textproduct: NWS Wilmington

WHAT HAS CHANGED

Discussions updated. Rain chances and forecast amounts have increased for Sunday night into Monday.

KEY MESSAGES

1) Showers and a few thunderstorms will precede a strong cold front which will move through the area this evening.

2) Expect a few periods of showers and isolated thunderstorms Monday through Tuesday.

DISCUSSION

KEY MESSAGE 1...Showers and a few thunderstorms will precede a strong cold front which will move through the area this evening.

KEY MESSAGE 1 DESCRIPTION... Dense mid and high level clouds produced via isentropic lift on the 320-325K surfaces could drop some light rain across eastern South Carolina this morning. Of greater concern is the possibility that deeper convective showers and thunderstorms may develop this afternoon once sufficient surface heating and destabilization has occurred.

Assuming surface temperatures creep into the low-mid 80s, the GFS and NAM suggest CAPE in the 1500-2000 J/kg range should develop this afternoon with little to no capping. Lapse rates should be reasonably steep from the surface up through 600 mb, then less impressive higher up. A weak shortwave in the northwesterly flow aloft and low level convergence in the vicinity of a prefrontal trough arriving from the north may be enough to trigger the development of showers and storms.

Bulk shear values near 20 knots could support loose multicell organization, but convective parameters don't look overly favorable for organized severe weather. Dry air entrainment could produce locally gusty winds if convection can become deep enough.

HREF and RRFS ensembles suggest the best potential for convection will be in the 2pm-6pm timeframe. The actual surface cold front should move southward across the area between 7-10pm, accompanied by a few additional elevated showers.

KEY MESSAGE 2...Expect a few periods of showers and isolated thunderstorms Monday through Tuesday.

KEY MESSAGE 2 DESCRIPTION... With a cold front well south of the region on Sunday, surface low pressure is expected to form and track along the boundary during Sunday night and Monday. Operational and ensemble guidance systems have shown a general upward trend in rainfall amounts over the last few runs, with the highest amounts in SC. With the region on the north side of a surface low, expect an area of light to moderate rain to develop and track eastward, mainly affecting the SC zones. Exactly how far north this rain reaches is hard to determine at this juncture, but rainfall amounts are expected to range from a trace to a few tenths of an inch.

Meanwhile, another cold front is expected to slide down from the north on Monday with scattered showers and isolated thunderstorms forming along and ahead of it. Uncertainty exists with regard to when those showers and isolated storms start to nose in from the north, ranging from the middle of the afternoon to the middle of the night. However, little to no shower activity is expected until this band comes down, so a sizable amount of our NC counties may end up dry but mostly cloudy for much of Monday. Rainfall amounts will vary with this band as precipitable water values surge up to 1.6-1.8", bringing localized heavy rain amounts as high as 1" in less than an hour to some areas while others in between the heavy showers see between a trace to perhaps a tenth inch of rainfall.

Finally, a sharpening shortwave trough is expected to pivot down from the north on Tuesday, helping to shove the cold front well to the south. Another round of showers is expected to accompany this shortwave, with the best chances for renewed development near the front itself. Thus, the position of the front will play an important role in who sees additional rainfall, although amounts will mostly be in the hundredths to perhaps a few tenths of an inch at most.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

We have high confidence VFR conditions will continue through the morning hours beneath thick mid and high level cloud cover. Some light rain is possible at the KFLO airport this morning but intensity should be too little to reduce visibility.

Of potentially greater concern is the potential for convective showers and thunderstorms to develop this afternoon, mainly in the 18-22z timeframe. Convection will move south to southeastward and has a moderate chance to bring visibility and wind direction/speed impacts to KLBT and KILM this afternoon. KFLO, KCRE, and KMYR have a somewhat lower potential for impacts. The actual surface cold front should arrive from the north during the evening, accompanied by a shift to northeasterly winds and a low potential for MVFR ceilings across South Carolina.

Extended Forecast...There is a moderate potential for MVFR/IFR conditions to develop in scattered afternoon and evening showers and thunderstorms Monday and Tuesday.

MARINE

Through Tonight...Generally light winds are expected today with veering wind directions as a cold front approaches the Carolinas from the north. Patchy showers could become more impactful as several models show scattered thunderstorms or even a short squall line possibly developing and moving southward after 2 pm. If storms were to develop, the most likely location and timing appears to be between Myrtle Beach and Cape Fear between 2 pm and 6 pm. Later models should be able to adjust that window.

Of equal or even greater concern to mariners will be the arrival of the cold front this evening. The front should pass Wrightsville Beach around 7 pm and Myrtle Beach around 10 pm, and will be accompanied by a shift to NE 20-25 kt winds with gusts to 30 kt possible. Seas should quickly build to 4-6 feet after midnight and a Small Craft Advisory remains in effect for tonight.

Sunday through Wednesday... Breezy SCA-level northeast winds on Sunday morning will gradually subside and veer to easterly as high pressure builds in and moves offshore during the afternoon, with 25 kts gusts expected to cease by early afternoon. As the high shifts increasingly offshore on Sunday night, expect winds to continue veering to southeasterly and remain in place through Monday at speeds between 5-15 kts. Another cold front is then expected to slide southward on Monday night, with winds turning to between northerly and northeasterly for Tuesday into Wednesday with speeds as high as 15-20 kts in the 0-20nmi zones between Tuesday afternoon and Wednesday morning.

Seas will subside through Sunday night into the 2-4 ft range in the 0-20nmi zones and 4-6 ft range in the 20-60nmi zones in tandem with the decreasing winds. Seas hold in this range into early Tuesday before the next front brings another round of elevated seas, with 6 ft seas possible in the 0-20nmi zones near and east of Cape Fear for a short time starting late Tuesday. A primary driver of the elevated seas will be northeasterly swells with a period between 8-10 sec.

ILM WATCHES/WARNINGS/ADVISORIES

NC...None. SC...None. MARINE...Small Craft Advisory from 8 PM this evening to 1 PM EDT Sunday for AMZ250-252-254-256.