Scientific Forecaster Discussion

000 fxus61 krnk 190001 afdrnk Area forecast discussion National Weather Service Blacksburg Virginia 801 PM EDT Thu Apr 18 2019 Synopsis... a strong cold front will approach from the west late tonight into Friday morning. This cold front will move across the mid- Atlantic and southeast states Friday before pushing to the east Friday night. An area of upper level low pressure lingers into Saturday resulting in unsettled weather before high pressure builds back in late in the weekend. && Near term /through Friday/... as of 300 PM EDT Thursday... Nice late afternoon across the region with mild temperatures prevailing within the warm sector as the residual warm front continues to lift northeast. This should allow the region to remain in a lull with only some passing mid/high clouds early this evening ahead of the next round of deep moisture to the west. Models continue to show an abrupt increase in moisture/clouds espcly later this evening with enough lift within the warm sector to support some increase in showers, mainly Blue Ridge after midnight, as the low level flow turns more southeast. Thus included pops a bit sooner over the southwest but only spotty showers at this point. Otherwise main concern will be toward daybreak over southwest sections as a band of residual deep convection heads east toward the I-77 corridor. This in advance of a deepening upper low sliding northeast out of the mid Mississippi Valley and ahead of an associated cold front dropping in with phasing northern stream energy. However this band looks to run into less instability given timing, so appears initial band likely more of a heavy rain threat with stronger storms just south closer to the surface wave. Therefore looking at higher likely/categorical pops southwest late tonight with mainly chance evolving elsewhere late. Southerly flow will also be increasing above the surface as a jet Max in advance of the upper low shifts east resulting in a 45-50 kt 850 mb jet across the west after midnight. However forecast soundings show a strong inversion around 4-5k feet that could limit mixing before the rain arrives. This may curtail the stronger speeds across Tazewell/Mercer counties to just a brief period toward dawn so wont hoist a Wind Advisory headline at this point but need to watch. Lows should be mild although could see values drop to the cooler side of guidance outside the ridges as winds diminish so included some 50s with low 60s ridgetops. Very active day in store on Friday as the upper low deepens and heads just west of the mountains while the surface cold front punches slowly east with a possible secondary wave enhancing convection overhead. Latest guidance just a bit slower but in turn stronger with instability along/east of the Blue Ridge Friday afternoon. This beneath very strong wind fields aloft that may tend to result in more parallel flow to bands later in the day as the upper low gains latitude. However models still remain inconsistent with the evolution espcly with timing, and the degree of deeper convection that should form along the leading edge of the earlier band once it starts to encounter some increase in instability beneath the very strong shear heading east of the mountains in the afternoon. This along with the possible second wave and another strong shortwave riding up from the Carolinas likely to result in a possible bowing line/qlcs with embedded supercells espcly if things do slow up a bit. Thus running with similar pops as previous including a potential secondary line along the actual cold front over the far west in the afternoon. Winds may also mix down just ahead of the convection to produce stronger gusts just ahead of the showers/storms out east where beefed up gusts to near advisory levels there as well Friday afternoon. Highs mostly in the 70s with 60s west where anticipate more clouds/showers sooner that should taper highs a bit. Flooding also another aspect pending the degree of bands per higher rainfall rates needed for flash flooding given some drying the past few days and green up out east. Latest models have decreased amounts a bit with the higher threat looking to be from the NC mountains northeast along the Blue Ridge to south of Highway 460 as very high precipitable waters for mid April advect in. However given lack of confidence in seeing heavy rainfall outside of bands, decided to forego an areal Flood Watch at this time and let the mid shift take another look. && Short term /Friday night through Sunday night/... as of 155 PM EDT Thursday... Upper low starts out in western Tennessee and moves up the western side of the central Appalachians into eastern Ohio by late Saturday. Sunday and Sunday night the low moves east toward New Jersey. Colder temperatures aloft reach southwest Virginia, northern North Carolina and West Virginia on Saturday with a residual pocket of pocket of cold air over Virginia and West Virginia increasing lapse rates lingering into Sunday. A strong cold front