Potential for Significant Severe Weather on Wednesday December 26, 2012

All,

Looking at the latest model guidance, it appears that a significant severe weather episode is likely over portions of the Carolinas during the afternoon and evening hours of Wed December 26.

Shortwave energy over the Intermountain West this afternoon will amplify as it digs southeast into the lower Midwest by sunrise Tuesday (25th), progressing east through the lower MS river valley by sunrise Wed (26th) as an attendant surface low strengthens over the Deep South and advects an atypically moist airmass northward from the GOMEX.

H7-H4 DPVA (17Z 12/24/2012)

Dewpoints @ 17Z 12/24/2012

12Z.12.24.2012.NAM.F36-72 Valid 00Z 26th – 12Z 27th

Although it would be extremely unusual for this time of year, a high shear / moderate instability setup appears likely, with MLCAPE values as high as 1000 J/Kg over the Carolinas Wednesday aft/eve.  Forecast soundings show initially weak /moist adiabatic/  lapse rates Wednesday morning.  However, a pronounced mid-level dry slot is expected to advect into the Carolinas atop the warm sector late Wednesday aft/eve, resulting in a relatively considerable amount of convective instability – which is likely to be released given strong layer-lifting associated with the progressive upper level low tracking from the TN valley into the Virginias.  A hybrid CAD wedge is expected to be in place across the Carolinas Wednesday morning, eroding from the south Wed afternoon as a strong southerly LLJ /atypically rich warm sector/ progresses into the region in advance of the deepening synoptic low upstream over the TN valley. Deep-layer shear is progged to be on the extreme side of the spectrum (60+ knots), as is low-level shear with 0-1 km SRH values likely at 300+ m2/s2 during the afternoon and early evening, especially in vicinity of the warm front lifting northward through the Carolinas into VA.

The attached forecast soundings/ hodographs for RDU illustrate the impressive thermodynamic/kinematic setup.

12Z.12.24.2012.NAM.RDU.F60-63 Valid 00-03Z 27th

It is difficult to pin down the timing/ mode/extent of the severe threat over the Carolinas Wednesday aft and eve – which will depend on the precise evolution of the synoptic low, evolution of MCS activity upstream over the Deep South, etc.  From a pattern recognition standpoint, a tornado outbreak certainly seems plausible.

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