Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 200446 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1246 AM EDT Mon Aug 20 2018 .SYNOPSIS... A front will remain stationary over the Mid Atlantic region overnight through Monday and then lift north as a warm front Monday night. Low pressure over the Mid Mississippi Valley on Monday night will track northeast into Canada by Wednesday morning, pushing a strong cold front through the region. Cooler and drier high pressure builds into the eastern United States for Thursday and Friday. && .NEAR TERM /THROUGH TODAY/... As of 905 PM EDT Sunday... Main concern continues with heavy rain producing convection across the north and east where analysis showing an axis of higher instability ahead of an initial wave aloft to the west. Latest evening soundings show quite a wet environment for showers to continue into the overnight given Pwats near 2 inches within a rather unidirectional but diffluent mid level flow aloft. This along with a weak surface front over the area likely to keep slow moving convection going with some back building still possible ahead of yet another stronger wave to the northwest. Moisture also trapped beneath the weak inversion aloft conducive to warm rain processes with showers tending to stay far enough east of the weak downsloping aloft. However latest short term models suggest that showers could redevelop off outflow even across the west per current convergence fields. Thus keeping high pops east and at least chance coverage west into the early morning hours. Otherwise should finally see showers fade late followed by more fog/stratus as the flow turns more easterly along and behind the front by morning. Dropped lows a bit espcly where have seen cooling via heavy rain from earlier with most falling into the 60s overall. Previous discussion as of 210 PM EDT Sunday... Water vapor loops showed two short waves that will track across southwest Virginia, southeast West Virginia and northwest North Carolina tonight and Monday. These short waves in combination with weak surface convergence will spark showers and thunderstorms. The first wave coming out of eastern Kentucky this afternoon will reach the Blue Ridge this evening. The next, more prominent, vort max tracks from Michigan into central Virginia by late Monday morning. Best areal coverage will be this afternoon and evening, but enough lift and instability to keep at least isolated showers and thunderstorms in the forecast into early Monday morning. Amount of cloud cover, position of the surface front, and location of any remnant outflow from today`s storms will impact where and when showers and thunderstorms will develop on Monday. No change in air mass tonight and Monday with moist surface dew points, deep moisture and precipitable water values around 1.5 inches. This will keep overnight lows mild. Will stay closer to the warmer guidance numbers for lows tonight. Starting off mild but will need to burn off the stratus on Monday so will lean toward cooler maxes for highs. Confidence was below average for coverage for precipitation tonight, above average for temperatures and average all other elements. && .SHORT TERM /TONIGHT THROUGH WEDNESDAY NIGHT/... As of 210 PM EDT Sunday... Look for a continuation of unsettled weather into Monday night as well, as a warm front near the VA/NC border begins to return northward. This will keep shower/thunderstorm chances going into the overnight along with abundant cloud cover. Most active day in the short-term period is Tuesday, as a potent surface low and parent upper-level disturbance moving across the Ohio Valley into the Northeast moves into a humid, conditionally unstable air mass. Increasing 500 mb height falls overspread the mid- Appalachians, Blue Ridge into the Piedmont region through the day and in such a thermodynamic environment, showers and thunderstorms should turn scattered to numerous in the afternoon to mid-evening hrs. While the actual cold front arrives well after peak heating more into Tuesday night/early Wednesday, convergence associated with a pre-frontal trough/theta-e gradient should help provide a corridor of focused lift aside from terrain. Effective-layer and deep-layer shear magnitudes are rather strong for mid-August with values between 30-35 kts over the northern NC counties to as much as 40 kts in the Blue Ridge and central VA Piedmont. While directional shear will likely weaken as winds veer with the northward passage of the overnight/early-day warm front, speed shear is pretty robust and would support at least organized multicells and approach values ordinarily associated with occurrences of supercells across the northern half. What is less clear is the degree of instability, which stems largely from uncertainty on when/if and the extent to which early-day cloud cover/warm frontal showers can thin out. This also affects the start time of convection, though forecast convective inhibition probably will not take much heating