The GFS/Euro models continue to portray a weaker system than the NAM in regards to the severe risk on Wednesday and Wednesday evening. While lower and upper winds remain strong with the system, the marginal instability is being enforced by warmer air in the mid levels along with added tropical moisture. Strong to severe limit winds remain the primary focus with any convection that does manage to take hold with the marginal instability. As stated below yesterday, I continue to feel the NAM is on the strong side of actual outcome with this system. Still, if more instability is realized and is able to entrain into the system, then severe weather will be more likely.
Original post - 9/9/14
A deepening low pressure system similar to last Friday will churn over the upper Mississippi Valley, Midwest and Great Lakes this Wednesday into Thursday. While the last storm center on Friday 9/5 had much better instability to work with, this one appears it will have better or stronger dynamics and wind fields, quickly turning clockwise winds at low heights (helicity) and powerful bulk shear force winds to compensate for weaker instability. Both bulk shear winds /Surface - 18,000 ft/ and heights as low as 925 MB /2500ft/ and 850 MB /5000ft/ are forecast to have winds estimated 50 - 70 knots. With that kind of wind just aloft, it won't take much to bring it down to the surface even with lesser instability this time.
This projected storm, better suited for late October or November with its projected depth (on NAM guidance which may be a bit strong), will take aim on the region Wednesday into Wednesday Night. Not unlike the last storm, the main threat will be damaging winds but in addition, there will be some risk of isolated tornadoes because of the spin in the lower atmosphere.
Maps are for Wednesday afternoon:
By Wednesday evening (below); the center of the low is slated to move into northern Michigan and while doing so, it will bring the most unstable air north into lower Michigan and thus, continuing the threat of severe weather into the first part of the night as the cold front swings across Lower Michigan into Southeast Lower Michigan.
Keep abreast of weather forecasts and potential of severe Outlooks, watches and warnings with this system especially if development continues to unfold as projected. The plotted model is that of the NAM which has been the stronger model with this system.
Making weather fun while we all learn, Bill Deedler -SEMI_WeatherHistorian