There has been a lot of discussion lately about a possible Predecessor Rainfall Event (PRE) that occurred yesterday (June 5, 2013) and weak Cold Air Damming (CAD) ahead of Tropical Storm Andrea. These are both processes that have occurred in previous landfalling tropical cyclone (TC) cases in the Carolinas, including TC Ernesto (2006). While the exact rainfall distribution of Tropical Storm Andrea will become clear in the next few days, it is worth first investigating: 1) Did a PRE occur over South Carolina on June 5, 2013? and 2) Did CAD occur over the Carolinas and Virginia on June 5-6, 2013?
1) Did a PRE occur over South Carolina on June 5, 2013?
In order to objectively determine whether or not the persistent region of convection over South Carolina on June 5, 2013 was a PRE, the four criteria proposed by Galarneau et al. (2010) for PRE identification need to be examined for this case.
1) Radar reflectivity values ≥35 dBZ within a coherent area of rainfall persisting for at least 6 h.
Base Reflectivity at Charleston, SC radar site from 1800 5 June – 0600 6 June 2013
Radar reflectivities ≥35 dBZ did occur over a 12 h period from 1800 UTC June 5 – 0600 UTC June 6 2013. Therefore, it appears Criteria 1 was met.
2) The average rainfall must be ≥100 mm (24 h)-1 over the entire life of the PRE.
24 h QPE Accumulation ending at 1200 UTC 6 June 2013. PRE region circled in white over South Carolina.
After examining the 24 h QPE Accumulation (Radar only – not Stage IV), it appears that the average rainfall was not ≥100 mm (24 h)-1 over the entire life of the PRE. However, the 24 h rain gauge measurements obtained from CoCoRaHS indicate that rainfall amounts ≥100 mm (24 h)-1 or ≥~4 in (24 h)-1 did occur in isolated spots over a broad region in the South Carolina Lowcountry.
24 h CoCoRaHS rain gauge measurements ending at 1200 UTC 6 June. Rainfall amounts ≥~4 in (24 h)-1 are indicated by orange and red dots.
3) There must be a clear separation on the radar imagery between the coherent area of rainfall and the TC rain shield.
WPC 0000 UTC 6 June 2013 Surface Analysis.
It certainly appears that there was a clear separation between the convection along the stationary boundary in South Carolina and the main TC rainfall shield located over central and southern Florida.
4) Deep tropical moisture directly associated with the TC must be advected away from the TC into the region of the coherent area of rainfall. (Click on image below to see loop.)
RUC precipitable water values from 1800 UTC 5 June – 0600 UTC 6 June 2013. Plots courtesy of InstantWeatherMaps.com
It does appear that deep tropical moisture was in place over South Carolina throughout the duration of the rainfall event. The slow, northward movement of the high precipitable water values from central South Carolina into central North Carolina at 0600 UTC 6 June does indicate that Tropical Storm Andrea might have played a role in advecting deep tropical moisture into the region of coherent rainfall.
In summary, most of the criteria appear to have been met for the heavy rainfall event over South Carolina on 5 June 2013 to be classified as a PRE. However, the second criteria (average rainfall must be ≥100 mm (24 h)-1) is only marginally met.
2) Did CAD occur over the Carolinas and Virginia on June 5-6, 2013?
Another topic of discussion yesterday (5 June 2013) was the development of a surface pressure ridge indicative of weak CAD and its possible implications on the track and precipitation distribution of Tropical Storm Andrea as it moves northward over the Carolinas. While the pressure ridge did appear in surface analyses, was CAD identified using the objective CAD detection criteria developed by Bailey et al. (2003)?
These criteria include:
- The mountain-normal Laplacian of sea level pressure must be negative and exceed in magnitude one standard deviation of the average of all the negative mountain-normal Laplacian values in the dataset.
- The mountain-normal Laplacian for potential temperature must be greater than zero.
- Sea level pressure must be greater at the center station relative to the end stations.
- The difference in the pressure along line D must be greater than 1.5 mb between either GSP and GSO or GSO and RIC, with higher values to the northeast.
- All requirements must be met for at least six consecutive hours on at least one of the mountain-normal lines (A-C).
These criteria are examined along four lines outlined below.
Surface stations and lines used in the objective CAD-detection algorithm developed by Bailey et al. (2003).
Looking at surface temperature and sea-level pressure observations from 2100 UTC 4 June – 2000 UTC 6 June 2013 (below), reveals that none of the criteria were met along the northernmost Line A and southernmost Line C. However, the criteria for CAD were met along Line B (North Carolina) from 1300 UTC – 2000 UTC 5 June 2013. This suggests that weak CAD occurred over North Carolina during the daytime hours of 5 June. Additionally, the difference in the pressure along line D was greater than 1.5 mb between either GSP and GSO or GSO and RIC, with higher values to the northeast for the entire 48 h period. The higher pressure to the northeast indicates the presence of a high pressure ridge extending southward as surface analyses indicated.
Analysis of CAD criteria from 2100 UTC 4 June – 2000 UTC 5 June 2013. Hours with CAD criteria met for 6 or more hours are highlighted in yellow.
In summary, when considering pressure observations alone, CAD occurred from 2100 4 June – 2000 6 June 2013 (Line D). However, when incorporating temperature observations, only weak CAD developed over North Carolina (Line B) from 1300 UTC – 2000 UTC 5 June 2013.
It will certainly be interesting to see what enhancements the PRE over South Carolina might make to total rainfall accumulations in that region after Tropical Storm Andrea passes. However, since CAD does not appear to be currently present (June 6), it appears unlikely that CAD will be a major player in modulating the precipitation distribution and track of Tropical Storm Andrea as it moves northward over the Carolinas.