Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 240156 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 956 PM EDT Mon Apr 23 2018 .SYNOPSIS... Low pressure will slowly move from the Tennessee Valley to the Carolinas through Tuesday. Prolonged southeast winds will keep the area wet through Tuesday night, when the low lifts to the Delmarva. As this system departs on Wednesday, two additional disturbances look to cross over or near our area Thursday into Friday. && .NEAR TERM /THROUGH TUESDAY/... As of 956 PM EDT Monday...Raised temperatures up some through the rest of the overnight, but this amounts to essentially temperatures remaining roughly steady-state. Mesonet observations indicate southeast wind gusts in the Wind Advisory area have been between 35-45 mph. 18z GFS appears to have better handle on low-level winds per comparison with Doppler VAD profiler data than does the bullish NAM, and there is still a small window of opportunity for further strengthening to occur until midnight. Thereafter winds should begin to ease. Current Wind Advisory through 2 AM looks valid at this point in time. As expected, rainfall rates have been pretty gradual with a largely stratiform precipitation character. Radar derived rain totals likely are still underestimating in the southern Blue Ridge with rain amounts through several sources in Watauga County on the order of 2 inches, resulting in some rises on the New River and likely into surrounding smaller creeks/streams. No current hydro problems noted per county officials in spite of these rains. Amounts taper to an inch or less as you get further away from the Flood Watch area. Still looking at additional rains tomorrow but don`t presently feel extension of the Watch is warranted attm. Finer- resolution guidance indicates another round of potentially steadier/heavier rains after 06z along/east of the Ridge as convection over GA weakens into a more stable environment and lifts north/northeast. These ideas generally handled in the forecast so no major changes needed to PoPs/Wx attm. Previous discussion issued at 225 PM follows... Southeast flow accelerating this afternoon, with gradient tightening between low pressure over Western TN and high pressure off the mid- Atlantic/Northeast coast. Getting some gusts over 40 mph in the higher terrain of NC, with 25 to 35 mph further north. Models are indicating stronger part of low level wind field arrives around 00z- 04z this evening, and have added western Greenbrier to the wind advisory. Also strong winds should linger past midnight, so extended the advisory til 2am Tuesday. The flood watch still looks good, see hydro discussion below for details. Looking at periods of rain working in from the south as plume of deep moisture drives northwest from the Atlantic through the overnight. Models in reasonable agreement in placing emphasis of higher rates over the southern Blue Ridge with light-moderate rain surround this. Latest high-res models are keeping the northeast piedmont dry until overnight/after midnight and leaned pops toward that solution with categorical over the southern CWA to chance in the far northeast and northwest. Toward morning, southeast flow at 8h continues but better lift starts shifting to the northeast with potential rain shadow effect taking shape over the mountain empire of SW VA into southern WV. Seems enough moisture transport to keep pops greater than 50 percent across the west, with 100 pops in the east. As the 8h low moves over the Mountain Empire in the afternoon, a pocket of lower rainfall works from southwest VA into central WV, while upslope flow in the east persists but weakens somewhat in the afternoon. Still looks like a good period of rain along/east of the Blue Ridge Tuesday. Some SBCAPEs greater than 500 J/KG start to get close to our NW piedmont of NC in the afternoon, and includes isolated thunder threat here, but overall looks too cloudy and wedged in for deeper convection. Kept lows tonight similar to previous forecast with mid to upper 40s across much of the area. Highs Tuesday will be cool again especially across the Blue Ridge, and Alleghanys, with upper 40s to mid 50s, with upper 50s to lower 60s expected west of I-77 in WV, and over the upper TN VAlley in SW Va, as well as southeast of a Lynchburg to Stuart line. && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY NIGHT/... As of 200 PM EDT Monday... The center of the slow moving upper low that would have been associated with our rainfall Monday and Tuesday, is expected to start Tuesday evening over eastern Kentucky and progress due east across the forecast area by early Wednesday morning. Showers will prevail through the night in association with the system`s steeper cold pool lapse rates. Early in the evening, the far southeast portion of the area will be very close to region of weak CAPE. SREF numbers keeps any amounts greater than 500 J/Kg just outside the forecast area. At this point, am leaning toward keeping the forecast simply as showers. On Wednesday, the upper low will begin a turn northeast in response to yet another shortwave trough heading southeast into NY/PA. The first upper low will lose its definition as it becomes enveloped within the second. Flow across our area will turn northwest on the back side of the system, trending the precipitation to be focused in two regions. The first will be an area of upslope showers across southeast West Virginia. The other will be showers on the western flank of the upper trough axis, still influenced by the steeper lapse rates of the associated cold pool. Close to adding thunder in the