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Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC
Issued by NWS Raleigh/Durham, NC
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436 FXUS62 KRAH 161549 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1149 AM EDT Tue Jul 16 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 1145 AM Tuesday... ...Dangerous and Near Record Heat To Continue... ...Heat Advisory Remains in Effect from Noon to 8 PM... An upgrade to Excessive Heat Warning was strongly considered for the Coastal Plain, given a continuation of record-breaking surface through 850 mb temperatures observed across the srn Middle Atlantic the past couple of days. Additionally, mean mixing ratio values have increased 2 g/kg at RNK and GSO since 12Z Mon, indicative of less mixing out of afternoon dewpoints today over the Piedmont, while near steady, 17 g/kg will favor persistence dewpoints over ern NC. Yet, multi-layered mid/high-level clouds have kept temperatures 2-7 F cooler on average than this time Mon; and these are likely to persist into the afternoon and may consequently yield slightly lower hourly and high temperatures, and heat index values, over all but the srn Sandhills and srn Coastal Plain. Regardless of whether the heat index values reach 108 or 110 F, it will remain dangerously hot; and appropriate precautions should be taken. .PREV DISCUSSION... /ISSUED 330 AM EDT Tue Jul 16 2024/ There is very little to no change in the synoptic pattern and antecedent airmass in place as subtropical ridging and Bermuda high pressure unite to sustain dangerous and near record heat across the region. Highs will be very similar to yesterday, in 95-100 range, with heat indices ranging from lower 100s in the NW Piedmont to as high as 109-110 across interior and eastern/southern portions. As such, Heat Advisory has been issued for generally the same climatological divisions across central NC. PWATs are progged to inch up slightly to around 2.2-2.3" and there remains a signal in the hi-res models that convective coverage should be higher than recent days as weak upper impulses emanating through the ridge and atop lee troughing could potentially help to drive a west to east moving cluster of showers and storms across central NC between 18 to 03z. Given the moist environment, some locally heavy rain rainfall and minor flooding is possible. Also, given the high CAPE/steep lapse rate environment, cannot rule out an isolated strong/severe downburst. Bulk of convection should dissipate by midnight. Balmy overnight lows in the mid to upper 70s, except for some lower 70s across the far northern Piedmont counties. && .SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/... As of 330 AM Tuesday... ...One Last Day of Dangerous Heat.... Moisture will continue to increase in advance of an upper trough and attendant cold front that will progress SEwd into the Ohio and Tn Valley late Wednesday-Wednesday night. The influx of ~2.5" PWATs into the area will result in an uptick in showers and storms Wednesday afternoon and Wednesday night as another parade of vort impulses spread through the region during max heating/ destablization. While the increase in cloud cover and PoPs will temper the dry-bulb temps more into the lower/mid 90 ranges, there will be less mixing out of afternoon dewpoints. Thus, we`ll see one last day of heat advisory criteria of heat indices in the 103-107 range across much of the same area. Deep layer shear is forecast to increase to ~25kts, which is higher than recent days. As such, SPC has introduced a level 1/marginal severe threat across the northern half of the forecast area, with wind the primary threat. Given the moist airmass and weak height falls starting to spread into the region , convection could linger well into the overnight hours. Lows in the 70s. && .LONG TERM /THURSDAY THROUGH MONDAY/... As of 400 AM Tuesday... Widespread showers and some storms are expected on Thursday as a cold front reaches central NC and slows down. Deep southwesterly flow aloft ahead of broad mid-level troughing that digs from New England into the lower MS Valley will continue to bring in above- normal PW values of 2 to 2.5 inches. So there will be plenty of moisture for this front to tap, along with moderate instability of around 1000-2000 J/kg. There will also be upper support as we will be in the right entrance region of a jet streak over New England and the northern Mid-Atlantic. The vast majority (80-90%) of GEFS/EPS/CMC ensemble members depict precipitation. Thus categorical POPs are justified, maximized with daytime heating in the afternoon/evening, though chance POPs still linger overnight. 1 to 2 inches are expected on average, highest SE, but locally higher amounts will be possible especially where slow-moving storms occur. WPC now has a slight (level 2 of 4) risk for excessive rain on Thursday across nearly all of central NC. Fortunately widespread severe weather is not a major concern given the weak shear, but locally damaging winds can`t be totally ruled out. The front will settle near the NC/SC border on Friday, and more heavy rain will be possible especially south in closest proximity to the front. Amounts may come down a bit overall as upper support wanes with the mid/upper trough lifting into Canada. But WPC still has central NC in a marginal (level 1 of 4) risk for excessive rain on Friday. The front looks to retrograde back NW a bit to near the VA/NC border from Saturday through Monday, wavering back and forth as it does so, and likely POPs continue each day. Overall rainfall amounts may continue to decrease slightly as Atlantic ridging builds back westward, and we actually get mild height rises. But rich moisture and moderate instability will still be in place. Ensemble mean QPF through the whole extended period is generally 2 to 3 inches, greatest south and east, and of course locally higher amounts will always be possible where storms occur. While so much rain for multiple days in a row will cause some flooding concerns, it will provide more welcome drought relief. It will also help bring down high temperatures to upper-80s to lower-90s on Thursday (near normal) and only 80s from Friday through Monday, with the official forecast tending toward the cooler end of guidance. Lows will be in the upper-60s to mid-70s. && .AVIATION /11Z MONDAY THROUGH SATURDAY/... As of 630 AM Tuesday... Shower and storm coverage this afternoon and evening is expected to be higher than the past couple of days, with a general consensus that a cluster of storms will move west to east across the area between 18 to 00z. Have added a prob30 for all TAF sites. Convection should dissipate between 03 to 06z. Outlook: Mainly VFR conditions are expected to continue away from diurnally-driven convection, which will gradually increase in coverage through Wed. A cold front will move into NC late Wed-Wed night, then become quasi-stationary over the region Thu-Sat. Higher chances of flight restrictions, both in and away from diurnally- maximized convection, will consequently result Thu-Sat. && .CLIMATE... Record High Temperatures: July 16: KGSO: 99/1915 KRDU: 101/1887 KFAY: 103/1937 July 17: KRDU: 100/1887 KFAY: 102/1932 Record High Minimum Temperatures: July 16: KGSO: 75/2010 KRDU: 77/2010 KFAY: 78/1992 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 noon today to 8 PM EDT this evening for NCZ007>011-023>028-039>043-073>078-083>086-088-089. && $$ SYNOPSIS...MWS NEAR TERM...MWS/CBL SHORT TERM...CBL LONG TERM...Danco AVIATION...CBL/Luchetti CLIMATE...RAH