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Long Island and Mid-Atlantic Record Deluge
Pattern and Analysis
By
Richard H. Grumm
and
Charles Ross and Elyse Colbert
National Weather Service State College, PA
1. Introduction
Islip, New York had a record rainfall of 13.27 inches on 14 August 2014, setting a new 24-hour
record rainfall for the state of New York (AP 2014, WSJ 2014)1. The larger scale synoptic
pattern produced heavy rainfall (Fig. 1) and flooding from Maryland into New England.
Extremely heavy rainfall; in excess of 192 mm; was observed near Baltimore, MD, in southern
New Jersey, and over Long Island. Over a two day period, the system brought heavy rains from
the Mid-Atlantic region to eastern Maine. Several sites had 6-hour rainfall totals exceeding the
200 year return period. The rain occurred in a tongue of high precipitable water air (Fig. 2)
associated with a slow-moving north-south frontal boundary and a weak surface wave which
propagated along the frontal boundary (Fig. 3).
The larger scale pattern, with a north-south frontal and moisture boundary (Fig. 2) had many of
the characteristics often associated with a Maddox Synoptic rainfall event type (Maddox et al.
1979). It will be shown that as the wave moved up the frontal boundary, strong easterly flow
developed north of the cyclone. On the time scale of hours, this event transitioned into a Maddox
Frontal system, with stronger easterly flow. Heavy rainfall events which transition from synoptic
to frontal types can produce record rainfall. For example, the record breaking rainfall 29-30
March event in southeastern New England (Grumm 2011) was associated with this transition.
Events of this type are difficult to forecast with numerical guidance due to grid resolution issues
and the need in models with grid spacing over about 11 km (Kain et al. 2008; Weisman et al.
1997) to use parameterization schemes to deal with instability and the implied convective
rainfall. The modeling gray area is horizontal resolutions of 5-11km. Convection allowing
models; with horizontal resolutions generally below about 5km; should perform better and
produce higher quantitative precipitation amounts than coarser models. However, these systems
have limitations and often fail to predict mesoscale heavy rainfall events or have both temporal
and spatial errors. These uncertainties and limitations require human interventions related to
flash flooding and heavy rainfall forecasts and warnings. Situational awareness in these events
requires knowledge of patterns in which heavy rain may occur and the ability to discern a
potential ordinary rainfall event from a potential record event.
1
Previous record 11.6 inches in 24 hours set 27-28 August 2011 in Tannersville, NY
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The use of standardized anomalies (SA: Hart and Grumm 1991; Grumm and Hart 1991; Graham
and Grumm 2010) applied to re-analysis data and forecast guidance (Junker et al. 2009) has been
demonstrated to have value in identifying potential record events. Du et al (2014) demonstrated
how this concept could be applied to ensembles to produce a simple extreme forecast index. Du
et al. (2014) used anomaly-based forecast concepts as tools to identify a potential record rainfall
event near Beijing, China. Another iteration of anomaly-based forecast and situational
awareness, leveraging the forecast probability distribution function (PDF) was demonstrated by
Graham et al (2013). Using the full PDF, the extreme events can be placed into percentiles and
in some events, the forecasts may be of the record event relative to the 30-year climatological
PDF.
This paper will show the pattern associated with the historic Long Island rainfall event and the
record rainfall in Maryland and New Jersey. The focus is on the pattern and the value of the
pattern and anomalies in improving forecaster situational awareness for this and similar extreme
events, which typically occur with short, if not minimal predictability horizons. A companion
paper will demonstrate the limitations of NWP in predicting events of this nature.
2. Methods and Data
Gridded CFSR data were used to show the large scale pattern relative to a CFSR-based 21-day
centered 30 year climatology (Hart and Grumm 2001). Plots show the field of interest and the
standardized anomalies in standard deviations from normal.
The stage-IV rainfall data was retrieved in gridded form from NCEP. Unless specified otherwise,
all plan-view images show accumulations using the 6-hourly data files. The point diagrams use
the hourly files for comparison to the point observations. METAR sites were used to get point
data at select locations. These point data tended to be higher than those in the gridded datasets.
Rainfall return periods in 1, 3, 6 and 12 hour intervals were examined using the U.S. Department
of Commerce Technical Paper No. 40 and the NOAA Atlas 14 Precipitation Frequency Data. For
select points such as Islip, Baltimore and Portland, the data were collected to compare to
METAR data. These data were also examined using the National Severe Storms Laboratory
(NSSL) Flooded Locations and Simulated Hydrographs Project (FLASH) webpage. These data
are available in near real-time providing return periods based on rainfall estimates. The return
periods at the FLASH site are computed relative to the NOAA-14 Precipitation Atlas data.
