An ALERT For Strong S-SW Winds From Late Friday Into Christmas Eve Morning. Wind Gusts of 30-50+ MPH Will Be Possible, Especially At Middle-Upper Elevations And With Any Mountain Waves.
The Potential For Heavy Rainfall, With Embedded Thunder Possible, Will Develop Into Christmas Eve Morning. Abundant Low-Level Moisture Will Linger Behind The Heavier Rain Shield Into Christmas.
Overnight Into Thursday Morning
Partly-mostly cloudy. Windy. SW-WSW winds 10-20 mph, with higher gusts, on mid-upper elevation mountain ridges & plateaus. Temps varying from 30s in the sheltered valleys to the 40s across exposed locations, then dropping into 30s toward morning in upper elevations ( to around freezing on highest peaks ). Wind chills in the 30s and 20s, except 10s in gusts on high peaks toward morning.
Thursday Afternoon
Partly cloudy ( increasing clouds late ). Winds shifting NW-N at 5-15 mph, with higher gusts. Temperatures varying from low-mid 30s in upper elevations to the low-mid 40s, tending to drop during mid-late afternoon. Wind chills in the 20s & 30s, except 10s at highest elevations.
Thursday Night Into Friday Morning
Mostly cloudy ( mid-high clouds ). Northerly winds shifting E-SSE at 5-10 mph on mid-upper elevation mountain ridges. Temperatures varying from upper 10s to lower 20s in the colder valleys to the upper 20s to lower 30s ( rising toward morning into the mid-upper 30s on high mountain ridges ).
Friday Afternoon
Mostly cloudy. Windy. SSE-SSW winds 10-20 mph, with higher gusts, especially across mid-upper elevation ridges and plateaus. Temperatures mainly in the 40s to lower 50s.
Friday Night Into Christmas Eve Morning
Cloudy & windy. Rain developing, with a chance for thunder, overnight into morning. Locally heavy rainfall. Winds SSE-SSW at 10-20 mph, with higher gusts, below 2700 feet. Winds SSW-SW 20-35 mph, with higher gusts, along mountain ridges above 2700 feet. Temperatures varying from upper 30s to mid-upper 40s. Areas of fog.
Areas of fog will be across the mountain area Christmas Eve Day into Christmas Morning. Widespread, dense fog will be likely at upper elevations in orographic clouds ( capping pilatus ). Dense fog could also develop at other locations. Remain alert for these conditions as abundant low-level moisture engulfs the area.
Christmas Eve Afternoon
Cloudy. Heavy rain & possible thunder giving way to showers. Areas of fog. Gusty SW winds decreasing to 5-15 mph, with higher gusts, along mountain ridges below 2700 feet. Winds SW-W at 10-20 mph on mountain ridges above 2700 feet. Temperatures in the 40s to around 50 degrees.
Christmas Eve Into Christmas Morning
Cloudy ( abundant low-level moisture ). A chance of showers-drizzle and areas of dense fog. SW-W winds 5-10 mph, with higher gusts, on mid-upper elevation mountain ridges. Seasonally mild with temps in the 40s to lower 50s.
Weather Discussion ( Wet Holiday )
Thursday Night Update
I have updated my forecast into the Christmas Holiday to increase wind speeds for late Friday into Saturday morning, with another strong low-level jet set to ROAR.
This also sets the stage for another rainfall enhancement episode with strong orographic forcing in the High Knob Massif-Black Mountain corridor. Rain amounts of 1.00″ to 1.50″+ are looking likely in this area, to include the City of Norton ( within the massif’s lifting zone on SW air flow ).
Again it must be stressed that abundant low-level moisture lingers after the heavier rains pass, such that fog, drizzle and a few showers are likely to remain factors into at least mid-morning on Christmas day.
The Wettest Area In Virginia – Climate Notes
Surface Vector Wind Composite MEAN For September 30-December 20, 2016
The pattern of low-level inflow into the mountain area has recently ( above ) been very close to long-term annual mean flow at the surface ( below ).
Annual MEAN Surface Vector Wind ( 1981-2010 Period )
The long-term 925 MB flow ( below ), which is often near the elevation of Wise, has also been nearly identical to surface flow observed during this wet pattern.
Annual MEAN 925 MB Vector Wind ( 1981-2010 Period )
The current pattern reveals one factor ( of which there are many ) that makes the High Knob Massif area the wettest in Virginia, with a MEAN SW flow observed in the surface-925 MB layer ( the lowest portion of the atmosphere which is orographically lifted into the middle-upper elevations ).
A MEAN SW Flow Into The High Knob Massif-Landform
In looking at air flow trajectories for any given location, it is very important in the Appalachians ( or any mountains ) to observe what lies UPstream. In this case, a short and/or long-term SW flow can stream into the High Knob Massif area from the Gulf of Mexico without having to first cross high mountains upstream ( between the Gulf and massif ). Much of the flow can actually stream up the open expanse of the Tennessee Valley to reach the High Knob Massif.
