Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 231922 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 322 PM EDT Fri Sep 23 2016 .SYNOPSIS... High pressure will build southward across the Great Lakes and into the Mid-Atlantic region this weekend, finally bringing drier air into the Carolinas. Moisture and rain chances will return by Tuesday as the high moves offshore. Another cold front will arrive from the west Wednesday, bringing the first dry, cool airmass of this Fall into the area. && .NEAR TERM /THROUGH TONIGHT/... As of 300 PM Friday...Surface low pressure is centered about 20 miles south of Georgetown, SC, and should settle farther south just off the South Carolina coast tonight. This feature has been on our weather maps for a good portion of our intern`s career now! Dense stratus took many hours to burn off earlier today, but once surface temperatures reached 80-81 degrees and uncapped the atmosphere, numerous showers have developed. Weak steering flow within the tropical airmass surrounding the low has led to another day of slow moving heavy rain. The combination of the best instability and low-level convergence has set up across SE North Carolina just inland from the coast. Rainfall rates over 1 inch per hour have so far remained on the west side of US Highway 17, fortunately not aggravating the areas which picked up 5-8 inches of rain and significant flash flooding today. The Flash Flood Watch for Pender and New Hanover counties will remain posted through this evening, at which point the inland showers and storms should be weakening and drifting southward (offshore) behind the departing surface low. With drying mid-levels but very rich low level moisture in place with light winds, fog and low stratus is again expected to develop after midnight. A check of GFS and NAM MOS performance over the past few days shows a persistent -2 degree bias in these products when verified against actual low temperatures, so our forecast lows are about 2 degrees above MOS consensus across the board. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 300 PM Friday...As our low drifts farther south Saturday and high pressure to the north begins to build in, the airmass should dry appreciably from the top down. Precipitable water values in the 1.8 to 2.0 inch range today should fall to 1.5 inches Saturday afternoon and to 1.25 inches Saturday night. Using 850 mb relative humidity as a guide, I am limiting showers Saturday afternoon to just the seabreeze convergence zone of Horry and Georgetown counties in South Carolina, anticipating the airmass will become too dry elsewhere to overcome the convective cap. With skies becoming mostly sunny by late morning, highs should reach the upper 80s inland, 82- 86 closer to the coast. High pressure north of the Great Lakes Saturday will build southward down the Mid-Atlantic coast on Sunday. This should push a cold front through the coastal Carolinas early Sunday. With a building ridge aloft and meager moisture, this front will probably come through dry with only an increase in low-level wind speeds expected. Winds behind the front Sunday should become east-northeasterly, perhaps picking up enough Atlantic moisture for scattered showers to develop and advect onshore. Since this is heavily modified Canadian air that will have spent a thousand miles or so over the warm western Atlantic, low-level lapse rates should be steep which may help support these shallow convective showers. Given the significant post-frontal influence off the Atlantic, I am not forecasting Saturday night and Sunday night low temperatures to fall quite as low as recent MOS guidance would indicate, particularly along the coast. && .LONG TERM /MONDAY THROUGH FRIDAY/... As of 300 PM Friday...High pressure will be to our north on Monday but rather than a wedge-like NE flow wind will remain out of the E. This will keep shower chances in the forecast as Atlantic moisture is handily advected ashore. They may tend to favor coastal areas though as mid level ridging keeps the upper levels of the column quite dry. This onshore flow will turn more southerly with the approach of a cold front on Tuesday, its passage slated for Tuesday night. Wednesday and Thursday could feature the strongest cool advection we`ve seen in a while and bring some unseasonably cool weather with Thursday night possibly slipping below 60 away from the coast. A wedge then sets up on Friday continuing the cool weather. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... As of 18Z...A surface low is S of KMYR with a weak trough extending northward along the coast. MVFR cigs will improve to VFR at KFLO/KLBT by mid afternoon. Showers will still be possible, and therefore will indicate VCSH. VFR cigs are expected this afternoon outside of precipitation at the coastal terminals. Confidence is higher for thunderstorms at the coastal terminals and if a shower/thunderstorm sets up directly over a terminal there could be an extended period of SHRA/IFR/VCTS as the steering flow is very weak. Due to low confidence of timing IFR this is not indicated in TAFs. Have included TEMPO groups indicating MVFR and will amd as necessary. Models suggest the surface low will fill and weak high pressure will build into the terminals overnight. With moist low levels, light winds and sct skies there is increasing confidence of dense fog development. The sub-VFR conditions should improve to VFR around 13Z. EXTENDED OUTLOOK...Mainly VFR. Chance for SHRA/TEMPO MVFR Sunday. Chance for SHRA/TSRA Tue through Wed. && .MARINE... NEAR TERM /THROUGH TONIGHT/... As of 300 PM Friday...Low pressure is centered about 20 miles south of Georgetown, SC, and should slip farther down the South Carolina coast overnight. Cyclonic flow will maintain mainly northeasterly winds overnight, with wind speeds generally 10 knots or less. The bulk of the showers and thunderstorms are located inland this afternoon with only isolated activity over the coastal waters east of Cape Fear. Models suggest that this evening the showers in the Wilmington area could slide south and off the coast, migrating farther offshore overnight. Seas are still 2-3 feet, mainly in easterly 12-second swell being produced by distance Tropical Storm Karl. Combined seas could build to as high as 4 feet in the Cape Fear area coastal waters tonight. SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 300 PM Friday...Surface high pressure north of the Great Lakes on Saturday will build southward down the Mid-Atlantic coast Sunday. This should push a cold front through the coastal Carolinas early Sunday. Winds both ahead and behind the front will be northeasterly, and since the front will come through dry the only change heralding the front`s arrival will be an increase in wind speed. Winds Saturday and Saturday night around 10 knots should increase to around 15 knots for Sunday. These winds, in combination with swells from Tropical Storm (or Hurricane) Karl out near Bermuda, will build seas to 3-4 feet with some 5-footers probably lurking not too far from 20 miles from shore near Cape Fear. LONG TERM /MONDAY THROUGH WEDNESDAY/... As of 300 PM Friday...Monday starts off with easterly winds as high pressure is centered well to our north. This flow will then veer as the day wears on as the high progresses eastward and a cold front approaches from the northwest. This boundary comes through Tuesday night and it will usher in an increasingly strong NE surge of cooler air. Ahead of the front on Monday seas will be quite manageable and any prefrontal increase in seas Tuesday should be quite gradual. This may not hold true with and following FROPA Tuesday night into Wednesday where headlines may become necessary. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...Flash Flood Watch until 9 PM EDT this evening for NCZ105>108. MARINE...None. && $$ SYNOPSIS...TRA NEAR TERM...TRA SHORT TERM...TRA LONG TERM...MBB AVIATION...RGZ

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