Download 5-6 January 2005 Ice Storm

5-6 January 2005 Ice Storm
by
Richard H. Grumm
National Weather Service Office
State College PA 16803
1. INTRODUCTION
The first week of January 2005 started
with unseasonably warm weather over
most of the eastern United States. The
conditions were not those that often
precede major ice storms. However, a
strong upper-level system moving out of
the southwest, a cold air mass to the
north with a modestly strong surface
anticyclone set up conditions conducive
for an ice storm. This weather system
would bring snow to the central Rockies
and northern plains as it moved eastward.
South of the snow, a band of ice would
extend from portions of Missouri
eastward to Pennsylvania and southern
New York State.
Across central Pennsylvania this would
be the largest ice storm in recent
memory. Ice accumulations of 0.25 to
over 1 inch would topple trees or cause
limbs and branches to fall on Wednesday
and Thursday the 5th and 6th of January
2005. This resulted in road closings and
power outages over many counties in
central Pennsylvania. Power outages in
hardest hit areas led to school closings
and even one day after the storm, single
schools in several school districts
remained closed due to the lack of power.
The ice affected the higher elevations of
the State down to the Maryland border.
This led to power problems along the
ridge tops as far south as Dauphin
County, despite that fact that at the
airports in the valleys, the surface
temperatures never reached 0C (32F).
Farther north, ice affected all elevations
but higher elevations were particularly
hard hit owing to the larger ice accretion
rates. Along the New York border, snow,
and areas of heavy snow were observed
with at least one report of 25cm (10
inches) of snow.
Due to the warm antecedent conditions,
the ice did not have a big impact on
surface transportation. Preceded by 3
days of much above normal
temperatures, roads and walk-ways were
relatively warm. The cold air filtered
into Pennsylvania as the rain began. In
most locations were ice was observe the
surface temperatures ranged from -1 to
0C (30-32C). This favored icing on trees
and grassy surfaces. Most road closures
were due to trees and tree limbs falling
and blocking roads.
This ice storm was particularly well
forecast by both the National Centers for
Environmental Predictions (NCEP)
Medium-Range Ensemble Forecast
(MREF) system and the NCEP ShortRange Ensemble Forecast (SREF). The
2m and 850 hPa 0C temperature
spaghetti plots and probability plots
were very effective in forecasting and
warning on this weather event.
This paper will serve to document the
ice storm of 5-6 January 2005. The focus
will be on the use of Ensemble
Prediction System (EPS) outputted data
in forecasting the event.
Figure 2 As in Figure 1 except 00-hour forecasts valid at
Figure 1 MREF 00-hour forecast of 850 hPa tempetures valid at 1200 1200 UTC 4 January 2005.
UTC 2 January 2005 showing 850 hPa temperatures and forecast
including spaghetti
departures from normal in standard deviations from normal.
plots and
probability plots were used.
In addition to the temperature of the
boundary layer, it is critical to have an
estimate as to whether the precipitation
will be in the form of liquid or solid
upon entering the boundary layer. For
this approximation, the 850 hPa 0C
isotherm was used.
The SREF doe implicitly produce rain,
ice pellets, snow, and freezing rain data.
At this time these data are not shown.
Figure 3 As in Figure 2 except MREF MSLP
forecasts and anomalies.
2. METHODS
All data were available in real-time via
the PSU-NWS data feed. All EPS output
images shown here were replicated using
the operationally available data and the
operationally deployed software. For ice
storms, determining the temperature of
the boundary layer is critical to
determine if the boundary will be cold
enough to cause rain to freeze. For this
purpose, EPS 2m temperature forecasts,
Some fields were displayed showing the
mean of the EPS forecasts for that
variable verse the 30-year climatology.
In these instances, the departures are
shown as standardized anomalies in
standard deviations (SDs) from the 30year normal. The reproductions
software used the same data and scripts
used in real-time. All real-time graphics
changes are based on improvements
made during the case study process.
Figure 2 shows the evolution of the 850
hPa temperature field over the United
States as of 1200 UTC 4 January 2005.
The 850 hPa temperatures are slowly
winter storm from the southwest
mountains to the East Coast.
