513 FXUS63 KBIS 091953 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 253 PM CDT Tue Oct 9 2018 .SHORT TERM...(This evening through Wednesday) Issued at 253 PM CDT Tue Oct 9 2018 Accumulating snowfall potential tonight through Wednesday south central and across the James River Valley highlights the short term forecast. The 12 UTC global suite is in agreement on surface low pressure across Kansas this afternoon propagating northeast into Iowa tonight, and western Wisconsin Wednesday afternoon. Wrap around precipitation with this system will spread into southern North Dakota this evening, continuing south central through the James River Valley through the day on Wednesday. The biggest challenge is how fast surface temperatures will cool to support a change over from rain to snow, in addition to snow ratios through the event. The 12 UTC HREF, and the NBM through its 18 UTC iteration suggest rain changing to predominately snow between 03-06 UTC for most areas. The area of most uncertainty is the James River Valley where lesser elevation may lead to a slightly longer time tonight for a full change over from rain to snow. Regarding snow ratios, favored smaller ratios through the event, given near freezing temperatures and increasing northerly winds leading to possible enhanced compaction. Snow ratios of 0-5:1 between 00-06 UTC, should slowly increase to 5-8:1 between 06-12 UTC, and slowly increase to near 10:1 Wednesday morning, before reducing into the afternoon. Using this pattern of ratios, the overall snowfall forecast is relatively unchanged with widespread 3 to 6 inch amounts expected across the Winter Weather Advisory area. The area that will have to be watched for a possible upgrade to a Winter Storm Warning are the glacial hills to the west of the James River Valley across western LaMoure and Dickey counties, into Logan and McIntosh counties. 6 to 8 inches of snow locally are certainly possible for these locations, and will monitor trends through the evening and into the overnight. Again the main sources of uncertainty are how fast surface temperatures cool, and resulting snow ratios. There is certainly large spread in forecast amounts, highly dependent on what snow ratio technique is used. How well snow is accumulating early will be a key indication as to whether higher amounts may be possible. Precipitation will decrease from west to east Wednesday afternoon and into the early evening. While significant blowing snow is not expected with northerly winds gusting 25-30kts given near freezing temperatures, low drifting and sticking of snow on roadways may further impact travel, in addition to reduced visibility with falling snow in strong winds. .LONG TERM...(Wednesday night through Tuesday) Issued at 253 PM CDT Tue Oct 9 2018 A short and relative warm up on Friday, followed by two clipper systems highlight the extended forecast. A relative warm up into the upper 40s and lower 50s, still well below average through, is forecast across most of western and central North Dakota on Friday ahead of a clipper system for Saturday. Strong northwest winds are possible with strong cold air advection and pressure rises in the wake of the clipper Friday night into Saturday. The 12 UTC global suite thereafter favors another possible clipper for Monday into Tuesday. && .AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon) Issued at 253 PM CDT Tue Oct 9 2018 Widespread IFR/MVFR stratus will continue across western and central North Dakota for the 18 UTC TAF cycle. A light rain and snow mix will continue southwest this afternoon, with light rain or drizzle south central and across the James River Valley. The next round of precipitation will spread across southwest and south central North Dakota this evening, and continue into Wednesday morning, decreasing from west to east Wednesday afternoon and evening. Precipitation is expected to be mostly snow southwest, with a mix of rain and snow further east, transitioning to all snow late tonight into Wednesday morning. IFR/LIFR visibility in snow is expected. Strong northerly winds with gusts around 25-30kts will develop south central, through the Turtle Mountains and James River Valley Wednesday. && .BIS WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 9 PM CDT /8 PM MDT/ this evening to 1 AM CDT /midnight MDT/ Thursday for NDZ023-025-036-037-045>048- 050-051. && $$ SHORT TERM...PA LONG TERM...PA AVIATION...PA