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Area Forecast Discussion
Issued by NWS Blacksburg, VA
Issued by NWS Blacksburg, VA
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115 FXUS61 KRNK 131444 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1044 AM EDT Tue Aug 13 2024 .SYNOPSIS... An upper level disturbance is passing across the area resulting in clouds and isolated showers, otherwise a large area of high pressure centered over the northern Mid-Atlantic will keep the weather mainly dry. The next widespread chance for rain arrives toward the end of the week as a low pressure system approaches from the Ohio Valley. && .NEAR TERM /THROUGH TONIGHT/... As of 1030 AM EDT Tuesday... Key Message: Mainly dry weather aside from cloud cover and a few showers. Highest pops for the afternoon favors NC piedmont. Upper disturbance is overhead and will move east of the are mid-late afternoon. Expecting current shower activity to wane or move east of the CWA...water vapor channel from satellite imagery already validating drying aloft. Lowered afternoon high temperatures along and south of the VA/NC border to account for thicker cloud cover there, otherwise previous forecast on track. Previous discussion... Just minor changes from previous forecast and still looks like a disturbance moves across overhead today and with some moisture convergence along/south of the VA/NC border, could see a few showers, possibly some thunderstorms. Otherwise high pressure will keep things dry but expect more mid and high clouds today mixed in with intervals of sunshine, along with scattered cumulus. Any showers should fade with dusk and clearing out tonight for most. Amount of fog Tue night will depend on how much coverage of rain occurs. Models hinting at some fog/lower vsbys along/south of the VA/NC border Wed morning. Highs today will run about 5 degrees below normal with lower 80s east to 70s west. Lows tonight closer to normal int the lower to mid 60s Piedmont to mid to upper 50s mountains. Forecast confidence above average except sky cover/storm coverage today. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/... As of 130 AM EDT Tuesday... Key Points: 1. Mainly dry on Wednesday. 2. Increasing chance of showers/storms on Thursday, but especially on Friday. 3. Near normal temperatures. A look at the 12 Aug 12Z NCEP Ensemble Mean 500 mb Geopotential heights shows a nearly stationary upper trough across the western Atlantic, extending from Greenland to the Canadian Maritimes. Additionally, a shortwave ridge shifts east from the Great Lakes to New England. Also, an upper trough progresses from the Northern Plains to the Great Lakes/Ohio Valley region. At the surface, high pressure will be centered over the Ohio Valley on Wednesday. This high progress eastward and be near the Canadian Maritimes by Friday. Additionally, a trough of low pressure over the Dakotas will head east deepen as a closed low over the western Great Lakes region by Friday. Output from the 12 Aug 12Z Ensemble Situational Awareness Table shows 850mb temperatures trending slightly higher through this portion of the forecast. On Wednesday, values are expected to average +14C to +16C over the region. By Friday, values of +16C to +18C are forecast. Precipitable Water values on Wednesday and Thursday are expected to range from a little under 1.00 inch to 1.25 inch. On Friday, expect values closer to 1.50 inch. The above weather scenario offers decreasing influence of dry high pressure Wednesday into Thursday. Thursday into Friday, the approach of a trough and exit of high pressure will help provide for a better southerly fetch of moisture into the region. With the better moisture, and daytime heating in advance of the approaching system, look for much better coverage of showers/storms on Friday as compared to the previous two days. Temperatures will be near normal through the period. Confidence in the above weather pattern is moderate to high. && .LONG TERM /SATURDAY THROUGH MONDAY/... As of 130 AM EDT Tuesday... Key Points: 1. Good coverage of showers/storms this weekend. 2. Less, but still scattered coverage, on Monday. 3. Near normal temperatures. A look at the 12 Aug 12Z NCEP Ensemble Mean 500 mb Geopotential heights shows slowly eastward moving upper rough which amplifies each day. On Saturday the feature is expected to be over the Great Lakes/Lower Ohio Valley region. By Monday, the axis will be near the spine of the central to southern Appalachians. At the surface, low pressure over the Great Lakes will progress eastward and be over the mid-Atlantic region by Monday. Output from the 12 Aug 12z Ensemble Situational Awareness Table shows 850mb temperatures around +18C on Saturday and Sunday, but closer to +16C to +17C for Monday. Precipitable Water values on Saturday are expected to be around 1.50 to 1.75 inch, 1.25 to 1.50 inch on Sunday and Monday. With the approach and arrival of upper trough and surface trough/cold front, look for increasing chances of showers/storms over the weekend. By Monday, while the surface front is expected to be to our east, the parent upper trough will still be over the region with its associated upper level cold pool, yielding steep lapse rates aloft. Showers/Storms will continue to be a possibility on Monday, but with less coverage than what is expected Saturday/Sunday. Temperatures will be near normal for this time of year. Confidence in the above weather pattern is moderate with timing of the surface cold front and upper trough axis the biggest question mark. && .AVIATION /15Z TUESDAY THROUGH SATURDAY/... As of 610 AM EDT Tuesday... Mainly VFR expected through the 24 hour valid TAF. Just some mid and high clouds and scattering of cumulus today. A few light showers possible this morning though no impeding on vsby. Appears we should clear out late tonight and fog to form in the favored LWB valley. Confidence lower elsewhere for fog, but models still hint at fog across southern VA into NC or along/south of a TRI-DAN line. Winds generally 5kt or less from the northeast to east. AVIATION OUTLOOK... Mainly VFR expected Wednesday and Thursday. An organized area of low pressure will approach the area by Friday bringing the potential for precipitation and widespread sub-VFR flight conditions which could last through Saturday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PM/WP NEAR TERM...PM/WP SHORT TERM...DS LONG TERM...DS AVIATION...PM/WP