3. Pattern and Standardized Anomalies
The larger scale pattern over the United States (Fig. 4) from 1200 UTC 11 August through 0000
UTC 14 August 2014 showed a highly amplified pattern with a strong ridge over western North
America and a sharp-deepening trough over eastern North America. The 500 hPa height
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anomalies were +2 to +3 in the west and the trough was -1 to -3 below normal in the east;
reaching its most negative anomalies as the rainfall event was peaking in the eastern United
States.
The developing trough (Fig. 4) pulled a plume of deep moisture (Fig. 2) into the region. Though
not shown, PW values over 65 mm and 2-3 were present over the southwestern Atlantic as the
trough deepened and the surface waves developed along the frontal boundary (Fig.3). The deep
moisture plume was pulled into the Mid-Atlantic region and southern New England. Strong
southerly flow at 850 hPa was present (Fig. 5). The north-south PW plume and strong southerly
flow was indicative of a Maddox Synoptic pattern. But ahead of the waves along the frontal
boundary, easterly flow developed. The 850 hPa winds and 850 hPa u-wind anomalies indicated
+2 to +3 u-wind anomalies in Maryland and New Jersey at 1800 UTC 12 August and 0000
UTC 13 August respectively. The low-level easterly wind anomalies peaked at -4 below normal
over Long Island at 0600 UTC 13 August then quickly lifted northward into southern New
England, where additional heavy rainfall was observed.
Another contributing factor in this event may have been the strong frontal system and the
injection of unseasonably cold air into the system from the west (Fig. 6). The 850 hPa
temperature anomalies were -2 to -4 below normal in the Ohio Valley (Fig. 6e) while a narrow
ribbon of +1 above normal air was pulled into the coastal regions ahead of the front. These data
imply strong baroclinicity in the larger scale pattern. Backward trajectories from a point near
Islip, New York implied that flow ahead of the frontal boundary ingested air from the
southwestern Atlantic (Fig. 7) and the air at 1500 meters originated near the 65 mm PW area off
the southeastern United States.
The heavy rainfall and the extreme rainfall over portions of Long Island were likely related to the
enhanced low-level flow and a coastal front. The latter feature is beyond the scope of this study
to examine. This case shows the value of anomaly-based analysis to identify patterns of
potentially high impact weather events.
4. Rainfall and Rainfall Return Periods
i.
Rainfall Patterns and Amounts
The slow moving frontal system produced heavy rainfall in relatively short 6-hour windows (Fig.
8). The heaviest rainfall in the Baltimore area fell in the 6-hour period ending at 0000 UTC 13
August 2014. In southern New Jersey, the heaviest rainfall fell in the 6-hour period ending at
0600 UTC 13 August, and over Long Island the heaviest rainfall was between 0600 and 1200
UTC 13 August. All the bands had a distinct southwest to northeast orientation until the system
entered New England and the bands become more north to south in orientation (Fig. 6d).
Though the 24 hour rainfall maximum at Islip ended at 1030 local (1430 UTC), it is rather
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amazing as to how little rain accumulated in the Stage-IV data on Long Island after 1200 UTC
(Fig. 10). The bulk of the rain fell in a short time window.
These rapid accumulations in short 3-6 hour windows produced remarkable rates which
accounted for 80 to 95% of the event total estimated rainfall (Fig. 1). The point data from the
stage-IV data (Fig. 10) and observational data showed the period of extreme rainfall at Islip and
Baltimore (BWI). The rainfall near BWI was very light from 0000 UTC then peaked
dramatically from 1500 to 1800 UTC. The intense rainfall was in a 3 hour window.
At Islip the gridded data implied the rain began at 0300 UTC and picked up in intensity at 06000700 UTC when over 9 mm rain fell. The gridded data then showed 5 hours of hourly rainfall in
excess of 18 mm and a peak value of 69.13 mm. Point data showed even higher amounts,
illustrating some of the limitations of gridded rainfall estimates. Summaries of the rainfall totals
in the Mid-Atlantic region, Long Island and southern New England, and New England are
provided in Tables 1-3. Many locations had over 4 inches (100mm) rainfall.
ii.
Rainfall Return Periods
Table 4 shows the observed 1- and 6-hour peak observed rainfall for the Islip, New York
METAR (KISP) compared to values obtained from the Extreme Precipitation Analysis webpage
(Northeast Region Climate Center). These data show that this was a statistically anomalous
event, with the rainfall rates exceeding those of a 100 year event2. The National Severe Storms
Laboratory (NSSL) Flooded Locations and Simulated Hydrographs Project (FLASH) site
compares radar derived return periods using the NOAA-14 Precipitation Atlas data. The
estimated 3-hour return periods over Long Island (Fig. 11) show the return period during the
event. A significant portion of Long Island experienced greater than 200 year 3-hour rainfall
return periods. The data show an extraordinary event over portions of central and western Long
Island. It is also interesting to note on the warm side of the boundary how light the rainfall was
over most of eastern Long Island.
