Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 280141 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 941 PM EDT Sun May 27 2018 .SYNOPSIS... High pressure centered offshore will maintain a moist southerly flow of air into the region for the next several days. Subtropical Storm Alberto moving into the Gulf of Mexico this weekend will move ashore and into the lower Mississippi Valley early in the week. Tropical moisture will stream north into the region making for very rain efficient showers and thunderstorms. && .NEAR TERM /THROUGH MONDAY/...
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As of 940 PM EDT Sunday... The bulk of the precipitation continues to be focused mainly along and north of Route 460. This trend will continue into the early morning hours of Monday. Meso-scale models continue to suggest this precipitation will dissipate within a few hours after midnight. Precipitation in advance of Alberto will start to spread north into the far southern sections of the region around daybreak Monday, however the trend has slowed. Have reflect this in the forecast during the forenoon hours on Monday. As of 655 PM EDT Sunday... The forecast through the evening hours has been updated to reflect a decreased chance of precipitation across the southern half of the region. The bulk of the precipitation will be situated along and north of Route 460 through midnight. After midnight, we are expecting a return of precipitation to the southern half of the region, especially as we approach daybreak Monday. As of 340 PM EDT Sunday... Scattered showers and thunderstorms have formed in the moist unstable air across our region with the better coverage across the higher terrain and along the Blue Ridge. Ample insolation has allowed MLCAPE values to climb to around 1500 to 3000 J/KG. The combination of upper trough over the northern Mid-Atlantic and tropical moisture streaming north from the Gulf of Mexico has resulted in a convergence axis along or north of our CWA border. With healthy PWATS, these storms will be very rain efficient rain producers. The HRRR and HiresW-ARW-East indicated that the greatest convection concentration will occur cross the northern half of our forecast area. The NAM and RAP generates precipitation across the entire forecast area. The GFS and ECMWF showers widespread storms with the heaviest north of our forecast area. The excessive rain fall had marginal potential across the entire forecast area with slight chance to our northeast across Washington area. We will need to watch for flooding issues across entire forecast area with deeper convection or training. The good news is that the storm motion is faster today around 15 knots compared to yesterday. One weak spots will be Tazewell county where some rain gages reporting 2-3 inch amounts Saturday. Attm will hold off on a Flash Flood Watch. Prefer to see where convergence boundaries will line-up and target those areas of high confidence as opposed to blanketing a larger piece of real estate. Low temperatures tonight will range from the upper 50s in the mountains to the mid 60s in the piedmont. Refer to the latest advisories from NHC on Alberto for track and its movement. Alberto comes ashore Monday morning and continues moving northward into southern Alabama by Monday evening. The flooding potential for Monday will depend on how much rainfall is accumulated from several days of locally heavy tropical rainfall and where the deeper convection forms. The greatest chances for showers and thunderstorms Monday will occur across southern portions of the forecast area. Decided to mention the rain could be heavy at times in the south. The Day Two excessive rainfall showed a sharp gradient with a slight chance to our south across North Carolina. Both the GFS and ECMWF showed richer moisture try to push into the southern edge of our forecast area before translating east to the coast. High temperatures on Monday will vary for around 70 degrees in the mountains to the upper 70s in the east.
