Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 211818 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 218 PM EDT Tue Mar 21 2017 .SYNOPSIS... A cold front will move through the Carolinas late tonight, accompanied by scattered showers and thunderstorms. Cold Canadian high pressure will build southward from the Great Lakes Wednesday night through Thursday night, perhaps bringing freezing temperatures to some inland locations. The high will move offshore Friday with a warming trend expected over the weekend, along with an increasing chance of showers or thunderstorms by Sunday. && .NEAR TERM /THROUGH TONIGHT/... As of 1030 AM Tuesday...Minimal changes for the day/afternoon period. Weak shortwave has pushed off the coast with a little bit of subsidence lingering in its wake. This should be sufficient to suppress any upward motion into the afternoon. Mainly concern would be along the sea breeze which should remain pinned to the coast by west to northwest flow just above the surface. Temperatures will run well above climo everywhere except the immediate coast, due to the aforementioned sea breeze, with westerly/downslope flow and abundant sunshine pushing highs in the upper 70s to lower 80s. Severe weather later this evening and overnight remains a possibility as a stronger shortwave and cold front drop in from the northwest. Discussion from 300 AM follows... Thereafter, concern turns towards potential severe weather late tonight. The aforementioned cold front will drop slowly southward, taking its time to move through the area as it becomes aligned nearly boundary parallel to mid-level flow. As this front sags into the CWA, a potent vorticity impulse rotating through the main longwave trough will move overhead overnight, and the interaction of this feature with the baroclinic zone below will spawn weak cyclogenesis and convection. The surface wave will be weak, only deepening once it gets offshore on Wednesday, but convection beneath the shortwave will affect the area tonight. SPC has much of our CWA in a MRGL risk for severe, and while surface based instability will be lacking due to a nocturnal event, other parameters suggest at least a chance for strong storms. Elevated CAPE of 600-1000 J/kg thanks to steep lapse rates above 7C/km between 800mb and 500mb will support strong updrafts capable of hail, while unidirectional bulk shear values rise to 30-40 kts, supportive of multicell structures. These multicell clusters will move quickly along the front from W to E, and downshear corfidi vectors suggest motion of up to 40 kts, with this forward momentum aiding the potential for strong wind gusts already in place by 400-600 J/kg of DCape. If this was an aftn event, would be supportive of a more significant severe risk, but very stable air at the surface will preclude more than isolated damaging winds, while decreasing updraft intensity and scattered nature of storm coverage limit a more widespread hail risk. The best chance for convection will likely be in the 8pm- 2am timeframe, with the highest risk for severe across the Pee Dee, but again, only isolated strong storms are expected. Once convection moves offshore, the weak surface low will drag the cold front further south, finally pushing through the entire CWA by Wednesday morning. Continued cloud cover and only a short duration of CAA behind the boundary will prevent rapid cooling, so mins Tuesday night will remain above normal, low 50s far north, to around 60 along the South Santee River. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... As of 300 AM Tuesday...A potent upper level disturbance should be exiting the Carolina coast around sunrise Wednesday. Surface low pressure along a southward-moving cold front will accompany the upper system eastward. While it was looking over the past couple of days like there would be significant clouds remaining behind the front within the frontal inversion for most of Wednesday, latest models show a somewhat drier airmass aloft which should allow clouds to fragment during the afternoon hours. Despite cold advection the strong late-March sunshine should be able to push temps well into the 60s if not near 70 degrees over the Pee Dee region, with slightly cooler readings forecast along the coast and over SE North Carolina. It`s worth noting there are still significant inter-model timing differences with the onset of cold advection Wednesday. Historically models have struggled with accurately depicting temperatures during a cold frontal passage around sunrise, and any meaningful delay in cold advection could lead to temperatures far surpassing our latest forecast Wednesday. Cold advection will definitely be ongoing by Wednesday evening. 