Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 232359 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 759 PM EDT Sun Apr 23 2017 .SYNOPSIS... A complex and slow moving storm system will bring heavy rain and 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 MONDAY/... As of 8 PM Sunday...Proverbial calm before the storms as on a smattering of light showers are moving across the forecast area at this time. Latest high-res guidance has backed off a bit on an overnight heavy rain scenario, but other convective indices still support the possibility of some spinners moving ashore. Previous discussion from this afternoon follows: Active period set to begin with anticipation of widespread showers and thunderstorms along with potential for severe weather and flooding. Front stalled south of Cape Fear will gradually lift north-northwest this afternoon and evening. Convection offshore is likely to shift onshore as the area of low level convergence shifts west. Forecast soundings and cross sections depict the strongest convergence setting up over inland SC this evening and overnight with a second enhanced area along the SE NC coast. Warm sector spreads over at least a portion of the forecast area as the boundary moves inland later this evening, setting the stage for severe weather. Risk is low, although SPC has bumped up a portion of the area from marginal to slight risk. Strong low level jet develops tonight, peaking around 40 kt, as the front heads inland. Additionally warm advection will bring about a slight increase in low level instability. Instability isn`t quite surface based but the surface stable layer appears shallow enough that it may not provide a buffer. The combination of increasing low level instability and 0-1km helicity approaching 300 m2/s2 suggest some storms will be rotating, posing at least some tornado risk (hence SPC increasing tornado prob to 5%). Storms will be moving quickly, storm motion will be around 25kt, but their high precipitation efficiency nature means any training storms will pose a flood threat. In addition to a deep warm cloud layer there will be an abundance of moisture in the region. Deep southerly flow has some success tapping into the plume of moisture spreading north from near the Bahamas, which helps push precipitable water values over 1.5 inches tonight. This is within shouting distance of all time highs for late April. Cloud cover and abundant moisture will keep lows overnight 10 to 15 degrees above climo in most areas. Extended periods of heavy rain will continue on Mon with flooding taking over for severe weather as the main threat. Helicity decreases somewhat as flow become a little more unidirectional, but there will be a slight increase in surface based instability. Cannot rule out damaging wind from the strongest storms and SPC has maintained a marginal risk for the area. The bigger concern is potential for very heavy rain for much of Mon and the high likelihood of flooding. Front will be stalled in the area as surface low tracks across SC. Deep southerly flow pushes precipitable water values over 1.7 inches tomorrow afternoon, pretty much all time highs for late April. Warm cloud layer remains deep and saturated, around 12k ft, which will again keep precipitation efficiency quite high. Further aiding storm development Mon will be strong divergence aloft swinging over the area in the afternoon, ahead of the 5h low/trough. Given the favorable conditions/set up it appears rainfall totals of 4 to 7 inches with locally higher amounts are on offer. Cloud cover and rain will keep highs Mon a little below climo.As of 3 PM Sunday...Active period set to begin with anticipation of widespread showers and thunderstorms along with potential for severe weather and flooding. Front stalled south of Cape Fear will gradually lift north-northwest this afternoon and evening. Convection offshore is likely to shift onshore as the area of low level convergence shifts west. Forecast soundings and cross sections depict the strongest convergence setting up over inland SC this evening and overnight with a second enhanced area along the SE NC coast. Warm sector spreads over at least a portion of the forecast area as the boundary moves inland later this evening, setting the stage for severe weather. Risk is low, although SPC has bumped up a portion of the area from marginal to slight risk. Strong low level jet develops tonight, peaking around 40 kt, as the front heads inland. Additionally warm advection will bring about a slight increase in low level instability. Instability isn`t quite surface based but the surface stable layer appears shallow enough that it may not provide a buffer. The combination of increasing low level instability and 0-1km helicity approaching 300 m2/s2 suggest some storms will be rotating, posing at least some tornado risk (hence SPC increasing tornado prob to 5%) Storms will be moving quickly, storm motion will be around 25kt, but their high precipitation efficiency nature means any training storms will pose a flood threat. In addition to a deep warm cloud layer there will be an abundance of moisture in the region. Deep southerly flow has some success tapping into the plume of moisture spreading north from near the Bahamas, which helps push precipitable water values over 1.5 inches tonight. This is within shouting distance of all time highs for late April. Cloud cover and abundant moisture will keep lows overnight 10 to 15 degrees above climo in most areas. Extended periods of heavy rain will continue on Mon with flooding taking over for severe weather as the main threat. Helicity decreases somewhat as flow become a little more unidirectional, but there will be a slight increase in surface based instability. Cannot rule out damaging wind from the strongest storms and SPC has maintained a marginal risk for the area. The bigger concern is potential for very heavy rain for much of Mon and the high likelihood of flooding. Front will be stalled in the area as surface low tracks across SC. Deep southerly flow pushes precipitable water values over 1.7 inches tomorrow afternoon, pretty much all time highs for late April. Warm cloud layer remains deep and saturated, around 12k ft, which will again keep precipitation efficiency quite high. Further aiding storm development Mon will be strong divergence aloft swinging over the area in the afternoon, ahead of the 5h low/trough. Given the favorable conditions/set up it appears rainfall totals of 4 to 7 inches with locally higher amounts are on offer. Cloud cover and rain will keep highs Mon a little below climo. && .