textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Rain chances were increased 10-20% for Saturday based on latest model trends.

KEY MESSAGES

1. Well above normal and humid with scattered showers and thunderstorms possible during peak heating hours 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 in the early to middle part of next week, particularly Tuesday night/Wednesday morning. Protection of sensitive vegetation might be necessary.

DISCUSSION

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

Summerlike weather pattern will continue thru Saturday, with a Bermuda high supplying warm, moist air into the region. Highs today and tomorrow will be 10-15 degrees above normal, while lows tonight will be 15-20 degrees above normal thanks to dewpts in the 50s to lower 60s. Fcst soundings show a little weak mid-level capping that should limit deeper convection this aftn, despite some sbCAPE. Guidance in decent agreement in scattered showers and isolated tstms developing across the Midlands, but staying mostly to our south. A few stray showers in the mountains will also be possible. Then, a seabreeze front will work inland from the SE overnight and could trigger some showers across mainly the Upstate and NE GA before daybreak Saturday. The moisture with that front will likely produce a stratus deck that will linger into the day Saturday, but should scatter out by early aftn. An upper trough will approach from the west and should help break the mid-level inversion enough to support better instability for diurnal convection. The 00z CAMs have trended a little more active, with the NAMnest continuing to be unimpressed. Have bumped up PoPs slightly for Saturday, but still undercutting the categorical PoPs seen in the latest NBM. Even if coverage overperforms, lack of shear should keep the severe threat low.

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.

A mature mid-latitude cyclone is expected to track across the Great Lakes Saturday evening and then over eastern Canada on Sunday. As the upper system lifts to the east, the associated mid/upper trof will become broader and weaker, but is still expected to have enough of a push eastward to move a cold front to the mountains late Saturday night and then across the western Carolinas and northeast Georgia on Sunday. Although precip should move in from the west ahead of the front Saturday night, the latest model guidance is generally in agreement that the front will not reach the mtns until after midnight, and then will move east off the mountains after daybreak Sunday, but should still exit to the east of the fcst area by sunset. The forcing associated with the front looks like it will be gradually weakening as it crosses the region on Sunday, but still looks like it should have enough support for a categorical precip prob. There is some concern that the weakening forcing does not bode particularly well for rainfall amount trends. For now, the rain looks beneficial to the extent that the mtns will likely see a half inch to inch, while areas east of the mtns might see half that. Suffice to say that the risk of flooding rain appears to be minimal. If anything, the rainfall amounts are more likely to go down. As for severe weather potential, the timing of the front relatively early in the day is unfavorable and the chances of getting sufficient sfc-based CAPE above 500 J/kg are very low apart from the southeast fringe in the afternoon. Deep layer shear also appears to be not quite favorable enough as it tops out around 25-30 kt. Note this still bears watching for another model cycle or two until we see how the CAMs want to handle the convection along/ahead of the front. As for the temps, Saturday night will be the last seasonally mild night ahead of the front. Highs Sunday get knocked down 5-10 degrees based on cloud cover and shower activity.

Key message 3: Cold air could bring frost or a freeze to parts of the NC mountains, foothills, and northwest Piedmont in the early to middle part of next week, particularly Tuesday night/Wednesday morning. Protection of sensitive vegetation might be necessary.

Continental high pressure should build into the area Sunday night and Monday in the wake of the front, and will get reinforced as a larger high moves across the Midwest to the Mid-Atlantic coast Tuesday and Tuesday night. Temps overall will drop back down on the cool side of normal for early April. Of concern are the min temps over the mtns early Monday and early Tuesday, but for now only the highest peaks have any chance of getting below freezing. A pressure gradient remains across the mtns both nights that should help to preclude frost formation except in some sheltered spots. The situation changes Tuesday night/early Wednesday with the shot of even cooler air from the reinforcing high. Even the mtn valleys have a 50/50 shot at reaching 32F and frost will be likely. Even locations east of the mtns across the foothills and northwest Piedmont of NC will have a chance for some frost if the wind goes light.

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.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: Overall, cigs have not been as low as the model consensus so far this morning. Nevertheless, plenty of MVFR cigs are expected to start the 12z TAFs. The stratus should scatter out by around midday to VFR. Isolated diurnal convection possible in the mountains mainly SE of KCLT, but chances remain too low to mention in any of the TAFs at this time. Winds pick back up from the S to SW by early aftn, then weaken again somewhat this evening. Confidence is fairly high that another round of MVFR to possibly IFR stratus develops overnight thru Saturday morning.

Outlook: Another round of MVFR to IFR stratus possible early Saturday morning, scattering out by early aftn. Scattered SHRA/TSRA and associated restrictions expected Saturday aftn to early evening, with a band of convection 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.