Big changes are coming to parts of the country as the week progresses.  First lets look at the current surface and 500mb maps…

Now the 24 and 120 Ensemble mean 500 mb forecast and relative means of predictability

A shortwave trough and trailing cold front that contained some of Isaac’s old energy (the rest may regenerate in the Gulf of Mexico in the next 24-48 hours as Nadine) tore across New England today leaving some serious flash flooding.  Elsewhere a vigorous cold front is pushing across the middle of the US.  Behind this cold front there will be cool Canadian air.  For example, here is a plot of the high temperatures for three Midwestern cities  from Tuesday September 4 through Saturday September   8.

On Friday and Saturday there looks to be some serious convection rolling through the lower plains and into the Ozarks.  This system will then tap into Gulf of Mexico moisture and develop a low pressure system along the east coast.  It looks a lot like a summer Nor’Easter wants to drench the Hudson River Valley.  This is the price for an extended period of high pressure.  Earlier forecasts indicated a deeper trough in the east but the pattern change will not be a permanent one.  By next week the trough is forecast to erode and high pressure will take control of the east.  A trough will try to deepen in the upper Midwest early next week, but with high pressure centered along the North Carolina coast and the desert southwest it will have a hard time pushing south.  There will be another cold front approaching in about 7 days.

Switching gears, El Nino continues to develop and it looks to be strongest in the Central Pacific.  This is consistent with previous El Ninos that have developed when the Pacific Ocean was in its cold, or negative phase. Later this week I’ll touch on El Nino and how it will impact this upcoming winter.