Weather Headlines

ALERT For A Rapid Temperature Drop Saturday Evening Into Sunday Morning With Rain Showers Changing To Snow Showers Amid Strong SW To NW Winds

Local downpours along a strong cold front will cause a gusty wind shift from SW-WSW to WNW-NW winds and rapidly dropping temperatures this evening into the overnight hours of Sunday morning.

Rain changing to snow showers will redevelop during the evening into the overnight as a upper-level disturbance passes across the southern-central Appalachians. Heaviest snow amounts with this vorticity maximum will occur to the north in West Virginia, especially in the central-northern highlands of WV where Great Lake moisture will enhance amounts, but some sticking snow is expected in higher elevations of the southern Appalachians.

Expect snowfall accumulations of less than 1″ below 3000 feet, with 1-2″ possible above 3000 feet, into the predawn hours of Sunday (24 November). Little to no accumulation is expected below 1800 feet.

The worst conditions are expected to develop at elevations above 3000 feet, with temperatures at highest elevations dropping into the low-mid 20s by Sunday morning. Wind chill factors will plunge into the single digits and 10s at upper elevations, with upper 10s & 20s chills at middle-lower elevations.

Some rime formation is expected above 3000 to 3500 feet into Sunday morning in the High Knob Massif and Black Mountain.

Former Alert

*ALERT For A Period Of Dense Fog Friday Evening As Cloud Bases Drop Into Middle Elevations Along And North Of The Tennessee Valley Divide And High Knob Massif

*While fog will be mainly present at upper elevations, above 3000-3300 feet, into early Sunday amid orographic capping pilatus (feeder clouds), a period of dense fog will also be possible into middle elevations (below 3000 feet) as cloud bases lower for a period Friday night on northerly upslope flow along and north of the High Knob Massif.

The longevity of any fog forming in middle to lower elevations Friday night into early Saturday will be dependent upon a developing inversion, and if it may be able to maintain fog near the surface in places where fog develops.

A Recap Of Recent Wintry Events_Lingering Color

A stormy and much wetter pattern is now looking to develop through the last week of November into the first week of December 2019.

Ensemble members show locally lesser and higher amounts, with much higher amounts on some individual members. This pattern has the potential to refill water supply reservoirs, serving the High Knob Massif area, during the next several weeks.

The GFS Model is now coming into alignment with the European Model group in predicting a series of storm systems, with significant precipitation, to impact the mountain region and Tennessee Valley during the final week of November and the first week of December.

While the majority of this precipitation is currently expected to be in liquid form, dual blocks over the Northern Pacific and North Atlantic (southern Greenland) will have to be monitored for the potential of lower heights and colder air that could contribute to additional frozen types at times despite a warmer, mean flow pattern.

A southwesterly flow regime, in the mean, across the southern-central Appalachians and Tennessee Valley will transport significant amounts of moisture from the Pacific Ocean and Gulf of Mexico.

Orographic forcing, as recently observed, will be a notable player as the orographic forcing season gets underway with vigor through coming weeks. Since precipitation recording began at Big Cherry Dam in 2007-08, in the High Knob Massif, December has been the wettest month of the year.

This has been associated with the annual increase in orographic forcing that develops during the Thanksgiving-Christmas Holiday period, when storm systems feeding upon thermal differences develop pressure gradients that drive strong winds into the mountains with development of positive-negative forcing couplets and mountain torques.

The mountains are not static, but push back against the atmosphere to generate torques that influence the magnitude and spatial distribution of precipitation and temperatures, such that orographic forcing is a vector phenomenon endemic to mountainous and hilly landscapes across planet Earth. They extend their influence and communicate to other portions of the atmosphere through waves that develop and ripple outward from their orographic forcing zones with transport of mass, energy, and momentum to places removed (sometimes very far removed) from their forcing.

The Winter Season Of 2019-20

The dawn of December will mark the beginning of Meteorological Winter 2019-20, so it is time to review some of the main players expected to impact the upcoming season (*).

*The following highlights some major forcing factors, but not nearly all the factors, and is given only as general guidance.

Beginning with the main climate system driver, the sun, Earth is currently within a deep solar minimum as 2019 is now poised to become one of the top years with respect to a lack of sunspots on the surface of the sun.

*National Center for Atmospheric Research Staff (Eds). Last modified 20 Aug 2013. “The Climate Data Guide: QBO: Quasi-Biennial Oscillation.” Retrieved from https://climatedataguide.ucar.edu/climate-data/qbo-quasi-biennial-oscillation.

A -QBO phase in combination with a solar minimum enhances the potential for stratospheric warmings, with a major Southern Hemispheric warming having already been observed above Antarctica.

