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Eastern United States Synoptic Rain Event 15-16 May 2014-Draft
heavy rain and flooding event
By
Richard H. Grumm
National Weather Service State College, PA
1. Overview
A deep 500 hPa trough with -4 height anomalies over the central United States (Fig. 1)
combined with a strong ridge with +2 height anomalies over eastern Canada, produced strong
southerly flow. The deep southerly flow brought a plume of high precipitable water (PW: Fig. 2)
air into the region along with strong 850 hPa southerly flow (Fig. 3) resulting in heavy rainfall
(Fig. 4) and isolated severe weather (Fig. 5). The heavy rainfall produced flooding in the MidAtlantic region from Virginia northward into New York State.
The pattern was well predicted by the NCEP models and ensemble forecasts systems. The NCEP
SREF will be used to illustrate the successful prediction of the larger scale pattern. The
precipitation forecasts showed some uncertainty as to the amounts and locations of the heavy
rainfall though all forecasts systems correctly predicted the potential for heavy rainfall in the
general region where the rainfall was observed.
The paper will provide an overview of the pattern, the resulting weather, and forecasts of the
rainfall. The brief, local severe weather which affected central Pennsylvania is also examined
including a mini-supercell which did not produce severe weather. The most significant impact on
the region was heavy rainfall with areas where 50 to 150 mm (2-6 inches: Table 1) of rain was
observed and the resulting flooding along streams and creeks.
2. Data and Methods
The large scale pattern was reconstructed using the 00-hour forecast of the NCEP Global
Forecast System as first guess at the verifying pattern. The standardized anomalies were
computed in Hart and Grumm (2001). All data were displayed using GrADS (Doty and Kinter
1995). The focus on the larger scale pattern was to show the standardized anomalies and how
they helped identify this event and the potential they have in anticipating similar high impact
weather events (HIWE).
The NCEP GFS, GEFS, and SREF were used to examine the forecasts of this event. As the
pattern was generally well predicted the focus is on the quantitative precipitation forecast (QPF)
produced by these systems.
The stage-IV data were used to get a first guess at where, when, and how much precipitation was
observed. These data were analyzed in planview with GrADS and in Python to make point
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diagrams showing the timing of the rainfall. Python was used to plot all-time series and plume
diagrams.
Radar imagery was obtained from the local AWIPS in State College and QPE data was examine
using the Stage-IV data.
3. Pattern and Weather
i.
pattern
The large scale pattern (Figs. 1-3) showed a deep trough to the west and a strong ridge to the
east. The resulting deep southerly flow allowed a surge of deep moisture and strong southerly
flow into the eastern United States. A nearly textbook Maddox Synoptic rainfall (Maddox et al.
1979) event type. There was some shallow cold air damming and easterly flow which likely
enhanced the lift over the frontal boundary in the Mid-Atlantic region contributing to the rainfall
amounts in some areas in excess of 5 inches.
The surface pattern, Figure 6 shows the surface pattern, which indicated a strong surface
anticyclone off the East Coast and a large anticyclone over the central United States. Several
relatively weak cyclones developed in the trough between the two anticyclones. Though not
clearly indicated, the strong easterly flow at low-levels pushed a marine layer into central
Pennsylvania on 14 May creating an enhanced frontal boundary over which the deep southerly
flow (Fig. 3) moved over. It is beyond the scope of this study to show these mesoscale details.
ii.
rainfall
The evolution of the precipitation shield as it moved over the eastern United States (Fig. 4).)
shows the areas of 25 mm or more QPF. The first 3-panels show the 12-hour windows of the
rapidly moving shield of heavy rain from 0000 UTC 15 May through 1200 UTC 16 May 2014.
