Area Forecast Discussion
Issued by NWS Wilmington, NC

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785 FXUS62 KILM 170457 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 1255 AM EDT Sat Mar 17 2018 .SYNOPSIS... A dry cold front will continue to slip south farther into South Carolina, stalling near Charleston tonight and returning north again Saturday morning. This same front will continue to waver across the Carolinas through early next week. A strong area of low pressure is expected to develop inland Monday night and Tuesday, bringing good chances of showers and thunderstorms and finally pushing the front back through Tuesday night. Colder temperatures will build in Wednesday through Friday of next week. && .NEAR TERM /THROUGH SATURDAY/... As of 900 PM Friday...Previous forecast looks on track as a weak cold front slips south farther into SC. Winds have shifted north and diminished just south of the border of NC/SC. Overall, expect near calm winds overnight with only some passing cirrus. The shallow cool air advecting in from the north will create a gradient in temps with mid 40s over northern tier counties to near 50 at the Grand Strand. Previous discussion: The cold front will shift very slowly southward this evening, encroaching upon a ridge of surface high pressure centered over the Peninsula of Florida. As this front sinks southward, winds will shift to to the north. The position of this front is the biggest question mark of the forecast period, although guidance has come into better agreement with its evolution. The front will sag southward through the overnight, its motion inhibited by becoming boundary-parallel to the mid-level flow. High res guidance suggests the front will sink nearly to Charleston late tonight before retreating quickly back to the north in response to subtle shortwave ridging and increasing southerly flow ahead of a shortwave and surface low development near the OH VLY. With mid-level flow remaining mostly zonal, the retreat of this front may be modeled too fast in guidance and have slowed the wind shift to the south and warming of temperatures a bit from the models. Of course, if the front doesn`t sag as far south as current thinking suggests, temps could warm very quickly as noted by the SPC HREF guidance, and it remains a tricky frontal position forecast for Saturday. At the same time, a weak mid-level impulse will push across the area during the aftn, noted both by PVA and increasing saturation in the 850-600mb layer. While PWATs only climb to around 1 inch, and very dry air exists above this layer, there appears to be enough agreement and moisture that some scattered showers are possible across the area Saturday aftn and eve. Have capped POP in low-chc and QPF should be less than 0.l inches, but this increasing moisture in the form of clouds/showers will also have the potential to limit warming south of the front. Have used a blend of guidance for highs Saturday, with low to even mid 70s likely far southern zones, cooling to the upper 60s in the northern tier. This will be a significant warmup from mins tonight dropping in the low to mid 40s. && .SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/... As of 300 PM Friday...Low-level water vapor and mosaic radar overlays depict well the 995 mb low swirling over Kansas this afternoon, with pre-frontal showers and storms raking eastward across a marginal risk area from Mississippi into Alabama. This low will track off Cape Fear as a 1010 mb low prior to daybreak on Sunday. The sensible weather implications, include a mild Saturday with increasing clouds, and a chance of showers in the afternoon and night, before a short-lived drying and cooling trend on Sunday. The onset of strong warm air advection will begin late in the period, as another powerful low moves into the central plains late Sunday. A fair amount of CAPE is present Saturday night, but dry air in the mid levels appears too deep and will likely cap growing Cu, but a few showers should debut. Dewpoints will peak in the mid 50s Saturday night. A low QPF system < 0.10" most areas. Sunday on the back side of the low, low clouds and cooler temperatures with tapering rain, a few periods of drizzle may be seen early Sunday, and Max-t about 10-15 deg cooler than Saturday. && .LONG TERM /MONDAY THROUGH FRIDAY/... As of 300 PM Friday...Both the GFS and ECMWF develop another cold and deep trough across the eastern U.S. by the end of next week. The particulars of how we get that trough here are different enough to produce sensible weather differences across the Carolinas Monday through Tuesday. A well-defined shortwave will move east across the Ozarks Tuesday. The GFS shows enough ridging off the East Coast to deflect this feature into Indiana/Ohio Tuesday, while the flatter flow shown on the ECMWF allows the shortwave to move into Virginia. Surface high pressure off the East Coast should be lifting a warm front northward along the South Carolina coast. On the