Area Forecast Discussion
Issued by NWS Blacksburg, VA

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504 FXUS61 KRNK 221724 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 124 PM EDT Fri Jul 22 2016 .SYNOPSIS... A Bermuda high will keep pushing warmer and more humid air into the region throughout the weekend and into the first part of next week. The combination of high air temperatures and humidity will create dangerously high heat index values, especially east of the Blue Ridge, with readings around 105 at times through the weekend. The heat and humidity will also create an unstable airmass, and a series of weak fronts moving through the region will help keep at least a small chance for showers and thunderstorms in parts of our forecast area for the next several days, with increasing chances as we get into next week. && .NEAR TERM /THROUGH TONIGHT/...
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As of 115 PM EDT Friday... Weak convergence and orographical lift along the southern Blue Ridge in North Carolina has allowed a few showers to pop up. These afternoon showers will be very slow moving and are capable of producing heavy downpours. As these storms decay, outflow boundaries may generate a few more storms into this evening across the High Country of NC and into the foothills. As of 705 AM EDT Friday... No notable changes to previous forecast other than minor tweaking of sky and T/Td grids in line with latest observations. No new guidance suggests any difference on where isolated convective chances will be this afternoon, mainly over NW NC. Recent HRRR runs which are in the ball park with latest convective activity in southern IL/IN and NW PA suggest some dying convection may barely reach northern fringes of fcst area by evening but there is little instability most of the day to support this with any degree of confidence. Other models have it dying sooner or are more focused on our western fringes later overnight. See no reason so far to adjust the slight chance PoPs this evening and overnight for the far west and far north part of the area. Previous discussion as of 415 AM EDT Friday... Surface high pressure centered near Bermuda but does nose westward into the heart of the central Appalachians this morning. Warmest 850mb temps in the eastern US under the huge upper ridge appear to be near the VA/NC/TN border area. Lots of dry air aloft, and cirrus blowoff from large convective complex over upper midwest is trying to sneak into western Appalachians but dissipates as it does so. This may have some influence on keeping patchy fog to a minimal area across the mountain valleys early this morning and not quite as dense. For the daytime Friday the 850mb thermal ridge shifts slightly eastward but still generally over the Appalachians and into the foothills. So max temps likely to be just about as warm in the foothill regions as farther east in the Piedmont, and guidance is in good agreement that highs in these areas will reach mid 90s, which is 5-6 deg warmer than yesterday. Still think that dew pts may mix out a little more than MOS values suggest, especially in the mountains like yesterday with dry air aloft. This will keep heat index values from being even higher, but still expect values in upper 90s by mid afternoon east of Blue Ridge. If dew pts remain in the upper 60s to near 70 vs lower 60s, then heat index values may briefly reach 100 or so. Max temps not really that close to records however. Dry air aloft and mixing down to surface will also keep instability somewhat limited and so believe it will be another mainly dry day across the fcst area, with possible exception of higher terrain of NW NC and far SW VA and into foothills of NW NC by early to mid afternoon, where several hi-res models suggesting isolated coverage at best. Another more organized cluster of convection, possibly MCS, is likely to form well to the north closer to frontal boundary and will try and drop south toward evening and overnight but with best instability axis on western slopes of Appalachians and farther west, indications are most of this will remain to our west and weaken by the time it gets near the fcst area. Did include some slight chance pops there for this evening and overnight, but will have to watch these far western areas as we get into the evening in case this area of convection is strong and survives longer and farther east, but not thinking this is a big concern at the moment. Likely to be even a few more mid to high clouds around overnight tonight, especially in the west, so fog even less likely and did not put it in the forecast, but if convection to north dies quickly and we remain mostly clear then patchy valley fog becomes more likely. Should be a bit milder Fri night given a few more clouds and dew pts jumping back up.
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&& .SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/... As of 215 AM EDT Friday... The center of a central U.S. upper ridge will make some progress westward on Saturday. This will allow for better chances for an upstream cold front to approach the area Saturday and stall near the area on Sunday. Showers and storms will be on the increase in advance of this front and in its proximity once it stalls. The region will remain on the hot and humid side of the front, so a cool down is not in the offing. By late Sunday night, a shortwave trough approaching through the upper Mississippi valley will help to buckle the stalled front north as a warm front. This will keep hot and humid conditions across the area, and briefly shift the main focus for additional showers and storms north of the area. By Monday afternoon however, the trough axis of this disturbance will be approaching the western portions of the area and act as a focus for additional showers and storms. The greatest concentration will be in the west. These higher chances will continue into at least Monday evening with some decrease by late Monday night as the trough axis moves east of the region. Temperatures during this portion of the forecast as still forecast to be above normal by 5 to 10 degrees for this time of the year. Heat index values are forecast to surpass 100 degrees for at least one hour across the far southeast portion of the region Saturday, Sunday, and Monday afternoons. Monday afternoon some heat index readings may briefly top 105 degrees between roughly South Boston, VA and Charlotte Court House, VA. We will continue to highlight the heat in the Hazardous Weather Outlook. && .LONG TERM /TUESDAY THROUGH THURSDAY/... As of 330 PM EDT Thursday... The large upper ridge will extend from the West coast to the East coast through Wednesday. 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. Moisture will slowly increase Monday and Tuesday ahead of the front. Afternoon and evening showers and thunderstorms are possible Monday into Midweek. There remains disagreement in model solutions with just how far south the front will make it. This makes it harder to place the best time of convection. Temperatures will be above normal for the start of the week and return to normal by end of the week. && .AVIATION /17Z FRIDAY THROUGH TUESDAY/...
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As of 105 PM EDT Friday... High confidence forecast to keep it dry and VFR most of the period at all terminals...with only exception being some brief MVFR to IFR fog at KLWB/KBCB during the early morning hours Saturday. Bands of mid and high clouds this afternoon may lingering into the overnight and will be the key to any fog formation. Will maintain persistent forecast with some high clouds under an upper level ridge limiting mountain valley fog to KLWB. Any fog Saturday morning will burn off quickly. Diurnal convection would be isolated and mainly along the southern Blue Ridge toward GEV/TNB this afternoon. Will still need to keep an eye on upstream convection over the Ohio Valley which may approach KLWB late this evening and perhaps this time could result in thicker cloud deck but confidence in getting any shower or thunderstorm activity through 18Z Sat morning is very low. Extended aviation discussion... Strong high pressure aloft centered across the Midwest will attempt to build eastward into the weekend, 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.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...MBS/SK NEAR TERM...RCS/SK SHORT TERM...DS LONG TERM...AMS/KK AVIATION...RCS/SK is the U.S. government's official web portal to all federal, state and local government web resources and services.