Area Forecast Discussion
Issued by NWS Wilmington, NC

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672 FXUS62 KILM 240613 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 213 AM EDT Thu Aug 24 2017 .SYNOPSIS... An approaching cold front will bring thunderstorms into Thursday, some of which may be strong. Canadian high pressure will bring an early fall-like feel Friday and through the weekend. We will be watching the tropics for possible development of low pressure off the Southeast coast early next week. && .NEAR TERM /THROUGH THURSDAY/... As of 915 PM Wednesday...The big convective line is weakening slightly as it approaches the coast of SE North Carolina. Ingesting the very humid but slightly cooler (and therefore initially capped) boundary layer is likely responsible for the storms` recent behavior. Although weaker, they should continue to sag southeastward and to the beaches in the next hour. Heavy rainfall, gusty winds, and frequent lightning are all expected. Redevelopment of storms off to the west from Marion and Johnsonville, SC all the way back through Timmonsville and Camden is a bit healthier than I would have thought two hours ago. Surface parcels are heavily capped back in this region, but recent SPC mesoanalysis graphics indicate central and eastern South Carolina is coming under the influence of a 250 mb jet streak located over the Mid-Atlantic region. Upper level divergence located within the right-entrance region of this jet is probably helping lift elevated parcels starting in the 5000-1000 foot AGL range. There`s no reason to expect this activity to dissipate given its disconnect from the boundary layer and the likely persistence of the upper level pattern. Likely (60-70 percent) PoPs have been expanded across a good portion of eastern South Carolina through early Thursday morning. Discussion from 730 PM follows... A synoptic cold front across central North Carolina extends from Greenville, NC to Fayetteville to near Charlotte and is dropping southward. A large cluster of thunderstorms has developed just ahead of the boundary, taking advantage of uncapped surface-based CAPE of 2500 J/kg. Precipitable water values near 2.0 inches normally doesn`t coexist with large DCAPE values, but in this case just large enough temp-dewpoint spreads exist above the boundary layer to support DCAPE as large as 1300 J/kg -- leading to the gusty winds we have seen in today`s storms. A gust to 55 mph was measured at the ASOS in Lumberton, NC, and trees were blown down in Dillon, SC. Evolution of this event through the evening is shown very well on the last few HRRR runs as well as the 18Z NAM. A massed-line of storms should work down into Wilmington and the Cape Fear region by late evening, joined by scattered showers and storms across the Myrtle Beach area. One weak upper level vort max is supporting our current storms, but a second vort digging across eastern Tennessee and western North Carolina later tonight may begin to back the mean flow just enough late tonight to cause activity to cease its eastward progress and hang up across the coastal waters and perhaps the coastal counties as well. On this latest update PoPs were increased to 70-100 percent for SE North Carolina, with QPF raised to 1+ inches in spots. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... As of 215 AM Thursday...A cooling trend back towards and then ultimately below seasonal readings will commence on Friday as high pressure builds in from the north and a series of waves move NE along a stalled frontal boundry offshore. These waves plus the strengthening high will gradually increase the NEly gradient, allowing daytime highs to drop back into the mid 80s throughout the forecast area on Saturday after a day of mid to upper 80s on Friday. Lows Saturday night will drop into the mid and upper 60s. The real difference will come from the drop in dew points, which will drop back into the lower and mid 60s by Saturday, allowing for a more autumnal-like feel for the first day of the upcoming weekend. A consensus of guidance keeps in slight chance pops for showers nearest the coast for both days in closest vicinity to the offshore front. