A Discussion of the 2017 Record Rainfall Year for WFO Mobile

Abstract

In 2017, The annual rainfall total in Mobile was ranked 8 th of 147 recorded years (1871-2018), while Pensacola, Florida was 2 nd of 138 years (1880-2018). In addition to a host of non-tropical locally heavy precipitation producing storm systems, three tropical systems directly and indirectly contributed to these totals (i.e., Tropical Storm Cindy, Hurricane Harvey, and Hurricane Nate). A total of ten significant heavy rain events, three tropical and seven non-tropical, over the course of 15 separate days accounted for 42% of the Mobile annual total and 41% of the Pensacola annual rainfall total. This study focuses and discusses both the flash-flooding and heavy rainfall aspects of one such non-tropical heavy rainfall case which occurred over Saraland, Alabama on May 20 th , 2017. The goal is to improve our overall forecast understanding of how these events potentially evolve, and how both the established pattern and associated ingredients work in concert to produce intense rainfall.

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A Discussion of the 2017 Record Rainfall Year for WFO Mobile
DA’VEL R. JOHNSON
NOAA/ National Weather Service Weather Forecast Office Mobile, AL
ABSTRACT
In 2017, The annual rainfall total in Mobile was ranked 8th of 147 recorded years (1871-
2018), while Pensacola, Florida was 2nd of 138 years (1880-2018). In addition to a host of non-
tropical locally heavy precipitation producing storm systems, three tropical systems directly
and indirectly contributed to these totals (i.e., Tropical Storm Cindy, Hurricane Harvey, and
Hurricane Nate). A total of ten significant heavy rain events, three tropical and seven non-
tropical, over the course of 15 separate days accounted for 42% of the Mobile annual total and
41% of the Pensacola annual rainfall total. This study focuses and discusses both the flash-
flooding and heavy rainfall aspects of one such non-tropical heavy rainfall case which occurred
over Saraland, Alabama on May 20th, 2017. The goal is to improve our overall forecast
understanding of how these events potentially evolve, and how both the established pattern and
associated ingredients work in concert to produce intense rainfall.
1. Introduction
The Gulf Coast region regularly ranks high
for annual rainfall, yet 2017 was an abnormal
year. The National Weather Service (NWS)
Weather Forecast Office (WFO) in Mobile
noted that the annual precipitation total for
Mobile, Alabama was 83.78 inches recorded at
the Mobile Regional Airport. In Pensacola,
Florida the rainfall total was 91.91 inches
recorded at the Pensacola Regional Airport for
2017. This represents the 8th and 2nd most
recorded precipitation in a given year for
Mobile and Pensacola respectively. For
perspective, the Mobile total was 17.6 inches
above the yearly average, while the Pensacola
total was 26.6 inches above normal. There were
30 days where flash flood warnings were issued
due to intense rainfall totaling 93 individual
warnings for the year. In addition, a total of 8
daily rainfall records were broken in both
Mobile and Pensacola.
TABLE 1. Top ten rainiest years in the Mobile Area
derived from the NOAA Reginal Climate Centers
1871-2018
Rank
Year
Total Precipitation
1
1881
92.32
2
1900
91.18
3
1912
89.86
4
1929
89.34
5
1975
86.58
6
1998
86.52
7
1947
84.20
8
2017
83.78
9
1983
83.46
10
1961
82.73
Heavy rain is often the precursor to
dangerous flash floods. For that reason,
understanding the causes of heavy rainfall helps
NWS forecasters better predict when and where

