Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 221723 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 123 PM EDT Sat Apr 22 2017 .SYNOPSIS... A slow moving cold front will sink south into North Carolina late tonight before stalling near the state line. A complex storm system will develop along this boundary and bring heavy rainfall and potentially strong to severe thunderstorms Sunday through Monday night. This system will lift away from the area Tuesday with a return to seasonable and dry weather by Wednesday. && .NEAR TERM /THROUGH TONIGHT/...
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As of 115 PM Saturday...Removed the 20 PoP for showers/storms this afternoon as there appears to be too much dry air at the top of the mixed layer. One of my favorite airmass convective parameters has become the LCL-LFC Relative Humidity on the SPC mesoanalysis webpage, and it`s showing values of only 50-60 percent where 80+ plus is usually associated with airmass convection. No other significant changes. Discussion from 1000 AM follows... Only minor tweaks needed to the forecast with this morning update. Plenty of dry air aloft will interfere with convection later today. The NAM soundings show convective inhibition at the top of the surface-based mixed layer that could act to stop any deep convection through the daylight hours. This opinion is shared by the ECMWF as well. On the other hand, the GFS, RAP and HRRR models show sufficient moisture at the top of the boundary layer extending upward for several thousand feet which could allow for isolated showers or storms to fire along and just west of the seabreeze front. I`ll leave the current 20 PoP in place for now. Discussion from 300 AM follows... One last very warm, nearly hot, day before conditions take a turn for the worse beyond this period. High pressure offshore will remain the dominant feature one more day, holding a cold front to the north at bay until late tonight. The surface high will remain anchored offshore beneath ridging across the western Atlantic Ocean today, but will slowly retreat to the east tonight thanks to height falls associated with a deepening mid- level trough dropping into the TN VLY late. With the surface high offshore, warm return flow will again flood into the Carolinas, while W/SW flow aloft keeps the mid-levels dry through the evening. This suggests another day with temps well above climo, and as 850mb temps soar towards 14C, late April insolation will drive highs well into the 80s, near 90 inland, with a robust sea breeze keeping the coast to around 80, mid 70s on that cool Brunswick County coast. While convection is not expected to be widespread today, slightly greater low-level moisture and marginal SBCape thanks to steep surface lapse rates should drive a more extensive Cu field than Friday, especially where the sea breeze provides additional lift. Have included a SCHC POP along the sea breeze this aftn, but otherwise expect enough dry air above the LCL to prevent CU from becoming deep enough to produce showers/tstms. Tonight, the aforementioned cold front will begin to sag south towards the ILM CWA as high pressure wedges down from the north behind it. GFS/NAM have come into good agreement pushing the front to about the SC/NC state line by Sunday morning, and the ECM has trended in that direction as well. As this front sags southward, the combination of surface convergence as well as isentropic lift atop it will drive increasing shower and isolated tstm chances late tonight. Have bumped POP to mid-chc across far northern zones where forecast profiles suggest rapidly decaying mid-level dry air, but again, most of the QPF is expected beyond this period into Sunday and Monday. The exact placement of this front will wreak havoc on the overnight min forecast, as locations south of the front will remain entrenched in warm flow, while cool NE flow develops north of the boundary, with cooling potentially aided by rainfall as well. Have tried to show a large gradient from S to N tonight, with mins only around 70 along the Grand Strand and South Santee River, dropping to as cool as 59 or so near St. Pauls. The placement of this front will be crucial to overnight mins however, and bust potential exists if it drops further south (or stays north) than currently progged.
