Cashiers / Highlands

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Forecast Discussion



... A couple of weak cold fronts will help drive scattered showers and storms each afternoon and evening through Wednesday. Some of these storms could pose a risk of severe weather today and Tuesday. The week will start out seasonably warm, but mild temperatures return Thursday and Friday behind the front.

NEAR TERM /THROUGH TONIGHT/... As of 1040 AM EDT Monday: CAMs, especially the HRRR, have been consistent with scattered thunderstorms developing along or near the escarpment late this afternoon and moving south across the area into the evening. 12Z RAOBs show an inversion in place with relatively steep lapse rates above. Guidance shows moderate instability developing as the clouds over the area dissipate and heating takes place. Low end moderate low level shear in place will lead to moderate effective shear over the area. This will lead to a couple of supercells along with some multicell clusters. Even though there will be a couple of supercells with mesocyclones, the tornado threat is low. However, large hail and damaging winds are more likely. The excessive rainfall threat is low as well, but localized heavy rain is possible. High temps look on track as the slower heating taking place is already baked into the numbers.

Otherwise, an upper level omega blocking pattern will remain centered over the northern Plains through the near-term period. East of the block, a large upper low will continue to spin just off the New England Coast and will move a mid/upper level short- wave southward and right over our area this evening. At the sfc, high pressure will gradually weaken over the region. Another round of diurnal showers and thunderstorms will likely develop later this afternoon/evening ahead of the above-mentioned upper shortwave. Convective initiation still looks somewhat delayed by the presence of a weak capping inversion that persists well into the afternoon. SPC's updated Day 1 Convective Outlook has pretty much all of our CWA in a Marginal Risk area for severe wx which seems reasonable given that most of the latest model guidance still generates 1000 to 2000 J/kg of sfc-based CAPE beneath a 500mb speed max producing up to about 35kts of 0-3km bulk shear at roughly 00z. The primary hazards still appear to be damaging straight-line winds and large hail. Most of the latest CAMs still depict the bulk of convective initiation over the NC escarpment, with discrete-looking cells moving into the Piedmont and Upstate thru the late evening with some fairly robust cells surviving into the late night. In addition, some support for rotating updrafts is expected over our area, but the lack of more pronounced low-lvl shear and the presence of fairly high DCAPE across the area should limit any tornado potential. Otherwise, PWATs should remain fairly low and storm motions should be fairly brisk. Thus, excessive rain/ hydro issues appear to be less of a concern than the severe risk.

SHORT TERM /TUESDAY THROUGH WEDNESDAY/... As of 115 AM Monday: Weak cold front most likely will have settled through most of the CWA by Tuesday morning. This reinforces northwesterly flow and extends it down to the surface, promoting better downsloping, so we expect above normal highs Tuesday, a few degrees warmer than Monday. The global models depict a clear moisture gradient across the CWA, such that the PBL becomes much deeper in areas well north of the boundary. This may put CCLs out of reach in our more northern zones, and thus essentially zero convective threat. On the other hand, the NAM doesn't develop the gradient and suggests most of the CWA would develop seasonably large SBCAPE, in most areas considerably higher than Monday. None of the models initiate more than isolated convection, however, with the weak vort axis being south of the CWA. Lee troughing or differential heating could be triggers, however. Will maintain slight chance to chance range PoPs mainly over our south half. Assuming we develop enough SBCAPE, we'd still likely see a damaging wind threat with similar moisture profiles aloft (NAM actually trends a little higher on sfc-mlvl delta-theta-e). Convective mode could be more pulsey given somewhat weaker shear; hail threat may be mitigated by the same token. SPC has introduced a well placed Day 2 Marginal Risk for a portion of NE GA and the Upstate.

Another cold front and attendant weak vort axis look to have pushed into KY/WV/VA by Wednesday morning; these features will shift southward into our CWA Wednesday afternoon or evening. The front itself looks to cool the midlevels, while temps higher aloft don't change much. Thus, models show shallow convection developing diurnally while it is still to our north, and subsequently propagates into the CWA. The previous front looks to remain stalled across GA/SC during the day, and diurnal convection certainly could develop along it during the afternoon. Our deterministic models do not yet agree whether that occurs within our southern zones, or south of the CWA entirely. This could however result in two separate rounds of precip for portions of the area, between Wednesday afternoon and Thursday morning. The activity along the front probably won't be of much severe or hydro concern, but if we do see diurnal convection develop along the old front, dry profiles and modest shear would suggest some severe threat similar to the prior two days, although instability does look to be at least somewhat lower. Max temps likely will be about the same as Tuesday's in our south, but should trend a few degrees cooler in the north if the front does arrive during the afternoon.

LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... As of 215 AM Monday: Precip associated with the front may wane to some degree with the loss of diurnal heating around or after 00z Thursday. However, forcing will ramp up as a much more clearly defined shortwave dives southward across the lower OH/TN valleys and rotates east past the southern Appalachians. A wave of PoPs thus will progress across the CWA Wed night and Thursday. Deep saturation is progged for a time, and PWATs peak around 1.3 to 1.5 inches. The current cycle of deterministic runs is in better agreement than on previous cycles, and confidence in timing has improved, with some interaction between the shortwave and front looking more likely. With the shortwave/front/precip being timed earlier, confidence is also higher for us to say we will experience cooler weather Thursday, a conservative estimate being 7 to 11 degrees lower. The GFS would put us in for a shocking 15 to 20 degree drop in some parts of the CWA; this is tied closely to its QPF response and seems to result from diabatic cooling given its depiction of a very deep mixed layer the previous day. Min temps Thursday night do look likely to drop into the lower 40s in higher mountain elevations.

Heights bottom out late Thursday or early Friday as the shortwave passes. Dry high pressure will migrate into the area in its wake; temps will moderate Friday but remain mild and likely a few degrees below normal. While the omega block doesn't totally break down for the remainder of the period, its influence weakens over the Southeast as quasi-zonal flow develops between shortwaves. Temps should return to about normal for Saturday and perhaps a tad above on Sunday under weak low-level ridging. That also will promote moisture return ahead of the next wave Sunday, when we expect our next chance for convection.