Scientific Forecaster Discussion

000 fxus62 krah 251917 afdrah Area forecast discussion National Weather Service Raleigh NC 315 PM EDT Thu Apr 25 2019 Synopsis... a weak surface front across Virginia will lift northward as a warm front later today. A stronger cold front will approach from the west tonight, and cross central NC Friday, triggering numerous showers and a few thunderstorms. High pressure will build in behind the front on Saturday. && Near term /through tonight/... as of 235 PM Thursday... The southern stream portion of the phasing full-latitude trough approaching from the west is playing a role in maintaining convection currently over West Alabama into S MS/la. While this convection will steadily weaken as it heads ENE, dampening in response to northern stream energy digging into the trough base over the mid miss valley, this initial shot of DPVA and the northern portion of this precip area still appears poised to cross central NC, necessitating chance pops spreading in overnight. Given the in situ relatively dry air in the lowest few km this evening, the risk for measurable precip will remain low through midnight, but will increase west-to-E late with deepening and strengthening SW flow and resultant increasing moist upglide and column moistening. Improving large scale ascent toward morning and minor uptick in forecast MLCAPE with increasingly favorable deep layer shear may support electrification late and will include a mention of isolated thunder, mainly west. The increasing cloud cover, higher dewpoints, and SW breeze overnight as compared to last night will equate to warmer lows, mainly low-mid 60s. -Gih && Short term /Friday and Friday night/... as of 315 PM Thursday... Showers and scattered storms moving into the West County warning area at the start of this period are expected to grow upscale and strengthen as they head east through Fri afternoon and encounter increasing diurnal destabilization within a background of strengthening deep layer shear. A nose of 50-60 kt mid level winds will spread in during the afternoon, just behind a 40-45 kt SW 850 mb jet and beneath strengthening upper divergence along and just ahead of the trough axis. These improving kinematics in conjunction with marginal to moderate destabilization should facilitate organization of convection into a wavy line or bowing segments capable of producing pockets of straight line wind damage, with a lesser risk of a weak tornado or two. The primary threat will be in the afternoon, decreasing by early evening as the surface cold front sweeps to our east and brings about late-night west-to-E clearing. Highs should range from the lower 70s NW, a minimal diurnal rise due to the clouds and precip, ranging to the upper 70s/near 80 east. With cold air advection Fri night, lows should be in the upper 40s to lower 50s. Frequent gusts to 25-30 kts are expected with SW surface winds ahead of the front, with gustiness lingering into early evening immediately behind the front as winds veer to west and west-northwest. -Gih && Long term /Saturday through Thursday/... as of 200 PM Thursday... Quiet weather for the majority of the long term with ridging aloft and surface high pressure building in over the deep south. This quickly moves offshore of the Georgia/SC coast on by Saturday night as the upper ridge flattens out and becomes more zonal. This will leave dry conditions with a westerly component to the winds over the Carolinas resulting in a really nice looking weekend with temperatures Saturday afternoon in the low to mid 70s and then climbing to the upper 70s to low 80s for Sunday as the surface winds swing around to southwesterly. By Sunday afternoon there is a low pressure system moving north of the area through the mid-Atlantic but it looks like most if not all of the precipitation associated with this feature will remain north of the Virginia border as a dry cold front moves across the area Sunday night. Going into early next week surface high pressure will now be northeast of the area for Monday and thus some cooler afternoon highs back into the low to mid 70s. It won't last long though as the high moves out to sea and sinks southward, brining southwesterly return flow back to the region and higher temps for Tuesday into the low to mid 80s. For Wednesday and Thursday, moisture advection does increase, both off the Atlantic and the Gulf and some slight chances move in particularly across the north and west as a frontal zone begins to approach the area with no real organization to it. An afternoon shower or thunderstorm will be possible. Highs in the low 80s. && Aviation /18z Thursday through Tuesday/... as of 200 PM Thursday... VFR conditions are likely to hold through at least this evening across central NC. While mid and high clouds will continue to stream over the area, potentially producing a few sprinkles, the low levels will remain too dry for much wetting of the ground through sunset. As a potent frontal system approaches from the west this evening and tonight, clouds will thicken with lowering bases, and MVFR cigs are expected to rule after 08z west (int/gso), after 10z central (rdu) and after 12z east (rwi/fay), although all terminals should bounce back up to VFR by late morning. A few showers are possible mainly after sunset tonight, but greater coverage of showers and storms, some strong, will arrive late tonight W, continuing through 18z as storms spread eastward. Isolated gusts up to 12-18 kts are possible this afternoon. Winds will be light from the SW tonight, then increase by mid morning Fri to sustained 12-18 kts gusting to 20-27 kts through the end of the taf valid period. Looking beyond 18z Fri, numerous to widespread showers and storms with a good chance of MVFR conditions will be ongoing Fri afternoon, along with brisk/gusty SW winds. A wind shift to west and northwest (still brisk/gusty) and clearing is expected as the cold front moves through central NC in the late Fri afternoon and early evening. Behind the front, VFR conditions are likely late Fri night through sun beneath high pressure. Another front approaching from the northwest may bring a risk of sub-VFR conditions late Sun night into Mon as it briefly stalls over the region. As this front heads back north late Mon and we get back into the warm sector, sub-VFR conditions with stratus/fog may return Mon night/Tue morning. -Gih && Rah watches/warnings/advisories... none. && $$

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