textproduct: Columbia

WHAT HAS CHANGED

Increasing concern for at least localized flash flooding as heavy rain develops along the front this weekend. Aviation discussion updated for 18z TAFs.

KEY MESSAGES

- 1. Front approaching the area leading to increasing chances for rain and cooler temps this weekend and into next week. Increasing concern for localized flash flooding.

DISCUSSION

.KEY MESSAGE 1: Front approaching the area leading to increasing chances for rain and cooler temps this weekend and into next week Increasing concern for localized flash flooding.

High pressure has shifted offshore with upper ridging continuing over the forecast area today. While there has been a modest moisture increase in the low levels, evident by dew points around 5 to 10 degrees higher than yesterday, much of this increase remains shallow with 12z sounding out of CHS showing dry air persisting in the mid levels. While some light showers are moving into the southern portion of the forecast area, capping and dry air aloft will limit convective coverage through this evening.

As the upper ridge axis shifts offshore into tonight, more significant moisture increase is expected with southwesterly flow developing aloft. HREF mean indicates PWATs will rise to above 2 inches across the forecast area, closer to 2.2 inches near the Coastal Plain. This will lead to widespread cloud coverage and chances for showers in the morning, although forcing will be generally weak so any showers will likely remain light. While instability will generally be weak with blended guidance indication probability of sbCAPE greater than 1000 J/kg around 20 percent or less. As a result, storm coverage may be somewhat isolated, with severe storms unlikely and moderate to heavy rain the main threat. Heaviest will likely be along the axis of the approaching boundary which higher resolution guidance generally favors in the northern half of the area, although the exact location for heaviest rain axis remains uncertain. Strong convergence along this boundary will likely continue into Saturday night and may lead to some general flash flooding concerns into the day on Sunday when upper level forcing strengthens as a robust shortwave likely moves through the area. With moisture anomalously high, the most likely scenario this weekend is widespread rainfall amounts greater than 2 inches, with blended probabilities of 60-70 percent. HREF LPMM indicates this higher axis of greater than 3 inches with localized spots of even higher amounts. Drier air will begin to move in from the north Monday with the axis of heavier rain shifting southward as an upper trough digs into the Northeast. Models have generally been favoring drier air into the middle of next week with cooler air behind the front.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

VFR conditions will prevail this afternoon and evening before scattered showers and storms develop Saturday.

Fair weather cumulus around 5 kft and light southerly winds around 5-7 knots continue before diminishing after sunset. A stray shower cannot be completely ruled out this afternoon and evening, mainly across the southern and eastern portions of the area, but confidence remains too low for inclusion. Increasing low-level moisture could lead to some restrictions tonight, though widespread impacts are not expected to be as significant as in previous nights, as cloud cover should increase with a front approaching the region.

Scattered showers and isolated thunderstorms will increase in coverage throughout the day Saturday, with periodic MVFR cigs possible. For now, PROB30 groups have been added between 12-18z for light showers, along with periods of BKN ceilings around 5 kft at all sites.

EXTENDED AVIATION OUTLOOK...Increasing chances of precipitation this weekend may lead to additional restrictions.

CAE WATCHES/WARNINGS/ADVISORIES

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