Area Forecast Discussion
Issued by NWS Wilmington, NC

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220 FXUS62 KILM 250549 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 149 AM EDT Tue Apr 25 2017 .SYNOPSIS... A complex and slow moving storm system will bring heavy rain and the potential for flooding tonight. This system will lift away from the area Tuesday with a return to dry weather and above normal temperatures during the mid to late week time frame. Expanding high pressure aloft and at the surface, will provide mainly clear skies with warm and dry conditions thru the upcoming weekend. High temperatures may break into the 90s away from the immediate coast during this weekend. && .NEAR TERM /THROUGH TODAY/... As of 8 PM Monday...Rain shield gradually retreating to the north with heavy rain and concurrent flooding threat now confined to Pender County. Expect current trend to continue with high-res guidance showing flash-flooding threat to end by around midnight or so, with more scattered showers thereafter. Previous discussion follows: Impressive rain event continues to unfold with 5h low and surface inflection over eastern GA/SC continuing to pump moisture into the region. Forecast soundings continue to keep precipitable water values near all time highs, over 1.7 inches, this afternoon and evening. Water vapor imagery starting to show hints of another Gulf/Western Atlantic moisture feed. A similar signature was evident in water vapor this morning as the moisture tap that ultimately ended up dropping copious amounts of rain across portions of SC developed. Based on the location of the surface and mid level low, should this signature develop further, significant rain and potentially flooding would develop across northeastern SC and southeastern NC. Strong divergence aloft and developing surface low will help pull the low level moisture feed over the front stalled along the coast. The combination of lift associated with the surface boundary and dynamical forcing should help maintain rainfall rates of 1-2 inches per hour across portions of the forecast area this afternoon and evening. The potential fly in the ointment remains the second surface low well east of Savannah. Guidance is split with the evolution of this low. The NAM depicts very limited interaction between the offshore low and the quasi-stacked low over the southeast. This scenario would lead to lower rainfall amounts through the late afternoon/evening hours and an earlier end to the bulk of the rainfall. The GFS/Canadian show much more interaction between these features, so much so that the GFS/Canadian have the surface low moving onshore close to Cape Fear while the NAM moves it onshore at Cape Hatteras. The scenario will hold onto the heaviest rain a few hours longer, with the bulk of the activity winding down around midnight. The 12Z ECMWF is almost a compromise of these two solutions and maintains heavier rains a little longer than the NAM. Continue to favor a wetter similar to a GFS/Canadian and ECMWF blend given satellite/radar trends and the fact that the 12Z NAM was already too far north (by a good bit)with the offshore low as early 15Z. Dry slot will eventually rotate around the 5h low overnight which will bring an end to the heaviest rain. The upper low will be slow to lift northeast and likely will not move across the area until midday Tue. Diurnal heating combined with the passage of the cold pool aloft will likely lead to development of some afternoon showers and possibly a few embedded thunderstorms. Despite linger cloud cover and moisture Tue temperatures will be near climo. && .SHORT TERM /TONIGHT THROUGH WEDNESDAY NIGHT/... As of 3 PM Monday...Stacked low pressure will be exiting the northeast NC coastal areas at the onset of the short term period. 500mb height increases will occur Wednesday and Wednesday night as a mid/upper ridge axis shifts into the Eastern United States. Much drier weather can be expected with time height cross sections showing a period subsidence through the column during this time. Temperatures will recover by Wednesday with cold air advection lacking in the wake of the aforementioned low. In fact, highs on Wednesday will likely range a couple categories above normal. Minimum temperatures will be above normal as well. && .LONG TERM /THURSDAY THROUGH MONDAY/... As of 3 PM Monday...Medium range guidance is fairly consistent with regards to the extended forecast. Longwave troughing will be in place out west with a cutoff low eventually developing with a southwest flow and strong ridging developing just off the southeast coast. At the surface it will be primarily high pressure anchored offshore. A weak front moving across well to the north will fade before even getting close to our area so a dry forecast continues. Temperature trends remain mostly intact well above normal throughout the period but moreso first half as heights fall slightly late in the period. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/...
