RE: Possible HSLC Event on 4 Jan 2015


This is a follow-up to Justin’s HSLC post.  I created a new post because the response box doesn’t allow (or seem to allow) multimedia content.

Thoughts: Based on anecdotal forecast experience and pattern recognition, my opinion is that environmental conditions do not appear favorable for severe weather on Sunday (very similar to Dec 24, 2014). Despite excellent low-level moisture (esp for early January) in the warm sector and favorable diurnal timing with the approaching cold front, pervasive cloud cover will limit insolation and, perhaps more importantly, destabilization via the presence/release of convective instability is unlikely given nearly saturated (moist adiabatic) mid-level thermal profiles and the absence of a sufficient release (layer-lifting) mechanism. 

Anecdotally, I strongly suspect that the presence of convective instability (and release thereof) is a crucial component of severe HSLC episodes.  In other words, a high shear low cape environment becomes a high shear moderate cape environment when convective instability is present and released.  In this manner, a high shear low cape environment east of the Appalachians can evolve into a high shear moderate cape environment more typical of warm sectors that develop in the lower MS river valley and Deep South.

A more detailed environmental analysis is presented below, along with a few images.

Brandon Vincent
NWS Raleigh, NC


Mid-level lapse rates Upper Left Panel Low-level Moisture/Wind in Lower Right Panel


GFS H5 height, MSLP and QPF in Upper Left GFS H5 height/vorticity in Lower Left NAM H5 height, MSLP and QPF in Upper Right NAM H5 height/vorticity in Lower Right

12Z 01/01 NAM Forecast Sounding Valid 18Z Sunday Afternoon

12Z 01/01 NAM Forecast Sounding for GSP Valid 18Z Sunday Afternoon


MSLP and Surface Wind/Dewpoints at 18Z on December 24, 2014

700-400 mb Vorticity Advection at 18Z on December 24, 2014

700-400 mb Vorticity Advection at 18Z on December 24, 2014

CHS raob on 12Z December 24, 2014

CHS raob on 12Z December 24, 2014

Synoptic Overview: Model guidance indicates that the parent upper trough /occluded surface low/ will be located over the Great Lakes when the attendant cold front progresses eastward into the Carolinas during the day Sunday.  Although both the GFS/NAM show additional shortwave energy approaching the southern Appalachians from the west Sunday afternoon, both models indicate the wave will provide only a glancing bout of DPVA to northwest portions of NC and lag well behind the cold front/warm sector, in addition to de-amplifying as it tracks northeast into the Mid-Atlantic.  It should be noted that the aforementioned shortwave energy will not move ashore the Pacific NW until Friday night and that the precise evolution of this feature remains rather uncertain.  Assuming the timing/track of this feature do not considerably change, upper level forcing over the Carolinas appears marginal at best, i.e. confined to small amplitude waves in southwest flow aloft.

Thermodynamic Overview: The GFS/NAM indicate weak low and mid-level lapse rates on Sunday with deeply saturated (i.e. moist adiabatic) thermal profiles during the afternoon.  Although diurnal timing appears favorable with regard to the cold frontal passage, cloud cover will significantly reduce insolation. Warm sector destabilization should largely be driven by advection, i.e. a higher theta-e airmass (sfc dewpoints 62-65F) surging inland from the Atlantic in advance of the front. Significant uncertainty persists with regard to the location of the retreating wedge front Saturday night into Sunday. Previous experience would suggest full wedge erosion /warm sector establishment/ is unlikely prior to the cold frontal passage in the far NW piedmont (i.e. winston W/NW to the blue ridge).

Kinematic Overview: Strong unidirectional (SW) deep-layer shear. Directional shear will be confined to the lowest levels in vicinity of the retreating wedge front.

About bvincentnws

Meteorologist National Weather Service 2003 - Current
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