Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 242338 AFDRAH AREA FORECAST DISCUSSION National Weather Service Raleigh NC 738 PM EDT Sun Oct 24 2021 .SYNOPSIS... High pressure will move off the mid-Atlantic coast this evening. An area of low pressure will develop off the SE coast late tonight and will track off the NC coast during the day on Monday as cold front sweeps east through the area during the evening and overnight hours. High pressure will then build into the area Tuesday and Wednesday. && .NEAR TERM /TONIGHT/... As of 340 PM Sunday... Surface high pressure will move offshore this evening as a cold front approaches from the west. Meanwhile, an area of low pressure is forecast to develop off the SE US coast late tonight as a weak shortwave trough(currently over the Gulf Coast region)eject ENE through the SE US and into NC by daybreak. Strengthening southerly warm moist advection in both the lower and upper levels and in concert with lift/forcing associated with DPVA moving into the region will lead to increasing multi-layer cloudiness overnight, including the potential for stratus(mostly likely across the coastal plain)and possibly even some fog a few spots. Additionally, increasing nocturnal destablization associated with the Atlantic maritime feed advecting inland into the coastal plain counties will support a slight to small chance PoPs towards sunrise. Milder overnight with lows 55-60. && .SHORT TERM /MONDAY THROUGH MONDAY NIGHT/... As of 340 PM Sunday... ...Late season severe weather possible late Monday afternoon and Monday night... ...However increasing model spread resulting in low forecaster confidence... A mid/upper level trough over the Ohio Valley will become increasingly negative tilted as it moves into the Middle Atlantic region through Monday night. Attendant sfc cold front will cross central NC during the evening and overnight hours. Meanwhile, in advance of this trough, a much weaker shortwave will move NE through the area through midday. The coastal low off the SE coast is forecast to undergo slow and gradual deepening as it tracks off the NC on Monday, followed by the potential for more rapid deepening(possible bombogenesis???)as it interacts/merges with the synoptic trough/cold front off the mid-Atlantic coast Monday night and into Tuesday. Some interesting differences have developed among the latest 12z/24 NWP model guidance, which is unfortunately resulting in growing forecast uncertainty, including the potential for severe weather Monday afternoon/evening. The biggest changes observed in the model guidance over the past 12- 24 hours and the primary source for decreasing forecast confidence and wide range of model solutions is related to the depiction of a more pronounced surface low off the NC coast. As a result of the stronger coastal low, models are exhibiting a strong convective response near/along the Carolina coast with pending ramifications inland, across central NC. The low-level wind fields will be especially sensitive to the evolution of the coastal low and associated convection, with some of the guidance showing significantly weaker and a drier/more westerly component which reduces/disrupts the moisture/instability transport into central NC. The latest 12z NAM and GFS are in such a camp, indicating a minimum of convective coverage and intensity across central NC Monday afternoon and evening as the strongest forcing moves off to our north and east(Carolina split). Meanwhile, we still have some guidance, including some CAMS that still show a well-organized line of showers and storms that will cross the area during the afternoon and late evening. Limited by weak mid-level lapse rates, as well as the potential for some rain- cooled air from pre-frontal shower activity during the late morning/early afternoon, instability is forecast to be weak, between 500-750 J/Kg MLCAPE. However, robust deep layer shear of 50-60 kt will support potential for some isolated damaging wind the main threat during the afternoon into the evening. Given the high uncertainty, will not make any big chances from previous forecast and will maintain likely to categorical PoPs Monday afternoon and evening, while continuing to advertise a chance for the potential strong to severe storms, with the best chance being across the western/NW Piedmont. Highs Monday in the mid 70s to lower 80s. Cooler behind the cold frontal passage Tuesday morning, ranging from lower 50s NW to mid/upper 50s SE. && .LONG TERM /TUESDAY THROUGH SUNDAY/... As of 300 PM Sunday... Strong upper level low off the Pacific NW coast will move onshore early this week and weaken into a deep trough as it moves across the western half of the US. By mid-week, the trough strengthens again and digs as far south as Texas with a redeveloping upper level low over the southern portion of the MS Valley. From there, the upper level low moves up across the OH Valley and into the Northeast by the weekend. Moisture values across central NC will remain low Tuesday and Wednesday, but will increase as the ridge is push offshore ahead of the strong trough, thus mostly clear skies for Tuesday and Wednesday with cloud cover increasing early Thursday morning. At the surface, high pressure builds in Tuesday behind the cold front that crossed the region Monday night. Tuesday will be sunny and breezy with winds from the NW and gusts ranging from 20 to 30 mph (stronger winds occurring in the NW Piedmont). Thursday, the strengthening surface low over the MS/TN Valley slowly treks up the Appalachian Mountain region through the rest of the week bringing chances of precip for the rest of the week. Models do diverge here a bit on the track of the low as well as the timing, thus medium confidence for the rest of the forecast. As the low moves NE, a warm front extends east across the southeastern states bringing warm moist advection to NC, which will increase the risk of thunderstorms across the region late Thursday into Friday. As this storm system is much stronger than the previous one it also has better dynamics with it as well. As southerly flow and deep moisture move in with the warm front it will be accompanied by PW values of 1.50 to 1.75 inches, which is about 1 inch above normal. Therefore rainfall amounts are expected to be higher than Mondays. With a limited amount of instability during this time frame, there could be enough forcing from such a deep low to make up for it. Shear values are also pretty strong with values in the 0-3km ranging from 40 to 60 kts and 60 to 75 in the 0 to 6 km. Thus, severe storms could be possible Thursday afternoon and evening. Again this all depends on the track and timing of the low, will need to take a closer look as the period gets closer. Temperatures this week will be near normal or slightly above with highs mainly in the 60s (with some low 70s in the S and SE) and lows in the 40s and 50s. && .AVIATION /00Z MONDAY THROUGH FRIDAY/...
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As of 735 PM Sunday... 24 hour TAF period: VFR condition will persist this evening through early tonight. Confidence is still on the low side with respect to timing and extent of sub-VFR conditions developing early Monday morning and chances for showers/storms through the TAF period. MVFR cigs are possible at all terminals between 10Z and 14Z Monday, while MVFR (maybe IFR) vsbys are possible at KRDU, KRWI and KFAY, though the extent of the latter may be limited to between the terminals. Also, as the strengthening low moves along the Carolina coast Monday, there will be a chance for some showers from south to north at the eastern 3 terminals, most likely at KFAY and then KRWI. During the late aft/eve the focus will shift to the western terminals KINT and KGSO as convection is expected to move through from the west ahead of an advancing cold front. Impacts at the eastern terminals will occur closer to or just after the end of the TAF period. Strong wind gusts and sub-VFR conditions are expected with the later convection. -KC Looking ahead: The convection should move east through the rest of the area by 06z Tuesday with dry VFR conditions returning Tuesday and Wednesday. Another storm system will likely bring adverse aviation conditions Thursday into Friday. -CBL
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&& .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...CBL NEAR TERM...CBL SHORT TERM...CBL LONG TERM...CA AVIATION...KC/CBL

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