The rainfall return period data for the Baltimore-Washington International Airport (KBWI: Table
5) shows the observed 1- and 3-hour peak observed rainfall compared to the NOAA Atlas 14
data. The overall rates and accumulations were not as extreme as those observed at Islip.
However the site did exceed the 100-year return period for 6-hour rainfall amounts. The
Plainview data near the peak of the event is shown in Fig. 12. The plainview data show the
focus of the heaviest rainfall south of the city of Baltimore and another area of heavy rainfall to
the south near Washington, DC.
Similar data is shown for the rainfall and return periods for Portland, ME in Table 6. These data
show that the 6-hour rates in Portland were just under those observed during a 500-year event
2
USDOC Technical Paper 40 does not categorize frequencies for return periods greater than 100
years
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and the 3-hour rates exceeded the 500 year return period. The planview data near the peak of the
event in the Portland area is shown in Fig. 13.
A summary for each METAR sites’ hourly rainfall is provided in the Appendix.
5. Summary
A slow moving frontal system, ingesting moisture from the southwestern Atlantic, produced
heavy rainfall and flooding from Maryland into New England. The strong low-level southerly
winds and strong easterly winds north of the low, which propagated up the frontal zone,
produced record rainfall in portions of Maryland and New Jersey and established a new 24-hour
rainfall record for the State of New York where 13.27 inches of rain fell.
The pattern of the rainfall shows how mesoscale this event was in both time and space. In most
areas, the heaviest rain fell in a 3-6 hour period. Across Long Island, where rainfall rates were
historically high, only a narrow band received the extreme rainfall rates, and areas on the warm
side of the boundary received significantly lower rainfall amounts. Deeper into the cold air, the
rainfall amounts fell off dramatically.
From a short-term forecast and warning perspective, this case demonstrates the value of knowing
the patterns in which heavy rains can occur. This case also shows the critical input rainfall return
period data can play in the near-term forecast process. At both the synoptic and mesoscale,
climate-based forecasts are useful and effective forecast tools.
This case shows the value of anomaly-based analysis to identify patterns of potentially high
impact weather events. When applied to forecast output from numerical weather prediction
models and ensemble forecast systems, these same concepts can be applied to anomaly-based
forecasts of high impact weather events. Probabilistic information relative to the forecast and
climatological PDF likely add significant value to this process.
The return period data Islip, Baltimore, and Portland show the value of return periods in the
forecast and warning process. Previous work on NOAA-14 data suggests rainfall rates in the 10
to 50-year interval produce nuisance floods. As the rates approach the 100-year event, the floods
become significant and rainfall rates exceeding 100-year and 500-year events tend to be recordbreaking and historic events. These data need to be better incorporated into the flood warning
process in real-time.
6. Acknowledgements
The Albany map for details on the rainfall, model and model initialization issues and confusing
Moses for NOAA.
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7. References
Associated Press and Wall Street Journal, 2014: Long Island Rainfall is 200-year weather event,
experts say, and similar stories. (http://online.wsj.com/articles/record-breaking-rain-floods-longisland-new-york-region-1407947088)
Associated Press, 2014: Flooding-Metro NYC Story. And similar stories.
Fritsch,J.M and L.A. Giordano, 1991: Strong Tornadoes and Flash-Flood-Producing Rainstorms
during the Warm Season in the Mid-Atlantic Region, Wea. Forecasting,6,437-455.
Du, J, R.H. Grum, and G. Deng, 2014: Ensemble anomaly forecasting approach to predicting
extreme weather demonstrated by extremely heavy rainfall even in Beijing. Chin., J. Atmos,
Sci,38(4): 685-699 (in Chinese)
Graham, R.A, T. Alcott, N. Hosenfeld, and R. Grumm, 2013: Anticipating a rare event utilizing
forecast anomalies and a situational awareness display: The Western Region US Storms of 18-23
January 2010, BAMS,2013.
Graham, R A. and R. H. Grumm, 2010: Utilizing Normalized Anomalies to Assess SynopticScale Weather Events in the Western United States. Wea. Forecasting, 25, 428-445
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.
Kain,J.S, S.J. Weiss, D. R. Bright, M. E. Baldwin, J J. Levit, G W. Carbin, C S. Schwartz, M L.
