Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC
Issued by NWS Raleigh/Durham, NC
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158 FXUS62 KRAH 171740 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 140 PM EDT Tue Sep 17 2024 .SYNOPSIS... An area of surface low pressure will drift west across northwestern South Carolina through tonight. The remnants of this system will linger and gradually dissipate across the mid- Atlantic through the end of the work week. && .NEAR TERM /THROUGH TONIGHT/... As of 1035 AM Tuesday... Regional surface observations and SPC mesoanalysis show the center of the surface low over upstate SC with an occluded boundary extending north-northeast towards the Triad and then back east- southeast through the Triangle and to Goldsboro and off the NC coast. This boundary is supporting a train of light showers with embedded moderate showers along and to its north where some weak instability (250-500 J/kg of MUCAPE) and strong low-level moisture transport exists. Satellite imagery confirms mostly low-topped updrafts comprised mostly of water droplets and some dry mid-level air spreading north of the eastern periphery of the stacked low to our west. Hi-Res guidance shows this regime should slowly spread northward towards VA through the morning into the early afternoon, but based on current observations, it may become stationary as diurnal heating to the south and diabatic cooling to the north lock in the occluded front; continuing to steady light rain and embedded moderate showers over the Triangle for most of the afternoon. Latest model guidance shows a tongue of 500-1000 MLCAPE spreading west along and north of the boundary this afternoon and may result in some localized deeper updrafts capable of moderate to perhaps locally heavy rainfall at times. The training nature of showers and isolated storms within the streamer of moisture may result in narrow swaths of higher rainfall totals and localized flooding mainly in urban and poor drainage locations along and north of the boundary. Rain rates may struggle today to produce any flooding within an hour, but steady moderate rain with pockets of heavier rates my produce flooding concerns where multiple hours of rain occurs. Some shower activity may continue into the overnight hours towards the Triad as the occluded low and mid-level forcing begins to wobble back east. Highs today will be tricky and depend on the frontal position and associated steady rain, but generally expect low 70s in the west and perhaps into the low 80s in the south and east. Lows should settle in the mid 60s overnight for most locations. && .SHORT TERM /WEDNESDAY/... As of 355 AM Tuesday... The surface and mid/upper low impacting the region over the past few days will weaken into Wednesday as it shifts north and east. A weak surface trough will extend from southeastern VA southwest into the remnant low circulation across western SC on Wednesday afternoon. Abundant moisture across the region will result in widespread low clouds and perhaps some fog to start the day on Wednesday morning. With some breaks in the overcast developing, especially in the southern and eastern areas, the air mass will becoming moderately unstable during the afternoon and support scattered showers and thunderstorms. With a very weak low level wind field and a unidirectional southwesterly flow in the mid levels don`t expect much in the way of strong storms. Afternoon highs will range from the upper 70s in the Triad to the lower 80s in the Coastal Plain. -Blaes && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... As of 345 AM Tuesday... Mid/upper troughing will extend down the Eastern Seaboard including into central NC from Wednesday night into Thursday. Meanwhile a surface low will linger off the northern Mid-Atlantic coast. This will bring one last day of upper forcing for ascent and above-normal PW values, so POPs on Thursday are generally in the 30-40% range, highest east and maximized during peak heating. There could be enough instability for a few storms. QPF amounts in the models are not impressive, so not concerned about an additional heavy rain threat, and looks fairly low impact overall. As a mid/upper ridge over the Plains begins to move east on Friday, it will begin to push the trough east and offshore. This will result in NW flow and much drier air moving into central NC, with PW values trending below normal. At the surface, high pressure will gradually nose down from SE Canada into the Mid-Atlantic from Friday into Monday. Overall this pattern supports limited rain chances at best, which is a significant change from last night when the models depicted the mid/upper low moving back south across the area. Now, they keep it and the associated surface low well to our east in the western Atlantic. Instability looks minimal to non-existent as well given the cool wedge of high pressure and NE flow near the surface. The best chance for clouds and isolated showers looks to be in the Coastal Plain closer to the low and where there will still be moist onshore flow in the low levels, but even there POPs are only slight. The GFS does try to bring a shortwave through the area on Sunday, so POPs are slight that day areawide. As for temperatures, highs should be within a few degrees of normal from Thursday through Saturday, in the upper-70s to lower-80s. Lows will gradually trend downward as the air mass turns drier, from mid- to-upper-60s on Wednesday night to upper-50s to lower-60s by Saturday night. On Sunday and Monday when the cool wedge of high pressure really takes hold, forecast highs are only in the 70s with lows in the mid-50s to lower-60s. If the raw GFS is to be believed, some places won`t even get above the 60s on these days, but would like to see more support from ensemble guidance before buying into this too much. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... As of 135 PM Tuesday... Light rain, with moderate embedded showers, and IFR cigs have been forming along a nearly stationary boundary stretching from the INT/GSO to RDU and south of RWI through early afternoon. Some brief improvement to low-end MVFR is possible at RDU, but IFR cigs would quickly return after sunset. Isolated thunder is possible at RWI for the next few hours as shower/storm coverage increases. Cigs along and north of the stalled boundary will likely lower well below alternate minimums and approach landing minimums by early Wed morning. Confidence is low on if/when cigs will lower at FAY. Gradual improvement is expected through the morning hours, slowest at GSO/INT. Scattered showers/storms will again be possible Wed afternoon with greatest confidence in the vicinity of INT and GSO. Outlook: An unsettled period will continue through Thursday with areas of overnight and early morning stratus and a risk of some afternoon showers and possibly a storm. Aviation conditions will generally improve for the weekend. -Blaes && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Blaes NEAR TERM...Swiggett SHORT TERM...Blaes LONG TERM...Danco AVIATION...Swiggett/Blaes