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Early May Cut-off low and Mid-Atlantic rains
By
Richard H. Grumm
National Weather Service State College, PA
Abstract:
A deep 500 hPa cutoff developed in the southern Plains on 3 May 2013. It produced a prolonged
period of unseasonably cold weather in the Plains to the Gulf States from 2-6 May 2013. There
were some reports of late season snow from this system in places were spring time observations
of snow are rare.
As this relatively long-lived cutoff moved northeastward, it produced bands of heavy rainfall in
the Mid-Atlantic region and southern New York. The rain fell in the northeast quadrant of the
cutoff as it slowly moved northward and merged with westerlies.
The initial rain bands in the northeast quadrant of the cutoff were relatively well predicted by the
NCEP SREF. The second surge of heavy rainfall across New Jersey and New York, though
predicted by the SREF was a relatively low probability outcome event.
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1. Overview
A deep 500 hPa cut-off low meandered across the southern United States and into the northeast
from 2-9 May 2013 (Fig. 1). Beneath this deep low 850 hPa temperatures were -3 to -4 below
normal (Fig. 2). This air mass brought a late season cold snap to the plains and reports of snow
well south for the month of May. The deep low (Fig. 1) moved beneath a blocking mid-latitude
ridge which was focused over northeastern Canada (Fig. 1a-c). As the ridge weakened with time
the close low moved northeastward into the Mid-Atlantic region and northeastern United States
before getting absorbed into the westerlies around 1200 UTC 10 May 2013 (Fig. 1i).
Bell and Bosart (1993) noted the amplification of a ridge over the western United States and
adjacent eastern Pacific during the evolution of a cut-off over the eastern United States. Note the
strong ridge in Figure 1a-c during the evolution of the cut-off of May 2013. The cut-off
Early and late season cut-off lows. The implied impulse in the northwest flow east of the ridge
axis was the feature which would deepen and become the deep cut-off over the southern Plains.
Visually, the process was similar to that outlined by Bell and Bosart (1993). The evolution
shown in Figure 1 is similar to the cut-off evolution shown by Nieto et al. (2005:Fig. 1) to
include the re-absorption into the westerlies during the final stage. The cut-off of 2-10 May 2013
was nearly poster child for their described evolution of cut-offs.
From a climatological perspective, cut-off development in the United States and North America
peaks in the months of July to September, with strong secondary maximum in April-May-June
and weaker signal in the autumn months of October-November-December (Nieto 2005:Fig 10).
The primary location of cut-off evolution over North America is off the coast of California
though a secondary maximum extends from the southern plain into eastern Canada (Nieto et al
2005:Fig. 8).
Cutoffs can impact the total precipitation and have been shown to produce heavy rainfall events.
Payer (2010) identified patterns of heavy precipitation (HP) and low precipitation (LP) cutoff in
the northeastern United States. Strong southerly flow with deep moisture and easterly flow tend
events tend to produce heavy precipitation events. Events with more northwesterly flow tend to
produce few significant precipitation events. Heavy rainfall in northwesterly flow behind a cutoff
is not a good location for heavy rainfall.
This paper will examine impacts of the enduring cutoff low of 3-9 May 2013. The key aspects
here are focused on the weather in the Mid-Atlantic region to include several days of locally
moderate to heavy rainfall. The persistent cutoff low and much of the associated cold and snow
in the plains and southern United States will likely be studied in more detail by others.
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2. Data and Methods
The large scale pattern was reconstructed using the Climate Forecast System (CFS:Saha 2010) 6hourly data and CFS based 30-year climatic means and standard deviations. The standardized
anomalies were computed in Hart and Grumm (2001). All data were displayed using GrADS
(Doty and Kinter 1995).
The Stage-IV rainfall data were used to get event total, 6,12,24 and longer rainfall estimates.
3. Pattern overview
The 500 hPa pattern from 1200 UTC 2 to 10 May 2013 (Fig. 1) showed a weakening cutoff low
over the Gulf States at 1200 UTC 2 May 2013 and a strong 500 hPa ridge off the West Coast.
East of this strong ridge a small wavelength trough was dropping into the plains (Fig. 1a). This
system cut-off over the southern Plains at 1200 UTC 3 May and the previous cutoff became less
descript over Florida (Fig. 1b). The second stronger cutoff had -3 to -4s height anomalies near
the low center as it drifted across the southern plains on 4-5 May (Fig. 1c-d) and began to fill in
as it reached the southeastern United States. The system turned northward up the East Coast and
began to get absorbed into the westerlies on 9-10 May. This cut-off clearly persisted from 3-9
May and became an open wave on 10 May 2013.
The deep cutoff low was associated with a pool of cold air at 850 hPa (Fig. 2). This cold air
brought -3 to -4s 850 hPa temperatures into the Deep South. NCDC daily low maximum
temperature data suggest 40 to 53 daily low maximum temperature records were set or tied from
2-5 May in the central and southern United States. Similarly, 32 to 66 daily record low
temperature records were set or tied from 2-6 May 2013. These data suggest that the cutoff
brought unseasonably cool weather to much of the central and southern United States during the
first week of May 2013.
The composite pattern and anomalies over the central United States (Fig. 3) shows that in the
mean the 500 hPa cutoff and cold 850 hPa temperatures persisted for most of the period covered
by the NCDC period of cooler than normal temperatures.
