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I’ve gotten a lot of questions about ensemble models, so I thought this would be a good time to address.

There are the so-called deterministic versions of the models, which you can think of as the headliner versions.  These are supposedly the best resolution with the best physic schemes.  In addition to the deterministic models, each major global model runs ensemble versions.  There is a control run, which is a slightly dumbed down version of it’s big brother deterministic.  Then the rest of the ensemble suite is comprise of model runs, which are initial perturbed slightly.  The idea is to slightly alter the initial conditions in a manner that represents the inherent uncertainties with weather observation and subsequent reanalysis.  So, in the case of the ECMWF (Euro) there are 50 runs in addition to the control run.  The motivation here is to eliminate the iterative effects of chaos with extended timesteps in the modeling.  What you typically see, long to mid range out, is a wide ensemble spread.  As you approach the event, in the short term, your hope is that the ensemble members begin to look like each other.

Let’s look at an example with the current setup.  These are side by side comparisons from the ECMWF ensembles from 5 cycles ago and on the right from the current 12Z runs.

First off, I should say…don’t take these totals literally.  The Euro treats any frozen precip as “snow”.  So for our upcoming event, we are expecting snow and sleet and freezing rain.  The quick and dirty rule of thumb of 10:1 snow to liquid ratios will not be valid.  These charts show 10:1, chop them at least in half.  Again, what’s more important for this discussion is how the ensemble members have come into agreement.  (in the charts time moves from left to right, and each bar at the top is the snow accum for each of the 51 ensemble members…the graph at the bottom compares the deterministic snow accum to the ensemble mean.)  Note  how the deterministic was much above the ensemble mean 5 runs ago.  Now they match nicely.  Also, note the uniformity of pink on the right in the current run vs. lots of blue and variance on the left.  More confidence.  All this being said, there are still a lot of big questions.  Answers are coming.  I promise!

The new 0Z models are rolling in.  I’ll put some comments later.

The outline for a major Carolina winter storm is coming into better focus.  Finally, the various models are beginning to converge on a unified solution.  Notice I said beginning to…not that they’re there yet.  At issue are perturbations in the southern branch of the jet, their magnitudes and their timing.

Here’s the basic outline.  The arctic front will slip thru NC early tomorrow AM and the colder air will begin to filter in.  There will probably be a couple of snow showers late.  Then the first wave ripples along the front during the day on Tues.  It now appears that most of the associated precip will fall south of the Triangle during the day…south of a line from Greenville to Charlotte to Fayetteville to New Bern in the form of a rain/snow mix to the south, changing to all snow where it precipitates hard enough and on the north of the band where the colder air is.  If there is a shift north with this band, it would bring accumulating snows in earlier than the overnight Tues/Wed timeframe for the real storm. (latest 18Z NAM is hinting at earlier arrival)

Then Wed morning the major system approaches out of the Gulf and rides north along the coast and just offshore.  It appears to be a set up for a formidable storm with high QPF (precip) and extensive winter weather headaches over a large chunk of real estate.  Lots of snow and ice in the South, riding all the way up the Eastern Seaboard.  I like the looks of the 12Z Euro, but haven’t seen it’s ensembles yet.  The 12Z GFS is too far out to sea and whisks the storm away (altho it is now beginning to correct back west).  The 12Z Canadian is, well, very Canadian, eh.  Very snowy and a thumping from SC to Maine with a 961mb low off Cap Cod overnight Thursday.

As I said at the beginning, there are still many questions unanswered and hopefully these next two model run cycles will unify the message.  So, at this point, what does this mean for sensible weather?  It’s a classic NC winter setup with best chances for snow to the north and west and a transition zone to sleet then freezing rain, then all rain at the coast.  Right now it looks like the Triangle will see snow beginning Wed AM, changing to sleet and then possibly to freezing rain.  And you say, “what about accumulations?”  to which I say “that question really isn’t all that important”.  What is important is that there will be a lot of precip.  And the fact that it will all be something wintry is reason enough to say this will be a high impact event.  That’s the bottom line.  It will be a major event for most of the state and beyond.  We’ll fill in details about p-types and quantities over the next couple days.

So…stock up on the essentials: bread, milk, beer…and we’ll watch this evolve.

It’s been a crazy day.  And while I’ve had time to look at a lot of data.  I haven’t had time to put two or three coherent sentences together until now.  And I’ve literally got to go now, but so may of you are asking for an update.  Here’s the skinny…

We are trying to predict a very complex situation for next week.  I’ll explain later, but here’s what we can say.  It is likely that NC will see winter weather.  There are good chances that the winter weather will be significant.  Timing is questionable, as are the number of systems.  Snow could arrive as early as Tues AM.  And there could be a prolonged duration to the precip.

