Scientific Forecaster Discussion

000 fxus62 krah 191302 afdrah Area forecast discussion National Weather Service Raleigh NC 800 am EST Tue Feb 19 2019 Synopsis... cold high pressure will move east across the Great Lakes into New England today through tonight. The high pressure will build southward into our region today into tonight. A storm system will approach from the southwest tonight then move NE across the region on Wednesday and early Thursday. && Near term /today through Wednesday morning/... as of 800 am Tuesday... Quick update to account for virga/light rain falling across the southern zones. Can't rule out some very brief, no impact, light sleet in these zones as well. Previous valid discussion... ..Winter Weather Advisory tonight through late Wednesday morning for the Piedmont triad region east along the Interstate 40/85 corridor and north to the Virginia border for light icing... ..a light coating of sleet is possible Tuesday evening, before light freezing rain dominates Tuesday night into mid- morning Wednesday in the advisory area... Elsewhere... a cold rain with a chance of a very light (trace) icing near the advisory area Tuesday night, mainly on elevated surfaces. A strong (1040 mb) and bitterly cold surface high pressure over the upper Midwest to the Great Lakes will move east to New York/PA tonight into Wednesday. The high pressure will build down the eastern Seaboard through tonight providing a source of cold dry air influx into our Piedmont damming region. The high is more than strong enough, and in a very favorable to produce a strong cad event beginning Tuesday night, maximized on Wednesday, then slowly (very, very slowly) break down later Thursday. The high will be rather progressive - moving off the New England coast Wednesday night. However, once precipitation develops and overspreads the Piedmont damming region Tuesday evening through the morning Wednesday, the cold nose will be maximized in the mid to upper 20s (at 1000 to 2000 feet agl) centered through The Triad region from central Virginia. The quantitative precipitation forecast to lock in these cad events typically is around 0.20 to 0.25 of an inch. It appears that there is enough agreement in that the quantitative precipitation forecast has locked in to an average of around 0.25 to 0.35 in our western Piedmont Tuesday night, with additional quantitative precipitation forecast of 0.25 into the day Wednesday. The highest quantitative precipitation forecast is expected to be in the Piedmont damming region during The Heart of the cad event, with lesser quantitative precipitation forecast to the east, with a delayed timing in the east as well. This should limit the icing chances to the northwest and north Piedmont (mostly the Interstate 40/85 corridor) Tuesday night into Wednesday, with lighter rain to the east (east and south of the Triangle very late Tuesday night and Wednesday. As for the precipitation type (p-type), partial thicknesses suggest light rain mixed with sleet at onset in the advisory area late Tuesday afternoon. As the rates increase by around dark into the evening, strong evaporative cooling will lead to sleet mixing with freezing rain for several hours. A light coating of sleet is possible but a very strong warm nose aloft will punch in overnight, which runs a top the cold nose at the surface leading to freezing rain between 06z and 12z/Wednesday. Since the cold nose maximizes late Tuesday night into early Wednesday, that is when some freezing rain accrual would be most likely depending on temperatures. This is a difficult temperature forecast as the NAM is coldest and favored due to the current data (strong cold high, favorable position, and bitter cold air associated with the high). The GFS would have essentially all rain, even in The Triad. Using forecast wet bulb temperatures (using the colder solution), we will continue our current forecast with light freezing rain in the aforementioned areas, especially the Piedmont triad region Tuesday night into mid-day Wednesday. Even if temperatures are slow to rise above 32 (which it may be 18z or so in the gso and int areas), the freezing rain accrual will become self limiting as warming aloft leads to warmer rain falling to the surface, and the supply of cold dry air into the damming region will have been cut off before 12z/Wed. We will carry 0.05 to 0.10 ice accrual in the advisory area of the Piedmont, after a trace to a light coating of sleet to start the precipitation Tuesday evening. Lows around 30 expected in most of the advisory area (with very slowly warming temperatures to 32 around 18z, then mid 30s by 21z. The travel issues should end by mid to late morning with the slow warming. To the east, periods of light rain, possibly mixed with some light freezing rain on elevated surfaces from Albemarle to Raleigh (areas closest to the the advisory area). Lows around 32 to 33, rising into the upper 30s by late day. Sensible weather for today... thickening cloudiness, chilly NE breeze 10 mph. Highs in the 40s. In the advisory area tonight, rain likely with a chance of a little sleet early. Then freezing rain. Lows 29-32. Ra in likely elsewhere outside the advisory area. Lows in the 30s. Freezing rain to rain in the advisory area by around noon Wednesday. Highs in the mid 30s. Elsewhere, periods of rain. Highs in the upper 30s to mid 40s. && Short term /Wednesday afternoon through Thursday night/... as of 400 am Tuesday... Most nwp systems blast the cad away Wednesday night and Thursday with the exception of the NAM. GFS MOS provides a high near 70 at gso and 80 Fay. While the cad should weaken, given the pattern and the lack of a well defined erosion mechanism, still feel it will remain in place longer and more resilient than much of the guidance says. Will undercut guidance and go with highs in the lower 50s in The Triad to the upper 60s southeast. Huge bust potential with the temperature forecast so stay tuned. Also, periods of light rain and drizzle will continue into Thursday night. Lows in the 40s/50s with areas of dense fog possible, as well. && Long term /Friday through Monday/... as of 447 am Tuesday... Yet another chilly high pressure will lead to cad Friday into Saturday, as a front will stall to our south and waves of low pressure track along the front. Another good chance of rain can be expected Friday into Saturday. The cad will not be as cold or deep as the predecessor. Lows generally in the 30s northwest to 40s south. Highs in the 40s except 50s southeast. Sunday and Monday, a storm track from the Great Lakes to southeast Canada and a strong frontal passage late Sunday will lead to a warm sector blowing our cad out Sunday. Highs should return to the 60s/70s with a good south-southwest flow ahead of the front. There is a good chance of showers and isolated thunderstorms Sunday into Sunday night. Partly sunny, breezy northerly winds Monday will occur behind the cold front. Highs will return to near normal, mid 50s to lower 60s. Lows in the mid 30s to mid 40s. && Aviation /12z Tuesday through Saturday/... as of 100 am Tuesday... VFR conditions are expected for much of the day, with lowering MVFR ceilings late. Deeper lift and cloudiness spread north from the deep south later today and this evening. Some light rain may accompany those lowering ceilings late today at kgso/kint, and perhaps as far east as kfay. Outlook: sub-VFR conditions and widespread rain will develop this evening. As warm and moist air aloft surges nwd a top cold, high pressure at the surface over the Middle Atlantic States. A prolonged period of cold air damming, characterized by widespread LIFR-IFR conditions, and periods of rain and low level wind shear, will result through Sat. Additionally, a period of freezing rain will likely occur at int and gso between 02z-17z Wed. && Equipment... krax is still suffering from poor data quality issues in the 0.5 and 0.9 degree elevation slices. This will impact several radar products, including rainfall estimates. Users should use the data with caution. We appreciate your patience as our engineers and technicians solve the problem. We will provide updates when they are received. && Rah watches/warnings/advisories... Winter Weather Advisory from 7 PM this evening to noon EST Wednesday for ncz007>009-021>025-038-039. && $$ Synopsis...Badgett

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