Good morning everyone.  A chilly day for the Eagles but it gives me some time to get this STORMcast Discussion out before the hysteria breaks at the beginning of the week when we might need aerosol sedatives to calm everybody down.  

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The Easier of the Two Forecasts - Saturday-Sunday

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​From the last Discussion, this weekend's storm was "number 3" on the docket and is playing out about as expected.  Around 7 AM Sunday, a strong low pressure center will pass us well to the west out near the PA/OH border.  Atmospheric pressure is forecasted to be around 978 mb which is respectable but the center of influence is focused to well our west.  However, as the storm ramps up, we'll see an associated warm front cross the region around Saturday morning that will advect significant amounts of warm air and moisture into the region.  At first, most of us will get a shot of snow but this will change over to rain for the metro area and points south and east by around dinner time Saturday.  This is reflected in the positioning of the NWS's Winter Weather Advisories to our north and west where the cold air will last longer and allow for a higher initial snow accumulation.  This storm will bring in quite a bit of precipitation between Saturday night and Sunday morning so don't be surprised if we pick up an inch of rain or so by the time it's all over.  Sunday afternoon should see the end of the precipitation as the lagging cold front barrels through.  This point will be pretty obvious as blustery W/NW winds will rapidly increase by evening.  Resultant temperatures will drop drastically with highs barely reaching freezing for the first half of the work week.  This temperature regime could also help set us up for an impressive midweek storm.

 

The More Difficult Forecast - Thursday   

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This is the event I'd like to get out ahead of before the armchair meteorologists grab hold of it and turn it into snowmageddon.  Recall that our jet stream for the past few storms has really not been all that conducive to helping a surface low turn into a decent-sized event.  It readily explains our shots of 2"-4" of wet snow and ice but little more.  This week, however, things look like that will change and we could see a healthy connection between lower and upper atmosphere for a downright kickin' Nor'easter if the pieces all fall together.  Long range (and now medium range) models are still bouncing around a lot but some type of snow event is slowly starting to come into focus for next Thursday. 

 

With the weekend storm out of the way, the picture is much clearer now and I think we're going to dodge this one unless some pretty significant course corrections take place between now and Thursday.  

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How This Has To Come Together

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By the beginning of the work week, models suggest a trough will start to dig into the central Plains and slide east.  Meanwhile, a surface low in the Gulf of Mexico (you know where it is) looks like it will also form and begin a track across the Southeast before returning over the ocean off the Carolinas.  By Thursday morning, this low is forecasted to be off the NJ coast (top image, green L).  Meanwhile, the trough in the upper atmosphere might approach this low from the west and tilt backwards (top image, green line) into what we call a "negative tilt".  If these two elements come close enough together, a chimney effect could be opened up between them and greatly intensify the coastal storm.  At the moment, I don't see a perfect connection but it does look like there might be some healthy coordination between them.

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Current forecast model projections still show both the upper level and surface features mentioned last week but their lack of connection seems much more pronounced this morning so it really doesn't look there will be much upper level support for the low.  This will greatly knock down the overall intensity of the surface system (top right). 

 

Also involved could be cold, Canadian high pressure to the north of the storm.  This high will be responsible for much of the cold air that will drain into the region early in the week and the position of the high could play a pivotal role in the storm's development.  If the high sets up westward of the storm, it could knock it out to sea and the impact on us could be very low.  It could also set up to the NE of the storm and help to block the storm's forward speed leaving the region with a prolonged and potentially heavy snowfall.  

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The Canadian high pressure settled in right on schedule and is definitely pumping in the cold air but models generally suggest it'll take up a position behind all of the activity along the coast which means it is more likely to help push the surface low near the Carolinas offshore and away from land.  Depending on your model of choice, the GFS pretty much has the system completely  missing the forecast area with the exception of the southern DE beaches while the NAM has a bit more northerly course bringing a touch of snow up into the metro area but not enough to write home about (center right).  Worst case scenario here might be a covering around Philadelphia but nothing further north (bottom right).  If, however, it looks like this might be possible, watch for Winter Weather Advisories to creep up closer to Philly.  I don't think this will be the case but you never know.  

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I've been watching this one since it first showed up on the models  Wednesday when the first snowfall estimate was something ridiculous like two feet for the metro region.  Since that time, models have wobbled by hundreds of miles back and forth with everything from a major snowstorm to almost nothing happening so for now, I'm in a watch and wait mode.  This morning's model run shows a pass to the south off the VA coast (middle image) but with a precipitation shield that spreads all the way to North Jersey.  Cold air will already be in place so if anything happens, it seems likely that most of it will be snow.  The current snowfall forecast is also shown at right (lower image).  Note the distinctive SW-NE trending swath of heavy snow associated with Nor'easters. 

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Well, I hope you understand how these situations solidify my aversion to long range winter storm forecasts.  They're worth watching but until we get within about 72 hours, any snow projections are highly suspect and of pretty limited value.  This is a perfect example.  ​​At least for now, the scope looks clear for activities later in the week.  If this changes, I'll post again as we get closer but as of right now, this will be the only update for Thursday's storm.  Have a nice week everybody.

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rs

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