event.

Surface analysis from 12Z Thursday, January 18, 2007



Satellite Imagery

The very fast southwesterly flow can be viewed in both the infrared and the water vapor imagery loops below. The fast flow is easily seen as well, illustrating the difficult nature of forecasting precipitation events associated with a strong southern stream.

A Java Loop infrared satellite imagery from 1215Z (715 AM EST) on Wednesday, January 17, through 2315Z (615 PM EST) Thursday, January 18, 2007 is available.

A Java Loop water vapor satellite imagery 1215Z (715 AM EST) on Wednesday, January 17, through 2315Z (615 PM EST) Thursday, January 18, 2007 is available.

Infrared satellite imagery from 1515Z on 
Thursday, January 18, 2007



Partial Thickness, Surface Wet Bulb Temperature and Surface Weather

The thickness of a layer of air is proportional to the layer's mean temperature. The warmer the layer, the greater it's thickness. The layer of air bounded by a pressure of 1000 - 850 MB is used by forecasters to monitor the average temperature in the lower level of the atmosphere (roughly from the surface to 5,000 ft). The 850 - 700 MB layer is closely followed by forecasters to monitor the average temperature in the layer of air (roughly from 5,000 to 10,000 ft) where elevated above freezing temperatures may exist. NWS forecasters at Raleigh have correlated the thickness of these layers as observed from weather balloon launches to the observed wintry precipitation types. The technique has proven to be helpful in anticipating the frequent precipitation type changes often associated with winter storm events in central North Carolina. The Java loop shows the changes in the thickness levels and their associated wintry precipitation types.

The image below depicts the partial thickness values for 1000 - 850 MB and 850 - 700 MB along with surface wet bulb temperatures and the observed weather at 11Z on Thursday, January 18, 2007.

A Java Loop of partial thickness, surface wet bulb and weather imagery from 06Z through 20Z on Thursday, January 18, 2007 is available.

Partial Thickness, Surface Wet bulb and Weather Imagery from 11Z on Thursday, January 18, 2007



Raleigh - KRAX Radar Imagery

Base reflectivity imagery from the Raleigh, KRAX WSR-88D at 1101Z (601 AM EST) on Thursday, January 18, 2007 is shown below. At this time, a mix of wintry precipitation was reported across much of central NC.

A Java Loop of KRAX Radar imagery from 0702Z (202 AM EST) Thursday, January 18 through 1759Z (1259 PM EST) Thursday, January 18, 2007 is available.

Raleigh, KRAX Radar imagery from 1101Z (601 AM EST) Thursday, January 18, 2007



The Micro Rain Radar

In a collaborative effort between the NWS Raleigh, NC and Dr. Sandra Yuter of the Cloud and Precipitation Processes and Patterns research group at NC State, a METEK Micro Rain Radar was placed on the roof of the building which houses the NWS Raleigh.

Picture of the METEK Micro Rain Radar that is on the roof of the Research III building on NC State's Centennial Campus - click to enlarge The Micro Rain Radar (MRR) is a vertically-pointing Ku-band radar that is commercially available from METEK Inc. The MRR was placed on the antenna array on southeast corner of the roof of Research III building on NC State's Centennial Campus. The radar unit is the black radar dish and assemblage that is shown in the top center portion of the picture on the left. The output from the MRR can be processed to provide the user with a vertical view of reflectivity and Doppler velocity. The high temporal and vertical spatial resolution of the MRR along with its unique data makes it an interesting tool to view precipitation type changes during winter storms. This winter was the first winter in which meteorologist at the Raleigh NWS office were able to view the data.



