Area Forecast Discussion
Issued by NWS Blacksburg, VA

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352 FXUS61 KRNK 230209 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1009 PM EDT Sat Apr 22 2017 .SYNOPSIS... A frontal boundary will stall out across our area through Sunday with a wave of low pressure moving along it. Low pressure will slowly track along the front through the Tennessee Valley and into the Carolinas. The front moves southeast by Monday, with high pressure building in Tuesday. && .NEAR TERM /THROUGH SUNDAY/...
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As of 1000 PM EDT Saturday... Convective threat is over, now we move into the flood threat of this large-scale synoptic system. First, the backdoor/wedge front has slide generally south and southwest of the CWA, with the possible exception of the far southwest corner of the CWA. Solid cloud cover and rainfall will ensure that this remains in place until the large-scale synoptic system moves out of the area Tuesday. The upper low that will be the main culprit behind the evolving pattern is currently located across the western TN Valley. The low will intensify and deepen as southern stream short wave energy translates from the desert SW into the existing upper low. As the upper low moves into the Midsouth/Southeast Sunday night, it will take on a negative tilt, slow, and spin slowly in a northeastward direction from GA/SC to off the VA coast in the Mon-Tue time frame. Thus, the best dynamics for heavy rainfall and training of echoes will be in the Sunday night/Monday morning time frame. HRRR and radar trends suggest that rainfall overnight may tend to be rather meager, and likely less than was previously expected. Therefore, have decreased rainfall amounts some from what was previously indicated. All threat of thunder has ended. There may again be another lull in the precipitation Sunday morning until the next wave of heavier rainfall arrives Sunday afternoon. Again, the main concern from dynamics/synoptic support will not arrive until Sunday night into Monday morning. Heavy rainfall potential may linger into the area through much of the day Monday. Other than decreasing rainfall amounts in the near term, am not making any changes to longer term rainfall or the flood watch. Temperatures will remain unseasonably cool Sunday into Monday as the wedge persists and is continually reinforced with rainfall and solid clouds cover along with northeast flow. As of 450 PM EDT Saturday... Severe thunderstorm watch remains in effect until 9 PM this evening for southeast portions of Forecast Area. Flood Watch has been posted for midnight to Monday evening for most of the region. Frontal boundary continues to slowly drop south across the region this afternoon, before it stalls tonight into Sunday. Scattered showers and thunderstorms will develop ahead and along the front. With the instability and shear, a few strong to severe thunderstorms capable of damaging wind gusts and large hail will be possible this afternoon and evening. Localized flash flooding may be possible this afternoon into tonight with any training of thunderstorms with heavy rainfall. Look for a southward shift in convection this evening into tonight, then as next upper trough moves east into Tennessee. A surface low will shift to eastern Tennessee, then eastward to South Carolina by Sunday night. This will result in a northward shift in the rain with the wedge reinforcing. For pops used a blend of HRRR and NAM. Low temperatures tonight will range from around 40 degrees in the northwest mountains of Greenbrier county to the lower 50s in the Piedmont. Cloudy and cooler Sunday with occasional rain, heavy at times. As the rain continues to fall, the ground will become saturated and the potential for flooding will increase with time. The widespread potential starts at midnight tonight and continues into Sunday. High temperatures Sunday will be cool with readings from the upper 40s in the mountains to the mid 50s in the Piedmont.
