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

The Discussion of Weather Events Daily for Philadelphia and Pennsylvania

  last updated: 09/14/2003 02:24 PM

Good Morning!  The short of the discussion today is rain coming for Saturday and again on Monday night.  The Saturday rainfall will be a general swath of precipitation spreading over the state like the last few rain events.  The Monday rainfall will be a line of thunderstorms sliding through (as mentioned in an earlier discussion). 

This morning's surface map and satellite combo shows a lazy stationary front/cold front in Canada draped to our distant north and a high pressure system approaching from the southwest.  Today will be the nicest, warmest day in memory (if your memory is sort of short.  The next system is sitting in Minnesota with convection draped to the south.  

Remember that the picture above shows the temperature of the surfaces the satellite is seeing. There is a slight grayness to the coloration over PA.  The visible satellite (below) shows the bright white cumulus cloud fields over the area (but being lower clouds, they are about the same temperature as the ground, so they don't show up well in Infrared. 

The overall upper level wind pattern hasn't yet changed much (looking back a week) so Saturday's system will again pass just to the south of the state with the heaviest precipitation along the southern 1/3rd of Pennsylvania again.  Eventually, there may be some flooding problems there!  Watch for watches and warnings by Saturday evening. 

A quick look at the 500mb winds (see earlier discussions for an explanation of what these mean) for the upcoming Wednesday compared to today show the story.  There overall troughiness of over the nation becomes somewhat more flatter (straight west to east) with ripples traveling along the broad pattern.  This will give PA some warmer temperatures and more thunderstorms when weather does happen. 

Yesterday's 500mb pattern and winds                                                    This upcoming Wednesday's map. 

The next image is the forecast for Saturday night at 8pm and shows the position of the low and the heavier rain according to a computer model named the ETA.  But this is a bit different from another model...

The GFS model (which is used for more long range views of the weather) shows the low a bit further south and moving a bit further east than the ETA above.  It also has less overall rain for Saturday.  The NWS forecasters are tending to rely on the ETA more for the weekend. 

Stay dry and enjoy the ever increasing breaks between storms and rising temperatures. 

Meteorologist John Ensworth, M.S. 

(This page sometimes contains sources from AccuWeather.com   www.accuweather.com ).