Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 202338 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 735 PM EDT Tue Mar 20 2018 .SYNOPSIS... A series of low pressure areas will move eastward along the surface front through Wednesday. An upper level low will sweep through the area late Wednesday through Wednesday night, finally allowing the surface frontal system to move east and out of our region. Cool high pressure will build into the area for Thursday and Friday. && .NEAR TERM /THROUGH TONIGHT/...
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As of 730 PM Tuesday... We will likely need a Winter Weather Advisory for at least 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. This is supported by the partial thicknesses forecast between 00z and 06z, which 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 35 at Roxboro and Mount Airy, forecast of 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 or lower in the mid levels. This would suggest that the late night and early morning hours will have wet snow as far south and east as much of the Piedmont including the 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 light 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 are continuing to evaluate.
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&& .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. Expect snow to begin to change back over to rain after 15z. 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 /18Z TUESDAY THROUGH SATURDAY/... As of 225 PM Tuesday... Residual cool airmass across the area will maintain predominantly IFR conditions over the TAF sites through 00Z, with LIFR returning along with widespread rain by 03Z. Visibility will remain mostly VFR as winds remain 6-10 knots over night from the NW-NE. Expecting rain to mix with, perhaps change over to snow in the Triad sites towards 09Z, concurrent with exit of upper short wave and arrival of deep cooler air. Precip will be tapering off to more showery nature by 12Z, with ceiling improving slowly by late morning, perhaps reaching MVFR by 18Z. VFR conditions are expected to return Wednesday night through Friday, with unfavorable aviation conditions possible on Saturday and into Sunday as a warm front approaches the area from the SW. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...CBL/KCP NEAR TERM...Badgett SHORT TERM...Ellis LONG TERM...MLM AVIATION...MLM is the U.S. government's official web portal to all federal, state and local government web resources and services.