Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 161859 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 200 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 /TODAY/... As of 1010 AM Tuesday... 12Z GSO depicts a deep sw flow over the region. This flow in response to an upper level disturbance and associated Arctic front approaching from the west. This sw flow coupled with a dry air mass will push temperatures this afternoon across central NC well into the 40s, with lower 50s probable across the southern counties. Mostly sunny skies this morning will giveway to increase mid-high level cloudiness from the southwest later this afternoon into this evening. && .SHORT TERM /TONIGHT THROUGH WEDNESDAY NIGHT/... As of 237 AM Tuesday... ...Winter Weather Advisory all of Central North Carolina 1100 PM tonight through 7 PM Wednesday for snow... Regardless of the "milder" afternoon, a lingering, very dry boundary layer associated with the departing arctic high pressure will set the stage for snow after midnight in the west, spreading late tonight and early Wednesday. Moderate to high confidence in 1-3 inches of snow for much of central NC later tonight through Wednesday. Models continue to converge on a snow event for much of central North Carolina tonight and Wednesday. A widespread 1 to 2 inches of snow is expected, with some locally 3 inch totals over the north-central and Northeast Piedmont and Northern Coastal Plain. This snow is fairly an atypical snow event for our region as it will be generated by a mid/upper system with a departing arctic high. Typically, for accumulating snow to occur with these typically moisture starved systems, the mid-level vorticity maximum must track just to our south and east. This appears that it is the case this time, with the models in good agreement. This will maximize the little moisture available with strong lift to produce precipitation. Given the dynamics and the expected evaporative cooling, it should be a situation that brings efficient snow accumulations, even with low QPF (0.1 to 0.3 of an inch now forecast). Snow/liquid ratios are forecast to be 10-1 at onset, becoming 15-20 to 1 with strong cooling aloft and at the surface as the precipitation ends Wednesday. Ground temperatures will be cold enough too as the precipitation will start in the west at night, and in the east by around daybreak. As far as precipitation type (P-Type), the models agree in snow as being the dominate type, with a very brief period of light rain at the onset in the SE. Even with highs in the mid 40s to lower 50s today (the very dry and chilly boundary layer will evaporatively cool quickly with the onset of precipitation in the west later tonight. Partials in the 1000/850 and 850/700 strongly support this scenario. The temperatures will quickly wet bulb to below freezing (20s). The area that may get more light rain at onset is in the SE Coastal Plain at precipitation onset Wednesday morning. This region is forecast to have a dew point recovery to near 32, with temperatures forecast in the mid 30s at onset (12z to 15z Wed). It may take a few hours of light rain for the precip to transition to snow in these areas. However, light snow is then expected as the mid level cool rapidly with the system moving overhead (dynamic cooling). The timing of snow in the west is between 100 and 400 am into the western Piedmont, and between 400 am and 700 am in the central, and 700 am and 1000 am in the east. A window of accumulating snow should last around 6 hours in most areas ending from west (noon or so NW) and elsewhere mid to late afternoon. Temperatures should begin warmest as the precipitation starts. So, highs for the calendar day should end up being before the snow hits Wednesday. Temperatures should start in the mid to upper 30s tonight with thickening and lowering clouds. Temperatures (according to wet bulb forecasts etc... ) should cool into the 20s during the evening. Readings may recover a degree or two in the Yadkin Valley (far west) as skies clear mid to late afternoon. However, the arctic air will be pouring in on increasing NW winds, so do not expect anything more than possibly 28 or 30 in the Triad around 300 or 400 PM, before falling quickly again by sunset. Readings in the Triangle may not recover from the mid 20s during the event Wed. afternoon, before falling into the teens at night. In the SE Coastal Plain Wednesday, expect any light rain to quickly change to snow between 12 and 15z. Then snow is likely through the day, ending by late afternoon. Highs in the upper 30s at onset, falling quickly into the lower 30s during the day. So, an Advisory for 1-2 inches is needed all the way through the Interstate 95 corridor again. Alternate scenarios include the potential for lower QPF, as these systems are often moisture starved. The snow accumulation forecast uses the most likely outcome when examining all available data. There is a 20-30 percent chance of lower snow totals, but 1 inch appears to be the low end for the Triangle area, and 0.5 in the Triad, with trace to 0.1 in Fayetteville and Goldsboro. Conversely, there is a chance (20 percent) of higher totals that those officially forecast. This would be 2 to 2.5 inches in the Triad, 3-4 from the Triangle to Halifax, and 1.5 inch in Fayetteville and Goldsboro. The P-type is high confidence of snow in all but the SE, with 70 percent chance of accumulating snow in the SE, after some light rain to start. Wednesday night... Rapidly clear skies along with the gusty NW winds will bring wind chills into play. Actual lows should be 10- 20 NW to SE, with wind chills zero to 10. && .LONG TERM /THURSDAY THROUGH TUESDAY/...
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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. 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. 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.
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&& .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 11 PM this evening to 7 PM EST Wednesday for NCZ007>011-021>028-038>043-073>078-083>086-088-089. && $$ SYNOPSIS...WSS NEAR TERM...WSS SHORT TERM...BADGETT/BSD LONG TERM...WSS AVIATION...WSS

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