Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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763 FXUS62 KRAH 220008 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 708 PM EST Sat Jan 21 2017 .SYNOPSIS... An elongated convectively-enhanced surface low will track across the Carolinas through early tonight, before lifting away from the Middle Atlantic coast late tonight. A potent upper level low and attendant low pressure system will track northeast from the Deep South into the Carolinas on Sunday, linger over the area on Monday, then gradually track offshore the Mid- Atlantic coast on Tuesday. && .NEAR TERM /THROUGH TONIGHT/... As of 345 PM Saturday... The warm sector supportive of severe convection is bounded to the west by an effective cold front related to a fast moving QLCS/squall line now stretching S and then SWwd through Srn GA and the FL panhandle, from a 1004 mb surface low near Augusta GA, and to the N by a warm front retreating slowly Nwd across central and E-central SC. This air mass is expected to be shunted Ewd and offshore as the related surface low races to the SC coast during the next several hours. To the north, an old surface low --but nonetheless a vigorous one with wake-low pressure falls of 5-8 mb in just minutes-- has become elongated from near Atlanta to Greenwood SC. This low will continue to move NEwd across south-central NC this evening, along a preceding warm/wedge front, while likely weakening as the earlier deep/severe convection "transfers" energy Ewd to the aforementioned lead frontal system. So while severe weather is not expected in central NC through tonight, and probably not even any thunder for that matter, an area of deformation-type showers --driven by both strong DCVA accompanying the primary MCV from the upstream MCS, and 925-850 mb WAA-- will pivot NEwd across central NC through mid-late evening. Upstream 3-hourly precipitation totals in this area of deformation showers suggests rainfall amounts in central NC are expected to range from around a quarter to half inch roughly from Albemarle to Raleigh, with lighter amounts on the NW edge of the precipitation/ over the NW Piedmont, and also in the dry slot across much of the Sandhills and Srn Coastal Plain. However, a renewed area of showers, and possibly a couple of thunderstorms, are likely to arrive in the Srn NC Piedmont around daybreak, in association with the residuals of what is likely to be another upstream MCS from the Deep South. Once this lead area of rain lifts NEwd, and probably out of the NE Piedmont and Nrn Coastal Plain by around 03Z, the majority of the night is expected to be measurable rain-free. However, the passage of the associated low pressure and rain, and following light Nly surface flow, will serve to reinforce the wedge airmass that has persisted over most of central NC well into this afternoon. As such, widespread very low overcast, areas of (some dense) fog, and mist/drizzle will envelop central NC overnight. Temperatures will hold within a few degrees of current readings, mostly in the 50s, to lower 60s along the Srn tier of counties. && .SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/... As of 320 PM Saturday... ...Severe storms possible, especially over southern and eastern NC, late Sunday.... There should be ongoing showers and possibly a few thunderstorms Sunday morning, associated with increasing upper diffluence ahead of a deepening upper trough over the Deep South and possibly an MCV from Lower Miss Valley convection tonight. Given the stable airmass over central NC today will hold tonight, the convection should be elevated and not pose a severe threat. The biggest concern will be how the early morning precip impacts destablization during the afternoon, as the upper trough closes off and a surface low deepens toward the southern Appalachians. The primary surface low will initially be over the mid-Miss Valley region, while a secondary low develops over northern Georgia. The two lows are then forecast to consolidate and deepen sub-990mb (-4 to -5 STD) somewhere in the vicinity of the TN/NC/VA border area Sunday night, an fairly typical pattern. Thus, confidence is still below average in the details of potential severe weather on Sunday evening. Strong upper level forcing will arrive after 20Z under increasing diffluence and 100+ meter height falls, not to mention the couple left exit/right entrance regions of the upper jet. Convection will increase from the south during the evening as low-level warm advection increases and a 50kt LLJ crosses SC and southeast NC. It seems pretty certain that a QLCS will cross northern FL, GA and SC, and the question continues to be how far north the line will extend as the warm front penetrates inland in the lingering wedge airmass over central NC. The deepening of the surface low just west of the mountains and the isallobaric response should help to draw the low/mid 60 dewpoint air into the the Piedmont, at least as far as US-1, resulting MLCAPE that could approach 1000 J/KG. This combined with a 100kt flow at 500mb and strong backed low-level flow would support some discrete supercells and a straight line wind/tornado threat, especially in the vicinity of the retreating warm front. It is also possible that the QLCS to our south disrupts moisture transport and mass fields to the north, and renders convection less robust. The main takeaway is that there is a lot in place for severe weather, but tonight`s precip and the mesoscale evolution to our south still leads to a fair amount of uncertainty. QPF on Sunday and Sunday night will be heavy in spots given the unseasonably moist airmass, but widespread flooding is not expected. One area to watch may be the northwest Piedmont Sunday night as the upper low pivots over the mountains and moderate is is potentially sustained for several hours. otherwise, the bulk of the deep convection is expected to depart to the east by 06Z Monday && .LONG TERM /MONDAY THROUGH SATURDAY/... As of 230 PM Saturday... The departing upper low will give way to height rises on Tuesday, with a brief stint of high pressure at the surface. The ridging aloft will quickly give way to increasingly cyclonic flow aloft as a broad trough develops over the eastern US through the end of the week. A shortwave moving out of the Southwest is forecast to lift and shear across the Midwest States Wednesday/Thursday, resulting in limited moisture/lift over our region as the cold front crosses the area early Thursday. The GFS and ECWMF are in pretty good agreement on the timing of the front, but the GFS brings a secondary wave through the base of the trough, resulting in anafrontal precip on Thursday. Confidence in the longwave pattern and trend from above normal temps Tuesday/Wednesday toward below normal is high, but the details of the midweek precip are still uncertain. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/... As of 705 PM Saturday... 24-Hour TAF period: High confidence that poor aviation conditions will persist through at least 15Z Sunday and likely through the TAF period at KINT and KGSO. The showers will continue to move NE through and out of central NC by roughly 04Z, with a brief and slight improvement in visibilities where rain is falling. Once the rain ends, expect mostly LIFR/IFR conditions to dominate the remainder of the night and into the day Sunday. There may also be a brief period of low level wind shear tonight between 00Z and 09Z at RDU/FAY/RWI, as a generally Sly low level jet surges northward ahead of the surface low, atop lingering light surface winds. Expect another round or two of rain to move through Central NC on Sunday, with some thunderstorms possible late in the day (best chances at KFAY and KRWI, worst chances at KINT and KGSO). There is still uncertainty with respect to if/when/and to what extent KRDU, KRWI, and KFAY will improve Sunday afternoon. -KC Looking ahead: Generally sub-VFR conditions will persist through Sunday night, as a powerful upper level low and deepening sfc low pressure system track across the TN Valley and Srn Middle Atlantic states. However, a brief lifting to MVFR or scattering to VFR will be possible south of the Triad terminals early Sunday afternoon, when a warm front is expected to retreat farther Nwd into central NC. There will also be episodes of convection, focused around both midday Sunday and again Sunday evening, the latter of which could become severe and produce damaging sfc wind gusts and perhaps an isolated tornado or two. The powerful upper level low and attendant sfc low will linger over the region Monday and result sub-VFR ceilings and scattered convection. Improvement should begin by Tuesday as the system tracks toward the Mid-Atlantic coast, though gusty NW winds are likely Tue afternoon. -MWS && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT TERM...SMITH LONG TERM...SMITH AVIATION...KC/MWS

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