textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant changes to the timing and expected rainfall amounts associated with the cold front tonight through Sunday.

Low temperatures continue to trend slightly upward during the early to middle part of the next week.

Updated for the the 06Z aviation discussion.

KEY MESSAGES

1. Well above normal and humid with isolated to scattered diurnal showers and thunderstorms possible today. 2. The passage of a cold front Sunday will bring the best rain chances 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 each night from Sunday night through Wednesday night. 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 today.

A Bermuda high will continue to influence our weather today, with highs 10-13 deg above normal and dewpts in the 60s, making it feel humid. Meanwhile, heights aloft will begin to fall, as an upper trough crosses the Midwest. The height falls should help steep mid-level lapse rates and erode a mid-level cap that was in place yesterday. Model consensus shows up to 1000 J/kg of sbCAPE by peak heating. Differential heating and perhaps some weak upglide should trigger at least scattered convection across the western half of the forecast area by early aftn, then drifting NE late aftn into the evening. The 00z CAMs are in decent agreement on the activity remaining unorganized. Bulk shear will remain rather weak, and overall forcing and modest CAPE should keep any severe threat low. But a few sub-severe strong storms may be possible. PoPs will range from 30% in the far eastern zones to around 80% in the northern NC mountains.

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

An upper trough will continue to progress from the Mid-MS Valley to the OH Valley Saturday thru Sunday, bringing a fairly strong cold front into the forecast area. Guidance is in really good agreement on the timing of the front and associated precipitation. This still means a late Saturday night thru early Sunday aftn time frame for convection. A lack of instability looks to be the limiting factor in any heavy rain and severe threats. The 00z CAMs are in decent agreement on a QLCS tracking across central KY/TN, but losing organization and overall weakening before even reaching the NC mountains around or just after midnight. So it is looking like a generally broad band of showers with a few embedded tstms crossing the mountains overnight, and enter the Piedmont around daybreak Sunday. The line will then continue to push east and largely exit the SE edge of the forecast area by sunset. There could be some increasing instability ahead of the front to allow convection to strengthen, but any uptick in severe threat looks to be to our east.

In addition to any non-zero severe threat, we could see some beneficial rainfall across the area, especially in the SW-facing slopes of the mountains. But with the lack of instability and the progressive nature of the convection, any excessive rainfall looks unlikely. Some favorable slopes could see 1-2" of rainfall in a 6-hour period, which given the drought, should not result in flooding. The rest of the area could see half an inch to an inch of rainfall.

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

The new model guidance continues to show a period of cooler and drier weather through at least the middle part of the week as an initial continental sfc high builds in behind the front early in the week, and then the cool air mass gets a reinforcement from a stronger high moving along in the northern stream Tuesday night and Wednesday. The second high will essentially bring a dry wedge to the fcst area from Wednesday through the rest of the week. The main concern continues to be the overnight low temps mainly over the mtns and I-40 corridor, however the new model blend continues the slightly upward trend. The probs for temps getting below 32F Sunday and Monday night are confined only to elevations above 5K ft and remain very low at any rate. Wind might also prove to be steady enough overnight to inhibit frost formation because of the persistent pressure gradient across the mtns. Opted not to introduce any frost wording yet because of this slow warming trend and because the central/southern mountain zones don't become active until tomorrow. Tuesday night still looks like the best chance to have frost/freeze issues after the reinforcing front crosses the region, but the latest blend keeps areas outside the northern mountains in the middle 30s, thus a freeze might no longer be on the table. Frost still looks like a decent bet, though. By Wednesday night, the air mass modifies enough to mitigate the concern.

As a reminder, 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, a frost advisory is a good bet across parts of the mtns and foothills early Wednesday. Unfortunately, this also sets up the confusing situation where the northern mountains might get below freezing but have no warning issued because those zones do not become active until 21 April. That being the case, we will have to let the fcst speak for itself.

With respect to fire weather, RH looks likely to drop below 30 percent across the area each afternoon from Monday through Wednesday, though with generally light winds. Fuel moisture will have to be monitored. Fire danger will likely increase during this period. Temperatures bottom out Wednesday around five degrees below normal, but will rebound to normal Thursday as the parent high moves off the Northeast coast. For the rest of the period, a gradual warming trend is expected with increasing moisture, which should improve the fire weather situation once again.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Guidance has backed off the low MVFR to IFR cigs this morning, but still expect at least some MVFR cigs across the Upstate sites developing right around or just before daybreak. The clouds may spread NE toward KCLT and KHKY, but guidance is mixed, so confidence is low. There is potential that the convective Cu field may start out below 3000 ft before lifting to 3500 to 5000 ft this aftn. As for convection, spotty showers have developed thanks to upslope flow near the NC/SC border. For now, will go with VCSH at KAND, where the best chance of seeing SHRA early this morning. Still expect scattered SHRA and TSRA to develop across the area this aftn, with PROB30s placed during time period of highest chances. Light S to SW wind tonight picks back up during the day, with low end gusts for the afternoon. A lull in convection expected this evening ahead of a line of showers and storms approaching from the west. A PROB30 for TSRA was added at KAVL, as there is a chance convection may arrive toward end of TAF period.

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

GSP WATCHES/WARNINGS/ADVISORIES

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