Sunrise 6:34 AM   Sunset 8:19 PM Moonrise 3:33 AM   Moonset 3:39 PM

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FXUS62 KGSP 041035 AFDGSP

Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 635 AM EDT Sat May 4 2024

SYNOPSIS
A front will stall over the region through the weekend resulting in periodic showers and thunderstorms. The best chance for widespread showers and thunderstorms can be expected today into tonight, with only scattered activity expected on Sunday. A general summertime pattern sets up early next week, which will keep scattered thunderstorm chances around each afternoon. Highs will trend well above normal by Tuesday, remaining well above normal through at least Thursday.

NEAR TERM /THROUGH TONIGHT/
As of 630 am EDT Saturday: Isolated/scattered convection continues within general low level confluence zone/coincident deep moisture plume within broad area of modest height falls. Convective coverage has been steadily increasing across the region over the past couple of hours, especially around the periphery of the CWA, and this trend is expected to continue, with activity gradually filling in across our area as a short wave trough steadily approaches from the Deep South. Eventually, categorical coverage of convection is expected from around late morning through the afternoon. Precipitable water values will remain very high...generally in the 90th-99th percentile of the daily climatology for our area. A weak backdoor cold front is expected to nudge into the CWA from the northeast later this morning, with the leading edge likely settling in the vicinity of the NC/SC border by this afternoon. This will provide an additional source of lift for convective initiation, and could be the focus for locally heavy/ perhaps isolated excessive rainfall this afternoon.
Deep S/SW flow will also pose the potential for training cells within persistent confluent zone. Heavy rain has been very spotty since Fri afternoon, so dry antecedent conditions remain the norm across the forecast area, but higher coverage of showers and storms combined with the above factors will result in a ramping up of the excessive rainfall threat today. Extensive cloud cover (max temps actually a little cooler than climo) and poor mid-level lapse rates will result in rather weak buoyancy through today, while shear parameters will remain weak, so the conditional probability of severe storms remains close to zero.

As the short wave trough finally drifts over the forecast area tonight, the zone of low level confluence and moisture plume will begin to shunt east of the area. However, this trend will be rather slowly evolving, so convective chances will remain elevated through at least the evening, before slowly winding down overnight. The threat for locally excessive rainfall threat will persist through at least the evening. Min temps will again be well above normal.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/
As of 230 AM EDT Saturday: A stationary front will be draped from the Ohio Valley to the Southern Plains Sunday into Monday, remaining well west and north of the western Carolinas. This will allow for SW'ly flow at 850 mb keeping an influx of rich gulf moisture in place across the region. Convective chances look to be much lower on Sunday compared to Saturday thanks to a weak upper trough gradually pushing east of the western Carolinas throughout the day. However, isolated to scattered showers and thunderstorms are still expected to develop throughout Sunday per the 00Z CAMs. Thus, have likely PoPs in place across the northern zones, with chance PoPs across the southern zones. Storm movement will continue to be slow on Sunday with only about 10-15 kts of shear in place so locally heavy rainfall will be possible, especially for areas that received rain on Saturday. The majority of guidance shows SBCAPE values ranging from ~800-1500 J/kg during peak heating Sunday afternoon. Model soundings from both the NAM and GFS show an inverted-V and mid-level dry air in place Sunday afternoon. Thus, an isolated severe storm or two cannot be entirely ruled out, with the main potential impact being damaging wind gusts. Shower and thunderstorm chances will show a slight uptick on Monday as upper shortwaves track overhead the western Carolinas. Went with categorical PoPs across the western third of the forecast area on Monday, with likely PoPs elsewhere.
The strong to severe storm potential and locally heavy rainfall threat return again on Monday thanks to continued weak shear and 800- 1500 J/kg of SBCAPE in place. Highs should end up a few degrees above climo through the period, with lows around 10-13 degrees above climo.

