23-24 July 2008 Flooding Rains

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An unusually strong upper low was forecasted to track through the Great Lakes and northern New England on 23-24 July 2008. All sources of deterministic and ensemble guidance, were consistent from run-to-run with the general evolution of the upper pattern several days in advance. Even 2 to 3 days prior to the event, guidance was offering increasing support for the potential for flooding rains across New York and New England. The predicted total rainfall amounts varied considerably, but the lower end of a broad spectrum of guidance predictions was 3.00-6.00 total of rain over a large area. We can begin by looking at the pure deterministic total-event precipitation forecasts from the NAM and GFS (Fig. 1), then look at various forecast parameters to see what could have given the best clues to what actually would happen.



Figure 1. Precipitation forecasts from a) the 22 July NAM12 total event precipitation and b) the 22 July GFS40 48 hour precipitation. Both these forecasts encompassed the entire event. Note the higher-resolution NAM was predicting a widespread 4-8 of rain, while the lower-resolution GFS was predicting a less widespread 3-5 of rain. The areal extent of the heaviest rain in each source of guidance was noticeably different as well.



Figure 2. 12Z 22 July NAM80 500 hPa heights and vorticity for a) 0000 UTC 24 July and b) 1200 UTC 24 July. Note the Upper low center was cut-off and centers of vorticity were rotating around the periphery of the upper low.



Figure 3. GEFS 1000 hPa PWAT and anomalies for a) 00Z 23 July valid 00Z 24 July, and b) 12Z 23 July valid 00Z 24 July. Note the PWAT 2-3 SD above normal over the northeastern U.S. Also note the anomalously low PWAT anomalies in the Midwestern U.S.


Figure 4. 00Z 23 July GEFS 850 hPa U and V winds and anomalies valid a) 00Z 24 July and b) 12Z 24 July. Note the V winds increased from 2-3 SD above normal to 4-5 SD above normal. These anomalous V winds helped transport moisture and provided low-level convergence for the production of heavy rain.



Figure 5. NAM80 12Z 22 July 850 hPa wind and wind isotachs valid a) 00Z 24 July and b) 12Z 24 July. Not the strengthening jet forecasted, similar to the GEFS.


Figure 5. GEFS 12Z 23 July 250 hPa U and V winds and anomalies valid 12Z 24 July and b) 12Z 22 July NAM80 300 hPa wind barbs and isotachs. Note the 3-4 SD jet core in the V winds and the northeastern U.S. is in the right-entrance region.



Figure 6. 12Z 24 July NAM12 850 hPa winds valid at a)12Z 24 July, b) 15Z 24 July and c) 18Z 24 July. Note the better detail and more precise track/timing of the low-level jet core through New England.




Figure 7. 12Z 22 July GFS 850 hPa wind barbs and isotachs valid at a) 18Z 23 July, b) 00Z 24 July, c) 06Z 24 July and d) 18Z 24 July. Note the definitive low-level jet core that develops and tracks through New England.



Figure 8. 12Z 22 July NAM80 PWAT valid a) 00Z 24 July and b) 12Z 24 July. Note the values at or larger than 2, which are extremely supportive of potential for flooding.



Figure 9. GEFS probability for 2.00 of rain and spread valid a) 18Z 24 July and b) 06Z 25 July. Note the consistent signal of >80% probability over much of New England and eastern NY.



Figure 10. Surface dew point forecast valid 18Z 24 July from the 12Z 22 July NAM80 and GFS40. Note the tight thermal gradient over NY resolved in the NAM80, suggesting the greater surface moisture would be confined to southern NY and southern New England. This dew point boundary acted as a relatively stationary warm front, and a focusing mechanism for low-level wind convergence and possible isentropic lift.



Figure 11. Plume diagrams from the 00Z 23 July MREF for a) Albany, NY, b) Binghamton, NY, c) Bradley, CT. Note the high values in eastern locations, with ranges between 2.00 and 4.50 inches predicted.



Figure 12. Plume diagrams from the 12Z 23 July MREF for a) Albany, NY, b) Binghamton, NY, c) Bradley, CT. Note the consistent successive model run predictions for 2.00 to 4.00 inches of rain.



Figure 13. Visible satellite imagery and lightning overlay from a) 1930Z 23 July, and b) 2330Z 23 July. Note the northern extent of the lightning in southern and central NY, where the stationary warm front existed.



Figure 14. Water Vapor satellite imagery from a) 0755Z 23 July, b) 1555Z 23 July, c) 2340Z 23 July. Note the upper cut-off low pressure center and deep drying associated with an upper impulse tracking around the southern periphery of the upper low. Also note the tropical connection of the moisture.



Figure 15. Event total rainfall (mm) from 00Z 23 July through 12Z 25 July. Roughly 25 mm = 1 inch. Note the rainfall distribution, much more like the GFS40 than the NAM80.


Severe weather reports for individual NWS offices, courtesy of The Iowa Environmental Mesonet division of Iowa State University Department of Agronomy


NWS Buffalo, NY



NWS Binghamton, NY



NWS Albany, NY



NWS Burlington, VT



NWS Taunton, MA



NWS Upton, NY