Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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598 FXUS62 KRAH 230008 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 808 PM EDT Sun Oct 22 2017 .SYNOPSIS... High pressure will extend across the eastern United States through tonight. Meanwhile, a strong frontal system will approach from the west and cross the Appalachians and Carolinas late Mon through early Tue. && .NEAR TERM /THROUGH TONIGHT/... As of 335 PM Sunday... Near term forecast on track as eastward drift of the mid-upper ridge offshore will allow a s/w currently crossing the southern Plains to deepen over the TN Valley tonight then swing newd across the lower OH Valley Monday. This feature will aid to strengthen sly flow over our region tonight. This flow will moisten the atmosphere, leading to increasing cloud cover, especially over the western Piedmont this evening, and over the remainder of the forecast area overnight-early Monday. Best isentropic upglide is still positioned primarily over the western-southern Piedmont overnight through early Monday, so the greatest threat for isolated showers will remain in vicinity of the Yadkin/PeeDee Valley after 06Z, and more so toward 12Z. Expect relatively mild conditions tonight with low temperatures varying from the mid 50s in the northeast where skies will remain partly cloudy until late, to the lower 60s southwest where thicker clouds will occur this evening. && .SHORT TERM /MONDAY AND MONDAY NIGHT/... As of 335 PM Sunday... Model difference from 24 hours ago is that the models are trending faster with system crossing our region Monday night. S/w over the TN Valley early Monday will lift newd into the lower OH Valley by Monday evening. This system takes a negative tilt, thanks to strong jet digging on the backside of the system, resulting in upper level heights of of 60-80m over central NC. Low level jet on the order of 40-50kts will supply abundant warm moist air, with precipitable water values of 1.5-1.8 inches expected by late Monday. Additionally, near term model guidance now depicting a sfc wave lifting nwd across the foothills/western Piedmont Monday evening. Expect shower coverage to increase Monday afternoon west of highway 1, encroaching upon the highway 1 corridor after 21Z, more likely toward 00Z, then crossing the coastal plain in the late evening/first half of the overnight. Now appears that the bulk of the showers will exit our far east-ne counties prior to 09Z. While kinematics quite impressive with bulk shear 40-50kts (highly supportive of organized convection), model guidance remains weak/anemic when it comes to low level instability as MLCAPE on the GFS are below 500 J/kg. Storm relative helicity (0-1km) on the order of 150-300 m2/s2 by Monday evening raises concerns for rotating updrafts. Expect a band of heavy showers to progress east-ne across central early Monday evening with isolated thunder probable close to the SC border. Due to strength of the low level flow, strong/locally damaging wind gusts expected along the band of showers, especially where any bowing occurs. Lack of low level instability may deter tornadic threat, though if this parameter were to increase closer to 800-1000 j/kg MLCAPE, then tornadic may be realized. If low level instability achieved, believe tornadic threat will be greatest over the southern and western Piedmont, since this region in closer proximity to the sfc wave. High temperatures Monday highly dependent upon shower coverage. If shower coverage becomes widespread by early afternoon across the west, then high temps may end up being a couple of degrees cooler. Due to expectation that showers will occur late in the day/early evening in the east, potential for temperatures along and east of highway 1 reaching well into the 70s. Expect warm/breezy conditions Monday evening, with cooler air not reaching the western Piedmont until after midnight, and likely not reaching the coastal plain until close to daybreak. && .LONG TERM /TUESDAY THROUGH SUNDAY/... As of 220 PM Sunday... Low level jetting and the associated deep moisture plume will be moving east of the Coastal Plain early Tuesday morning, with drier and cooler air beginning to filter in as the moisture tap cuts off. Cloudiness will be diminishing, with stratocumulus lingering primarily across the northern tier associated with the H85 trof which will be moving across during the afternoon/evening. Cold air advection will be delayed until the surface front exits the area towards sunset, allowing highs to reach mostly lower 70s, with some upper 60s in the northwest. The cool air settles in Tuesday night and Wednesday, with mins mostly in the mid 40s Wednesday morning warming to mostly low and mid 60s in the afternoon. The main long wave trof axis will cross the area Wednesday night as it rotates across the Great Lakes into New England. The ensuing northwest flow will reinforce the cool air and perhaps produce some scattered very light showers in the lee of the mountains, but it will be short-lived as the progressive pattern rapidly transitions to a short wave ridge and rising heights in the late week. Lows Thursday morning will be our coolest of the period, falling to the upper 30s to lower 40s, with highs Thursday topping out 5-7 degrees below normal in the 60 to 65 range. The short-lived ridging shifts offshore and ensuing southerly return flow induces a modest warming trend into the weekend, with highs from 65 to 70 on Friday, and upper 60s to lower 70s on Saturday. Our next front will be approaching from the west, perhaps as early as Saturday night, with associated showers and a cool down into the 60s likely on Sunday. && .AVIATION /00Z MONDAY THROUGH SATURDAY/... As of 805 PM Sunday... 24-Hour TAF period: Not super high confidence with respect to the timing and height of low cigs expected to develop overnight, but for now expect MVFR/IFR cigs at KINT and KGSO, with MVFR/VFR cigs elsewhere through Monday morning. A few isolated showers may develop in the Triad before daybreak, but expect the better coverage and higher intensity showers and storms to move into that area around noon, continuing through roughly midnight. A similar pattern will follow at KRDU, KFAY and KRWI, with slightly later start and end times. There is high confidence for strong to severe thunderstorms to move through the region during the latter half of the 24 hour TAF period, with strong gusts and heavy rain likely to result in reduced visbys. Cigs will likely remain in the MVFR range throughout the duration of the storms. Winds will become strong and gusty after sunrise, mainly out of the south-southeast at 10-12 kts sustained, gusts of 18-25. Higher gusts are possible with the storms. Looking ahead: Showers and thunderstorms will likely continue, at least into Monday night, ending earliest at KINT (around midnight) and latest at KRWI (by 10Z). Gusts from Monday aft/eve should abate overnight, with the exception of a storm moving over a terminal. There could be some lingering sub-vfr conditions through daybreak Tuesday, but expect VFR conditions to dominate thereafter. The exception will be a brief instance of MVFR ceilings Wednesday as an upper level trough passes overhead. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...WSS SHORT TERM...WSS LONG TERM...mlm AVIATION...KC

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