Area Forecast Discussion
Issued by NWS Wilmington, NC

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287 FXUS62 KILM 220947 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 336 AM EDT Sat Apr 22 2017 .SYNOPSIS... High pressure will remain offshore and extend back into the Carolinas today. A slow moving cold front will sink south into the area late tonight before stalling. A complex storm system will develop along this boundary and bring heavy rainfall and potentially strong to severe thunderstorms late in the weekend and through Monday night. This system will lift away from the area Tuesday with a return to seasonable and dry weather by the middle of next week. && .NEAR TERM /THROUGH TONIGHT/...
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As of 300 AM Saturday...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 /06Z SATURDAY THROUGH WEDNESDAY/... As of 06Z...VFR forecast through the valid period. High pressure offshore will keep a cold front to the north at bay, but increased moisture this aftn will permit at least SCT aftn diurnal CU at VFR levels. A few showers and even isolated tstms are possible, mostly along the sea breeze, but are not expected to impact any of the terminals. SW winds will remain above 5 kt through daybreak, and then increased to 10-15 kt, with gusts over 20 kt likely at the coastal terminals behind the sea breeze. A local backing to the S/SW is also forecast at ILM/CRE/MYR this aftn. Winds will ease slowly after dark, and the aforementioned cold front will begin to sag southward. Guidance has sped up the timing of this fropa, but will likely still not see a wind shift or any showers across NC terminals until just beyond this valid period. However, VFR cigs will slowly develop from north to south overnight tonight. Extended outlook...MVFR/IFR cigs possible late Saturday night into Sunday. Periods of MVFR and IFR are likely Sunday and Monday in showers and thunderstorms. VFR re-developing on Tuesday. && .MARINE... NEAR TERM /THROUGH TONIGHT/... As of 300 AM Saturday...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 from 7 AM EDT this morning through this evening for SCZ054-056. NC...Beach Hazards Statement from 7 AM EDT this morning through this evening for NCZ106-108-110. MARINE...None. && $$ NEAR TERM...JDW SHORT TERM...RJD LONG TERM...SHK AVIATION...JDW

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