Area Forecast Discussion
Issued by NWS Blacksburg, VA

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063 FXUS61 KRNK 211156 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 756 AM EDT Sat Oct 21 2017 .SYNOPSIS... The influence of high pressure over the area will continue to maintain dry conditions across the region this weekend. A strong cold front will approach and then cross the region Monday through Tuesday, bringing good coverage of showers and some gusty winds. High pressure returns to the area by the latter half of the upcoming week. && .NEAR TERM /THROUGH TONIGHT/... As of 230 AM EDT Saturday... Surface high pressure was centered over the eastern panhandle of West Virginia. An upper level ridge of high pressure extended from a parent high near Charleston, SC, north-northwest over our region, and farther to over Michigan and western Ontario. Over the course of the next 24 hours, both the surface high and upper ridge will make gradual progression eastward, placing the centers of both off the coast of the Carolinas by late Saturday night. The result across our region will a trend toward increasing southerly flow across the area. This flow will help advect milder air with higher dew points into the region such that high temperatures today will be slightly, by a degree or two, milder than those realized on Friday, especially western sections of the area. Low temperatures tonight will average a few degrees milder than those expected this morning, especially across western sections. While we are expecting some short-lived patchy mountain and river valley fog this morning, the potential increases tonight with the increase in dew point temperature and winds still remaining on the light to calm side. Sunshine today will be abundant again, but with increasing potential for some cirrus clouds to work their way into the area, especially in the west, from mid-day and onward. This trend will continue through tonight across a greater expanse of the region. && .SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/... As of 415 AM EDT Saturday... The strong upper ridge that has been over the area for the past several days will finally shift east to along the NC coast by the end of the day Sunday. The upper flow will become increasingly southerly/meridional as a deep trough develops across the TN Valley in response to the combination of a southern stream upper low moving northeast from the Mid-South and a northern stream upper trough moving southward from the north central U.S. Although clouds will be on the increase as Gulf moisture is drawn northward as well as high clouds from the rapidly increasing PVA/WAA aloft, temperatures will still be warm with southerly flow and 850mb temps hovering in the +14C to +16C range. Monday, will bring a marked deterioration in the weather as the southern stream upper low/trough takes on a negative tilt, begins to lift northeast and phase with the northern stream trough approaching from the northeast. The ECMWF phases these two together as the upper low moves out of the southeast U.S, while the GFS is a bit slower, still indicating that the systems will not phase until they reach the Mid-Atlantic. While some light rain will develop during the day Monday, especially along the southern Blue Ridge thanks to increasing moist southeast upslope flow, a marked increase in showers will begin around or just after sunset Monday evening, tracking across the CWA from southwest to northeast overnight and into Tuesday morning. In fact, the heaviest precipitation should be during the early morning hours Tuesday. Occasional heavy rain showers are expected with PWATS around 1.6, well above normal for late October, and strong PVA/dynamics as the southern stream upper trough takes on a negative tilt and begins to phase with the northern stream upper trough. QPF should average in the 1.5 to 2.0 inch range for much of the CWA. Only brief, minor flooding problems are expected given the recent long dry spell, even though WPC has outlooked our NW NC mountain counties for a slight risk of excessive rainfall and much of the remainder of the CWA west of the Blue Ridge for a marginal risk of excessive rainfall. With respect to the thunderstorm threat, there is a small window of opportunity for thunderstorms early Tuesday morning into mid-morning Tuesday, mainly southeast of a line from UKF-ROA- LYH. During this time frame the combination of a strong 50+kt 850mb LLJ, upper diffluence, weak instability on the order of 100-400 J/kg CAPE, and notable shear will spread across the area in association with the LLJ and trough phasing. Still cannot completely rule out the potential for a QLCS and potential brief spinups within the line across mainly Piedmont/Southside areas. We are not currently outlooked, but this remains something that will need to be watched closely. Otherwise, the bulk of the heavier precipitation will shift east-northeast by afternoon leaving scattered showers in its wake as the northern stream upper low digs southward into the area. A dry slot may limit the bulk of the showers to areas west of the Blue Ridge and north of I-64 at this point. Temperatures will continue well above normal again Sunday, but that will be the last day of highs in the 70s as cloud cover, precipitation, and eventually strong cold advection bring more seasonal conditions to the area. On the other hand Monday morning and Tuesday morning will, however, be very mild and on the order of 20-25 degrees above normal thanks to the cloud cover, showers, and strong southerly flow. Afternoon temperatures will be held mostly in the 60s, or near normal. && .LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 445 AM EDT Saturday... On Wednesday, the upper trough will begin to shift east of the region, but keep cold advection and upslope across the region. This will result in some continued threat for showers across mainly the Alleghanys southward into the western mountains of NC. Possibly early in the morning, low-level temperatures and thickness values will support a mix of rain/snow in the higher terrain, but nothing supports any accumulation or much QPF at this time. With regard to the remainder of the period, all of the extended models have trended much more progressive with the upper trough, lifting it out of here as quickly as Thursday while another upper trough amplifies and moves east from the Midwest. West to southwest flow aloft will return to the area and allow temperatures to begin to moderate as quickly as Thursday. Wednesday will be a rather chilly raw day, especially across the mountains where rain/snow showers and gusty northwest winds are expected as the upper trough reaches its deepest point. Thursday-Friday will see an end to the precipitation and a moderation in temperatures as the upper trough quickly lifts out of the region. Despite the intrusion of cold air, 850mb temps generally remain above zero through the period, a widespread freeze is still not anticipated. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
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As of 745 AM EDT Saturday... High pressure will remain across the area through the TAF valid period yielding primarily VFR conditions. The exception will be a few hours around daybreak this morning and Sunday morning of some mountain and river valley IFR/MVFR fog, mainly west of the Blue Ridge. Any sub-VFR conditions will quickly improve to VFR a couple of hours after sunrise. Winds will be mostly calm or light and variable through the TAF valid period. High confidence in ceilings and visibilities through the TAF valid period, except medium confidence during the 09Z-13Z time frame. Low to medium confidence in wind direction, high confidence in light wind speeds through the TAF valid period. Extended Aviation Discussion... A deep upper trough and associated strong cold front will move into the region from the west by Monday with sub-VFR conditions in rain/showers along with increasing wind. Colder, blustery, unsettled weather with perhaps largely MVFR conditions across the mountains will linger through the later half of next week.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...DS NEAR TERM...DS SHORT TERM...RAB LONG TERM...RAB/WP AVIATION...AMS/DS/MBS/RAB

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