textproduct: Columbia

WHAT HAS CHANGED

Little change in the thinking with a locally heavy rainfall threat this afternoon and evening. Aviation discussion updated for 18z TAF.

KEY MESSAGES

- 1. Scattered to widespread showers and storms expected this afternoon, continuing into the early overnight. A couple strong to marginally severe storms will be possible along with isolated spots of flash flooding. Lingering moisture leading to scattered to widespread afternoon storms Tuesday and Wednesday.

DISCUSSION

Key Message 1: Scattered to widespread showers and storms expected this afternoon, continuing into the early overnight. A couple strong to marginally severe storms will be possible along with isolated spots of flash flooding. Lingering moisture leading to scattered to widespread afternoon storms Tuesday and Wednesday.

Overall, there has not been much change in the outlook for the rest of the today based on 18z observations and updated hi- res guidance. A slow moving surface front is gradually working south through the FA and will eventually stall across the I-20 corridor and CSRA. Mean sfc-400mb flow out of the southwest should advect PWAT's 1.5-2 standard deviations above normal (~2.20-2.40"), creating very moist profiles. With surface dewpoints remaining in the low to mid 70s, mean HREF soundings indicate SBCAPE values between 1500-2000 J/kg should develop through the day, less than previous days mostly due to cooler surface temperatures ranging from the low 80s north to the mid to upper 80s south. With more moist adiabatic profiles, DCAPE is progged to be less than previous day's as well. An uptick in kinematics from the mid level low should bring increased shear (EBWD ~20-25 kts) though, so a few strong to marginally severe storms remain possible during the afternoon/early evening with the main hazard being damaging wind gusts from precip- loaded downdrafts. Activity likely carries into the overnight, but the severe risk is expected to diminish during this period.

The bigger talking point is the heavy rainfall potential through the afternoon and evening. The 12z guidance has followed well with current observations, continuing the threat for locally high rainfall amounts later today. These moist adiabatic profiles, long/skinny CAPE, a nearly stationary front, and the aforementioned PWAT's should act to bring quite efficient rainfall rates and the potential for training convection. The HREF 24hr QPF LPMM product indicates a fairly widespread 0.50-1.0" is a reasonable expectation, but it does indicate spots of 2-5" being possible, which is further supported in deterministic CAMs and the 12z REFS LPMM; even 6hr LPMM indicates over 5" possible in highly localized spots. There is lesser confidence in the exact placement of higher QPF amounts with some spatial variability amongst deterministic CAMs, but with the frontal boundary expected to nearly stall in the southern FA, this could be an area to watch more closely. Areas in the southeastern Midlands near eastern Bamberg County into Orangeburg and southern Calhoun Counties saw 1-3" of rainfall from Sunday's activity so these spots would be the most sensitive to additional higher amounts. Overall, isolated instances of flash flooding will be possible if these higher amounts are realized.

Scattered shower and thunderstorm chances will likely continue Tuesday and Wednesday as ensemble meteograms indicate PWAT's near to over 2" remain in place through at least the first half of Wednesday. As the mid level low slowly retrogrades, drier air may begin pushing into the Pee Dee and northern Midlands Wednesday, bringing lower rain chances here. Cooler than normal temperatures are also expected to persist through Wednesday before gradually warming during the second half of the week back to near normal. Typical diurnal rain chances could make an appearance late in the week into next weekend, but there is generous spread in LREF cluster analysis at this point.

AVIATION /18Z MONDAY THROUGH SATURDAY/

A mix of MVFR and VFR conditions are expected this afternoon and evening, with showers and storms helping that. Widespread MVFR and IFR ceiling restrictions are expected tonight.

The slow moving front is currently sagging southward across the area. Cooler low-level air and northeasterly flow north of this feature has led to a morning filled with MVFR restrictions, with these slowly lifting now. Expecting convection to quickly fill in along this front, with heavy rainfall expected in any thunderstorm that passes over a site. The best chances for storms in the next few hours are at the Columbia sites, with the Aiken, Augusta, and Orangeburg sites seeing convection either develop or move in later this afternoon and evening. Restrictions are expected with any storm that does develop. Variable winds are forecast to remain in place this afternoon and into tonight, with potentially more persistent northeasterly flow arriving tonight. Given expectation of rain coverage today & already moisture rich airmass in place, guidance is quite aggressive with cig forecast tonight, showing IFR developing quickly after sunset. This makes sense from a pattern recognition standpoint and have the forecast reflecting such. Expecting that MVFr/IFR cigs will hold on into the midday on Tuesday.

EXTENDED AVIATION OUTLOOK...A good chance for afternoon showers and thunderstorms with associated restrictions is expected through at least Tuesday.

CAE WATCHES/WARNINGS/ADVISORIES

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