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&& .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/... As of 230 PM EDT Sunday... NHC forecasts bring Alberto ashore Monday morning and continues the center on a NNW track into southern Alabama by Monday evening while weakening the system to a tropical depression by Tuesday morning over north central Alabama. Forward speed then increases as the NNW track continues with the depression in southern Indiana by Wednesday morning and Michigan by Thursday morning. Our forecast area will remain in deep southerly flow through this entire period with extremely high moisture for late May. The 12z/27 RNK sounding PWAT of 1.42 inches is near `all-time` high for the date (1995-2014 SPC climatology) and chances are very good that this type of moisture remains over the region for next few days and beyond. As mentioned in previous discussions the forecast NAEFS anomalies for PWAT are 2 to 2.5 standard deviations above normal through much of the week. Bottom line is any showers that develop are will be efficient rain producers and storms that lingers over a particular location or small basin more than about 1-2 hours are liable to produce minor flooding and 2-3 hours could initiate flash flooding. Day 3 Excessive rainfall outlook from WPC shows Slight Risk (10-20 percent chance)for exceeding flash flood guidance 12z Tuesday through 12z Wednesday. This does not provide much confidence in issuing a Flash Flood Watch given the lack of focus and the shotgun pattern of flash flood guidance (FFG) in place across our region. 3-hour FFG ranges generally from 1.5 to 3.0 inches with the lower guidance mainly in the mountains. Will hold off on a Flood/Flash Watch as the pattern is too diffuse in terms of timing and location. A short-fused Watch targeted to developing organized convection may make more sense than what could be a 3-day (or longer) Watch. As Alberto lifts into the Tennessee Valley late Tuesday into Wednesday a Marginal Risk for Severe will exist with the possibility for isolated tornadoes in the northeast quadrant of the weakening system as well. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 300 PM EDT Sunday... A generally wet pattern is expected to continue late week into next weekend as the remains of Alberto lift northward into the northern Great Lakes. But then the forecast becomes interesting as 12z/27 GFS shows intense upper low over the western North Atlantic blocked by ridging south of Greenland. This forces the upper low over the Great Lakes back to the southeast with a broad area of low pressure over the Mid-Atlantic that deepens with time over VA/NC area providing continued chances for rainfall. The 00z/27 ECMWF shows similar longwave pattern with the Greenland block but develops surface low pressure somewhat further north and east, while 12z/27 CMC closer to GFS. Obviously pattern details cannot be descried with any real confidence at this range but the pattern certainly fits the definition of `unsettled` through the extended period. Used mainly Super blend for the grids during this time frame. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... As of 730 PM EDT Sunday... Showers and a few thunderstorms have been come concentrated across the northern half of the forecast region. Conditions are primarily VFR except under the heavier showers/storms. As the night progresses, look for fog to develop with visibility`s trending IFR/MVFR. Visibilities will improve to VFR around or a little after 13Z/9AM Monday. Precipitation will return to the region from the south on Monday as moisture streams north on the east side of Alberto. Winds will be calm or light and variable through the period. Medium confidence in ceilings, visibilities and winds during the taf period. .Extended Aviation Discussion... Wet pattern anticipated through the upcoming week with periods of at least diurnally driven sub-VFR conditions associated with deep convection. Moist low level southerly winds will also favor lower layers of cloudiness at night, especially along the spine of the Appalachians with potential for both MVFR Cigs and early morning MVFR visibilities from mist/haze. The tropical system over the Gulf of Mexico may begin to impact the region mid-week. The unsettled weather with showers and thunderstorms may continue into the end of the week. && .HYDROLOGY... As of 315 PM Sunday... Significant focus in hydro concerns as discussed in portion of the Near, Short and Long term discussions above. Alberto and associated effects will be with us through the upcoming week in one form or another. The question of Flood or Flash Flood Watches will be with us during much of this time period as mentioned earlier but the notion of holding onto what could be a very long-fused Watch seems almost counter to producing real awareness of any flood threat. There may be opportunities for more short-fused Flash Flood Watches tied to a more discernible threat such as banded features or persistent clusters of storms. That said, the ground is fairly saturated in many places but as noted above very erratic coverage. River flooding cannot be discounted especially on smaller fast- reacting streams such as the upper Roanoke, Watauga, or even something like Mason Creek. All basins will have to be monitored closely. The latest MMEFS ensembles, mostly from 00z/27 or 06z/27 are painting some a somewhat less robust response on the bigger rivers than the previous set of runs with minor to moderate flooding generally in the 30 percent or less chance in most large river basins. The experience with MMEFS has been that is tends to overdo the probabilities of reaching high stages at these extended forecast time periods. Still, the Dan River basin once again under the highest threat for flooding but that would be a late week time frame. && .EQUIPMENT... NOAA Weather Radio, All Hazards, WZ2500 at Wytheville, VA, broadcasting on a frequency of 162.45 mHz has been fixed. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...KK/PM NEAR TERM...DS/KK SHORT TERM...PC LONG TERM...PC AVIATION...DS/KK/PM HYDROLOGY...PC EQUIPMENT...DS

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