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... As of 300 PM Wednesday...A stationary front will be located well offshore and S of the area Sat with an area of low pressure positioned on the front, but well offshore. High pressure will be ridging across the Carolinas from New England. The pressure gradient between these two features will keep a fairly brisk NE wind in place across the Forecast Area through the weekend. This will ensure the cool air is continually reinforced and highs during the weekend should only be in the lower to mid 80s and this will be coupled with comfortable humidity levels. Model profiles continue to be quite dry with precipitable water values below climatological normals and thus will not include any mention of a shower or thunderstorm during the weekend. This pattern looks to hold through Sun. The flow becomes more onshore and the column begins to moisten Mon and Tue. There may also be some mid- level shortwave energy impinging on the area from the SW. Will paint more in the way of cloudiness as a wedge likely sets up and introduce the potential for some showers and perhaps a thunderstorm. The probabilities for precipitation will be highest at the coast. The heat is not to be found, even early to the middle of next week with highs only in the lower to mid 80s. Lows will be in the mid 60s to near 70 through the period. As a caveat, we will be watching the tropics early next week to see if a weak disturbance is able to develop somewhere off the SE coast. Confidence in any development is very low as is its eventual track. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... As of 06Z...Messy night continues with an area of heavy rainfall and thunderstorms progressing across the SC terminals. This area of rain is expected to slowly migrate E/SE through the next few hours, eventually moving offshore and eroding. However, the Myrtles will likely see at least TEMPO IFR in TSRA while FLO/ILM experience MVFR in showers. A cold front west of the terminals will begin to migrate E/SE as well, and IFR stratus is forecast to develop behind the boundary as it shifts through LBT/FLO overnight. Due to the presence of this front, winds will be variable at all the terminals overnight, with gusts likely confined to convection at CRE/MYR. After daybreak, stratus will slowly erode and the front will very gradually shift SE, exiting the area only late on Thursday. This will persist light and variable winds, possibly becoming SE at the coast with the sea breeze for a time this aftn, while taking on a NE direction inland. Late morning CU will lift only to MVFR much of the day leading to the development of scattered showers and tstms once again. These will be most likely along and east of the front, but coverage and intensity is expected to be lighter than what occurred tonight. As the front pushes away finally by the end of the period, VFR will gradually develop as NE winds usher in drier air to the region. Extended Outlook...MVFR ceilings could linger, especially near the coast Friday. VFR through the weekend. Showers may increase beginning Monday as a coastal low passes well offshore. && .MARINE... NEAR TERM /THROUGH THURSDAY/... As of 915 PM Wednesday...Storms are moving rapidly toward the beaches of SE North Carolina. They are weakening but are still capable of producing some hefty wind gusts as they move offshore. Please reference the latest Marine Weather Statement for details. Additional storms are developing across central and eastern South Carolina and will likely affect the areas south of Cape Fear closer to midnight and into the early hours of Thursday morning. No changes have been made to synoptic forecast reasoning through the night. Discussion from 730 PM follows... A cold front currently across central North Carolina is moving southeastward and should reach Cape Fear by daybreak Thursday. Steady southwesterly winds around 15 knots ahead of the front should become disturbed by late evening as a large line of thunderstorms across the interior Coastal Plain moves out across the waters. Variable wind gusts in and near these storms should settle down to a weaker, more westerly average wind after midnight. Mariners planning to be out on the waters, especially north of Myrtle Beach, should closely watch these storms now through midnight. SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... As of 215 AM Thursday...A gradually tightening gradient between high pressure building in from the north and a deepening trough of low pressure east of the waters will lead to steadily increasing NEly winds through the period, with Small Craft Advisory conditions likely developing on Saturday for 20 to 25 kt winds and 4 to 6 ft seas. The period will start off a bit more benign, with 10 to 15 kt winds from the NE and 2 to 4 ft seas on Friday morning. LONG TERM /SUNDAY THROUGH MONDAY/... As of 300 PM Wednesday...Persistent and brisk NE winds will likely require a Small Craft Advisory for much of the period. The tightening pressure gradient will be the result of high pressure ridging down into the Carolinas from the N and an offshore front and area of low pressure. NE winds will be around 25 kt for much of the period. Seas will be 5 to 7 ft and perhaps up to 8 ft by Mon. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...None. && $$ SYNOPSIS...REK NEAR TERM...JDW SHORT TERM...REK LONG TERM...RJD AVIATION...JDW MARINE...REK/RJD/JDW

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