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Synoptic
Overview –
A deep
500 mb trough moved east from the Central United States with its axis
extending from the Ohio Valley to the Central Gulf Coast by early Saturday
morning January 22 (Figure 3). Strong shortwave energy rounding the base of
this trough led to cyclogenesis off the East Coast of Florida by 12Z
Saturday morning. (Figure 4).
The low then quickly moved northeast well off the North Carolina
coast. Light precipitation moved onshore over Carteret County and the
southern Outer Banks by 13Z (9 am) Saturday morning (Figure 5). The 12Z
Newport/Morehead City sounding (Figure 6) was dry initially above the surface,
but the atmosphere quickly saturated as light precipitation moved onshore
during the morning. As the low continued to deepen, widespread snow
overspread the southern coastal sections, eastern sound counties, and
southern Outer Banks by 18Z (1 pm) with snowfall rates of 1 to 2 inches per
hour observed (Figure 7).
Figure 3. 500 mb Heights/Vorticity
12Z (7 am) Saturday January 22, 2011.
Figure 4. A 1020 mb high pressure
center to the north provided cold air for Eastern North Carolina as low
pressure formed off the northeast coast of Florida and moved well off the
North Carolina coast.
Figure 5. Light snowfall approaches
the southern North Carolina coast at 13Z (9 am) Saturday morning January
22, 2011.
Figure 6. The 12Z Newport/Morehead
City MHX Sounding shows plentiful dry air above 950 mb, but the atmosphere
rapidly moistened as light precipitation moved onshore.
Figure
7. At 18Z (1 pm) Saturday January 22, 2011, low pressure well east of
Charleston, SC is strengthening with widespread snow covering much of
coastal Eastern North Carolina.
Frontogenesis
is a very important and common atmospheric process that refers to a change
in the magnitude and orientation of a thermal (temperature) gradient at a
level or in a layer due to horizontal changes in the total wind. This
change alters thermal wind balance, which forces a vertical motion response
in the atmosphere which can result in mesoscale bands of enhanced snowfall.
The 850-700 mb Frontogenesis forecast at 18Z Saturday is illustrated below
(Figure 8). Note the enhanced axis of frontogenesis near the coast where
higher values of snow were observed. Heavy snow banding was observed on
radar around 3 pm or 20Z (Figure 9).
Figure 8. 850-700 mb Frontogenesis
magnitude valid 18Z ( 1pm) Saturday January 22,
2011.
Figure
9. Radar Reflectivity from the Newport/Morehead City (MHX) radar at 1940Z
Saturday January 22, 2011 showing heavy snow bands near the coast.
The
precipitation ended over the central Outer Banks by early evening as the
low accelerated to the east-northeast away from the region. Very cold
temperatures followed Saturday night as skies cleared over the fresh snow.
The minimum temperature at the Newport/Morehead City Forecast Office
dropped to 9 degrees Sunday morning, the second coldest low ever recorded
at the site since the station moved from Cape Hatteras in 1994.
Models/Forecasts
Forecast challenges for this system included:
1.
This event developed offshore and affected only the
Newport/Morehead City (MHX) CWA (County Warning Area).
2.
There were no upstream surface observations and radar
data to observe, as the system affect only near-coastal areas.
3.
Snowfall arrived faster than predicted by models and
heavy snowfall developed rather quickly. Determining the best model QPF
forecasts were difficult given model inconsistencies.
With
regards to the position and strength of the surface low pressure, the
models were fairly similar, as the 00Z Saturday January 22th run of both
the GFS (Figure 10) and NAM12 (Figure 11) both showed the low well east of
the South Carolina coast, with the NAM12 being a bit stronger. The GFS
verified better as of 18Z Saturday showing a 1000 mb low center, with a
1001 mb center observed well east of South Carolina.
Figure 10. GFS model PMSL forecast
verifying at 18Z (1 pm) Saturday January 22, 2011.
Figure
11. NAM12 PMSL forecast verifying at 18Z (1 pm) Saturday January 22, 2011.
Despite
the relatively similar forecasts in the position and strength of the
surface low, inconsistency in the model QPF/snow amount forecast developed.
For example, the 00Z Saturday run of the GFS a band of 2 to 4 inch snow
amounts along the southern North Carolina coast (Figure 12), while the 06Z
NAM12 forecast, issued 6 hours later, showed little snow in the same area
(Figure 13).
Figure 12. The 00Z GFS forecast of
12 hour snowfall amounts ending at 00Z (7 pm) Saturday night.
Figure
13. The 06Z (1 am) Saturday NAM12 model verifying at 21Z (5 pm) Saturday afternoon
shows almost no snow over Carteret, Craven or Pamlico counties, but did
show 1 to 2 inches near Cape Hatteras.
The GFS
snowfall amounts verified better, better showing the axis of higher
snowfall amounts near the coast. The ECMWF was similar to the GFS with low
strength and placement, and showing at least light snow accumulation along
the coastal counties. Additionally, the Hydrometeorological
Prediction Center (HPC) predicted a swath of 1 to 2 inches of snow right
along the immediate coast with no snowfall inland. These model differences
led to uncertainties in forecasting the amount and placement of heavier
snowfall.
Advisories/Warnings
The
initial Winter Weather Advisory was issued at 343 PM Friday for Carteret
County and the Outer Banks counties of Hyde and Dare, for 1 to locally 2
inches of snow. The advisory was from 1 PM until 10 PM Saturday. At 3:37 AM
Saturday, with precipitation already developing off the South Carolina
coast, the beginning of the Advisory was moved up to 10 AM,
and Craven and Onslow counties were added. Snow was observed over Carteret
County beginning just before 9 AM Saturday morning. At 12:17 PM, Mainland
Hyde, Pamlico and Beaufort Counties were added to the Winter Weather
Advisory. The Advisory was upgraded to a Winter Storm Warning for Carteret,
Craven, Pamlico and Mainland Hyde counties at 1:43 pm as bands of heavy
snow continue to move across the coastal and adjacent counties. As the
heavier snow moved toward the Outer Banks, the advisory for Outer Banks
Hyde and Dare counties was upgraded to a Winter Storm Warning through 10
PM. As the snowfall ended over Onslow County, the advisory was cancelled
there at 5:46 PM. Shortly thereafter, at 6:03 PM, Warnings were cancelled
for Craven, Pamlico and Beaufort counties. As the snow ended from south to
north, the Winter Storm Warning for Mainland Hyde and Carteret counties was
cancelled at 7:29 PM, and then was cancelled for Outer Banks Hyde county at
9:18 PM, before expiring at 10 PM for Outer Banks Dare county. Numerous Local
Storm Reports (LSR’s) and Public Information Statements (PNS’s)
were issued throughout the course of the day to provide the latest snowfall
amount information.
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