Area Forecast Discussion
Issued by NWS Wilmington, NC

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399 FXUS62 KILM 200402 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 1202 AM EDT Sun May 20 2018 .SYNOPSIS...
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Scattered showers and thunderstorms can be expected much of the upcoming week, as humid air of tropical origin remains across the region. Rain chances will increase late Wednesday into Thursday, as a cold front slowly crosses the coast. High pressure to the north, may bring a brief drying trend Friday and Saturday.
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&& .NEAR TERM /THROUGH SUNDAY/...
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As of 845 PM Saturday...The tropical moisture plume is currently getting shunted westward. This a result of Bermuda high pressure also ridging westward and onshore across South Carolina. Will re-orient this plume, via the latest edited hier Pops channel, across coastal SC and and into NC. The HRRR is indicating a nocturnal increase in convective activity along this plume overnight into Sun daylight morning hours that could affect the eastern FA with additional pcpn. The threat for widespread heavy rain is over with but still could see isolated heavier showers later overnight. Tonights lows have been tweaked hier by 1 to 2 degrees, especially at the coast where 70s dewpoints will be common. Previous....................................................... As of 3 PM Saturday...Tropical moisture will continue streaming over the region into the evening, maintaining high rainfall chances for the next several hours. Widespread showers are likely with isolated embedded thunderstorms into the evening hours. Later this evening a subtle westward shift in the surface and mid-level ridge off the Southeast coast will disrupt the tropical moisture feed. It will also lead to an increase in mid-level subsidence over the area and shunt the vorticity stream farther west. These factors should bring about a 24 hour or so period of decreasing precip chances despite precipitable water levels still running between 1.6 and 1.8 inches. South to southwest flow overnight will weaken somewhat but boundary layer winds will remain 15 to 20 kt. This should keep fog from being an issue in most areas overnight. Drying trend continues on Sun although showers and thunderstorms will remain a possibility, especially in the afternoon. Precip chances drop into the chance range Sun with the best chances looking to be along the SC coast. Weak low level convergence and some weak divergence will support some convection in this area. Across the forecast area there could be breaks of sun, which could produce differential heating boundaries as well as help to increase diurnal instability. Any storms that develop will continue to have the potential to produce very heavy rain for brief periods of time. Warm cloud layer depth will be around 11k ft. While not as high as today, this is still sufficient to produce very heavy rainfall. Slow storm motions will increase the flooding potential on Sun, especially in the afternoon. Forecast soundings drop storm motion under 10 kt by SUn afternoon. Highs will be in the lower 80s, but mid 80s will be possible with breaks of sunshine.
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&& .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/... As of 300 PM Saturday...Subtle improvement forecast for the beginning of the week. Persistent Bermuda High will expand westward causing the weak upper low to the west to weaken. This occurs in conjunction with advection of drier air from the east, with PWATs dropping below 1.5 inches for the first time in several days. These two features combined with at least weak subsidence as the ridge expands, will bring a decrease in shower activity during the short term. Southerly, tropical, flow will still be capable of producing convection, mostly diurnally forced inland with nocturnal increase possible at the coast, but total coverage will be less than the past several days. Less cloud cover and the increased thicknesses will allow highs to climb into the mid 80s inland, low 80s near the coast, on Monday. Mins both nights will remain well above normal, around 70 as the humid airmass persists. && .LONG TERM /TUESDAY THROUGH SATURDAY/... As of 300 PM Saturday...Guidance differs considerably in the extended with how much rain may fall during the middle of the week, leading to lowered confidence in this part of the forecast. The GFS seems to be the wettest Tue-Thu, while the CMC/ECM are much drier across the Carolinas. The synoptic setup is comparable through each model, with the key difference being the timing and duration of moisture return as model PWATs