EPAWA Winter 2014-2015 Outlook
Outlook effective dates: December 1st, 2014 through March 31st, 2015We are at that time of year which everyone either embraces or dreads, and that is by far our most popular season, and historically our time to shine at EPAWA. And that of course is winter. What makes this year a little more anticipated than the last, is the fact that we are coming off of one of the snowiest winters for some in recorded history. Can we do it again? Well… a lot have certainly speculated that another busy winter is coming up. It will be difficult to replicate what we achieved last year, with some areas receiving over 200% of what “normal” snowfall is expected to be. This outlook will outline our thoughts on how this year will play out. The science behind the prediction, the breakdown of what we expect in monthly departures from normal with regard to temperatures and precipitation, and of course, our snowfall outlook for this year by location.
The Winter Forecast Factors:
State of the ENSO:From the above, we expect this Winter to have an ONI index in the months of December through February to reflect a weak El Nino (+0.5 to +0.6) with a margin of error of +/- 0.3. There are some indications of warming in ENSO regions 3.4 and 4 would suggest a weak El Nino is currently developing. This, combined with some other factors will determine where we are headed this winter. The ENSO is a big driver, but many other factors play into the outlook. However statistically, using Philadelphia as a point of reference, the snowiest winters (average) occurred in weak El Nino years more so than every other climate state, as highlighted in red in the chart below, courtesy of the National Weather Service, Mount Holly, NJ:
Climatological Factors:We matched up analogs to the following factors, each of which are present or projected for this Winter in the table below. Matched most closely to a cold Pacific Decadal Oscillation (PDO), a warm Atlantic Multidecadal Oscillation (AMO), a warm Eastern Pacific Oscillation (EPO), and above average October Northern Hemispheric snow cover (as was the case this year).
|Winter Year||Warm PDO||Easterly QBO||El Nino||Above Average Snow cover October Northern Hemisphere|
|1976-1977||YES||YES (~ -15)||YES (Weak)||YES|
|1986-1987||YES||YES (~ -10)||YES (Moderate)||NO|
|2002-2003||YES||West to East||YES (Moderate)||YES|
|2009-2010||YES||YES (~ -16)||YES (Moderate)||YES|
The North Atlantic Oscillation (NAO)The illustration below is a simple graphic that shows what a positive and negative NAO can do for our area: Positive Phase: Negative Phase:
Pacific Decadal Oscillation (PDO)Both of the oscillations above play a very important role with cold air availability and track for our winter storms. To get an idea of what these oscillations will do for the winter we look at several factors with the most important one being the sea surface temperature anomalies. When we have warm water surrounding Greenland it is an indication of higher heights being favored which coincides with a negative NAO. The PDO is much more simple and we can clearly see that we will once again be in a warm phase….a text book warm phase. This shows the warm water piling up along the Western US coast extending up into Alaska and leads to a ridge over these locations.