As the 500 hPa cutoff moved into the Mid-Atlantic region it produced several bands of rain to
the north and east of the 500 hPa low center (Figs. 4-5). The stronger bands of rain on 7 and 8
(Figs. 4c-d and 5a-b) May were associated with deep south-southeasterly flow on the north east
side of the cutoff. As the flow became more westerly and northwesterly on 9 May (Figs. 5c-d)
the rainfall amounts were considerably lower. The more notable periods of rain were observed in
the mountains of North Carolina (Fig. 4b-c) and Virginia (Fig. 4c & 5a) and southeastern New
York (Fig. 5b).
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The heavy rainfall over the 48 hour period from 1200 UTC 7-9 May (Fig. 6) showed that there
were several distinct and persistent rainbands that brought locally heavy rainfall to the MidAtlantic region. For the most part the bands were oriented from southeast to northwest over
North Carolina, Virginia, Pennsylvania, New Jersey and New York.
4. Forecast
The large scale pattern was generally well predicted with 4-7 days of lead-time and is not shown
here. The focus here is on the NCEP SREF forecasts for initial bands of rainfall on 7-8 May
2013 on the northeast side of the 500 hPa cut-off low. The 6 SREFs predicted a band of
moderate to heavy rainfall to impact the mountains of Virginia between 1200 UTC 7-8 May
2013 (Fig 6).
Forecasts for the second period of heavy rainfall from 0000 UTC 8-9 May (Fig. 7) showed the
correct general region though rainfall over 25 mm was forecast as a 40% outcome. The region to
be affected and the pattern of the rainfall was well forecast but the SREF underestimated the
rainfall.
5. Summary
A From a climatological perspective, cut-off development in the United States and North
America peaks in the months of July to September, with strong secondary maximum in AprilMay-June and weaker signal in the autumn months of October-November-December (Nieto
2005:Fig 10). The two cut-off of April and May 2013 occurred during the secondary maximum
time of year.
Lasting 5-6 days the cut-off of 2-8 May 2013 was a relatively long-lived cut-off. Most cut-offs
only lasted about 2 days (Nieto et al 2005). Previous research suggested subtropical cut-off were
shorter-lived than polar cut-offs. The intrusion of cold air may be a key factor in some of the
longer-lived polar cut-offs. The 7-day event of May 2013 showed the importance of cold air in
maintaining this long-lived cut-off.
Figures 4,5, and 6 all suggest that the favorable location of heavy rainfall is in the northeastern
quadrant of a slow moving 500 hPa low. During this warm season example, the heavy rain fell in
bands oriented from south-southeast to north-northwest. There was clearly no heavy rainfall in
the westerly flow and northwesterly flow quadrants of the 500 hPa cutoff low.
6. Acknowledgements
7. References
Bell, Gerald D., Lance F. Bosart, 1993: A Case Study Diagnosis of the Formation of an Upper-Level
Cutoff Cyclonic Circulation over the Eastern United States. Mon. Wea. Rev., 121, 1635–1655.
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Doty, B.E. and J.L. Kinter III, 1995: Geophysical Data Analysis and Visualization using GrADS.
Visualization Techniques in Space and Atmospheric Sciences, eds. E.P. Szuszczewicz and J.H.
Bredekamp, NASA, Washington, D.C., 209-219.
Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season
Weather Events: Preliminary Findings. Wea. and Fore., 16,736–754.
Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic
scale events objectively. Mon. Wea. Rev., 129, 2426–2442.
Nieto, Raquel, and Coauthors, 2005: Climatological Features of Cutoff Low Systems in the Northern
Hemisphere. J. Climate, 18, 3085–3103.
doi: http://dx.doi.org/10.1175/JCLI3386.1
Saha, Suranjana, et. al., 2010: The NCEP Climate Forecast System Reanalysis. Bull. Amer. Meteor.
Soc., In Press (DOI: 10.1175/2010BAMS3001.1).
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Figure 1. CFS 500 hPa heights (m) and height anomalies (standard deviations) in 24 hour periods from a-i) 1200 UTC 2-10 May 2013. Contours every 60
m and standardized anomalies as in color bar. Return to text.
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Figure 2. As in Figure 1 except for 850 hPa temperatures (C ) and temperature anomalies. Return to text.
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Figure 3. CFS based composite pattern focused over the central United States from 1200 UTC 2 May through 0000 UTC 6 May 2013. Return to text.
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Figure 4. Stage-IV 12 hour accumulated precipitation (mm) valid for the periods ending at a) 0000 UTC 6 May, b) 1200 UTC 6 May, c) 0000 UTC 7 May, d)
1200 UTC 8 May 2013. Values in mm as in color bar. Return to text.
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Figure 5. As in Figure 4 except for 8 and 9 May 2013. Return to text.
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Figure 6. As in Figure 4 except for total estimated precipitation from 1200 UTC 7 May though 1200 UTC 9 May
2013. Return to text.
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Figure 7. NCEP SREF forecasts showing the probability of 25mm or more QPF for the 24 hour period ending at 1200 UTC 8 May 2013 from NCEP SREFS
initialized at a) 0900 UTC 5 May 2013, b) 1200 UTC 5 May 2013, c) 0900 UTC 6 May, d) 2100 UTC 6 May, e) 0300 UTC 7 May and f) 0900 UTC 7 May 2013.
Return to text.
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Figure 8. As in Figure 7 except for the 24 hours ending 0000 UTC 9 May 2013 and forecasts from a) 0900 UTC 6 May, b) 2100 UTC 06 May, c) 0300 UTC 7
May, d) 0900 UTC 7 May, e) 1500 UTC 7 May, and 2100 UTC 07 May 2013. Return to text.
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