textproduct: Columbia

WHAT HAS CHANGED

Updated Key Message #1 to reflect latest trends. Aviation discussion updated for 00Z TAFs.

KEY MESSAGES

- 1. Periods of moderate to heavy rain expected this evening into Thursday where severe weather will be possible.

- 2. Rain chances pick back up late week and into the weekend.

DISCUSSION

Key Message 1: Periods of moderate to heavy rain expected this evening into Thursday where severe weather will be possible.

As of 9:25PM: An area of showers and thunderstorms is now approaching the western edge of the forecast area. A few storms were severe warned across northeast GA into the SC upstate, but have since decreased in intensity as they have pushed eastward. There is still some potential for a few strong to severe storms across the area tonight given modest instability and strong deep layer shear as a shortwave is moving into the area. Moisture is still streaming into the area on deep southwest flow with SPC mesoanalysis indicating PW values approaching 1.5-1.7". CAM solutions are showing multiple waves of showers and storms moving through the area through the overnight period. Model forecast soundings become increasingly saturated through the column as we move through the overnight period, and despite fast storm motions, expect heavy rainfall to occur with the strongest storms as they move through. There is some expectation that convection will weaken as it moves through the area late tonight into early Thursday as stability increases, and some of this is supported by decreasing lightning activity in the convection across the upstate. The remainder of the forecast remains on track.

Overview as of 1:40PM: Based on recent hand surface analysis the cold front is now draped across the Tennessee Valley down into far northern MS/AL and toward northwest LA. The base of the upper trough is continuing to move eastward out of the southern Plains, but a 100+kt jet streak is gradually shifting toward the Tennessee Valley with 50-65 kts of mid level flow currently overspreading the Southeast as seen in the latest CAE VAD profile. Continuing the remainder of the afternoon and into the evening, moisture advection will continue to intensify where a couple showers or a storms could move into the northern part of the CWA, where a storm on the strong to severe side is possible. Late this evening and overnight is when coverage is expected to increase, bringing the potential for heavy rain and a couple strong to marginally severe storms. The heavy rain risk carries into Thursday where another round of strong to marginally severe convection could be possible in the late morning to afternoon hours. These periods of heavy rain bring a flash flooding threat limited to mainly urban and flood prone areas.

Rainfall: There has not been a significant change to the rainfall outlook. As PWAT's surge to 1.75-1.90" and a 35-50 kt LLJ sets up later this evening with overspreading upper support ahead of the surface front, periods of heavy rain will become possible. This risk for heavy rain carries into Thursday as multiple rounds of convection are expected. Overall, blended guidance mean amounts have increased a bit toward 0.75-1.25" across the area, which seems reasonable considering the near climatological max PWAT's/IVT and possible training of convection. Due to possible training convection REFS and HREF LPMM QPF shows pockets of 2-3" of rainfall mainly across the northern FA, possibly toward the CSRA. Overall, the risk for widespread Flash Flooding remains low, but the combination of possible training thunderstorms and very efficient rain rates will bring the risk for isolated spots of Flash Flooding mainly for urban and flood prone areas.

Severe Threat: The potential severe threat remains rather complex as the first round of convection is expected this evening into the overnight period before another will be possible late Thursday morning and into the afternoon. This second round will be highly dependent on the progression of the first. For this first round of convection, recent CAM's and paintballs from the HREF and REFS show coverage gradually increasing across the Upstate late this afternoon and early this evening before shifting into the northern FA later this evening. Some breaks in cloud cover are noted currently in satellite imagery in eastern GA and near the western CWA border, which may allow slightly greater instability to build, but still, instability with this first round is expected to generally be limited to MUCAPE under 800 J/kg. Solid shear is already moving in place per the CAE VAD profile with near 60 kts of 0-6km shear and 20 kts of 0-1km shear as 60-70 kts of mid level flow overspread the region. One change with the outlook is if a storm can get going before any instability wanes overnight, there is a non-zero tornado risk as VAD 0-3km SRH is already around 100-150 m^2/s^2. This risk is mainly confined to the far northern FA. Instability should continue to wane overnight with weakening lapse rates so the main risk with any more organized segment/cluster should be heavy rain along with the marginal risk for strong to severe wind gusts.

As has been discussed, this second round of possible strong to severe storms later Thursday morning into the afternoon will be heavily dependent on this first round. Overall, guidance is favoring the plume of deep moisture shifting eastward through the afternoon and is favoring lingering cloud cover and shower/storm activity into the later parts of the morning, keeping a more stable profile. This is noted in recent BUFKIT soundings where the RAP profile at OGB depicts some inhibition around 825 mb that likely would prevent robust surface based CI. If the morning activity clears a bit faster or greater recovery is seen than expected, this second round could bring storms with the main hazard of strong to severe winds. It is worth noting the 12z REFS does bring fixed layer STP values up to 0.5-1.0 in the far eastern Midlands and parts of the southern CSRA as strong shear will likely remain in place, shown in hodographs where sfc-1km SRH is upwards of 100-200 m^2/^2, so a non-zero tornado risk is possible here, but there is very low confidence in this risk materializing. Overall, trends with the first round will need to continue to be monitored to gain more confidence in any potential second round of convection.

Key Message 2: Rain chances pick back up late week and into the weekend

PoP's remain low through Friday as a quick moving surface high moves in, but PWAT's move over an inch again into Saturday and early next week. A shortwave may aid in bringing slight chance to chance PoP's overnight Friday into Saturday morning, but the next main rain chance seems to hold off until at least Sunday afternoon into Monday. Global models still differ some in timing for this late weekend/early week system, but the synoptic pattern favors more rain chances.

AVIATION /00Z THURSDAY THROUGH MONDAY/

VFR conditions to deteriorate into MVFR late tonight as rain showers and clouds associated with a cold front moves into the region.

A rather active TAF period is expected as a cold front moves east across the western Carolinas. A few showers and storms are already ongoing to the north and west of our CWA, with impacts expected to gradually increase later this evening and continue through the overnight hours. Initial activity is expected roughly between 02-06Z, although timing remains a bit uncertain given ongoing convective development and organization upstream. Early activity may be isolated to scattered and favor northern terminals, though any terminal could experience brief heavy rain and short lived visibility restrictions. By around 07-09Z, activity becomes more widespread as embedded showers and storms continue into Thursday morning, with occasional lulls possible.

A few storms could become strong to severe overnight, with strong winds as the primary hazard. Lower ceilings and reduced visibility are likely with showers and storms, with conditions dropping to MVFR or IFR at times. Some improvement in visibility is possible late Thursday morning, but ceilings are expected to remain below 1 kft, keeping IFR conditions in place into the early afternoon. Winds will remain occasionally gusty, with sustained speeds around 7-12 knots. Flow stays southerly tonight, then shifts westerly Thursday as the front pushes through. Cloud cover lingers through much of the day before gradually clearing late in the period. EXTENDED AVIATION OUTLOOK...System brings a chance for restrictions once again with showers and thunderstorms into Thursday afternoon. Break in the rain possible on Friday. Additional rain chances this weekend.

CAE WATCHES/WARNINGS/ADVISORIES

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