Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 212357 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 657 PM EST Sat Jan 21 2017 .SYNOPSIS... A weak warm front south of the area will continue to be the focus for impulses to travel along overnight into Sunday, resulting in added rounds of rainfall tonight through Sunday afternoon. The front should lift north into the region Sunday night as a stronger area of low pressure approaches from the west. This low pressure system will slide slowly east across across the area Monday before exiting Monday night, allowing for drier weather under high pressure for Tuesday. && .NEAR TERM /THROUGH SUNDAY/... As of 305 PM EST Saturday... Active near-term forecast period begins tonight continuing into a good part of the short-term period due to multiple rounds of steady rainfall to affect the Blacksburg forecast area. Late-afternoon surface analysis depicts a 1003 mb low near Atlanta, enhanced to some extent by ongoing strong convective line marching across southern Georgia, with eastward-trailing warm front from the low center to a position from Greenville/Spartanburg SC to just north of Florence SC to Wilmington NC. Lingering in-situ wedge lies north of it across our area, keeping most of our forecast area in the cool, cloudy side. Aloft, diffluent mid-level regime with increasing 500 mb height falls approaching far southwest Virginia. Composite radar mosaic reveals band of steady rain along the northern periphery of warm front/mesolow advancing to the northeast, its northern extent from Knoxville TN to near Charlotte NC. For Tonight: First band of steady light to at times moderate rain to progress primarily across the Grayson Highlands, southside Virginia and northwestern North Carolina between roughly 21z and 04z per high- res HRRR and RAP. Highest PoPs are focused across that general area, with a lower/gradient in PoPs north to northwest essentially west of the Blue Ridge. Appears that the northern fringe of negative Showalter indices snakes its way into those aforementioned southern areas. Included slight chance Thunder mention accordingly, but I did not want to hit that harder given the best potential for convection remains well south of our forecast area into central/western NC on southward. Rainfall amounts for tonight range from a few hundreths in the northwestern part of the CWA to around four-tenths along the North Carolina Piedmont into southside Virginia. In the wake of the rain, indication from most guidance is that the wedge/CAD rebuilds back in with light east flow. Thus for the after midnight period, I`ve again introduced patchy fog for most of our forecast area as we`ll be waiting for the second, more significant/widespread round of rain approaching from the southwest. Forecast lows in the mid 40s to low 50s under persistent overcast and areas of rain showers. For Sunday: Upper trough closes off in the lower Mississippi Valley early Sunday morning. Aforementioned warm front then lifts north across much of our forecast area. This focuses a period of moderate rainfall with rainfall rates enhanced somewhat by (1) convection - elevated negative Showalter Indices advect into our central portion of our CWA per NAM/GFS and (2) potential for southeasterly upslope enhancement in the foothills and Piedmont areas in Virginia and North Carolina. SPC maintains a Marginal/5% severe and Slight/15% severe Sunday, primarily across our Piedmont NC counties into the southside of Virginia due to strong vertical wind profiles. However only very marginal if any surface-based CAPE with much of the already modest instability being elevated. Potential is there for thunder but given the limited amount of instability owing to early- day CAD, not thinking we`ll see any severe. A conditionally greater risk for stronger cells remains well to the south from central NC to FL and as reflected in SPC`s Day-2 Convective Outlook. Rainfall stands to be the heaviest during the morning into early afternoon hours. Interesting that to some varying extent amongst the 12z models there is something of a lull in the rain during the afternoon. So while PoPs start high-Likely to Categorical, I`ve tried to show a reduction in PoPs to reflect a break in rain, though they are still high-Chance (40-50%) and only stand to increase again from SW - > NE Sunday night. All in all, anticipate a wet Sunday with at least some rise on area rivers and streams. See the Hydro section which speaks more to the hydrologic aspect given a few days of at least steady rains (spanning into the short-term period). Looking for highs Sunday to range from the mid 50s to around 60. && .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/... As of 245 PM EST Saturday... Next round of heavier rainfall including possible embedded stronger storms across the far south will be ongoing Sunday evening as very strong lift/diffluence arcs north ahead of deep low pressure over the western Carolinas. This will likely bring the heaviest period of rainfall given more convective nature under 1.25 inch PWATS just ahead of the advancing warm front and subsequent possible triple point just off to the southwest. However just how much of the south and/or east can work into the warm sector remains in question, with models showing the low level wedge in the wake of earlier rainfall holding near the VA/NC border before weakening by early Monday. Since shear is quite impressive and strength of warm advection stronger than usual, per such a stacked system given the upper low passing near the Blue Ridge, opting to include more thunder mention espcly southeast into Sunday night. Also cant rule out at least some elevated rumbles elsewhere overnight espcly Sunday evening southern Blue Ridge where seeing a brief but strong upslope component. Otherwise pops returning to categorical into early Monday across the region with lows in the 40s but perhaps rising some early on. Upper dry slot may act to cut off showers across the east Monday morning as the center of the upper low drifts into western VA and eventually to the east/northeast of the area by Monday night. This likely to shift bands of heavier rainfall over the west during Monday as the deformation zone develops aided by wrap around moisture behind the exiting upper low. However uncertainty remains espcly by Monday with the track of this system including the degree of more concentrated heavy rain, and where precip may be more convective resulting in higher rates. Models have again shifted east with QPF totals given more convective nature over the southeast and more in the way of stratiform rain closer to the low at this point. This on top of previous