Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 211457 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 957 AM EST Sat Jan 21 2017 .SYNOPSIS... A warm frontal boundary situated across the Gulf Coast states will move north today, moisture increasing across the southern Appalachians with the potential for rain as far north as southern West Virginia and Virgina by this evening. There will be several surges of moisture that will flow northward from the Gulf Coast this weekend, with the potential for the rain to be moderate to heavy at times. The heaviest of the rain will occur Sunday night into Monday as a bonafide area of low pressure develops over the Mid-Mississippi Valley and moves northeast across the area. && .NEAR TERM /THROUGH TONIGHT/...
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As of 957 AM EST Saturday...Dense fog still continues this morning essentially from I-81 east into the Piedmont with visibilities running between one-half and one-quarter mile. Little improvement has been noted since this morning in spite of sunshine trying to induce fog-top mixing. As I noted this morning, the fog itself was very shallow and this is apparent to some extent in area webcams at elevation where skies were mostly clear above the fog layer. While visibility should improve through the morning, we`ll be seeing increasing mid and high clouds racing northeast into the afternoon on 60 kt mid- level southwesterly winds (per 12z RNK RAOB) casts some question as to how quickly visibility will fully clear. Though it may only intermittently meet dense fog criteria during that period, I`ve opted to extend the Dense Fog Advisory through noon. Otherwise, I`ve adjusted/sharpened PoPs up for the mid to late afternoon per a blend of recent HRRR, RAP and 12z NAM output. Large shield of convective precip now across TN/GA looks to advance northward in a degraded sense by mid to late afternoon. Per consensus in high-resolution guidance, this would really just be restricted to our North Carolina counties through 00z Sunday, with only a slight/low Chance level PoP for parts of Virginia. Previous near term discussion issued at 445 AM Saturday follows... Areas of dense fog were common from the New River and Greenbrier Valleys, east across the Blue Ridge and into the Piedmont of Virginia and North Carolina. The airmass over the eastern 2/3rds of the forecast area is stable with shallow wedge of cool air, clear skies, and moist ground helping to promote the fog. The HRRR model suggests the fog will be stubborn to dissipate, lingering as long as 10AM. Today`s challenge will be high temperatures. Fog this morning and increasing clouds from late morning into the afternoon may keep temperatures from reaching their full potential in spite of warmer air aloft. As such, went 2 to 5 degrees blo the MOS forecast highs. Never the less, readings will still be 8 to 15 degrees above normal for Mid January. There are a couple of record highs that may be tested today, Bluefield 60 and Blacksburg 59. However, much will depend on how fast the fog dissipates and the clouds increase this afternoon as to whether these numbers can be tied or exceeded. Today`s rain threat will come from the Gulf Coast Region. A strong upper level disturbance, short wave trough, was ejecting northeast from the Lower Mississippi Valley. Widespread showers and Thunderstorms will be the result across much of the Deep South, but some of this activity is forecast to surge as far north as southern West Virginia and Virginia this evening, although not expecting any thunderstorms this far north. Per the west-east orientation of the warm front along the Gulf Coast, think the bulk of the heavy rainfall and thunderstorms will remain well south of our region. QFP amounts across our forecast area will be significantly less with models suggesting a quarter inch or less from this particular surge of moisture. Onset looks to be mid to late afternoon today with the rain lasting for about 4 to 6 hours before ending. Greatest potential for rain will be east of the Interstate 81 corridor with probabilities increasing substantially from eastern Tennessee and North Carolina southward. Once the rain threat abates during the late evening we will once again be left with a moist ground and the potential for at least patchy fog. Do not think we see any widespread clearing so the probability for dense radiation cooling induced fog appears to be less. Another surge of moisture will arrive late tonight into Sunday. The upper trough that is over the western United States is littered with short wave energy from storm systems which continue to pound the west coast. Yet another piece of energy will eject northeast late tonight with rain overspreading the area by daybreak Sunday, a precursor for more to come later Sunday into Monday, each wave having the potential to produce higher amounts of rain due to the increasing available moisture flowing north from the Gulf Coast Region. By Sunday, a bonafide area of low pressure will develop over the Mid-Mississippi Valley, a feature that will have to be closely watched as it may focus a much larger fetch of rain into our region leading to the potential for excessive rainfall.
