Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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514 FXUS62 KRAH 210555 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 155 AM EDT Wed Mar 21 2018 .SYNOPSIS...
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Cold surface high pressure will build into the region from the north tonight as a storm system develops over southern Appalachians. This storm will move off the coast this morning, and will develop into a Nor`easter as it tracks up the Delmarva coast today. Cold high pressure will build into NC Wednesday night through Friday.
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&& .NEAR TERM /THROUGH TONIGHT/... As of 940 PM Tuesday... We will issue a Winter Weather Advisory for portions of the northern Piedmont, including the Triad for the overnight and early Wednesday period. We are evaluating the observational and model data and it appears that one band of elevated convective precipitation develop/overspread our region in the next few hours. This band is expected to lift north of the region later tonight into VA. Much of this precipitation is expected to be rain, with the exception of a change to a mixture of snow/rain, then wet snow over the far northern Piedmont as the heavier precipitation rates arrive. Some convection may produce some hail, but small enough and accumulations should not affect roads with this initial band. A chance to snow may occur with this band from Roxboro and Henderson northward. Then later tonight, another band or two of precipitation is expected to rotate into the region. Partial thicknesses forecast between 00z and 06z, are expected to fall from 1315m and 1570m in the lower and mid levels - to near 1300m/1555m by 06z/tonight. This is an average of the model data. The current observational data suggests that the lower levels are already colder than forecast over the northern zones where wet bulbs are already 34 at Roxboro and Henderson, forecast was for 39-40. After 06z, all levels are forecast to cool into the "mostly snow" on the Partial Thickness Universal Nomogram throughout the NW and N Piedmont where thicknesses are forecast to dive into the 1290s/1530s. This suggests that the late night and early morning hours will have wet snow as far south and east as much of the Piedmont including the NW Triangle area. Accumulations will be dependent on snowfall rates late tonight, as wet bulb temperatures should be able to fall to 31-32 with the snowfall over the Piedmont (most all models show this). Therefore, due to the snowfall occurring at night through mid-morning, a bit more in the way of accumulation is certainly possible (1-2 inches). If the temps do fall to 31 or 32, then some slick spots will certainly be possible. We will have the advisory as noted below, running from 1 to 11 am. QFP of only 0.1 to 0.25 expected, with around or less than 9 or 8:1 snow/liquid ratios suggests very wet and slushy accumulations. To the south and east of the Advisory area, partials suggest a mix with snow and rain later tonight, with a change to snow showers on Wednesday morning. Essentially only a small, slushy snow accumulation is expected on the grassy surfaces with temperatures in the mid 30s from Albemarle to Raleigh and Halifax. For FAY to GSB, only rain showers are expected at this time. && .SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/... As of 410 PM Tuesday... As an upper level low pressure system crosses the northern portions of the state, chances for a precipitation mix or changeover to snow will increase for areas mainly north of US-64. Looking at forecast soundings, there is plenty of moisture in the dendritic growth zone accompanied by sufficient lift to produce some snow as thickness values drop across the Triad, northern Piedmont, and possibly the northern coastal plain. Timing: Expect precipitation to change over to snow in the Triad beginning at 6z and all areas north of US-64 by 9z. Expect most areas north of US-64 and east of I-95 to change over to snow by 12. Accumulation: Much of the best lift with this system is expected to come through prior to the changeover to snow, but there is a very small window between 6z and 9z where there is some model indication of potentially higher snowfall rates possible in the Triad that maybe able to bring about a little more accumulation but we are still expecting less than an inch (confident) and probably less than half of an inch of accumulation (moderate confidence) in this area. If we are going to bust the forecast on the high side, this is where it will be. All points east of the Triad that are north of I-85 should see less than a half inch of accumulation. Areas south and east of I-85 should see very little to no accumulation. Impacts: At this time we do not expect many impacts across the CWA. If there are to be travel impacts they would be in the area north of I-85 and more likely north of I-85 in the Triad area. Otherwise, a combination of warm ground temperatures, lackluster snowfall rates, and limited timing should combine for no travel impacts across much of the area south of the I-85 corridor. Temperatures will top out in the low 40s across the north with low 50s in the south. && .LONG TERM /THURSDAY THROUGH TUESDAY/... As of 230 PM Tuesday... The upper trof will shift offshore, with ensuing deep northwest flow settling in through the weekend to maintain unseasonably cool temperatures. Initial surface high pressure building in on Thursday and Friday will be dry, with increased sunshine allowing mins in the morning to fall below freezing across the north to mid 30s south on both Friday and Saturday mornings. A lot of divergence in model solutions this weekend, as a short wave will be racing southeast in the mid level flow, with deep cooling of the airmass with potential precipitation breaking out on Saturday and continuing through Sunday. This wave could produce some mixed precip across the northern tier where CAD airmass will be deeper, but will let this ride for now given low confidence. The cool airmass will remain entrenched over the area on Monday, with modest warming and perhaps some sun Tuesday as upper ridging amplifies up the east coast. High temperatures all the way through the extended will be hard-pressed to climb out of the mid 50s over most of the area, with some 60s across the southern tier dependent on the extent of the CAD airmass. Low temperatures will be mostly in the mid to upper 30s, with perhaps some lower 40s in the south and east. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
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As of 200 AM Wednesday... A closed upper low will move across the Carolinas through midday. Wrap-around deformation band associated with the upper low will pivot eastward across the area between 09 to 18z. As mid-level temps cool, rain will mix with or change to snow at KINT/KGSO/ and likely KRDU between 09z and 12z. A mix of light snow and rain showers are expected at KRWI between 12 to 15z, while KFAY should remain mostly all rain. Expect IFR to MVFR ceilings to persist with visbys decreasing to IFR with any moderate snow, mostly likely at KINT and KGSO. Additional shortwave disturbances, diving down the backside of the trough will support widely scattered showers, mostly in the form of rain, across the area during the afternoon. However, ceilings are expected to slowly improve through the afternoon with a return to VFR conditions everywhere by the evening. Additionally, gusty NW winds into the upper teens to lower 20s are expected during the afternoon. Outlook: VFR conditions are expected through Friday. Next storm system will likely impact the area Saturday through Sunday, producing widespread sub VFR conditions in rain, with the possibility of a winter mix Saturday night/early Sunday. Stay tuned.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 1 AM to 11 AM EDT Wednesday for NCZ007>009-021>025-038-039. && $$ SYNOPSIS...CBL NEAR TERM...Badgett SHORT TERM...Ellis LONG TERM...MLM AVIATION...CBL is the U.S. government's official web portal to all federal, state and local government web resources and services.