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As of 06Z...Low confidence forecast as a weak low pressure moving across the coast will create varying cigs and IFR along with periods of showers. Weak low nearly due east of CRE will meander to the NE through the overnight before lifting away slowly during Tuesday. On the north side of this surface low, a warm front is pushing NW and inland from the coast, while a cold front moves offshore and a secondary surface trough pushes SE off the SC coast. The inland terminals are locked into IFR cigs and expect IFR/LIFR to persist into Tuesday morning. The forecast is trickier at the coast where breaks in cigs are expected until the low pulls away and the IFR can flood back towards ILM/CRE/MYR. However, as cigs break, fog is likely to quickly develop, so periods of IFR are also expected at the coastal terminals, with LIFR/VLIFR possible due to the very saturated surface. Additionally, periods of showers, some of which may be briefly heavy, are expected overnight at all terminals. Winds will remain northerly most of the night at speeds 10 kts or less. After daybreak, surface low will slowly pull away but cool northerly flow will keep stratus entrenched with only a slow lift to MVFR Tuesday night. Still quite a bit of uncertainty into how quickly cigs will lift as dry air tries to advect into the region, and have been pessimistic with the ceiling forecast. Showers and isolated tstms are also possible during the aftn/eve creating periods of MVFR or IFR vsbys. Late in this valid period VFR is expected to develop at all terminals as cigs finally erode, with any fog development likely after 06Z on Wednesday. Extended outlook...IFR or lower conditions possible in fog and stratus Tue night and early Wed. VFR Thursday through Saturday.
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&& .MARINE... NEAR TERM /THROUGH TUESDAY/... As of 8 PM Monday...Latest obs show Small Craft Advisory criteria being met over the waters, with 8 ft seas and winds of 20 to 25 kts. Advisory will continue through at least midnight and there is a possibility that it may be extended a few hours. Previous discussion follows: Strong south to southeast flow will continue into the evening hours before winds become southwest in response to surface low lifting north. Gradient will remain pinched with speeds hanging around 20 kt into early Tue. Gradient starts to weaken after midnight as the surface low moves off to the north. Southwest flow drops to a solid 15 kt for the bulk of Tue. Seas running 4 to 8 ft may increase slightly during the next 6 hours or so but then winds shifting to southwesterly and trending down will allow seas to start following a downward trend. Seas across SC waters will drop below 6 ft around midnight but NC waters will take a little while longer. However, by Tue morning all seas should be under 6 ft running 3 to 5 ft through the end of the period. SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/... As of 3 PM Monday...Offshore flow will prevail in the wake of exiting low pressure Tuesday night. The weakening offshore flow will back to a southwest to south direction during Wednesday as the sea breeze pushed inland. Generally light southwest flow will prevail by Wednesday night with high pressure far off the Southeast U.S. coast and lower pressure across the MS valley. Expect a larger range of seas initially with the exiting low and an offshore trajectory. Seas will be 3 to 4 ft, primarily in swell, through the bulk of the short term period. LONG TERM /THURSDAY THROUGH SATURDAY/... As of 3 PM Sunday...Essentially a summertime pattern for the coastal waters as Bermuda High pressure will be the main feature. South to southeast winds of 10-15 knots will prevail Thursday through Saturday. Significant seas will be 2-4 feet. && .ILM WATCHES/WARNINGS/ADVISORIES...
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SC...Flash Flood Watch until 8 AM EDT this morning for SCZ017-023- 024-032-033-039-053>056. NC...Flash Flood Watch until 8 AM EDT this morning for NCZ087-096- 099-105>110. MARINE...Small Craft Advisory until 8 AM EDT this morning for AMZ250- 252-254-256.
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&& $$ SYNOPSIS...DCH NEAR TERM...REK/JDW SHORT TERM...DCH LONG TERM...SHK AVIATION...JDW MARINE...DCH/REK/III/SHK

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