Characteristics of the MRR on the Roof of Research III

Transmit Frequency - 24.1 GHz (1.24 cm wavelength)
Transmit Power - 50 mW
Beam width - 2 deg.
Height resolution - 150 m
Averaging time - 30s
Measured variables - height, spectra, drop spectra, radar reflectivity, rain rate and fall velocity.
Dimensions - 0.6 m x 0.6 m x 0.6 m


Introduction into the MRR Output

The Cloud and Precipitation Processes and Patterns research group at NC State have produced a software package to view two facets of the MRR output. The application displays both the reflectivity in dBZ and the Doppler velocity. Both of these datasets are displayed with a vertical axis depicting height in thousands of feet and a horizontal axis depicting time.

The reflectivity data can be used to determine precipitation intensity and potential bright banding. In the imagery shown in this summary the cooler colors (blues and greens) represent lighter reflectivities with the warmer colors (yellows into oranges and reds) depicting a higher reflectivity.

The velocity data can be used to determine precipitation type based on the fall velocity of the precipitation particle. Snowflakes have a much slower fall velocity than sleet or rain (including freezing rain). The location where a slower fall speed changes into a faster one can be inferred as the melting layer.


Analysis of the Event with MRR Data

The MRR is a vertically pointing radar and the data it presents is only appropriate for locations near the radar, this is especially during evolving complex events. The text and imagery below provide details about the rapidly changing precipitation types at the surface during the event and the corresponding data from the MRR (map of the MRR location at the NWS Raleigh and the RDU airport).

Annotated and zoomed in MRR reflectivity and velocity image between 0930Z and 1030Z - click to enlarge Precipitation was first noted by forecast staff at the NWS Raleigh at about 950Z on January 18, 2007. The precipitation was first reported at RDU (Raleigh-Durham International Airport) which is located about 10 miles northwest of the NWS Raleigh, at around 1000Z. In the three hours leading up to the arrival of the precipitation, the cloud layers rapidly lowered and thickened with the ceiling falling from 12,000 feet to 3,000 feet in a little over 3 hours.

The precipitation initially fell as sleet (15 hours of observations at RDU) and then a mix of sleet and freezing rain. The sleet implies that snow falling aloft melted and then refroze before reaching the surface or that rain drops froze before reaching the ground. The cooling required to produce sleet can result from a liquid rain drop simply traveling through a colder or drier layer of air. If you look closely at the MRR output, you can see that the first time that the radar reflectivity reaches the surface is just before 1000Z, which is coincident with the time of the first surface reports of precipitation.

Looking closely at an annotated and zoomed in MRR reflectivity and velocity image between 0930Z and 1030Z you can see where the fall velocities increase dramatically between 4,000 and 3,500 feet indicative of the melting layer. A zoomed in skew-T display of the RUC dev2 analysis at 10Z for RDU shows a warm nose centered at 900 MB with the temperature warming above freezing at 875 MB or around 4,000 feet. During the next hour the precipitation fell as a mix of freezing rain with some sleet mixed in. Surface temperatures were in the mid 20s.

As the precipitation intensified between 1030Z and 1100Z the warm nose indicated by the dev2 analysis at 11Z for RDU begins to erode. Initially the cooling produced by evaporation and then the cooling produced by melting worked to trim back the warm nose. In addition, dynamic cooling played a role in cooling the layer and mitigating the initial impact of warm air advection. Dynamic cooling is the cooling produced by upward vertical motion, or in other words, the cooling of a parcel that results from decreasing pressure. A zoomed in skew-T display of the RUC dev2 analysis at 11Z for RDU shows a diminished warm nose with temperatures very close to freezing. An AMDAR aircraft sounding from a plane landing at RDU at 1120Z shows that the warm nose has cooled sufficiently to a near freezing isothermal layer. The mixed precipitation reported across the Raleigh area changed to all snow at around 1100Z.