Canadian Snow cover
October:Having flashbacks to last November! Early last November the snow cover rapidly filled in over Canada and helped to erase the warmth forecast by the models very quickly. This year we had a nice build up that coincide with a category 5 Super Typhoon to erase the warm forecast for November. Snow cover is a very important variable for the winter forecast and is centered around the Arctic Oscillation. When the Arctic Oscillation is in a negative phase we get more heat transported to the poles with areas of ridging forcing the Polar Vortex to split or move out from the Arctic Circle. This results in the wavy pattern with many troughs and ridges – making things fun with cold air displacement and winter storms. During the positive phase the opposite happens with the Polar Vortex keeping the cold air bottled up in the higher latitudes. Snow cover plays a huge role in determining the arctic oscillation: The record snow build up over Siberia is forcing the jet stream to head south and weaken the Polar Vortex. This increases poleward heat flux for the stratosphere to warm keeping the lower heights favored into the middle latitudes. This leads us into the Quasi-Biennial Oscillation which has a say in this also…
The Quasi-biennial Oscillation (QBO)The QBO is a jet stream in the tropical stratosphere that oscillates between westerly and easterly. In the negative phase the winds are easterly and usually stronger and last longer. This helps enhance something called the Brewer-Dobson Circulation. The negative values in the Fall are generally a good indication for rapid snow cover build up through the stratosphere warming from the enhancement to this circulation. If the circulation is too strong it could have an adverse effect and keep the cold bottled up in the higher latitudes and favor a strong Pacific Jet. Looking at observations we can clearly see this time the negative QBO allowed rapid snow cover build up. We may have to deal with one more period of perhaps an acceleration of the Pacific Jet but it does not appear to greatly impact the winter outlook. The current QBO value is -23.86, as indicated in the chart below, but here are indications that when we see the data come in for next month, it will rise to about -17 or so: This chart below taken from wxrisk.com shows that a rising QBO also favors the Negative NAO as talked about before this:
Late November into DecemberWe are expecting the pattern to relax as we head towards Thanksgiving and enter December. We can see why on the observations right now. We have had a noticeable decrease in convection over the Maritime Continent and we are increasing convection over the equatorial Atlantic. This is forecast by the models fairly well and show this to be the start of our next MJO system that will march eastward. As it marches eastward we will enter some warmer phases towards the end of the month into December. This warming does not look to last too long or do any damage to the ongoing forecast but we are expecting a warmer start to December with a colder end. This should balance out the month to near normal conditions. November 7th IR imagery: Euro Forecast Phase 4 composite for December: The tropical forcing shifting eastward will also send westerly winds across the western ENSO regions. This should provide for another rapid drop in the Southern Oscillation Index values and send an amplification to the pattern. This amplification should help us get winter started around the middle of December. It is during this timeframe that +AAM anomalies will be moving poleward favoring a temporarily breakdown of the western US ridge and an enhanced pacific jet. Worries may exist that this could further decrease SST anomalies around the Gulf of Alaska, taking away a favorable set up for ridging along the Western US coast. We don’t think this will occur unless this is a much more prolonged pattern then expected. But with the stratosphere extending its warming over to Alaska, we see one more push of the Pacific jet. The image below shows this warming extending eastward into Alaska, which would make sense for the ridging to ease as we head towards the end of the month.
Winter highlights by month:
Active storm track and precipitation departures:The active track promotes increase in coastal storms and more intense storms than last year as a whole. A battle zone may set up many times this Winter where split somewhere within our coverage area will be a tough rain/snow/ice line to deal with, making our job calling individual storms that much more difficult. Warm Atlantic Sea Surface Temperatures (SST) promote the notion of bigger coastal systems. But that could also lead to storms closer to the coast and more rain/mixing there, which would favor heavier snows falling inland. As winter progresses and the SSTs cool in the Atlantic, coastal tracks should be shifting more east. Conversely, far NW areas are less likely to get into as big snow events, as the strongest lifting will be favored over the coastal plain. This will lead to a significant sinking motion in central PA and strong lifting in eastern PA and New Jersey. Naturally, mesoscale dynamics will change with each storm, and will be handled with much more accuracy in the short range. The balance between the two favor a snowy winter in the interior, but not too far interior. The outlooks by month and as a whole are more weighted by location based on statistical probability.
Breakdown of temperature and precipitation departures from normal by month:December we are going with near normal precipitation and near normal temperatures, and then above normal precipitation for most from January through March. January is the coldest month, followed by February as a close second. Late January into February looks like it will once again yield the highest chance for much above normal precipitation and snowfall. March keeps the wetter theme, but most of the analogs we used call for a turn on a dime to warmer than average temperatures in March. When that occurs will have to be monitored in the shorter term, and snow can still occur in the month of March, especially earlier in the month.
