NWS Area Forecast Discussion
Area Forecast Discussion National Weather Service Chicago/Romeoville Illinois 1005 AM CST Monday february 6 2012 .Discussion, 341 AM CST Stratus and fog once again the main forecast challenges. Broad stratus deck across the upper Mississippi Valley continues to spread east and should overspread the entire county warning area by daybreak this morning. Visibilities under the stratus deck drop rapidly behind the leading edge of the stratus to less than a half mile or so, but then gradually improve generally into the 1 to 3 mile range. Out ahead of the stratus, portions of east central Illinois and northwest Indiana have already seem areas of dense fog develop and expect this area to expand under clear skies, thus have hoisted a dense fog advisory. If trends hold, may see these areas improve some after stratus overspreads the area later this morning. Surface winds this morning have already transitioned to a wsouthwest flow ahead of expansive area of high pressure spanning the central and southern Plains. Weak warm advection aloft will act to strengthen low level inversion this morning, trapping any moisture underneath. Most guidance tries to scatter/mix out low level moisture this morning, but believe that guidance is too optimistic and that stratus will hang around most of the day. Continued cooler than guidance temperatures this afternoon based on stratus lingering through the day. To the north, a cold front dropping south out of Canada will cross the area tonight. Moisture with this system is very shallow, and dont see much in the way of precipitation with the front. Behind the front broad stratus deck expected to push into the county warning area. Mid level dry layer persists, but models do suggest deeper mid/upper level saturation as upper wave pushes through with height falls overspreading the area. May even potentially be brief period under jet coupling Tuesday afternoon enhancing synoptic scale lift. Should ingredients come together, may see seeder/feeder setup develop Tuesday allowing for light snow across the area. Otherwise, expect to see decent lake effect setup develop Tuesday with northeast boundary layer flow steering precipitation into northern Illinois. Long fetch will be favorable, but overall thermodyNAMics are rather marginal with inversion heights topping out around 5kft. So while minor accumulations are possible, not looking at anything too crazy at this point. Cold airmass begins to moves into the area Tuesday night behind cold front with 850mb temperatures dropping to near -10c by Wednesday. Maximum temperatures Wednesday will be more seasonable, even slightly cooler than normal in the low 30s, but the big cool down will hold off until Friday and Saturday as reinforcing shot of cold/arctic air makes its way south. This morning, ecmwf and GFS in decent agreement bringing -20c air into the area, though run-to-run consistency has not been good on how far south to bring it. Models begin to break down west coast block over the weekend with a warmup in store for early next week, but will have to see how well models handle meridional pattern and do see some potential for cold air to linger if the blocking pattern holds. Bmd && |
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