textproduct: Greenville-Spartanburg

SYNOPSIS

Temperatures remain unseasonably warm through Saturday. Rain chances also increase through Saturday until a cold front pushes through the area. Temperatures return to near-normal Sunday into early next week under building high pressure.

NEAR TERM /THROUGH TONIGHT/

As of 6:45 AM Friday:

Key Message 1: Some potential for areas of dense fog this morning, mainly over the lower Piedmont/Lakelands.

At daybreak, a sfc analysis reveals a lingering pool of drier air along/N of I-85 that was hindering the northward development of fog. South of I-85, fog was slowly developing and will be locally dense, particularly near the lakes. At this point, a Dense Fog Advisory is looking like less of a possibility. The fog potential will end by midday due to better mixing.

Key Message 2: Precip chances increase from the west this afternoon with the arrival of deep moisture and forcing.

A short wave lifting out of the mid/upper trof over the Plains today will carry low pressure over the Great Lakes and into eastern Canada, which in turn will drag a cold front across the TN Valley region this afternoon. Mid/upper forcing will improve quickly during the early afternoon ahead of the boundary and should spread eastward across our region through the late afternoon and evening. Isentropic lift at low and mid-levels also improves across the area along and N/W of I-85 this afternoon while deep layer moisture moves in from the west. So, in spite of the meager precip along the boundary early this morning, the increase in forcing should help precip to blossom ahead of the front across the mtns/foothills at least. Precip chances improve from the west as a result, but this was already handled fairly well in the ongoing forecast, so only minor changes were made, mainly to impose a bit of a delay based on the latest runs of the HRRR. There could be a few rumbles of thunder because of some weak elevated instability. The situation continues to improve tonight as the boundary lays out across the region, so a high precip prob still looks warranted. The QPF looks manageable at this time, especially considering how it has been relatively dry lately. If there would be an outside chance of a problem, it would be in the southwest-facing slopes in the little TN basin in southwest NC.

SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/

As of midnight Friday morning:

Key message 1: Heavy rain may continue Saturday morning in the southwestern forecast area.

Although the best frontogenetic forcing still is likely to occur over the mountains and upper Savannah River Valley prior to 12z Saturday, 850mb winds will remain in a favorable SW'ly direction through the morning and may actually increase in speed between daybreak and noon, continuing to provide upslope forcing into the southwest facing mountain ridges and to some degree the Escarpment. 900mb frontogenesis (basically very near surface for the higher mountains) is shown to increase on the NAM and GFS after 12z, so the potential for heavy rainfall will persist, with steering flow oriented roughly parallel to that axis. PWATs still will be near their peak value (+3 SD per ensemble mean) over the area. Hence any bands of heavy rain ongoing at daybreak will take time to diminish, and new bands could develop. Of note, the higher res deterministic models are not showing much potential for rates greater than an inch per hour; the main concern is for a longer duration rain event with totals slowly building and causing rises on creeks and streams, and that appears most likely in our far SW NC counties and the adjacent portion of GA/SC. In light of recent dry weather being a big mitigating factor for flood risk, the WPC Marginal Risk looks appropriate for the western CWA and embedded Slight for the mountains near and southwest of the Great Smokies. Not confident enough for a Flood Watch at this time.

Key message 2: Convection Saturday afternoon could pose an isolated risk of damaging winds.

Surface cold front looks unlikely to cross the mountains until Saturday evening, but deep saturation continues to be shown over the area through the day, so lapse rates remain poor, although strong southwesterly WAA is expected to bring temps to the upper 60s over most lower elevations northwest of I-85, and lower 70s southeast. Dewpoints already will be in the upper 50s in the morning and should push past 60 by midday. SBCAPE remains small, no better than 200-300 J/kg, in most guidance solutions, though the probability of nonzero SBCAPE has increased in recent cycles of several models. Screaming 40-50 kt winds at 850 mb will offer sufficient 0-3km bulk shear to anticipate a small threat of damaging winds with low-topped convective storms, even if no thunder results (though thunder chances have increased with the mean SBCAPE). The orientation of the shear vectors is less than ideal w.r.t. the expected orientation of the front, and the CAMs that go out thru Saturday are not producing much in the way of updraft helicity. We also lack the strong 500mb trough that is known to be an important factor in high-shear low-CAPE tornado threat. Nevertheless, SPC has a Marginal Risk area encompassing the CWA Saturday, and that looks reasonable. Even if convection does not produce a spin-up, have included forecast wind gusts above NBM on the assumption that rain or convective downdrafts could bring down strong winds of 30-40 mph.

