Temperatures have run at well above normal levels so far this month for much of the Midwest, Appalachians and Northeast sections of the country, but big changes are coming during the second half of October; especially, from the Midwest to the Appalachians. Mid-Atlantic cities such as Philly (+7.0 degrees), DC (+6.4) and New York (+6.6) have all run at well above normal temperature levels in October and so have the Midwestern locations of Chicago (+7.1), Detroit (+6.8) and Minneapolis (+6.0). While it will be quite hard to negate all of that above-average warmth in the Mid-Atlantic during the second half of the month despite the likelihood of experiencing the coldest air yet this season, dramatically colder-than-normal temperatures in the Midwest could manage to do just that for those locations.
An unfolding weather pattern change will bring about a deep upper level trough of low pressure centered over the Upper Midwest beginning over the upcoming weekend (GFS forecast map below, courtesy NOAA) and this will open the door for cold air intrusions into the country for the latter stages of the month possibly resulting in the first lake-effect snow for the Great Lakes region and the first freeze for many locations. The 500 millibar height anomaly map (above) for this weekend from last night’s GFS ensemble computer forecast model run depicts an overall pattern featuring a deep trough in the Midwest (blue region) and strong ridge of high pressure along the west coast (red region) which combine to drop cold air from central Canada into the Upper Midwestern states [map courtesy Penn State e-wall]. It is not out of the question that snow even reaches as far east as the Appalachians by the end of the month depending on the exact timing of the cold air outbreaks with potential east coast storminess.