Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 171801 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 101 PM EST Sun Dec 17 2017 .SYNOPSIS... High pressure will settle east across the southeastern US through the weekend. A series of upper level disturbances will move through the region through mid week. && .NEAR TERM /THROUGH TONIGHT/...
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As of 1040 AM Sunday... Visible satellite and 12Z GSO upper air data suggest mid level clouds may have a greater impact than earlier forecast on high temperatures over the nwrn and nrn Piedmont, where highs may struggle to hit 50 degrees. Otherwise, filtered sunshine through considerable high cloudiness that will yield to thickening and lowering mid-high ceilings by this afternoon, should allow temperatures to range into the middle to upper 50s over the srn RAH counties. KFCX radar data indicates virga has already reached as low as 4 thousand ft AGL in the past hour over the Triad, so sprinkles or a slight chance of barely measurable rain is on track area-wide for the afternoon, especially across the nrn and nwrn Piedmont where glancing mid level forcing will be relatively maximized, and into the evening hours over the sern counties. Previous discussion as of 315 AM Sunday follows... Southern stream trough currently over the Oklahoma Panhandle will undergo significant de-amplification over the next 24 hours, shearing apart within the confluent flow as it tracks NE and east through the Ohio Valley during the afternoon and then off the Mid- Atlantic States late this evening, tracking along the western and northern periphery of the mid/upper level ridge in place across the SE US. While the better forcing/lift associated with the weakening trough will remain well north of the area, the associated low to mid-level moist plume, fed by a SWLY 50 kt mid-level jet, will spill east atop the mid/upper level ridge in place. This will result in thickening and lowering cloud ceilings during the afternoon and evening. There could be fairly widespread virga that may reach the ground in the form of patchy light rain or sprinkles by the late afternoon and through the evening/overnight hours. This moisture plume will quickly shift offshore shortly after 06z, with the potential for skies clear out late tonight, especially across the NW Piedmont. The thickening cloud cover could potentially temper afternoon highs, especially across the western Piedmont where earlier onset of thicker cloud cover would coincide with peak afternoon heating. Highs ranging from lower 50s NW to mid/upper 50s south. Lows tonight will be noticeably milder than previous nights, especially across the south, where clouds may linger. Lows ranging from mid/upper 30s NW to mid 40s SE.
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&& .SHORT TERM /MONDAY AND MONDAY NIGHT/... As of 315 AM Sunday... As we await for the closed low over the SW US to eject eastward, the Carolinas will be in-between waves/systems on Monday, under the control of the flat subtropical high ridging north into the SE states. Deep westerly flow will result in a fair wx day with dry conditions along with a brief return of sunshine Afternoon temperatures will moderate into the 60s everywhere, under light southerly winds with the development of a surface trough in the lee of the mountains. Mild overnight lows in the 40s. && .LONG TERM /TUESDAY THROUGH SATURDAY/... As of 212 AM Sunday... Below average confidence continues in this part of the forecast, but generally models are starting to trend toward one another, increasing confidence from previous days. The temperature forecast will be somewhat of a roller coaster, but mainly above to well above normal for mid-December. Best chances for rain will be Tuesday night through Wednesday and again Saturday and Sunday (next weekend). Through Wednesday: The upper level wave (the deamplifying low/trough) exiting the Southwest US on Tuesday will move eastward through the Midwest toward the Mid-Atlantic. Meanwhile, the flow over Central NC will be nearly zonal Tuesday and Tuesday night ahead of the approaching wave. The model differences with this wave as it moves into the region continue with the latest runs as the EC still holds onto a slightly stronger low/wave while the GFS continues to be more open/weaker. Regardless, both models indicate likely rainfall moving into the area from the west-southwest on Wednesday, lingering into Wednesday night. At the surface, a Piedmont trough sets up on Tuesday, keeping the better warm moist advection south of the area. South-southwesterly flow increases over the region Tuesday night into Wednesday as the surface low over the ARKLATEX moves eastward, impinging on the Carolinas and increasing the warm moist advection into Central NC. The placement of the surface low varies between the NAM, GFS and EC, thus so does the southward progression of the approaching cold front toward/into the area. The track of the low will also have an impact on the precipitation forecast as well, but chances increase farther south. Temps Tuesday/Tuesday night will be well above normal, highs in the mid 60s and lows in the low to mid 40s. Wednesday highs will depend on the location of the low and advancement of the front, generally in the mid 50s to low 60s. Wednesday Night through Saturday: In the wake of the upper wave and surface low, the upper level ridge will again extend northward into the Carolinas. A surface cold front will sink through the region, setting up a brief wedge Wednesday, which could stick around (primarily in the far NW) into Friday. The more interesting weather feature will be the upper level trough the digs southward through the Rockies and into Texas, swinging through the Midwest through Friday. A reflection of the upper level system will be extremely evident at the surface as a strong low develops over the Midwest/Plains on Thursday. The surface cold front associated with this low will strengthen as it progresses eastward with a strong Arctic high digging south out of Canada in it wake and increasing south-southwest flow ahead of it. As of the latest model runs, the front appears to get hung up over the Appalachians Friday night into Saturday, however the strong southwesterly flow into Central NC ahead of it will result in increasing chances for rain on Saturday. The coolest day of the period will be Thursday with the wedge/high lingering over the region. However, expect temperatures to moderate ahead of the next system to well above normal again by Friday and Saturday. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/...
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As of 1255 PM Sunday... A mid level trough and associated mid-high level moist axis will lift rapidly enewd across the srn-cntl Appalachians (and NC) through this evening, with an associated thickening and lowering of ceilings down to between 5 and 10 thousand ft for a several hour period late this afternoon and this evening. While most associated precipitation will evaporate before reaching the ground (virga), some light VFR rain or sprinkles will be possible from west to east between 20Z and 03Z Mon. Skies will also generally clear from west to east thereafter/early tonight, although an area of lower level clouds -- in the low VFR to high VFR range-- are expected to expand/develop enewd across SC and srn NC later tonight through early Mon. This cloud area may result in a short period of MVFR ceilings around 2500- 3000 ft at KFAY in the several hours centered around 12Z Mon. Outlook: The nwd retreat of a warm front, and leading edge of a more humid air mass, will result in a medium chance of advection fog, and/or VLIFR stratus, Mon night-Tue morning, particularly at FAY. There will then be a high chance of rain and sub-VFR conditions, heaviest and lowest at FAY, late Tue night through Wed night, with the passage of a low pressure system across the sern U.S.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...CBL NEAR TERM...MWS/CBL SHORT TERM...CBL LONG TERM...KC AVIATION...MWS

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