Area Forecast Discussion
Issued by NWS Blacksburg, VA

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872 FXUS61 KRNK 221404 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1004 AM EDT Sun Apr 22 2018 .SYNOPSIS... High pressure will stay anchored from the Great Lakes to the Middle Atlantic through Sunday night, while low pressure shifts from Arkansas to eastern Tennessee. This low will then track over the Ohio Valley on Wednesday. && .NEAR TERM /THROUGH TONIGHT/... As of 950 AM EDT Sunday... Mid to high level cloud cover continues to thicken and spread eastward across the region. This trend will continue through the afternoon hours. Have adjusted the hourly sky cover grids to reflect the latest satellite imagery across the area with this coverage advecting eastward along the 500 mb flow. For temperatures, dew points and winds, have only needed to tweak values to better reflect the current conditions, and kept the trends heading into and through the afternoon the same. Have added a small chance of light rain to parts of the Northern Mountains of North Carolina. The 12Z/8AM HRRR offers a solution that paints a hundredth or two in this area during the afternoon, and adding a slight POP matches well with the POP/wx grids GSP has along our common border in that area. As of 300 AM EDT Sunday... For today, an upper low will be in Arkansas, while an inverted trough extends northeast into Kentucky, from a surface low over Mississippi. Meanwhile, over the Mid Atlantic, two surface high pressure systems will join forces to keep rain out of the area for one more day. We will see an increasing in high clouds as moisture from the upper low rides over the merged surface ridge. Despite thickening high clouds, temperatures across the mountains will be slightly warmer than normal with highs in the mid to upper 60s, with the exception of higher ridges being in the upper 50s. The foothills and piedmont counties will experience slightly cooler than normal temperatures today. With a cool light southeast breeze, highs in the east will range in the upper 60s to lower 70s. Clouds will continue to lower tonight as the upper low moves over eastern Tennessee and closer to the area. Saturation of the environment will take some time as the dry high over the Mid Atlantic holds off rain from entering the area until later Monday morning. Some light rain from southeast upslope flow may fall by sunrise, but the bulk of the heavier rain is not scheduled until later in the day. With moisture continuing to pump into the region overnight, temperatures will remain mild with lows ranging in the 40s. && .SHORT TERM /MONDAY THROUGH WEDNESDAY/... As of 150 AM EDT Sunday... Upper low becomes vertically stacked and drifts from the Tennessee Valley into the Ohio Valley during this time frame. At the surface a triple-point low forms Tuesday then moves northeast to the Virginia coast Tuesday night and Wednesday. Upper pattern is somewhat blocky with a ridge preventing much eastward movement of the upper low. But timing of precipitation similar to last several runs with steady rain filling in from southwest to northeast across southwest Virginia, southeast West Virginia and northern North Carolina on Monday. This a favorable scenario for a classic upslope heavy rain event along the Blue Ridge and foothills to the east. With deep and very strong isentropic lift and upslope bringing precipitable water values around 1.5 inches into the region, anticipate 0.5 to 1.0 inches of rain every 6 hours from 18Z/2PM Monday through 18Z/2PM Tuesday. Will stay on the cool side of guidance for maximum temperatures on both Monday and Tuesday due to very limited solar insolation and a strong in-situ wedge. Once the surface low reaches the Virginia coast, surface winds will become west then northwest eroding the wedge. At this time, models have this occuring on Tuesday night. Not particularly meaningful cold air advection behind this system but there may be enough of a downslope component to the wind along with the deeper moisture pulling out of the area, that there may be some breaks in the clouds east of the Blue Ridge on Wednesday. Breaks or not, will still have Wednesday warmer than Tuesday. && .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... As of 320 PM EDT Saturday... The upper low over the Great Lakes will get caught up in the northern stream jet, open as a wave, and eject eastward with the prevailing flow. Its associated cold front/trough axis will cross our region, and help bring winds around the the northwest Wednesday into Wednesday night. This will result in our winds shifting northwest, and allowing for upslope rain showers across the west, and decreasing coverage of precipitation east of the Blue Ridge. Our weather pattern will remain quite progressive through the remainder of the forecast period. Two additional northern stream shortwave troughs are expected to zip through the region. Each will bring a return of showers to the area, with the greatest concentration across the mountains. The first will be Wednesday night into Thursday. The second will occur either Friday or Saturday, as model agreement isn`t as strong on this second one. Our forecast will reflect the quicker of the two solutions with enough colder air arriving in its wake for the potential for some snow showers along the highest peaks and ridges of southeast West Virginia, south into the Northern Mountains of North Carolina. Temperatures during this portion of the forecast will trend colder with readings by Saturday some five degrees below normal. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... As of 725 AM EDT Sunday... VFR conditions should be the rule through the forecast period as high pressure is firmly in control. Expect increasing high clouds today from the west. Over the course of the day, increasing moisture will lead to a thicker cloud deck and some lowering of ceilings. Winds will mainly be light and variable or even calm through the morning hours. During the day, winds should predominantly be from the south-southeast, however speeds should remain under 8 knots. There exists some possibility for light rain in the southwest section of Virginia and northwest North Carolina late in the TAF period, but should remain south of KBLF and KBCB through 12Z/8AM Monday. High confidence for VFR conditions through the bulk of the TAF period, with uncertainty growing near the end for small potential to introduce MVFR conditions in the NW NC mountains. Extended Aviation Discussion... Clouds will thicken and lower throughout the day Monday with widespread light to moderate rain and MVFR/IFR ceilings and visibilities are expected Monday night and Tuesday. Strong winds are expected on the west side of the central Appalachians on Monday and Monday night. Southeast wind gusts up to 40 knots area possible at KBLF Monday and Monday night. The low pressure system is expected to track northeast to the Virginia coast on Wednesday. Low clouds and showers are possible across the mountains. A cold front is forecast to move across the region Thursday, bringing rain to the entire area. && .HYDROLOGY... As of 530 AM EDT Sunday... Collaborated with WPC on covering the southern Blue Ridge in a Marginal/Slight Risk of Excessive Rainfall for Monday through Tuesday night. Total rainfall amounts will range from 1.5 to 3 inches with locally higher amounts along the Blue Ridge. If this amount of rainfall in realized some small streams and creeks will likely flood, then mainstem rivers will rise. Based on the expected rainfall the lower reaches of the Dan and Roanoke Rivers and along the James River from Lick Run to Scottsville have to potential to rise to Action Stage. While the official river forecast does not yet contain the expected rainfall, this discussion highlights an above average potential for flooding within these river basins. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...RCS NEAR TERM...DS/RCS SHORT TERM...AMS LONG TERM...DS AVIATION...AMS/RCS HYDROLOGY...AMS is the U.S. government's official web portal to all federal, state and local government web resources and services.