December 2014 Temperatures:
December 2014 Precipitation (liquid equivalent, not snow):
January 2015 Temperatures:
January 2015 Precipitation (liquid equivalent, not snow):
February 2015 Temperatures:
February 2015 Precipitation (liquid equivalent, not snow):
March 2015 Temperatures:
March 2015 Precipitation (liquid equivalent, not snow):
The EPAWA Winter Snowfall Map:The Winter starts off on the average side, but picks up in a hurry in Jan/Feb. We are expecting 50-75% of these totals to occur January 15th to February 15th this year:
SELECT CITIES AND TOWNS EXPECTED SNOWFALL
|City/location||Average annual snowfall at this location||EPAWA expected snowfall for 2014-2015 winter at this location|
|State College, PA||46″”||46″|
|Mount Pocono, PA||48″||67″|
|Mount Holly, NJ||20″||30″|
|Atlantic City, NJ||17″||24″|
|Cape May, NJ||16″||22″|
Every year we include wildcards that could increase or decrease amounts listed above, and this year our wildcards favor the above map totals to be ABOVE rather than below. The wildcards are listed below:1) Bigger Snow Events: Warm Atlantic waters combined with the likelihood of the negative NAO dominating this winter it opens the door for some big time winter storm events. It would only take one to really skew the numbers but if we have two like in 09-10 this could really exceed snowfall expectations. Another ingredient will be the southern stream that will be enhanced by our weak El Nino favoring the Miller A type storms coming up from the Gulf. 2) December goes cold and snowy: If this relaxation in the pattern does not occur we may have an increasing likelihood of more snow than anticipated for December with colder temperatures. Seeing how the pattern reverted back to a +PNA look in November makes one think that any attempt to change this may be short-lived given the warm classic PDO look to the SST anomalies. 3) March and November: March on all of our analogs suggest a very warm March. This makes sense given the cold air we expect to come in January and February, but we are not really expecting much contribution from March in the snowfall department so any snow in March or November will be “bonus”. The cold in November will make for a few interesting storm systems but too early to know if they will convert into accumulating snowfall. 4) Pacific Jet/QBO: If the QBO does not rise and continues to fall it may increase the threat for a more zonal look to the jet stream. But this is number 4 for a reason since I really doubt it occurs. We will see this type of pattern late November into December (as of right now) but I have my doubts that it hangs around long as the supporting cast of observations and teleconnections don’t support it. (5) Teleconnections. The most ideal situation for cold and snow as illustrated below is for a -EPO, +PNA, -AO, -NAO combination. If this is the dominant pattern for this upcoming Winter, even snow lovers may be waiving the white flag begging for mercy. (6) Solar activity. Lower solar activity like last year is occurring again this year, which promotes a warmer stratosphere, and a warmer stratosphere usually indicates a colder eastern half of the United States. Higher solar activity results in more UV rays escaping through the ozone layer and cooling the stratosphere. This allows us to warm in the troposphere (our weather).
What can go wrong?Blocking potential is not realized, and the Pacific jet dominates with a general west to east flow (zonal) across the country. Although unlikely, we have to allow for that possibility. Also a possibility is the QBO does not rise from its current state, and stays strongly negative. This would also promote milder conditions with less snow. The Arctic Oscillation (AO) according to the SAI index is expected to be predominantly negative, but this is not always a guarantee for cold to come to the Eastern US. Although we listed several variables here, the wildcards favor our projected snowfall estimates to be too low rather than too high. It only takes one or two mega storms to skew the totals…
“Plain English” summary
- In comparison to average, we expect most areas to feature well above normal snowfall, favoring central and SE areas
- Temperatures will not be as cold as they were last year, and will not be as prolonged in duration when cold snaps or warm-ups occur
- Less frequency of storms overall compared to last year, but when they do hit, they hold greater potential to produce larger amounts at one time
- Odds favor not nearly as much snow as last year, but certainly well above normal
- Snow depth will not be as great as last year with up and down fluctuations of cold and mild periods
- Greater potential for blocking unlike last year, making coastal storms slower, bigger, and more robust