textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Revised wording of Key Message 2. Slightly increased concern for severe weather and localized heavy rainfall threats Wednesday evening.

Rain chances increase as another cold front could cross the area again to start next week.

KEY MESSAGES

1. Seasonable temperatures but low humidity and breezy conditions today, with some minor concerns for increased fire danger in the afternoon. 2. A cold front moving through the region Wednesday and Thursday looks to bring showers and thunderstorms to our area. An isolated threat of large hail or damaging wind may develop. Localized heavy rainfall is possible with any of the thunderstorms, but the threat of flooding is low, except possibly in areas that receive multiple rounds of heavy rainfall. 3. Drier conditions return for the majority of the weekend before another potential cold front increases rain chances for the start of next week.

DISCUSSION

Key message 1: Seasonable temperatures but low humidity and breezy conditions today, with some minor concerns for increased fire danger in the afternoon.

Similar to Monday, western Atlantic surface high pressure will extend inland over the Southeast, with slightly cyclonic upper level flow carrying weak perturbations over the CWA, and patchy high altitude cloud cover moving in as a result; a few CAMs even depict some sprinkles over the mountains to match, so very small PoPs return there late in the day. Despite the cyclonic flow and front approaching from our west, heights over our area remain on a net increase, and temperatures in most spots will trend slightly warmer compared to Monday. Once again a very dry layer is present in the midlevels, and assuming the warm temperatures will allow mixing into that layer, it is prudent to lean toward the low end of guidance for afternoon dewpoints. Most spots verified around if not a little below the 10th percentile of NBM on Monday despite SW'ly flow, so will be using the 10th percentile again today, with our eastern Piedmont areas mostly looking to dip to 25-30% RH. The surface gradient will be a bit stronger as the frontal system in the mid-Miss. Valley inches closer, and mixing should also bring down gusts of 25 to 30 mph at times this afternoon. Localized areas in NC/SC may meet objective wind/RH criteria for an Increased Fire Danger Statement, though coordination with state fire agencies would be needed in light of spring greenup largely having occurred. Of note, a few RAWS automated fuel moisture observations did dip below 7 percent on Monday, so dead brush/leaves may still be dry, hence the expected coordination.

Moisture increases at 850-700mb tonight as cold front moves across the lower Ohio Valley, and some elevated instability develops over East TN as a result. A few light showers are possible over our northwest half, primarily over the mountains and foothills, late tonight into early Wed morning.

Key message 2: A cold front moving through the region Wednesday and Thursday looks to bring showers and thunderstorms to our area. An isolated threat of large hail or damaging wind may develop. Localized heavy rainfall is possible with any of the thunderstorms, but the threat of flooding is low, except possibly in areas that receive multiple rounds of heavy rainfall.

Profiles moisten aloft over our area Wednesday with moist southwest flow out of the Gulf, ahead of the surface cold front which is expected to cross the eastern half of TN over the course of the day. Low-level lapse rates remain poor through the morning, particularly over our I-85 corridor. Any weakly organized elevated convection that reaches the NW half of the area in the morning appears likely to gradually diminish as it progresses east of the mountains, probably not posing much if any severe threat despite increasing effective shear. The threat of heavy rain appears low with this phase of the event, also. Though PWATs will increase well above normal, we will lack a strong forcing mechanism, though WAA and some convectively generated upper divergence will be in play. REFS 6-hr PMM values are less than a quarter inch Wednesday except for higher amounts in/near the Smokies, where available moisture will be greatest and some upslope enhancement may occur; REFS has 40-60% probs for a half inch or more in that area.

Surface-based instability does develop on most models during the afternoon as capping layer mixes out, with the greatest values over the western half of the area where 500-1000 J/kg looks likely to form. HREF and REFS probs of 500 J/kg are still pretty low at peak heating in the east, due to lapse rates in the moist environment being only marginally favorable. CAM response is not terribly exciting over our area during the daytime hours, perhaps owing to continued lack of a strong forcing mechanism within the CWA. However, there appears to be a signal for development along an axis south of the front in southern TN or north AL/GA--where better instability and upper divergence are seen during the day--which propagates into our western areas by late afternoon. 0-6km bulk shear appears likely to exceed 50 kt over the CWA by that time. CAMs do develop some updraft helicity streaks with the incoming convection, and the simulated reflectivity has a supercellular look on the HRRR and HRW-FV3. Thus our severe threat will ramp up during the afternoon from west to east, further increasing after 00z Thu when the front crosses the mountains and jet streak becomes more favorably positioned for dynamic lift over our area. Hodographs are largely unidirectional in the afternoon and evening, with boundary layer winds being veered and, especially in our east, LCLs a bit high to expect a significant tornado threat. Severe hail would be a primary threat with any elevated storms in the afternoon/evening; there is some potential for upscale growth and a wind threat is also possible in our western zones. SPC Day 2 Marginal Risk now extends across the NW half of the area. Furthermore, per CAMs, a few swaths of 2" total rainfall may also result overnight into Thursday morning. Can't rule out some training of cells due to the orientation of shear w.r.t. the front. With dry antecedent conditions that would appear necessary in order for any localized flash flooding to materialize. The slow progression of the front suggests it may not totally clear our lower Piedmont zones by Thursday afternoon, and ensemble SBCAPE probs are actually higher at that area/time than Wednesday afternoon/evening, with similar shear parameters and more dry air aloft. SPC Day 3 Marginal brushes that part of our area accordingly.

Key message 3: Drier conditions return for the majority of the weekend before another potential cold front increases rain chances for the start of next week.

High pressure slowly builds in Friday, behind the departing system from Thursday. By Saturday, a shortwave could increase rain chances briefly but confidence is low at this time. Towards the start of next week, long range guidance starts to signal for another shift in the synoptic flow with a ridge building out west and a trough swinging eastward. Model guidance depicts moisture slowly returning over the weekend ahead of the frontal boundary as light southerly winds prevail. This looks to be the next rainfall opportunity, but is too far out in the forecast to pinpoint any details at this time. Temperatures should also remain warm and closer to normal for May.

AVIATION /07Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: VFR. Light SW winds should continue early this morning with only FEW cirrus seen. Good mixing will promote development of gusts by 16-18z, with frequent gusts generally around 22-26 kt this afternoon, with brief peak gusts likely closer to 30 kt for the day. Direction should remain from the SW. A few cumulus may be seen at around FL060, and cirrus will increase during the day. Continued breezy SW winds after sunset with midlevel cigs developing after midnight.

Outlook: Periodic restrictions are likely with next frontal system Wednesday and Thursday, with continued gusty winds and a threat of TSRA. Low forecast confidence Friday into next weekend but restrictions cannot be ruled out at that time.

GSP WATCHES/WARNINGS/ADVISORIES

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