Weisman, K K. Droegemeier, D B. Weber, and K W. Thomas, 2008: Some Practical
Considerations Regarding Horizontal Resolution in the First Generation of Operational
Convection-Allowing NWP. Wea. Forecasting, 23, 931–952.
Maddox,R.A., C.F Chappell, and L.R. Hoxit. 1979: Synoptic and meso-alpha aspects of flash
flood events. Bull. Amer. Meteor. Soc., 60, 115-123.
Weisman,M.L, W. C. Skamarock, and J. B. Klemp, 1997: The Resolution Dependence of
Explicitly Modeled Convective Systems. Mon. Wea. Rev., 125, 527–548.
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Figure 1. Stage-IV gridded rainfall estimates for the period of 1200 UTC 12-14 August 2014. Values in mm as in the color bar to the right.. Return
to text.
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Figure 2. Climate Forecast System (CFS) reanalysis of precipitable water and precipitable water anomalies in 6-hour increments
from a) 1800 UTC 12 August through f) 0000 UTC 14 August 2014. Return to text.
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Figure 3. As in Figure 2 except for mean-sea level pressure and pressure anomalies. Return to text. Return to pattern section.
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Figure 4. As in Figure 2 except for 500 hPa heights (m) and 500 hPa height anomalies in 12-hour increments from a) 0000 UTC 11 August 2014
through f) 0000 UTC 14 August 2014. Return to text.
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Figure 5. As in Figure 2 except for 850 hPa winds (kts) and v-wind anomalies. Return to text.
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Figure 6. As in Figure 2 except for 850 hPa temperatures and temperature anomalies. Return to text.
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Figure 7. Hysplit backward trajectories for a point near Islip, NY valid at 1200 UTC 13 August 2014. ARL trajectory site used to compute the trajectories.
Return to text.
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Figure 8. As in Figure 1 except for the accumulated rainfall in 6-hour windows for the 6-hour periods ending at a) 0000 UTC 13 August 2014
through d) 1800 UTC 13 August 2014. Values in mm as per the color scale. Return to text.
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Figure 9. As in Figure 8 except for the period as the rains moved into New England from a) 1200 UTC 13 August through d) 0600 UTC 14
August 2014. Return to text.
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Figure 10. Stage-IV accumulated rainfall (mm:Green) and 1-hourly rainfall (mm:Gray) at Islip
and Baltimore-Washington airports. Return to text.
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Figure 11. FLASH image of radar-based 3-hour rainfall rate return periods, in years as per the color bar. A loop of these data is available here.
Return to text.
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Figure 12. As in Figure 11 except for over the Baltimore area at 1900 UTC 12 August 2014. A loop is available. Return to text.
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Figure 13. As in Figure 12 except for return periods ending at 0330 UTC 14 August 2014 for eastern Maine. A loop is available here.
Return to text.
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Storm Total Rainfall from the
Mid-Atlantic
8/12 - 13/2014
GREEN HAVEN, MD
10.32
Storm Total Rainfall from Southern
New England and Long Island
8/13/2014
Storm Total Rainfall from
Northern New England
8/13/2014
ISLIP AIRPORT, NY
13.27
PORTLAND, ME
6.44
12.57
MILLVILLE, NJ
DUNDALK, MD
8.94
8.75
HOLBROOK, NY
FREEPORT, ME
6.22
WEST ISLIP, NY
11.50
GRANBY, MA
5.62
NEWPORT, NJ
8.60
BAY SHORE, NY
11.35
NEWBURY, NH
5.26
ROMANCOKE, MD
PUMPHREY, MD