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&& .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... As of 440 PM EDT Saturday... The upper low tracks southeast and will be off the coast Monday night before making a turn to the northeast on Tuesday. Wedge at the surface remains in place through Monday night then pressure lower enough as the surface low reaches the North Carolina coast to bring winds around to the north then northwest and erode the wedge. Models showed a prolonged period of decent easterly winds which will aide in widespread to heavy rain Sunday night. The low is far enough offshore that a bulk of the moderate to heavy rain will move east of Lynchburg and Danville by Monday morning. However, there is enough moisture wrapping around the low that chance of showers in the foothills and piedmont extends through Tuesday. Stayed close to MAV guidance for temperatures through the period. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... As of 100 PM EDT Saturday... Brief period of upper ridging and higher 500MB heights Tuesday night and Wednesday. By then the long wave trough deepens in the central United States putting the region in broad southwest flow Thursday through Saturday. Models were showing varying degrees of strength of the southeast ridge. Low pressure tracking northeast into the Great Lakes will move a front into the Ohio and Tennessee Valleys on Thursday. This boundary stalls on Friday before another low moving out of the southern Plains pushes a warm front into the Ohio Valley on Saturday. This pattern keeps the rain potential to the west of the Mid Atlantic states. Temperatures will be above normal Wednesday and Thursday. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/... As of 750 PM EDT Saturday... Poor aviation conditions expected through the TAF valid period. Upper low across western TN will move southeast into GA, then slowly light northeast with a negative tilt Mon-Tue to off the Mid-Atlantic coast. High pressure anchored over the northeastern U.S. will combine with the upper low and surface low pressure to the south to result in a prolonged period of widespread rain, low clouds, and induced fog across the forecast area through the TAF valid period and beyond. Look for IFR-LIFR cigs generally in place at all TAF sites through the TAF valid period. Visibilities will be dependent on areas of heavier rainfall, but prevailing MVFR visibilities in -RA BR can be expected, with periods/pockets of IFR visibilities. Higher terrain will see more prolonged periods of IFR and even LIFR visibilities, but for the TAF sites proper, generally MVFR visibilities are expected through the TAF valid period. Winds will be NE 7-10kts with low end gusts likely across the Piedmont through the TAF valid period. High confidence in cigs through the TAF valid period. Medium to high confidence in vsbys through the TAF valid period. High confidence in wind dir/speed through the TAF valid period. Extended Aviation Discussion... Poor aviation conditions expected to continue through Monday. Some improvement will begin late Monday, but more likely into Tuesday as the upper low moves northeast of the area and the surface winds come around to the northeast as opposed to the northeast. This will allow ceilings to improve to MVFR and visibilities to VFR as the rain comes to an end or at least becomes much more showery in nature. VFR conditions are expected Wed-Thu as high pressure, but surface and aloft, build over the area. Another weather system will bring showers and possibly thunderstorms into the area late in the week. && .HYDROLOGY... As of 330 PM EDT SATURDAY... Flood watch has been posted for much of the region from Midnight tonight until Monday evening. Despite the semi- wet week, antecedent conditions are fairly dry across the eastern 2/3 of the CWA and generally about normal in the west. Moderate drought is still depicted on the U.S. Drought Monitor across much of the piedmont and Abnormally Dry over most of the Blue Ridge and surrounding area with near normal conditions in the west. This suggests that we can absorb more water than might be typical. 3-hour flash flood guidance from the River Forecast Centers ranges generally from about 1.5 inches in the west up to around 3 to 3.5 inches in the west, reflecting well the drier conditions east of the mountains. A slight risk for convective rainfall exceeding flash flood guidance was issued by WPC earlier for Day 1 and Day 2. Pockets of minor advisory-type flooding cannot be ruled out in convective storms which will be more likely tonight and Sunday and further south in the CWA where instability may be much higher. Any training convection over the same basins could produce pockets of flash flooding. At this point the river forecast is purely QPF based. Model QPFs have shown some decent run-to-run consistency over the last few cycles providing more confidence that this will fulfill expectations. Current WPC QPF in the day 1-3 period (this afternoon through early Monday) period is generally about 2 to 4 inches, with the bulk of it falling Sunday and current WFO grids are close to these numbers. The prolonged nature of the rainfall (48 to 72 hours) will lessen the risk of serious river flooding (and flash flooding) as runoff will be more spread out and less efficient. The two best St. Louis University CIPS analogs to this event are April 9-12, 2003 and March 28-30, 2010 both of which featured upper lows over the southeastern U.S. Both events resulted in minor to moderate river flooding on the Dan River but not on the New, James, upper Tennessee or upper Roanoke Rivers. Ensemble river forecasts from the GEFS ensemble are also highlighting the Dan and portions of the Roanoke basins as the most likely to flood, suggesting a greater likelihood for moderate or higher flooding but the NAEFS has been consistently showing lower probabilities. && .RNK WATCHES/WARNINGS/ADVISORIES...
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VA...Flood Watch through Monday evening for VAZ007-009>018-022-023- 032>034-043>046-058-059. NC...Flood Watch through Monday evening for NCZ001>006-018>020. WV...Flood Watch through Monday evening for WVZ042.
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&& $$ SYNOPSIS...WP NEAR TERM...KK/RAB SHORT TERM...AMS LONG TERM...AMS AVIATION...AMS/KK/RAB HYDROLOGY...KK/MBS/PC

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