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Locally heavy rainfall and return Periods
By
Richard H. Grumm
and
Charles Ross
National Weather Service State College, PA
1. Overview
Two heavy rainfall events affect the United States between 8 and 10 September 2014. The first
event occurred over Arizona (Fig. 1) on 8 September and the second event occurred over
Missouri on 10 September (Fig. 2). Both events were associated with deep plumes of moisture
with above normal precipitable water (PW) and strong low-level winds in the moisture field.
Deep moisture and moisture flux have been associated with heavy rainfall and these two events
were fairly proto-typical heavy rainfall events.
Both events developed rapidly and had periods of intense convection which produced relative
rapid rises water levels in streams and urban areas. Flooded streets and communities made the
news and social media in the Phoenix area on 8 September 2014 (AP 2014a). Reports in excess
of 3 inches of rain closed interstates and close many roads in the Phoenix Metropolitan area (AP
2014b). The NWS recorded a new all-time 24-hour rainfall record of 2.99 inches in Phoenix
breaking the old record of 2.91 inches set in 1933. The 24 hour rainfall event was larger than the
climatological mean, 2.71 inches, which falls during the summer entire summer season.
The rainfall in north central Missouri was more widespread than the Arizona event and produced
higher rainfall amounts. The reports of flooding and flooding impacts were not as severe (AP
2014c) as those reported in the Phoenix metropolitan area.
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.
Both events are examined from the perspective of the overall pattern, using standardized
anomalies to see where key parameters often associated with flooding may have contained a
signal. The NOAA-14 data are used to show how the return periods of flood data could be of
value when forecasting flash flooding in urban and rural areas.
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2. Methods and Data
The NCEP 13km rapid refresh 00-hour forecasts were used to reconstruct the pattern associated
with both events. These data provided 1 hour analysis and were used in lieu of the coarser
Climate Forecast System Reanalysis data which are available in 6-hour increments on a 0.5
degree grid. The fields, such as PW, mean-sea level pressure and winds are shown relative to the
departures from normal.
The NOAA Atlas 14 Precipitation Frequency Data were obtained from the Multi-radar multisensor (MRMS) Flash site. This site provides a range of data to include the return period in 1, 3,
6, 12, 24 and the maximum return period. An example of the 3-hour return periods at 0500 UTC
10 September is shown in Figure 3. These data show a larger region where the return period
exceed 10 years with embedded regions where the rainfall return periods for the 3-hour period
were between 75 and 100 years. Limited knowledge on the use of these data suggest nuisance
flooding is observed with return period over about 50 years and significant flooding often is
observed when the return periods exceed the 100 year period. Many severe flooding events, such
as the Long Island floods of August 2014 (Grumm 2014) have return periods in the 200 and
higher range.
The rainfall images in plain view were produced using the Stage-IV data and plotted using
GrADS.
3. Pattern and Standardized Anomalies
i.
Arizona event
The 500 hPa pattern in 6-hour intervals from 1200 UTC 7 to 1800 UTC 8 September 2014 show
a weak ridge over the southwestern United States and a tropical cyclone of the Baja (Fig. 4) with
implied southwesterly flow into southern California and Arizona. There was a region of deep
moisture and anomalous PW off the Baja where the PW anomalies are in excess of +5 above
normal. Some of the deep moisture was present over the southwestern United States and Arizona
where PW anomalies were +3above normal (Fig. 5).
In the presence of the deep moisture over Arizona, the RAP showed a surge of strong
southeasterly winds over Arizona between 0600 and 1200 UTC September 2014. The heavy
rains fell during the period of strong southeasterly winds (Fig. 6) when the v-wind anomalies
peaked near +3 above normal, not with the 850 hPa southwesterly flow to the west.
ii.
Missouri event
The large scale pattern over the United States on 8-10 September showed the weak ridge over the
southwestern United States (Fig. 7) and a short-wave with modest negative height anomalies
moving into the central United States. The heavy rainfall in north-central Missouri was
associated with the enhanced flow over the ridge and the short-wave.
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A plume of high PW air was present over the central Plains with PW anomalies on the order of
+3above normal (Fig. 8) over portions of Kansas, Nebraska, Iowa, and northern Missouri. In
this region of high PW air a strong 850 hPa low-level jet was present (Fig. 9). This low-level jet
peaked near +5above normal between 0400 and 0600 UTC 10 September in the region of high
PW air.
At the surface, the strong boundary had a developing surface cyclone (Fig. 10). This feature
moved along the frontal boundary and deepened as it moved northeastward. The pressure
anomalies in the RAP were in the -3 below normal range by 0800 UTC.
4. Rainfall and Rainfall Return Periods
i.
