Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 230829 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 429 AM EDT Sun Apr 23 2017 .SYNOPSIS... A cold front will stall across the area today. A complex and slow moving storm system will bring the potential for flooding through Monday night. There is a small risk for severe weather as well. This system will lift away from the area Tuesday with a return to dry weather and above normal temperatures mid and late week. && .NEAR TERM /THROUGH TONIGHT/... As of 300 AM Sunday...Complex situation begins to unfold today leading to what will likely be a widespread heavy rainfall event beginning late tonight. A flash flood watch has been issued beginning at midnight. Cold front has sagged into the CWA and is currently nearly along the SC/NC state line. This front will continue to slip southward into the morning hours before stalling across our SC zones. The front will then likely begin to lift back to the north slowly in response to shortwave ridging ahead of a deepening mid-level closed low dropping into GA. It is this feature that will primarily be responsible for the deteriorating weather through Tuesday. Initially, as the front wavers in the vicinity, it will create a complex temperature and wind forecast locally. Locations that remain north of the front today will be cloudy and cool, with highs only around 70, while areas south of the boundary will warm into the low 80s with more sunshine. This tight gradient just across the CWA leads to bust potential if the front moves differently than currently forecast, but guidance is in pretty good agreement that only the southern few counties will warm much today. At the same time, the aforementioned upper low will deepen and shift SE from western TN this morning into SE GA tonight. Ahead of this, S/SW flow will intensify while diffluence aloft increases. Although it is late April, this sets up an isentropic lift scenario, and rain showers will gradually increase across the area from late morning through early evening. While isentropic lift is usually more supportive of stratiform rain than convective showers, enough elevated (surface) instability will exist north (south) of the front for isolated tstms as well. Total QPF into tonight is expected to be light however. Things deteriorate rapidly thereafter, and the flood watch begins at midnight. As the upper low deepens and drops further SE, LLJ at both 850mb and 700mb increase from the south causing column saturation and PWATS rising to 1.5 inches, near the highest all-time for the date. Additionally, increasing confluence is noted within these layers, and moist advection will occur both from the GOM and the western Atlantic Ocean, in an environment characterized by increasing MLCape to 300-500 J/kg which will support very heavy rainfall rates. High-res guidance suggests a "wall" of heavy rain with embedded tstms will move into the western counties late, aligned closely with the best LLJ and moist advection. QPF through Monday morning will range from up to 1.5 inches west of i-95, to around 0.25 inches along the Cape Fear coast, with much more expected beyond this period. While a lower end threat than flooding, note very high helicity values along the wavering front this evening and tonight approaching 300 m2/s2 in the lowest 1km, especially inland. With SBCape minimal expect a very limited tornado threat, but would not be surprised to see some rotating updrafts in any stronger convection tonight. SPC has added a 2% tornado prob to their MRGL risk tonight across most of SC, noting that if buoyancy can increase above current progs, the tornado risk may be higher, especially in the presence of the increasing LLJ late. && .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... As of 300 AM Sunday...A very wet period Mon and Mon night with widespread heavy rain and scattered thunderstorms necessitates a Flash Flood Watch for our entire CWA. We are forecasting a storm total of 3 to 6 inches of rain with the heaviest of rainfall ending late Mon and Mon night. Although the ground is dry and can absorb a great deal of runoff, it is the rainfall rates that may result in flash flooding. Urbanized areas and areas of poor drainage will be especially prone to flash flooding. Rivers will see dramatic rises and many will exceed bankfull during the week. Deep low pressure will be located across Georgia Mon morning. Slow moving low pressure is expected to move offshore Mon eve and then turn up the Carolina coast, following a frontal boundary that will be stretched along the coast. The center of the low should be in the vicinity of Cape Fear Tue morning and Cape Hatteras Tue eve. As the low approaches, it will draw deep moisture northward and this moisture can be seen pooling N on satellite across portions of Cuba, Florida and the Bahamas. Precipitable water values will reach climatological maximums and they will remain near these levels for an extended period of time given the slow movement of the system. Isentropic lift will be strong with deep moisture running up and over the frontal boundary. Mixed layer CAPE values of 500 to 800 J/kg should support convective elements which will be capable of dumping 2 or more inches of rain in a short amount of time. Cells will tend to train along the front which will