Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC
Issued by NWS Raleigh/Durham, NC
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852 FXUS62 KRAH 111430 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1029 1M EDT Thu Jul 11 2024 .SYNOPSIS... A cold front will stall out over the Coastal Plain this morning. This front will push back westward into the Piedmont later today, but will generally hold over the western and central Carolinas through Sunday, as it slowly weakens. && .NEAR TERM /TODAY AND TONIGHT/... As of 1029 AM Thursday... Ongoing stalled convection has largely stayed east of our area as the cold front has finally caught up to the pre-frontal trough just east of our border. This convection has produced some good rain rates, with most efficient cells producing 2-3 inches per hour. The atmosphere is quite juicy out that way, with PWATs over 2 inches. The instability axis (which quickly drops off with westward extent into our area this morning) which slowly shift westward through this afternoon with diurnal heating. Associated rain chances should pick up with time later today and spread west towards the US-1 corridor through tonight. Increasing convergence along the quasi-stationary front and sea breeze should focus much of today`s early convection to our east. It`s not entirely clear how far west the better convergence will extend this afternoon especially given rather week wly flow aloft. However, the overnight HREF (00/06Z runs) probabilities for exceeding 2 in/hr rain rates do maximize to our east this afternoon and think the highest QPF through 00Z should stay east of our CWA. With time this evening/overnight, onshore moisture flux, as noted in IVT model fields, should increase especially along and east of US-1. Additionally, weak mid-level impulses will pivot across this area later tonight into the overnight period. As such, scattered to numerous showers and storms will likely increase in coverage tonight, and blossom further west through the early Friday morning time period. While instability will wane some overnight, PWAT will remain 120 to 150 % of normal (>2 inches) area wide. Given forecasted freezing levels >15,000 feet, tall-skinny CAPE profiles, warm-cloud layer depths > 12,000 feet, and very slow mean-cloud layer flow (~5 kts), any convection that develops tonight could be capable of producing highly efficient downpours. Despite quite dry soils, this type of atmospheric environment could easily encourage efficient warm-cloud processes and rain rates capable of producing flash flooding. Biggest concerns would be over urban areas, but any efficient, slow- moving storm could produce localized flooding even in rural areas. Will continue to assess guidance trends, but may need to consider a Flood Watch with the afternoon update. Highs today will still push the upper 80s/lower 90s but with lower humidity for much of the day. Overnight lows will dip into the lower to mid 70s. && .SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/... As of 340 AM Thursday... * Heavy rainfall leading to localized flooding remains possible across north-central and eastern portions of central NC through Fri into Fri night. Confidence is growing that we`ll see high rain chances across central NC from late tonight through Fri. As the surface front sits roughly steady with subtle wavering over the W Piedmont, the aforementioned low level low center expected to be near the SC/GA coast early Fri will continue a drift to the WNW, propelled inland by a mid level inverted trough to its W well off the Southeast coast as it too tracks WNW around the deep/strong anticyclone over the NW Atlantic, all of which will help to draw copious moisture into the eastern half of NC. Deterministic models agree on a continued deep low-mid level SE flow riding up and over the surface front, including a 20-30 kt low level SSE jet, as anomalously high PWs (likely surpassing daily records) around 2.5" become focused through the central and eastern Carolinas. With the combination of slow storm motions (10-15 kt steering flow at 850-500 mb), weak DPVA from the mid level trough moving in from the SE, marginal SBCAPE over our area, high integrated vapor transport into E NC, a projected very deep warm layer (LCL-0C) over 4 km, and weak moist upglide over the surface frontal zone, heavy rainfall remains a high concern over much of our area. If these model signals persist in later runs, a flood watch may be needed for late tonight through Fri. Rainfall totals of 1-2" are expected over much of our area, particularly over the N Piedmont, W Sandhills and the Coastal Plain, and the LREF has a 60-90% chance of an inch or more from FAY through the Triangle toward Kerr Lake and Lake Gaston. By Fri night, the surface front will hold through central NC while the mid level trough/low weakens while tracking northward from E NC to the Delmarva/NJ. With a reduction in forcing for ascent including lessening moist upglide, we should see rain trending downward in coverage and intensity from S to N overnight, and will depict a trend back down to just chance pops Fri night. Expect below normal highs from the upper 70s N to low 80s S, followed