Marine Weather and Tide Forecast for Barnwell, SC

KEY MESSAGES
- 1. An active weekend ahead as Hurricane Isaias moves inland to our west. Confidence in the extent of impacts increasing with periods of heavy rain bringing an isolated flooding threat.

DISCUSSION
Key Message 1: An active weekend ahead as Hurricane Isaias moves inland to our west. Confidence in the extent of impacts increasing with periods of heavy rain bringing an isolated flooding threat.

Overview: Moderate to heavy rainfall is likely as Hurricane Isaias passes to our west on Saturday and into Saturday night.

Discussion: Hurricane Isaias remains in a hurricane in the central Gulf this afternoon, in line with forecast expectation so far. Guidance continues to be in fairly good agreement on it intensifying quickly over the next 24-36 hours as it treks northeastward and begins interacting with an upper level trough pushing eastward out of Texas. While continuing to decrease in the 00z and 12z model suites, there is still some spread between the AI & Physics- Based models on how far east Isaias eventually makes landfall. Regardless, the impacts to the Midlands and CSRA are expected to be relatively unchanged regardless of track with our main feature being a secondary axis of moisture and forcing.

Isaias will overspread the area with widespread high cloud cover over the next couple of days as the system interacts with a remnant trough over Texas. This trough will continue to shift the Isaias northeastward and eventually northward as the northern/southern stream of the jet streak merge over the Mid-Atlantic and everything gradually amplifies. The right entrance region of this upper level jet streak is forecast to overspread much of the southeastern CONUS by Friday afternoon, helping moisture advection to begin going to our south. As Isaias move northward, it will begin interacting with the overall synoptic pattern & is expected to expand in size, pushing up against a low-level high pressure system & develop a strong easterly branch of the low-level jet that extends across our area. This feature, in some form or fashion, is expected to overspread the area during the day on Saturday. As it does so, significant PWs on the order of 2-2.5 standard deviations above normal (2.25"+) will overspread the region.

The synoptic pattern associated with this, featuring high PWs, a broad entrance region of an upper level jet, onshore 850 hPa flow atop a strong surface high pressure, the associated warm frontal and isentropic lift zone, should favor widespread and heavy rainfall on Saturday. Guidance has finally acquiesced and in general has steadily ramped up precip totals across SC and eastern GA, falling in line with the aforementioned meteorological and climatological expectations. While some "dry" slot does develop across parts of the interior Southeast as a sort of coastal low, or more focused trough along the warm front and surface boundary forms, this does not appreciably reduce rainfall totals like previous guidance has suggested.
Extremely impressive PWATs and an easterly 925-700mb jet over 50 knots combine to produce a broad shield of IVT and associated strong isentropic lift across the northward lifting surface boundary and warm front. This strong IVT and WAA develops by Saturday morning, continuing through early Sunday, all within a plume of 2.25" PWATs and a loosely unstable warm sector south of the boundary and recipe for at least periods of heavy rain for the area.

While ticking up, rainfall totals still vary a bit across guidance, but the distribution and potential has come into better focus and agree much better with the meteorological setup. In general a widespread 2-4" looks likely for much of the area, with LREF and AI ensembles now in much better agreement. The 10-90th percentile ranges are expectedly large however and this will likely remain the case until the event is unfolding. But a reasonable worst case for rainfall (90th percentile) is generally 3-5" for our area. In reality, the footprint of the rainfall will likely look like a mix of the 10th and 90th together, some areas underachieving while others over. These tropical high PWAT, weakly unstable events tend to produce banding and sporadic rainfall footprints that develops along mesoscale features within the strong low level flow. While widespread flooding is not expected given how dry the area has been, any locations that push towards the higher end scenario will exceed flash flood guidance.

While a secondary hazard, the tornado potential is also worth noting. Impressive hodographs south of the warm front with a 50 knot 850mb jet surge north within the bulk of the deepest PWATs. Instability will not be impressive despite LCLs scrapping the ground however, but it will not take much from a convective updraft and stretching potential to produce a few tornadoes within this environment. So a few weak short lived tornadoes across the area remains possible, primarily late Saturday afternoon into early Sunday.

Overall, while we will see some heavy rain and breezy winds, Isaias looks to remain well west of the region, and its greatest impacts are forecast to remain there as well. However, that doesn't mean that we won't see impacts here locally. Keep an eye on the forecast over the next couple of days!

AVIATION /18Z THURSDAY THROUGH TUESDAY/
VFR conditions expected throughout the forecast period.

Widespread mid-upper level cloud will continue to overspread the area today and tonight and this will prevent any sort of fog- stratus concerns for Friday morning. Winds will remain fairly light and variable the remainder of Thursday into Thursday night before strengthening out of the northeast Friday morning. Rain chances will hold off until after the TAF period with cigs remaining high VFR until late Friday night.

EXTENDED AVIATION OUTLOOK...The threat of restrictions and rainfall increases Saturday morning as tropical moisture surges into the Southeast with Isaias. Strong gusty northeasterly winds likely throughout Saturday and Sunday along with periods of heavy rain and LIFR conditions.

CAE WATCHES/WARNINGS/ADVISORIES
SC...None.
GA...None.