increase the flood threat. In addition, cold air aloft, 500 mb temps as low as minus 19, will skirt the area on Mon. The deep moisture should preclude any glimpses of sunshine which would further steepen lapse rates and lead to an increased hail and wet microburst threat. At this time, the threat from flooding is higher than the severe weather risk. However, the severe weather risk is non-zero and SPC has included the area in a marginal severe weather risk on Mon. Low pressure will be pulling away from the area on Tue and drier air will slowly work into the region on the backside, ending the showers. Partial clearing will likely hold off until late Tue or Tue night as weak ridging begins to build across the Southeast states. Of course even partial clearing will foster low clouds and stratus to fill in overnight Tue given the saturated soils and light winds. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... As of 300 PM Saturday...Fair weather is expected for the long term period. Temperatures will be above normal. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/... As of 06Z...Moderate confidence forecast with IFR likely at the start of the period, and possible again late tonight, especially at the NC terminals. Cold front sinking slowly southward has crossed LBT/ILM and will do so at FLO shortly. This front may reach the Myrtles as well before stalling, and any terminals that have FROPA overnight will also experience a wind shift to the NE and at least TEMPO IFR stratus. Cigs have been bouncing a bit upstream, so opted to leave IFR at TEMPO with low-MVFR predominant, but may need AMD`s if IFR settles in like the NAM soundings suggest. A few showers are possible north of this front overnight as well, but most rainfall will hold off until daybreak. This front will waver in the vicinity during Sunday, creating a complex ceiling forecast. Terminals that remain north of the front may stay MVFR/IFR all aftn, while those that become south of the boundary will likely lift to VFR. Attm expect only the Myrtles to lift to VFR, with MVFR remaining elsewhere. Additionally, showers and isolated tstms will become increasingly widespread, especially Sunday night, with MVFR and possible IFR in any stronger convection. Winds will fluctuate considerably as well, E/NE 10 kts or less north of the front, and S/SE 10-15 kts south of the boundary. As showers and tstms increase in coverage late in this valid period, even those terminals that reach VFR will drop back to MVFR with cig restrictions. Extended outlook...Periods of MVFR and IFR through early Tuesday in low stratus, showers, and thunderstorms. VFR thereafter. && .MARINE... NEAR TERM /THROUGH TONIGHT/... As of 300 AM Sunday...Cold front dropping south across the area this morning will slow and eventually stall, creating a wind shift to the NE with speeds increasing to 10-15 kts across only the NC waters this morning. South of the boundary, southerly winds will remain through the daylight hours, also around 10-15 kts. However, this front will begin to lift back to the north tonight, moving onshore, and turning winds around to the SE across all waters at speeds of 15- 20 kts with higher wind gusts. While current wind speeds are not forecast to reach SCA thresholds, the combination of a 10 sec easterly swell and an amplifying S/SE wind wave to 6ft/6sec will drive seas up from 2-3 ft this morning to 4-6 ft late. This has prompted an SCA to be issued, beginning at 10pm tonight and persisting beyond the near term. SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... As of 300 AM Sunday...A Small Craft Advisory is in effect for all waters through Mon and for the northern waters through Mon night. Widespread poor visibility is expected in heavy rain and strong to severe thunderstorms Mon and Mon night as low pressure moves offshore of Georgia and then slowly up the Carolina coast. The showers will end from S to N during Tue as low pressure along the coast moves far enough to our N. Seas will be up to 4 to 7 ft Mon/Mon night. Seas will remain elevated on the backside of the low Tue and Tue night with perhaps 5 and 6 ft seas lingering across the northern waters. Winds Mon will be SE ahead of the low, then the direction will become variable with low pressure and a frontal boundary across the area. As the low pulls N of our latitude, the wind direction will become NW late Mon night and Tue with NW winds persisting through the end of the period. Sustained winds will be no higher than 15 to 20 kt, but strong to severe convection will likely produce higher winds. LONG TERM /WEDNESDAY THROUGH THURSDAY/... As of 300 PM Saturday...Light south to southwest winds are expected with seas 4 ft or less. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...Flash Flood Watch from late tonight through Tuesday morning for SCZ017-023-024-032-033-039-053>056. NC...Flash Flood Watch from late tonight through Tuesday morning for NCZ087-096-099-105>110. MARINE...Small Craft Advisory from 10 PM this evening to midnight EDT Monday night for AMZ254-256. Small Craft Advisory from 10 PM this evening to 8 AM EDT Tuesday for AMZ250-252. && $$ NEAR TERM...JDW SHORT TERM...RJD LONG TERM...RAN AVIATION...JDW is the U.S. government's official web portal to all federal, state and local government web resources and services.