SNOW THREAT FOR THE EAST COAST INCREASING???

full update at 230 pm AFTER the 12z GFS ENSEMBLES and op Euro comes out

In case you did not get a chance to see the column or blog entry from yesterday afternoon (which was actually posted early yesterday evening) please take a look and read it carefully. That's a very important concept which needs to talk about with regard to how forecasts are supposed to change according to the data.

Let's talk about the potential for significant winter weather over East Coast after January 15. There certainly is a lot to talk about and the excitement in the various weather forums has definitely shifted from ..."well it's never going to get cold"... to "wow look at that snow threat/ pattern...".

So let's lay some ground rules here. First lets talk about the potential with this front stalling over the Tennessee Valley or the Southeastern states and some sort of wave development on the front for several days now.

The operational Run of the GFS 0z is keying on a particularly strong shortwave which at Day 7 is located over the southwest US. Because we still have a flat Ridge over the Gulf Coast and Florida... the interaction of this shortwave as its tracks east causes LOW pressure wave development and overriding precipitation across the upper Delta the Tennessee Valley into the Midlantic states and lower New England Jan 17-18. That's the general idea.
day 7
http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/fp0_168.shtml
day 9
http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/fp0_216.shtml
DAY 10
http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/fp0_240.shtml

Now that being said let's talk about a couple different aspects of this. It's been said that one of the worst things is to be in the bull's-eye for the heavy precipitation or heavy snow or hurricane track at 168 hours or 120 hrs etc . While that is generally true it's missing a more important point.

As you can see from this link only 3 of the 11 GFS ensembles at 0z 216 hrs support of any sort of wave development over the Southeast in a manner which is similar to the operational 0z GFS. That should tell you that the operational GFS scenario is not likely to occur based upon the data we see at this time.
http://www.meteo.psu.edu/~gadomski/ENSPRS_0z/f216.html

However this is where the use of ensembles becomes very important and a lot of weather hobbyists weenies and even some forecasters miss this concept. If you are using the GFS ensembles from 0z Tues and say its NOT a serious threat b/c 8 of the 11 show no wave development over the Southeast... you would be correct in a technical sense. However to make such a definitive or declarative statement at 192 hrs is to be using the ensembles as a deterministic forecasting tool which is not what they are designed for.

The fundamental concept with ensembles forecasting is to use them in stochastic manner NOT a deterministic one. We have to follow the TREND in the GFS ensembles with each new 0z and 12z run .... because on the next GFS ensembles and instead of saying only 3 members showing the wave development... we might see 4 and then on the next run it might see 5 .... and on the next 6 members ... well you get the idea.

Remember the KEY feature here is the s/w in the southwest US. If there is NO s/w hanging back over the southweest states AFTER the front moves through the East coast there will be NO big winter event for the eastern US.

The second point with regard to this potential event... does the POSSIBLE winter scenario match the seasonal trend?

Well since we have been so mild for so long it's hard to say but one thing we do know is that in a weak El Niño you have a lot of activity in the Subtropical Jet ( STJ) as strong shortwave tack underneath the Western Canada Ridge and these S/W move into the southwest US. So in the overall idea or sense seeing a strong shortwave hanging back over the southwest US.... that at some point gets kicked eastward because more energy is coming in from easstern Pacific.... Well that does make sense and does match the pattern !!!

Now remember folks we are dealing with a subtropical short wave as the KEY feature we have to focus on. That means over the next several days --even on the GFS ensembles -- the GFS is going to suppress the feature because the GFS does not know how to handle strong shortwaves in the STJ. This is what we talk about when we referred to the cold bias. It over does the northern branch so strongly that the system ends up getting crushed on the Model.

Classic case of this? Take a look at the 6z OP GFS ... as you can see clearly the KEY shortwave is still in place at Day 6-7 but as you go past that point
http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/06/fp0_168.shtml


-- and remember AFTER 180 HOURS is when the model resolution truncates into the much coarser model grid-- so the cold air bias of the op- GFS really kicks in and we see the southwest s/w getting crushed as it moves East. Even so we can still see some sort wave a flat one of Low pressure moving off the North Carolina coast at 228 hrs.
http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/06/fp0_228.shtml

Therefore it is going to be a BIG mistake to simply to look at the operational GFS runs over the next several days... see the s/w or surface Low being crushed by the over done Polar Jet... and say "it's not going to happen...'

Just like last week when the operational GFS overdid to amounts of arctic air coming down the West Coast and over develop a PIG SE US Ridge that was supposed to develop over the Southeast and delay the arctic front arriving in the east Until Jan 17-18...

It's the same sort of GFS model problem. Only now we arew moving back to the more typical cold bias we are all use to with the OP GFS ... with southeast US Low pressure getting crushed or supressed .

CMC ensembles DO offer some support as well
CM01 CM02 CMO3 CM08 CM12 CM16
http://www.meteo.psu.edu/~gadomski/ENSCMCPRS_0z/f216.gif