textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated for the Aviation part of the discussion.

KEY MESSAGES

1. Mild temperatures and low relative humidity continue on Saturday, mainly east of the mountains. 2. Widespread rain is expected Saturday night through Sunday night. This will mainly be a beneficial rainfall with little to no hydro impacts expected. 3. A trend toward a dry and very warm pattern is expected next week.

DISCUSSION

Key message 1: Mild temperatures and low relative humidity continue on Saturday, mainly east of the mountains.

An area of high pressure centered over the central Appalachians at present will continue to shift eastward through Saturday, keeping rain chances suppressed and fire concerns elevated. Guidance from model soundings show a large column of dry air over the area, but with minimal winds, especially in the boundary layer. For fire concerns, surface winds look to be light through the evening hours and become calm overnight.

By Saturday, the axis of high pressure slides eastward, allowing for southwesterly surface winds and an uptick in moisture. This should help to increase the dewpoint and narrow the window for near-critical RH. Expect afternoon RH in the 25-30% range and light winds of 5-10 mph east of the mountains. Temperatures should also be a few degrees warmer than today ahead of a low pressure system bringing rain Saturday night, hopefully contributing to an improvement in ongoing drought conditions.

Key message 2: Widespread rain is expected Saturday night through Sunday night. This will mainly be a beneficial rainfall with little to no hydro impacts expected.

A short wave trough/compact upper low will move out of Texas early Sunday, and across the Deep South through Sunday night. Attendant cyclone is forecast to weaken as it moves across the Gulf Coast states on Sunday, before secondary cyclogenesis occurs near Cape Hatteras Sunday evening. Forcing and moisture attending the first cyclone will be sufficient to bring widespread rain to the forecast area from late Saturday night through Sunday evening...warranting 100 PoPs in all locations. Robust moisture flux from the Gulf will support precipitable water values of over 1.0"...which would be around the 99th percentile with respect to climatology will support an environment favorable for heavy rainfall. On the other hand, the surface low will track south of the CWA...while in-situ cold air damming will become established over the CWA as precip falls into an initially cool/dry air mass. Thus, instability will be non-existent, while forcing profiles will be steadily weakening coincident with the weakening of the cyclone. As such, rainfall rates are forecast to be rather tame, and forecast storm total rainfall is in the 0.75-1.0" range, with some locally higher amounts along the southern escarpment of the Blue Ridge. Due to the unseasonably cool weather that has impacted the region for much of the past month, the ground over the mountains is more impervious than is usual for this time of year, but rainfall amounts should nevertheless be too tame to cause anything more than very isolated, nuisance hydro issues, especially in light of ongoing drought conditions. Forecast max temps are expected to be more than 5 degrees below normal on Sunday due to the in-situ CAD.

Key message 3: A trend toward a dry and very warm pattern is expected next week.

Within a quite active upper air patter, a low amplitude, long wave ridge is forecast to develop west of the region next week. A quick return to above normal temps is expected on Monday as in-situ cold air damming undergoes a quick demise and heights rise aloft. A broad warm sector will become established over the Southeast through the week, with the warming trend continuing over our area through Thursday. From Wed night through Friday...forecast max temps are more than 15 degrees above climo, while forecast mins are only a little shy of where maxes should be for this time of year. A cyclone passing over the Great Lakes late in the week may drag a frontal boundary toward our area late in the week, but current indications are that forcing and moisture will be insufficient for significant precip chances, and only token, mostly slight chance PoPs are advertised Thu night and Friday.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: VFR at all terminals through the TAF period, with only thickening cirrus to mid clouds on Saturday afternoon as the next system makes its approach from the west. Light/var wind overnight will probably be calm at times, then when the wind returns Saturday morning, expect light S to SW. Any restrictions will hold off until closer to daybreak Sunday.

Outlook: Precipitation chances and associated restrictions return Saturday night into Sunday, with widespread rain contributing to low ceilings through Sunday and Sunday night. Drier conditions and VFR should return for the beginning of the week.

GSP WATCHES/WARNINGS/ADVISORIES

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