Missouri rainfall
The rainfall over Missouri (Fig. 11) peaked in the period ending at 0600 UTC 10 September
(Fig. 11b) and no significant rain was observed after 1200 UTC 10 September (Fig. 11c). The
short window of heavy rainfall, in the 3-6 hour timeframe is a common characteristic of many
heavy rainfall events. A plot of the Stage-IV data hourly rainfall accumulations showed the
heavy rainfall began around 0400-0500 UTC and that the rainfall ended at 1200 UTC. The total
rainfall at this point was 208 mm (8 inches).
The 3-hour return period data showed heavy rainfall with a return period in northwest Missouri
of over 100 years north of Andrew County at 0230 UTC. Heavy rain developed to the east and
by 0400 UTC there were two areas where the return periods exceed 100 years1 including the
region north of Andrew County and a new area in Grundy County, Missouri. Areas of heavy
rainfall continued with over 100 year return periods through 0600 UTC after which the rainfall
began to diminish.
Based on news reports the flooding was significant but historic.
ii.
Arizona rainfall
The heavy rainfall in southern Arizona and near Phoenix began after 0600 UTC (Fig. 14) and
peaked in the 0600 to 120 UTC timeframe. The hourly rainfall return periods showed 50 to 100
year returns by 0900 UTC 8 September southeast of the City (Fig. 15). By 1030 UTC 50 to 75
year return period hourly rates were observed over the Phoenix metropolitan area. The heavy
rainfall area kept moving to the northwest and exited the metropolitan area by 1130 UTC (Fig.
6). The 3-hour return periods peaked at 1130 UTC (Fig. 17) and diminished rapidly thereafter.
Peak return periods were near 100 years. The maximum rainfall return period rates (not shown)
indicated some small regions of the metropolitan area had 100 to 200 year return periods.
1
The shades of purple, often 3 are not easy to view in this document.
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The return period verse rainfall in tabular form is shown in Table 1. The 3 and 6-hour rainfall
amounts were in the 10 to 100 year period at this point and the 24 hour rainfall was just over a
100 year event.
5. Summary
Two heavy rainfall events affected the United States on between 8 and 10 September 2014. The
first event affected the Phoenix metropolitan area producing widespread urban flooding. The
second event occurred in more rural environments of north-central Missouri. The impact of the
flooding, as reported by news media suggested the 2-3.5 inches of rain in the desert and urban
environment was of greater impact than the 5-9 inches of rain in the rural regions of Missouri.
Both events were associated with strong low-level flow and above normal PW. In both cases PW
and v-wind anomalies were on the order of +2 to +3 above normal. The high PW associated
with the strong flow likely contributed to the potential for a high end rainfall in each region. Both
events were mesoscale in nature and convectively driven.
In the Missouri event, a deep surface cyclone developed along the moisture and thermal
boundary. This evolution was similar to the intense wave along the boundary which brought the
record rainfall to Long Island on 13 August 2014 (Grumm and Ross 2014). This may be
indication and short-term forecast tool where the low-level flow is enhanced by the evolution of
the wave. Strong forcing such as this and the locally intense low-level jet should be predicted
with high resolution models. The only data available for this case locally was the 13km RAP (not
shown) which some signal in its short range forecasts of the evolution of the cyclone, radar, and
QPF.
The return periods in these two events (Tables 1-3) show that most of the rainfall return rates
were in or near the 100-year timeframe. The points selected were picked based on good rainfall
gauge data and return period data. These data do not represent the maximum return periods
during the events. The goal was to compare observed rainfall from a gauge to the return period.
The return periods in these events were slightly lower than those observed in the Baltimore and
Long Island floods of 13 August 2014 (Grumm and Ross 2014). The 3-hour return periods
during the Baltimore floods were on the order of 200 years and during the Islip event 1 and 6
hour return periods exceeded the 1000 year return periods (Grumm and Ross 2014).
In this case in the urban and desert environment with 2-3.5 inches of rain and 75 to 200 year
return periods for the rainfall produced significant flooding which made the National news. From
a reporting perspective the Phoenix event with lower rainfall amounts and lower return periods
appeared to be a more significant and higher impact event than the rural Missouri event.
These data continue to show the potential value in return periods in gaging the potential severity
of flooding during heavy rainfall events.
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6. Acknowledgements
7. References
Associated Press, 2014a: Arizona Governor declares state of emergency due to flooding. AP, 8
September 2014 and similar stories.
Associated Press, 2014b: Phoenix floods and heavy storms set all-time rainfall record for a single
day. AP, 8 September 2014 and similar stories.
Associated Press, 2014c: Heavy rains flood parts of Missouri. AP, 11 September 2014.
Du, J, R.H. Grumm, 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.
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.
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Figure 1. Stage-IV rainfall (mm) over the southwestern United States from 1800 UTC 7 September through 0600 UTC 9 September 2014. Return
to text.
