textproduct: NWS Wilmington

SYNOPSIS

Tropical Storm Chantal will approach the South Carolina coast tonight, strengthening into a tropical storm before landfall. The largest impact on the Carolinas will be locally heavy rainfall with potential for isolated flooding tonight through Sunday and strong rip currents and dangerous surf. Unsettled weather returns during the middle of next week after a brief break in widespread activity late Monday into Tuesday.

NEAR TERM /THROUGH TONIGHT/

New christened but still poorly organized Chantal centered east of GA coast. Sat imagery shows some westerly shear that will relax with time this period, the shearing come from an upper low to the storm's west, but this feature will begin to retrograde. From there steering flow will be able to impart a slow northerly motion (the storm currently meandering with poorly defined initial motion) which will naturally mean a steady deterioration in conditions late today and moreso tonight into Sunday. For all of the specifics including a plethora of graphics check out our webpage (www.weather.gov/ilm) or for our Chantal-specific briefing at www.weather.gov/briefing.

Our current QPF forecast paints a max of 4-6" along and south of the NC/SC state line, remaining east of I-95. Raised a Flash Flood Watch for areas that included the areas of highest rainfall. Probabilistic guidance of amounts exceeding 6" are nearly nil so flooding may only be localized and handled with a warning or two. With most of the rain falling on a Saturday night of a holiday weekend in a heavily populated and touristy area that is the Grand Strand felt that some extra notice was prudent. Current forecast would have moderate to occasionally heavy rainfall continue through the period, with much of this hinging upon the storm's eventual uncertain path (some guidance trending much slower, which was also a last minute nudge supportive of the Flood Watch).

SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/

Impacts from Chantal will be ongoing at the start of the period over portions of southeastern NC. The main concerns during this time would be heavy rain and the potential for localized flooding. Urban areas along the coast have the highest chance of seeing heavy rainfall, but any communities along the coast should be paying close attention to our rainfall forecast.

Chantal moves off to the north and east of our area on Monday and leaves a warm and humid air mass in its wake. Models show that Chantal will be slow to progress out of the CWA, so flooding could remain a concern through a good chunk of Monday. Flood Watch continues through Monday due to this potential.

LONG TERM /MONDAY THROUGH SATURDAY/

Moisture continues to pool across the eastern Carolinas on Tuesday, but weak subsidence as Chantal exits to the northeast should limit convective coverage to isolated or widely scattered storms. If any convection does develop, weak steering flow and saturated soils could quickly produce localized flooding, especially in the hardest hit areas.

Weak troughing over the central US and a broad shortwave over the mid Mississippi River Valley will promote SW flow across the southeastern US later this week. Amble moisture transport and solid mid level lapse rates should encourage widespread showers and thunderstorms each afternoon. The main trough axis approaches on Friday which should denote the peak in activity.

The forecast gets a bit unclear next weekend in a chaotic upper level pattern and weak surface flow. There does not appear to be any significant form of dry air advection during this time, so while PoPs decrease, we should still see scattered diurnally driven convection.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Chantal to dictate the forecast with lowered flight categories for most of the period, save for a near term lack of cloudiness due to some sinking air on the western periphery of the storm, which still includes a leading line of heavier showers that will lead to lowered VSBY and CIG before sunset. Thereafter from east to west will be the true and widespread decline in both. Thunder is tougher to diagnose...not normally characteristic of tropical system but with this leading edge of sun almost 1500J/Kg of CAPE has developed. This window aside, thunder should be the exception and not the rule with just rain expected. The FM groups late in the period only address wind direction change as the circulation approaches from the south.

Extended Outlook... Sub-VFR conditions expected through Sunday night due to low ceilings as tropical low moves onshore across SC. Intermittent flight restrictions possible Monday through Wednesday due to daytime convection and potential early morning fog/stratus.

MARINE

Through Sunday... TS Chantal still showing slow/erratic movement. As the mid level low to its west retrogrades tonight the system should move to the northward. This will also relax the shear and the wind field should increase and/or become slightly more symmetric. Expect a deterioration of both wind and seas through the period. TS Warning now in effect for all waters.

Sunday Night through Wednesday... Swells from Chantal will be just offshore overnight and gradually lifting northward with the center of the storm. SC offshore waters will gradually see conditions begin to improve Sunday night into Monday as southeasterly swell becomes a bit more tame and mixed with a long period easterly swell of around 1 foot. Better conditions on Monday with the disturbed sea transitioning to the summer baseline of SE and E swell. Tuesday and Wednesday will see predominately SE swell (2-3 feet at around 7 seconds and 1 foot around 15 seconds). Showers and thunderstorms see a brief minimum on Monday evening and early Tuesday before better coverage during the middle of next week.

TIDES/COASTAL FLOODING

Other - Rip Currents: Increasing south-southeasterly fresh swell with wave periods averaging 7-8 seconds will feed onto the beaches today in advance Tropical Depression Three. This wave group should produce a high risk of rip currents at most area beaches. Conditions should deteriorate further Sunday as the tropical system approaches landfall on the southern South Carolina coast when breaking wave heights in the surf zone could approach 6 feet south of Cape Fear. A High Surf Advisory is now in effect.

ILM WATCHES/WARNINGS/ADVISORIES

NC...Tropical Storm Warning for NCZ106-108-110. Flood Watch through Monday evening for NCZ099-109-110. High Risk for Rip Currents until 8 PM EDT this evening for NCZ106- 108. Beach Hazards Statement until 8 PM EDT this evening for NCZ110. High Surf Advisory from 6 AM to 8 PM EDT Sunday for NCZ110. SC...Tropical Storm Warning for SCZ054>056-058. Flood Watch through Monday evening for SCZ054-058-059. High Risk for Rip Currents until 8 PM EDT this evening for SCZ054- 056. High Surf Advisory until 8 PM EDT Sunday for SCZ054-056. MARINE...Tropical Storm Warning for AMZ250-252-254-256.