Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 300157 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 957 PM EDT Wed Mar 29 2017 .SYNOPSIS... Relatively cooler and drier high pressure will build in from the north tonight thru Thursday then shifting offshore Thursday night. Showers and thunderstorms are expected late Thu night thru Friday as milder and more humid air returns ahead of an approaching cold front. The cold front will sweep across the area and offshore late Friday night. Some of the thunderstorms may be strong to severe during Friday. The weekend should be dry as high pressure takes hold. By early next week, the next low pressure system and associated cold front will produce showers and thunderstorms as it crosses the area. && .NEAR TERM /THROUGH THURSDAY/... As of 930 PM Wednesday...Will go with partly cloudy skies thruout for the overnight period due to high level clouds moving over the low amplitude upper ridge axis that nearly lies overhead. Some mid-level ac could also affect the SC portions of the ILM CWA. Will also have to keep an eye on the low stratus clouds that may expand northward into the ILM SC Counties overnight. With an active ENE to E wind overnight, radiational fog should not be an issue. In addition, no POPs are in the fcst for tonight. Very little if any tweaking needed for tonights lows, unless, the deeper cloudiness expands northeast across the ILM NC CWA. Previous..................................................... As of 3 PM Wednesday...Surface high pressure extending south across the eastern Great Lakes and into the Carolinas is coincident with a deep mid and upper level ridge. All of these ridging features will traverse the area from west to east overnight. Precipitable water values in the 0.8 to 0.9 inch range tonight are in the 60-75th percentile for this time of year, indicating this is not a particularly dry airmass. Plenty of daytime cumulus that has developed west of the seabreeze should dissipate this evening, however enough moisture should remain trapped beneath the subsidence inversion up around 6500 feet AGL that I don`t think we`ll see cloud- free skies for any significant length of time. Given the presence of this moisture and also low-level winds expected to not go calm tonight due to the distant position of the surface high, I have bumped forecast lows up by a degree or two to 54-57 over NE South Carolina, and 50-55 over SE North Carolina. As the deep layer ridge moves offshore Thursday, winds will veer more southerly above the surface. Gulf moisture arriving in the 850- 700 mb layer during the afternoon could become thick enough to yield a few showers back in the South Carolina Pee Dee region before sunset. This should not be a substantial precipitation event during the day however -- that will hold off until later Thursday night into Friday. Thursday`s highs should range from 70-74 inland, with mid to upper 60s within 5-10 miles of the coast. && .SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/... As of 3 PM Wednesday...Southern stream shortwave moving across the Mississippi Valley Thu night into Fri will lift a warm front across the area early in the period. Front is likely to be accompanied by showers and possibly an isolated thunderstorm but strong storms seem less likely given the evening timing and weak dynamics. Best chance for widespread rainfall still appears to be between 09Z and 15Z Fri, prior to the cold front moving into the area. Although the timing is less than ideal there will be an abundance of deep moisture, precipitable water values rise from around 1" Thu evening to 1.5" Fri morning, and plenty of forcing. Height falls and low level convergence ahead of the front should help produce deeper convection, which will have a 40-50 kt low level jet to tap into. The biggest limiting factor will be the lack of surface based instability given the early morning timing. Cannot rule out strong or severe storms but at this point the potential is on the low side, hence SPC`s marginal risk. Do think heavy rain is almost guaranteed for a couple hours Fri morning and will continue with inherited categorical pop. Southerly flow in the wake of the warm front Thu night and the first part of Fri will keep temps above climo. Lows will be in the lower 60s Fri morning with highs around 80 Fri afternoon. Cold front pushes across the area during Fri with guidance showing a rather impressive dry slot wrapping around the system aloft. Low level moisture will linger into early afternoon and may support some light rain/showers, but the significant precip should end by noon. Although the cold front passes later Fri there is not much in the way of cold air behind the front and lows Fri night will still end up about 10 degrees above climo. