Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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539 FXUS62 KRAH 230717 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 315 AM EDT Mon Apr 23 2018 .SYNOPSIS... Strong high pressure will remain nearly stationary or slowly drift off the New England coast through tonight. Low pressure approach from the Tennessee Valley today and tonight, then track slowly east over SC on Tuesday, then NE across eastern NC late Tuesday and Tuesday night. && .NEAR TERM /THROUGH TONIGHT/...
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As of 315 AM Monday... Increasing easterly winds and timing/amount of rain are the main issues today and tonight... Current projections suggest that East winds will increase to 20 to 30 mph late this afternoon and tonight, with gusts to 35 mph. QPF on the order of 1 to 1.5 inches expected through 12z/Tuesday. Severe thunderstorms not expected through 12z/Tuesday. The mid/upper low near Memphis is forecast to slowly drift NE toward southern KY by 12z/Tuesday. The associated surface low is expected to slowly weaken and move east through the Tennessee Valley and weaken with time, with secondary development over SC by 12z/Tue. An impressive 40-45kt SE low level jet will bring the plume of deep subtropical moisture from the Caribbean and SW Atlantic NW into our region. Rain is expected to continue to expand northward with time today, reaching the southern and western Piedmont between 12z and 15z, then the western Sandhills and rest of the Piedmont between 15z and 21z. The rain will be light to start, then gradually pick up in intensity through the day from SW to NE. The steady rain should reach into the far NE counties before sunset today. Rain, some heavy is expected for all areas tonight. As far as the impacts, the wind will be an issue beginning by early afternoon as the easterly winds increase to 15-20 mph. The winds will increase through the afternoon and evening as the H85 SE low level jet increases to 40-45kt. This jet will help drive and increase surface winds to 20-25 mph with gusts to 30-35 mph as mixing occurs with the steady rainfall. The main timing of these stronger winds appears to be 21z/today through much of the night. With the newly leafed out trees, the heavy rain and wind gusts may down some limbs and we will mention this in the Hazardous Weather Outlook. The other impact from this system will be the rain, potentially heavy at times. There continues to be signal within many of the ensemble solutions that there may be a minimum of QPF over a portion of inland NC (possibly the southern Piedmont of NC / Sandhills). This is due to the favored maximums of QPF to be in the upslope areas of western NC and along the coast. The maximum (if if it develops along or near the coast) may be convectively driven from a secondary low pressure circulation along the coastal front. If this develops, it may focus the convection and heaviest rain there and there may be a shadow or minimum of QPF to the NW of this area. Even if this shadow or minimum occurs, there should still be "healthy" totals in the 1 inch range, with 1.5 to 2.5 inches in the upslope or higher Piedmont areas to the NW, but focused even more so along the east slopes of the Blue Ridge. Locally 3+ inches may occur there. The higher QPF (locally 1.5 to 3 inches in 12-24 hours) would still fall short of the Flash Flood Guidance criteria. Therefore, with the uncertainty in the heavier QPF to fall over all the region and with the FFG being rather high after the drying out period this past week, we will simply mention the chance of some minor flooding in urban and poor drainage areas in the Hazardous Weather Outlook. Severe thunderstorms are not expected through at least 12z/Tuesday as the entire area will have widespread stratiform rain that will keep the instability at near zero. The warm/coastal front will still be south and southeast of our region by 12z/Tuesday, with the only chance of convection expected to be elevated on the cool side of the boundary. Sensible weather includes: Rain developing/overspreading central NC (west this morning, east this afternoon). Highs upper 50s west, to near 70 east. Winds increase to 20-30 mph late today and tonight with rain. Lows in the 50s to near 60 (SE).
