Scientific Forecaster Discussion

000 fxus62 krah 192031 afdrah Area forecast discussion National Weather Service Raleigh NC 331 PM EST Tue Feb 19 2019 Synopsis... cold high pressure centered to our north will extend southward into the area tonight through Wednesday, as a southwest flow aloft draws plenty of moisture up and over the cold air. This moist flow is ahead of a storm system that will approach from the southwest before stalling over the region through the end of the week, bringing persistent unsettled weather. && Near term /through Wednesday night/... as of 325 PM Tuesday... ..Winter Weather Advisory tonight through late Wednesday morning for the northern and northwestern Piedmont including The Triad region for light icing... ..added Warren County to the Winter Weather Advisory... A strong and cold 1040 hpa high pressure system centered across the eastern Great Lakes this afternoon will move into eastern New York/PA tonight and weakens as it reaches the New England coast late Wednesday. The high is of sufficient strength and in a favorable location to establish a cad regime tonight that will extend into Thursday. Dry air has extended south with surface dew points in the single digits across NY, the teens in PA into northern Virginia and the 20s across northeastern NC. A strong upper level trough near The Four Corners region this afternoon will move into the Central Plains as a strong southwesters flow aloft extends across the Southern Plains and southeast. In addition, a strengthening low and mid level southerly flow will provide ascent and moisture resulting in lowering ceilings and this afternoon as precipitation expands and develops across the area. Latest nwp guidance suggests that the amount of cold and dry air at the surface may be more limited than previous forecast guidance. This is seen in a dprog/dt of NAM surface T and TD. A band of light precipitation driven by mid level warm advection continues to stream east across our southern tier. This precipitation is being driven by elevated processes and the precipitation is falling out of elevated ceilings of 8kft or higher resulting in lots of virga and spotty amounts. Expect some sleet to mix in at times this afternoon and early evening, mainly at the onset as the precip cools into the dry air below the cloud deck. Isentropic lift ramps up this evening in the 295 to 300k layer resulting in a region of low and mid level ascent and subsequent warm advection precipitation. The precipitation will overspread the region from southwest to northeast this evening and overnight. Although the forecast bl temps may have warmed, forecast soundings suggest a burst or brief period of wet snow and sleet will mix in with the rain this evening across The Triad and then spread northeast into the Virginia border counties. The snow and sleet should be short lived and will fall with surface temperatures above freezing resulting in limited accumulations. Given the rates may be impressive for a short period, have added a limited coating of snow/sleet to the forecast in these areas. A strong southwest flow aloft will push a strong warm nose across the area shutting down the snow potential quickly this evening. The precipitation will produce some wet bulb cooling as the surface layer becomes saturated. Not surprisingly the NAM is the coolest guidance with the wet bulb freezing line pushing south into the I-85 corridor which should serve as the farthest extent of any freezing rain potential. The GFS is notably warmer in the surface layer and partials. Will lean toward the cooler NAM solution given its skill to handle these patterns more skillfully. This will support a changeover from rain to freezing rain as temperatures fall to 32 or 31 in the damming region after several hours after midnight from Albemarle to Raleigh to Warrenton northwestward. Given the marginal surface temperatures, the light precipitation amounts, some lulls in the precipitation and moderate temperatures ahead of the event, expect ice accumulations will be limited and mainly confined to locations near and north of I-85 where a light glaze is possible with radial ice accumulations of 0.08 inches or less. Temperatures will slowly moderate after daybreak on Wednesday as the parent high pressure system moves away. This will allow the freezing rain to diminish from south to north and end during the mid morning hours. Lows will range in the 30 to 32 range in the northern Piedmont and the mid 30s elsewhere. An impressive cad regime will be in place on Wednesday and continue into Wednesday night with periods of light rain continuing.The most widespread rain will shift northeast during the day with lingering areas of drizzle and mist. Highs on Wednesday will range in the mid 30s across The Triad and