Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 181356 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 956 AM EDT Tue Jul 18 2017 .SYNOPSIS... An upper level trough should head overhead today and could spark isolated showers and thunderstorms in the mountains. High pressure will bring drier conditions later in the week. Upper level ridging should make it quite hot and humid by Friday and the upcoming weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 954 AM EDT Tuesday...Made a few changes to the forecast. First, raised temps and dewpoints to get those better in line with current observations and expected trends over next 3 hrs. The more significant change is to increase coverage of isolated to scattered thunderstorms predominantly west of the Blue Ridge, using a blend of the 06z 3-km NAM and the past couple HRRR runs. You`ll note the 1330z SPC Day-1 Convective Outlook now highlights the Blue Ridge corridor in a 5% severe probability/Marginal Risk. This morning`s RNK, GSO and IAD RAOBs all show residual steep mid- level lapse rates between 6.5-7.2 degrees C per km. Couple other things worth noting include weaker capping, freezing levels around 13 kft AGL, PWATs a little higher than yesterday and poor lower to mid-tropospheric wind fields. The latter is a result of being on the northern side of a upper level trough becoming increasingly cut off from the westerlies evident in morning WV imagery essentially along the Blue Ridge/Shenandoah Valley. Anticipate further heating of the boundary layer will yield moderate to perhaps strong instability values this afternoon. Forcing is very limited - pretty much along localized convergence and differential heating zones, but with less of a cap than yesterday, it`s doubtful much lift will be needed. The aforementioned higher- resolution models key in on two general areas today for storm development: (1) the Alleghany Highlands in eastern WV/VA into southern Shenandoah Valley, and (2) the central/southern Blue Ridge from roughly far southern Roanoke County in VA to Watauga/Wilkes Counties in NC. East of the Blue Ridge into the Piedmont, left either dry or slight chance PoPs given little in the way of progged lift. Coverage may be limited to widely scattered at best between 2-7 pm in these areas, but storms could pulse quickly to strong/locally severe limits. Slow moving nature to storms may allow for localized heavy downpours, with gusty winds and small hail being a conditional possibility in the strongest of reflectivity cores. Other than that, no other changes were needed. Previous discussion issued at 430 AM follows... Clearing skies, light winds, and recent rainfall from Monday afternoon will produce early morning valley fog. The fog will quickly dissipate after sunrise and give way to sunshine with cumulus clouds developing by midday. With the upper level trough heading east of the CWA, dynamic forcing and wind shear will be quite weak. Thus, lift will have to come from orographical effects and differential heating. Mesoscale models insist that convection will fire along and west of the Blue Ridge in the afternoon, but the amount of coverage appears scattered at most. Some of the convection could drift towards the Piedmont by the evening but eventually weaken after sunset. SPC does not have our region highlighted for any severe weather today. High temperatures today were nudged upward given the prevailing bias of guidance being a couple degrees too cool over the past few days. Lows for tonight were bumped downward to reflect high pressure building in from the west overnight as the upper level trough pushes offshore. Dewpoints also appear to drop a couple degrees as well. Clearing skies and calm winds later tonight should provide another opportunity for valley fog by early Wednesday morning, but better chances for development will be where any rainfall occurs from Tuesday afternoon and evening. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/... As of 430 AM EDT Tuesday... The upper low pressure trough over the region early in the week will evolve into a weak cutoff upper low that will initially meander toward the Virginia coast through Thu, then southward along the NC/SC coast toward the weekend as high pressure aloft from the Midwest builds eastward into the central and southern Appalachians. This will result in an increasingly stable air mass across the region resulting in less convective development Wed-Thu. Would expect any convection Wednesday to be confined to the mountains from differential heating and toward the eastern parts of the Piedmont closer to the departing upper low. On Thursday, look for little to no convection as the CWA is basically removed from any forcing mechanism and strongly capped with ridging aloft. Any convection will likely be confined to the mountains