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Thursday, February 21, 2013

Finding The Rain/Snow Line Critical For Weekend Forecast

The 12z European Forecast Model (ECMWF) displaying Sunday morning.
Notice a much weaker low compared to yesterday.  1000 mb compared to 992 mb (See yesterday's blog  post).

I debate over this with every storm…When do I release the dreaded forecast snow map? Once you put a number out there, you can’t take it back.  People always remember the high end of your forecast range too.  If I show 6”-12” inland, 3”-6” for the shore, someone in Bridgeport will write me an e-mail and ask me what happened to the foot of snow I was forecasting.  

Usually I don’t release snowfall maps until I feel confident that it will not need further tweaking (normally within 24-26 hours).  But today, 48 hours from the storm I feel compelled to tell you my thoughts.  Other meteorologists are putting out forecast maps earlier and earlier.  And I don’t want my viewers to feel like I’m “holding out” on them. I am competitive and I want my viewers to feel satisfied after watching my forecast (without switching over to another station at 11). 

That being said, I’m not as confident as I want to be with my snowfall forecast.  But I want to give you my latest thoughts so you’re up to date on the latest trends, even if the forecast might change down the line.

The massive winter storm that has been all over the news in the middle of the Nation is NOT the storm heading our way.  But as this storm dies it will lend its energy to a new storm developing off the mid-Atlantic coast.  This will be more of a typical Connecticut winter storm with mainly snow in the hills, a mix in the interior and mainly rain for the shoreline.  FINALLY!  All the storms so far this winter have had a bullseye in the southern half of the state.

Snow/sleet will break out in the afternoon on Saturday (light to start) before changing over to rain late afternoon into Saturday night. Rain will then change back to a wintry mix after midnight into early Sunday morning from northwest to southeast.  The heaviest snow is over by afternoon. But lingering snow showers are possible all the way through Sunday night. Accumulations will hinge on when the changeover to snow occurs and how heavy the snow is into Sunday morning. For those forecast details, we need to know the storm’s track and the strength of the storm’s low.

But right now it doesn’t look like the storm will be strong enough to overcome a mild easterly and southeasterly flow during the storm. This means a period of rain or sleet will significantly cut down on snow accumulations.  While this is still likely to be a messy storm, it will not be a big snow producer for us.  For areas north of the mass pike, jackpot!  All snow is expected to fall in these areas and it’s just a matter of figuring out precip amounts.

I think the shoreline will get the lowest accumulations.  The northwest and the northeast hills will get the most snow.  This might even be an elevation event (valleys get less than hill tops). Right now I’m thinking 3”-6” for the northern half of the state with locally higher amounts in the hills and a coating to 3” for the southern half of the state.  How confident is this forecast? Eh.  The rain/snow line is so critical and a small change in the storm will greatly alter accumulations. But this seems like a good place to start and it just so happens to be in line with the European forecast model.  Coincidence? No way.  The Euro has been so consistent.  I’m riding it until I get burned.

Preliminary Snowfall Forecast...subject to change!

ECMWF snowfall based on 10:1 snow to liquid ratio

ECMWF total precip. Notice western CT has lower precip
amounts than eastern CT. That's why NE CT could get the most!
Surface temps stay above freezing the entire event according to the Euro

1 comment:

fred said...

I love your forecasts but people need to realize that we are in a pattern that is very hard to pin down exactly. Just a few miles difference in the storm track can make a huge amount of difference in accumulation totals. I appreciate you fretting over providing us the best possible forecast that you can given all the variables that you have to work with.