8.47
7.83
ISLIP, NY
10.20
BRUNSWICK, ME
5.25
DEER PARK, NY
8.40
KENNEBUNK, ME
5.16
CENTREVILLE, MD
PASADENA, MD
7.60
7.55
MASSAPEQUA, NY
8.20
WELLS, ME
5.02
RONKONKOMA, NY
8.20
BIDDEFORD, ME
5.00
LITTLE EGG HARBOR TWP, NJ
7.50
WANTAGH, NY
7.84
GRAY, ME
4.97
MANAHAWKIN, NJ
7.25
FARMINGVILLE, NY
7.73
SACO, ME
4.97
STAFFORD TWP, NJ
DEALE, MD
7.25
6.90
MERRICK, NY
6.81
COVENTRY, RI
4.95
CENTEREACH, NY
6.71
POWNAL, ME
4.92
CHURCHTON, MD
6.56
ST. JAMES, NY
6.11
RAYMOND, ME
4.90
LEAMINGS MILL, NJ
BWI AIRPORT, MD
6.40
6.30
LIDO BEACH, NY
5.30
SOUTH SUTTON, NH
4.78
OLD BETHPAGE, NY
5.27
DEERFIELD, NH
4.74
ESTELL MANOR, NJ
6.17
BELLMORE, NY
5.20
WEST FALMOUTH, ME
4.55
CENTREVILLE LANDING, MD
MARLTON, MD
6.06
5.92
SMITHTOWN, NY
5.17
WAYNE, ME
4.54
PORT JEFFERSON STATION, NY
5.10
SUNDERLAND, MA
4.37
MANASQUAN, NJ
5.91
EAST FARMINGDALE, NY
4.93
NEW GLOUCESTER, ME
4.33
POINT PLEASANT, NJ
FORT WASHINGTON, MD
5.90
5.66
GARDEN CITY, NY
4.75
YARMOUTH, ME
4.33
PLAINVIEW, NY
4.70
CUMBERLAND CENTER, ME
4.30
ATLANTIC CITY, NJ
DUNKIRK, MD
5.58
5.53
LONG BEACH, NY
4.64
NORTHAMPTON, MA
4.29
MADISON, CT
4.50
LEWISTON, ME
4.27
UPPER MARLBORO, MD
5.44
MONTVILLE, CT
4.45
DURHAM, ME
4.22
BALTIMORE, MD
5.42
MEDFORD, NY
4.45
CANTERBURY, CT
4.04
BRICK TWP, NJ
5.40
MOUNT SINAI, NY
4.40
PINEWALD, NJ
5.27
EAST NORTHPORT, NY
4.40
HANOVER, MD
5.25
SYOSSET, NY
4.01
NORTH VINELAND, NJ
5.22
FRANKLIN TWP, NJ
NORTH LAUREL, MD
5.19
5.13
BRICK TWP, NJ
ELKRIDGE, MD
5.10
5.04
Table 2. Rainfall data for reports over 4
inches in the New York Metropolitan area
and southern New England. Rainfall in
inches from NWS Public Information
Statements. . Return to text.
JESSUP, MD
5.03
SILVER SPRING, MD
5.03
BRIDGETON, NJ
5.00
BARNEGAT, NJ
5.00
Table 1. . Rainfall data for reports over 5
inches in the Baltimore Washington,
DC.. Rainfall in inches from NWS Public
Information statements. . Return to
text.
Table 3. As in Table 2 except for central
and eastern New England Sites. Return
to text.
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KISP –
Islip, NY
Observed
1 Hour
6 Hour
5.34
12.92
1 Year
PF
Estimate
0.90
1.85
10 Year
PF
Estimate
1.57
3.37
100 Year
PF
Estimate
2.60
5.95
500 Year
PF
Estimate
3.72
8.86
Time Window
0900-1000 UTC 13 August 2014
0600-1200 UTC 13 August 2014
Table 4. Islip, NY ASOS. 1-hour observed total 0900 to 1000 UTC and the 6-hour observed total 0600 to 1200 UTC August
13, 2014 compared to the NRCC Extreme Precipitation Analysis webpage for 1, 10, 100, and 500 year return periods.
Return to text.
KBWI –
Observed
BaltimoreWashington
International
Airport
3 Hour
4.97
1 Year
PF
Estimate
10 Year
PF
Estimate
100 Year
PF
Estimate
1000
Year PF
Estimate
Time Window
1.51
2.71
4.20
6.04
6 Hour
1.86
3.32
5.32
7.98
1600-1900 UTC 12 August
2014
1300-1900 UTC 12 August
2014
5.75
Table 5. Baltimore-Washington International Airport. 3-hour observed total from 1600 to 1900 UTC and 6-hour estimated
1300-1900 UTC compared to the NOAA Atlas-14 volume 2, 10, 100, and 1000 year return periods. Return to text.
KPWM – Observed
Portland,
ME
Jetport
2 Hour
4.21
6 Hour
6.06
1 Year
PF
Estimate
10 Year
PF
Estimate
100 Year
PF
Estimate
500 Year
PF
Estimate
Time Window
1.00
1.53
1.82
2.96
3.16
5.45
4.67
7.34
0100-0300 UTC 14 August 2014
2100-0300 UTC 13-14 August
2014
Table 6. Portland International Jetport. 2-hour observed total from 0100 to 0300 UTC and 6-hour estimated 2100-0300
UTC compared to the NRCC Extreme Precipitation Analysis webpage for 1, 10, 100, and 500 year return periods. Return to
text.
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Appendices of summarized METAR hourly rainfall used to estimate return periods and produce imagery.
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