January 18, 2007 Winter Storm|
...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...
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.
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
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
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