and only isolated at best. Friday may offer a slightly better opportunity for convection across northern parts of the CWA and the mountains mainly for two reasons. First, the upper ridge will be sagging slightly to our south, allowing west to northwest flow aloft to impinge upon the region and a weakening cap in those areas. Second, the air mass across the region will be quite hot and conditionally unstable, especially when considering it will likely be one of the hottest days of the year with temperatures in the mid 80s west to upper 90s east and dewpoints ranging from the 60s west to the lower 70s east. At this point it appears any convective complexes or MCSs will be held well north of the area, but the increased heat and instability and slightly better upper dynamics acting upon the hot/humid/unstable air mass should spark more convection than will be the case Wed or Thu. Temperatures will be averaging well above normal through the period with mid to upper 80s west of the Blue Ridge, except near 80 in the higher elevations of the mountains to the mid and even a few upper 90s across the Piedmont as 850mb temps average in the +20C to +22C range during this period. Minimum temperatures will be well above normal as well ranging from the mid 60s west to the mid 70s Piedmont areas. No significant change to the heat indices advertised yesterday for Friday, still just above 100 degrees across the east central and southeast parts of our Piedmont, roughly the area between LYH and DAN. Will continue mention in the HWO and eHWO. && .LONG TERM /SATURDAY THROUGH MONDAY/... As of 430 AM EDT Tuesday... Weather pattern continues to become more active over the weekend into early next week as the upper ridge breaks down and the westerlies encroach upon the region from the north. The pesky upper low, which originated from the upper trough over our region earlier in the week, will meander about the southeast states and then could potentially become drawn up in a developing eastern U.S. trough later next week. Increases in convection during this period will be a result of better forcing from enhanced westerlies, strong diurnal heating and differential heating and moisture deepening from the south as a result of the upper low across the southeast states and a tendency to pull in tropical/Gulf air. Thus, there are several synoptic-scale factors that warrant the chance to high chance range pops currently advertised. However, having said this, an MCS does not appear in the cards at this point as the track for such should remain north of our CWA. 850mb temperatures peak Friday around +24C, but fall back only slowly through this period into the +20C to +22C range. Increased cloud cover and greater coverage of showers and thunderstorms will contribute to lower temperatures overall. Instead of mid 80s west to upper 90s east, look more for lower 80s west to mid 90s east. Heat indices will still be in the 100-103F range across the central and southeast parts of the Piedmont, generally the same areas seeing such on Friday. Will continue to highlight in the HWO and eHWO. && .AVIATION /14Z TUESDAY THROUGH SATURDAY/... As of 745 AM EDT Tuesday... Seeing some early morning fog at the usual locations, which is expected to burn off quickly. Generally VFR conditions are expected to prevail through the remainder of the TAF valid period until late tonight/early Wednesday. Some cumulus/stratocumulus development is likely by the afternoon, and there is a potential for isolated showers and thunderstorms as an upper trough lingers over the area. The limited coverage and extent of this convection makes it too uncertain for any mention of thunder, but VCSH was given to BCB as that site has the highest chance for any activity compared to the other TAF sites. KLWB would probably be the next most likely site for activity, but did not include yet. Whatever activity does form should vanish by Tuesday night. Late tonight/early Tuesday, look for the usual mainly river valley fog with the potential for KLWB to go to LIFR and less possibility of such also at KBCB and possibly IFR at KLYH and KDAN. Aviation Extended Discussion... The upper level trough will head offshore by Wednesday, which will open our region to a large upper level ridge moving eastward from the Plains. Convective activity should become even more sparse as capping takes place aloft. Overall, it appears that VFR conditions will primarily reign through the daytime hours. There could be some MVFR/IFR fog and ceilings during the early morning hours in the cooler mountain valleys like LWB. Hot and humid weather will prevail through the end of the week. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PW NEAR TERM...AL/PW SHORT TERM...RAB LONG TERM...RAB/WP AVIATION...PW/RAB

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