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&& .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 300 AM Saturday...A front will likely lie in close proximity to the Forecast Area on Sun and may actually bisect the area. A deep area of low pressure, cut off from the main flow, should be located across the mid south Sun morning. The upper low is expected to move slowly to the SE across Tennessee and Georgia and should be moving offshore of coastal Georgia during Mon and then up the Carolina coast to a position near Cape Fear by first light Tue. The surface low should take a similar path. The coastal system will be pulling slowly north of the area during Tue. This scenario results in a very wet period for the eastern Carolinas with strong lift and deep moisture. We are forecasting rainfall amounts of 1 to 3 inches with perhaps higher amounts, especially across the Cape Fear Region. This will not be enough to cause widespread flooding, but showers and thunderstorms with heavy rain may cause localized ponding, especially in urban areas. The severe threat is expected to be isolated with limited instability. Mixed layer CAPE values should reach 500 to 1000 J/kg Sun. Guidance shows effective shear values have increased over 24 hours ago with 0-6 km bulk shear of 40-50 kt. However, overall wind fields remain very modest. Then on Mon, a pocket of cold air with 500 mb temps down to minus 16 deg C, skirts the area. We are expecting little if any glimpses of sunshine on Mon to help further steepen lapse rates. However, should any convection be able to attain sufficient depth, severe weather will become a risk to include large hail and we will be watching for this possibility in later forecasts. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 300 PM Friday...The extended period will be active early on. As for Wednesday through Friday mid level ridging will develop across the southeast as deep troughing develops out west. By Tuesday morning the surface low should be near Cape Hatteras with the mid level feature not too far behind. POPs will be trending down Tuesday. The remainder of the week should be dry with above normal temperatures trending even higher as time goes on. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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As of 18Z...High confidence in VFR conditions through at least 06Z-08Z tonight. Breezy southwest winds associated with the seabreeze could gust to 25 knots at ILM, MYR, and CRE now through sunset. Attention will then shift to a cold front dropping southward across eastern North Carolina after midnight. This front could arrive in LBT as soon as 06Z, ILM-FLO as soon as 09-11Z, and the Myrtle Beach airport 11Z-13Z, accompanied by a shift to northeasterly winds. IFR ceilings will develop behind the front and may linger into at least noon local time on Sunday. Showers will also develop behind the front with occasional visibility restrictions possible. Extended outlook...Periods of MVFR and IFR Sunday afternoon through early Tuesday in low stratus, showers, and thunderstorms.
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&& .MARINE... NEAR TERM /THROUGH TONIGHT/... As of 1000 AM Saturday...A strong seabreeze circulation should develop early this afternoon as inland temperatures soar well into the 80s but air temps over the coastal waters remain in the lower 70s. This should push wind speeds to a solid 20 knots at the beaches this afternoon with gusts to near 25 knots at times. An exercise caution headline may be issued on the next update if these winds appear to last more than a few hours. Current seas consist of mainly a 5-second wind wave which will be quite uncomfortable for small craft through the day. Discussion from 300 AM follows... High pressure offshore and a cold front to the north will keep the waters sandwiched in a pinched gradient through tonight. Winds will persist from the SW at 15-20 kt much of the near term, although a local backing along with slightly higher speeds is expected within the near shore sea breeze circulation. Late in the period, the cold front will drop into the northern waters and the gradient will begin to relax, so a decrease in wind speeds along with a shift to the E is possible for AMZ250 and AMZ252 before morning on Sunday, but the SC waters will remain SW at 10-15 kt. Although a long period easterly swell will exist in the spectrum today, likely from post-tropical storm Arlene, the predominant wave group will be a SW 5 sec wave, driving seas to 3-4 ft with isolated 5 fters. At this time no cautionary statements are expected, but it will get close to SCEC criteria this evening. SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 300 AM Saturday...A backdoor cold front may reach portions of the waters before stalling on Sun. The weather across the marine environment will be wet and stormy for much of this period. Slow moving low pressure well to our west Sun will take a track to the southeast, moving offshore of Georgia before turning up the Southeast coast Mon, reaching the Cape Fear area near first light Tue morning. Mariners should expect poor visibility in showers and thunderstorms with perhaps the worst of the visibility restrictions developing Sun night and persisting through Mon night. The wind direction ahead of the low should be mainly SE and S, Sun through Mon. On the backside of the low, the wind direction will shift to N Mon night. The strongest winds are expected to develop on the backside of the low and may reach 20 to 25 kt later Mon night. Wind speeds earlier in the period will be up to 15 to 20 kt. Seas will be 3 to 4 ft Sun with seas building up to 4 to 5 ft across the southern waters Sun night and up to 6 ft Mon night. Across the northern waters, seas will build up to 6 ft Sun night and up to 7 ft Mon night. Seas may be higher throughout on Tue. LONG TERM /TUESDAY THROUGH WEDNESDAY/... As of 300 PM Friday...A complex storm system will continue to move up the coast Tuesday and with the nature of the storm and the extended time frame, confidence is low for this forecast. The flow will turn offshore Tuesday. As of now, speeds should remain below flag criteria, but an extended period of somewhat churned up seas may allow heights to eclipse six feet. Both elements should subside considerably Wednesday. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...Beach Hazards Statement until 8 PM EDT this evening for SCZ054- 056. NC...Beach Hazards Statement until 8 PM EDT this evening for NCZ106- 108-110. MARINE...None. && $$ SYNOPSIS...RJD NEAR TERM...TRA SHORT TERM...RJD LONG TERM...SHK AVIATION...TRA

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