textproduct: Columbia

WHAT HAS CHANGED

Higher chance for thunderstorms overnight Wednesday into early Thursday morning. Aviation discussion updated for 18Z TAFs.

KEY MESSAGES

- 1. Periods of moderate to heavy rain are expected starting overnight Wednesday, continuing into Thursday with severe weather possible.

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

DISCUSSION

Key Message 1: Periods of moderate to heavy rain are expected starting overnight Wednesday, continuing into Thursday with severe weather possible.

Overview: A front will begin nearing the FA overnight Wednesday into Thursday morning as strong upper support and moisture advection overspreads the region as well. This is expected to bring moderate to heavy rainfall across the region starting as early as overnight Wednesday, continuing into the afternoon hours Thursday. As has been discussed previously, a conditional threat for severe weather will exist with this system. Fairly breezy winds will be possible through the day Thursday as well where a Lake Wind Advisory may be needed.

Rainfall: Robust moisture advection is expected to begin shifting into the area late Wednesday into Thursday morning ahead of a front as a 35-45kt LLJ ramps up and PWAT's reach near 1.75-1.90". Enhanced convergence ahead of the front and a nearing upper trough should increase rain chances overnight Wednesday into Thursday morning where heavy rainfall will be possible due to the aforementioned features along with deep warm cloud depths (over 11,000 ft) and at least some weak instability. Overall, forecast rain amounts have not shifted too much where a widespread 0.50-1" seems reasonable, but as CAM guidance reaches further into Thursday, spots of higher amounts will be possible. This is seen in recent 12z CAM's where spots of 1-2.25" of rainfall can be seen, mainly where any convection tracks. These amounts are further supported in the most recent NBM 5.0 run where 25th percentile QPF amounts are around 0.30-0.75", with the 75th percentile showing areas of 1-2.25". Probabilities for greater than 1" of QPF through Thursday night have risen toward 40-60%, mainly along and north of the I-20 corridor. As mentioned in previous discussions, the overall Flash Flooding risk is low with antecedent dry conditions, but isolated spots of Flash Flooding mainly in urban or flood prone areas will be possible where convection moves over due to higher rates.

Severe Threat: The main change in severe weather chances comes late Wednesday into Thursday morning. As the upper trough progresses eastward, enhanced mid level flow around 50-65 kts should begin overspreading the FA with the aforementioned deep moisture moving into place. The mean 12z HREF solution, along with multiple CAM's (including 12z HRRR, NAM3km, and RRFS) show a broken line of convection shifting into the Upstate Wednesday evening before moving into the FA late Wednesday night, continuing southward into the early parts of Thursday morning. With enhanced mid level flow and a strengthening LLJ moving in, strong deep layer shear on the order of 50-65 kts should shift in place overnight. Considering the time of day, weak instability is expected with MUCAPE generally under 800 J/kg. Therefore, a rather conditional and marginal severe threat is possible late Wednesday into Thursday morning, which has been further supported in recent HRRR Neural Network, CSU, and NCAR AI/ML guidance. Overall, the main risk will be heavy rain as discussed in the previous section, but if a more robust segment of convection can maintain itself (mainly from the strong kinematics), strong to marginally severe wind gusts could be possible more so toward the northern FA. Behind this activity and before the front moves through, redevelopment of convection could be possible late Thursday morning into the afternoon as strong deep layer shear remains in place, but this threat is also conditional. A couple of failure modes exist in the afternoon however with model guidance favoring lingering cloud cover through the day, deep moisture gradually shifting out of the FA and toward the Coast, and the belt of enhanced mid and upper flow shifting northward. If instability can be realized, another round of strong to marginally severe storms could be possible, mainly toward the eastern Midlands and into the Pee Dee region.

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

There continues to be a trend toward a more progressive pattern the remainder of the forecast period across global models and ensemble guidance. There remains uncertainty in if drier conditions could win out in the Friday and Saturday time frame with weak shortwaves passing so at least slight chance PoPs seem reasonable during this period. There is a bit more consensus that another trough will move in early in the week with deeper moisture returning and increased rain chances.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

VFR conditions expected to prevail over the next 24 hours.

Moisture continue to be on the increase across the area, allowing for the development of some scattered mid-level cumulus this afternoon. This should dissipate towards sunset, leaving mainly increasing cirrus clouds moving in from the west overnight and through much of Wednesday. Winds will be out of the south through the period. Daytime wind speeds both this afternoon and again for Wednesday will generally be around 10 knots, with gusts between 15-20 knots. Winds overnight tonight should see the gusts diminish, but the sustained winds will still remain up between 5-8 knots, although there could be a brief period around sunrise in which winds drop below 5 knots. Either way, the combination of increasing cirrus and wind speeds remaining strong enough overnight should be enough to to help keep fog formation at bay. Any rainfall potential ahead of an approaching system on Wednesday will hold off until after the 18z period.

EXTENDED AVIATION OUTLOOK...System brings a chance for restrictions once again with showers and thunderstorms starting Wednesday evening into Thursday.

CAE WATCHES/WARNINGS/ADVISORIES

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