Ensemble Forecasts of the Blizzard of 22-23 January 2005
By
Richard H. Grumm
National Weather Service
State College Pennsylvania
1. Introduction
A fast moving upper-level disturbance
produced a wide area of snow, and heavy
snow from the Great Lakes to the
northeastern United States from 21 to 23
January 2005. This storm has become
known as the blizzard of 2005 snowfall
produced by this storm over the 3 day period
is shown in Figure 1. Though hard to infer
from the image, Areas of intense snowfall, in
excess of 10 inches (25 cm) were observed
west of Lake Michigan in Illinois, in southern
Michigan and south of Lake Ontario in New
York. Very heavy snowfall, with over 20
inches (50 cm) and in some instances 30
inches (75 cm) was observed in southern
New England. This heavy snowfall set the
city of Boston up for its snowiest January on
record.
Though there were several medium-range
forecasts of this system 4-7 days in
advance, the exact details of the snowfall
areas remained elusive. Most forecast
systems showed a clipper-like low which
had the potential to produce heavy snow
from the central Appalachians to the East
Coast. Within 4 days of the event, some
forecasts of extremely heavy snow were
indicated. Clipper-type lows are not normally
associated with extremely heavy snowfalls
in the eastern United States. This storm was
forecast to be an exception to this general
rule.
Though the medium and short-range models
did forecast a significant East Coast snow
storm, they missed the details. This storm
showed many errors typical of recent large
snow storms along the East Coast. As in the
Presidents Day Weekend storm of 16-17
February 2003 (Grumm 2003), the storm
was initially not forecast to affect New
England with significant snow. This storm
shared other characteristics with previous
storms in failing to produce significant snow
far enough northward. Whether these
apparent problems reflect systematic errors
or represent anecdotal evidence is
uncertain. It is clear that several large East
Coast storms in recent years were initially
forecast not to affect the regions farther
which were ultimately affected by the storm.
This storm was a relative success for both
the deterministic and ensemble forecast
systems (EPS) in that they correctly forecast
a significant winter storm; they correctly
forecast the meso- features associated
with this storm, and the correctly forecast
the general timing of the event. This event
also was a failure in that the areas of heavy
snow in New York and New England were
poorly forecast with any significant lead
times. It is beyond the scope of this paper to
determine if this was an initial conditions
issue or is related to model physics issues.
This paper will serve to document the
Blizzard of 2005. It will also demonstrate the
strengths and the weaknesses of the current
National Centers for Environmental
Predictions (NCEP) Medium- Range
Ensemble Forecast (MREF) system and the
Short-Range Ensemble Forecast (SREF)
system. The emphasis is on how these two
systems correctly forecast a significant
snowstorm, but failed to capture and
correctly depict the details of the heavy
snow areas.
2. Method
All SREF and SREF data were archived in
real-time. All images were recreated after
the event using GrADS (Doty and Kinter
1992). Standard displays of spaghetti and
spread are shown. The spread is normally
shown with gray shading. The spaghetti
plots show distinct contours for each
parameter. Lower panels for some fields
show the ensemble mean with color shading
showing the ensemble mean in standard
deviations from normal, referred to as
standardized anomalies (Grumm and Hart
2001;Hart and Grumm 2001).
Precipitation displays show the critical
thresholds over a time period, normally 24
hours and 0.50 inches of quantitative
precipitation forecast (QPF). Upper panels
normally show the probabilities and the
consensus of the specified thresholds.
Lower panels show the position of the
specified threshold for each EPS member
and shading shows the ensemble mean
QPF.
The spaghetti plots follow the standard
ensemble concepts outlined by Sivillio et al
(1997). This paper is a good reference point
Figure 1 Large view of the snow fall observed during the blizzard of January 2005. Snowfall shows totals in inches from 20-23
January 2005. Lower panel has zoomed in view of southern New England and southern New York.