The above graphic shows the deviation of actual observed temperatures at 10 MB above the South Pole (black line) versus long-term climatological values (grey line). Warming began suddenly during late August-early September, fitting the name of Sudden Stratospheric Warming (SSW) which in this case was a major event.

This has been responsible, in part, for dryness and wild fires across Australia making current news headlines.

A strong stratospheric warming, with temperature increases of up to 80+ degrees Fahrenheit at 10 MB above the Northern Hemisphere, is being predicted by the GFS Model group heading into the first week of December 2019.

Stratospheric warming is being driven by North Atlantic wave 1 forcing, with upward propagation and wave breaking (releasing mass, momentum, and energy) from the troposphere into the stratosphere in combination with upward wave activity flux (WAF) from Siberian snow cover as highlighted by Cohen to initiate the beginning of troposphere-stratosphere-troposphere coupling through coming weeks.

Judah Cohen highlights in his blog that snow cover extent during October was above average across Siberia, which favors stronger Siberian High Pressure and subsequent weakening of the polar vortex via upward wave activity flux (WAF) and feedback with tropospheric-stratospheric-tropospheric coupling leading to a negative Arctic Oscillation and colder than average conditions across mid-latitudes of the Northern Hemisphere.

Eurasian Snow Cover Variability And Links To Stratosphere-Troposphere Coupling

This is complicated and subject to variations depending upon the nature and positioning of forcing factors, both within and outside the arctic region and across the Northern Hemisphere.

The Polar Vortex is now predicted to weaken, with magnitude and extent of this yet to be determined along with resulting impacts for any given region.

As warming occurs, the polar vortex weakens and wind speeds decrease. During a Sudden Stratospheric Warming (SSW) event the direction shifts to easterly in direction. Warmings and SSW’s increase meridional flow across the Northern Hemisphere, enhancing transport of arctic air southward.

Although there is no official El Nino or La Nina running in the equatorial Pacific Ocean, the Southern Oscillation Index (SOI) has been consistently in decline as above average sea surface temperature (SST) anomalies are concentrated mostly in the central Pacific Ocean (versus eastern Pacific in classic +ENSO).

This is a Modoki El Nino (+ENSO) signal which has, from a climatological perspective, favored colder than average conditions in eastern North America and the eastern USA.

A positive Indian Ocean Dipole (+IOD) phase acts to enhance any positive ENSO that may be present during the same time.

The PNA has mostly been in its positive phase (+PNA) during November, which has been anomalously cold, following the mainly -PNA phase of October 2019 that featured much above average temperatures.

Warm sea surface temperatures, relative to average, within the Gulf of Alaska favors a positive PNA oscillation pattern and downstream troughing across the eastern USA, but the positioning and extent of western Pacific ridging is critical.

Blocking near the Aleutians and across Greenland was a persistent feature through Summer 2019 to the present, which contributed to major melting of the Greenland ice sheet (continental glacier).

The continuation of warm sea surface temperatures around Greenland would favor high latitude blocking to continue over this region of the North Atlantic.

Snow and ice extent across North America and Eurasia are near decadal averages, which is now ready for southward expansion of a more long-lived nature. Expansion across the northern USA will aid southward movement through the end of 2019, with initially wet and stormy conditions (featuring mainly rain) in the south.

If positive snow cover anomalies develop, as models currently predict, across eastern and southeastern Canada into December this will be a strong signal for later wintry conditions across the eastern USA and Appalachians versus if the main positive snow cover anomalies were to develop across western Canada.

Feedback associated with positive snow cover anomalies force a natural southward progression of the baroclinic zone (and main storm track) over time, since storm systems feed upon thermal contrasts developing across snow cover-bare ground gradients.

The Bottom Line

Many factors currently favor a colder than average winter season during 2019-20, with stormy conditions across much of the nation (especially in the Great Lakes to Appalachians corridor of the eastern-northeastern USA).

The potential for severe winter periods are likely given highlighted factors, but this will depend upon how the forcing from each of these different teleconnections (Modoki +ENSO, -QBO, SST Anomalies, Polar Vortex changes, snow cover, and more not highlighted here) interact at any given time during coming weeks to dictate the magnitude and extent of these periods.

Long-term forecasts remain elusive given complex feedbacks within and between components of the climate system, with short-term events in both space and time acting to alter conditions immediately that then can force subsequent changes which may skew a month or season away from a prediction made prior to development of the short-term event(s).

This is an evolving situation, as always during the Holiday season, so check back for later updates as conditions change across the Northern Hemisphere.