Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 130942 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 442 AM EST Mon Nov 13 2017 .SYNOPSIS...
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A cold front will progress eastward through the first part of Monday. A ridge of high pressure then builds in from the northwest later tonight into early on Wednesday. Though mostly cloudy skies are expected for tonight, clear skies and dry conditions should prevail for Tuesday. Late in the week turns more active with a couple of frontal passages later Wednesday and an even stronger one on Saturday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 400 AM EST Monday... The Appalachians, Blue Ridge and Piedmont this morning awakens to an extensive overcast. Intervals of patchy fog being most common in the New River Valley, Blue Ridge foothills and into the VA and upper NC Piedmont. Stuck between a steady rain shield and approaching cold front which is just beginning to approach Mercer and Tazewell Counties. Region is stuck in a damp, light-flow regime, with trapped moisture underneath dry mid-level air seen on water vapor imagery. These trends will change little through the morning commute, leading to persistent areas of fog/mist and low clouds. Have issued a Special Weather Statement to address low visibilities impacting the Monday morning commute. We`ll be awaiting the westerly windshift with FROPA to help mix out the fog/mist. Visibilities should stay above dense fog advisory criteria, but may for short periods of time drop as low as one-quarter mile. Otherwise, for today the forecast challenge is timing when we can shake free of the low clouds and its effect on temperatures. BUFKIT forecast soundings reveal that trapped moisture may linger in the Piedmont until around noon, while locales in southeast WV, the mountains of NC and much of southwestern VA likely remains blanketed in overcast even into tonight. These trends will keep temperatures on a very slow climb until the afternoon in the Piedmont, while climbing maybe a couple degrees all day along and west of the Blue Ridge up into southeastern WV. Modest cold advection takes place with FROPA, further tempering much warming. Should see a wind shift to west around 10-15 mph with the frontal passage, along with spotty rain showers in our far western counties in WV/VA this morning. Winds gradually abate into the evening. Overall a better second half of the day in the Piedmont and foothills, while more pessimistic in the western mountains of VA, WV and NC. Trended temperatures closer to colder MET guidance, which leads to highs in the mid/upper 40s and into the lower to middle 50s in the VA/NC Piedmont, southside and foothills. Into tonight, surface high becomes positioned over central Ohio with ridging across the region. It`s likely that stratus will still continue in the western mountains early, with at least brief clearing from the Blue Ridge east. There may be enough moisture left to wring out some sprinkles or even flurries in the Greenbrier mountains at elevations above 3500` where profiles turn sub- freezing, but nil accumulations. As flow turns to a northerly/northeasterly after midnight, look for the onset of wedge conditions leading to increased cloudiness immediately along the Blue Ridge into the foothills in NC/VA. Despite 850 mb temperatures of 0 to -2C, surface temps cool more slowly with effects of cloudiness. Indicated lows mainly in the 30s, lowest in the west and toward the upper 30s in the Piedmont, Southside and foothills.
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&& .SHORT TERM /TUESDAY THROUGH THURSDAY NIGHT/...
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As of 400 AM EST Monday... Zonal, highly kinematic flow with numerous embedded short waves will remain in place across the region through this period. Tuesday, the forecast area will be located between the stronger short wave moving off the east coast that will bring the rain Monday and the next in the series of short waves moving east- southeast from the Northern Plains. Meanwhile, a weak dry upper trough will be present over the forecast area Tuesday with weak high pressure at the surface evident from southeast Canada into the southeast U.S. Thus, Tuesday should bring slightly moderating temperatures to near normal levels along with increasing amounts of sunshine compared to Monday. The air mass behind the weather systems this week, at least until the weekend, will remain largely of Pacific origin with no intrusions of Arctic air expected. Any upslope rain/snow showers across the Alleghanys are expected to end Monday night. By Wednesday afternoon/evening, the aforementioned upper trough will be shifting southeast into the region from the Ohio Valley. An associated cold front will move into the forecast area during the afternoon/evening with a line of showers expected to accompany the front into the western parts of the CWA or near the WV/VA border by Wed evening. As the upper trough moves by to our north Wed night/Thu morning, the front will stall to our south near the NC/SC and TN/GA borders. While a few light showers may make it across the Blue Ridge, little precipitation is expected with this system into the VA/NC Piedmont or even Southside VA, leaving much of the area dry again by Thursday, with the exception of the western mountains where upslope clouds and showers may linger. Again, there is no appreciable change in air mass and with a westerly flow, temperatures will remain near to even slightly above normal seasonal levels during the midweek. This will leave high temperatures mostly in the 50s west to the 60s east with lows in the 30s to lower 40s. Thursday we will find the forecast area once again sandwiched between the midweek upper trough with stalled weak front near the NC/SC border yet and weak high pressure evident across the eastern U.S. Skies should be partly to mostly sunny with seasonable temperatures continuing. Rainfall amounts through midweek are expected to remain below 1/4 inch, near none across the Piedmont.