there is potential for large amounts of rain and
subsequent flash flooding.
TABLE 2. Top ten rainiest years in the Pensacola
Area derived from the NOAA Reginal Climate
Centers 1880-2018
Rank
Year
Total Precipitation
1
1953
93.32
2
2017
91.91
3
1947
90.32
4
1995
89.45
5
2009
88.30
6
1937
87.58
7
2005
87.32
8
1881
86.02
9
2014
83.17
10
1964
82.96
Studies on the subject of heavy rainfall
across the United States often arrive at the same
conclusion when it comes to the causes of these
events. A study by Maddox et al. (1979)
showed that some common features associated
with heavy rain and flash floods are: the
development of convective storms, abnormally
high moisture content, training storms over the
same area, and other synoptic and mesoscale
factors. Additionally when it comes to the Gulf
Coast and the Southeast United States, tropical
systems contribute large portions of the overall
yearly rainfall total in a relatively short amount
of time. In a study by Schumacher (2005) which
examined the characteristics of extreme rain
events, he notes that generalizations made
about rainfall totals and their distributions in the
Southeast can vary widely in accordance to
tropical activity. With regards to tropical
activity, the 2017 Atlantic Hurricane Season is
considered a hyperactive season, Goldenberg et
al. (2001). Locally, the NWS Forecast Office in
Mobile was impacted directly and indirectly by
three significant tropical events Tropical Storm
Cindy, Hurricane Harvey, and Hurricane Nate.
These systems bolstered the precipitation totals
FIG. 1. GOES-16 Infrared Satellite Image of
Hurricane Nate on October 7th, 2017 from the
Cooperative Institute for Research in the Atmosphere
(CIRA).
for June, August, and October adding to the
annual total and underscoring the year to year
variability in rainfall along the Gulf Coast
region. Finally, a study performed by the
National Severe Storms Laboratory looking at
the climatology of heavy rain events showed
that heavy rainfall and flash flooding is often
associated with rain rates greater than 1 inch per
hour (in. h-1) which last for an extended period
of time (Brooks 1999).
In 2017, a total of ten significant heavy rain
events, three tropical and seven non-tropical,
over the course of 15 separate days accounted
for 42% of the Mobile annual total and 41% of
the Pensacola annual rainfall totals. The
following section will focus on one such non-
tropical heavy rainfall event in particular where
flash flooding occurred within WFO Mobile’s
County Warning Area (CWA). The purpose
being to expand upon how NWS meteorologists
assess heavy rainfall potential.
2. May 20th, 2017 Heavy Rainfall Event in
Saraland, Alabama
By examining the Saraland, AL flash flood
event in May and its local effects, it becomes
possible to understand some of the common

causes of heavy rainfall. A forecaster will
typically implement four steps prior to the onset
of a heavy rain event.
 Assess the synoptic & mesoscale moi-
sture content as it compares to clima-
tology
 Determine whether there is the potential
for continuous convective storms
 Examine antecedent conditions
 Investigate where the moisture is being
transported
Using these steps, forecasters are able to eval-
uate flash flood likelihood. It is through this
lens in which this case will be examined.
Prior to the onset of the heavy rain in
Saraland, at 12Z on May 20th a mid-level trough
with its axis centered over eastern Colorado
sparked a convective line extending from the
Southern Plains to central Texas (Fig. 2).
FIG. 2. The 500mb chart for May 20th, 2017 at 12Z.
The isohypses are contoured in black, 500mb
temperatures are dashed and contoured in red, and
wind speeds are shaded in blue. Image taken from the
Storm Prediction Center (SPC) Mesoscale Analysis
webpage.
As the center of the trough moved slowly to the
northeast, the remnants of the convective line
travelled eastward into the Lower Mississippi
River valley and would act as a focal point for
the heavy rain later on in the evening (Fig. 3).
FIG. 3. The Southern Mississippi River Valley
radar mosaic for May 20th, 2017 at 12Z. Star denotes
the location of Saraland, Alabama Image taken from
NEXLAB College of DuPage website.
At the surface, the environment out ahead of the
line was primed for convective storms. Along
the Gulf Coast, dew points were well into the
70s (Fig. 8). This ample low-level moisture in
addition to temperatures reaching the low 80s
by midday contributed to mixed-layer convec-
tive available potential energy (CAPE) values
above 2500 J/kg.
Looking at the 12Z sounding analysis from
WFO New Orleans (Fig. 4) acts as an approx-
imation for assessing the localized moisture
content. The sounding shows a moist and
buoyant overhead air-mass with a precipitable
water value of 46.31 mm or 1.82 inches. When
compared to the Storm Prediction Center’s
sounding climatology, the May 20th 12Z profile
ranks above the 90th percentile for precipitable
water based on a 91-day moving average for the
period of record which dates back to 1948. This
quantifies the atmosphere overhead as being
abnormally saturated; moreover, any storms
that form in this environment would have heavy
rain potential because of the excess moisture.

FIG. 4. WFO New Orleans/Baton Rouge Skew-T log-P diagram from the KLIX Slidell radiosonde for May
20th, 2017 at 12Z. Image taken from the University of Wyoming Atmospheric Sounding Archive.
By 18Z, the first storms initiated within the
Mobile CWA (Fig. 5) solely due to diurnal
heating and buoyancy owing to the lack of any
notable inversion layer aloft. These storms were
moving north at 20 knots. Meaning that as
storms formed along the coast, they moved
northward continuously across southern
Alabama. This continual training of storms
served to moisten the ground ahead of the main
convective-line which by 18Z had reached the
Louisiana/Mississippi boarder. The 850mb
level is often used to access the low-level
moisture transport which is calculated by
multiplying the 850mb wind in m s-1 by the
mixing ratio in g g-1 then using a scaling factor
of 100 (Junker 1999). A relative maximum can
be seen when examining the moisture transport
vectors over southern and central Gulf Coast in
Figure 9. This further justifies the exceptional
potential for additional heavy rainfall develop-
ment in the evening.
FIG. 5. KMOB Radar image for May 20th, 2017 at
18Z.
Radar estimates show nearly 2.5 inches had
fallen by 22Z over areas south of Citronelle and
near Mount Vernon. At 23Z the convective line

