1. Case Study of Torrential Rain and Flood in the Merrimack River Valley in May 2006 Jian-Hua Qian University of Massachusetts Lowell Jian-Hua Qian University of Massachusetts Lowell Acknowledgement: Alex Brown, Joel Dupont, Lance Frank, Chris Hampton, Bill Leatham, Kurtis Malone, Todd Miller, Shawn Moynihan, Chris Roller, Zongyao Yang

2. The Results of Expanding the Domain in a WRF Model Run April 25, 2013 The 2006 Flood of the Merrimack River in Lowell  Like most river valleys, the Merrimack River valley prone to minor flooding.  However, on May 15, 2006 heavy rainfall caused extreme flooding the Merrimack River raising the river more than 8 feet above flood stage in Lowell, MA.  This extreme event caused severe property damage, forced evacuations and in nearby Haverhill, MA their main sewage pipeline broke causing 35 millions gallons of raw sewage per day to be dumped into the Merrimack 2

3. Terrain of Northeast US Observed Rain

4. UMass Lowell at the north end of the University Ave Bridge over the Merrimack River

5. View of Merrimack River from the University Ave Bridge in Lowell MA in May 2013

7. Synoptic Scale Anomalous negative NAO Blocking High Pressure in Eastern Canada Cut-off Low Pressure over Great Lakes Diffluent flow Duration of event

8. North Atlantic Oscillation Extremely negative NAO Leads to blocking 2006 5 7 -0.595 2006 5 8 -1.444 2006 5 9 -1.922 2006 5 10 -2.028 2006 5 11 -2.019 2006 5 12 -1.818 2006 5 13 -1.604 2006 5 14 -1.275 2006 5 15 -1.045 2006 5 16 -0.838 2006 5 17 -0.846 2006 5 18 -0.811

9. Rex Block A “Rex Block” formed due to negative NAO Idealized Case May 2006 Case

10. Diffluent Flow Divergence at the 500 mb level Gives rise to upward vertical motion We all like upward vertical motion!! Right?

11. Jet Level A not so typical; but still favorable Jet Position developed. Divergence at the 300 mb level; contoured in yellow (especially one the 14th) May 13th 12zMay 14th 12z

12. Negatively tilted a couple times during period

13. Water Vapor Satellite Image Valid 12Z 14 May 2006 Note the “Rex-Block” evident over eastern North America, and the attendant deformation zone extending from Hudson’s Bay through the central US. Also note the “atmospheric river” of subtropical moisture streaming onshore into New England. 13 NOAA

14. Observed total Precipitation & average 700 hpa mixing ratio and winds on may 13-15

15. Low Level Moisture Mixing ratio plots highlight stream of moisture May 13th 12z May 14th 00z May 14th 12z May 15th 00z

16. Low Level Moisture Flow around High & Low pressure = consistent SE wind. Lots of Atlantic Moisture picked up

17. FNL - Default Settings 6 day 12 hr – Valid 5/18/2006 1200 UTC http://www.ncdc.noaa.gov/sotc/hazards/2006/5 = 127 mm DELTA (MAX TOTAL) 0 – 127 mm LESS Simulation by Mesoscale Model WRF Approx 7 Day Comparison Ending 5/18/2006 WRF Observation

18. Conclusions Extremely negative phase of the NAO leads to blocking Due to blocking over North Atlantic a Rex Block develops; giving rise to a cut-off Low Pressure over Great Lakes region Upper level divergence at both the 500 hPa & 300 hPa level provide a favorable region for upward vertical motion Combined low level flow around both pressure centers draws in copious amount of Atlantic moisture into Southern & Central New England Negatively tilted 500 hPa low and trough (in NW-SE direction) favors rain Moisture transport from the Atlantic to the Merrimack WRF successfully simulated the Mother’s Day flood