Key message 3: Cold front will bring windy conditions Saturday night and Sunday, which could overlap with low relative humidity Sunday afternoon.

CAA will set in Saturday night with leading vort axis blowing thru the area, and sfc gradient begins to increase across the mountains, and high elevations will see gusts pick up accordingly, potentially reaching advisory criteria by daybreak Sunday. Enough moisture could linger along the TN border for flurries with very light snow accums not out of the question in high elevations thru midday. The main shortwave passes during the day Sunday, which ushers in a secondary front and veers winds solidly NW. Temps in the higher mountains should remain nearly steady through the day or fall slightly, while areas east of the mountains will see a normal diurnal curve, with colder air offset by downsloping. Dewpoints will tank following the trough axis, and gusts to nearly 30 mph are likely at times in the Piedmont. There could be a brief period of overlap between the wind and low RH, though both conditions become less concerning for fire wx quickly with sunset in the Piedmont, though it wouldn't take much to worsen conditions during the afternoon. We'll be coming out of the rainy period, so fuel moisture may not be quite low enough to warrant any fire weather headlines as it is, but that is something that will need to be monitored Sunday.

LONG TERM /MONDAY THROUGH THURSDAY/

As of 1250 AM Fri:

Key message: Dry and seasonably cold Monday and Tuesday. Passing trough brings back chances for precip Wednesday and Thursday.

Mostly clear with winds becoming light Monday as sfc high centers over the Gulf/Atlantic coastal plain. Max temps end up about a category below normal for the first time in a while. RH likely to bottom out at or below 30 percent in much of the area that afternoon, though with the light winds fire wx concern may be mitigated. SW flow develops Tuesday; temps and dewpoints rebound although low RH again may occur. Height falls are reinforced aloft as clipper-like trough digs into the Upper Midwest circa Tuesday. Models continue to show this trough deepening near the Great Lakes and bringing another cold front to the southern Appalachians Wed or Thu. Most models still show the trough activating the coastal front, and while the deterministic GFS/EC/GDPS now depict the incipient low distant enough from our area to preclude precip in our portion of the Piedmont, a few ensemble members from each model still do, and so NBM reflects small PoPs there. Temperatures would be warm enough for only rain outside the mountains, where snow would be predominant where temps are cold enough. Light mountain accums appear possible.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: A southerly flow of moisture returning from the Gulf continues to force the development of a low cloud deck that was steadily dropping down into the IFR to LIFR range from south to north. The atmosphere has revealed a preference for a ceiling restriction instead of a vis restriction because of a lingering pool of drier air along/N of I-85 at daybreak, so we have cut back on the fog at most terminals with the exception of KAND. Most terminals should prevail IFR for the better part of the morning, with temporary LIFR. Radar shows a few light returns over the Upstate, and a larger area of precip well off to the west. The latest guidance suggests that precip will be delayed a bit into the mid/late afternoon, so we ramp up with a PROB30 followed by a TEMPO for rain restrictions after 20-22Z or so. During this time, guidance suggests that the ceiling will lift to VFR for a few hours, which seams reasonable. Wind should bend more SW for the afternoon. Around sunset, the low clouds will return along with the arrival of the steady precip. At some point in the evening, an MVFR to IFR ceiling will return, with widespread rain and restrictions from late evening onward.

Outlook: Restrictions will persist thru most of Saturday. Flight conditions improve late Saturday into Sunday behind the front.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...None. SC...None.