Area Forecast Discussion
Issued by NWS Wilmington, NC

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035 FXUS62 KILM 241510 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 1110 AM EDT Mon Apr 24 2017 .SYNOPSIS... A complex and slow moving storm system will bring heavy rain and likely flooding today through Monday night. There is a small risk for severe thunderstorms as well. This system will lift north northeast, away from the forecast area Tuesday with a return to dry weather and above normal temperatures during the mid to late week period of this week. Expanding high pressure aloft and at the surface, will provide mainly clear skies with warm and dry conditions thru the upcoming weekend. High temperatures may break into the 90s away from the immediate coast during this weekend. && .NEAR TERM /THROUGH TUESDAY/...
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As of 11 AM Monday...Heavy rain along with showers and a few embedded thunderstorms continue across much of the forecast area this morning. Low pressure aloft slowly moving southeast across southern GA will continue pulling deep moisture into the region today. Moisture feed extending from off the FL coast, as seen on satellite and radar, will continue spreading over portions of SC this morning and into early afternoon. Some of these areas have already seen significant rainfall and have been reporting flooding. This area of enhanced moisture should rotate north and east as the 5h low moves into eastern GA this afternoon. Meanwhile surface low developing north of the Bahamas, visible as a swirl on satellite imagery, and its associated moisture will continue moving north today. The interaction between the 5h low over the southeast and this low/moisture will be key to the magnitude of the afternoon/evening/overnight rain event. The latest satellite imagery suggests at least a portion of this moisture will be pulled in by the 5h low. The dry slot that had been prominent on water vapor has been eroding in recent imagery. The 12Z NAM does not depict a lot of interaction between these features and has the bulk of the heavy rain ending around 00Z. However, looking at the 15Z location of the surface low north of the Bahamas in the NAM compared with reality suggests the NAM is too quick with the movement of the surface low. This suggests a better chance the low and its moisture will have at least some interaction with the system aloft. Other than tweaking the first few hours of the forecast for latest radar/satellite trends have not made a lot of changes to the forecast. Previous discussion from 330 AM Monday follows: Developing situation tonight may be more typical of mid- fall than late- spring, but is certainly a classic setup for a heavy rain event (think Oct 2015, Sep 2010) locally. While we are not expecting 10-20" of rain like the 2 events mentioned, a 24-hr period of very heavy rainfall begins now, and widespread 3-6" of rain is being forecast, with locally heavier amounts probable. Anomalously deep upper low currently across AL will dig SE through this aftn before slowly lifting north to be off the SC coast by Tuesday morning. At the surface, a weak low pressure over SE GA will drift north across the eastern Carolinas through today, while a secondary but very important low off FL will be captured and absorbed into the primary low tonight. Additionally, a weak warm front/coastal trough aligned from the primary surface low to off the Grand Strand will waver and lift slowly NW today. So how does this all come together to create a flash flood threat today? As the 500mb low pivots SE, deepens, and begins to tilt negatively, upper diffluence increases both due to winds exiting the upper speed max, and directional ventilation NE of the upper feature. Beneath that, the primary surface low will aid in creating lift, while increasing 850mb and 700mb LLJ transports moisture from the GOM and, becoming more dominant with time, the Atlantic Ocean. The 850mb LLJ dominates through today as strong WAA develops on anomalously strong easterly winds, further enhancing lift, while confluence will likely drive a pivoting band of increasingly heavy rainfall across the region (similar to Oct 2015). This easterly jet becomes even more important as it pulls the surface low near FL and its associated tropical- esque moisture (PWATs to 1.75 inches, would break the record high for the date) into the Carolinas, and crosses orthogonally the coastal front. A strong easterly LLJ funneling deep tropical moisture atop a coastal front is a clear heavy rain signal (like Sep 2010), and this will be even further aided by MLCape of 300-500 J/kg as mid-level lapse rates steepen beneath height falls in advance of the upper low. While true convective rainfall will likely be confined to areas south, along, or immediately north of the coastal front, elevated instability will allow for heavy rainfall to continue even well north of the boundary. This will become increasingly the case late today and tonight, and the heaviest rainfall is likely from this aftn through the overnight hours. While a pivoting band of heavy rainfall will likely be the primary rain maker today, increasing coverage of showers and tstms will develop across the area, and have carried categorical POP for all locations. Total QPF will range from 3 to 6 inches, with locally higher amounts probable especially just NW of the coastal trough, or in any training convective elements. Our soils are dry, and fuels will absorb a lot of the rain, but the flash flood watch remains in place as extremely heavy rainfall rates at times will produce localized flooding. If that isn`t enough, there remains a non-zero severe threat, especially tonight, as shear profiles suggest the chance for storm organization. With limited instability do not anticipate much of a wind threat, and hail chances are even smaller, but high helicity as noted by strongly curved hodographs could support an isolated tornado as elements cross the coastal front. The total severe threat is low however, and flooding remains by far the primary concern. The upper low starts to fill and lift NE on Tuesday, causing the surface low to weaken as it becomes a vertically stacked system. Drier air begins to advect from the W/SW behind this, and most of the heavy rain will be out of the area by late morning Tuesday. With 500mb cold core of -14C to -17C moving overhead Tuesday, diurnal showers are possible through the day, and have carried SCHC POP for this potential. However, do not expect these to be heavy and should be scattered at worst, and will fade with loss of heating in the evening. High temperatures today will be warm only south of the coastal front, rising into the low to mid 70s along the SC coast, but only reaching the upper 60s inland. Highs on Tuesday will be a more uniform low 70s, although the slow erosion of cloud cover may keep highs a bit cooler inland. Mins tonight will range from the upper 50s well inland, to the low 60s near the coast, just slightly above normal in what is an otherwise very abnormal pattern.
