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Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC
Issued by NWS Raleigh/Durham, NC
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317 FXUS62 KRAH 170656 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 250 AM EDT Wed Jul 17 2024 .SYNOPSIS... A strong subtropical ridge will gradually weaken over the Southeast though mid-week. A cold front will settle into the Carolinas on Thursday, then become quasi-stationary while weakening into the weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 250 AM Wednesday... Water vapor imagery this morning depicts a low-amplitude trough over the Great Lakes/northeast. More locally, a de-amplified mid-level ridge remains off the southeast coast. At the sfc, a trough was observed extending north from the southern Piedmont through the northern Piedmont. The upper trough will lift into the northeast this afternoon through Thursday. While associated upper forcing and shear will maximize to our north, some mid-level impulses embedded within the swly aloft will trickle across central NC this afternoon. These features will help to initiate scattered showers and storms along a pre-frontal trough that will set up just west of the US-1 corridor. Storms will generally flow from west to east through the afternoon before diminishing with loss of heating tonight. Bulk-layer shear will peak around 20 kts, but spread relatively uniform across central NC. A little better shear may develop across the NC/VA border late afternoon, which may provide a bit better environment for a few organized clusters to possibly develop. If any stronger storms can develop, isolated damaging wind gusts may be possible today. Additionally, forecasted hodographs/supercell composite fields do indicate a non-zero chance for perhaps a few supercells to develop across the central/northern Piedmont into the northern Coastal Plain. Not overly concerned with tornado potential today given relatively high LCLs. However, stronger gust potential may exist with any isolated rotating updraft that could form later this afternoon in the areas mentioned. Overall though, the severe threat today should be relatively localized. A few residual showers/storms may persist into the overnight period, but overall, rain chances should wane overnight. Max temperatures will soar into the mid 90s again today. Afternoon dew points along and east of the sfc trough (roughly along and east of US-1) will likely remain in the lower to mid 70s. West of the boundary, dew points may mix out into the upper 60s. Given the current position on the sfc trough mentioned above, as well as latest high-res model trends, decided to expand the Heat Advisory today a few counties westward. Overnight lows in the mid to upper 70s are expected once again. && .SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/... As of 315 PM Tuesday... ...A heat advisory has been issued for the Coastal Plain on Wednesday from 12 PM to 7 PM... Potentially the final day of the oppressive heat is expected on Wed for central NC. Low-level thicknesses and 850mb temperatures subtly cool with glancing influence from a broad mid/upper trough as it pivots across southern Ontario through the period. The result should be highs not-as-hot as previous days, but still reaching into the mid/upper 90s by early afternoon with forecast heat index values, in the absence of storms, expected to rise above 105 once again in the Coastal Plain. Westward extent of the Heat Advisory remains uncertain as surface dew points mix out in the west and increased deep layer moisture will result in increased cloud cover and earlier storm initiation. The Heat Advisory was issued where greatest confidence in dangerous heat will be possible before storm initiation and may need to be expanded westward to include the Triangle with subsequent updates. Showers/storms are again expected to be in greater coverage compared to previous days as better moisture and slightly perturbed mid-level flow spreads into the area. The airmass by early afternoon will be hot and humid with MLCAPE 1500 to 2500 J/kg and moderate DCAPE around 1000 J/kg. Marginal storm organization will be possible as the upper trough pivots eastward and 15 to 25 kts of perturbed mid- level flow will leak east of the mountains and over the northern portions of the forecast area. Pulse-like airmass storms will be possible south of Raleigh with wet downbursts the most likely hazard. From Raleigh northward, some storm clusters congealing along common cold pool may result in isolated increased wind threat. && .LONG TERM /THURSDAY THROUGH TUESDAY/... As of 216 PM Tuesday... * Widespread rainfall expected Thursday into Friday * Unsettled weather continues this weekend into early next week * Cooler temperatures from Friday through early next week The forecast remains on track for widespread showers on Thursday as a slow moving cold front arrives in central NC, stalls, and remains in the vicinity through the weekend. The surface boundary will be slow to move through the area given front-parallel flow aloft from the southwest and as such, showers and thunderstorms will be slow moving/prone to training over the same areas. PW`s will range from 1.75 to 2 inches immediately ahead of the front and showers will certainly be efficient rain producers. Marginal amounts of instability ranging from 500 J/KG in the NW Piedmont to upwards of 1500 J/KG will be present ahead of the front although deep layer shear will only peak around 15-20kts and the severe weather threat appears to be limited. Taking a look at the grand ensemble of Canadian, GFS, and EC members, anywhere from 80-90 percent of members advertise measurable precip across the area late Thursday morning into the afternoon/evening hours, with a handful of members suggesting more than 2 inches of rain Thursday afternoon through Friday morning. the percentile NBM QPF is also in this range as well, and PoPs will be increased a bit during this period. Exact QPF amounts are still a challenge to pin down but the highest amounts should fall across the central/eastern portions of NC. The front will stall across eastern NC on Friday, then slowly retreat northwestward through the afternoon/evening. While the day may start off mostly dry, showers will quickly redevelop across the Coastal Plain and Sandhills by mid morning, then expand northwestward as the front shifts back over the NW Piedmont. Once again, nearly 80-90 percent of ensemble members indicate precip during this period and PoPs will be bumped upward through early Saturday morning. It`s conceivable an additional 1-2 inches of rain could occur during this period although it`s hard to say with certainty where this secondary QPF axis will set up as the front retreats through the area. Regardless, Thursday and Friday should be the wettest days of the forecast with periods of heavy rain and flooding possible. Temps during the day Thursday will remain near or above normal ahead of the front, turning noticeably cooler on Friday given cloud cover and rain with highs generally 5-8 degrees below normal. Looking at Saturday through Tuesday, ridging over Bermuda will gradually strengthen and shift westward, resulting in a return to a fairly typical July distribution of precip chances with the highest chances across the western Piedmont and a secondary focus area from sea breeze convection migrating inland. Temperatures will remain below normal this weekend, gradually moderating closer to normal by early next week. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... As of 206 AM Wednesday... VFR conditions will prevail for much of the 24 hour TAF period. However, scattered showers and storms will develop and spread west to east this afternoon through tonight. Associated brief sub-VFR restrictions could develop at any terminal during this period. Any lingering showers/storms should diminish overnight. Outlook: Early morning stratus looks possible across the I-95 corridor Thursday. A cold front will sag into and stall across the region Thursday through Saturday which will increase the chances for sub-VFR conditions through this weekend. && .CLIMATE... Record High Temperatures: July 17: KRDU: 100/1887 KFAY: 102/1932 Record High Minimum Temperatures: July 17: KGSO: 75/2013 KRDU: 80/1887 KFAY: 77/2019 July 18: KGSO: 76/1986 KRDU: 79/1887 KFAY: 77/2019 July 19: KGSO: 75/2016 KFAY: 77/2023 July 20: KGSO: 75/2019 KRDU: 76/2020 && .RAH WATCHES/WARNINGS/ADVISORIES... Heat Advisory from 11 AM this morning to 8 PM EDT this evening for NCZ008>011-024>028-040>043-075>078-084>086-088-089. && $$ SYNOPSIS...MWS NEAR TERM...Luchetti SHORT TERM...Swiggett LONG TERM...Leins AVIATION...Luchetti CLIMATE...RAH