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Mid-Atlantic heavy rainfall event
of
15-16 October 2014
By
Richard H. Grumm
National Weather Service State College, PA
1. Overview
A deep 500 hPa trough (Fig. 1) moved across the central United States from 13-16 October
2014. Ahead of the trough, a strong 500 hPa ridge was present over the western Atlantic. The
flow between the deep trough and downstream ridge allowed a surge of warm moist air into the
Mid-Atlantic region (Fig. 2) with a strong southerly 850 hPa low-level jet (LLJ: Fig. 3). The
strong southerly flow resulted in a widespread rainfall event (Fig. 4). Rainfall totals exceed over
24 mm over large portion of the Mid-Atlantic region with enhanced south-to-north bands of
heavier rainfall. Some locations received between 100 and 128 mm of rain.
The strong south-to-north PW surge, the strong 850 hPa southerly winds, and the orientation of
the rainfall axis implied that this event was Maddox-Synoptic rainfall event (Maddox et al.
1979). This event type has been shown to be one of the more prolific heavy rainfall event types
in the eastern United States (Grumm 2011; Grumm and Holmes 2007; Grumm and Hart 2001;
and Junker et al 1993) and when this event type sets up with high PW and V-wind anomalies the
events are often associated with locally heavy rainfall and flooding. Flooding is also related to
the antecedent conditions, which in this case, the widespread heavy rain fell on the heels of a
prolonged dry period producing no significant flooding.
Effective forecasting of extreme weather events (EWEs) and high impact weather events
(HIWE) at near and longer terms is a critical aspect of tying weather to decision making. At
longer ranges (1-5 days) ensemble forecasts tools aid in pattern recognition and when used in
conjunction with reanalysis climate (R-Climate) and model climate (M-Climate) they can aid in
quickly identifying potential EWE or HIWEs. A good starting point is a summary of the
anomalies for key patterns (WR-SATABLE). The summary of key fields in the eastern United
States from the North American Ensemble Forecast System1 (NAEFS: Cui et al... 2014;Cu et al.
2012).
The NAEFS SA Table (Fig. 5) standardized anomalies indicated a period of high V-winds, high
moisture as indicated by the specific humidity (q), precipitable water (PW), and integrated
vertical moisture transport (IVT). Other items on the table indicated a deep trough with the
1
The NAEFS is a calibrated ensemble produced from the combination of the NCEP Global Ensemble Forecast
System (GEFS) and the Canadian Global Ensemble Forecast System (CMC-EFS). The Canadian ensemble uses the
GEM to produce the forecast and the core of the GEFS is the GFS. Each system contributes 20 member to produce
a 40 member ensemble.
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approaching system. The return intervals suggested at some levels, the V-winds and IVT were at
the tails of the 30-year period of record. Most parameters showed a return period of 2-5 years.
The message was a relatively high end weather event in the eastern United States but not a EWE
event. The V-winds (Fig. 6) were highly anomalous in the 850 to 500 hPa layer. The 850 hPa Vwind anomalies in the eastern United States peaked at +4.6SD above normal with a return period
of 30 years, the length of the climatology. The 850 hPa winds valid at 1200 UTC 15 October
2014 (Fig. 7) show the strong southerly jet in terms of the standardized anomalies and the return
periods during the time of heavy rainfall from western Virginia into central Pennsylvania.
This paper will show the pattern associated with the heavy rainfall event of 15-16 October 2014
in the Mid-Atlantic region. This high-end rainfall event produced little impact due to relatively
dry antecedent conditions. The event occurred in a pattern conducive to heavy rainfall and had
standardized anomalies which implied a high probability of a significant rainfall event. The focus
of this paper is on the quantitative precipitation forecasts and how the SREF and HRRR forecasts
may be used in shorter ranges during an event.
2. Methods
The NCEP 3km HRRR and 16km SREF were used to show the evolution of the rainfall and
precipitation. The HRRR had the added value of showing the “evolution” of the radar and
convection. The 0.5 degree CFSR was used to construct the larger scale pattern.
Radar data was retrieved from the multi-radar/multi-sensor (MRMS) site to evaluate and make
loops of the evolving convection and rainfall.
The rainfall images in plain view were produced using the Stage-IV data and plotted using
GrADS.
The NAEFS situational awareness table and anomalies were retrieved from the NWS Western
Region site.
