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    Two-Part Series Winter Forecast-Busier Winter Season Compared to Last Year

    By Paul Pastelok, AccuWeather senior meteorologist
    September 27, 2012, 4:27:06 AM EDT

    Winter 2012/13 Temperatures

    350x233_09231353_wintertemps922


    Winter 2012/13 Precipitation

    350x233_09221545_winter1213precip922


    Key Points:

    1. Higher snow potential for the East this year, compared to last year with the lower and central Appalachians above normal. 2. Southeast is wetter this winter, compared to last winter, and cool. 3. More active southern jet and warmer-than-normal water temperatures this year can contribute to bigger storms along the East coast. 4. Mixed signals for the Plains and Midwest but leaning toward near- to above-normal temperatures and below-normal snowfall. 5. Northwest and northern Rockies will be drier than normal. 6. Temperatures will run above normal in the northern Rockies, while the southern Rockies may end up below. 7. A more active southern jet could help feed moisture into the Gulf Coast region and Southeast.

    The 2012-2013 Winter Season is expected to be a busier year, especially in the East where most locations had very little snowfall last year in the heart of the season. Last year was also a mild winter for many areas extending from the northern Plains to the Northeast. A higher use of snow equipment and higher fuel bills compared to last year is expected for the East.

    For the East:

    Snowfall should average above normal for the lower and central Appalachians extending toward the Hudson Valley and southern New England. We believe that snowfall will average closer to normal for the interior Northeast through northern New England. Here is our snowfall map for 2012-13 season which includes the fall and spring season. A good portion of the snowfall in the Rockies will occur in the fall and more spread out during the winter season.

    Snowfall Map

    350x233_09231354_wintersnowfall922


    The Southeast can be wetter than normal with cooler-than-normal temperatures. A split jet stream pattern is quite common in any El Nino phase. The storm track may be dominated by the southern branch the first part of the season, until the polar jet increases and strengthens with greater opportunities for phasing mid- to late season. Therefore, we expect more widespread rain than snow events early in the season or storms tracking off the Southeast coast and out to sea. The second half may feature more storms turning up the East coast. The question still remains how much blocking sets up this year and who most often is impacted by the blocking.

    For the Central U.S.

    For the northern and central Plains, snowfall is expected to be below normal with near- to above-normal temperatures. With a weak El Nino, there is too much variability to make a call farther east into the Midwest and Ohio Valley. 1977-78 was a snowy season in Chicago with snow amounts over 80 inches, but in 2006 snowfall was in the 20s; both were weak El Nino events. Strong El Ninos are drier for the Midwest and Ohio Valley, so we will allow for near normal in this region for now.

    If we see again an absence of deep snowcover in November and December across the Plains and Midwest and a not-so-cold source region, temperatures may again start the season off above normal.

    For the Rockies and West:

    The fall season could be very active across many areas of the West, particularly the interior West into the Four Corners region. Many weak El Nino falls showed above-normal snowfall for this area. Also, the Northwest could have normal to above-normal moisture in November, especially first part, meaning a good snow buildup in the mountains. However, the southern jet will hold, staying well to the south going into winter. The northern jet will generally be absent to the Northwest, located north of Alaska and heading down into central and eastern Canada. The intensity and frequency of events for the Northwest and northern California will lower and so will the amount of rain and snowfall in December and January. Cold will surge early south into the Rockies but expected to shift east during the winter.

    Analogs:

    The ENSO signal is weak and is expected to get weaker during the course of the winter into the spring. Although ENSO may have some impact on the overall winter season, it may not be the main driver. Other signals like the PDO, PNA, NAO, AO, AMO and summer season matches are important. The QBO is also an important signal which defines the upper tropospheric and stratospheric winds. Some say the positive to slightly negative QBO trend was a major force for the warm winter last year along with a spike in solar output that led to a positive NAO for most of the season and less blocking than anticipated. Right now, the QBO is strongly negative which could be the reason for the change to a more negative NAO during the summer and early fall period. This supports the idea for periods when the NAO can go negative and blocking can develop.

    Here is a chart that shows a combination of weak and moderate El Nino winters, their ENSO trends and all other indices mentioned above with exception of the QBO. The three letters for each column in the ENSO section are months (Ex.: DJF is December, January, February). Plus and minus symbols indicate positive and negative indices. Three symbols in a block is the monthly average for December, January and February.

    Analog Chart for 2012/13 Winter season

    590x441_09221555_winter2012analogs


    This season, we expect a weak El Nino, cold (negative) PDO which could weaken somewhat, neutral to slightly positive PNA and a positive AMO. The NAO is a tougher forecast. We will see a mainly negative NAO overall for the fall, possibly easing to start the winter season, then becoming neutral to negative during the middle and late season. Negative NAOs can lead to blocking. There was less-than-normal blocking last year and what blocking that did occurred, most of the cold and nasty winter weather took place on the side of the globe.

    So using the analog chart above, I have created a map below that shows the temperature departures for all these years.

    Long Analog Package Temps and Precipitation

    350x317_09221707_winterlonganaltemps922


    But keep in mind, the intensity of ENSO can create different results. I have noticed that most moderate El Ninos produced a stronger ridge into western Canada leading to a more amplified pattern compared to weaker El Ninos. Colder-than-normal temperatures occurred in the Southwest and Southeast. This means that the ridge was placed far north with an undercutting jet into the Southwest extending to the Southeast persistently. In addition, drier and milder conditions occurred across the northern Plains. However, in weaker El Nino Years, on average, colder-than-normal air took over the northern Plains. The Southeast and Southwest were not as cool. The mean ridge in the Northwest and western Canada was not as strong leading to farther south polar jet. This gives more opportunities for phasing with the southern branch of the jet stream. Take a look at the comparison below of all weak El Nino winters compared to moderate El Nino winters.