to overcome. Instability is also limited by poor mid-level lapse rates. A conservative estimate based on the NAM/GFS along with some clouds would offer a marginally to moderately-unstable environment with seasonably strong wind shear. Strong to potentially severe thunderstorms are possible in this regime capable of strong/damaging winds, with hail a secondary but non- zero possibility simply because the winds at mid/upper levels are pretty strong. SPC has a Marginal/5% severe probability shown for Tuesday. Sustained storms should produce heavy rainfall as well with PWATs around 1.6-1.8", supporting the Marginal excessive rain outlook, but quick storm motions limits duration and may otherwise preclude flash flooding. In a general sense, areas potentially more vulnerable are those preconditioned by rain leading up to Tuesday. Will continue to message Tuesday as a day to pay close attention to in the HWO. Westerly flow as the upper trough axis passes should help to clear the region from clouds, except for a period of stratus overnight along the western Appalachians. Substantial air mass change in store for Wednesday as the cold front moves eastward early in the day. 850 mb temperatures should fall into the low teens Celsius by afternoon, with dewpoints falling into the 50s/low 60s from dry advection and mixing of dry air aloft. It may also prove to be a somewhat breezy day with modest pressure rises and 850 mb winds around 20-30 kts. If there is a threat for showers it may be across the southern half of the area as the 850 mb front crosses the region in the afternoon but won`t offer anything worse than slight chance. Really though, after mountain stratus thins out Wednesday looks to all-in-all be nice weather-wise with highs in the 70s/lower 80s and comfortable humidity levels. Ridge builds in overnight and leads to quiescent weather for the evening hrs as well. Overall forecast confidence this period is moderate, though is high on the cooler/drier conditions expected for Wednesday. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 210 PM EDT Sunday... Thursday and Friday look quite pleasant across the region, as large area of high pressure and upper-level ridge builds into New England and maintains a surface ridge/dry wedge across our area. Plentiful sunshine and 850 mb temperatures should remain in the lower teens, and dewpoints in the 50s should really make for ideal conditions for any outdoor plans. Forecast confidence for the latter half of the week is high. The weekend appears to be a little more unsettled as a weakness in the upper-level ridge begins to avail itself across the mid-Atlantic Region. May see some showers and thunderstorms develop in the southern half of the area on the wedge boundary Saturday night and again Sunday. Pattern appears to turn warmer and more typical of mid- summer with convective chances limited to terrain for early next week. Forecast confidence for the weekend/early next week is moderate. && .AVIATION /06Z MONDAY THROUGH FRIDAY/... As of 1243 AM EDT Monday... Showers and storms continue across parts of the region espcly southern/eastern sections where convection remains slow to move. Given lingering instability and moisture along with a front around, would expect coverage to linger into the overnight along residual outflow boundaries. Thus kept in either a prevailing or vicinity mention all locations with TEMPO for lower cigs/vsbys within isolated shra/tsra across the region. Once convection does fade a bit, will see more of a light easterly component develop just behind the front resulting in areas of sub-VFR in stratus and fog through early Monday. This could also produce IFR/LIFR that may last into mid morning Monday with at least MVFR cigs possible as well through midday if not Monday afternoon. This cloud cover, position of the surface front, and location of any remnant outflow from today`s storms will impact where and when showers and thunderstorms will develop on Monday. For now appears best chances will be across southern and western sections where may see more heating closer to the southern side of the front. High confidence of extensive IFR to LIFR ceilings overnight and MVFR to IFR fog. Extended Discussion... The frontal boundary will move northward as a warm front Monday night then a stronger cold front moves through the Mid Atlantic region Tuesday. This will produce widespread showers and thunderstorms with the associated MVFR to IFR ceilings and visibilities. High pressure will build in Wednesday and will be overhead by Thursday. VFR conditions are expected with this cooler and drier air mass through Friday. MVFR to IFR fog is likely each night in the along the typical river valleys, especially west of the Blue Ridge. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AMS NEAR TERM...AMS/JH SHORT TERM...AL LONG TERM...AL AVIATION...AMS/JH/RAB is the U.S. government's official web portal to all federal, state and local government web resources and services.