Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 162019 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 318 PM EST Tue Jan 16 2018 .SYNOPSIS... A strong Arctic cold front will approach from the west, crossing our region very late tonight and through Wednesday morning. High pressure will build into the Carolinas Thursday, then remain anchored over the Deep South through the weekend, leading to a warming trend. && .NEAR TERM /THROUGH WEDNESDAY NIGHT/... As of 318 PM Tuesday... Summary and Main Points: NWP models have trended wetter over the past couple of runs, which has boosted forecast confidence that the strong dynamic support aloft associated with the vigorous shortwave trough will support a quick burst of significant snowfall across central NC between 12 to 18z Wednesday, with an increasing likelihood of seeing a broad snow swath of 2 to 4 inches, with locally higher amounts of 5 to 6 possible across the interior portions of the RAH CWA. As such, have upgraded all but the 2 most SE counties(Sampson, Wayne)to a Winter Storm Warning. Due to the west-east difference in onset and ending timing of precip, will segment the warning/advisories maps, with an earlier start in the west and later ending in the east. Details: The well-advertised blast of Arctic air, that`s currently supporting daytime highs in the teens and 20s across central and eastern TN, is well on it`s way. There has been little change in the model timing of the frontal passage through the area; between 06z-09 through the NW Piedmont, to 12 to 15z across the southern Coastal Plain. On the heels of this front, the high-amplitude positively-tilted trough that currently extends from SE Canada back into the Southern Plains, will assume a neutral tilt as it traverses the region on Wednesday, a signal of the strengthening deep layer lift(coupled ascent from strong s/w dynamics, vigorous jet divergence and deep layer f-gen)as depicted by bufkit soundings that shows a classic cross-hair signature(moderate to strong lift intersecting with the dendritic growth zone(-10C to -18C))across central NC between 12 to 18z. As such, expect the ana-frontal precip band to fill in and blossom across the area during that time frame. So, it`s not a surprise to see that models have trended wetter over the last 24 hours, with average liquid equivalents now in the 0.20 to 0.40"range. While we could see a brief period of rain or rain-snow mix at onset, especially east of the US 1 where precip will move in after daybreak, deep layer cooling will result in a quick change-over to snow, with high snow:liquid ratios of 12:18 expected in the 3 to 6 hour window of strongest forcing, similar to the upstream ratios we saw across Middle Tn and Central-Eastern Tn earlier this morning. This would support a broad swath of 2 to 4 inches(0.5-1.0"/hour rate)across interior sections of the state, with localized higher amounts of 5 to 6 inches certainly possible given the favorable set up for banding precip. Elsewhere, along the western and SE fringes, 1 to 3 inches are possible. Aggressive drying aloft will overwhelm the area as early as late morning in the west to mid afternoon in the east. This would support a brief transition of freezing drizzle before ending, resulting in only trace ice amounts. The drying and subsidence aloft is so strong and abrupt, that western portions of the forecast could see sunshine by the afternoon, indeed a quick hitting system. Additionally, the Arctic high building in the Southern US in the wake of this system will support very cold temperatures and wind chills Wednesday and Wednesday night. Daytime highs will likely occur during the early morning at most locations, with wet-bulb cooling and CAA leading to a quick drop into the 20s by midday. NWLY wind gusts of 15 to 20 kts will make it feel even colder, producing wind chills in the teens and 20s. Lows Wednesday night are expected to be in the teens, with wind chill values approaching the single digits across much of central NC. Given these frigid temperatures, any snow or slush on roads and bridges/overpasses will freeze, making for treacherous road conditions Thursday morning. && .SHORT TERM /THURSDAY AND THURSDAY NIGHT/... As of 200 PM Tuesday... In the wake of the exiting system Thu, expect blustery and cold conditions with afternoon temperatures 10-15 degrees below normal. Dependent upon the extent of the snowpack, min temps Thu night may end up a few degrees colder, possibly ranging from the mid teens to around 20. && .LONG TERM /FRIDAY THROUGH TUESDAY/... As of 200 PM Tuesday... The extended forecast will feature a warming trend across the Carolinas as the upper level pattern transitions from a highly amplitude pattern with a west coast ridge and east coast trough, to one of feature a broad, low amplitude trough across the entire U.S. Moderation in the atmosphere begins in earnest Friday and continue through the weekend. The dry air mass coupled with a wly flow should result in a wide diurnal temperature swing of 25-30 degrees. Highs Friday will be close to normal in the upper 40s and lower 50s, then warm well into the 50s Saturday, and near 60-lower 60s Sunday. The mild conditions will persist into early next week as the flow in the mid-upper levels remain zonal. An upper level disturbance will be lifting well to our nw Monday. This system`s attendant cold front will approach and cross our region Monday and Monday night, increasing the threat for scattered showers. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... As of 100 PM Tuesday... There is a high likelihood that VFR parameters will persist across central NC until 03Z Wednesday. After 03Z, an approaching Arctic cold front will get closer to central NC and cause areas of precip to develop. This precip falling through the dry sub cloud layer will initially evaporate before reaching the surface but will aid to lower ceilings into the MVFR category after 06Z Wed in the Triad, and between 09Z-13Z elsewhere across central NC. The precipitation will initially start out as rain but will quickly change over to snow. the snow will pick-up in intensity across the west after 09Z, and across the east after 12Z, with visibilities 1-2SM common. The snow will taper off over the western Piedmont after 15Z, and by early-mid afternoon across the eastern counties. Sfc winds will be light and variable through 10Z, then increase out of the north with gusts 18-23kts probable. Aviation conditions will improve west-to-east late Wednesday and Wednesday night as a drier colder air mass overspreads the region. VFR parameters expected Wednesday night through Sunday. && .RAH WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 4 AM to 9 PM EST Wednesday for NCZ078- 089. Winter Storm Warning from 11 PM this evening to 4 PM EST Wednesday for NCZ007-021>025-038>040-073>076-083-084. Winter Storm Warning from 4 AM to 9 PM EST Wednesday for NCZ008>011- 026>028-041>043-077-085-086-088. && $$ SYNOPSIS...WSS NEAR TERM...CBL SHORT TERM...WSS LONG TERM...WSS AVIATION...WSS is the U.S. government's official web portal to all federal, state and local government web resources and services.