Figure 4d shows the 24-hour total, the window when most of the rain fell over Pennsylvania and
the Mid-Atlantic region. The rain shield had an abrupt western edge and an embedded northsouth band of 50 mm with a 100 mm area within it over east-central Pennsylvania. It should be
noted that two private rain gauges in Spring Mills, PA recorded around 150 mm ( 6inches) of
rain just a few 10s of kilometers west of the 100 mm (4 inch) area outlined in the Stage-IV data.
These data show the oft observed character of synoptic rainfall events including the sharp northsouth axes of heavy rainfall and the sharp edges of the heavier rainfall.
iii.
Pennsylvania severe weather
The focus here is on three storms, two of storms which produced severe weather in central
Pennsylvania and a mini-supercell. The mini-supercell, which developed near Lewistown, PA,
and moved to the north-northeast shown near its peak (Fig. 7) was not associated with any
known damage. The first severe storm was a bow-echo structure which produced damage north
of Harrisburg, PA. The second severe storm was along a line of storms and heavy rain which
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developed a break in the line and a strong rear flank downdraft near Lock Haven around 2300
UTC. This storm produced some wind damage and a short-lived tornado.
The mini-supercell storm which produced wind damage over southern Clinton County and east
of Lock Haven, PA is shows (Fig. 8) shows a hook in the reflectivity and a ZDR arc around
mesocyclone (see black arc outlining the feature). The base winds (V) show the circulation
associated with the low-level mesocyclone. This well-structured mini-supercell has not been
associated with any wind damage, though it contained many features found in stronger and more
meteorologically significant supercells.
The bow echo developed over Cumberland County (Fig. 9) south of Newville and had over 40kts
of wind associated with at 2212 UTC. The bow feature in the reflectivity became more
pronounced as the feature moved eastward. The final image, at 2249 UTC shows the distinct
bow feature well southwest of Amity Hall, which is just north of Duncannon where wind damage
was reported.
The second known severe storm developed in a line of showers and thunderstorms. It affected an
area just west of the area affected by the mini-supercell about an hour later. At 2300 UTC (Fig.
10) the storm showed a classic “broken-S” pattern, a region where a mesocyclone often develops
along a line of thunderstorms (orange arrow) and contains strong outbound velocities (white
arrow) peaking over 50 kts. Not the low values of ZDR in the region of the strong velocities in
the upper right panel. Higher values of ZDR were present to the east, near Loganton. Figure 11
shows the line as the mesocyclone at was still growing and the break in the line was not
complete. The low ZDR region was present near the emergent mesocyclone, the outbound winds
were weaker at this time and the break in the line, the “broken-S” was evident a few kilometers
northwest of Mackeyville, PA. This storm produced wind damage and a brief tornado on the east
side of Lock Haven, PA around 2300 UTC 15 May 2014.
4. Forecasts
Forecasts of the larger scale pattern were generally good with about 3-5 days lead-time (not
shown). This is revealed by the GEFS QPF (Fig. 12 ) which show the north-south oriented band
of rainfall to exceed 50 mm. Forecasts issued prior to 0000 UTC 11 May had some issues with
the location of the pattern and rainfall. Due to divergent solutions on the exact location and
timing of the rainfall, forecasts issued on and after 1200 UTC 13 May showed the higher
probability of 50 mm or more QPF. These data show the 24-hour window in which the heavy
rainfall was observed. Higher QPF values were indicated when these GEFS forecasts we’re
viewed in 30 to 36 hour windows.
The NCEP SREF (Fig. 13) showed a similar north-south axis of heavy rainfall from the central
Appalachians into southern New York. The axis of the SREF heavy rainfall was west of that
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produced in the GEFS and similar to the GEFS, showed the heavier rainfall with higher
probabilities as the forecast horizon decreased.
A comparison of select 0000 UTC GFS, and GEFS with 0300 UTC SREF data are shown in
Figure. 14 and 1200 UTC GFS, GEFS with 0900 UTC SREF are shown in Figure 15. These data
show similar forecasts by each system with regards to a north-south axis of heavy rainfall and the
general area to be impacted by the rainfall. The GFS had higher QPF amounts than the SREF and
GEFS though most of this could be attributed to averaging 21 members in the two EFSs. All of
these data suggest a well predicted pattern, a synoptic pattern, though the details as to where the
higher QPF would fall to have been more of a forecast issue.