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Figure 2. As in Figure 2 except for 1800 UTC 9 to 1200 UTC 10 September 2014 over Missouri. Return to text.
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Figure 3. Flash site 3-hour return rainfall return period for the 3-hour period ending at 0500 UTC 10 September 2014. Return to
text.
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Figure 4. 13km RAP 500 hPa heights and height anomalies over the southwestern United States in 6-hour increments from a) 0000 UTC 7 September
through f) 0600 UTC 8 September2014. Return to text.
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Figure 5. As in Figure 4 except for precipitable water and precipitable water anomalies. Return to text.
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Figure 6. As in Figure 5 except for 850 hPa wind and v-wind anomalies. Return to text.
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Figure 7. As in Figure 4 except for 500 hPa heights over the US from a) 1200 UTC 8 September 2014 through f) 0000 UTC 11 September 2014.
Return to text.
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Figure 8. As in Figure 7 except for precipitable water and precipitable water anomalies every 2 hours from a) 0000 UTC 10 September through f)
1000 UTC 10 September 2014. Return to text.
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Figure 9. As in Figure 8 except for 850 hPa winds and v-wind anomalies. Return to text.
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Figure 10. As in Figure 8 except for mean sea level pressure and pressure anomalies. Return to text.
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Figure 11. Stage IV data showing accumulated rainfall for the 6-hour periods ending at a) 0000, b) 0600, c) 1200 and d) 1800 UTC 1 September
2014. Return to text.
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Figure 12. Hourly plot (gray) of Stage-IV data for a point near 40.20 north and -92.47 in north-central Missouri for the period
of 0000 to 2400 UTC 10 September 2014. Return to text.
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Figure 13. 3-hour return periods from the Flash site valid top to bottom at 0230, 0400, and 0600
UTC 10 September 2014. Return to text.
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Figure 14. As in Figure 12 except for focused over Arizona at a) 0000, b) 0600, c) 1200 and d) 1800 UTC 8 September 2014. Return to text.
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Figure 15. As in Figure 13 except for 0900 UTC showing initial heavy rainfall. Image enlarged to show the region better than latter times (below). Return to text.
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Figure 16. As in Figure 13 except for 1 hourly return periods in Arizona at 1030 and 1130 UTC 8 September 2014. Return to text.
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Figure 16. As in Figure 15 except for 3-hour return periods valid at 1130 UTC. Return to text.
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KPHX –
Phoenix Sky
Harbor
Airport
1 Hour
Observed
1 Year
PF
Estimate
10 Year
PF
Estimate
100 Year
PF
Estimate
1000
Year PF
Estimate
Time Window
1.02
0.56
1.20
1.90
2.65
3 Hour
2.03
0.67
1.36
2.17
3.12
6 Hour
3.09
0.81
1.56
2.41
3.35
24 Hour
3.30
1.08
2.10
3.26
4.57
0900-0900 UTC 8 September
2014
0900-1200 UTC 8 September
2014
0900-1500 UTC 8 September
2014
Calendar Day 8 September
Table 1. The 1-hour observed total from 0900 to 1000 UTC, 3-hour observed total from 0900 to12900 UTC and 6-hour
observed total from 0900-1500 UTC compared to the NOAA Atlas-14 volume 1, 10, 100, and 1000 year return periods. Blue
shading shows where 100 year returns were met. Return to text.
KIRK –
Kirksville,
MO Regional
Airport
2 Hour
Observed
1 Year
PF
Estimate
10 Year
PF
Estimate
100 Year
PF
Estimate
1000
Year PF
Estimate
Time Window
5.08
1.60
2.73
4.21
5.91
6 Hour
8.63
2.18
3.78
6.07
8.82
0400-0600 UTC 10 September
2014
0400-1000 UTC 10 September
2014
Table 2. As in Table 1 except for Kirksville, MO Regional Airport . 2-hour observed total from 0400 to 0600 UTC and 6-hour
Observed from 0400-1000 UTC compared to the NOAA Atlas-14 volume 1, 10, 100, and 1000 year return periods Return to
text.
KCDJ –
Chillicothe
Agri-Science
Center
3 Hour
Observed
1 Year
PF
Estimate
10 Year
PF
Estimate
100 Year
PF
Estimate
1000
Year PF
Estimate
Time Window
3.00
1.77
3.27
5.43
8.07
6 Hour
4.98
2.14
3.91
6.53
9.78
0400-0700 UTC 10 September
2014
0400-1000 UTC 10 September
2014
Table 3. As in Table except for Chillicothe, MO Agri-Science Center . 3-hour observed total from 0400 to 0700 UTC and 6-hour
estimated 0400-1000 UTC compared to the NOAA Atlas-14 volume 1, 10, 100, and 1000 year return periods. Return to
text.
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Appendix of 13 August return period data:
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.