rainfall would likely bring the highest threat for flooding pending exactly where added bands set up, so keeping mention in the HWO and hydro section of the AFD for now. Highs Monday remain tricky with potential for values to zoom to around 60 Piedmont while staying in the 40s northwest, with low/mid 50s in between. Axis of deeper moisture should continue to fan out to the west before shrinking along the higher western mountains Monday night as the low and associated lift depart. However increasing upslope northwest flow in the wake of this system likely to keep showers going western third overnight, while some drying and cutoff in pops takes shape east of the Blue Ridge. Temps aloft also appear too warm for much snow so leaving out mention except highest elevations late Monday night. Main concern overnight will again be with strong winds as the low heads offshore and the pressure gradient between high pressure to the south increases. Latest input of strong subsidence, brief cold advection and a lowering inversion into local scheme data off the GFS, still suggests potential wind advisory criteria mainly NC mountains. Therefore have again included a mention in the HWO there overnight. Otherwise running with diminishing pops overnight with only chance pops lingering north/west late and little elsewhere. Lows mostly 30s to lower 40s. && .LONG TERM /TUESDAY THROUGH SATURDAY/... As of 215 PM EST Saturday... Strong upper trough will be lifting out on Tuesday making way for a brief period of shortwave ridging that will slide east and across the area later Tuesday into Wednesday night. May still be some upslope driven rain and high elevation snow showers Tuesday morning before much drier air slides in from the southwest under a nose of surface high pressure that will extend north from the Gulf coast. This should bring clearing skies Tuesday afternoon with sunny skies on Wednesday before clouds increase again later Wednesday night ahead of the next upstream cold front. Winds should remain quite strong into midday Tuesday with potential advisory level speeds espcly NC mountains into far southwest VA. Will be the mildest period for quite some time with highs 40s mountains to upper 50s east Tuesday, followed by 50s to lower 60s Wednesday under warmth aloft and westerly flow. Return to more winterlike conditions will ensue by weeks end with a cold frontal passage by Thursday ahead of a broad longwave trough that will drop south into the region and persist into next weekend. However latest guidance not nearly as strong or deep with this feature making for more Canadian type air vs. much colder thickness seen yesterday that was supportive of Arctic air. However will see persistent cold advection develop by Thursday with weak passing shortwaves gradually helping to bring in colder air aloft by Saturday. A band of showers possible along/behind the front Thursday into Thursday night although iffy given only sheared southern energy. Therefore will only include a low pop shower mention including possible western snow showers overnight. Otherwise will evolve to mostly upslope driven clouds and periodic light snow showers far west, to mainly clear/sunny east Friday into Saturday, under a rather dry and more westerly flow driven environment. Highs mostly 30s to around 40 mountains to mid 40s east into the weekend. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/... As of 658 PM EST Saturday... Poor flying conditions expected during the taf period with widespread IFR/LIFR in low clouds,fog and rain. This evening into tonight, flight categories will lower with patches of rain and drizzle. Pocket of fog in the east will degrade ceilings and visibilities to IFR and LIFR conditions at KROA,KLYH and KDAN. While in the west at KBCB, KLWB and KBLF expect VFR conditions to lower to MVFR in rain and fog. Rain will pull out of the east early tonight. Then, in-situ wedge develops in behind the rain again focusing a period of IFR to VLIFR conditions due to stratus and fog. Second wave of steadier rainfall then looks to build from the southwest during the morning hours Sunday, with coverage of rain covering a larger portion of our forecast airspace along with a few rumbles of thunder. Due to low forecast confidence in specific airports being affected by thunder, did not mention in the TAF at this point, but the best shot would be at KLYH, KDAN and KROA. Flight categories IFR to VLIFR through 00z Monday, due to low ceilings, with visibilities 3-6 SM in rain briefly heavy at times. Winds generally light and variable (trending southeast) through most of the TAF period. Low confidence in ceilings,visibilities and wind during the taf period. 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. High pressure prevail for Tuesday night and Wednesday night. A band of MVFR showers possible along/behind the front Thursday into Thursday night. && .HYDROLOGY... As of 310 PM EST Saturday... There will be several rounds of rainfall affecting the Blacksburg hydrologic service area beginning tonight and lasting through Monday. The greatest amount of rain over a 24-hour period looks to be on Sunday with 1 to 2 inches of rain across most of the forecast area, with lighter rainfall amounts expected tonight across the southern tier of counties/Dan and Yadkin River basins and again Monday along and west of the Appalachian Divide into southeast West Virginia, the Mountain Empire of Virginia and into parts of the New River Valley. Using a model blend, heavily weighting WPC in that blend, forecast rainfall amounts tonight through Sunday range from 1.5" for most of southeast West Virginia and the Alleghany Highlands, to well over 2 inches along the east-facing slope of the Blue Ridge. Current expectation is for gradual main-stem river rises with faster/greater response on smaller rivers/creeks, along with some potential for ponding on roads and in areas of poor drainage. Rainfall rates do not appear excessive at this point. The only river forecast point projected to go just into Minor flood stage is the Dan River at South Boston. That may be a worst-case, and it would take a greater amount of rain over a shorter period (more convective rain rates) to increase the risk for minor flooding across the forecast area. Given that confidence is low in flooding, a flood watch has been issued at this time and would rather wait to see what tonight`s rainfall brings before that is considered. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/PM NEAR TERM...AL SHORT TERM...JH LONG TERM...JH AVIATION...AL/KK HYDROLOGY...AL is the U.S. government's official web portal to all federal, state and local government web resources and services.