Ashevillewx 2018-2019 Winter Weather Outlook

October 14, 2018 Hunter Ward

Projected El-Nino Conditions Spell Wet Winter For Southeast

Forecasting winter in the southeast is never an easy task, but there are methods to foretell what nature could have in store. Meteorologist use sea surface temperatures in various locations of the Pacific, along with many other measures to forecast where storms will originate during the winter months. If warm trade winds blow towards the Eastern Pacific during winter, warm water will be upwelled.. and storm development will occur in this region (El Nino conditions). If these prevailing winds blow away from the Eastern Pacific, cooler, more settled weather persist in the Eastern Pacific (La Nina conditions). There is a growing consensus that a weak El Nino will develop this year, creating unsettled conditions in the Eastern Pacific. So what does that mean for the Southeast?

Current Sea Surface Temperatures

Weak El Nino & The Southeast

El Nino years typically produce active weather during the winter for the Southeast, and with ample rainfall activity already this year, its easy to see why excessive moisture will be possible this winter. So what exactly is El Nino, why does it send more moisture into the Southeast compared to La Nina years? The simple answer is more heat! More heat in the specific 3.4 Nino region. So if more heat occurs there compared to other areas, the predominate winds flow that direction in order to balance. With warming occurring off the Mexico & Central American Coasts, these winds flow towards the US, while cold water is upwelled of the South American Coast.

You can see in the graphic above that when storm development occurs, and a cold jet stream dips down in the winter.. a winter storm can develop. That is what I am projecting will occur this winter across the Southeast. I am highlighting years like 2006-2007, and 2009-2010 for comparison. In the charts below, you can see that 2006-2007 was a Weak El Nino, & 2009-2010 came in as a moderate El Nino.

In WNC during 2006-2007 there were several notable cold shots. February was especially brutal with temperature barely getting out of the 40’s for the first 2 weeks. There was also noticeable warm shifts as well during 2006-2007, a characteristic of El Nino Years. 2009-2010 was an exceptional year for snowfall around Western North Carolina with the most notable event being on December 18th, 2009 where Asheville received over a foot of snow. This year is noted as a moderate El Nino, and is comparable to this year due to how wet it was going into that winter. Asheville and many Southeast locations were well above average in regards to rainfall, and fall was relatively mild temperature-wise.

2018-2019 Winter Weather Projection

With an El Nino setting up, and excessive moisture prevalent this year.. I feel comfortable calling for an above average snowfall year in the southeast. So find your location, and multiple your average snowfall by the percentage shown. For example Asheville Averages Aprox 13” of snowfall each year, so I am expecting 18” of total snowfall this year. I believe that 2 large storms are possible, and 3-5 minor snowfall events will occur. So what other factors go into this forecast and its verification? Upper level features like the NAO and AO play a huge part.

What is the NAO?

The NAO or North American Oscillation is a large scale atmospheric feature that dominates storm patterns. Here’s NOAA’s Definition..

The North Atlantic Oscillation (NAO) index is based on the surface sea-level pressure difference between the Subtropical (Azores) High and the Subpolar Low. The positive phase of the NAO reflects below-normal heights and pressure across the high latitudes of the North Atlantic and above-normal heights and pressure over the central North Atlantic, the eastern United States and western Europe. The negative phase reflects an opposite pattern of height and pressure anomalies over these regions. Both phases of the NAO are associated with basin-wide changes in the intensity and location of the North Atlantic jet stream and storm track, and in large-scale modulations of the normal patterns of zonal and meridional heat and moisture transport, which in turn results in changes in temperature and precipitation patterns often extending from eastern North America to western and central Europe. Strong positive phases of the NAO tend to be associated with above-normal temperatures in the eastern United States and across northern Europe and below-normal temperatures in Greenland and oftentimes across southern Europe and the Middle East. They are also associated with above-normal precipitation over northern Europe and Scandinavia and below-normal precipitation over southern and central Europe. Opposite patterns of temperature and precipitation anomalies are typically observed during strong negative phases of the NAO. During particularly prolonged periods dominated by one particular phase of the NAO, abnormal height and temperature patterns are also often seen extending well into central Russia and north-central Siberia. The NAO exhibits considerable interseasonal and interannual variability, and prolonged periods (several months) of both positive and negative phases of the pattern are common.

NOAA

So, if the NAO can go negative a few times this year as the Polar Jet is dipping down, the Southeast could be in store for a few snowstorms. Currently looking at the NAO projections, it appears we will see it dip towards the negative around the end of October. Long range models also so an interesting period around WNC at the end of October, but I will get to that in a few. Another factor to observe that will provide cold air this winter is the AO or Arctic Oscillation.

What Is The AO Index

NOAA defines the AO as:

During the winter of 2009-2010 many cold shots moved through the Southeast, and with El Nino conditions in place storm development was frequent. Locations like Asheville received over 40” of snowfall, and many in the Southeast saw above normal snowfall. If the AO can go negative this year for an extended duration, I believe we will also see a -NAO develop and create a large Southeast snowstorm. El Nino conditions will send storms towards the Southeast this year, and an active polar jet will intertwine on occasion, making many snow lovers in the South very happy.

Looking Ahead

Long range global models such as the CFS paint a relatively cold picture to end October. Below you can see that colder than average temperatures are expected through the end of October, and with the NAO possibly going negative.. snowfall will be possible in the high elevations of WNC.

November temperatures though looks to run around average in the early part of the month, but could be fairly warm around Thanksgiving compared to average.

These can change though, so check back soon for forecast updates as more model data comes in.. and get those coats ready for a cold winter!