3. RESULTS
i.
antecedent conditions
Figure 1 shows the 850 hPa
temperatures and the anomalies
observed in the MREF system on 2
January 2005. These data show a wide
area of above normal 850 hPa
temperatures over the eastern United
States and adjacent Canada at this
time. Largest temperature anomaly
(+2SD) was over the eastern Great
Lakes at this time.
Figure 4 As in Figure 3 except showing 850 hPa winds
with a) U-wind anomalies and b) V-wind anomalies.
returning toward normal in the
northeastern United States. Slightly
below normal 850 hPa temperatures
were over the northern plains. The data
show an extensive quasi east-west
baroclinic zone spanning from the
Rocky Mountains to the east coast. On
the south side of this border, above
normal air was present at 850 hPa. The
baroclinic zone was serving as the
boundary between the cold and warm air
and along this boundary and north of this
boundary heavy rains, ice, and heavy
snow were observed.
Figure 3 shows the massive surface
anticyclone in the northern plains and
the Pacific storm over California and
Nevada. These two systems would move
eastward together producing the major
Figure 4 shows the resulting 850 hPa
winds with the U and V wind
anomalies. Not surprisingly, strong Uwinds were observed over the
northern plains and along the eastern
slopes of the Rocky Mountains
between the two pressure systems.
The 850 hPa U-winds were over 4SDS below normal in Wyoming.
Ii MREF forecasts
The 850 hPa temperature forecasts from
the MREF initialized at 1200 UTC 2
January valid at 0000 UTC 06 January
2005 is shown in Figure 5. This time
was selected because it was about the
mid-point of the forecast precipitation
event. Note the 850 hPa 0C line, in the
mean, was over north-central
Pennsylvania. The 850 hPa temperatures
were forecast to be above 0C over the
southern half of the state. Though not
shown, most of northern Pennsylvania
had a 80-90% chance of being at or
below 0C at this time (see appendix).
Figure 6 MREF forecasts of 2m temperatures
initialized at 1200 UTC 2 January 2005 valid at
0000 UTC 6 January 2005.
The corresponding 2m temperatures
valid at the same time are shown in
Figure 6. These data show the low-level
cold air was forecast to penetrate south
of the Pennsylvania-Maryland border by
this time. At the 80% level, the surface
was forecast to be cold enough for
freezing precipitation as far south as
central Pennsylvania with the 40%
contour penetrating northern Maryland.
This implied low-level cold air with
warm air aloft, Figure 5, suggested a
potential ice storm. There was only a
40% chance that 850 hPa temperatures
would be at or below freezing at 850 hPa
from south-central Pennsylvania into
Maryland (Fig. A1).
Figure 5 MREF forecasts of 850 hPa temperatures initialized
at 1200 UTC 2 January 2005 and valid at 0000 UTC 06
January 2005. Upper panels shows 0C isotherm forecasts from
each ensemble member. Lower panel shows the consensus
forecast and the departure of this field in standard deviations
from normal.
Model precipitation forecasts showed a
high probability of 0.5 to 1.0 inches of
precipitation (not shown). Forecasts
from 0000 and 0600 UTC 2 January
were quite similar to these forecasts and
are not shown.
MREF forecasts maintained this
baroclinic zone and the cold air
progressing farther south than the cold
air at 850 hPa with successive forecasts.
For brevity these forecasts are not shown.
iii.
SREF Forecasts
Figure 7-9 show select SREF forecasts
initialized at 2100 UTC 3 January 2005.
Figure 7 shows the 24-hour probability
of 12.5 mm (0.5 inches) of QPF for the
24-hour period ending at 1200 UTC 06
January and the spaghetti plots and
spread. These data show the SREF was
forecasting a large area of 1.0 and
greater rainfall over Pennsylvania. Most
of the State was in a high risk for
receiving 12.5 mm or more QPF.
Combined with the forecasts of freezing
to sub-freezing temperatures, near the
surface, in some EPS members down to
the Maryland border with the 850 hPa
mean isotherm farther north, suggesting
a strong ice storm potential.
Figure 7 SREF forecast of 12.5 mm(5 inches)
of precipitation initialized at 2100 UTC 3
January 2005 for the 24-hour period ending
1200 UTC 06 January 2005. Upper panel shows
the probability and the consensus position of the
12.5 mm contour. Lower panel shows the
consensus QPF and the individual ensemble
members position of the 12.5 mm contour.