Figure 2 MREF forecasts initialized at 0000 UTC 18 January 2005 showing a) MSLP (hPa) forecasts valid at 1200 UTC
Sunday 23 January 2005 and b) 24-hour precipitation for the threshold contour of 0.50 inches of QPF. Pressure shows
spaghetti plots and spread in the upper panel. The lower panel shows the ensemble mean and the departure of this field in
standardized anomalies. The precipitation panel shows probability of exceeding 0.50 inches of QPF and the mean position of
the 0.50 inch contour. The lower panels shows the spaghetti plot of each EPS members forecast position of the 0.50 inch
contour and the EPS mean QPF for the 24 hours ending at 1200 UTC 23 January 2005. 
to those learning to use ensembles.
3. Results
a) MREF Forecasts
MREF forecasts as early as 16 January
2005 showed signals for a snow event along
the East Coast. However, the overall
structure showed a fast moving cyclone
affecting the Mid-West and the Mid-Atlantic
region. It was a slow evolution toward a
significant snow event and most forecasts
appeared to emphasized the Mid-Altantic
region as the focus for the heavy snowfall. A
few MREF images will be presented to show
the evolution of these forecasts.
a.1) 18 January Forecasts
Forecast from 0000 UTC 18 January 2005
are shown in Figure 2. These forecasts
show off the Carolina coast at 1200 UTC 23
January 2005 with most of the precipitation
(Figure 2 right) over the southeastern United
States. This run was too fare south with the
cyclone track from the upper-Mid West to
the coast. Though not shown, the CMC
forecasts at this time showed a higher threat
of snow farther north. Subsequent 0600 and
1200 UTC MREF runs began to converge
on a more northern storm track and a
northward shift in the precipitation shield
(not shown).
a..2) 19 January 2005 forecasts
Figure 3 shows the MREF forecasts
initialized at 0000 UTC 19 January 2005.
The panels on the left show the MSLP
Figure 3 MREF forecasts initialized at 0000 UTC 19 January 2005 showing a) MSLP (hPa) forecasts valid at 1200 UTC
Sunday 23 January 2005 and b) 24-hour precipitation for the threshold contour of 0.50 inches of QPF. Pressure shows
spaghetti plots and spread in the upper panel. The lower panel shows the ensemble mean and the departure of this field in
standardized anomalies. The precipitation panel shows probability of exceeding 0.50 inches of QPF and the mean position of
the 0.50 inch contour. The lower panels shows the spaghetti plot of each EPS members forecast position of the 0.50 inch
contour and the EPS mean QPF for the 24 hours ending at 1200 UTC 23 January 2005. 
forecasts (hPa) and the panels on the right
the QPF forecasts. Of interest is the cyclone
off the Mid-Atlantic coast. In the mean, this
cyclone was forecast to be -1.5SDs below
normal. This implies good agreement
between ensemble members. The spread,
in the upper panel is on the order of 2-4 hPa
with some dispersion in the position of the
surface cyclone. A strong anticyclone is
seen to the west. Though not shown, this
cyclone was forecast to track out of Canada,
across the Mid-West and to the Mid-Atlantic
coastal area.
The resulting QPF is shown in Figure
2(right). These data showed a high
probability of precipitation; forecasts
suggested cold air in place for snow, based
on 2m and 850 hPa temperatures; for the 24
hours ending at 1200 UTC 23 January 2005.
Note 1 member forecast the threat into
central New England, based on Figure 1,
suggesting that at least one member caught
the potential of snowfall farther north. The
focus at this time was for precipitation along
the coast. Early POP forecasts showed the
track of high threat running from Minnesota
to Maryland but are not shown.
These forecasts followed a trend which
began in the 0600 UTC 18 January 2005
MREF forecast cycles showing a farther
northward track and a significant threat for
snowfall in the Mid-Atlantic region. Figure 4
shows the QPF for times the 24-hour
periods ending at 0000 and 1800 UTC 23
January 2005. These two figures shows the
threat for precipitation as the wave moved
across the Mid West and the threat for snow
into southern New England for the forecasts
Figure 4. MREF forecasts initialized at 0000 UTC 19 January 2005 showing the probability of
exceeding 0.50 inches of QPF for the 24 hour periods ending at 0000 and 1800 UTC 23 January
2005.
ending at 18000 UTC 23 January 2005.
Again, note that at least one member
showed the threat for at least 0.50 inches of
QPF into central New England for the period
valid ending at 1800 UTC 23 January 2005.