east on Wednesday, but the best lift looks to remain just east of the forecast area. As we progress Wednesday night through Thursday night, our region will remain within a progressive pattern, influenced by both the northern and southern stream jets. First there is the question as to how quickly the NY/PA upper low will head northeast, taking its influence with it. While there are some minor differences within the deterministic models, a clear consensus is that by daybreak Thursday all should be dry, at least for a little bit of time. As early as Thursday afternoon, we will need to start examining our next potential weather maker. This one is another shortwave trough, but within the southern stream this time. Guidance sweeps the system from the Lower Mississippi River Valley on Thursday afternoon, to a location near GA/SC by daybreak Friday. The path and timing vary such that either this feature will have no influence on our weather, or the southern half or third of the region will experience showers on its northern flank Thursday night, potentially starting as late Thursday afternoon across the far southwest section of the forecast area. Our forecast will reflect a toned down version in terms of POPs that the wetter solutions have for our region. During this portion of the forecast, high temperatures will trend milder to readings near normal for this time of the year. Low temperatures will trend slightly cooler to readings slightly below normal. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 335 PM EDT Monday... On Friday, we will still have the question as to the degree of influence the southern stream shortwave trough has for our area. A lot will depend upon how quickly yet another northern stream system progresses eastward, inviting a turning northward of the southern system. If in fact it does turn north Thursday night, scattered showers will continue to impact the area on Friday. The aforementioned northern stream system is progged to move through the area Friday night into Saturday morning. Models differ as to whether this will be a quick moving clipper, or be a bit beefier with an actually associated closed or open low that crosses our region. Either way, the the forecast detail this far out in time will be the same, one of forecast scattered showers. Once this system clears the area by Saturday night, the overall synoptic pattern changes. For Sunday into Monday, expect an upper level ridge to approach, amplify, and then be centered over the area. This will result in a forecast that has two consecutive days of no precipitation. Temperatures will start this portion of the forecast a few degrees below normal, but trend milder, especially for Sunday and Monday. By Monday temperatures are forecast to average about five degrees above normal. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/... As of 750 PM EDT Monday... With wedge of high pressure in place east of the mountains and isentropic lift increasing, expect showers to continue to spread into the region from the south with cigs trending toward IFR conditions. With slow motion of the closed low moving out of the Mississippi valley expect rain and IFR conditions to linger through through Tuesday. Believe the wedge barrier jet will keep winds a bit gusty at KLYH and KDAN through the period. While the southeasterly low level jet running over the top of the wedge is quite robust, very moist and stable boundary layer conditions will inhibit winds mixing down. The most likely locations for stronger wind gusts will be at the higher elevations in the west to include the airfield at KBLF. Extended Aviation Discussion... Tuesday afternoon the low will be situated overhead so rainfall may start to fade in the southwest around BLF after 18z Tuesday then shifting northeast through Tuesday evening. Expect sub-VFR through the afternoon, and into Tuesday night/early Wednesday. Winds turn to the northwest, by late Tuesday night into Wednesday and allow for the precipitation to trend showery in the west. The weather pattern will remain unsettled Thursday into Friday as a series of disturbances crosses the area, each with the potential for providing showery precipitation and localized sub-VFR conditions. && .HYDROLOGY... As of 240 PM EDT Monday... No changes need to the flood watch, which remains in effect through 400 PM Tuesday. Forecast amounts from this morning through Tuesday showing 2-4" along the southern Blue Ridge from near Meadows of Dan to Blowing Rock. Looks likely that the areas around the Watauga River in NW NC as well as streams flowing off the Blue Ridge are going to need to watched. Ensembles for the rivers are still lagging though the SREF is a bit robust in bringing flooding threats later on to the Dan River, as well as portions of the Greenbrier and New. Other ensembles as well as the RFC are keeping all rives at or below flood stage. Something to watch once we see how much falls, but rises along the main stems, including the New and Dan Rivers are likely, as well as portions of the Roanoke. Given lack of deep convection, flash flooding appears less likely. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Flood Watch through Tuesday afternoon for VAZ015>017-032. Wind Advisory until 2 AM EDT Tuesday for VAZ007-009-015. NC...Flood Watch through Tuesday afternoon for NCZ001>003-018-019. Wind Advisory until 2 AM EDT Tuesday for NCZ001-018. WV...Wind Advisory until 2 AM EDT Tuesday for WVZ042-043-508. && $$ SYNOPSIS...WP NEAR TERM...AL/WP SHORT TERM...DS LONG TERM...DS AVIATION...MBS/WP HYDROLOGY...WP

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