in the mid to upper 40s across the sandhills and southern coastal plain. Temperatures will be steady or rise a degree or two on Wednesday night under overcast skies and periods of light rain, drizzle and mist. && Short term /Thursday through Thursday night/... as of 325 PM Tuesday... Most nwp systems continue to blast the cad away on Thursday with the exception of the NAM. The in-situ damming regime is nibbled on the east and from the west from subsident flow east of the mountains. Most of the statistical guidance pushes highs in the 60s and 70s, even in The Triad where this should be way overdone. While the cad should weaken, given the pattern and the lack of a well defined erosion mechanism, still feel it will remain in place across the Piedmont longer than much of the guidance says. Will undercut guidance and go with highs around 50 in The Triad to the mid 60s southeast. && Long term /Friday through Tuesday/... as of 330 PM Tuesday... A quasi-stationary cold front will sag south through NC on Friday before inching back north Saturday evening. This will allow numerous weather features to track from SW to NE across central NC, introducing rather persistent pops from Friday morning through Sunday afternoon. Model quantitative precipitation forecast through this period varies from around a half inch across the coastal plain to just over an inch across the northwestern Piedmont, with locally higher amounts possible. Depending on how much precipitation falls earlier in the week, this may enhance the river flood potential late week, although both the naefs and gefs has backed off on this threat over the last few days. The persistent rainy pattern looks to come to an end on Sunday evening at the hands of a progressive frontal passage expected to slide west to east across the mid Atlantic, with high pressure likely to fill in on its heels Sunday night through Monday. Another storm system begins to develop over the MS river Delta early Tuesday, likely to introduce showers to the forecast as early as Tuesday evening. Temperatures will be a bit of a roller coaster (with rather high bust potential) through the long term period, varying greatly thanks to several fropas, cad events, and persistent cloud cover. A rather robust cold air advection regime on Friday behind a backdoor cold front should allow for generally falling temps through the day Friday with cold air damming conditions strengthening into early Saturday. The parent high will shift offshore over the weekend allowing southerly flow to reestablish warm air advection, beginning to erode the cad Saturday night/Sunday. Have trended a bit below guidance both days, not sold that the erosion mechanisms will be efficient enough to completely gouge out the colder/denser airmass out of The Triad region prior to the front crossing the region sun evening. Expect highs in the upper 40s/low 50s on Friday and Saturday with low to mid 70s on Sunday (assuming a complete cad erosion in the warm sector). Behind the front, temps will rise into the upper 50s/low 60s. Low temps should remain seasonable, and above freezing through the period. && Aviation /20z Tuesday through Sunday/... as of 1240 PM Tuesday... VFR conditions continue across central NC, with said conditions likely to persist through sunset Tuesday. Cigs (currently above 10kft) will gradually lower as the period continues, with MVFR cigs/vsbys likely to accompany the first rain showers this evening into the overnight hours. Some icing will be possible, especially across The Triad terminals (kint/kgso), between 08z/3am - 15z/10am tomorrow, as a mixture of rain/fzra/pl impacts the northern most Piedmont sites. Can't rule out a brief period of freezing rain at krdu around sunrise Wednesday. Precipitation should return to all liquid form through Wednesday afternoon and likely persist much of the day, with IFR/LIFR cigs/vsbys present under continued cold air damming conditions. Outlook: at least intermediate sub-VFR conditions expected to continue into the weekend as a series of weather systems spread showers across central NC. This includes, a prolonged period of cold air damming characterized by widespread LIFR-IFR conditions, and periods of rain and low level wind shear, will persist Wednesday and Thursday, with more intermittent cats likely through Saturday night. && Equipment... electronics technicians have completed work on krax, which seems to have fixed the data dropouts at the lower beam heights. We will continue to monitor the data over the next few days to ensure accuracy. && Rah watches/warnings/advisories... Winter Weather Advisory from 7 PM this evening to noon EST Wednesday for ncz007>010-021>025-038-039. && $$ Synopsis...jb near term...blaes

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