Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 180304 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1104 PM EDT Thu May 17 2018 .SYNOPSIS... A stationary frontal extends from central West Virginia into central Virginia. Meanwhile, an area of low pressure was lifting north lift north through the southern Appalachians. Warm and humid conditions will continue into the weekend with widespread and numerous showers and thunderstorms through Friday becoming more scattered over the weekend. && .NEAR TERM /THROUGH FRIDAY/...
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As of 1100 PM EDT Thursday... The frontal boundary will linger over the region tonight with a gradual drift to the north as weak impulses of short wave energy aloft move up form the south in deep, moist tropical flow. Timing of features and loss of diurnal heating will allow for a brief break in heavy rainfall before another band pushes up from the southeast late tonight with a renewed threat for locally heavy rain. Rainfall will continue to be very efficient and it will not take long for slow moving cells to produce excessive rainfall so will maintain the Flash Flood Watch for the entire area as is. Previous discussion... Numerous to widespread showers will be common across the forecast area through Friday. Synoptic pattern supports slow moving deep convection with mean wind of 10 kts or less. Main focus for heavy, and potentially excessive rainfall, will reside along a stationary front vcnty of I-64, this front the focus for persistent convergence. In addition, a pseudo-tropical low will move north into the southern Appalachians tonight. This feature, although becoming less discernible in the model data with time, should track directly overhead providing dynamic support in addition to a sustainable fetch of southeasterly low level flow into the east side of the Appalachians. Based on the combination of all aforementioned features, think its prudent to blanket the entire forecast area with a flash flood watch. Counties adjacent to the Blue Ridge and along the frontal boundary will be most susceptible per the better convergence and low level lift. However, tendency of convective elements to drift away from the Blue Ridge makes it hard to discount potential even for the piedmont, and especially for the urban areas where a torrential downpour will quickly compromise the drainage infrastructure. With respect to temperatures, the abnormally humid conditions will lead to well above normal minimum temperatures in the 60s to even near 70 across the Piedmont, with high temperatures mostly in the 70s mountains to the lower 80s Piedmont. Keep in mind that normal low temperatures this time of year are in the 40s with highs in the 70s to near 80. Thus, the minimum temperatures will be averaging 15-20 degrees above normal through the weekend.
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&& .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/... As of 330 PM EDT Thursday... The item of focus for this portion of the forecast will be a slowly northward moving upper level low across the Lower Ohio/Tennessee River valleys and copious moisture that will be streaming north on its east side. This moisture will be impacting our region with moderate to heavy rain showers and some thunderstorms through Saturday night. A Flash Flood Watch is in effect for the first half of Friday night for entire region. As we progress into and through this part of the forecast, where, and how much, precipitation falls, and additional expected precipitation, will dictate where, and if, any additional watches may be needed through Saturday night. There will be a brief lull in the precipitation late Saturday night into early Sunday morning as the upper low washes out and gets caught up in the northern jet. In its wake will a brief period where a shortwave ridge will be over the area, all the while an area of low pressure begins to develop across the Central Plains states. This low will begin to head eastward Sunday night. A return of precipitation is expected as early as Sunday afternoon across western sections of the region where surface heating, and a beginnings of a return of low level moisture will coincide. Sunday night, with the loss of daytime heat, precipitation coverage will decline. However, scattered showers will still be possible in the west, within the region of best moisture return and dynamics aloft in advance of approaching low pressure. Temperatures during this part of the forecast will start around five degrees above normal for Saturday, but warm to around five to ten degrees above normal on Sunday. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 130 PM EDT Thursday... On Monday, an area of surface low pressure is expected to be heading east through Indiana along a stationary front that extends into Pennsylvania. The low`s associated cold front will trail southwest into eastern Texas, all while its parent upper low tracks through the western Great Lakes region. High pressure will still be anchored in the western Atlantic, east of the GA/FL. For our region, plenty of moisture streaming north between the cold front and the high pressure will be realized in form of at least a mostly cloudy sky and an abundance of showers and some thunderstorms. Monday night into Tuesday, the surface low is expected to have reached KY/OH/WV with the stationary front over PA sinking south as a backdoor cold front. The cold front associated with the surface low is progged to stall and take on warm front characteristics from roughly Missouri into Kansas. While there initially be a decrease in coverage of precipitation across the region Monday night, most of this decline will be due to loss of heating. Western sections of the area, closer to the approaching low, will be the most likely to continue to see some isolated to scattered showers, and few storms, through the night. On Tuesday, coverage is expected to increase again with the return of daytime heating, and increased dynamics as the system draws closer. Tuesday night night into Wednesday, look for the low to wash out, but the back door cold front will continue to inch south through, then south of the area, stalling over GA/SC by Thursday morning, and remaining there into Friday. Behind the front, high pressure will build into our region. The result for our region will be another decrease in coverage Tuesday night with the loss of heating. On Wednesday, scattered showers will be confined to primarily the southern third of the area. Wednesday night into Thursday, limited, if any, precipitation is forecast for the area with the push of high pressure into the area. Temperatures during this portion of the forecast will average about ten degrees above normal on Monday, and trend cooler each day. By Thursday, temperatures will average around five degrees above normal. && .AVIATION /03Z FRIDAY THROUGH TUESDAY/... As of 745 PM EDT Thursday... Generally poor flight conditions are expected at TAF sites this period. Lingering frontal boundary combined with impulses of short wave energy in a tropical airmass will generate showers and thunderstorms with heavy rain. Timing of features will be difficult, but mesoscale models are in the ballpark so will lean on them heavily and amend as necessary with a preference to leave lower conditions prevailing when more activity is expected to move in. Indications are for a break in precipitation late tonight but expect IFR cigs to remain in place. Another wave of convective precipitation is expected to push in from the southeast toward daybreak and continue to affect TAF sites through the latter portion of the valid period as the frontal boundary starts to push back to the north. .Extended Aviation Discussion... The pseudo-tropical low and the lingering frontal boundary will begin to drift north of the CWA later Friday and especially into the weekend. This should bring about a slight decrease in rain coverage with activity trending more scattered and diurnal in nature as we go into the weekend and high pressure ridging is noted across the southeast states. However, still feel that chances for showers/thunderstorms and associated aviation issues will be greater than normal for this time of the year, at least through Saturday with periods of sub-VFR possible. Fog and low clouds will be common during the late night and early morning hours as well. After more isolated storms on Sunday, appears another weak cold front may bring increasing coverage of showers and thunderstorms again on Monday. && .HYDROLOGY... A Flash Flood Watch is in effect for the entire CWA through Mid-Night Friday night. The main concern through Friday will be for localized flash flooding, as well as flooding on small creeks, streams, and smaller rivers as a result of heavy tropical rain showers and thunderstorms expected across the area. Rainfall during the past few days has been spotty and highly variable, but as low pressure passes overhead Friday, the coverage is expected to increase. Flood Guidance suggests 2 inches or more of rain in less than 3 hrs may lead to flooding, and if this sort of rainfall occurs in less than an hour, flash flooding would be possible. WPC is concerned about excessive rainfall, and especially for those areas along a stationary near the Interstate 64 corridor. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Flash Flood Watch through Friday evening for VAZ007-009>020- 022>024-032>035-043>047-058-059. NC...Flash Flood Watch through Friday evening for NCZ001>006- 018>020. WV...Flash Flood Watch through Friday evening for WVZ042>044-507- 508. && $$ SYNOPSIS...PM NEAR TERM...MBS/PM SHORT TERM...DS LONG TERM...DS AVIATION...MBS/PM HYDROLOGY...PM

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