Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 132347 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 647 PM EST Fri Jan 13 2017 .SYNOPSIS... A cold front will move south, then stall across northern North Carolina Saturday before returning north as a warm front late Saturday night. This front will then oscillate north and south across the region into early next week keeping an unsettled and wet weather pattern in place. By the middle of next week, a stronger frontal system will finally approach from the west and move the pesky front out of the area. && .NEAR TERM /THROUGH SATURDAY/...
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As of 640 PM EST Friday... Forecast updates this evening are focusing on the very near term of the forecast. Will leave any portion of the forecast dedicated to the time period of the Freezing Rain Advisory alone for now. May revisit closer to 10pm when additional model data are available. For this evening, have made only minor tweaks regarding cloud cover, temperature and dew point. Have adjusted values based upon the latest observations and expected trends through the evening. Light rain is arriving across parts of the western sections of the region. The ongoing precipitation timing forecast looks on track. As of 310 PM EST Friday... A cold front will slowly move into North Carolina this evening. Cooler temperatures will be felt behind this front but temperatures will remain warmer than normal with morning lows ranging from the upper 20s from the Greenbrier and Southern Shenandoah Valleys to the upper 30s towards the NC/VA border. The coldest air associated with this front will remain with the center of high pressure currently located over the Great Lakes. This high pressure center will track east keeping the very cold air north of the Mason-Dixon line. Some freezing temperatures will move south once the core of the cold air moves east of the Appalachains tonight. These freezing temperatures are expected to move into the Greenbrier and Southern Shenandoah Valleys and Allegheny Highlands early Saturday morning. Winds will remain out of the east through the day Saturday, so these areas will only warm into the mid to upper 30s. These easterly winds will also keep areas east of the Blue Ridge cool with temperatures closer to normal values (low to mid 40s). Looking at forecast soundings, this cold air will be shallow, possibly 2000 feet thick. Therefore this cold air in easterly flow will not move west across the mountains Saturday. With no cold air intrusion, southwestern Virginia and northwestern North Carolina will likely warm into the the lower 50s Saturday. There remains a baroclinic zone from the Ozarks to the Southern Ohio Valley tonight and Saturday. Weak impulses along this zone will bring rain into the mountains late tonight into Saturday. Most of the models have the higher rainfall amounts through the period north of I64. This just so happens to be the area where temperatures will drop below freezing, thus creating a threat for freezing rain Saturday morning. First of all, models are moving moist warm air over cold temperatures possibly over doing amounts. Also, this type of signature could be an area where more snow or sleet falls as compared to freezing rain. Looking at forecast soundings, the cold air will be shallow south of I64, approximately 2000 feet. North towards Charlottesville, the depth of the cold air is around 3000 feet thick. With this in mind, we are thinking Ptype south of I64 will be predominately rain changing to freezing rain with a light glaze on elevated surfaces to include overpasses. North of I64, a snow/sleet mixture changing to freezing rain by sunrise with accretions possibly up to a tenth of an inch on elevated surfaces and overpasses. We are not thinking much will coat the roads with our recent warmer than normal weather the past couple of days. Now lies our problem within the freezing rain advisory. The GFS is the warmer model and keeps all precipitation as rain. Low level thicknesses are above 1315m and 85H temperatures are +2C. The NAM is the coldest model with 85h temperatures near 0C and low level thicknesses ranging from 1295 to 1310 in the advisory area. The NAM also has a strong warm nose, pushing 7C air over surface temperatures in the upper 20s. The ECM is in between the GFS and NAM. With differing models, RNK, WPC and neighboring offices are leaning towards the colder NAM. Our thinking is, it is better to have an advisory and nothing happen than have something happen and no headlines. Therefore we will keep the current freezing rain advisory for tonight and Saturday as is.
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&& .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/... As OF 300 PM EST Friday... The high amplitude split upper flow will remain across the nation during the weekend. Trough in the southwest continues with a ridge in the Southeast. A wavy frontal boundary will remain across our region Saturday night through Sunday night. In general, the cooler temperatures associated with the boundary will prevail in the north and on the east side of the Appalachian Divide. Meanwhile, warmer air over the Tennessee Valley will push north. An isolated pocket of freezing rain may be possible Saturday night into Sunday morning in Northwest Greenbrier county. Coverage will be limited and only on the coldest elevated surfaces. But overall, Sunday morning is forecast to be a few degrees warmer compared to Saturday. Low temperatures will range from the lower 30s in northwest Greenbrier county in Southeast West Virginia to near 50 degrees in the southwest mountains in North Carolina. During the day on Sunday, the front lifts northward with the best chance of rain in the west. High temperatures on Sunday will warm to near 40 degrees in the northern mountains to the lower 50s along the southern Blue Ridge. The NAM continues to hint that slightly cooler air will drift back into the area Sunday night as the front gets a little nudge to the south. If this turns out to be the case, then some of the higher elevations along and north of I-64 could see some freezing rain/drizzle Sunday night. It would be low QPF and mainly on elevated surfaces. Low temperatures Sunday night into Monday morning will range from the lower 30s in the northern Mountains to the mid 40s in the southwest mountains. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As OF 300 PM EST Friday... Frontal boundary across the area Monday morning lifts north into New York Tuesday and reaches Maine by Tuesday night as strong upper low moving across the central U.S. amplifies ridge downstream. Cold front approaches from the west on Wednesday and moves into the CWA on Thursday. On Thursday night into Friday, the boundary slowly moves north across the region. There are differences in timing of frontal systems with GFS faster than ECMWF. Leaned towards superblend for Medium range. Monday will average near normal to 15 above in the west, then high temperatures will run about 10- 20 degrees above normal Tuesday into Friday. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... As of 1215 PM EST Friday... Generally deterioriating to poor aviation conditions expected through the TAF valid period as a backdoor front sags southward through the region, then stalls just south of the NC/VA border, returning northward slowly Sat-Sun. An upper disturbance and stalled front will return rain to the region later this evening and overnight. Cigs will remain/trend toward IFR-LIFR in the west and become generally MVFR in the east as this area of rain spreads east overnight. As cold air spreads southward from the northeast overnight, a brief period of freezing rain will be possible at KLWB early Saturday morning. Winds generally WNW-NW this afternoon becoming NE-ENE this evening into Saturday. Medium to high confidence in ceilings through the TAF valid period. Medium to high confidence in visibilities through the TAF valid period. Medium to high confidence in wind speed/direction through the TAF valid period. Extended Aviation Discussion... A baroclinic zone will remain anchored across the region sandwiched between a strong upper ridge over the southeast states and an active jet stream from the southwest U.S. into the northeast U.S. Thus, an unsettled, wet weather pattern is expected to remain in place through the extended periods. The frontal boundary may lift north enough by Monday for a brief period of mainly VFR, but a stronger front will arrive By Tuesday with more showers. Generally looking at MVFR much of the period, although periods of VFR as well as IFR-LIFR can be expected at times through the period. Overall, not the best weather for aviation. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Freezing Rain Advisory from 4 AM to 4 PM EST Saturday for VAZ018>020-023-024-035. NC...None. WV...Freezing Rain Advisory from 4 AM to 4 PM EST Saturday for WVZ044-507. && $$ SYNOPSIS...RCS NEAR TERM...DS/RCS SHORT TERM...KK LONG TERM...KK AVIATION...RCS

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