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&& .SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/... As of 345 AM EST Saturday... During this portion of the forecast, the feature we will be watching is a closed upper low that will track from the mid-Mississippi Valley eastward, and then northeastward across our region. In advance of this system, low level flow with Gulf of Mexico origins will advance across our area. Contained within this southwest flow will be individual shortwave troughs ahead of the main upper low. A generous rainfall is expected during this portion of the forecast. Anticipate distinct rounds of precipitation. The first will be during the day Sunday, primarily the morning hours. After perhaps a six hour decrease in coverage Sunday afternoon into early Sunday evening, precipitation on the northeast flank of the parent upper low is expected to arrive across the area. As Sunday night progresses, the center of the upper low is progged to move northeast over the forecast area. Guidance offers solutions that place first the northern and the western flank deformation zones over the region, with western sections of the area having the best chance for continuous moderate rain heading into Monday morning. By Monday afternoon and evening, the center of the low is expected to heading northeast of the region with the flow over our region becoming northwest. Latest guidance continues to ramp up the 850mb winds across the region, maintaing upslope precipitation in the west with wind speeds and gusts potentially reaching at least wind advisory levels at the highest elevations across at least the Northern Mountains of North Carolina. Another concern with the stronger winds will be a greater chance of downed trees without typical wind thresholds for doing so thanks to what will become very saturated soils across the region. Rainfall amounts within the Sunday through Monday night time frame are expected to average between 1.50 and 2.00 inches, with locally higher amounts near 2.50 inches along the crest of the Blue Ridge and other higher peaks where a southeast upslope flow amplifies precipitation production. With this amount of precipitation expected over a multi-day period, localized flooding, as well as, rises or rivers and streams towards the end of the period, or slightly after the end of the period cannot be ruled out. This concern will continue to be mentioned in the Hazardous Weather Outlook. Cold air advection will also establish itself across the area, with some of the precipitation in the west transitioning to light snow across the higher elevations of southeast West Virginia, south into the Northern Mountains of North Carolina late Monday night into Tuesday morning. Upslope rain/snow showers will continue into Tuesday night but with decreasing coverage through the night. Winds will decrease as the low level flow veers southwest as the axis of an upper ridge shifts east of the region. Isolated thunderstorms are possible at times during this portion of the forecast. Currently we are watching Sunday and Sunday evening as potential time frames across primarily the southern third of the region based upon model forecast Lifted Indices slightly below zero, and surface based CAPE of a few hundred J/KG. Temperatures during this portion of the forecast will average fifteen to twenty degrees above normal through Tuesday. Cooler, but still above normal, temperatures are expected Tuesday night. && .LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 415 PM EST Friday... Shortwave ridging continues Wednesday into Wednesday night with weak surface high pressure extending north into the region. This should result in a period of dry/mild weather for midweek with highs well above normal and likely 60s east of the mountains. This will begin to mark the end of the period of above normal temps and bring the reality of winter back as a longwave trough pushes southeast and across the region by the end of the week. Cold front preceding this upper trough will cross the region on Thursday which will be the transition day to colder air that arrives on Friday. Weak impulse sliding up ahead/along the boundary could produce a band of rain/showers by Thursday morning before things switch to a mainly mountain upslope snow shower regime under cold northwest flow within the cyclonic trajectory by Friday. Expect highs by then to only range from 30s west to low/mid 40s east under blustery northwest breezes. && .AVIATION /15Z SATURDAY THROUGH WEDNESDAY/... As of 700 AM EST Saturday... Poor aviation conditions expected through 14z/10AM with IFR/LIFR along and east of the Appalachian divide...widespread fog present from the mountains to the coast. West of the Appalachian Divide (from BLF and west) conditions were VFR. Areas with fog will see a gradual improvement late morning into early afternoon to at least MVFR and possibly VFR. Any VFR will be short lived per increasing moisture from the gulf coast region. Showers and Thunderstorms will encompass much of the southeast States today, some of this activity reaching as far north as southern WVA and southern VA this evening. Arrival of rain will bring lowering cloudbases and vsbys. Models indicate a break in the rain tonight before another surge of moisture arrives daybreak Sunday. In spite of the break in precip tonight, both cigs and vsbys will remain low per redevelopment of fog and stratus. Winds through the 24 hour Taf period are forecast to remain under 8kts and will be near calm at times. Extended Aviation Discussion... A slow-evolving but potent storm system affecting a good part of the central and southeast CONUS will lead to a potentially long period of sub-VFR conditions through Monday with stratus, fog, and periods of rain, in addition to thunderstorms. Once the storm passes on Tuesday, conditions will improve with respect to cigs and vsbys, but it will become breezy with northwest winds gusting to 30 kts behind the departing storm on Tuesday. && .CLIMATE... As of 305 PM EST Thursday... Record warm Mins for January 21 Blacksburg....42 in 1954 Bluefield.....47 in 1999 Danville......56 in 1954 Lynchburg.....51 in 1927 Roanoke.......51 in 1959 Record highs for January 21 Blacksburg....59 in 1954 Bluefield.....60 in 1999 Danville......68 in 1959 Lynchburg.....72 in 1932 Roanoke.......74 in 1932 && .RNK WATCHES/WARNINGS/ADVISORIES...
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VA...Dense Fog Advisory until noon EST today for VAZ010>014-016>020- 022>024-032>035-043>047-058-059. NC...Dense Fog Advisory until noon EST today for NCZ002>006-019-020. WV...Dense Fog Advisory until noon EST today for WVZ044-507.
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&& $$ SYNOPSIS...PM NEAR TERM...AL/PM SHORT TERM...DS LONG TERM...JH AVIATION...PM CLIMATE...AMS

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