textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant changes to forecast trends through the weekend.

Temperatures have trended slightly warmer early next week and frost or freeze conditions appear slightly less likely overall over the southern parts of the forecast area.

KEY MESSAGES

1. Well above normal and humid with isolated to scattered diurnal showers and thunderstorms possible through Saturday. 2. The passage of a cold front Sunday will bring the best chance for rainfall at any time over the next week. The risk of flooding rain or severe weather appears to be low. 3. Cold air could bring frost or a freeze to parts of the NC mountains, foothills, and northwest Piedmont early next week. The most likely time would be Tuesday night. Protection of sensitive vegetation might be necessary.

DISCUSSION

Key message 1: Well above normal and humid with isolated to scattered diurnal showers and thunderstorms possible through Saturday.

Bermuda high lingers over the region through Saturday keeping the summerlike pattern around. Well above normal and humid conditions will linger over the region thanks to southerly flow and dewpoints ranging from the 50s to 60s. Highs each afternoon will end up 10-13 degrees above normal with lows tonight ending up around 20 degrees above normal. Shower and thunderstorm activity will remain isolated today, with the best chances expected across the North Carolina mountains and south of I-85. Convective chances may increase on Saturday but confidence is low as the 12Z CAMs are still not in agreement regrading coverage of convection tomorrow afternoon and evening. The 12Z HRRR shows scattered to widespread convection developing across much of the GSP CWA while the 12Z NAMNest shows mostly dry conditions outside isolated convection across the mountains during peak heating tomorrow. With confidence on PoPs remaining low, capped PoPs to 65% west of I-77 and 50% along/near the I-77 corridor. Severe storms are not expected thanks to very low shear in place, but a few strong, sub-severe storms cannot be entirely ruled out during peak heating hours Saturday.

Key message 2: The passage of a cold front Sunday will bring the best chance for rainfall at any time over the next week. The risk of flooding rain or severe weather appears to be low.

Mature midlatitude cyclone will track north of the Great Lakes late Saturday, then into northern Quebec Sunday. Trailing cold front will advance across the Southeast. Though convection generally will wane nocturnally Saturday evening, NAM and HRRR suggest some new development may occur in prefrontal convergence zone over NE GA and the southwesternmost NC mountains; thus retained higher PoPs in the evening than we had on earlier cycles. No major changes to expectations as far as frontal timing with this forecast package. The front itself is still expected to move into the NC mountains after midnight Sunday morning, progressing east. PoPs peak east of the mountains during the day Sunday, still in the categorical range, with any precip most likely ending before 00z Mon. Forcing will weaken as the front crosses the CWA and so the greatest QPF is forecast in the southwestern mountains and Escarpment, additionally due to the potential for Saturday evening convection being greatest there. LPMM QPF from the HREF suggests localized spots along the south-facing ridges could see 1" or more in 6 hours, but that product tends to be on the high end of possible outcomes. NBM probs of 1"/6h in the same areas are no better than 25%, for what that is worth. Still seems reasonable to sum up the QPF potential east of the mountains as being about half as much. Still not expecting much if any flood threat but also not likely to make a big improvement in soil moisture or drought conditions.

The other aspect of the front is the potential for strong to severe thunderstorms, but that too continues to be low, with small instability being the main factor; early morning arrival in the mountains is unfavorable, but the CAPE values progged upstream also are not all that impressive. We also lack classic synoptic elements of a high-shear low-CAPE setup. 0-1km and 0-3km shear are not quite enough to expect an appreciable threat. Upper divergence associated with right entrance region of jet streak should provide some degree of forcing for ascent--one reason we retain high PoPs east of the mountains--though the core of the jet will remain well north of the CWA, and we will lack the strong DPVA at 500 mb noted as an ingredient for HSLC. The front will be passing the NC/SC Piedmont near peak heating, so some destabilization is expected though less than 500 J/kg. While the the HRW-FV3 and NAM-3km develop some new convection along the front, its depictions are unimpressive so far, in terms of severe threat.

Key message 3: Cold air could bring frost or a freeze to parts of the NC mountains, foothills, and northwest Piedmont early next week. The most likely time would be Tuesday night. Protection of sensitive vegetation might be necessary.

Still expecting temperatures to trend back to near or slightly below normal Sunday night through Tuesday via the continental high pressure building in behind the front. Temps have trended slightly warmer with the latest NBM and now only a few areas of the northern mountains reach the mid-30s where frost might become a concern. Gusty winds Sunday night are expected to limit localized radiational effects that would result in cold spots, and also limit frost potential; the exception might be areas that are more sheltered from northwest winds. A reinforcing high will move into the upper Midwest Monday night, then center over the Northeast US Tuesday night. CAA may strengthen in the mountains Monday night. Gusts will be lower at that time but still breezy enough to limit radiational cooling by the same tokens. We now expect Monday night's temps will be similar to the previous night. The reinforcing high will however cooler daytime maxes Tuesday in our far north and then allow temps to fall cooler in all areas Tue night, when frost/freeze appears more likely in the mountains and perhaps parts of the NC Piedmont I-40 corridor.

The frost/freeze program will be activated on 5 April for the mountain zones along and southwest of I-26 and also for the NC Piedmont I-40 corridor (Greater McDowell east to Rowan, and north). It is already active in areas to the south. As the forecast stands, some combination of frost adv/freeze warn is a good bet across parts of the mtns and foothills early Wednesday. Unfortunately, this also sets up the confusing situation where the central and southern mountains might get a freeze warning because those zones are active, but the northern mountains will not get a warning in spite of being even colder because the program does not become active until 21 April.

With respect to fire weather, RH looks likely to drop below 30 percent across the area Monday through Wednesday afternoons, though with generally light winds. Fuel moisture will have to be monitored. Fire danger will likely increase during this period. Temperatures will rebound Thursday as the high moves off the Northeast coast, with dewpoints trending higher slightly as well. Thus RH may not be as much of a concern that day. There appears a small chance of a coastal low developing by Thursday night, but an even smaller chance (per ensemble members) of such a low bringing precip this far inland.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Mostly dry through the 18Z TAF period outside of the potential for scattered SHRA/TSRA across the SC terminals and KCLT tomorrow afternoon. Confidence on PROB30s for TSRA remains very low as high-res guidance sources are not in good agreement regarding the coverage of activity tomorrow. Maintained dry condition at KAVL and KHKY as these terminals are unlikely to see SHRA/TSRA develop before the end of the 18Z TAF period. VFR through late tonight before MVFR to IFR cigs return during the morning hours Saturday. Cigs should lift to VFR levels by the late morning or early afternoon hours outside of SHRA/TSRA. Wind direction will remain mostly S/SE through tonight, becoming more S/SW tomorrow. Isolated low-end intermittent gusts are possible this afternoon and again tomorrow afternoon.

Outlook: A cold front brings better chances for SHRA/TSRA and associated restrictions Saturday night into Sunday across the terminals. High pressure builds into the region behind the front Sunday night into early next week allowing dry and VFR conditions to return.

GSP WATCHES/WARNINGS/ADVISORIES

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