will cross from west to east across the area Friday night dropping 850mb temperatures to below zero west of the Blue Ridge by Saturday morning, with NAM running cooler than the GFS. Temperatures start to moderate on Sunday. Cold front will be the back edge of the more intense showers and thunderstorms. Models show a brief dry slot behind the front then showers filling back in with the upper low from around Bluefield Saturday morning spreading to the Piedmont Saturday evening. Challenging to determine if lapse rates and instability will support isolated thunderstorms Saturday. && Long term /Monday through Thursday/... as of 1245 PM EDT Thursday... Once upper low exits the northeast United States pattern returns to broad southwest flow and weak 500mb ridging over the east through Thursday. Consensus is fair in the long range guidance with no significant synoptic features impacting the mid Atlantic region this time frame. High pressure following the deep surface low fills in from the southwest with the high over centered over the Carolinas by Monday night. 850 temperatures warm back into the +10 to +14 range on Monday and remain similar through Thursday. Temperatures will be above normal through the period. Low level moisture trapped under the upper low will just be exiting southwest Virginia Monday morning. Baroclinic zone from the northeast United States across the Ohio Valley may get close enough by Wednesday morning to increase cloud cover north of I-64. Cold front may drop into northern Virginia Thursday with more clouds and a chance of precipitation but confidence is low at this time. && Aviation /00z Friday through Tuesday/... as of 750 PM EDT Thursday... VFR conditions remain in place at this hour and that should hold for another six hours or so before conditions begin to deteriorate rather quickly. For now, just high/mid clouds with ceilings at or above 100 through the evening with no visibility restrictions. Immediate aviation concern would be the wind with gusty southerly winds lingering into the evening hours more than usual as a strong storm system and the tightening gradient approaches from the west. Look for south-southeast-south-southwest 10-15kts with gusts 20-25kts through at least 04z. Some locations may not even see much decrease in winds after 04z because of low-level jet increasing overnight. Main concern for the rest of the taf valid period will be the deep upper low and associated occluded front approaching from the west and then moving over the region slowly from west/southwest to east/northeast Friday. Expect multiple rounds of convection given good dynamics and reasonable thermodynamics. The first line of convection should reach the western border of the County Warning Area around 08z or so and impact kblf, kbcb, and to a lesser extent (because of weakening extent northward) klwb. Possible brief period of gusty winds on leading edge of convection and brief heavy rain. This line will be in a diminishing state due to early morning timing. Note, there may also be some southeast upslope induced showers along the Blue Ridge from ktnb to kroa/kmtv overnight/early Friday before the convection arrives bringing MVFR ceilings. During the day, with the upper low overhead and its associated cold pool and dynamics, redevelopment of bands or parallel lines of convection is likely, especially east of the Blue Ridge. Locally heavy rain will be the main threat, but the potential for stronger cells and qlcss remains with the threat for damaging wind gusts and isolated tornadoes, mainly east to southeast of the Blue Ridge. The greatest threat for severe will remain east and south of our County Warning Area it appears at this point. Have included thunder at all taf sites during the afternoon Friday, but the greatest concern for severe thunderstorms would be kroa, klyh, and especially kdan in the 18z-00z time frame. Ceilings and visibilities during the day, and especially in rain showers and thunderstorms, will average mostly MVFR with brief periods of IFR in heavier rain and associated fog. Winds will remain south-southeast-south-southwest until late in the day at speeds of 8-12kts with gusts 20-25kts. /Confidence in forecast parameters/ ceilings - moderate, visibilities - moderate, winds - moderate to high, thunderstorm threat - moderate to high. Extended aviation discussion... The upper low will slowly circulate and drift northeast of the area Saturday. This will bring a deformation zone over the area leaving Saturday with scattered to numerous showers and isolated thunderstorms with periods of MVFR ceilings and visibilities for the most part, especially west of the Blue Ridge. Winds will be gusty from the west and northwest behind this front on Saturday and Saturday night. VFR conditions will return Sunday into Tuesday under weak high pressure and a relatively flat flow aloft. && Rnk watches/warnings/advisories... Virginia...none. NC...none. WV...none. && $$ Synopsis...jh

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