Clearly, there was sufficient information to
alert the public of the threat for the potential
heavy snow as far north southern New
England to include the city of Boston.
a.3) 20 January 2005 forecasts
The 20 January MREF forecasts continued
to show the threat for a precipitation event,
mainly snow from the Mid-West to the East
Coast (Fig. 5). The POP and QPF is shown
for the 24 hour periods ending at 0000 and
1200 UTC 23 January. The earlier times
show the high threat for the Chicago area
and portions of Ohio. The latter forecasts
show the high threats for the Mid-Atlantic
region. It is interesting that 2 ensemble
members continued to show a threat farther
north into New England suggesting some
members were capturing the more northern
cyclone track and the more northern
precipitation shield with this system.
Figure 6 shows the MSLP and 850 hPa wind
forecasts valid at 1200 UTC 23 January
2005. The much below normal easterly
winds were a significant sign of a potentially
significant event. Grumm and Hart (2001)
showed the impact of much below normal
easterly winds with winter storms. Though
the QPF values were low, the signal with the
jet suggested the potential for a significantly
above normal event. Though not shown, the
single GFS runs showed the 850 hPa jet to
be as much as -5SDs below normal over the
region. Other fields showed similarly high
anomalies.
b) SREF forecasts
The QPF forecasts for the 24 hour periods
ending at 0000 and 1800 UTC 23 January
2005 from forecasts initialized at 0900 UTC
Figure 5 As in Figure 4 except for MREF forecasts initialized at 0000 UTC 20 January 2005 for
time 24 hour intervals ending at a) 0000 and b) 1200 UTC 23 January 2005.
21 January 2005 are shown in Figure 7.
These data show a high probability of 0.50
or greater QPF over the Chicago and
Washington DC areas on the 22nd. Later the
forecasts suggest a high probability of 0.50
or greater QPF for southern New York and
extreme southern New England. There is a
sharp cut-off of precipitation to the north.
Local maximum in the consensus forecasts
show some areas of over 1.0 inches of QPF
and several SREF member forecast over 1.0
inches of QPF (not shown). Based on
temperatures profiles, forecasts indicated
10:1 snow ratios along the southern edge of
the storm and 20:1 ratios in the colder air.
This the 0.50 area of QPF implied 10 inches
of snow over most areas. Snow ratios are
beyond the scope of this paper.
Figure 8 shows SREF 850 hPa wind
forecasts valid at 0600 UTC 23 January
2005 from forecasts initialized on the 21st at
0900 UTC. These data show a significantly
above normal low-level jet feature. Over
New Jersey the forecast showed easterly
wind anomalies in excess of -5SDs below
normal. The anomalous jet extended well
westward with the are of -3 to -4SD
anomalies covering most of Pennsylvania
and the area of -2 to -3SD below normal
easterlies extending into Illinois. This
suggests two things first, a very anomalous
and strong easterly jet was forecast and
second, there was general agreement
between forecast members to produce such
large anomalies in a 42-hour forecast. The
latter point suggests the SREF may have
lacked spread in the forecast members.
The SREF MSLP forecasts from the 0900
UTC 21 January cycle are shown in Figure
9. These data show generally good
agreement with the primary features
including the anomalously strong surface
anticyclone. The only area of spread is
associated with secondary cyclone along the
coast. This is due to the effects central
pressure and placement differences
between SREF members.
Figure 6 MREF forecasts initialized at 0000 UTC 20 January 2005 valid at 1200 UTC 23 January 2005 showing a)
MSLP forecasts and b) 850 hPa winds. The 850 hPa winds show the ensemble mean and the U(upper) and V (lower)
wind anomalies in standard deviations from normal. MSLP shows spaghetti and spread in the upper panel and the
ensemble mean and standardized anomalies in the lower pane..