by lows once again in the low-mid 70s. -GIH && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... As of 235 AM Thursday... Heights will rise on Sat as the broad troughing at mid-levels from Fri shifts east mainly along the NC coast. West to southwest flow at 500 mb will ensue thanks to a weak trough axis stretching from the Great Lakes into the Midwest. While this would normally reduce storm chances, most model guidance shows that the low-level circulation from Fri will be located roughly over eastern NC. While it will be in a weakened state, a low-level convergence axis will still be present, draped from NE to SW from roughly Roanoke Rapids to Wadesboro. At the surface, there may be a weak surface low as well along the Coastal Plain or along the coast. Forecast soundings show relatively weak CIN and MLCAPE of 1200-1500 J/kg. This along with the convergence axis and sea-breeze should favor another day of scattered showers and storms. Chances would appear highest along and SE of the convergence axis, roughly along/east of US-1. Coverage should diminish after sunset with loss of heating. Highs should warm into the upper 80s/90 in the east to low 90s W. On Sunday, the flow at mid-levels roughly stays the same with westerly flow. The low-level convergence axis is forecast to shift offshore, with any surface boundary from Sat washed out by this time. This along with lower PWATs should favor less overall chances of storms. The best chance appears in the Sandhills to central Coastal Plain as guidance shows a fairly robust sea-breeze potential. Highs should warm with more sunshine and rising low-level thicknesses to the low and mid 90s. Heat indices will also get close to advisory criteria in the Triangle and Coastal Plain between 100 and 105. Precipitation chances become more isolated in nature and more diurnally driven Mon through Wed. Storms appear to be mainly focused along the western Piedmont/Foothills along a persistent lee trough setting up at the surface, as well as along the sea-breeze over the southern Coastal Plain. Rain chances, however, look limited thus far with forecast soundings showing a capping inversion. Downslope flow and lowered PWATs may also limit coverage. The next best chance of showers/storms may not come until late Wed or Thu as ensembles show a potential cold front approaching. Outside of this, it will be hot and humid, with highs in the mid to upper 90s to even some lower 100s over the Triangle. Lows will also be rather muggy and warm in the mid/upper 70s, which could challenge some record high mins. Dewpoints should mix out into the upper 60s W to low 70s E in the afternoon under the westerly downslope flow, however heat indices look to still be rather unbearable between 103 to 109 along/east of US-1. The prolonged hot weather will increase the risk of heat- related illnesses. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... As of 640 AM Thursday... VFR conditions are expected to mostly dominate central NC terminals through this evening, although a downturn to IFR is likely starting later this evening, esp over central and eastern TAF sites. A band of storms which has formed just east of the forecast area in the last hour or so is just E of a front which has shifted into the Coastal Plain. These showers and isolated storms are expected to expand and begin to edge westward later today, as the front pushes westward back into the Piedmont, and brief sub-VFR conditions with erratic winds are possible in and near any storm. The westward push of the front will result an east-to-west trend toward IFR cigs and scattered showers and storms starting around 00z-02z at RWI/FAY and around 03z-04z at RDU. INT/GSO should remain VFR with dry weather through 06z, before dropping to IFR around 09z. Surface winds will be mainly from the N and NE today, with a few gusts to 15-20 kts through early afternoon, then shifting slowly east-to-west to be from the ENE or E from this evening through tonight. Looking beyond 12z Fri, sub-VFR conditions and numerous areas of showers and isolated storms, including locally heavy rain, will prevail Fri morning through Fri night at all sites, as the westward moving front slows then stalls over the W Piedmont or Foothills, where it will linger into the weekend. Shower/storm chances will persist, mainly each afternoon through evening and primarily in the E Sat/Sun. Mon currently appears mostly dry with only isolated storms. Morning sub-VFR fog and low clouds are possible mainly E early Sat/Sun with fairly low chances Mon. -GIH && .CLIMATE... Record High Temperatures: July 15: KGSO: 97/1974 KRDU: 99/1997 July 16: KGSO: 99/1915 KRDU: 101/1887 KFAY: 103/1937 Record High Minimum Temperatures: July 11: KGSO: 75/1992 KRDU: 77/1981 KFAY: 76/2019 July 12: KGSO: 76/2011 KRDU: 78/2011 KFAY: 80/1935 July 13: KGSO: 75/2017 KRDU: 78/2017 KFAY: 78/2017 July 14: KGSO: 77/1981 KRDU: 77/2017 KFAY: 80/2017 July 15: KGSO: 74/2019 KRDU: 76/1992 KFAY: 78/2015 July 16: KGSO: 75/2010 KRDU: 77/2010 KFAY: 78/1992 && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Hartfield NEAR TERM...Luchetti SHORT TERM...Hartfield LONG TERM...Kren AVIATION...Hartfield CLIMATE...RAH