textproduct: Charleston

WHAT HAS CHANGED

Aviation section updated for the 12z TAF issuance.

KEY MESSAGES

- 1) A widespread significant rainfall event will occur across southeast Georgia and southeast South Carolina through Saturday.

- 2) Minor coastal flooding possible with the Friday and Saturday evening high tides along the SC coast.

DISCUSSION

KEY MESSAGE 1: A widespread significant rainfall event will occur across southeast Georgia and southeast South Carolina through Saturday.

For today, the pattern will feature zonal flow aloft while at the surface the forecast area will sit just north of a stationary boundary that runs east/west across south GA back along the entire Gulf Coast. Today is expected to bring periods of scattered showers across the region in the presence of some subtle isentropic ascent and some prominent low-level ThetaE ridging. This activity could start in the morning and then continue periodically through the day is a rather disorganized manner. Model soundings do show some pockets of weak instability, but the expectation is that we will see mostly showers with only the threat of an isolated thunderstorm. Rainfall amounts will likely be spatially variable due to the showery nature of the precip. Rainfall totals should be in the 0.10-0.25" range in most areas, though a few pockets of 0.25-0.50" will be possible.

As we transition into Friday evening and overnight, attention turns to the much needed and long-awaited widespread significant rainfall event. The arrival of a nice combination of strong forcing for ascent and deep moisture will combine to produce an expanding footprint of widespread rainfall beginning Friday night and likely maximizing across the forecast area Saturday morning into early afternoon. There is a strong upper divergence signal in the right entrance region of 125+ kt northern stream jet coinciding with a corridor of precipitable water values surging into the 1.4-1.6" range which would be on the order of the 90th percentile for 02/12z at KCHS according to the SPC sounding climatology. Increasing intensity and coverage of rainfall will spread in from the west through the late night hours and not taper off until late Saturday afternoon and into the evening.

There is actually good agreement and consistency regarding rainfall totals across the suite of deterministic and ensemble model data. There is high confidence in widespread 1.00-1.50" rainfall totals for the Friday evening through Saturday evening time period. Probabilities of >1" are on the order of 80% or higher, >1.5" 50-70%, and >2" generally 20-40%. This represents a solid much needed rainfall given the current drought situation, but it also looks like rainfall rates will be low enough to keep the threat of flash flooding very low. There could still be some minor flooding issues in low-lying and poor drainage areas across the Charleston and Savannah metro areas as pockets of periodic moderate to heavy rain move through. Guidance continues to depict the potential for a band of heaviest rain likely setting up across our far inland tier and across the SC Midlands. Confidence in the exact placement of this band is low as model solutions continue to fluctuate this band further north/south at times. This band is where the best chances of seeing 2" or more will reside.

Also good news is that model guidance continues to suggest no surface instability with this event, yielding no severe threat. There are signs of elevated instability which could produce some elevated convection and also result in pockets of more intense rainfall and hourly rates.

KEY MESSAGE 2: Minor coastal flooding possible with the Friday and Saturday evening high tides along the SC coast.

Astronomical tide levels will peak today due to the full moon. Continued NE winds will maintain a notable positive tidal anomaly for the next 24-48 hrs. Last evening's high tide anomaly was +1.2 ft, so if we project that into this evening, we could touch 7.2 ft MLLW in Charleston, which would necessitate a Coastal Flood Advisory. It's a bit more marginal for Saturday evening since the winds will tip to a more NNW direction which should bring down the anomalies. We'd need +1.1 ft to reach the 7.0 ft minor coastal flood threshold, which may be a stretch based on that wind forecast.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

The 12z TAF period begins with VFR conditions at KCHS, KJZI, and KSAV. Radar imagery shows that shower activity is beginning to increase across the area, but not near the terminals yet. Better coverage of showers is expected into the afternoon. These showers could produce some periodic MVFR conditions. Late in the TAF period (likely after 00z), steady rainfall will increase from the west and also bring widespread ceilings lowering into the MVFR and eventually the IFR range. Once IFR conditions arrive with the increasing rainfall coverage, IFR will remain through the end of the TAF period.

Extended Aviation Outlook: An extended period of flight restrictions from low ceilings and reduced visibilities will continue through Saturday. VFR conditions will likely return Saturday night into Sunday.

MARINE

A brief period of marginal Small Craft Advisory conditions is possible over the SC nearshore and GA offshore waters Saturday afternoon into Saturday night as surface low pressure develops along the SC coast. At the moment, we only have a few hrs of 25 kt wind gusts, and seas possibly reaching 6 ft over far outer portions, so an advisory would be very marginal.

CHS WATCHES/WARNINGS/ADVISORIES

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