A more detailed discussion tomorrow.  But if you’d like quicker brief updates, follow me on Twitter @trextrex14.

A very quick post to say that the 12Z runs were very bullish on the a significant winter storm for central NC.  The Euro deterministic was very excited about a whole mixed bag of wintry mess…and a lot of it.  See the accumulated snowfall map (don’t read all of this as snow), which looks like a glacier.

And not only that, there is great ensemble agreement in a wintry scene of some sort at RDU.  Nearly all the ensemble members print accumulating snow or ice.  See here…

Of course this is a loooong time off.  At this point, we wait, watch and think about contingencies.

Oh yeah, and there might be a flurry tonight.  Enjoy.

It’s a crazy pattern that we’re in now with an active southern branch of the jet stream.  There are lots of little disturbances blipping by every couple days, and the fun is trying to figure out which one(s) will blow up into a decent storm.  And then figuring out even if it does mature into a surface cyclone whether or not it continues out to sea.

The best guess on the weekend is that a weak system sends a couple of flurries/sprinkles our way tonight.  Then the following system holds a better chance for some development on Saturday with a cold yuck rain…best chances from the Triangle and east.

But the more interesting event is mid next week.  Over the past couple of cycles, there’s been growing model and ensemble support for a significant winter weather event.  We’re 5-6 days out, so at this point it’s just something to watch as it comes better into focus.

Maps, charts and nerdy stuff to come…

I hope everyone enjoyed the snow, even if we were robbed of a couple inches.  And what better thing to do than look ahead to the next event…

Things begin to warm up such that a beautiful weekend is in store with temps in the mid to upper 50s Sat and well into the 60s on Sun before rain comes in late.  Then there’s a cool down, and second storm, mid week that we’re again on the warm side.  But maybe the third storm is the charm for next weekend with something brewing out of the gulf for another NC snow special.

Obviously, a long way off and just something to keep an eye on for now.

Things still look on target for snow to begin in the Triangle between 3:30 and 4:30pm.

Let’s not get ahead of ourselves before the first flake falls, but some of the short-term/high-res ‘nowcast’ modeling is indicating the potential for higher totals in the the Triangle, especially Wake county.  Let’s not up our totals just yet, but just say that the 12Z Euro came in a little wetter, and now the RAP is more bullish.  So while the large totals are still a longer shot, there are decent chances on the table for a major event this far west.

Just for fun, and because I know you children like pictures…here is the RAP snowfall accumulation map that assumes a ratio of 10:1.  Remember that I’m expecting ratios at 12 to 15:1.  So, if this solution verified you can bump the Raleigh value of 7-8″ up to 9″-12″.  (for reference the first purple contour is 7″, the dark blue is 3″…again, assuming a 10:1 ratio)

Again, I show you this kind of stuff just for fun.  This is not a forecast.  Just an example of one of many products that the meteorologist has to ingest before coming up with a forecast.

Happy Snow Day!

In a continuation of the theme from last night, it would seem that southern sections (like FAY) will be influenced by a mid-level intrusion of warmer air such that a period of sleet and possibly some freezing rain will reduce their snow totals.  At the same time, this influx means more overall moisture transport farther westward.  So, what looked like a sharp back edge to the snow totals a couple of days ago, now looks more broad.  There still looks to be a relatively sharp axis where generally light snows will ramp up to a band of heavy snow in northeastern NC.  There is some concern about just how dry this arctic air mass is and the loss of some snow totals that will go to saturating it.  Fair enough. Just another of the questions that linger as we get warmed up in the pregame.

Here’s my new outline…5-7″ in central Raleigh, tapering to 3-4″ in Durham, back to 1-3″ in INT/GSO.  Then going east, we ramp up from Rocky Mount to the Tidewater where it’s 8″ up to 12″+.  For the second storm in a row, the big winners are in northeast NE.

You see the theme here…farther east more snow…farther south more sleet.  We’ll see how this all plays out this afternoon after kick-off.

Not much in the way of changes in the thinking for the event.  A couple of points, tho…maybe, in general, more moisture farther west, which also implies a little more mid-level warm air in the south and at the coast.  This means maybe bumping snow totals from the Triangle and west, and looking for more ice at the coast and having to worry about a sleet zone in the south.

For now, i’m happy to leave my outlined totals where they are until I see what the Euro looks like when it comes in after 1am.  In a previous life I probably would be making changes now, but there’s still plenty of time to reevaluate to the upside on snow near the Triangle and adding ice south.

Last post was before the Euro came in.  Thought you’d like to see the comparison of snow totals from last night’s run (top) to this AM’s (bottom).  Note that this uses a standard 10:1 ratio for snow conversion, so you might expect a little higher totals on the western edge if the the QPF verified.