Overall, these forecasts showed the threat
and captured the regions susceptible to the
threat. However, when compared to Figure
1, these forecasts failed to capture the
heavy snowfall observed farther north in
New York State and central New England.
iii) SREF initialized at 2100 UTC 21 January
Forecasts initialized at 2100 UTC 21
January 2005 showed several significant
differences than the previous forecasts. This
is best illustrated by the MSLP forecast
which showed the cyclone at 23/0000 UTC
off the Delmarva and New Jersey coastal
areas (Figure 10a). The cyclone was
forecast to be deeper and farther north. The
forecasts valid 6 hours later showed a
strong cyclone (992 hPa) in the ensemble
mean south of Long Island, New York (Fig.
10b).
This resulting northward shift pushed the
anomalous low-level jet northward (Fig. 11)
now focusing the inflow and the cold
conveyor belt over southern New England
and into southeastern New York State.
These forecasts represented a dramatic shift
northward in the region threatened by heavy
snow as seen in Figure 12. Note the
increased probabilities of 0.50 inches or
greater QPF with the clipper as it zipped
from Illinois to the coast and the marked
northward shift in the snow shield over New
York and New England. The impact of the
cyclone track shift was dramatic.
6. Conclusions
The Blizzard of 2005 struck the Mid West
and the East Coast from 20 to 23 January
2005. This storm brought heavy snow from
the upper Mid West to the northeastern
United States. Heavy snow was observed in
Figure 7 SREF forecasts initialized at 0900 UTC 21 January 2005 showing 24-hour accumulated
precipitation for the 24-hour periods ending at 0000 and 1800 UTC 23 January 2005. Upper panels
show the percentage of exceeding 0.50 inches of QPF and the consensus position of the 0.50 inch
contour. The lower panels show each members location of the 0.50 inch contour and the consensus
QPF.
Chicago, Detroit, Philadelphia, New York,
and Boston. Snow fall totals in excess of 12
inches were common in the Mid West and
along the East Coast. A rare event when a
Clipper-like cyclone can produce such high
snowfall totals. Overall, the NCEP and CMC
EPS’s performed well with this event,
providing long-lead times for a potential
snow storm with a high probability of heavy
snow in close proximity to where the snow
was observed.
Two distinct problems were observed in this
event. First, all forecasts, including the
MREF and SREF, initially had the storm
track and heavy precipitation shield too far
south compared to what was observed.
However, the signals were well forecast.
The QPF forecasts along the northern edges
of the storm were particularly poor for
interior New York State and New England.
In the SREF, the more northward track shift
at 2100 UTC 21 January produced pushed
the precipitation shield farther north. The
cause of these more northward positions of
the cyclone, wind anomalies, and resulting
precipitation shield may have been due to
the impacts of initial conditions and possibly
the impacts of convection in the models.
Zhang et al (2003) showed how the upscale
impacts of model convection can impact the
larger scale forecasts. How convection
affects the development and track of the
surface cyclone and the evolution of the
precipitation shield is an potential avenue of
future research.
The MREFs by 0000 UTC 19 January began
to show, with as many as 2 member, the
potential for heavy snow into central New
York and New England. This implies to
some measure that the initial conditions may
have picked up on the pattern that made this
Figure 8 SREF forecasts initialized at 0900 UTC 21
January 2005 showing EPS mean 850 hPa winds and a) U
wind anomalies and b) V wind anomalies in standard
deviations from normal.
event as the forecasts appeared to capture
the outcome which was observed. Why the
SREFs appeared to lack similar spread and
missed the more northward precipitation
shield and cyclone track is an intriguing
question. It may be due to initial conditions
and/or model physics. But the impact on the
precipitation shield was dramatic.
Despite the under forecasting of the
precipitation amounts and the northward
extend of the precipitation shield, the models
forecast the event quite well. They produced
signals which should have, and in many
instances did, alert forecasters to the
potential of a significant winter storm. The
strong low-level 850 hPa jet in both the
longer range MREF (Fig 6) and shorter
range SREFs (Fig. 8) implied an unusually
strong low-level jet. A feature well know to
be associated with significant winter storms
and heavy rainfall in the eastern United
States (Grumm and Hart 2001). Though not
shown, the 250 hPa wind anomalies were
on the order of -2.5 SD at below normal in
the SREF. Stronger anomalies were found
Figure 9 SREF forecasts initialized at 0900 UTC 21 January
2005 showing EPS mean MSLP (hPa) forecasts valid at 0000
UTC 23 January 2005. Upper panel shows select contours and
spread and lower panel shows the ensemble mean and the
departure of this field in standard deviations from normal.
in single deterministic runs. The strength of
the upper-level jet was significantly above
normal in this event. The 850 hPa winds
were in the nearly historic levels as forecast
by the SREF, comparable to the Blizzard of
19'78. However, the upper-level anomaly
was weaker than the -3.85SD anomaly at
250 hPa which persisted for over 30 hours in
that event (Stuart in review).