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&& .SHORT TERM /TUESDAY THROUGH TUESDAY NIGHT/... As of 300 PM Sunday... Overview: Compact closed upper level low over the Lower MS Valley Monday morning will drift slowly NNE across the TN Valley and into the southern/central Appalachians through Tuesday night. Primary surface low associated with this system will remain beneath the mature cyclone and eventually fill with a secondary low at the triple point expected to track NEWD through the Savanna River Valley and into eastern NC Tuesday afternoon/evening, before lifting up along the Delmarva coast Tuesday night and into Wednesday. Rainfall Amounts/Impact: While strong mid/upper level ridging extending along the western Atlantic/Mid-Atlantic coast may initially delay the spread of rain NEWD into the area Monday, rain will become widespread over the entire CWA by late Monday afternoon/early evening. Owing to the slow movement of the upper low across the region, along with a very strong Atlantic maritime feed between the offshore high and the mature cyclone to our west, a widespread soaking of 1 to 2 inches, with localized higher amounts can be expected. The heaviest rainfall is expected to occur Monday night through Tuesday morning, with another round of convective rainfall possible Tuesday afternoon as the surface low and attendant warm sector retreats north into southern/southeastern NC(more on that below). With area rivers and creeks running high from recent heavy rainfall over the past week, this additional rainfall may be enough to result in some localized/minor flooding. Severe Potential: Models are in good agreement that the secondary/triple point low and attendant warm front/sectors(sfc dewpoints surging into the mid 60s) will lift north into southern/southeastern NC Tuesday afternoon. CAD in place across central NC should keep the the warm sector confined to southeastern portions of NC, with mid-level dry air intrusion likely to result in sufficient breaks in cloud cover to allow for partial insolation by the afternoon. Models indicate warm sector destabilization of 500 to 1000 J/Kg of MLCAPE across the eastern Sandhills and southern Coastal Plain counties. Effective shear of 35-40kts will be sufficient to support a few strong to severe storms Tuesday afternoon/early evening with the presence of the dry air aloft (models showing dCAPE values of 750 to 1000 J/Kg) likely to enhance downdraft potential with any strong updrafts that develop. Additionally, with the low tracking NE along the warm/wedge front, cannot rule out an isolated tornado. Stay tune. Temperatures: Rain spreading SW to NE into the area, along with widespread and lowering cloud deck and the development of cold air damming over the NC Piedmont will will keep temps on the cool side Monday. Highs ranging from mid to upper 50s across the NW Piedmont to upper 60s/near across the NE coastal plain counties where rain will be last to set in. Highs Tuesday will be complicated by the ongoing CAD and the exact location/track of the secondary low and attendant warm sector lifting NE into the area. Highs ranging from upper 50s/ near 60 over the northern Piedmont to mid 70s across the southern Piedmont, southern Sandhills and southern Coastal Plain counties. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... As of 245 PM Sunday... Unsettled weather is expected through the middle and end of the work week, with mean mid level troughing dominating over the eastern states, and generally more clouds than sun, at least through Fri. The first shortwave trough (responsible in part for the Mon/Tue rain) will be sitting overhead early Wed before lifting off to the NE, culminating in a phasing with a strong polar stream wave over the Northeast states Wed night. Models suggest that we may see enough low and mid level moisture (focusing on 850-700 mb) for a few late-day showers with heating, however the overall forcing for ascent will be small, and expect no more than chance pops. Expect temps to stay slightly below normal, with thicknesses about 10 m under normal and reduced insolation. The next shortwave, which will be located over NE/KS early Wed, will cross the Gulf states and swing NE over the Carolinas Thu night into Fri. As this feature will be deeper and provide stronger forcing for ascent along with an opportunity for moisture return, we should have higher rain chances, targeting late Thu through Thu night, and some of this may be heavy, as the GFS shows a PW over 1.5". After dry weather late Wed night through the first half of Thu, will bring in good chance pops Thu afternoon through Thu night. Temps should again be a bit below normal figuring in the cloudiness and cooler thicknesses. Then, a deep mid level trough from central Canada through the Upper Midwest early Thu should be our next weather-maker. The GFS/Canadian are in fair agreement on a deep low just to the W or NW of NC late Fri, crossing the Carolinas Fri night or early Sat, although uncertainty and model spread grows at this range. Will keep a chance pop, higher N closer to the cooler mid levels and better lapse rates aloft, Fri/Fri night extending into early Sat. We should see subsidence and drying behind this wave for Sat afternoon through Sun, and will trend toward dry weather and fewer clouds as high pressure builds in from the west. -GIH && .AVIATION /06Z MONDAY THROUGH FRIDAY/... As of 1245 AM Monday... MVFR to VFR CIGS will lower to IFR TO LIFR CIGS and IFR VSBYS with rain and fog later today and tonight. Winds will increase from the east at 15-20kt with gusts to 30kt this afternoon and tonight. Looking beyond 06z Tue: Poor aviation conditions will dominate all sites through Tuesday. Mostly IFR CIGS and IFR to MVFR VSBYS with areas of rain and showers are likely on Tuesday, before tapering off Tuesday night. Sub-VFR stratus/fog may linger into Wed morning. VFR conditions should return Wed, although a few showers are possible Wed afternoon, and again Thu into Fri, as a series of upper level troughs cross the region, bringing unsettled weather and perhaps brief periods of sub-VFR conditions. && .RAH WATCHES/WARNINGS/ADVISORIES... NC...None. && $$ SYNOPSIS...Badgett/Hartfield NEAR TERM...Badgett SHORT TERM...CBL LONG TERM...Hartfield AVIATION...Badgett/Hartfield

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