5. Summary
A strong frontal system ahead of a deep late season trough to the west and a strong ridge to the
east brought a surge of deep moisture into the eastern United States resulting in heavy rain in the
Mid-Atlantic region. Due to a lack of deep instability, severe weather was generally a low-level
issue with this Synoptic-type heavy rainfall event. The heaviest rains fell in a 24 hour period
between about 1800 UTC 15-16 May 2014 (Figs. 4 & 16) from Virginia into central
Pennsylvania (Table 1). The heavy rainfall produced flooding along rivers and streams in the
Mid-Atlantic region with approximately 34 points reaching or exceeding flood stage (Table 2).
Similar to many synoptic events, the heavier rainfall fell in a general north-south oriented band
in deeps southerly flow in a plume of air with above normal precipitable water. This is another
case which demonstrates the value of anomaly based analysis and forecasting in identifying high
impact weather events. This and other cases show the value of anomaly based forecasting in
near, short, and medium range forecasting.
In this event, the relatively accurate prediction of the anomalous plume of deep moisture and
strong low-level southerly flow resulted in relatively useful and accurate QPF forecasts in the
NCEP deterministic models and ensemble forecast systems. This relationship between the strong
signal in the anomalies and the high QPFs should provide some modicum of confidence
information to forecasters predicting heavy rainfall and potential flooding events. The NCEP
guidance shown here implied a relatively impressive lead-time on the order of 3-6 days.
The heavy rain produced rapid rises on several creeks and streams in central Pennsylvania such
as Shermans and Penns Creek (Fig. 17). The black curves show the rapid rises on both creeks.
Forecasts of the flooding were relatively slow in showing the flood potential. Similar to many
previous events, when widespread rainfall over a basin exceeds 75 mm ( 3 inches), flooding
along smaller streams and creeks often occurs.
6. Acknowledgements
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The Pennsylvania State University for data access and providing students for research and case
study projects. Ron Holmes for processing river stages and forecasts. Charles Ross for
consultation and editing Elyse Colbert for summarizing observations and rainfall observations.
Charlie Chillag for data on river flooding and stages.
7. References
Bodner, M. J., N. W. Junker, R. H. Grumm, and R. S. Schumacher, 2011: Comparison of
atmospheric circulation patterns during the 2008 and 1993 historic Midwest floods.
Natl. Wea. Dig., 35, 103-119.
Doswell,C.A.,III, H.E Brooks and R.A. Maddox, 1996: Flash flood forecasting: ingredients
based approach. Wea. Forecasting, 11, 560-581.
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.
Durkee, JD., L Campbell, K Berry, D Jordan, G Goodrich, R Mahmood, S Foster, 2012: A
Synoptic Perspective of the Record 1-2 May 2010 Mid-South Heavy Precipitation Event.
Bull. Amer. Meteor. Soc., 93, 611–620. doi: http://dx.doi.org/10.1175/BAMS-D-1100076.1
Junker, N. W., R. S. Schneider and S. L. Fauver, 1999: Study of heavy rainfall events
during the Great Midwest Flood of 1993. Wea. Forecasting, 14, 701-712.
Maddox,R.A., C.F Chappell, and L.R. Hoxit. 1979: Synoptic and meso-alpha aspects of
flood events. Bull. Amer. Meteor. Soc., 60, 115-123.
flash
Stuart, N., Grumm, R. (2009) The Use of Ensemble and Anomaly Data to Anticipate Extreme
Flood Events in the Northeastern United States. Nat. Wea. Digest 33: 185-202.