    Weak El Nino Year Winter Season Temperature Departures

    350x320_09221548_winterweakelninotemp


    Moderate El Nino Year Winter Season Temperature Departures

    350x320_09221643_wintermodelninotemp


    So you may say, why does our forecast match closer with the moderate El Nino case compared to the weak El Nino case? Because the forecast is based off more than just ENSO. Take a look at the chart again above. Look at the PDO. There were just as many weak and moderate El Nino years that had a negative PDO. The Pacific signals of the PDO and PNA could have a stronger influence on the pattern this winter than the Atlantic signals. However, it's the AO which was negative for most of the winter for the weaker El Nino years compared the moderate El Ninos which still concerns me.

    Shorter Analog Package

    350x313_09221708_wintershortanaltemp922


    Models:

    The models, overall, have been inconsistent in run-to-run updates. The European, Japanese and IRI forecasts all had very different looks to the winter season this month compared to last month. In addition, they all have different solutions amongst them. The Japanese had the colder-than-normal departures only along the Gulf Coast and Southeast from August and now has it for much of the nation, including the northern Rockies and Northwest. The IRI trended chillier for the northern Plains and warmer for the southern Plains, but I do like the IRI precipitation forecast for the winter season.

    IRI Precipitation forecast

    350x373_09221710_winteririprecip


    The European seasonal outlook did a complete flip from August to September as far as the upper pattern look. The August outlook, based on anomalies not the actual mean contours, had a more positive PNA and negative NAO look. This shows a more amplified pattern with a greater chance of dryness in the Northwest and storms in the East. However, the September outlook flipped and shows the strong positive anomalies shifted toward the western Gulf of Alaska and a neutral to negative PNA look and a neutral to positive NAO (higher positive anomalies in central Atlantic, lower anomalies northeast Canada and southern Greenland). You do not need a negative NAO to get the storms this season in the East. Forecasting too much cold, based on this look, may not be right. So, we have a compromise on temperatures in the Northeast. Remember, the European trend last year in September and confirmed even greater in October was the best model for last winter.

    Sea Ice and Solar Impact:

    The sea ice extent this season peaked out the lowest on record, beating 2007. It has been talked about in many papers, but not clearly proven yet on a short data base, that more open water near the Arctic can contribute to more blocking. More open water and shifting of ice can alter wind patterns around the arctic (basically influencing the AO signal). The ice extent was very low going into last winter, but only some blocking occurred and that was mainly over the other side of the pole. So there are no guarantees, but this ready to contribute to a potentially stormy outlook for the eastern U.S. this year.

    Sea Ice Map

    590x472_09221715_pro13yearyearice


    According to NASA, the current prediction for Sunspot Cycle 24 gives a smoothed sunspot number maximum of about 76 in the fall of 2012. We are currently well over three years into Cycle 24. The current predicted and observed size makes this the smallest sunspot cycle since Cycle 14 which had a maximum of 64.2 in February of 1906. The cycle is expected to peak in May of 2013. The cycle did show some life in July with a few strong solar flares, but overall it is expect to be low output cycle. A strong output cycle with an El Nino pattern can lead to extreme warming. We saw a glimpse last winter when there was a solar flareup in the middle of the season which could be a reason for unusual mild and low snowfall season. In my opinion, solar impact on this winter will be minimal either way due to low output predicted. If anything, this may favor the colder idea.

    Solar graphic

    350x262_09221719_cycle24


    Risks and Confidence:

    1. Modeling run-to-run updates have been all over the place, particularly the Japanese, CFS and even to a degree the European. Last October, the European came out with an outlook that pretty much described all last winter in a nutshell. Unfortunately, this forecast has been sent out before that update, but if there are any big changes on the European that we feel we need to change in our forecast, this will be taken care of in the 180-day forecast at the end of October and an update to the actual seasonal section in November.

    2. We have higher confidence in the Southeast and Middle Atlantic this season along with the Northwest. We have lower confidence for the Midwest and northern Plains based on the huge spread in temperatures and snowfall amounts that can occur from different El Nino seasons. We have moderate confidence in the Northeast. Our temperatures are near normal overall, but we do feel it is colder in January and February. If we have deep snowpack and strong blocking, then we are not cold enough. But if we little snowpack by the middle of the season and less blocking during this time, then our forecast should be good.

    3. For the northern Plains in December, we are leaning warmer than normal from many of the analog years. We are basing this forecast on less blocking and being a more typical December with marginal cold air masses rather than prolong arctic outbreaks. We are not expecting the same departures as last year.

    Summary:

    The winter season of 2012-13 should be colder than last winter with a bigger threat of large storms in the eastern U.S. The coldest temperatures, compared to normal, should be over the interior Southeast and southern Appalachians. December, overall, should not be a cold month. The rest of fall into the early winter should be snowy in the southern and central Rockies, then shift to the east in the middle and late parts of the season. This should lay down the foundation for more cold in the East during the middle of the season. The Northwest should be drier than normal. Precipitation will run higher than normal along the Gulf coast and into central and northern Florida. This also is an area to watch for possible severe storms from time to time. Below are our maps for the month-to-month breakdown.

    For Friday, I will break down December, January and February better and show individual maps including a mean snow day map.

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