Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 230309 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1009 PM EST Tue Jan 22 2019 .SYNOPSIS... Arctic high pressure over the region will drift offshore through this evening, allowing a slightly warmer and more moist air mass to overspread the region. An approaching storm system will bring unsettled weather to central NC Wednesday into Thursday. Drier and colder weather will arrive for the end of the week. && .NEAR TERM /TONIGHT/... As of 1000 PM Tuesday... The Winter Weather Advisory has been extended in time, through 16Z/noon for the nw Piedmont, where the highest probability of measurable freezing rain and/or drizzle of up to several hundredths of an inch are expected, and where a developing in-situ cold air damming regime will likely cause temperatures to remain colder and and supportive of light glazing longer. There remains the potential for spotty light glazing to the east of those areas, from Stanly Co. newd across Randolph, Alamance, Chatham, Orange, and Person Co., where we will highlight that patchy and lower confidence risk with a Special Weather Statement (SPS). Observational trends will need to be monitored overnight for consideration of a short-fused expansion of the Advisory in those counties. 1038 mb Arctic surface high pressure centered about 60 miles off the sern VA coast this evening will continue to extend wwd into NC through early Wed, while drifting toward and just north of Bermuda. A developing and strengthening WAA regime on the wrn periphery of, and rising isentropically-atop, that dry air ridge characterized by 02Z surface dewpoints in the single digits to teens in cntl NC, has caused an area of 1000-4000 ft ceilings to develop from ern GA and SC nwd into w-cntl NC during the past several hours. It will be that developing and deepening low level saturation that will likely produce an axis of light precipitation into the srn and wrn NC Piedmont late tonight-early Wed - a process that will take some time and probably not materialize until after 09Z owing to the preceding Arctic dry. Indeed, some light radar returns have been recently noted from near GSP to AVL, though none has yet been reported at nearby AWOS/ASOS. Opposing physical processes will be at work as those clouds and precipitation expand newd. Initially, downward IR from the expanding low clouds, and some light ssely surface stirring, will cause temperatures to rise as the clouds move overhead tonight, as has been observed already as far east as HFF, SOP, and SCR in the past couple of hours, where earlier upr 20s have been replaced by 32 degree readings. The aforementioned development of light precipitation should then cause surface temperatures to diabatically cool as they trend toward the wet bulb temperatures in the upr 20s to near freezing. As was noted earlier, the highest probability of overlap of sub-freezing surface wet bulb temperatures and light precipitation will exist over the wrn Piedmont, near the Yadkin River. And should that measurable precipitation indeed materialize as expected, the colder NAM/NAM Nest/HiRes NMM/HiRes ARW simulations, for temperatures holding in the 30s over the nw Piedmont for most of Wed, will likely prove most realistic. What could go wrong: If the precipitation is too light and/or spotty, then incomplete wet-bulbing would result in surface temperatures hovering aoa freezing, even in the Advisory area. That is considered a lower probability, but next most likely solution versus the official forecast discussed above. That alternate scenario is more likely to the east of the Advisory area (in the SPS counties), where the expected patchier and lighter nature of the precipitation there was the primary reason we elected to not expand the Advisory ewd. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY/... As of 345 PM Tuesday... The icing threat will persist in the far NW sections until just past daybreak, although models indicate temps rising above freezing in these areas soon thereafter, not unreasonable given that the parent high will be pushing offshore and in an unfavorable location to provide a replenishing of cold dry air into our area, rendering it vulnerable to the strengthening and deepening warm surge aloft. Pops through much of the day will patchy and light as we await stronger forcing for ascent with the potent but dampening mid level shortwave trough moving in from the WSW. Expect widespread showers to move into the W CWA by early Wed evening, with coverage peaking overnight in tandem with passage of vigorous upper divergence, and waning by lunchtime Thu as the trough axis and surface cold front move through. One to two inches of rain is possible, given the strong focused lift late Wed night into Thu morning, and a few urban flooding problems are possible. Expect very gradual clearing W to E late Thu with downslope low level flow and drying aloft. Highs Wed will range from the mid 40s NW to the mid 60s SW, followed by a minimal drop or steady/rising temps Wed night within strong warm air advection. Highs from the low 50s to mid 60s Thu should occur early, with steady or slowly falling temps during the afternoon. -GIH && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... As of 230 PM Tuesday... Quiet weather shaping up for much of the long term forecast. Rain will move out of the area Thursday night which will lead to a period of dry and cold weather with below featuring below normal temperatures through the end of the work week and into the weekend. Synoptically, a long wave upper trough will remain over the eastern CONUS with a continental surface high over the Midwest that will promote cold air advection through the weekend. High temperatures during this time frame will be generally in the 40s with lows in the 20s. Wind chills are not expected to be much of a factor as they winds will be light and variable during the overnight hours. Looking past the weekend, models are trending towards taking a coastal low pressure system further offshore and likely have no impact on central NC weather on Monday but perhaps a rain shower or two on Tuesday in the extreme east as it moves up the coast and eventually further out to sea. A little warmer on Monday with highs in the upper 40s to low 50s. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... As of 750 PM Tuesday... Surface high pressure remains centered just east of the VA coastline this evening with cloudy skies just moving over INT and GSO. This evening surface high pressure is forecast to pull east with isentropic upglide commencing. This can actually already be seen in South Carolina with widespread MVFR to VFR conditions. Wednesday morning, VFR ceilings will slowly lower to MVFR then IFR as upglide continues. Also at this time, light returns will likely appear on the radar across INT/ GSO and temperatures will likely be below freezing (meaning freezing rain). Temperatures will likely remain just below freezing possibly into mid-morning before rising above. The other concern here will also be the increasing low level flow. The NAM strengthens the low level jet across the northwest with 2 kft AGL wind actually above 50 kts at times Wednesday morning. The wind is then weaker further to the south and east. Late Wednesday morning into afternoon the wedge front will lift northwest allowing a rise in ceilings towards KFAY and KRWI. Northwest of the wedge front widespread LIFR ceilings and visibilities will be likely. For now have just trended the forecast this direction. By the end of the TAF issuance the wedge front will likely only be just northwest of RDU. Looking beyond 00z Thur: The wedge front will likely stall between KINT and KGSO as another strong system approaches the terminals. Widespread rain and sub-VFR conditions will continue with winds turning from the northwest behind the front. Drier air will arrive with high pressure behind the front, and VFR conditions are likely from late Thu through Sun. && .RAH WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from midnight tonight to 12 PM EST Wednesday for NCZ021-022-038. && $$ SYNOPSIS...Hartfield NEAR TERM...MWS SHORT TERM...Hartfield LONG TERM...Ellis AVIATION...Haines

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