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&& .LONG TERM /FRIDAY THROUGH SUNDAY/...
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As of 420 AM EST Monday... The strongest weather system of the week will arrive during this time frame. The parent driving system will be a strong upper trough moving through south central Canada, but taking on a negative tilt as it moves into the eastern Great Lakes. The trailing cold front will reach the forecast area Friday night into Saturday morning. The warm front to our south may begin to spread some light precipitation into the western mountains in advance of the main front Friday night/Saturday morning. The front will arrive in the form of a narrow yet strongly forced line of showers, perhaps even a QLCS. Have not included thunder just yet, but with Brooks-Craven SIG SVR values creeping up toward 2500, will need to consider with later forecasts. Overnight/morning timing is not favorable for thunder as much either. Some adjustments with timing and intensity of this late week system remain likely as the models in general have not had a lot of run-to-run consistency or consistency among each other on this system, especially with respect to timing. The 00Z ECMWF is nearly 12-18 hours slower than the 00Z GFS, 12Z Sat for the front to come through the RNK CWA per GFS, but not until 00Z or even 06Z per ECMWF. But, these trends are largely opposite of what was displayed with yesterday`s model runs. Another notable difference today is the pattern that evolves in the wake of the Fri/Sat system. Yesterday, the ECMWF and GFS were at odds with the resultant pattern. While the ECMWF carved out a deep upper low across the northeast U.S. dominating the weather for the eastern 2/3rd of the U.S. with colder than normal temperatures and cyclonic flow, the GFS wanted to transition quickly back to a zonal, more progressive flow. However, today, the GFS/Canadian/ECMWF are all in remarkable agreement in depicting a deep upper trough across the east/northeast U.S. into the first half of next week, similar to what the ECMWF had yesterday. This will lead to below to well below normal temperatures in the extended periods (i.e., going into next week) with lows in the 20s and highs in the 30s and 40s west to around 50 in the Piedmont. Precipitation will be below normal with a predominate drier northwest flow aloft. One issue that remains a concern will be gusty northwest winds behind the front with strong CAA/significant pressure rises, and deep low pressure developing across the northeast U.S. For areas along and west of the Blue Ridge, appears solidly in the Wind Advisory category, with the potential for a High Wind Warning at the higher elevations in northwest NC into southwest VA.
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&& .AVIATION /09Z MONDAY THROUGH FRIDAY/... As of 1130 PM EST Sunday... Flying conditions will remain quite variable overnight with sub-VFR conditions lingering a while longer in the east before the shield of rain exits shortly after midnight. Any residual spotty rain should also taper to drizzle or fog over the mountains and elsewhere later tonight as a weak cold front arrives from the northwest. Slight drying behind the boundary may give way to periods of VFR mountains overnight excluding the KLWB-KBLF corridor where upslope will make for IFR/MVFR through early Monday. Weak downsloping northwest flow should also allow for conditions to gradually transition to VFR east of the Blue Ridge Monday morning, while the weak NW flow results in a continuation of IFR conditions with a saturated boundary layer keeping some rain or drizzle around through Monday morning over the far western sites. Could see enough drying to result in VFR from KBCB/KROA east Monday afternoon while at least MVFR cigs persist at KBLF/KLWB per latest HREF output. Weak high pressure builds in Monday night with VFR continuing east of the mountains while sub-VFR may linger west of the Blue Ridge under low clouds until early Tuesday at this point. Extended Discussion... VFR conditions then anticipated Tuesday into early Wednesday. Next chance for possible sub-VFR conditions is with a cold front later Wednesday/Wednesday night time frame. Returning VFR conditions behind the front on Thursday and under weak high pressure Friday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL SHORT TERM...RAB LONG TERM...RAB/WP AVIATION...AL/JH/MBS

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