ECMWF, the shortwave is close enough geographically to the front to induce surface low pressure to develop along the front. This scenario would lead to better rain chances here on Monday, plus cooler temperatures with a slower northward progression of the front. Strong upper level northwesterly winds across the Rockies and Plains will carve out an increasingly deep trough across the lower Mississippi Valley Monday night into Tuesday. Surface low pressure should intensify across the southern Appalachians in response to this deepening trough, and the warm front should lift northward through North Carolina allowing us one full day in the warm sector Tuesday. Highs should reach well into the 70s Tuesday with breezy SW winds, along with waves of showers and thunderstorms possible. SPC`s day 4 (Monday) severe weather outlook includes a risk area across GA/TN/AL, and we`ll monitor for the potential for strong storms here as this system propagates into the Carolinas Monday night into Tuesday. Tuesday night the strong surface low inland should move eastward and off the Virginia coast. The trailing cold front will move through our area with colder air building in on northwesterly winds. Cold air aloft with the trough won`t allow for quick clearing, and it`s possible we`ll maintain a cloudy/showery pattern through Wednesday, along with much chillier temperatures. Once the upper trough clears offshore Thursday we should dry out, but may also have to deal (yet again) with potential freezing temperatures as the surface high builds in from the west. For now I`ve forecast lows as cold as the mid 30s across much of our area for Thursday night, but with light nighttime winds it`s quite likely there will be some areas below 32 degrees given this pattern. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
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As of 06Z...VFR is expected through this forecast period. A cloud deck of between 6000 and 8000 feet is expected on by Saturday afternoon and evening. Winds will be light overnight but as a old front pushed northward wins will increase from the southwest to 10 to 15 knots with gust in the 20 to 25 knot range. Extended outlook...Periods of MVFR Sunday through Wednesday.
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&& .MARINE... NEAR TERM /THROUGH FRIDAY/... As of 900 PM Friday...Winds were gusting still at the outer buoys up to 20 kts, but overall winds and seas were on the downswing as weak cold front eases its way southward through the waters tonight. Previous discussion: This front will be accompanied by a wind shift to the N/NE with a weak surge to 10-15 kts briefly, before the front lifts back to the north Saturday. This will bring a return to a S/SW wind direction, with speeds increasing gradually through the day to around 20 kts late and becoming gusty through the aftn hours. Seas of 3-5 ft will fall on the decreasing winds to 2-3 ft, and although these amplitudes will be low, the spectrum will likely become confused as a variety of wave groups exist. During Saturday, a SW wind wave will again become predominant, with seas climbing to 3-5ft, and then likely above SCA thresholds just beyond this period. An SCA may be required before Saturday is over. SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/... As of 300 PM Friday...A difficult marine period, as low pressure brings gusty pre-frontal SW winds, then gusty N winds Sunday as the low moves offshore of Cape Fear by sunrise. At a minimum, a `Exercise Caution` statement will be needed Saturday night and early Sunday. If frequent gusts of 25 KT are expected, then an `Advisory` will be needed. Conditions will improve, but not until Sunday afternoon, with NE winds 10-15 kt, so still a bit bumpy offshore, and north of Cape Fear. Seas may reach 6 feet across the outer waters Saturday evening. No TSTMS expected except along and east of the west wall of the Gulf Stream. LONG TERM /MONDAY THROUGH WEDNESDAY/... As of 300 PM Friday...It will be a stormy start to next week with a couple areas of low pressure expected to affect the weather across the Carolinas. The first low is expected to develop along a warm front moving north along the South Carolina coast Monday. This low will slip offshore Monday night. A second much stronger area of low pressure is expected to develop inland Tuesday, drawing the warm front north of our area and allowing breezy southwesterly winds to develop. As this low moves offshore Tuesday night it will push the front back through the area as a cold front, with colder northwesterly winds expected to develop for Wednesday. Given this pattern, there is increasing likelihood we`ll need a Small Craft Advisory for Tuesday and Tuesday night for winds and seas. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...None. && $$ NEAR TERM...JDW/RGZ SHORT TERM...MJC LONG TERM...TRA AVIATION...RH is the U.S. government's official web portal to all federal, state and local government web resources and services.