LONG TERM /TUESDAY THROUGH FRIDAY/
As of 200 AM EDT Saturday: Weak upper ridging builds into the Carolinas on Tuesday while an upper shortwave tracks overhead the forecast area. This will act to keep shower and thunderstorm chances around across most of the forecast area. Have the highest PoPs (likely) confined to the NC/TN border, with chance PoPs elsewhere (the exception being the far southern tier of the CWA where there are no mentionable PoPs). With shear values expected to range from 25-35 kts and SBCAPE values around 1500 J/kg on Tuesday afternoon, isolated strong to severe storms seem plausible. Tuesday night into Wednesday weak upper ridging will continue building across Carolinas. This should generally lead to lower convective chances compared to Tuesday. Thus, capped PoPs to chance across the western two-thirds of the forecast area. However, since cloud cover is currently expected to be lower on Wednesday, global models depict SBCAPE ranging from 1500-2500 J/kg during peak heating, with shear still generally ranging from 25-35 kts. Thus, diurnal strong to severe storms will be possible again on Wednesday. At the sfc, a cold front will slowly progress eastward out of the central US but will remain well west of the Carolinas. This will lead to W/SW'ly 850 mb flow in place Tuesday into Wednesday, allowing temps will climb into the mid to upper 80s east of the mts and across the mtn valleys each afternoon. A few locations along I-77 in the southern NC Piedmont may even see highs reach 90 degrees on Wednesday. Thus, highs should end up around 6-12 degrees above climo. Lows Tuesday night and Wednesday night should end up around 12-15 degrees above climo thanks to some lingering cloud cover and SW'ly sfc winds.

The aforementioned cold front will continue trekking eastward Thursday into Friday leading to better shower and thunderstorm chances for the western Carolinas as well as the potential for locally heavy rainfall (especially for areas that received rainfall earlier in the week). Convection will be in place well ahead of the front Thursday into Friday so have chance PoPs painted across the entire area. The only exception is the NC mtns where likely PoPs are in place for Thursday. The actual FROPA itself should track across the western Carolinas Friday afternoon into early Friday evening.
The front will allow shear values to increase slightly, ranging from 35-45 kts. Guidance shows SBCAPE values ranging from ~1500-2000 J/kg on Thursday afternoon so the potential for strong to severe storms will return once again. Depending on how fast activity pushes south and east on Friday, the potential for strong to severe storms may return again Friday afternoon (mainly along and east of I-85). Temps will climb back into the mid to upper 80s Thursday afternoon across the mtn valleys and east of the mtns. A few locations along I-77 in the southern NC Piedmont may once again see highs reach 90 degrees.
Slightly cooler temps can be expected on Friday behind the cold front, with highs only climbing into the lower to mid 80s east of the mtns. Lows Thursday night will remain ~12-14 degrees above climo, becoming ~5 degrees above climo Friday night behind the cold front.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/
At KCLT and elsewhere: Very messy aviation forecast this period, as unusually high moisture levels for the time of year combined with weak lift and instability will yield periods of convection and cig/ visby restrictions through much of the period. This pattern is such that confidence in forecast trends is low...even in the 3-6 hour window
Scattered convection
primarily SHRA is ongoing across the area this morning, warranting VCSH at all sites to begin this forecast cycle. The convective potential as well as the likelihood of lowering cigs is expected to steadily increase through the morning, warranting tempos for SHRA and IFR/MVFR conditions.
Thunderstorm chances will steadily increase as well, peaking mainly during the afternoon, when tempos transition to TSRA. Cigs are forecast to settle to at least MVFR by late morning, while IFR is primarily confined to tempos. The potential for TSRA is expected to continue well into the evening hours, and Prob30s for TSRA are carried at most sites after 21Z. While intensity and coverage of convection is expected to begin diminishing during the evening, at least VCSH is warranted through the end of the period as conditions will remain very moist. Any modest improvement in cigs/visby this afternoon will be reversed by late evening...with all sites expected to see IFR/LIFR cigs along with 2-5SM BR settle in overnight.

Outlook: Enhanced moisture will combine with a weak/stalled front through early next week, resulting in a period of active/mainly diurnal convective weather each day. There will also be the potential for restrictions associated with widespread fog and/or low clouds each morning.

GSP WATCHES/WARNINGS/ADVISORIES
GA...None.
NC...None.
SC...None.

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