With a near freezing isothermal layer established aloft and an area of somewhat heavier precipitation moving over RDU, a change over to accumulating snow arrived between 1100Z and 1300Z. The heaviest snow was reported at RDU between 1214Z and 1239Z when visibilities were at 3/4 of a mile.
Annotated and zoomed in MRR reflectivity and velocity image between 1130Z and 1500Z - click to enlarge
Looking closely at an annotated and zoomed in MRR reflectivity and velocity image you can see the enhanced reflectivities located between 1,500 and 4,000 feet that occur between 1150Z and 1240Z. These enhanced reflectivities suggest that snow aloft is melting in a layer beginning at around 4,000 feet and continuing down to near 1,500 feet. The skew-T display of the RUC dev2 analysis at 13Z for RDU shows the temperature profile climbing above freezing at approximately 875 MB.

The intensity of the snow began to relax after 1239Z with the visibility at RDU improving from 1 to 2 miles between 1239Z and 1251Z and the low overcast layer at 1,200 feet becoming broken at 1,500 feet. By 1302Z, the precipitation had changed to a mix of sleet and snow and the visibility had increased to 6 miles.

The MRR velocity imagery shown in the lower part of the MRR image shows that the fall velocity was relatively constant and low through about 1230Z. This image is consistent with snow or snow that undergoes very limited melting aloft. After around 1230Z, the fall velocities increase notably but the area of higher reflectivities is still maximized in the bright band. The higher reflectivities do not extend completely to the surface which suggests that the snow is melting aloft but it is still reaching the surface as wet snow or snow that partially melted but refroze. Just before 1300Z the velocities increase even more and the higher velocities extend all of the way to surface, this is consistent with snow that melts aloft and refreezes into sleet. Observations at RDU and at WFO RAH suggest that sleet mixed in with the snow at around 1300z.

Typically sleet is a transitionary precipitation type and that was demonstrated in this event with the sleet being observed for around a half hour (between 1302Z through 1336Z at RDU) before the precipitation transitioned to freezing rain at about 1345Z at NWS Raleigh. During the transition period, the warm nose became more pronounced and deeper. An AMDAR aircraft sounding from a plane departing RDU at 1400Z shows that the temperature profile has warmed at low levels with the temperature exceeding freezing at around 875 MB. Similar details can be seen in the skew-T display of the RUC dev2 analysis at 14Z for RDU. The MRR velocity imagery shows the melting layer increasing in height with time as the event unfolds. The precipitation continued to fall as freezing rain until around 1630Z.


Java Tool to View MRR Data with Pop up Windows of Radar and Temperature Profiles

Move your cursor over the inserted section between the MRR reflectivity and MRR velocity data below to view pop up windows with radar data from KRAX along with temperature profiles from AMDAR aircraft soundings and RUC dev2 analysis data. The pop up windows will match times with the MRR data allowing you to compare both horizontal radar data (KRAX) along with the vertical radar (MRR).



Archived Text Data from the Winter Storm

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
RDUAFMRAH - Area Forecast Matrices
RDUHWORAH - Hazardous Weather Outlook
RDUNOWRAH - Short Term Forecast
RDUPFMRAH - Point Forecast Matrices
RDUPNSRAH - Public Information Statements (snow/ice reports among other items.)
RDUWRKDRT - Soil Temperature Data from the NC State Climate Office
RDUWSWRAH - Winter Storm Watch/Warning/Advisory
RDUZFPRAH - Zone Forecast Products


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NWS RAH Web Camera Loop

Click on the image below to load a loop of web camera imagery from NWS Raleigh during this winter storm. The imagery is looking toward the north from the third floor of the Research III building on N.C. State's Centennial Campus. The loop includes imagery from 937Z (437 AM EDT) through 1429Z (929 AM EDT). The images are at approximately 5 minute intervals.

The specific still image below is from 1202Z (702 AM EDT) which is around the time of the heaviest snow.

 Click on the image below to load a loop of web camera imagery from NWS Raleigh - Click to enlarge



Selected Photographs of the Winter Weather Event

Photos courtesy of Jonathan Blaes and John Hamilton.
(Click the image to enlarge.)


