Area Forecast Discussion
Issued by NWS Blacksburg, VA

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526 FXUS61 KRNK 230854 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 454 AM EDT Sat Jul 23 2016 .SYNOPSIS... Strong high pressure aloft to the west will continue to bring increasing heat and humidity over the weekend. A series of weak surface cold fronts approaching from the northwest should gradually bring an increasing threat for showers and storms early next week. && .NEAR TERM /THROUGH TONIGHT/... As of 235 AM EDT Saturday... Strong upper ridge west of the mountains will continue to expand east over the next 24 hours with heights slowly building as a piece of the high noses east into the Mid-Atlantic. Resultant 850 mb temps near +24C or better will accompany the ridging this afternoon as decent subsidence remains in place despite lingering rather moist pwats. This should bring one of the hottest days seen so far with mid/upper 90s east and 80s to perhaps touching 90 in spots over the mountains. However dewpoints again look to drop off enough east of the ridges in the afternoon despite only weak flow to preclude heat advisory headlines at this point. Still could see an hour or so where indices touch 103-105F piedmont, espcly if the flow ends up more southwest and dewpoints stay up, so continued HWO mention for now. Otherwise given little support aloft, expect more isolated convective coverage at best, with storms driven by differential heating/weak convergence mountains and perhaps the eastern lee trof. Latest suite of solutions suggest best low chances across the northwest NC mountains and possibly the Va Highlands, with shotgun 20ish pop elsewhere for isolated afternoon pulse storms at this point. Any residual convection should fade with loss of heating with another warm overnight in place as higher dewpoints return and debris clouds linger before clearing late. Will leave in some low pops mainly west early on for more outflow driven shra/tsra, otrw cutting to patchy fog overnight with lows from the mid 60s valleys to the low/mid 70s east. && .SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/... As of 415 AM EDT Saturday... This period will begin with a strong elongated upper ridge extending across the entire U.S. through the mid-latitudes from the desert southwest into the central Atlantic. The westerlies remain to our north about 500 miles while a weak pseudo-tropical upper low meanders westward through the Midsouth. For Sunday, this leaves our region with very weak dynamics, mostly ridging aloft with heights hovering near 549dm. One piece of the upper ridge is actually progged to be centered across the VA/NC Piedmont. As was the case Friday, differential heating will likely result in isolated to scattered diurnally driven showers/thunderstorms. Most favored areas will be across the NW NC mountains and into southwest VA. Cannot rule out an isolated shower/thunderstorm in other areas, but conditions are not favorable. For Monday, heights begin to drop slightly as the upper ridge shifts off the NC coast and a trough moves through the Great Lakes toward New England with a weak trough extending into the Mid- Atlantic region. An associated weak front will drift toward the PA/MD/WV border by evening, then slowly southward overnight. Shower/thunderstorm chances may be slightly better Monday during the evening in the far west/north, but overall it looks like the remnant upper ridge will dominate one more day and keep most of the CWA dry, especially the Piedmont closest to the upper ridge. By Tuesday, the heights continue to slowly lower into the 588 dm range as a series of weak upper troughs gradually return the region to a pattern we have seen repeatedly this summer, namely northwest flow aloft. One center of the upper ridge will remain over the western U.S. with the one that was over our region Sun- Mon shifting off the southeast U.S. coast. The aforementioned weak front will continue to drift very slowly south and should be near the NC/VA border by Wed morning. Shower and thunderstorm chances should increase substantially Tuesday in response to the lower heights, weak upstream northwest flow short waves, slight cooling aloft, and the proximity of the front. Dynamics overall will remain very weak, so instability/thermodynamics will be the main driving force for the convection. SPC has advertised a marginal risk near the I-64 corridor late Monday, but for our CWA Tuesday should offer a better bet for isolated strong to severe thunderstorms. Temperatures will remain warm to hot through the period, but edge back a few degrees by Tuesday. Sunday should be the hottest day across the region as 850mb temps peak out around +24C under the 594 dm upper ridge. Even though guidance has consistently been a few degrees too warm most days this summer, mid to upper 90s lower elevations of the Piedmont to the lower to mid 80s mountains seems reasonable in comparison to previous events where the 850mb temps reached these high a levels. SPC climatology page shows that +26C is the warmest 850mb temperature for BCB, which was reached during the infamous heat wave/derecho day of June 29, 2012. So it is quite possible that BCB could reach the 90-degree mark over the next few days for the first time since July 2012. We were in the mid 90s back then. By Tuesday, max temps should drop back into the lower to mid 90s with increased cloud cover, showers, and moisture content. Low temperatures will remain a good 5-10 degrees above normal through the period. && .LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 430 AM EDT SATURDAY... The broad west-east oriented upper ridge gets nudged further southward by broad troughing across central/eastern Canada. Upper heights will continue to gradually lower. More importantly will be a series of weak short waves embedded in the upper flow that will traverse the region and impinge upon a weak front that will remain stalled in/near the area. This should prove to be an increasingly active convective period with scattered to numerous showers/thunderstorms each day. Models present varying solutions as to when the best chances for showers/thunderstorms will occur within this period, but the overall consensus supports high chance pops during the afternoon/evening and low chance pops/night and mornings through the period. Too much noise and chaos in the signal at this point to specify much further, so have broad brushed pops with the previously mentioned methodology. Day-by-day details are of the mesoscale level and yet to be resolved by synoptic-scale models. The greatly increased convective coverage and associated clouds and moisture will result in max temperatures returning to more normal levels mostly in the 80s to around 90 Piedmont. Min temps will remain well above normal levels, mostly in the mid 60s west to lower 70s east. The large upper ridge will extend from the West coast to the East coast through Wednesday. Towards the end of the week, there is a gradual lowering of H5 heights as ridge splits and develops a Bermuda ridge and desert SW ridge. In the northern stream, a shortwave will drop southeast out of eastern Canada and crosses through New England on Tuesday. A frontal boundary will settle across the Mid-Atlantic and Southeast Wednesday into Thursday. A wave of low pressure along the front will slow the progression of the boundary. The placement of fronts are always a challenge in the summer. Low level moisture will continue to increase into the end of the period. Afternoon and evening showers and thunderstorms are possible Tuesday into Midweek. Temperatures will be above normal early next week and return to normal by end of the week. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/... As of 1250 AM EDT Saturday... Continued mainly VFR this period with fog being the only real issue late tonight into early Saturday. As such will stay with the previous forecast of bringing KLWB to LIFR around 08z/4AM, while including a brief tempo for MVFR at KBCB. Should see good flying conditions prevail on Saturday under scattered/broken cu including isolated convection, mainly along the southern Blue Ridge espcly toward KGEV/KTNB. However uncertainty too great to include any shra/tsra mention given likely spotty nature to convection. Extended aviation discussion... Strong high pressure aloft centered across the Midwest will attempt to build eastward into Sunday, then retreat back to the west early next week. Our region will remain on the eastern periphery of the upper high and thus subject to weak disturbances in northwest flow aloft tracking around the Great Lakes and into the Mid-Atlantic. Moisture and instability will be sufficient for isolated pop up thunderstorms, but a slightly better coverage will be possible early next week as weak front moves into the area, stalls, and lingers over the region for several days. Hot and humid conditions east of the Blue Ridge this weekend which may impact density altitude. Late night and early morning fog will still be possible at the usual sites almost every day, and those chances increase if any rain occurs during the afternoon or evening at any of these sites. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/RCS NEAR TERM...JH SHORT TERM...RAB LONG TERM...KK/RAB AVIATION...JH/RCS/SK/WP

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