While there is still time to view this storm system we wanted to get the word out about it since this system has the potential to create some travel issues along with the potential for power outage issues. Those power outage issues essentially apply to south of the PA /MD border where colder air is going to be trapped in against the mountains at the surface often referred to as Cold Air Damming or CAD..
A brief look at the guidance and we see the GFS is among the warmest of the guidance ..
What we see is high pressure in prime position located to the north of Maine..However..that high pressure begins to slide to the east as the storm system tracks up towards the Lakes region. A secondary does pop along the coast ..however..it never becomes stronger then the primary. As long as the primary tracking to the lakes region remains the strongest it causes a strong push of WAA or Warm Air Advection precipitation across the region. With cold air at the surface thanks to the high pressure ..precipitation starts as snow...with the mid and upper levels warming which we will show on the ECM from 12 Z ...this snow turns over to Ice and eventually to rain. The amount of ICE will depend on how fast or slow the cold air erodes at the surface..
Looking at the ECM we are just going to look at 96 hrs as this is when the heaviest precipitation is across the region.
And the temperatures ..
The ECM is arguably colder then the GFS. Tracking is quite similar with a primary towards the lakes and a secondary developing. However ..the ECM is a higher resolution so it is able to pick up on the CAD sig better then the GFS. So on the ECM you have snow that changes over to ice and a longer duration ice event. Matter of fact in locations such as ABE for example the ECM is all frozen.
The GGEM is similar with what it shows to the ECM..
So the GGEM shows a forecasting nightmare with snow and ice across the region with the only real change over to rain coming in at the end and along the coast.
We find it best in these type of scenarios to take a blend of the models and factor in climo into the forecast as well along with experience. When you have a track to the west with a stronger primary ...and a secondary that develops...two things usually happen.. One the precipitation arrives earlier then forecasted and secondly.. the WAA is stronger then modeled.
So we have come up with the following scenario map based on the guidance over the last couple days..
The red shading is where we expect primarily ice to occur and this is where we think that region could end up with power outage problems.
The next area which is pink is the winter mix zone where we expect precipitation to start off as snow /sleet and change to ZR or freezing rain but as warmer air filters in aloft we think that a changeover to plain rain will occur. Noteable exceptions to this could be NE PA and NW NJ and as far south as the Lehigh Valley where colder air could stay trapped at the surface and keep the precipitation as ice longer. To the north of that we have lighter snows ..mainly because the heaviest precipitation on the ECM looks to go east northeast from the frame that we have posted. And finally along the coast...plain rain..It could start as snow or sleet but a quicker transition to rain should occur there as temperatures are warmer.
We will update this should it become necessary!
We want to start this off by showing the EPO. There is no question that the EPO is negative and there is no question that the negative EPO has been responsible for the colder outlook on November. However..we have to duly note that while the EPO is negative ... this first week of December starting with today is going to average above normal temperature wise.. You can see this clearly when you look at Day 3 high temperatures along the east coast...
Temperatures into the 50s to near 60. As of writing this it is presently 52 degrees @ ABE ..53 @ PHL and 54 in Baltimore MD and 53 in NYC.. The point is that we still have the negative EPO in place but we are also going to be experiencing warmer then normal air temperatures. So this tells us that the negative EPO alone is not driving the weather pattern.
Another certainty we know is that we are not getting any help from the Madden Julian Oscillation or the MJO..
We are presently in the COD or the Circle of death where the MJO has little influence on the weather pattern.
What we do see is that the ECM wants to bring the MJO out into phase 3.. and then phase 4
Which would indicate a turn to milder weather in the longer range (talking around mid month)
Phase 3 and phase 4 are warmer phases of the MJO. However..we also know that these forecasts are not all that reliable either ..despite the ECM being one of the better performing weather models. However..it is what the model shows.
Even the GFS shows the MJO wanting to come out in phase 3. So if this were to occur we would be looking at milder weather across the conus.
Still yet another certainty we know is that the PNA (refer to image at top) or image below is quite negative and forecasted to remain quite negative.
A negative PNA indicates two things.
1. A negative PNA indicates a Southeast Ridge. And now we see the answer for the milder weather the next few days despite the - EPO.
This is shown quite well on the 12 Z ECM
2 . A negative PNA indicates troughing into the central and western part of the country. Again you can see this quite well on the ECM @ day 5.
You see in that image the Southeast ridge in place and you also see the troughing into the central and western US.
AS LONG AS THE PNA IS NEGATIVE =A SOUTHEAST RIDGE over all will be involved.
So essentially what you have is a battle between the - EPO which puts ridging into the Aleutian region and the - PNA which indicates a Southeast ridge.
So what does that leave us with on the east coast? It really depends on what teleconnection is more dominant at the time (since right now there is no help from the MJO) ... It means we are going to have times of warmer weather and times of colder weather ..which is the reason why we have gone with essentially average temperatures for the winter. It sets up a gradient pattern and depending on what side of the gradient pattern one ends up on will determine what precipitation type one deals with in sensible weather.
Also with a Southeast ridge in place you run the risk of storm systems cutting to our west... we see this on the 12 Z ECM...
Just looking at that image we see the troughing into the central US and the west with the ridging into the east which is forcing the storm system to track to our west which leads to a event starting as frozen in PA but turning over to rain as the mid levels and upper levels warm up.
What about the AO and NAO?
We know these images change with every 00z model run but presently the AO is positive ..however forecasted to go to neutral and then members become divided with some taking it negative and some taking it positive..
NAO is shown positive right now but expected to turn negative (east based) .. we really want a west based negative NAO to deliver the snow that so many people are looking for.
So basically we can expect times of warmer weather (-PNA southeast ridge) and times of colder weather (when the negative EPO allows the colder air to bleed into the east ..aka behind cold fronts etc) .. Storms potentially tracking to the west of the region (aka Southeast ridge) starting as frozen but changing to rain ... with temperatures ending up average when all is said and done.
Now..there is a question mark to mid month because the ECM ensembles are showing the - EPO breaking down and switching to positive (and you can kind of see this happening in the first image where it is rising from its extremely negative state) and a GOA low taking shape. If this happens then with the - PNA being so negative it would flood the country with mild air from the PAC..
So essentially it is a wait and see to see how things shape up come mid month.
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