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&& .SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/... As of 330 AM Sunday...No POPs this period. The FA will finally have a chance to dry out this period. The closed upper low over northeast NC at the start of this period, will lift north- northeast, further away from the ILM CWA, during this period. Residual clouds early Tuesday night, will clear out by Wednesday morning leaving only diurnal cu to affect the FA on Wed. Ridging aloft will become the dominate upper feature affecting the FA Wed thru Wed night and the accompanying subsidence aloft will keep a lid on any convection that tries to develop during Wed. Sfc ridging extending from well off the SE U.S. coast will temporarily ridge into the area Wed and Wed night. Will need to monitor for possible fog both Tue night and Wed night given plenty of standing water likely due to the heavy rains during the near term period. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 300 PM Sunday...The mid level pattern will shift initially into broad troughing across the central U.S. to a more amplified version of this pattern by the end of the period. With very strong ridging expected to develop across the southeast by the end of the period, expect dry and very warm conditions to develop. At the surface an elongated cold front associated with the initial broad trough makes a run to the east but quickly loses steam and never makes it into the area and in fact remains well to the northwest. Beyond this its all Bermuda High pressure. Temperatures, above normal for the start of the period quickly rise to close to ten degrees above average and the possibility of some highs into the 90s inland. && .AVIATION /14Z MONDAY THROUGH FRIDAY/... As of 12Z...The wavering coastal front lies along the immediate coast from just south of Bald Head Island southwest and onshore in the vicinity of Cape Romain. Not much inland movement if at all, is expected with this front. Overall, winds from now into early this evening will hold from the NE to ENE north of the front, and from the ESE to SE south of this front. The coastal terminals may see gusts up to 20 kt depending how much into the warm sector wavers northward. Will see convective type pcpn develop over the atlantic waters and push northward thruout the day. Subtropical plumes of moisture from the Atlantic may result in training of pcpn across southeast NC and Northeast SC and across local terminals. As the pcpn moves north of the coastal front, pcpn will transition to a more stratiform look to it with embedded thunder. Have basically kept IFR conditions thruout all terminals with tempo groups to identify any thunderstorm activity and possible LIFR conditions. Some improvement to occur during the 06Z-12Z timeframe as the closed low moves overhead shunting the moisture plumes east of the ILM CWA. Extended outlook...Periods of MVFR and IFR through early Tue in heavy rain, thunderstorms, and low stratus. IFR or lower conditions possible in fog and stratus Tue night and early Wed. && .MARINE...
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NEAR TERM /THROUGH TUESDAY/... As of 11 AM Monday...Southerly flow will continue increasing, peaking at 20 to 25 kt this afternoon and evening as weak surface low consolidates over southeastern SC and slowly lifts north. Combination of enhanced wind speeds and onshore flow will keep seas over 6 ft into tonight. Minimal changes for the morning update. Previous discussion from 330 AM Monday follows: Coastal front will push inland today as SE winds increase from 10-15 kts this morning, to 15-25 kts this aftn and tonight. A surface low pressure will drift from SE GA up to the Cape Fear region tonight, and then weaken across the Outer Banks Tuesday. This will cause winds to shift from SE to SW overnight into Tuesday, and then veer further to the West late on Tuesday. Speeds during this time may be highly variable depending on the exact placement of the surface low, but expect a general 10-15 kts once winds turn around to the SW. Although these winds only briefly reach SCA thresholds, the ongoing SCA remains unchanged through 8am Tuesday. This is due primarily for seas rising from 3-5 ft this morning, to 4-8 ft tonight, with a 7-8 sec SE wave group dominant the spectrum. On Tuesday as the winds shift SW to W, the SE wave deamplifies while the period lengthens, and a 6 sec SW wave develops. This will allow wave heights to fall back to 3-5 ft and any headlines should be allowed to expire on time Tuesday morning. There is also likely to be widespread showers and tstms today and tonight, with torrential rainfall creating severely restricted visibility at times. SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/... As of 330 AM Monday...This period will be highlighted with the closed upper low and it`s associated sfc low, lifting north- northeast, pulling further away from the local waters. An offshore wind will highlight Tue night into early Wed. During Wed thru Wed night, sfc ridging from well off the Southeast U.S. coast will extend inland just south of the local waters. This will result in a S to SW wind regime for the end of this period. The sfc pg remains weak or somewhat relaxed thru this period with speeds generally 15 kt or less. Significant seas will peak at the start of this period, followed by a slow subsiding trend. The east-southeast ground swell at 8 to 9 second periods will dominate the significant seas. Could see near shore locally produced wind waves due to the sea breeze circulation during Wed aftn thru early evening. LONG TERM /THURSDAY THROUGH FRIDAY/... As of 300 PM Sunday...Departing low pressure to the northeast and troughing in the central U.S. will leave a weak pressure gradient in place through Friday. For Thursday and Friday expect weak south to southwest flow as the overall flow mimics a summertime pattern. Significant seas will trend in a similar summertime pattern direction with 1-3 feet both Thursday and Friday.
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&& .ILM WATCHES/WARNINGS/ADVISORIES... SC...Flash Flood Watch through Tuesday morning for SCZ017-023-024- 032-033-039-053>056. NC...Flash Flood Watch through Tuesday morning for NCZ087-096-099- 105>110. MARINE...Small Craft Advisory until midnight EDT tonight for AMZ254-256. Small Craft Advisory until 8 AM EDT Tuesday for AMZ250-252. && $$ SYNOPSIS...DCH NEAR TERM...III SHORT TERM...DCH LONG TERM...SHK AVIATION...DCH MARINE...

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