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Figure 1. GFS 00-hour forecasts of 500 hPa heights (m) and 500 hPa height anomalies (standard deviations) in 12 hour increments from a) 0000 UTC 14 May 2014
through f) 1200 UTC 16 May 2014. Return to text.
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Figure 2. As in Figure 1 except for precipitable water (mm) and precipitable water anomalies in 6-hour increments from 0600 UTC 15 May through 1200 UTC 16 May
2014. Return to text.
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Figure 3. As in Figure 2 except for 850 hPa winds and wind anomalies. Return to text.
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Figure 4. Estimated rainfall for amounts of 25mm or greater from the stage-IV rainfall data for the 3 12-hour periods ending at a) 1200
UTC 15 May, b) 0000 UTC 16 May, and c) 1200 UTC 16 May 2014 and d) the 24-hour accumulated rainfall over the period ending at
1800 UTC 16 May 2014. The 24 hour period covers the heavy rainfall over Pennsylvania and the Mid-Atlantic region. . Return to text.
Return to rainfall portion.
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Figure 5. Storm reports from Storm Prediction Center. Colored by type. Return to text.
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Figure 6. As in Figure 3 except for MSLP. Return to text.
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Figure 7. Base reflectivity (Z) and base velocity (V) focused over Centre and southern Clinton counties showing the evolving
storm at (top) 2018 UTC, 2123 UTC and 2137 UTC. The hook echo is clear in the latter two images and at least weak rotation
is evident in each image. Return to text.
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Figure 8. KCCX 4 panel valid at 2137 UTC 15 May 2014 showing (clockwise from upper left) base reflectivity (Z), differential reflectivity (ZDR),
correlation coefficient (CC), specific differential phase (KDP). Note ZDR arc in the upper right panel. The ZDR arch is drawn to the left of the feature to
highlight it. Return to text.
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Figure 9. As in Figure. except for 2212, 2239, and 2249 UTC. Return to text.
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Figure 10. KCCX radar showing 0.5 dgree base reflectivity, ZDR, CC, and winds at 2303 UTC 15 May 2014. THe orange arrow show the break or “brokenS” in the reflectivity data, the pink arrow shows the ZDR minimum in the region of strong velocities deonted by the white arrow in the lower left panel.
Return to text.
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Figure 11. As in Figure 10 except valid at 2253 UTC and the CC is replaced by the storm relative velocity. Return to text.
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Figure 12. GEFS forecasts showing the probability of 50 mm or more QPF in the 24-hour window ending at 1800 UTC 16 May from GEFS initialized at
a) 00000 UTC 10 May, b) 0000 UTC 11 May, c) 1200 UTC 11 May, d) 1200 UTC 12 May 2014, e) 1200 UTC 13 May, and f) 1200 UTC 14 May 20144. Data
include the probability of exceeding 50 mm and contours in 25 mm increments. Return to text.
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Figure 13. As in Figure 11 except for SREF QPF probabilities initialized at a) 0300 UTC 13 May, b) 2100 UTC 13 May, c) 0300 UTC 14 May 2014, d) 0900 UTC
14 May, e) 1500 UTC 14 May, and f) 0300 UTC 15 May 20l14. Return to text.
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Figure 14. As in Figure 12 except showing 24 hour accumulations (GFS) and probabilities of 50 mm or more QPF in the GEFS and SREF from a) GFS initialized at
0000 UTC 14 May, b) SREF initialized at 0300 UTC 14 May, c) GEFS initialized at 0000 UTC 14 May, and c) GFS initialized at 0000 UTC 15 May, b) SREF initialized
at 0300 UTC 15 May, c) GEFS initialized at 0000 UTC 15 May. Contours are every 25 mm. Return to text.
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Figure 15. As in Figure 14 except for 1200 UTC GFS and GEFS and 1500 UTC SREF on 14 and 15 May 2014. Return to text.