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... As of 300 AM Wednesday...In the wake of this system, dry weather and above normal temps are expected for the weekend as mid- level ridging builds across the area and surface high pressure to our N ridges S. Attention then will turn westward as next potent southern stream system along the Gulf Coast Sun night lifts to the NE and drags a warm front to the N. This will again bring deep moisture into the Carolinas, and with that, showers and thunderstorms early next week. Timing differences have decreased slightly and it looks like the greatest risk for showers and thunderstorms will be Mon night. Showers and thunderstorms should be increasing from the SW and W Mon afternoon with the risk decreasing from W to E during the day Tue as broad low pressure consolidates offshore of the Mid- Atlantic states Tue and Tue night. Drier air will work into the eastern Carolinas during Wed. && .AVIATION /02Z THURSDAY THROUGH MONDAY/... As of 00Z...VFR conditions thru the 00Z valid 24 hr TAF Issuance period. However, 1 of the favorite aviation models indicates a possible IFR ceiling below 1k ft during the pre-dawn Thu hours across the Myrtle Beach sites. Until additional models indicate this possibility or model run to run consistency, will just indicate a FEW/SCT007 for just the MYR Terminal at this time. Ridging will extend down the east coast of the U.S. resulting in a somewhat tightened sfc pg across the area. As a result, looking at NE to ENE winds at 5 to 10 kt tonight, hiest across the coastal terminals. The sfc pg will tighten-some Thu aftn veering the winds to the ENE to ESE at 10 to 15 kt. From Thu midday thru Thu evening, will see VFR ceilings initially that slowly drop to MVFR thresholds by or during early Thu evening. Extended outlook...Areas to widespread MVFR/IFR conditions later Thu night thru Fri afternoon due to showers and strong to severe thunderstorms. Possible MVFR showers Fri night. VFR conditions Sat thru early Mon. Possible MVFR/IFR conditions late Mon due to the approach and passage of a low pressure system. && .MARINE... NEAR TERM /THROUGH THURSDAY/... As of 930 PM Wednesday...The main high pressure center id located over southeast Canada tonight. The ridging from it`s center extends down the U.S. East Coast, with the ridge axis nearly overhead. The result is not a true NE wind, rather than a good Easterly component within that wind field. Overall, ENE to E winds at 10 to 20 kt due to a somewhat tightened sfc pg. Significant seas will run 3 to 5 ft ILM NC Waters and 2 to 4 ft for the ILM SC Waters. A slow decaying southeast ground swell at 9 to 10 second periods continues to help keep overall seas somewhat elevated. The latest spectral density charts from NDBC still indicate plenty of power associated with the swell when compared to the 4 to 6 second wind driven waves. Previous................................................ As of 3 PM Wednesday...A ridge of high pressure extends southward across the Great Lakes and into the eastern Carolinas. This ridge should move eastward and off the coast late tonight and early Thursday morning. With the center of the high so distant from the Carolinas we will not have a period of light winds. In fact, wind speeds will increase to a hefty "moderate" this evening: 15-20 kt with seas building to 4-5 feet away from the coast. A `Small Craft should exercise caution` headline will be appended to the forecast through midnight north of Cape Fear. Thursday as the ridge moves farther off the East Coast wind directions should veer more southeasterly with time with speeds averaging 15 kt or less. SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/... As of 3 PM Wednesday...Onshore flow Thu night will become southerly very early Fri as a warm front lifts north of the waters. Gradient starts to tighten behind the front with southwest flow increasing to around 20 kt by Fri morning. Southwest flow remains a solid 20 kt Fri before passage of cold front brings a slight reduction in the gradient, dropping offshore flow to 10 to 15 kt late Fri night. Seas build Thu night in response to increasing southwest flow. At the start of the period seas will run 2 to 4 ft but quickly build to 4 to 6 ft overnight, peaking in the 4 to 7 ft range later Fri. Offshore flow behind the exiting cold front will drop seas below 6 ft around midnight. Anticipate at least a 12 to 18 hour stretch of SCA conditions during the period. LONG TERM /SATURDAY THROUGH MONDAY/... As of 300 AM Wednesday...Small Craft Advisory conditions possible late on Mon and Mon night. Low pressure will be intensifying south of New England Sat morning. Its attending cold front will be offshore by the start of this forecast period. Low pressure will continue to intensify as it moves E and then NE during the weekend. High pressure centered across eastern Canada will build south across the Carolina waters during the weekend with the ridge axis moving offshore Sun night. Next in series of potent southern stream systems will be approaching from the W on Mon. Its attending warm front is expected to move across the waters Mon night. NW winds will generally dominate Sat although an inverted trough poking N may cause winds to back, especially across southern waters ahead of reinforcing cold and dry push. Winds late Sat night and Sun in the wake of this push will be NE. The wind direction will veer to E later Sun afternoon and then ESE Sun night. The wind direction will become SE during Mon. The highest wind speeds during this period are expected on Mon when they are expected to increase to 20 to 25 kt late day and Mon night. Wind speeds Sat through Sun night should be mainly 5 to 15 kt. Seas will be 3 to 4 ft Sat morning, subsiding to mainly 2 to 3 ft Sat night and then 1 to 2 ft Sun. There will be a tendency for backswell to slow the slow the rate of subsiding seas through the first part of the weekend. Expect seas will reverse higher late Sun night and Mon, possibly reaching Small Craft Advisory levels Mon night. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...None. && $$ SYNOPSIS...TRA NEAR TERM...DCH/TRA SHORT TERM...III LONG TERM...RJD AVIATION...DCH

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