The SREF forecasts are of higher
horizontal resolution than the MREF.
However, the forecasts only go out to 63
hours, limiting the use of these forecasts
to periods of 0-2.5 days ahead of an
event. This finer resolution forecasts are
ideally suited for watch and warning
products, when the model is performing
well. Forecasts from the 3rd and 4th are
examined from the watch and warning
perspective.
iv. SREF forecast 2100 UTC 3 January
2005.
The heavy precipitation and the thermal
profiles implied suggest the SREF
provided at least a 2.5 day lead-time on a
potentially large ice storm. Though not
shown, the SREFs forecast the 2m 0C
isotherm to work southward as the
precipitation moved in. The 0C isotherm
was forecast to be over north-central
Pennsylvania in most EPS members by
1200 UTC 5 January reaching its farthest
extent to the south between 0000 and
1200 UTC on the 6th.
v. SREF forecast 0900 UTC 4 January
2005.
SREF forecast initialized at 0900 UTC
on the 4th continued to show the strong
ice storm threat. These products were
critical in the warning process as the
forecasts for the initial ice accretion
potential was within 24 hours of the
availability of these forecasts. Due to the
critical timing, more data will be shown
for this time period.
The anticipated QPF for the period
ending at 1200 UTC 6 January 2005 is
Figure 8 As in Figure 7 except SREF forecasts of
0C 2m isotherm showing spaghetti and spread and
probabilities.
shown in Figure 10. These data show a
high probability of 12.5 mm (0.5 inch)
of precipitation. The consensus forecasts
shows most of Pennsylvania would
received more than 14mm (0.60) with a
wide area of 25 mm (1inch) over
western portions of the State.
The 2m and 850 hPa temperatures
during the precipitation period are
shown in Figures 11 and 12 respectively.
Shortly after the onset time, the 2 m
temperatures were forecast to be near 0C
over central Pennsylvania. Few members
showed the 2m temperature contour
pushing into southern portions of the
State.
The forecast valid at 0000 UTC 6
January, about the mid-point in the 24hour QPF forecasts shown in Figure 10,
the 2m 0C line was forecast to be close
to the Maryland border (Fig. 13) with an
area of disagreement over the southern
sections of Pennsylvania. The 850 hPa
temperatures were forecast to be around
0C over central portions of the State (Fig.
12).
Figure 9 As in Figure 8 except 850 hPa 0C forecasts valid
0000 UTC 06 January 2005.
Figures 12 and 13 show there was
disagreement and the solutions clustered
toward two solutions. Several members
clustered toward a colder solution with
the 0C line at 2m extending to or south
of the Pennsylvania border. Other
members clustered in north-central
Pennsylvania (Fig. 13). There was
similar clustering with the 850 hPa
temperatures (Fig. 12). The probability
density function (PDF) shown here
implied the solutions were not normal,
but there existed at the very least, a
bimodal solution. The warm air over the
cold air implied an ice storm from
central Pennsylvania southward into
northern Maryland. However, the
differences implied a difficult forecast
over the region with a large degree of
uncertainty.
Figures 14 and 15 show the MSP and
850 hPa forecast respectively. These
data show a weak anticyclone to the
north and an area of below normal
pressure to the south. At 850 hPa, the
850 hPa temperatures were forecast be
Figure 10 As in Figure 7 except SREF forecast of
12.5 mm(5 inches) for forecasts initialized at 0900
UTC 4 January 2005. position of the 12.5 mm
contour.
above normal. Though not shown, the
precipitable water values along and
south of the baroclinic zone were
forecast to be 2-3 standard deviations
above normal in the deterministic
models and the MREF (not shown).
vi. SREF forecast 2100 UTC 4 January
2005.
The 2m and 850 hPa temperature
forecasts valid at 0000 UTC 6 January
from forecasts initialized at 2100 UTC 4
January 2005 are shown in Figures 16
and 17. These data show continued
disagreement with the southern extent of
the 2m 0C contour. The forecasts
Figure 11 SREF forecasts initialized at 0900 UTC 4
January 2005 valid at 1200 UTC 5 January showing the
spaghetti plots (upper) with variance and the probabilities
(lower) with the consensus position of the -20,0 and +20C
contour.
suggest a high probability that any
precipitation over central and southcentral Pennsylvania would result in
mixed precipitation or freezing
precipitation. Due to the bi-modal
solutions presented, there was a larger
degree of uncertainty displayed by these
forecasts.
