Marine Weather and Tide Forecast for Topsail Beach, NC

UPDATE
Aviation discussion updated for 12z TAFs below. Showers and thunderstorms continue to develop across the area and move fairly swiftly off to the northeast. Made some minor changes, mainly to keep up with pcp.

NEAR TERM /THROUGH TONIGHT/
Confidence is low as to when we will see convection over any one spot today, but confidence is high that we will see continued convective development in a very moist and unstable atmosphere.
Looking at soundings and moisture profiles, deep layer moisture through much of the day will maintain pcp water values up over 2 inches. The steering flow should veer from SW to W through the day driving convection eastward toward the coast. S-SW sfc winds will be higher than yesterday with gusts up to 20 kts or so in the aftn as gradient tightens a bit with front approaching the Carolinas from the west. Temps in the low to mid 70s this morning will reach into the 80s again today with dewpoint temps in the 70s maintaining very warm and muggy conditions. Temps tonight will be in the low to mid 70s again.

Area remains in deep SW-W flow between H5 ridge centered to our S to SE, near the northern Bahamas and a deep trough moving east from near the Tennessee valley Sat morning toward Kentucky/West VA by Sun morning. Minor perturbations will continue to track from SW to NE between these 2 systems. Axis of best shortwave energy should shift south and east through tonight and therefore should see convective development shifting closer to the coast. Initiation will be due to sea breeze and localized boundaries, plus differential heating. By tonight, should see some drier air moving in from the west in the low to mid levels above the sfc, and axis of best moisture should shift near the coast or offshore. Overall, expect widespread cloud cover and intermittent shwrs and tstms with potential for brief very heavy rain and gusty winds.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/
The subtropical ridge will extend across Florida to south of Bermuda. North of this ridge across the Carolinas, westerly upper level flow will contain a number of subtle upper disturbances and at least one not-so-subtle one. It's the passage of this stronger disturbance on Sunday where we're focusing high forecast chances (60- 80 percent) for showers and thunderstorms.

An old front will remain stalled across Virginia or northernmost North Carolina. Our airmass should stay warm, humid, and moderately to strongly unstable with MUCAPE 1000-2000 J/kg surging to near 3000 J/kg both afternoons. SPC has areas closer to the front outlooked in a "marginal" risk both days mainly for damaging wind. However even this far south enough 0-6 km bulk shear (20 kt) may exist to create organized multicell clusters with some potential for gusty winds.
700 mb winds forecast in the 20-25 kt range imply a healthy west to east storm propagation vector.

Daytime temperatures aren't an easy forecast as just a couple hours of direct sun could send temps spiking into the 90s given 850 mb temps near +18C and seasonally high sun angles. I've got upper 80s to near 90 forecast for most areas. Nighttime lows should range from 73-77, warmest on the beaches.

LONG TERM /TUESDAY THROUGH FRIDAY/
For Tuesday through Thursday the subtropical ridge should expand northward off the Georgia and South Carolina coast. This, plus low pressure developing across the Plains should help lift the old stalled front back to the north. The steady stream of upper level impulses should shift to our north allowing more sun and even hotter temperatures to develop. Away from the seabreeze influence highs should rise into the lower 90s Tuesday and mid 90s Wednesday and Thursday. Combined with dewpoints likely to stay in the 73-76 range even during mid afternoon mixing, heat indices could exceed 105 degrees -- especially on Wednesday and Thursday. We'll add this information to the Hazardous Weather Outlook and mention a Heat Advisory is possible each of those days. Convection will become more diurnal and scattered in nature with drier mid level conditions expected.

By Thursday night into Friday the 00z GFS, Canadian, and ECMWF show an upper trough swinging through the Great Lakes into New England, driving the next cold front southeastward and into the Carolinas.
It's possible this will be accompanied by an uptick in convection although models aren't showing any compelling increase in deep moisture at this time.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/
Persistence forecast in place for aviation concerns the next 24 hours. With a very moist and moderately unstable airmass in place, expect convective development to continue. Tough to pinpoint exact timing, but included Prob30 groups for mainly MVFR ceilings and vsbys in thunderstorms. S-SW Winds will be a bit stronger today with higher gusts in the afternoon. Overall expect intermittent convective development through the day with brief periods of sub-VFR in shwrs/tstms.

Extended Outlook...Brief restrictions due to intermittent thunderstorms are possible through Wednesday.

MARINE
Through Saturday night...Persistent SSW flow around Bermuda High will continue but will increase up to 15 to 20 kts with some higher gusts today as gradient increases with approach of low pressure toward the Carolinas. The persistent and increasing southerly push will drive seas up to 3 to 5 ft. A longer period SE swell, up to 8 to 9 secs, will mix in.

Sunday through Thursday...A front will remain stalled across Virginia or northern North Carolina Sunday through Monday, enhancing local coverage of showers and thunderstorms. Bermuda high pressure will maintain a moderate southwesterly wind across the area with speeds generally around 15 knots with afternoon/evening increases to near 20 knots possible. Seas should consist of a 5 second wind chop with a smaller 8 second southeast swell beneath, totaling 3-4 feet across the area.

Low pressure developing across the Plains states Tuesday and Wednesday should lift the stalled front north and away from the Carolinas. This should have the effect of reducing the number of showers and thunderstorms, although with a hot and humid airmass in place there will almost certainly be scattered storms developing along the seabreeze each day and over the coastal waters late at night. Southwest winds should continue through the period with speeds generally in the 10-15 kt range except stronger nearshore each day with the seabreeze.

ILM WATCHES/WARNINGS/ADVISORIES
NC...None.
SC...None.
MARINE...None.