It's Been One of Those Winters

So I've held off posting until now in the hopes that, given time, the picture of what is to happen with this mid-week storm would clarify.  I hoped that the models would arrive at a consensus and that the biggest problem with the forecast: the absence of a strong cold air source during the start to middle of the storm -  would also present itself more clearly and allow me to make solid conclusions.

But - nothing like that has happened.  Nothing.

To have a good "Miller A" Nor'easter - the type of snowstorm that is likely to dump 6+ inches of snow (or more) on our area, you need a few ingredients:

- A persistent cold air source.  This is provided by a strong high pressure over New England of the sort we've had for the past few days.  The problem?  It's moving east.  The further east it moves the more the clockwise flow around the high stops funneling cold air from the north and begins to funnel warmer air from the east and southeast.  So this is something of a question right now.

- A surface low pressure system near or over the Gulf Coast.  This we have - no problem here.

- An upper-level low moving out of the Plains states and phasing with the surface low somewhere in the southeastern United States.  Most all of the models agree this will happen although there are variations with respect to timing.  All of the important models have the phase happening well before the storm enters our region.  This is a good thing.  You'll remember that the majority of the problems we've had with storms and forecasts this year have been due to the difficulty in predicting the exact time and location of the phase and that the phasing has happened almost overhead which has caused dryslot problems.  Not this time.

- Favorable storm track.  Too close to the area and you have mixing problems: sleet or even rain mixes in with the snow and keeps accumulations down.  Too far off the coast and you get missed by the juiciest part of the storm and that keeps snow totals down.  The models are still in disagreement about the final track except that all the important ones agree that the track will be somewhere east of here.  In this case especially - with the cold air and temperatures being borderline at best - a small change in storm track will probably mean a huge change in what we experience. 

So - where are we with this thing?

To begin with, the storm forecast positions are slowing down.  What was once a Tuesday night into Wednesday storm now looks like a Wednesday morning - Thursday morning storm.  This has been a trend for the last couple of days so I wouldn't be surprised to see things slow down even further as we get closer to the event.

Now lets look at some of the big players in the model world and where they stack up.

We'll begin with the forecast frame for 30 hours from now (36 hours from 12Z - the run time of the model) or about 7 PM EST Tuesday night.  Here are both the NAM and the GFS for comparison at the same time frame.  The NAM will be on the top, the GFS on the bottom.   Remember that you can click on any image in this blog to see the full-size version.

I want you to keep an eye on two things.  First, notice that the position of the low (NAM just south of the Florida Panhandle, GFS right over it) is pretty similar in each model as is the precipitation field.  Heavy rains are breaking out from Georgia into South Carolina and severe thunderstorms are on their way into Florida.  Clearly there is a lot of energy and moisture available to this storm. 

But notice, also, that blue line that runs from the Missouri/Arkansas area northeastward towards the mid Atlantic.  That's the 0-degree isotherm and it can be used to approximate where the precipitation would remain snow (north and west of that line) and where it might change to rain (south and east of that line).

Notice that the NAM is quite warm - forecasting our entire area to be above freezing even at this frame while the GFS is slightly colder and actually paints the 0-degree line right over our area. 

Onward to 42 hours - around 1 AM Wednesday morning.

The storm is strengthening and is moving to the northeast.  Heavy precipitation is falling in Florida and along the Georgia and Carolina coasts.  We still have no precipitation in either model though it's beginning to break out just to our south in the GFS version of events.

Again the low position is pretty similar in both models but the precipitation field is more inland on the NAM bringing heavier rains to interior Georgia and South Carolina.  For us, note the 0-degree line.  The NAM now has it right over us.  The GFS continues with a colder forecast showing the line now sagging south of us putting us in a regime where snow could be expected once the precipitation starts falling.

6 more hours and its 7 AM Wednesday morning:

In the NAM, precipitation is still not quite over us but in the GFS it has begun to do something in our area.  Again the 0-degree line is right overhead in both models making the precipitation type a very difficult call.  The storm positions are a little different now with the NAM showing the center around the GA-SC border on the coast and the GFS slightly faster: along the NC-SC border at the coast.

6 more hours and it's 1 PM Wednesday afternoon.

We have ourselves a storm.  Both models agree on that and both have a fairly powerful one located somewhere near the VA-NC coast.  However, the other thing both models have for us is precipitation type issues.  The NAM is really warm with the 0-degree line now bisecting Pennsylvania while the GFS has it closer to us but still to the north.  This would argue for at least a mix of snow and rain if not all rain.

To 7 PM Wednesday evening we go:

The storm is quite intense now and just to our east.  It's powerful enough that it's drawing its own cold air down with the counterclockwise flow around the low and the 0-degree line creeps southward again.  Significantly more precipitation exists over us in the GFS forecast when compared to the NAM at the point where this takes place.

6 hours later - 1 AM Thursday morning:

Here we see a bomb of a storm with the GFS center position slightly to the west of the NAM position.  However, the NAM keeps the precipitation going for us with as the upper level energy rotates through.  The 0-degree isotherm hovers right overhead on the NAM however.  The GFS is colder - the 0-degree line is to our south - but much drier with just a little leftover precipitation in our area.


So, as you can see, even with a reasonable consensus on track and strength, two of the biggest models are still quite different in terms of thermal and precipitation profiles.  Following either of these models strictly is probably a bad idea but the general idea in either case is for a day when snow may begin in the morning on Wednesday and change to or mix with various other types of precipitation including sleet and rain.  Towards the evening, precipitation would change back to snow and if that upper energy swings through like the NAM suggests, we could see snow falling through most of Wednesday night into early Thursday morning.

Thermal profiles are tough for models to forecast and a lot is probably going to depend on your locale.  Further north and west is the better chance for this to remain all snow however it's also a place where less precipitation may fall. 

Accumulations are difficult to predict as some of the potential could be lost to liquid precipitation instead of accumulating snow.

I should note that other models have a similar story but a couple of them hold onto the snow longer.  One thing is certain:  any snow that does fall will be wet and heavy.

This has been a difficult year for forecasting.  In every case it seems that one element has been missing: phasing, precipitation field or, in this case, source of cold air.  That makes for difficult forecasts and this is one of the toughest.

The afternoon models are running (as is the 12Z Euro).  If they contain any surprises, I'll post them.