It should be noted that the National Weather
Service office (WFO) in Taunton,
Massachusetts made the following
observation during the event: “The first
blizzard since the April fools storm of
1997 has blanketed the area with a
top 5 historic snowfall... inside 24
hours... along with high winds and
bitterly cold temperatures”. Clearly,
this was one of the most significant storms
in recent history along the East Coast.
These data suggest that an extreme winter
storm forecast index (EWSFI) could be
Figure 10 SREF forecasts initialized at 2100 UTC 21 January 2005 showing EPS mean MSLP (hPa)
forecasts valid at a) 0000 UTC and b) 0600 UTC 23 January 2005. Upper panel shows select
contours and spread and lower panel shows the ensemble mean and the departure of this field in
standard deviations from normal.
produced from both ensemble forecast and
deterministic forecast data to gauge the
threat for a significant snow storm. These
results also suggest that the models had
some difficulty getting the details write of
the system and missed the more northerly
cyclone track. However the more
significant problem is focused on the QPF
forecasts. How we employ EPS QPFs in the
forecast process is still a significant
problem and clearly, the ensemble mean
and probabilities should be used with
caution and knowledge about uncertainty.
The fact that 20% of the MREF members
showed the threat for heavy snow farther
north was a signal. This system was
forecast within the realm of the ensembles.
How this information was used is
significant. The author used these data to
change a flight from Washington Dulles to
San Paulo, Brazil from the 22nd to the 21st
and avoided the storm and any potentially
delays. The question arises (adopted from
Ken Mylne, UKMO), “ would you risk
missing a trip if there was a 20% chance
that the weather would cause a delay or
cancellation of the flight you were on?”
7. ACKNOWLEDGEMENTS
John LaCorte for snowfall data and
maps. Mike Evans, Neil Stuart, Walt
Drag, and Jeff McQueen for insights
and observations.
8. References
Doty, B. and J.L. Kinter III, 1992: The
Grid Analysis and Display System
(GrADS): A practical tool for earch
science visualization. Eighth
International Conference on Interactive
Information and Procession Systems,
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Grumm, R.H., and RE Hart. 2001:
Standardized Anomalies Applied to
Significant Cold Season Weather
Events: Preliminary Findings. Weather
and Forecasting,16, 736–754.
Grumm, R. H., and Hart R., 2001:
Anticipating heavy rainfall events:
Forecast aspects. Preprints, Symp. on
Precipitation Extremes: Prediction,
Impacts, and Responses, Albuquerque,
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Hart, R.E., and R H. Grumm. 2001: Using
Normalized Climatological Anomalies to
Rank Synoptic-Scale Events Objectively.
Monthly Weather Review,129,2426–2442.
Hart, R., and Grumm R. H., 2001a:
Anticipating heavy rainfall events:
Climatological aspects. Preprints, Symp.
on Precipitation Extremes: Prediction,
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Sivillo J.K, JE. Ahlquist and Z Toth.
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Primer. Weather and Forecasting,12,
809–818.
Zhang, F, C.Snyder, and R. Rotunno,
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Figure 11 SREF forecasts initialized at 2100 UTC 21
January 2005 showing EPS mean 850 hPa winds and a) U
wind anomalies and b) V wind anomalies in standard
deviations from normal.
Figure 12 SREF forecasts initialized at 2100 UTC 21 January 2005 showing 24-hour accumulated
precipitation for the 24-hour periods ending at 0000 and 1200 UTC 23 January 2005. Upper panels show the
percentage of exceeding 0.50 inches of QPF and the consensus position of the 0.50 inch contour. The lower
panels show each members location of the 0.50 inch contour and the consensus QPF.