3. SREF QPF
The SREF did reasonably well with the larger scale pattern (not shown) and thus simulated a
south-to-north axis of heavy rainfall indicative of a Maddox-Synoptic rainfall event (Fig. 8). The
corresponding ensemble mean and spaghetti plot for these forecasts (Fig. 9) showed the rainfall
was focused with little rain ahead of the implied north-south band. These forecasts show the
most recent 6 SREF cycles available as the heavy rain began over central Pennsylvania.
The 0900 UTC SREF would have been available for a forecast update by approximately 1200
UTC. The rainfall predicted for the 12-hour period ending at 0000 UTC 15 October (Figs. 10 &
11) show the updated forecasts which include the 0900 UTC SREF. The radar image at 1200
UTC (Fig. 12) implies that a band of intense rainfall was already on the eastern edge of the
forecast area of heavy rain indicated by the SREF (Figs. 10 & 11). These forecasts verses the
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radar implied that the western edge of the rainfall may have already occurred and was potentially
over. The HRRR is shown in the following section as an example on how to address these issues
in real-time.
The latter half of the event is shown using the updated 1500 UTC 15 October SREF (Figs. 13 &
14). The 1500 UTC SREF (Fig. 13a) implied an eastward shift to the QPF shield and the axis of
heavy rainfall. This shift was slow but progressive in each successive forecast and the value of
the most recent forecast. The 1500 UTC with the high probability of heavy rain in eastern
Pennsylvania and central New York reinforced what the 1200 UTC and latter 1500 UTC (Fig.
12) radar implied.
The next section examines the value of the HRRR to improve upon the subtle signals provided
by the SREF.
4. HRRR
The 3km NCEP HRRR is presented here to illustrate some limitations of courser models which,
are updated in 6-hour increments and require time to spin-up convection and precipitation. It
would be prohibitive to show all the forecast cycles and their variations. Thus, a mix of some
output from individual run and a mix of forecasts from successive runs all valid at the same time
are presented.
Six successive 1200 UTC HRRR forecasts valid 1300 through 1500 UTC 15 October 2014 (Fig.
15) show the estimated evolution of the radar. These data emphasize weak to few echoes over
most of western Pennsylvania where SREF forecasts indicated the potential for additional heavy
rainfall through 1800 UTC and possibly through 0000 UTC 16 October. The intense echoes in
the HRRR were all displaced to the east of the region of heavy rain the SREF was focused on.
This was in relatively good agreement with the 1500 UTC composite radar (Fig. 12).
The total QPF produced by the 1300 UTC HRRR (Fig. 16) showed the rainfall which might fall
in each time window. These data show that the axis of heavy rain was east of the SREF region
and the back edge of the rain was east of the region still within the SREF 25 mm area. In the
period of 1400 to 1900 UTC heavy rain was forecast by the HRRR in areas some early SREF
members were slow to bring in the heavy rain (Fig. 8 & 9).
An examination of 6 HRRR runs valid at 1800 UTC 15 October (Fig. 17) indicate considerable
run-to-run variability with the more intense areas of high reflectivity over central and eastern
Pennsylvania. The 1800 UTC verifying radar (Fig. 18) shows that the overall intensity of the
event decreased around 1800 UTC with a few stronger elements in southeast Pennsylvania. The
2100 UTC image indicated that this area of rain intensified and moved into northeast
Pennsylvania and New York and another vigorous mesoscale feature was coming out of
Maryland into Pennsylvania. A loop of these data (not shown) implied that this feature brought
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another round of rain to southeast Pennsylvania and this was the last significant band of the event
in Pennsylvania.
5. Summary
A significant but sub-EWE rainfall event affected the Mid-Atlantic region on 15-16 October. The
larger scale pattern and the anomalies indicated the potential for a higher end event with at least
3 days of lead-time. As the event unfolded the NCEP SREF provided useful guidance as to the
areas to be affected by rain and potentially heavy rain. However, the details and rapid evolution
of the event were better captured using the high resolution rapid updating 3km NCEP HRRR.
This implies an ensemble to high resolution forecast funnel approach to this and similar events.
The NAEFS data shown here provided important guidance relative to the larger scale pattern.
Only the 12 October forecasts were presented thus implying a 3-day predictability horizon of a
significant weather event based on the standardized anomalies and return periods. However, a
more thorough examination of the data suggested that this events predictability horizon was on
the order of 3-6 days.