The 24-hour precipitation forecasts are
shown in Figure 18 for the period ending
at 1200 UTC 6 January 2005. These data
show an increase in the probability of
12.5 mm (0.5 inches) of precipitation
over the region and an increase in the
overall amount of precipitation.
Virtually the entire state was forecast to
Figure 12 As in Figure 11 except valid at 0000 UTC 6
January showing 850 hPa forecasts.
see at least 12.5mm of precipitation and
the consensus 25mm (1 inch) contour
now covered most of the state with the
37.5 mm (1.5 inches) contour covering
the westernmost portions of the state.
These forecasts suggested heavy snow
along the New York border and a major
ice storm over portions of central
Pennsylvania.
4. CONCLUSIONS
An ice storm struck central Pennsylvania
on the 5 and 6th of January 2005. This
ice storm was relatively well forecast by
both NCEP EPS including the MREF
system and the SREF system. These
good forecasts helped produced useful
Figure 13 As in Figure 11 except valid at 0000 UTC
06 January 2005.
watches and warnings for the potential
ice accretion problems.
The heaviest ice was observed along the
ridge tops from near the Maryland
border northward. Photographic
evidence showed ice accumulations in
excess of 1 inch on trees, tree branches,
and grass in the higher elevations of
west central Pennsylvania. This storm
was quite devastating in central
Pennsylvania and was the most recent
significant ice storm since the Janaury
1994 ice storm.
Prior to the ice storm, conditions were
relatively warm over the region. These
pre-existing warm conditions likely
contributed to the lack of icing on major
Figure 14 SREF forecasts initialized at 0900 UTC 04
January 2005 valid at 0000 UTC 06 January 2005
showing MSLP forecasts. Upper panels shows
spaghetti plots and spread, lower panel shows
consensus forecasts and departures from normal in
standard deviations from normal.
Figure 15 As in Figure 14 except showing the 850 hPa
forecasts valid at 0000 UTC 06 January 2005.
roads. Most of the damage was due to
the accretion of ice on trees, wires, and
other objects. Falling trees and tree
limbs produced most of the damage and
led to most of the observed power
outages.
The warm air over the cold air implied a
potential ice storm from central
Pennsylvania southward into extreme
northern Maryland. The probability
density function (PDF) shown here
implied the solutions were not normal,
but there existed at the very least, a
bimodal solution. The colder, more
southern solution was a lower
probability outcome solution as seen in
the 0C 2m temperature spaghetti and
probability charts. The probability of ice
down to the Maryland border was only a
20% outcome, but the 60% outcome was
only a few 10s of kilometers to the north.
Due to the potential scope of the icing
problem, extending warnings into the
low probability areas in the 40-60%
ranges was a prudent idea.
Figure 16 As in Figure 12 except valid initialized at
2100 UTC 4 January 2005.
When forecasting potential severe
weather events, it is important to
consider the impact if the public is not
notified and a low-end event occurs. As
an example, there it was believed that to
an 80% probability, that Sumatra would
block a tsunami headed for Thailand.
Thai meteorological institute came up
with a 20% probability that the wave
would not be blocked by Sumatra. With
a low probability (20%) outcome of such
an event, no actions were taken. would
Figure 17 As in Figure 13 except valid initialized at
2100 UTC 4 January 2005.
be blocked. In general terms “The more
severe the event, the more it is generally
necessary to warn in terms of risk and
probability”.(quote Ken Mylne, UKMO).
5. ACKNLOWEDGEMENTS
WMO for information on ensembles,
risks, and tsunami information.
Specifically Persson Anders and Keny
Mylne.
6. REFERENCES
Figure A1 MREF forecasts of 850
temperatures initialized at 1200 UTC 2
January 2005 valid at 0000 UTC 6
January 2005.
Figure 18 As in Figure 10 except SREF forecast of 12.5 mm (5
inches) for forecasts initialized at 2100 UTC 4 January 2005.