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Eastern North American Subtropical Ridge and heat of May 2010
Richard H. Grumm
National Weather Service
State College, PA 16803
1. Introduction
A large subtropical ridge developed over the
eastern United States during the week of 24
May 2010 (Fig. 1). This feature was
associated with some interesting, if not
unique weather over much of the eastern
United States and Canada. Warm moist air
transported poleward of the ridge produced
convection and mesoscale convective
complexes over Canada which moved over
northern New England. Dry conditions and
lightning produced fires in Quebec. Under
the ridge, the warm air produced record high
temperatures over southern Canada and the
eastern United States (Table 1).
At the height of the event, daytime highs
exceeded 90F from Chicago to Hartford.
Many high temperature records were set in
central New York and southern New
England on 26 May 2010. Locations in
Ontario and Quebec saw high temperatures
in the 80s and 90s from 25-27 May 2010.
This large subtropical ridge brought
atypically warm weather to eastern North
America. The 850 hPa temperatures were
forecast to exceed 20C over portions of New
York and New England on 26 May 2010.
These forecasts proved to be quite accurate
and rare readings over 20C at 850 hPa were
observed at several locations including
Albany, New York. Not surprisingly many
locations in eastern New York and New
England set or tied record high temperatures
and several sites recorded the earliest record
high so early in the season.
This paper will document the conditions
associated with this impressive subtropical
ridge. The focus is on the pattern and the
anomalies. Additionally, forecasts are shown
to demonstrate that this event was relatively
2. Methods
The 500 hPa heights, 850 hPa temperatures
and winds, other standard level fields were
derived from the NCEP GFS, GEFS, and the
NCEP/NCAR (Kalnay et al. 1996)
reanalysis data. The means and standard
deviations used to compute the standardized
anomalies were from the NCEP/NCAR data
as described by Hart and Grumm (2001).
Anomalies were displayed in standard
deviations from normal, as standardized
anomalies. All data were displayed using
GrADS (Doty and Kinter 1995).
The standardized anomalies computed as:
SD = (F – M)/σ ()
Where F is the value from the reanalysis
data at each grid point, M is the mean for
the specified date and time at each grid point
and σ is the value of 1 standard deviation at
each grid point.
Model and ensemble data shown here were
primarily limited to the GFS and GEFS. The
NAM and SREF data were also available for
use in this study. Displays will focus on the
observed pattern and some forecast issues
associated with the pattern.
For brevity, times will be displayed in day
and hour format such at 26/0000 UTC
signifies 26 May 2010 at 0000 UTC.
3. Results
Figure 1 shows the 500 hPa pattern at 0000
UTC from 20-28 May 2010. By 24/0000
UTC the 500 hPa anomalies were over 2SDs
above normal from Illinois into Ontario and
Quebec. A close 5880m contour was present
in these data. This feature shifted northward
over southern Canada by 25/0000 and begin
to weaken by 27/0000 UTC. The ridge
appeared to redevelop or retrogress to a
position over the northern plains by 28/0000
UTC (Fig. 1i).
The 850 hPa temperatures (Fig. 2) showed
above normal 850 hPa temperatures
associated with the tropical ridge. A closed
20C contour was evident over Illiniois at
24/0000 UTC and closed 22C contour was
evident at 25/0000 UTC. This pocket of
warm air followed the ridge eastward and
was over western New England by 27/0000
UTC. Thermal anomalies ranged from 2 to
3SDs above normal at 850 hPa with this
pocket of warm air. The air clearly modified
by 28/0000 UTC (Fig. 2i).
The precipitable water analysis at 0000 UTC
20-28 May is shown in Figure 3. These data
show the surge of high PW air north and
west of the subtropical ridge. This is a
common feature associated with many heat
episodes. PW anomalies over 3SDs above
normal were present in Canada south of
James Bay 24-25 May 2010. This area
shifted eastward on 26 May (Fig. 3g). Not
surprisingly during this period MCS activity,
was observed over Quebec and into northern
New England. The traditional “ring of fire”
with a subtropical ridge had been displaced
well north and east of its traditional location.
