Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 300720 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 320 AM EDT Sun Apr 30 2017 .SYNOPSIS... High pressure offshore will maintain a warm and humid southerly flow of air across the region through tonight. A strong cold front will then move through the area Monday with showers and thunderstorms, followed by some cooler, but near seasonal temperatures through the first part of next week. && .NEAR TERM /THROUGH TONIGHT/... As of 240 AM EDT Sunday... Upper ridging offshore will remain in control today but gradually weaken into tonight as height falls slowly push east ahead of the complex upper trough to the west. This will again keep much of the area in overall subsidence into the afternoon with very little support aloft for much more than isolated to widely scattered convection given weaker instability and still some capping aloft. Latest guidance again remains quite limited with any shra/tsra coverage later on espcly given early clouds and more of a southerly trajectory today. Appears best areas of progged weak convergence along the southern Blue Ridge/foothills where more differential heating possible per morning low clouds, and over the far northwest just south of a passing faint wave aloft. This closer to the latest Cams ensemble and SPC WRF solutions so after trimming back coverage early, keeping 20/30 pops in these spots for mid to late afternoon pop up storms. Otherwise expecting residual clouds to fade to more sunshine with heating, but may get stuck southern/eastern sections a bit longer espcly given current satellite trends. Expect this along with the more southerly flow and cooler temps aloft to keep highs lower, mainly 80-85, although mostly 70s at elevation. Upper low will pass well to the northwest late tonight in turn pushing a trailing cold front toward the region by daybreak Monday. Models show a steady increase in the jet aloft ahead of the boundary overnight with low level convergence developing southwest sections within a surface southeast trajectory that will quickly veer southwest with height. This in conjunction with an axis of higher PWATs should spell a redevelopment of shallow convection southern Blue Ridge after dark with coverage spreading north overnight as weak lift/moisture advection increase along a north/south theta-e lobe. However soundings show moisture quite shallow, so appears mainly showers vs much thunder, initially driven by more of an upslope type setup with instability fading shortly after sunset. Thus only running with low to mid range pops over all but the extreme eastern sections late as guidance usually a bit fast and overdone with this regime espcly given much deeper convection to the west near the front. Otherwise will see warm advection driven southerly winds ramp up over the western ridges overnight, but appears not enough for a headline across the Mtn Empire region for now per limited mixing via clouds. Elsewhere another very mild overnight with clouds increasing and lows mostly in the 60s. && .SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/... As of 210 AM EDT Sunday... A cold front will cross the area on Monday. Run to run model solutions have been fairly consistent with the timing and the associated convective parameters. Surface based cape increases to just shy of 2000 J/KG and surface based LI`s drop to around -5 or -6 by the afternoon. A very strong southerly 850mb low level jet may maximize around 40 to 50 kts just in advance of the front, prompting notable low level shear, at least in terms of low level speed shear, if not low level veering shear. The Storm Prediction Center is still prompting a marginal risk of severe weather in association with this front. By Monday evening, most of the area will be on the back side of the front, with lingering isolated to scattered showers and some storms along and east of Lynchburg to Danville line. These too should exit the region by the late evening. Northwest winds behind the front will be on the gusty side through the night and into Tuesday. Some of the higher elevations could experience gusts on the order 35 to 45 mph, especially during the Monday night time period. Dry weather is expected into Tuesday night as high pressure build into and then over the area. Winds will weaken as the high moves overhead. On Wednesday, wind will back to the south in advance of an approaching area of low pressure through the western Gulf Coast states. Enough daytime heating and moisture convergence near the crest of Blue Ridge may prompt a few showers from near Galax, VA, southwest into the Northern Mountains of North Carolina. By Wendesday night, a warm front will have developed over the area and lifted north into the Ohio Valley. Look for patchy light rain to develop, especially over western and northern sections of the forecast area. Temperatures during this portion of the forecast will start about 10 degrees above normal on Monday, and be near normal by Wednesday. && .LONG TERM /THURSDAY THROUGH SATURDAY/... As of 250 AM EDT Sunday... Larger differences in the synoptic pattern show up in the different models for this portion of the forecast. WPC leaning heavily on the ensembles with keeps a progressive amplified eastern trough. Will have highest probability of precipitation on Thursday through Thursday night. Then with lower confidence due to the large spread in the guidance will have lower probability of precipitation by Saturday and Sunday. Will be keeping maximum temperatures below normal for Thursday through Saturday. && .AVIATION /07Z SUNDAY THROUGH THURSDAY/... As of 105 AM EDT Sunday... Quite varied conditions likely into the overnight as an axis of residual moisture from earlier convection to the south lifts northeast across the area. This will result in a mix from mainly mid/high clouds over northeast sections to periods of lower strato- cu over the south and west where moisture may eventually lower into stratus late. Where clouds are less also expecting to see locally dense fog develop with MVFR vsbys or worse in the valleys espcly around KLWB, and perhaps KLYH toward daybreak. Otherwise followed closer to most guidance in bringing MVFR to ocnl IFR cigs into KDAN overnight and possibly KBCB/KROA/KBLF as low level moisture spreads north. Elsewhere keeping cigs mainly VFR but low confidence in just how much low cloud canopy will spread up espcly east of the Blue Ridge by morning. Lower cigs may again linger southern and southwest sections through midday Sunday under more of a low level south/southeast trajectory before heating/mixing helps scour out most low cigs during the afternoon. Convective chances again appear quite isolated Sunday afternoon and certainly not enough to include any mention in the TAF forecast for any particular airport at this point. Otherwise a bit more breezy with southerly winds perhaps gusting to 20 kts or better Sunday afternoon espcly along the ridges. Extended Aviation Discussion... As a low pressure system approaches from the west late Sunday night, the potential for showers and thunderstorms will increase over the western mountains by early Monday. The cold front associated with this system should reach the Appalachians Monday and depart offshore by Tuesday morning. As such, expect the best chance of MVFR conditions during the frontal passage from west to east during Monday. VFR conditions return behind the front on gusty westerly winds Tuesday followed by continued good flying weather under high pressure Tuesday night into much of Wednesday. However moisture may return to the mountains by late in the day Wednesday with local MVFR possible in isolated showers by the end of the day. By Thursday next system should be impacting us with showers and a few storms, so sub-VFR is likely. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/MBS NEAR TERM...JH SHORT TERM...DS LONG TERM...AMS/DS AVIATION...JH/MBS/WP is the U.S. government's official web portal to all federal, state and local government web resources and services.