The SREF had the generally areas correctly forecast as to where the rain would fall. This implies
the SREF correctly predicted the larger scale pattern and had the forcing to lift the warm moist
air. The slow but progressive shift in the QPF field (Fig. 13 and Fig. 14) in the SREF implies that
the short-term forecasts often produce more accurate forecasts and in short-term forecasting new
guidance can be of great value. Unfortunately the SREF is updated every 6 hours which limits
the value of the SREF during an on-going heavy rainfall event. This implies that there should be
considerable value on short-time scales of rapidly updated forecasts.
The qualitative comparative analysis of the HRRR (Figs. 15-18) and radar implied that the
HRRR was able to depict some of the mesoscale evolutions of the precipitation event. The exact
details were not perfect. These data also suggested that the rapid updated HRRR with a hot-start
could provide useful information to change the QPF forecasts provided by the larger scale
models and ensemble forecast systems. In this event, the HRRR suggested the SREF was too wet
too far west and too dry to the east. The successive HRRR runs may be of value improving upon
these forecasts. Juanzhen et al. (2014) showed the improved QPF skill in short ranges when
using rapidly updated convective allowing model. This simple case study appears to show that
this can be applied to the operational SREF using the HRRR.
Clearly, more quantitative analysis is required and there is a significant amount to learn about
effectively using high resolution guidance. Additionally, the run-to-run differences require a
storm scale ensemble. NCEP has plans to produce a HRRR-E (Ensemble) using a mix of 1-3
HRRR runs per hour and lagging in several previous forecast cycles to produce such and
ensemble.
6. Acknowledgements
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WR SA data provided by Trevor Alcott. NCEP provided the HRRR runs and Geoff Manikin
provided details on the HRRR and future plans on the HRRR. NAEFS references were provided
by Bo Cui of NCEP.
7. References
Cui, B., Y. Zhu, Z. Toth and D. Hou, 2014: "Development of Statistical Post-processor for NAEFS"
Submitted to Weather and Forecasting (in process)
Cui, B., Z. Toth, Y. Zhu and D. Hou, 2012: "Bias Correction For Global Ensemble Forecast"
Weather and Forecasting, Vol. 27 396-410
Grumm, R.H. 2011: Mo Record Maker rain event of 29-30 March 2010. NWA,Electronic
Journal of Operational Meteorology,EJ4.
Grumm, R.H., and R. Hart, 2001a: Anticipating Heavy Rainfall: Forecast Aspects.
Preprints, Symposium on Precipitation Extremes, Albuquerque, NM, Amer. Meteor. Soc.,
66-70.
Grumm, R.H., and R. Holmes, 2007: Patterns of heavy rainfall in the mid-Atlantic. Preprints, Conference on Weather Analysis and Forecasting, Park City, UT, Amer. Meteor.
Soc., 5A.2.
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.
Juanzhen Sun, Ming Xue, James W. Wilson, Isztar Zawadzki, Sue P. Ballard, Jeanette
Onvlee-Hooimeyer, Paul Joe, Dale M. Barker, Ping-Wah Li, Brian Golding, Mei Xu, and
James Pinto, 2014: Use of NWP for Nowcasting Convective Precipitation: Recent Progress
and Challenges. Bull. Amer. Meteor. Soc., 95, 409–426.
doi: http://dx.doi.org/10.1175/BAMS-D-11-00263.1
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.
Morris L. Weisman, Clark Evans, and Lance Bosart, 2013: The 8 May 2009 Superderecho:
Analysis of a Real-Time Explicit Convective Forecast. Wea. Forecasting, 28, 863–892.
doi: http://dx.doi.org/10.1175/WAF-D-12-00023.1
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Figure 1. CFS 500 hPa height and standardized anomalies from a) 0000 UTC 13 October through f) 0000 UTC 18 October 2014. Contours
every 60 m and anomalies as in color bar. 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 for a) 0000 UTC 15 to f)
0600 UTC 16 October 2014. Return to text.
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Figure 3. As in Figure 2 except for 850 hPa winds (ms-1) and v-wind anomalies. Return to text..
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Figure 4. Stage-IV rainfall in(mm) for the period of 0600 UTC 13 to 1200 UTC 16 October 2014. Return to text.
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Figure 5. View of the standardized anomaly version (left) and the of the return period (right) in years of the key
parameters from the 1200 UTC 12 October 2014 NAEFS. The table was intentionally truncated at 174 hours. Return to
text.
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Figure 6. As in Figure 5 except for the standardized anomalies and return periods of the anomalies for the V-winds at
each level. Return to text.