From a sensible weather impact, Figure 4
shows the high temperatures on 24-26 May
2010. Temperatures above 90F extended
from the Gulf States into the upper Midwest and into Canada with readings over
90F just south of James Bay.
Figure 5 shows a GOES-IR image valid at
25/1831 UTC. Generally clear skies were
present under the massive subtropical ridge.
The surge of high PW air over along the
western edge of the ridge was producing
convection over Illinois and Wisconsin. A
ridge-runner MCS was visible over Quebec
and northernmost Maine. The surge of high
PW air, wrapping over the ridge produced
this atypically northward are of MCS
Figure 6 shows an MCS moving out of
Ontario into northern New York at 27/0015
UTC. To the east of this MCS a smaller
developing MCS was visible. This eastern
MCS would produce severe weather from
Vermont to Long Island as it moved
southward. The lower panel in Figure 6
shows this feature over Connecticut and
Long Island at 27/05045 UTC.
Thus the GEFS may pay a penalty with only
6-hourly data resolution.
The overall pattern for the subtropical ridge
was well predicted by then NCEP ensemble
forecasts systems (EFS) to include the
Global Ensemble Forecasts System (GEFS)
and the short-range ensemble forecast
system (SREF). Figures 7-10 shows select
4. Conclusions
The 500 hPa pattern and 850 hPa
temperatures were well predicted by the
GEFS with high confidence (Fig. 7) from
the 23/0000 UTC GEFS. Figure 8 shows 9
selective forecasts of 850 hPa temperatures
and the standardized anomalies. These data
show the consistent forecasts of as a strong
subtropical ridge and 2-3SD 850 hPa
temperatures over New York and New
England. Many aspects of this event were
well predicted, to include the retrogression
but the focus here is on the warmest day.
The SREF forecasts from showed both high
confidence (Fig. 9) and a closed 18C
contour at 26/1800 UTC. The 9 select SREF
forecasts all showed this warm air. The data
shown are at 2100 UTC because that was the
time the SREF had the maximum 850 hPa
temperature forecasts. Temperatures were
slightly lower at 27/0000 and 26/1800 UTC.
Figures 11-12 show SREF threats of
variables associated with warm episodes.
These data show that the strong 500 hPa
ridge, warm 850 hPa temperatures, warm
2m temperatures and high PW air with the
subtropical ridge were relatively well
A large subtropical ridge developed on 24
May 2010 and persisted through 27 May
2010. This feature produced a closed 5880m
contour over southern Canada and 2SD
above normal height anomalies. At the
surface, high temperatures over 90F were
common beneath this feature and the pocket
of 20 to 22C air at 850 hPa.
This large subtropical ridge produced or tied
many high temperature records over eastern
North America. Additionally, this feature
produced convection over northeastern
Canada and MCS activity over an area
where such activity is quite rare, even in
summer, let alone in late May. Clearly, this
strong tropical ridge pushed high PW air and
conditions favoring deep convection well
north of the climatologically favored areas.
The “ring’ of fire associated with subtropical
ridges was pushed well northward with this
unusual system.
As this event ended, the large scale ridge
retrogressed. As it moved or redeveloped to
the west, the track of MCS activity moved
westward too. The record heat in New
England ended with as strong MCS that
moved nearly north-to-south trough the
region. This ridge-roller (Galarneau and
Bosart 2006) or ridge-runner produced a
cluster of many wind and hail reports in
Vermont and Massachusetts across
Connecticut and Long Island.
This event was well predicted by the NCEP
models and EFS. Only EFS data were shown
here. The key to this forecast was the
subtropical ridge. Subtropical ridges,
(Galarneau et al. 2008) play critical roles in
warm episodes and modulating the
distribution of convection.
5. Acknowledgements
Walt Drag for records in New England.
Lance Bosart for conference paper access,
and the Albany map for discussions on
many aspects of this event.
6. References
Galarneau, T.J and L.F Bosart 2006: Ridge
Rollers: Mesoscale Disturbances on the
periphery of cut-off anticyclones. AMS
Severe Local Storms symposium, Atlanta,
Galarneau, T. J., Jr., L. F. Bosart, and A. R.