vary. The GFS may be a bit too wet, but deserves at least some credence due to the pattern remaining similar to the past week, and the moisture tap from the tropics remaining in place. Will maintain CHC POP Tue-Thu, with Tue likely being the driest of the 3, and Thursday potentially the wettest thanks to a cold front which will drop into the region and serve as a focus for convection. Regardless of how many showers develop, there will be a good chance for showers/iso tstms each day Tue-Thu with temps slightly above climo for highs, and well above for lows. Front will weaken and dissipate friday which should lead to lowered POP chances as high pressure tries to build into the region for at least one day. However, this drier air may again be short-lived. All extended models suggest a low pressure of tropical origin will develop near the Caribbean and move slowly in that region. This will again send a plume of tropical moisture towards the Carolinas bringing a return to unsettled and humid conditions with continued slightly above normal temperatures. && .AVIATION /04Z SUNDAY THROUGH THURSDAY/... As of 00Z...Stagnant tropical pattern will continue through the period though the tropical connection through FLA and beyond looks to be weakening on satellite and radar attm. While there still is potential for on and offer showers through the period, activity should be more off and on than it was today. Even though MVFR conditions are expected to dominate through the period intermittent IFR is possible tonight with intermittent VFR possible after sunrise. Extended Outlook...Periods of MVFR/IFR cigs/vsbys associated with tropical type showers and tstorms through the period. Activity will be more numerous along the coast pre-dawn hours thru mid-morning hours each day, then translate to the inland terminals midday-early evening. Possible break in the action on Mon, but rain returning Tue and Wed. Finally an air mass change on Thu. && .MARINE...
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NEAR TERM /THROUGH SUNDAY/... As of 845 PM Saturday...Extended the SCA from midnight to noon Sunday. The SE-S winds that extend across a large fetch from well upstream to and across the local waters, has resulted in significant seas in the 4 to 7 foot range with 8 to 9 second periods dominating. The sig. seas, power wise via spectral density plots, are largely made up of the SE ground swell exhibiting 8 to 9 second periods due to the spoken fetch. It will take a bit more time for seas to lower even though winds across the local waters remain below SCA thresholds. Coverage of nocturnal showers and isolated tstorms will increase during the pre-dawn hrs and persist into the daytime hrs. Vsby will temporary lower to 1 nm or less within the heavier pcpn. Previous...................................................... As of 3 PM Saturday...Small Craft Advisory remains in effect for all zones this afternoon. Current hazard runs through midnight and latest wind/wave forecast meshes well with this. The Bermuda High starts to expand west this evening, which weakens the gradient and helps shift winds to more of a southerly direction. The decreasing wind speeds should allow seas to fall under 6 ft late tonight. Winds become more southwest during Sun, but speeds will be under 15 kt. Seas 3 to 6 ft through tonight will drop to 3 to 5 ft after midnight and by the end of the period will be 2 to 4 ft. SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/... As of 300 PM Saturday...High pressure offshore will continue through early next week although with slightly stronger ridging in place. This will persist S/SW winds across the waters, but a weak gradient will keep speeds at around 10 kts. These light winds will allow a SE swell due to long duration fetch around the offshore high to be the predominant wave group, with seas of 3-4 ft expected throughout. LONG TERM /TUESDAY THROUGH THURSDAY/... As of 300 PM Saturday...Continued high pressure offshore will drive S/SW winds of 10-15 kts across the waters Tue and Wed. Early on Thu, a weak front will drop into the area turning winds to the W/NW Thu morning, before this front dissipate leaving a weak gradient and variable winds through the end of the period. Seas of 3-4 ft will continue Tue/Wed thanks to a SE swell and SW wind wave. However, as the winds ease and become variable Thursday at the same time the swell begins to deamplify, wave heights will drop to around 2 ft late.
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&& .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...Small Craft Advisory until noon EDT Sunday for AMZ250-252-254- 256. && $$ SYNOPSIS...MJC NEAR TERM...DCH/III SHORT TERM...JDW LONG TERM...JDW AVIATION...RAN

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