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Figure 7. The 850 hPa V-winds displayed with the standardized anomalies
(upper) and return periods (lower) . Data from WR-SATABLE. Return to text.
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Figure 8. NCEP SREF forecasts of the probability of 25 mm or more QPF in the period from 0900 UTC through 1800 UTC 15
October 2014. SREF forecasts initialized at a) 2100 UTC 13 October, b) 0300 UTC 14 October, c) 0900 UTC 14 October, d) 1500
UTC 14 October, e) 2100 UTC 14 October, and f) 0300 UTC 15 October. Contour is the ensemble mean 25 mm contour and
the shading showed the probability in the color scale. Return to text.
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Figure 9. As in Figure 8 except for the SREF ensemble mean QPF (shaded) and each members 25 mm contour, if present. The
thick black line is the ensemble mean 25 mm contour. Return to text.
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Figure 10. As in Figure 8 except for the probability of 25 mm for the 12-hour period ending at 0000 UTC 16 October 2014 from
the 6 most current SREF members initialized at a) 0300 UTC 14 October, b) 0900 UTC 14 October, c) 1500 UTC 14 October, d)
2100 UTC 14 October, e) 0300 UTC 15 October, and f) 0900 UTC 15 October. Return to text.
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Figure 11. As in Figure 10 except for the ensemble mean QPF (shaded) and each members 25 mm contour and the ensemble
mean 25 mm contour. Return to text.
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Figure 12. Composite radar from the NMQ site valid at 1200 and 1500 UTC 15 October 2014. Return to text.
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Figure 13. As in Figure 10 except showing QPFs for the 12 hour period ending at 0600 UTC 16 October and SREFs initialized every 3-hours
from a) 0900 UTC 14 October through f) 1500 UTC 15 October 2014. Return to text.
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Figure 14. As in Figure 13 except for the ensemble mean QPF (shaded) and each members 25 mm contour. Return to text.
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Figure 15. NCEP 3km HRRR initialized at 1200 UTC showing composite reflectivity (dBZ) in hourly increments from a) 1300 UTC through f) 1800 UTC 15
October 2014. Return to text.
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Figure 16 As in Figure 15 except showing 1300 UTC HRRR total accumulated precipitation in the time periods defined in the header of each image. Rain is in
millimeters. Return to text.
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Figure 17. As in Figure 15 except showing the HRRR radar valid at 1800 UTC 15 October from individual HRRR forecasts initialized hourly from a) 1200 UTC
through f) 1700 UTC 15 October 2014. Return to text.
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Figure 18.. As in Figure 12 except radar at 1800 and 2100 UTC 15 October 2015. Return to text.
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Figure 17. As in Figure 16 except accumulated QPF from the 1700 UTC 15 October 2014 HRRR. Return to text.
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Appendix: Local flood issues and return periods not used in text contributed by Charles Ross
Newspaper Article: http://thetimes-tribune.com/news/flash-flood-warning-issued-for-lackawanna-parts-of-susquehannacounties-1.1771752
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Rainfall and flooding analysis:
This is another good case to use the NSSL FLASH (http://flash.ou.edu/) website to look for areas where
Annual Return Intervals (ARI) may be of value. In this case, the heaviest rainfall fell across extreme
northeast Pennsylvania and southern New York State. The 3-hour ARIs were unimpressive, however
there was indication of a significant rainfall in the 6-hour ARI (figure x), with returns varying between 2
and 30 years. Recent case reviews (http://cms.met.psu.edu/sref/severe/2014/10Sep2014.pdf) have
shown that true flash flooding usually begins with ARIs greater than 50 years (preferably over 100
years). In this case the ARI returns indicate that at least minor flooding was likely with this event. As
noted above, antecedent conditions were also quite dry heading into this event, something that is not
accounted for using ARIs, but is when using Flash Flood Guidance produced by the MARFC. Looking at
Flash Flood Guidance issued by the MARFC for this area we note that 3 and 6-hour values are close to
4.0 inches. The return period figure shows the gage adjusted radar precipitation estimates for this area,
showing that rainfall is close to the flash flood guidance. The combination of using FFG and the ARI data
from the Flash website indicates that at least minor flooding was likely occurring.
It is worth noting that Flash Flood Warnings were issued and NWS KBGM, the they did receive reports of
flooding. A news article from the Times-Tribune mentions mostly minor flooding in the region.
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