Low cloud deck associated with the cold front that moved across Raleigh on Tuesday, January 16 - Click to enlarge           Light snow falling across North Raleigh before daybreak - Click to enlarge           Light snow falling across North Raleigh before daybreak  - Click to enlarge

Light snow falling across North Raleigh around daybreak  - Click to enlarge           A light snow accumulation can be seen across N.C. State's Centennial Campus - Click to enlarge           A light snow accumulation can be seen across N.C. State's Centennial Campus - Click to enlarge

A light snow accumulation can be seen across N.C. State's Centennial Campus - Click to enlarge                 A light snow accumulation can be seen across N.C. State's Centennial Campus - Click to enlarge                 A light snow accumulation can be seen across N.C. State's Centennial Campus - Click to enlarge

Photo of snow accumulation on Business 85 near the border of Randolph & Guilford Counties, photo courtesy of John Hamilton - Click to enlarge           Photo of snow accumulation on a bridge over Business 85 near the border of Randolph & Guilford Counties, photo courtesy of John Hamilton - Click to enlarge           Photo of snow accumulation near the border of Randolph & Guilford Counties, photo courtesy of John Hamilton - Click to enlarge


Final Thoughts

Forecasters were able to use several new tools during this event...

    The Micro Rain Radar (MRR) provided a tremendous amount of observational data and insight concerning the precipitation type and intensity during the event.

    Traditional datasets such as 6 or 12 hourly RAOB's, conventional WSR-88D radar, and surface observations can provide insight into the precipitation type during an event but there are limitations to their utility. The high degree of temporal and vertical resolution of the MRR provides forecasters with an opportunity to dig deeper and understand the processes affecting precipitation type. This new tool allows forecasters to monitor smaller features that are often unnoticed but can have a tremendous impact on the resulting precipitation type at the surface. The observation of physical processes occurring aloft provides forecasters an opportunity to provide improved short term forecasting and now casting. With the ability to observe the physical processes aloft, forecasters should have a better opportunity to evaluate the evolution of the event as it relates to model guidance and the forecasts. In this event, despite the fact that the snow was falling rather heavily and accumulating, forecasters could observe the warming of the mid levels of the atmosphere and they could remain confident that the change over from snow would occur shortly. It is this added information and confidence that can keep a forecaster from making a erroneous adjustment to the forecast.

    AMDAR aircraft soundings from CLT, GSO, and RDU at around 40 minute intervals between 1030Z and 1200Z proved invaluable in showing warming aloft (with a southwesterly 50 knot jet near 850 MB at CLT). This confirmed the trend toward rain/freezing rain early that morning which gave forecasters confidence in the going forecast and kept forecasters from over-reacting to the sudden onset of snow. The AMDAR aircraft data was mentioned in the 1033 AM AFD.

    Forecasters used web cameras, primarily from NC Department of Transportation, to monitor conditions across portions of central North Carolina, especially across the Triad and Triangle regions.

    Model thickness values plotted on the p-type nomogram worked very well and assisted the forecaster in making the P-type forecast. Forecasters also utilized the surface wet bulb temperatures to discern rain versus freezing rain potential.

Other items to note...
    None of the models indicated measurable precipitation in the northern half of the RAH CWA before 12Z/18. In situations that feature a very strong (160-180kt in this case) west to east jet axis overhead or just to our north, sudden changes in cloudiness and precipitation patterns and intensity are possible. With a straight jet, greatest lift is at the front end and sometimes well ahead of the wind speed maximum (as seen at 300-200MB). Also, the satellite and radar patterns seem to suggest a possible standing wave “bump” in the Piedmont that could have aided in the development and maintenance of precipitation over a focused area. In such situations, some allowance is advised for the chance of a quicker and more intense onset of precipitation than model indications.

    Partial thickness values and patterns were very similar between the GFS and NAM and worked very well using the nomogram in defining the precipitation type.


Case Study Team

Michael Strickler
Brandon Vincent
Rod Gonski
Phillip Badgett
Jonathan Blaes


For questions regarding the web site, please contact Jonathan Blaes.


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