textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Adjusted tonight's forecast based on latest trends.

Updated for the the 00Z aviation discussion.

KEY MESSAGES

1. Well above normal and humid with isolated showers and thunderstorms possible during peak heating hours through Saturday. 2. Cold front Saturday night and Sunday brings the best chance for rainfall for any time over the next week. 3. Frost and/or freeze conditions possible in the mountains in the early to middle part of next week.

DISCUSSION

Key message 1: Well above normal and humid with isolated showers and thunderstorms possible during peak heating hours through Saturday.

It's only spring but the early summertime pattern remains in full swing thanks to a Bermuda high centered in the western Atlantic extending over the Southeast through Saturday. With low-level southerly flow remaining in place over the region, well above normal temperatures and humid conditions will stick around. Breezy winds should help offset the humidity somewhat. Highs each afternoon will run 10-15 degrees above normal with lows each night running 15-20 degrees above normal. 12Z CAMs show fairly anemic coverage of showers and thunderstorms each day. Coverage should remain isolated through Friday, before possibly becoming more scattered on Saturday. Confidence on PoPs remains low on Saturday as the 12Z NAMNest is not excited about coverage. The NBM continues to be way too bullish regarding PoPs on Saturday so lowered PoPs to chance this forecast cycle. If later runs of the HRRR start to line up with the 12Z NAMNest, PoPs may need to be lowered further. So, don't get your hopes up for a wetting rain on Saturday, especially if you live east of the mountains.

Key message 2: Cold front Saturday night and Sunday brings the best chance for rainfall for any time over the next week.

A maturing low pressure system will track north of the Great Lakes late Saturday, with trailing cold front subsequently being pulled across the Southeast. Still expecting the stronger dynamic lift to remain well north of the CWA. Frontal precip probably will reach the NC mountains early Sunday morning, with chances peaking later in the day from west to east. QPF from the GFS and EC ensembles have both remained about the same or increased slightly on the last couple of runs; deterministic models have generally trended slightly upward with QPF over a few cycles. Saw fit to maintain categorical PoPs generated by NBM on Sunday, though capped values at 80-90% over the lower Piedmont which will be more prone to downslope effects and perhaps weaker moisture advection owing to the timing. Per LREF ensemble QPF, at least 0.50" total precip looks likely to result from the front in the Smoky Mountains and along the immediate TN border further northeast in our mountains. However, the chance of even 0.25" precip is around or less than 50% in the areas southeast of I-85.

There appears a small but nonzero chance for a line of convection to make a run from East TN into the NC mountains in the predawn hours Sunday, but with 0-3km shear vectors only marginally favorable for tornadic circulations to develop within a line. Lapse rates are not very good and the timing not favorable anyway so our severe threat would appear very low, but worth keeping an eye on for now. The slightly slower fropa timing allows PoPs to extend later into the afternoon Sunday east of the mountains, and should permit more time for diurnal destabilization to occur there. Nevertheless CAPE looks likely to peak at less than 500 J/kg over most of the Piedmont, generally closer to 100 J/kg. 0-3km shear looks likely to decrease to 25 kt or less as the better instability develops, so the severe threat would appear even lower at that time/area.

Drier and cooler air will begin filtering into the CWA by Sunday afternoon. Maxes that afternoon should be cooler than Saturday, back to near normal in the mountains but still a few above normal in the I-85 corridor.

Key message 3: Frost and/or freeze conditions possible in the mountains in the early to middle part of next week.

Continental high pressure will build across the southern Appalachians Sunday night, with precip chances remaining near zero from Monday morning through Wednesday night. Min temps Sunday night fall back to around normal for early April. A freeze can't be ruled out in some higher northern mountain elevations, though with gusty winds the likelihood of frost is relatively low. Winds diminish Monday when max temps end up slightly below normal. Lighter winds Monday night suggest frost is a bit more likely in the mountains wherever temps reach the mid-30s, but conditions could also be too dry. By Tuesday night into Wednesday, the cool pattern is reinforced as upper trough reaches New England, with 1030+ mb high filling in behind. This may result in a dry onset CAD event, and results in a greater chance of a freeze for much of the NC mountains. Temperatures look likely to reach the mid-30s in the NC I-40 corridor both Tue and Wed nights, but confidence is low whether the surface will be moist enough for frost.

The frost/freeze program is activating on April 5 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.

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.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: Showers have ended and Cu slowly dissipating. S to SE winds slowly diminishing as well. These trends continue through the evening. Guidance showing agreement on MVFR stratocu developing/moving in before daybreak with IFR stratus near daybreak. LIFR cigs will be possible. That said, the guidance is unsure on whether cigs develop at KAVL. Have gone SCT for now to show the potential. Cigs lift to MVFR by mid-morning but don't scatter out until noon or shortly thereafter. Isolated diurnal convection possible, but chance too low for the TAFs at this time. Winds pick back up from the S to SW through the day Friday.

Outlook: Isolated SHRA/TSRA and associated restrictions possible through Saturday, with better coverage expected Saturday night into Sunday as cold front tracks across the terminals. Dry high pressure returns behind the front Sunday night through Tuesday.

GSP WATCHES/WARNINGS/ADVISORIES

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