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... As of 300 PM Sunday... Early Monday night there should still be some impressive rainfall rates being observed across the area, mainly NC counties. Some dryer mid level air should be impinging into SC zones. The 06-12Z Tuesday time frame is a bit more problematic. Granted this is the the tail end of the event but with the impressive antecedent rainfall observed this will still be a window of great hydrologic vulnerability especially if mesoscale/convergent streamers are still coming ashore. The GFS is very agressive in bringing in the dry slot and would actually imply that rainfall is done area-wide (and probably is over much of SC). Other guidance not so agressive and WPC forecast still showing a 12hr bullseye into the Cape Fear. Will preserve this slower sense of timing but with lower amounts in deference to the GFS, which looks very good from a conceptual model point of view. Needless to say the interaction with the main synoptic system and the disturbance coming out of Bahamas will be hard to resolve for NWP, the latter poorly sampled by upper air observations. A minor severe weather risk persists into Monday night with mid level temperatures so cold, but the threat remains marginalized by weak wind fields and instability through the lower half of the troposphere. By Tuesday morning the occluded upper low will be rotating into the region, captured by the upper low, filling, and then turning northeastward as it weakens. The channel of impressive moisture flux will be well to our north and only lighter wrap-around showers will remain. && .LONG TERM /WEDNESDAY 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 teh 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 /00Z MONDAY THROUGH FRIDAY/... As of 00Z...High confidence in IFR for our inland terminals for low cigs through virtually the entire TAF period, with somewhat lesser confidence in IFR VSBYs in showers for the same period. A little trickier along the coast for the first 3 hours as cigs will hover right around 1 kft before dropping more deeply into IFR territory. Once that happens, have high confidence in IFR continuing for cigs for the next 6 to 9 hours. Moderate confidence periods of IFR/MFR for vsbys in showers through the TAF period for the coastal terminals. Extended outlook...Periods of MVFR and IFR through early Tue in heavy Rain and thunderstorms and low stratus. IFR or lower conditions possible in fog and stratus Tue night and early Wed. && .MARINE... NEAR TERM /THROUGH MONDAY/... As of 8 PM Sunday...Small craft advisory remains in effect for all zones. Latest obs show 4 to 5 ft seas with NE winds of 15 to 20 kts. Previous discussion from this afternoon follows: Northeast flow across much of the waters this afternoon will become east-southeast tonight as front lifts north-northwest. Increasing low level jet and tightening gradient, as low develops on the front over GA/SC, will lead to increasing winds late tonight and Mon. Winds will back to more easterly late in the period as surface low emerging from the Bahamas approaches from the southeast. Although onshore flow may remain just under 25 kt the onshore component combined with a 3 to 4 ft swell will build seas over 6 ft late tonight and Mon. SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... As of 300 PM Sunday...Advisory in effect Monday night if not longer as not only surface and upper low move overhead but a swell hits the areas from a system that had lifted out of the Bahamas. This system phases in quickly with the western low by Monday evening. Pinning down wind speed and direction will be tricky during the Tuesday predawn hours depending on exactly where the system heads as its movement will be erratic for a time before it starts to accelerate northeastward Tuesday as Tuesday progresses. Seas will be slower to abate especially in light of the swell/wind wave interaction. The advisory will likely be lowered late Tuesday or Tuesday night, earlier south than north possibly. LONG TERM /WEDNESDAY 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 Wednesday and essentially the remainder of the period. For Wednesday a weak southwest flow will be in place with ten knots or less. For Thursday and Friday expect similar conditions with the southwest flow a bit better defined into basically a summertime pattern. Significant seas will trend in a similar summertime pattern direction with residual 3-5 feet early dropping to 1-3 feet Thursday and Friday. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...Flash Flood Watch from midnight EDT tonight through Tuesday morning for SCZ017-023-024-032-033-039-053>056. NC...Flash Flood Watch from midnight EDT tonight through Tuesday morning for NCZ087-096-099-105>110. MARINE...Small Craft Advisory until midnight EDT Monday night for AMZ254-256. Small Craft Advisory until 8 AM EDT Tuesday for AMZ250-252. && $$ SYNOPSIS...99 NEAR TERM...REK/III SHORT TERM...MBB LONG TERM...SHK AVIATION...REK MARINE...REK/III/MBB/SHK

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