Back To Home
The Story Behind the Weather - By Forecaster John Ensworth M.S. - 

The Discussion of Weather Events Daily for Philadelphia and Pennsylvania

 Last updated: 01/21/2004 11:51 AM
 

Good Afternoon Philadelphia!   

    I'm glad to announce that these discussions will continue until at least the end of February (it's a short month)! 
    I have the necessary server space (especially with missed days!).  

In short: After a short jaunt above freezing tomorrow, we'll stay in the freeze with snow possible almost any time. 

 In long: Looking at the Model Output Statistics, this forecast product shows, moment by moment, the weather expected for Philadelphia through Friday 7am.  I've added a dotted light green line at 32F.  

We'll climb to about 37F for a while tomorrow afternoon, then begin a long decline again (more on that at the bottom).

We have quiet weather around here because of the high pressure system that is in place over the eastern US.  Our next snow maker is getting itself together in the Great Lakes right now. 

The high is sitting under boring zonal flow on the 500mb map (this is the flow aloft at about 18,000 feet above sea level with winds traveling west to east over the continent parallel to the dark black lines).  The front and low over the Lakes is being powered by the deep cold trough/low that is now rolling south out of Canada.  Note it's further west than the earlier shots of cold air.  Watch out Great Plains!

The visible satellite picture shows white stuff on the ground (or in the sky) as white stuff.  And dark bare ground or open ocean as dark. I've outlined an area that is a bit fuzzier.  The places to the right of the red line are gray/white because of snow cover.  How do I know the stuff to the left of the red line is clouds (or at least thin clouds...since you can see land forms under the fuzzy stuff in Pennsylvania)?

You need the infrared satellite picture to do that. Cold clouds or ground show up as green and blue while reds are warm ground or ocean. I simply outlined the green areas below on the picture above.  See how different these satellite views are? They both have their own story to tell. 

The National Weather Service is forecasting the approaching Great Lakes front to begin spreading light snow into the state (almost to Philadelphia) by 10pm tonight...

The national map at that time shows a warm front has passed (giving us our warm Thursday temperatures) and the body of the front and low still to our west (this map is for 7am Thursday). 

Then more snow moves in by Thursday afternoon (with rain showers to the south where the temperatures are just a bit too warm). 

At that time (Thursday 7pm) the front has swept over the area and the cold air returns rapidly.  See how far south the front has swept in the Mississippi Valley and Ohio valley?

Let's look at the next week of nation wide temperatures and track this next invasion of cold air. 

Wednesdays highs show below freezing highs from eastern Montana, through the Dakotas, the Great Lakes and down into Ohio and Pennsylvania (and all places northward). 

Boom! Thursday's highs shows freezing highs spreading rapidly down to northern Missouri, Illinois, Indiana, W. Virginia and Pennsylvania (and points northward).  Even single digit highs are invading around the lakes. Brrr!

For Friday, the freezing temperatures retreat a bit northward but fill in more solidly in the New England states.  Pennsylvania is still frozen. 

Saturday, a NEW shot of very cold air (near zero highs) pushes down into the Dakotas and Minnesota and very cold temperatures (single digit highs) invade the northeast third of Pennsylvania. 

Sunday the northern tier of states across the nation see the invasion of very cold air (even further west!).  We are on the edge of climbing out of freezing temporarily. 

And REAL cold invades Colorado, Idaho, Nebraska and Iowa (and points northward).  I guess we'll let the rest of the nation take the hit this time even though it has to pass through here when it leaves next week.  Brrrr. 

Stay warm, drive safely and I'll see you here Thurs and Friday this week.  I'm traveling Saturday and will take that day off.  

I'll be back tomorrow!

Meteorologist  John Ensworth

Surface Station sky cover color key: