A Case Study of the 13-14 February 2007 Extended Sleet Event
Potter, Matthew S.; Selmer, Patrick A.; Clark, Richard D.
Department of Earth Sciences
Millersville University of Pennsylvania
Abstract
Methods
Conclusions
A major winter storm affected much of the Mid-Atlantic and
Northeastern United States on 13-14 February 2007. For an
extended and nearly uninterrupted period of 15 hours
between approximately 23 UTC 13 FEB and 16 UTC 14
FEB, precipitation in the form of sleet (ice pellets) fell over
much of south central Pennsylvania, northern Maryland, and
parts of New Jersey. This research investigated the factors
contributing to this unusually long precipitation event. The
influence of a persistent easterly low-level jet, which helped
to maintain a consistent pool of cold air in the near-surface
layer, was an important factor in maintaining precipitation in
the form of sleet. Temperature and humidity profiles from the
NAM, RUC, and GFS were analyzed to determine the
mechanism by which this extended sleet was sustained.
Bourgouin’s method of determining precipitation types was
implemented in the model soundings to show that sleet was
favorable during the times of interest.
Using Bourgouin’s Method of determining precipitation type, various modeled sounding data was used to determine if sleet was the
most favorable type of precipitation to be occurring during the storm at various point (station) locations and times (with the exception
of the GFS, which had a time step of three hours, the models used time steps of one hour). To validate this information that was
gleaned from the modeled data, surface observations, METAR and CoCoRaHS reports, were used. Radar data was used to indicate
how long and when it was precipitating at the various station locations. A computer program was written and used to graphically show
and calculate some useful characteristics of the wind in the vertical at each of the different point locations. BUFKIT was used to look
at various meteorological parameters in the modeled sounding data such as relative humidity (subsaturated regions in the warm
layer), wind direction, temperature, and others. Upper air and surface analysis maps were used to look at the synoptic features and
progression of the storm.
The locations of highest total amounts of sleet, as suggested
by the model data, are KMDT (Harrisburg, PA), KHGR
(Hagerstown, MD), KMRB (Martinsburg, WV), KRDG
(Reading, PA), KABE (Allentown, PA), and W54 (Westminster,
MD). These locations matched well with historic known areas
to receive the most amount of sleet. It was difficult to validate
the times at which sleet fell because the spatial and temporal
coverage was not as good as we hoped that it would be for
the CoCoRaHS and METAR reports. The relative humidity
profiles showed no subsaturation in the warm layer;
subsaturation would aid in the maintenance of sleet as
described by Hanesiak and Stewart [2]. This could have been
attributed to the lack of spatial resolution of the data in the
BUFKIT program. Warm air advecting in from the mid-levels
provided a consistent warm layer (warmer than 0°C) for
precipitation to fall as liquid, before refreezing in the boundary
layer. The most important factor in this extended sleet event
was a strong, low-level easterly jet. This jet, which was
ageostrophic in nature (see figure below and to the right) was
important in the maintenance of a cold, moist boundary layer
conducive for precipitation in the form of sleet. As the
reconstituted low pressure deepened and moved up the coast
of New Jersey the low-level jet broke down and winds at all
levels shifted to northerly, ending the extended period of sleet
and changing the remaining precipitation to snow. This
changeover of precipitation type (according to Bourgouin’s
Method of determining precipitation type [1]) corresponding to
the change in winds is generally observed in the model data
and is consistent with the actual timing of the low off the coast
of New Jersey.
A map showing the point locations of modeled data used in the investigation (left side) and example of one particular model (RUC) sounding at a particular time (6 Z
Feb 14th) for Reading, PA during the event (right side)
Overview
20
18
14
12
10
GFS
NAM
8
Eta
6
4
2
Number of total hours favorable for sleet for the 0 Z 14 February
2007 run of three different forecast models according to
Bourgouin’s Method for determining precipitation type [1]. The
Stations are oriented geographically from Northwest to Southeast.
KMIV
RUTG
KEWR
KPHL
KPNE
KTTN
KMMU
KILG
KMTN
KABE
KRDG
W54
KMDT
KMRB
KAVP
KHGR
0
KIPT
Average resultant (vector wind) wind magnitude as a function of
pressure for model levels in the 0 Z 14 February NAM model run for
KMDT (Harrisburg, PA) whose direction (according to the
meteorological coordinate system) was between 0º and 100º. The
labels on the top right-hand corner of the graph are: the maximum
resultant wind magnitude included in the calculation of the average
resultant wind calculations and the pressure level at which it
occurred, and the time range (inclusive) where wind directions were
between 0º and 100º. This is the whole graph; above where the line
stops there were no wind vectors with directions between 0º and
100º. The low-level maximum seen in this graph was pretty indicative
of all other models at all other stations.
16
KUNV
Hours favorable for sleet
A strong anticyclone (1037 mb) was centered over eastern
Ontario providing low-level cold air in the Northeast United
States on February 13. During the day a low pressure center
(1000 mb) was approaching the Appalachian Mountains.
Starting out of snow, warm southerly flow at the mid-levels
changed precipitation to sleet and freezing rain in the
southern part of our focus area by evening. During the
nighttime hours, all parts would change to sleet and freezing
rain. By 09z February 14, all of the focus area was
experiencing heavy sleet/freezing rain, and southeastern
sections were changing over to rain. At this stage of the event
the Appalachian low had since reconstituted itself over the
Carolinas and moved over the Delmarva with a pressure of
993 mb. Heavy sleet would continue into the morning hours of
the 14th before finally ending as a quick period of snow as the
wind direction became north-northwesterly at all levels. In the
end, a widespread area of 2-4 inches of sleet fell in northern
Maryland, south-central Pennsylvania, and parts of New
Jersey .
Hours Favorable for Sleet by Station, 0 Z February 14th
Model Runs
KAOO
NEXRAD Radar composite image from 8 Z 14 February 2007 [3].
Results
Photograph of accumulated sleet
and snow near Trenton, NJ (KTTN
on map above and to the left)
0 Z 14th FEB 925 mb geopotential
height and temperature analysis
overlaid with arrows indicating the
primary wind direction at 925 mb
and 700mb over our focus area
References
[1] Bourgouin, Pierre. "A Method to Determine Precipitation Types."
Weather and Forecasting 15 (2000): 583-92.
[2] Stewart, Ronald E., and John M. Hansiak. "The Mesoscale and Microscale
Structure of a Severe Ice Pellet Storm." Weather Monthly Review 123
(1995): 3144-162.
[3] Martin, Raymond Jr. C. "Snow and ice storm, February 13-14, 2007 - Satellite
Imagery." New Jersey Expressways and Tollways. Web. 07 Jan. 2010.
<http://www.njfreeways.com/weather/2007/14-Feb-07-SatelliteImagery.
html>.