Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 191945 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 345 PM EDT Sat May 19 2018 .SYNOPSIS... Periods of heavy rainfall will continue tonight, especially along the coast. High pressure offshore will expand westward into early next week bringing a subtle decrease in shower and thunderstorm coverage Sunday and Monday. A return to a tropical airmass with widespread showers is expected during the middle of next week. A cold front will drop into the Carolinas Thursday. Drier air may finally work into the area late next week. && .NEAR TERM /THROUGH SUNDAY/... 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. && .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 /20Z SATURDAY THROUGH THURSDAY/... As of 12Z...Stagnant tropical pattern will continue through this evening. Current convection affecting most of the region with predominately MVFR conditions with brief periods of heavy rain. Winds will be a bit more westerly this morning but should back to the south again by mid morning. Ceilings again will be fluid, bouncing around from a mid cloud ceiling to near IFR. Convection will continue through the overnight hours. 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. && .MARINE... NEAR TERM /THROUGH SUNDAY/... 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. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...Small Craft Advisory until midnight EDT tonight for AMZ250-252- 254-256. && $$ SYNOPSIS...JDW NEAR TERM...III SHORT TERM...JDW LONG TERM...JDW AVIATION...III MARINE...

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