Scientific Forecaster Discussion

000 fxus61 krnk 180909 afdrnk Area forecast discussion National Weather Service Blacksburg Virginia 409 am EST Mon Feb 18 2019 Synopsis... a cold front will move east across our region this morning. Drier weather will push into our area this afternoon into tonight. Moisture slowly returns from the southwest on Tuesday. By Tuesday night into Wednesday, there is enough colder air and moisture over the region for the strong potential for a significant winter event. && Near term /through tonight/... as of 409 am EST Monday... Rain will be ending from southwest to northeast this morning. Models in good agreement that deeper moisture exits southwest Virginia, northwest North Carolina and southeast West Virginia by 15z/10am. However, low level moisture will remain banked up again the western slopes of the central Appalachians today and tonight. Surface through 850mb winds turn to the west then northwest following a cold front this morning. Temperatures trend will be non- diurnal in the mountains as higher elevations drop behind the cold front then have little recovery until all the clouds today. In the foothills and Piedmont where The Wedge is nearly gone temperatures will continue to rise slowly and downsloping winds behind the front, as well as some sunshine will balance out the cold air advection. Will be cutting back on the cloud cover east of the Blue Ridge tonight. No significant changes to forecast lows for tonight. && Short term /Tuesday through Wednesday night/... as of 409 am EST Monday... Posted a Winter Storm Watch from Tuesday afternoon into Wednesday afternoon because of the potential for significant ice and snow accumulations. A strong (1040mb) area of high pressure will move east into the northeast United States, this will result in cold air wedging down the Appalachians. This is a favorable position for a strong cad event for our region. As a winter storm lifts northeast across our region, precipitation will spread north over this shallow dome of cold air, producing a wintery mixture of snow, sleet and freezing rain across the area. The storm is still evolving and the amounts and precipitation types will likely change over time. Utilized wpc quantitative precipitation forecast and mixed_precip_topdown_ensemble tool with GFS to calculate ptype, and the amounts of qpf,ice and snow. Total snow accumulations will vary from light amounts in the Piedmont to heavy snows in southeast West Virginia and Alleghany Highlands of Virginia. The highest amounts of snow will occur in western Greenbrier where 4 to 8 inches are possible. Ice accumulations will range from a light glaze to around one quarter of an inch. The onset time of the precipitation Tuesday was a little slower on the model solutions. Despite decent isentropic upglide over the cold dome, strong low-level cold and dry air advection may delay of the saturation of the airmass. High temperatures on Tuesday will range from the the mid 30s in the mountains to the mid 40s in the Piedmont. The moisture advection and lift are maximized Tuesday evening into Wednesday morning. This is the time period where we will have the highest snow and ice accumulations. Low temperatures Tuesday night will vary from the mid 20s in the mountains to the lower 30s in the Piedmont. Pockets of freezing rain will continue Wednesday morning into Wednesday afternoon until the warm air wins out and push north across our region. High temperatures Wednesday will generally be from the mid 30s in the east to the mid 40s in the western mountains. Periods of rain will continue Wednesday night with low temperature readings from the lower 40s in the west to the lower 30s in the east. && Long term /Thursday through Sunday/... as of 430 PM EST Sunday... Very wet and unsettled weather continues with a positively tilted mean trough in the western U.S. And a broad subtropical ridge remaining anchored off the southeast U.S. Coast. This will keep a conveyor belt of deep moisture anchored over the area along with an active baroclinic zone. Temperatures will start out below normal on Wednesday, then creep to above normal levels by the end of the week as the baroclinic zone is nudged a tad further north thanks to the peristent subtropical ridge off the southeast U.S. Coast. Indications are that the baroclinic zone will slip south of the area briefly as high pressure builds to our north on Thursday, to provide a small opportunity for the heavier rain to shift south of our area. However, another significant wave emanating from the southwest U.S. Trough is progged to push the baroclinic zone back north into our area Saturday with more heavy rain and flooding possible at that time. At any rate, temperatures will finally warm enough with southwest flow to erode the returning wedge, that the main focus through this period will be flooding, not winter weather. Wpc days 1-7 precip outlook currently highlights areas exceeding 4" mainly in western NC, eastern TN, and southwest Virginia. Areawide, expect at least 2-3", with local amounts of 4-5 inches likely. Please remain abreast of the threat of flooding and river flooding with later forecasts as their is an increasing threat for areal flooding and river flooding, especially during the later half of the week. As noted above, with the region increasingly in the warm sector during this period, temperatures will average above normal and remain mostly above freezing. && Aviation /09z Monday through Friday/... as of 1155 PM EST Sunday... IFR/LIFR a high likelihood until frontal passage. Front to pass across forecast area from west to east, preceded by rain, and followed by partial clearing and increasing winds. Flight conditions are expected to improve Monday...transitioning to VFR east of the Blue Ridge. Lingering MVFR cloudiness is anticipated across the mountains. Frontal passage will bring a wind shift to the northwest with frequent gusts throughout the day Monday to around 25 kts. Winds will gradually subside Monday night. Clouds across the mountains are also expected to dissipate Monday night. Forecast confidence in the above scenario is moderate. Extended aviation discussion... Lingering mountain cloudiness is expected to maintain MVFR cigs across the mountains through Monday night. Brief break in the wet weather is expected Tuesday, conditions primarily VFR. Tuesday night and Wednesday another low pressure system will bring sub-VFR conditions as well as providing another opportunity for winter precipitation. A frontal boundary will remain close to the region Thursday into Friday keeping precipitation and the potential for sub-VFR conditions in the forecast. && Rnk watches/warnings/advisories... Virginia...Winter Storm Watch from Tuesday afternoon through Wednesday afternoon for vaz007-009>020-022>024-032>035-043>047. NC...Winter Storm Watch from Tuesday afternoon through Wednesday afternoon for ncz001>003-018-019. WV...Winter Storm Watch from Tuesday afternoon through Wednesday afternoon for wvz042>044-507-508. && $$ Synopsis...ds/kk

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