850 mb temps plunging to near 0C could bring surface temps close to freezing across portions of SE North Carolina. After three freezes last week the last thing area berry farmers need is another freeze...but it`s looking quite possible. Average winds in the Sfc-1000 foot layer should remain 15-20 kt with fairly steep low-level thermal profiles, suggesting advection rather than radiation will dominate this possible freeze event. Dewpoints plunging deep into the 20s will make frost too unlikely for mention in the forecast. Thursday: we are ignoring the NAM and its impact on the SREF for the 00z cycle, instead going with the dry GFS, ECMWF, and Canadian models. A region of 700 mb moisture will overspread the region, particularly during the morning hours, however plenty of sub-cloud dry air and lack of any meaningful isentropic lift within the moist layer aloft should preclude precipitation. There will be an impact on temperatures from the clouds however, with highs probably holding in the lower to mid 50s for most areas. Another chilly night is expected Thursday night as winds die away and radiational cooling overcomes developing warm advection to again drop lows into the 30s across much of the area. Pender and Bladen counties in SE North Carolina again stand the best chance of seeing freezing temperatures. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 300 AM Tuesday...High pressure off the East Coast should produce a warming trend through the period. Highs in the 70s and lows in the 50s are expected through the period. The best chance of showers or t-storms will probably be late Sunday as a weakening upper system shears out through the Great Lakes but drags a ribbon of moisture and modest lift through the Carolinas. It`s getting close to that time of year where daytime instability could become a large enough factor to pop some showers or t-storms, and Sunday might be our first bout of that for the year. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/...
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As of 18Z...VFR conditions will prevail through 00Z before disturbance aloft and surface cold front moving into the area lead to development of showers and thunderstorms. Although some of the storms could be strong, especially across inland SC, the potential for damaging winds seems less while hail remains a little more of a concern. Overall chances for severe weather remain low but occurrence cannot be ruled out. Regardless of damaging winds or hail do expect to see brief gusty winds ahead of the cold front crossing the area as well as occasional lightning and the possibility of heavy rain. Showers may linger through 12Z Wed but intensity will be weakened. Brief periods of IFR will be possible between 02Z and 08Z but do not anticipate any extended IFR or lower vis/ceilings through the overnight. Low clouds and showers may linger into mid morning Wed, especially along the coast before reinforcing cool/dry air moves into the region on the back of increasing northerly winds later Wed. Extended outlook...VFR expected through the period.
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&& .MARINE...
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NEAR TERM /THROUGH TONIGHT/... As of 1030 AM Tuesday...Pinched gradient between high pressure sinking south of the waters and advancing cold front to the northwest will lead to southwest flow around 15 kt this afternoon and evening along with gusts in excess of 20 kt. Continued and increasing southwest flow this afternoon will build seas this morning from 2 to 4 ft to 3 to 5 ft prior to the passage of the cold front. Winds become variable as the front moves into the area overnight. Convection moves over the waters just ahead of the boundary, worsening conditions. Front pushes south of the area overnight and west-northwest flow becomes established and starts increasing as the boundary exits. SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... As of 300 AM Tuesday...Low pressure just off the coast should be driving a cold front southward across the South Carolina coastal waters when the sun comes up Wednesday. The low is not expected to strengthen appreciably, however the strong Canadian high moving across the Great Lakes will more than make up for the modest strength of the low. Strong northeast winds should develop by Wednesday night and continue through most of Thursday. A Small Craft Advisory will almost certainly be needed for this period for 25+ knot winds and 6-8 foot seas across open waters exposed to northeast winds. By Thursday night the high should move off the Mid-Atlantic coast, with winds veering a bit more easterly and diminishing in speed. LONG TERM /FRIDAY THROUGH SATURDAY/... As of 300 AM Tuesday...Canadian high pressure off the East Coast will weaken through the period, assuming a position only a bit farther north than the typical summertime Bermuda High by Saturday. Light easterly winds Friday should become more southerly Saturday.
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&& .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...None. && $$ NEAR TERM...III SHORT TERM...TRA LONG TERM...TRA AVIATION...III

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