textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The aviation forecast was updated to reflect the impending 06z TAF issuance.

Rainfall totals were updated for Sunday's rain event. They have trended slightly downward, but have not changed significantly.

KEY MESSAGES

1. Dry conditions continue today, and isolated critical RH support another afternoon of marginal fire weather issues. 2. Widespread rainfall is still expected on Sunday, but hydro concerns essentially nonexistent given ongoing drought conditions across the region. 3. A trend toward a dry and very warm pattern is expected next week.

DISCUSSION

Key message 1: Dry conditions continue today, and isolated critical RH support another afternoon of marginal fire weather issues.

Sprawling 1025mb surface high centered over eastern North Carolina continues to dominate the pattern tonight, maintaining dry and quiet conditions across the entirety of the western Carolinas and northeast Georgia. Despite the high's location to our east, basically no discernible moisture return has developed. 00z RAOBs depicted an extremely dry air mass up to the 500mb level, and there's little indication we'll see much improvement over the next 12 to 24 hours.

Marginally-low relative humidity is expected to develop again Saturday afternoon as a shallow mixed layer develops by late morning, tapping into dry air aloft. Were flow aloft a bit stronger, one might expect dewpoints to crater during peak heating, but with light flow through the depth of the troposphere, there won't be much to incite better mixing at the top of a weakly-capped PBL. So, most locations east of the mountains can expect afternoon RH to drop into the 25-30% range, but the likelihood of RH <25% is fairly low outside the I-77 corridor. Fire danger will be nonzero, but limited by light winds of only 3-6kts across the region. RH will rebound by late evening, as daytime mixing abates and we start to get better moisture return ahead of the next active system.

Key message 2: Widespread rainfall is still expected on Sunday, but hydro concerns essentially nonexistent given ongoing drought conditions across the region.

Guidance is in good agreement that a progressive shortwave trough will make tracks out of the Desert Southwest Saturday and quickly traverse the southern CONUS...driving a mature surface low into the lower Mississippi Valley by late tonight. All indications are that the low will already be in the process of occluding by Sunday morning, but a very strong WAA regime on the leading edge of this system will nonetheless pump ample moisture into the area...resulting in the onset of rainfall around or shortly after dawn Sunday for the western Carolinas. GFS, NAM, and HREF profiles are supportive of in situ cold-air damming east of the NC mountains...but the system's overall track is far enough south, and the cool-sector air mass modified enough by the time of onset, that even within the wedge temperatures will only be in the 40s. So, an all-rain forecast remains a near certainty, with the exceptions of very high ridgetops in the Smokies and Balsams, which could feasibly wet-bulb down cold enough to see brief flurries at the onset Sunday morning.

PWs ranging from 1-1.5" across the area and decent synoptic forcing during the day Sunday will support efficient precipitation, resulting in rainfall totals of 1-1.25 inches for much of the area...likely even more in favored upslope zones across the Blue Ridge Escarpment. The 00z HREF doesn't quite extend out far enough to capture the entire event, but the impending 12z cycle will, and will provide more insight into high-end totals. In general, though, hydro issues appear unlikely given most of the forecast area is currently under D2-D3 drought conditions. Ensemble hydrographs further support this assessment. Both the 00z REFS and HREF support ~25% chances of marginal 100-150 J/kg sbCAPE across the extreme southern tier of the forecast area - likely too little instability if it even materializes for a severe risk, but perhaps enough for a rumble of thunder and brief, localized convective rain rates. Elsewhere, guidance is in good agreement that the wedge will remain too entrenched for any surface-based instability.

Operational models remain in good agreement that as the z500 trough axis shifts to our east Sunday night, coastal cyclogenesis will occur over the NC Outer Banks. This will have the impact of helping usher in a postfrontal CAA regime that much quicker, eroding the in situ wedge in short order and bringing a swift end to rainfall through early Monday morning.

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

Cold Air Damming wedge quickly breaks down Monday as the parent surface high shifts off the New England coast and the passing upper shortwave and attendant surface low slide off the Carolina coast. Thereafter, broad and flat upper ridging becomes established from Mexico into the Gulf with an elongated belt of westerlies extending from the Desert Southwest to the Appalachians. Rising heights along with a northward developing warm sector will foster a return to well above average temperatures with afternoon highs climbing into the mid to upper 60s by mid week followed by low to mid 70s by Thursday and Friday. A frontal boundary may slowly drop through the Ohio Valley and towards the area, but will be oriented parallel to the entrenched westerlies. Thus, confidence is low as to how close to the area the boundary will make it. Nonetheless, a few stray warm advection showers cannot be discounted by late week, but this will be the exception and not the rule with most locations remaining dry.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: VFR continues tonight and Saturday as high pressure dominates the pattern. Following a period of mostly SKC conditions overnight, expect gradually-thickening cirrus through the day on Saturday. Calm wind overnight will become a light S/SW wind during the day. High VFR ceilings should develop Saturday night, but no prevailing restrictions are expected to develop through the end of the 06z TAF period. For KCLT, a PROB30 for onset of light rainfall after 08z Sunday morning was included; for all the other terminals, no mention of precipitation within the 24-hour TAF window. Expect that to change with the 12z TAFs.

Outlook: Precipitation chances and associated flight restrictions will develop early Sunday morning. Widespread rain will result in low ceilings - probably IFR or worse - well into Sunday night. Dry conditions should return Monday and persist through the first part of the week.

GSP WATCHES/WARNINGS/ADVISORIES

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