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LOCATION
Spring Mills
2 WSW DEER RUN
AMOUN
T
6.00
5.50
COUNTY
STATE
Centre
PA
PENDLETON
WEST VIRGINIA
CITY OF
1 SSE CHARLOTTESVILL
CHARLOTTESVILLE
VIRGINIA
5.22
WINTERGREEN
NELSON
VIRGINIA
5.19
NEWPORT
PERRY
PENNSYLVANIA
5.00
2 E MYERSVILLE
FREDERICK
MARYLAND
4.65
1 NW STERLING PARK
LOUDOUN
VIRGINIA
4.50
CASHTOWN 1S
ADAMS
PENNSYLVANIA
4.41
2 WSW POTOMAC
MONTGOMERY
MARYLAND
4.40
SOUTH MOUNTAIN
FRANKLIN
PENNSYLVANIA
4.28
8 W CHARLOTTESVILLE
ALBEMARLE
VIRGINIA
4.28
PURCELLVILLE
LOUDOUN
VIRGINIA
4.25
1 NNE WASHINGTON GRO
MONTGOMERY
MARYLAND
4.25
1 SSE WEST MCLEAN
FARIFAX
VIRGINIA
4.23
1 WSW CAVETOWN
WASHINGTON
MARYLAND
4.22
WILLIAMSPORT
LYCOMING
PENNSYLVANIA
4.20
WEST SPRINGFIELD
FARIFAX
VIRGINIA
4.20
CASHTOWN
ADAMS
PENNSYLVANIA
4.20
2 N DARNESTOWN
MONTGOMERY
MARYLAND
4.18
E OAKTON
FAIRFAX
VIRGINIA
4.16
2 W CHARLOTTESVILLE
ALBEMARLE
VIRGINIA
4.16
3 NNE SMITHSBURG
WASHINGTON
MARYLAND
4.12
SUMMERDALE
CUMBERLAND
PENNSYLVANIA
4.10
COUNTRYSIDE
LOUDOUN
VIRGINIA
4.10
BURKE
FARIFAX
VIRGINIA
4.10
2 N WOLFSVILLE
FREDERICK
MARYLAND
4.10
3 SW FAIRPLAY
WASHINGTON
MARYLAND
4.08
2 WNW STANARDSVILLE
GREENE
VIRGINIA
4.05
2 SE LYDIA
GREENE
VIRGINIA
4.05
1 SSW ORANGE
ORANGE
VIRGINIA
4.02
FRONT ROYAL
WARREN
VIRGINIA
4.00
7 NNW MADISON
MADISON
VIRGINIA
4.00
Table 1. List of locations, counties, and States where point data equal to or exceeding 4.00 inches (100
mm) of total rainfall. There were too many reports, over 120, with 3.00 to 3.99 inches (75mm) to
record easily. The Spring Mills reports came from an individual and a Newspaper report. Return to text.
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Figure 16. Stage-IV data for a point near Spring Mills, PA and a point to the east from the Stage-IV data. The heaviest rainfall
in the data was just east of this point. Return to text.
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Crest Statistics
Flooding and Crests - 05/16/2014 - 05/20/2014
First flood of 1 - May, 2014 and Seventh flood in 2014
Number of MARFC Forecast Points Flooding - 33 + 1 point flooding more than once - 34
Number of Floods Cresting in Minor Range - 21
Number of Floods Cresting in Moderate Range - 12
Number of Floods Cresting in Major Range - 1
Number of Floods Cresting in Missing Range - 0
MARFC Power Ranking - 91 (Minor = 1 - Moderate = 5 - Major = 10 - Missing =1)
E = Estimated
Table 2. Summary of Flooding in the Mid-Atlantic Region. Data provided by the Mid-Atlantic
River Forecast Center (MARFC). Return to text.
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Figure 17. River Stages (black) and forecasts for a point on Sherman’s Creek near Sharman’s Dale and Penns Creek, at Penns
Creek. Forecasts are in colored lines. Return to text.
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