Using Standardized Anomalies to Identify Significant Heavy Rain Events

Jason KrekelerAnd

Richard H. GrummNational Weather Service State College, PA

Motivation

• Record rainfall with remnants of TS Lee– Look at key features of this event– Pattern and persistence of the pattern

• Pattern similar 9/30 – 10/1 2010 event and others– Compare these events

Bloomsburg PA

City Island in Harrisburg

Swatara Creek in Hersey

Hersheypark, Comet Hollow

Rainfall from remnants of TS Lee

Rainfall totals from 9/29-10/2 2010

Lee 500 hPa pattern

Surface

Precipitable water

Sep 29-Oct 2 500 hPa Pattern

Surface pattern

Precipitable water

How do these events compare?

• Similarities– Deep trough in eastern US– Anticyclone in North Atlantic– Surge of tropical moisture along frontal boundary

• Differences– Deepening surface low vs cut off upper level low– T bone vs north/south bands– duration

Persistance

• Both events featured a 24 hour period of intense rainfall– Deep southerly flow– High precipitable water– Jet entrance regions– Downstream blocking ridge

Heaviest 24 hour periodSep 30th 2010

Heaviest 24 hour periodSep 7th 2011

Sep 30 2010

TC Lee remnants and Katia

Sep 7 2011

The resulting Rainfall Patterns

Comparison

• Similar rainfall pattern- north/south bands• Sep 7, 2011-– Higher rainfall totals– Stronger frontal boundary– More focused band of high pw

• Sep 30,2010 – broader area of heavy rain– Stronger southerly flow– Ridge further off the coast

PW Plume Comparisions

Comparison with LLJ

Conclusions

• Large scale patterns similar but differences in the details

• Persistence of the pattern• Similar pattern of rainfall but differences in

magnitude and scale• If TC Katia was not there…?– Would the LLJ been broader/stronger with

broader area of high PW and heavy rain

Bloomsburg PA