PRE and Cold Air Damming ahead of Tropical Storm Andrea?

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

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.

24hr_QPE_Andrea_PRE_circled

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

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.

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

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).

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 are highlighted in yellow.

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.

Advertisements

About fjdalewx

Graduate student at North Carolina State University.
This entry was posted in TC and Boundary QPF. Bookmark the permalink.

3 Responses to PRE and Cold Air Damming ahead of Tropical Storm Andrea?

  1. Pingback: PRE and Cold Air Damming ahead of Tropical Storm Andrea … | Lavanaut

  2. Phil says:

    One of the most important questions that I would ask would be why it rained 4-7 inches along the eastern slopes of the Blue Ridge into the Foothills of western NC Wednesday night and Thursday? This may have been a PRE. It was NOT associated directly with the TD.

    I do see signals of lingering in-situ or possibly hybrid CAD extending mainly along the VA Shenandoah Valley through the ROA valley to near the NC/VA border on Thursday evening. However… all indications were that there was not any lingering CAD over the Foothills of NC. Winds were SE.

    Looking back… there appears to have been a weak in-situ/hybrid damming episode over the western Piedmont of NC on Wed afternoon (as a bit of light rain developed) and kept temps in the 60s much of the day. This occurred as the surface high (parent) was shifting off the NEngland coast. This high was marginal for strength (less than 1025 MB) and location (way too far north of PA/NY state) to deliver or maintain CAA or DAA into the NC damming region. Cool dry air advection from the NE into the VA/NC damming region had already ceased before the light rain developed over the NC Piedmont Wed aftn. Just enough dry air was in place(55-60 dew points) that when clouds/then spotty rain dev. during the early aftn… it kept temps in check. To confirm weak CAD… the surface winds (light) did back to the NE-N when the light rain fell for several hours at MWK/UFK down to HKY/MRN. There must have been just enough rain (up to a tenth of an inch) to get the diabatic cooling processes rolling. However… this was a fleeting CAD due to the parent high (lacking strength/depth/and position) to deliver enough CAA/DAA and too progressive to support the continuance of CAA into the Damming region.

    For these reasons, I would call this a weak in-situ/hybrid combo event (most likely only in the Foothills… possibly into the KGSO/KINT areas Wed afternoon… then only in W-N VA Thu).
    Clearing late afternoon Wed allowed temps to recover into the lower to mid 70s at KGSO.

    As for Thu and Thu night… the CAD broke over the NC foothills as the SE flow and dew points in the upper 60s arrived during the afternoon. By evening… the only lingering CAD was up near CHO and SHD where the low level winds were still light N… extending into the ROA and New River Valley… where winds were light NE.

    The CAD boundary may have played a sig. role in the heavy rain of nearly 7 inches over parts of Surry County Wed night and Thu. The H85/h7 warm front appears to have been over the CAD boundary supporting the heavy rain.

  3. Jonathan Blaes @ WFO RAH says:

    Thanks for the post Jordan and for the additional comments Phil.

    From what I recall, the CAD event was still lingering in the Triad early on Thursday morning but it eroded by late morning. Winds were light southeasterly at the time and a low overcase with ceilings less than 200 feet and visbilities less than a mile or two were present after daybreak but the rain was scattered and intermittent at best and the wedge retreated northward.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s