Aiyyer, 2008: Closed anticyclones of the
subtropics and middle latitudes: A 54-yr
climatology (1950-2003) and three case
studies. Synoptic–Dynamic Meteorology and
Weather Analysis and Forecasting: A
Tribute to Fred Sanders, Meteor. Monogr.,
No. 55, Amer. Meteor. Soc., 349–392.
Fields (KBDL)
Comments and notes
Tied 20 May 1996 record
for month. Broke 1965
reading of 94. Warmest
since 2 August 2006
when it was 100F.
Warmest May reading
earliest 94F or greater
Broke record set 26 May
tied 1946 record
Broke record from 26
May 1978
86 in 2007
Ties 1965 record
Ties 1965 record
breaks 86 set in 2007
Table 1. Selected records by site and State set on 26 May
Figure 1. JMA 500 hPa heights and height anomalies valid at 0000 UTC from (a-i) 20 to 28 May 2010. Heights in meters every 60m. Anomalies in standard
deviations from normal in 1 standard deviation units. Return to introduction.
Figure 2. As in Figure 1 except for 850 hPa temperatures ( C) and temperature anomalies. Temperatures every 2C. Return to pattern.
Figure 3. As in Figure 1 except for precipitable water (mm)n and precipitable water anomalies. Return to pattern.
Figure 4. High temperatures for 24, 25 and 26 May 2010.
Figure 5. GOES IR image valid at 1831 UTC 25 May 2010. Local archive was limited to the
eastern United States. . Return to satellite
Figure 6. As in Figure 5 except valid at 27 May at 0015 and 0545 UTC. Return to satellite.
Figure 7. GEFS forecasts initialized at 0000 UTC 23 May 2010. Left side shows 850 hPa temperatures with each members 0,8,and 16C contour and the
spread. Lower panel shows the ensemble mean 850 hPa forecasts and the standardized anomalies. The right side shows 500 hPa heights. Upper right
shows each member’s 5460 and 5760m contours and the spread about the mean. Lower panel is the ensemble mean field and the standardized anomaly.
Return to text. .
Figure 8. GEFS forecasts of the mean 850 hPa temperatures from all 21 members and the standardized anomaly of this field. Temperatures every 2 C valid
at 0000 UTC 27 May 2010. Initialization times are a) 0000 UTC 23 May, b) 1200 UTC 23 May, c) 0000 UTC 24 May, d) 0600 UTC 24 May, e) 1200 UTC 24
May, f) 1800 UTC 24 May, g) 0000 UTC 25 May, h) 0600 UTC 25 May and i) 1200 UTC 25 May 2010. Return to text. .
Figure 9. As in Figure 7 left side except for SREF 850 hPa forecasts of 850 hPa temperatures and anomalies valid at 2100 UTC 26 May
from forecasts intialzied at (left) 0900 UTC 25 May and (right) 2100 UTC 25 May 2010. Return to text. .
Figure 10. As in Figure 8 except for SREF forecasts valid at 2100 UTC 26 May 2010 from forecasts initialized at a) 0900 UTC 24 May, b) 1500 UTC 24 May, c)
2100 UTC 24 May, d) 0300 UTC 25 May, e) 0900 UTC 25 May, f) 1500 UTC 25 May, g) 2100 UTC 25 May, f) 0300 UTC 26 May and i) 0900 UTC 26 May 2010.
Return to text. .
Figure 11. SREF forecasts initialized at 0300 UTC 24 May 2010 showing the probability of a) 1.5SD or greater 500 hPa height anomalies., b) 2.0 SD 850 hPa
temperature anomalies, c) 2.0SD or greater precipitable water anomalies. and d) 2.5SD or greater 2m temperature anomalies. ,
Figure 12. As in Figure 11 except showing a) probability of 500 hPa heights over 5840m, b) 850 hPa temperatures over 18C, c) precipitable water greater than
20mm, and d) 2m temperatures over 22C.