The Record South Carolina Rainfall Event of 3-5 October 2015:

NCEP Forecast Suite Success story

John LaCorteRichard H. Grumm

andCharles Ross

National Weather Service State College, PA 16803Presenter

Motivation

Record Rainfall impacted South Carolina Wide areas of over 10 inches and 24 inches of rainfall Enduring rain high rates were over days not hours Historic and damaging flooding

Pattern Ideal for Heavy Rainfall Strong easterly flow, cut-off, and high moisture

Forecasts were Useful and Valueable GEFS and SREF Some GFS and HRRR

Rainfallpeak 48 hour period and event

Courtesy Russ Schumacher

CFSR PatternIdeal for heavy rainfall

Long easterly fetch

Big view 500 hPa of the lumbering lowDeeper low to east associated with Joaquin

GEFS 36 HR QPFEnsemble mean (gray) and each members 150 mm/6 inch contour (color)

36Hr QPF

GEFS Forecasts 100 mm in 36 hoursProbability of 100 mm contour (4 inches)

D-3 D-2

D-1

GEFS Forecasts 150 mm in 36 hoursProbability of 150 mm contour (4 inches)

D-3 D-2

D-1

GEFS Things to ponder

The GEFS focused the rainfall over South Carolina

The GEFS had high QPF amounts But we need context for this QPF Relative to the GEFS internal climate Was the GEFS rainfall extreme or normal?

The answers to life's persistent questions, Guy Noir Private eye

What is an extreme rain event in the GEFS?

GEFS M-ClimateRainfall in GEFS compared to internal GEFS-R climate

72 hour GEFS QPF vs M-Climaterecord in model could be record in model atmosphere real atmosphere

M-Climate Says VERY SIGNIFICANT

SREF 36 HR QPFEnsemble mean (gray) and each members 150 mm/6 inch contour (color)

SREF 36 hour Probability of 100 mm (4”)

SREF Things to Ponder

The SREF focused the rainfall over South Carolina

The SREF had high QPF amounts But we need context for this QPF Relative to the SREF internal climate Was the SREF rainfall extreme or normal?

The answers to life's persistent questions, Guy Noir Private eye

What is an extreme rain event in the SREF? Beats me! No climate exists for the SREF!!!

GFSBig rainfall (inches) in 24 and 48 hours

GFS24 hour QPF (inches) vs 24 hour 100 year ARI

HRRR 6-hour QPF vs 6 hour 100 year ARI

When did it rain hardest? (6hr rainfall)

Summary

Record Rainfall impacted South Carolina Widespread areas of over 10 inches and 24 inches of rainfall Enduring rain high rates were over days not hours Historic and damaging flooding

Pattern Ideal for heavy rainfall Strong easterly flow, cut-off, and high moisture

Forecasts were useful and of value from all NCEP systems. Probably one of the finest set of forecast products NCEP has

produced Good lead-time of record event on the order of 3-5 days Valuable short-term guidance!

The Record South Carolina Rainfall Event of 3-5 October 2015:NCEP Forecast Suite Success story

John LaCorteRichard H. Grumm

andCharles Ross

National Weather Service State College, PA 16803

ABSTRACT

 

Record setting rains affected South Carolina from 3 to 5 October 2015. The combination of an intense period of heavy rain on 4 October and a long duration of rain event combined to produce rainfall totals between 10 and 24 inches and historic flooding across a large swath of South Carolina.

This paper will document the success of the NCEP forecast suite in predicting the heavy rainfall which affected South Carolina. Long range forecasts from the NCEP Global Forecast System (GEFS) are presented showing forecasts from 30 September through 2 October. The GEFS forecast a high probability of over 4 and 6 inches rain over South Carolina in several 24 hour periods. Though these values are far lower than observed rainfall, these forecasts were in the tails of the GEFS quantitative precipitation forecast climatology implying that the GEFS was forecasting a record event relative to its internal climatology.

As the forecast horizon decreased, the NCEP GFS and SREF forecasts are presented. Similar the GEFS, the SREF showed a high probability of over 4 inches of quantitative precipitation over the same region. The 13km NCEP GFS is presented showing the quantitative precipitation and the ratio of the quantitative precipitation to the 24-hour 100 year ARI data. These forecasts consistently showed that the GFS was forecasting rainfall amounts of 125 to 200% of the 24-hour 100 year ARI values.

Short range High Resolution Rapid Refresh quantitative precipitation is shown relative to the 6-hour 100 year ARI data. These forecasts may have been valuable in defining areas of short-duration high intensity rainfall.

This paper will focus on the success of the NCEP guidance forecasting the heavy rain, the relatively predictable nature of this event with over 4 days of lead-time of the event, and the value of using climatological data in identifying potential high impact and record rain events.