had reached the Mississippi/Alabama border
(Fig. 6). Low-level winds along the line were
out of the northwest while winds ahead of the
line were out of the south allowing for enhanced
surface convergence. The surface convergence
then redeveloped convection along the line
generating storms with torrential rain rates of
2.75 in. h-1 over Saraland, Alabama (Fig. 10).
FIG. 6. KMOB Radar image for May 20th, 2017 at
23Z.
These high rain rates prompted WFO Mobile to
issue four separate warnings covering most of
Mobile and Baldwin Counties while also
including parts southern Washington and Clark
County. There were several reports of street
flooding in the Saraland community (Fig. 7)
with many roads blocked off by emergency
officials. Even after the line moved eastward
into the Florida Panhandle areas over Alabama
were still under the trailing stratiform rain until
after 06Z. By the end of the heavy rain event,
radar estimations show areas near Saraland and
Mount Vernon had seen nearly 5 to 7 inches of
rainfall over the course of 15 hours (Fig. 11)
with 6.44 inches measured at the University of
South Alabama mesonet site in Saraland.
3. Conclusion
Intense rainfall events similar to the case in
Saraland occurred throughout the year and in
addition to the three tropical systems contri-
buted to the 2017 record precipitation. A
common predictive parameter used to examine
the May 20th 2017 case in Saraland is moisture.
FIG. 7. Car partially submerged in a Hibbett
Sport parking lot in Saraland, Alabama. Photo
credited to News 5 WKRG.
Forecasters can access heavy rainfall potential
by asking question about atmospheric moisture.
How much moisture is currently in the
atmosphere? How does the amount of moisture
compare to climatology? Where is the moisture
being transported? How much moisture has
already reached the surface? By studying the
answers to these questions, it becomes possible
to forecast heavy rainfall events and prepare the
public for flash flooding. Understanding what
contributed to the 2017 record rainfall year and
why it was unique is essential to improving
forecast techniques and skill. Studies like these
act directly towards accomplishing the National
Weather Service mission which is “to provide
weather… forecast and warnings for the protect
protection of life and property and the
enhancement of the national economy.”
Acknowledgments. Thanks and acknowle-
dgements are extended to the Weather Forecast
Office in Mobile for providing the resources to
conduct this project and to the NWS volunteers
Caitlin Ford and Dillon Blount for inspiring me
to compete this research.

REFERENCES
Brooks, H. E., and D. J. Stensrud, 2000: Climatology
of heavy rain events in the United States from
hourly precipitation observations. Mon. Wea.
Rev., 128, 1194–1201.
Goldenberg , S. B. , W. L. Christopher , M. Alberto ,
and W. Gray . 2001 . The recent increase in
Atlantic hurricane activity: Causes and impli-
cations. Science 293: 474 – 479.
Junker, N. M., R. S. Schneider, and S. L. Fauver, 1999:
A 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-a scale aspects of flash flood
events. Bull. Amer. Meteor. Soc., 60, 115–123,
https://doi.org/10.1175/1520-0477-60.2.115.
Schumacher, R. S., and R. H. Johnson, 2006: Charac-
teristics of U.S. extreme rain events during 1999–
2003. Wea. Forecasting, 21, 69–85, https://
doi.org/10.1175/WAF900.1.

APPENDIX
Additional Figures
FIG. 8. Surface temperatures and dew points for May 20th, 2017 at 12Z. Image taken from the Storm
Prediction Center (SPC) Mesoscale Analysis webpage.
FIG. 9. The 850mb moisture transport chart for May 20th, 2017 at 22Z. The 850mb isohypses are shown in
black, 850mb theta-e are contoured and dashed in green and the moisture transport are shaded. Image
taken from the Storm Prediction Center (SPC) Mesoscale Analysis webpage.

FIG. 10. KMOB One hour precipitation radar estimation for May 20th, 2017 at 0039Z.
FIG. 11. Radar derived 24-hour rainfall totals for May 20th to May 21st, 2017. Image captured from the
Advanced Hydrologic Prediction Service (AHPS) Precipitation Analysis webpage. Star denotes the
location of Saraland, Alabama.