On the other hand, by dramatic contrast, a short and/or long-term SW flow reaching Mount Rogers-Whitetop must cross MANY high mountains upstream ( between the Gulf and Mount Rogers-Whitetop ). In this case many repeated up-down slope motions have occurred to extract moisture from the air mass by the time it reaches what has always been assumed to be the wettest area in Virginia ( * ).
*Since Mount Rogers & Whitetop Mountain are the two highest in Virginia, most everyone assumed they were also the wettest given their higher elevations. However, as has also been noted by other researchers ( such as Baker Perry and Charles Konrad ) elevation is only a single factor of MANY that collectively dictate the precip regime of any given area within the southern Appalachians.
A MEAN SW Air Flow Into Mount Rogers-Whitetop Mountain
The following are RAW data files containing precipitation observed by the AFWS ( IFLOWS ) System since September 30 ( at 7 AM ) for both Eagle Knob of the High Knob Massif and Whitetop Mountain. The bulk of this falling since late November 2016.
It should be noted that BOTH rain gauges have undercatches due to strong winds, with estimated values being on the order of 20-30% given direct comparison of such gauges to hand-measured NWS rain gauges ( i.e., we do a direct comparison between the IFLOWS at Big Cherry Dam and a NWS rain gauge, and during many past years I have compared the Eagle Knob IFLOWS to both NWS and other rain gauges ). In other words, both of these gauges have likely caught only 70-80% of what has actually fallen ( these undercatches increase greatly for snow, but snowfall has been very limited to make this an excellent period for comparisons ).
Even if the Whitetop Mountain undercatch is 30% versus 20% on Eagle Knob the difference holds ( actually increases a little ) between the two sites. This is not a fluke, as per my observations during the past few decades this is intimately connected to air flow trajectories shown above and the up-stream air flow differences between each site and the Gulf of Mexico moisture source.
**Note I have never directly compared the Whitetop Mountain IFLOWS to a NWS rain gauge. However, on Eagle Knob, I have measured differences of up to 60% or more on undercatches in rainfall during extreme wind events. The IFLOWS loss on heavy and/or prolonged winter storm events can often be greater and more extreme than the most extreme wind driven rain events.
Apart from just looking at two wind swept peaks, I have complete confidence that the High Knob Massif area is the wettest in Virginia given many rain gauge records from a variety of elevations and exposures over long-time periods versus a similar set of sites in the Mount Rogers-Whitetop area of southwestern Virginia.
Previous Discussion & History Of Past Christmases
Rain and abundant low-level moisture will make the Christmas Eve to Christmas Morning period WET, NOT white in 2016.
For lovers of snow, and memories passed, I have included a local history of Christmases dating back to 1963 when only Clintwood and Wise data was available. I began adding in more information during the 1980s and 1990s when I started my climate research in the High Knob Massif area.
Rime Coated High Country Above Powell Valley – December 19, 2016
The above rime forming in the traditional manner via orographic forcing with upsloping across mid-upper elevations into December 19. Freezing fog on December 20-21; however, formed via radiational cooling and near calm conditions at lower-middle elevations with an apparent combination of hoar frost and rime. Hoar frost being by strict definition a deposition from gas to solid, while rime represents deposition from liquid to solid. Supercooled vapor was present in both cases ( water can exist in a liquid state in clean air down to temperatures of -40 Celsius, or even lower in certain conditions, before it spontaneously begins to freeze ).
*A lingering inversion, the same basic one that said we would get very little snowfall with more mix and fog initially, has been responsible for this shift into lower elevations as drier air aloft mixed downward across higher elevations.
Abundant surface deposition in local parts of the Great Valley indicated an abundance of nucleation centers for crystal growth, likely from impurities in the air. This may have supported both hoar frost and non-dynamic rime formation ( since rime formation is typically a dynamic process occurring with cold air advection and wind as air is physically forced to rise by the terrain in higher, mountainous locations of the Appalachians ). Around 10 rime formation days have occurred in the High Knob Massif this month, with rime deposition on trees & vegetation being part of an important secondary moisture source ( along with fog drip from trees ) that adds substantially to the annual moisture budgets of locations like the lofty basins of Big Cherry Lake and High Knob Lake.
Some 18 of the past 28 years have featured snow cover in upper elevations of the great High Knob Massif, with the High Knob Lake basin being the area that I have used for general record keeping. A white Christmas being one with at least 0.5″ of snow depth ( since we officially round snow depth to the nearest EVEN whole inch ).
Whitewater Rapids On Little Stony Creek of High Knob Massif – December 19, 2016
As my brief history of the past reveals, there have been many wet Christmas Holiday periods since 1963. This year adds another to the record, like in 2015, to keep whitewater rolling on steep creeks draining the High Knob Massif.
NAM 12 KM Model Total Rainfall Forecast To 7 AM Christmas Morning
While models are not yet set on the exact axis of heaviest rainfall, it is clear that another moderate-heavy round of rain is set to develop across the Tennessee Valley into parts of the western Appalachian front range ( with past climo on this type of flow field supporting this basic solution ).
GFS Model Total Rainfall Forecast To 7 AM Christmas Morning
An abundance of low-level moisture is likely to engulf the area and linger again, partially being trapped beneath yet another low-level inversion. So caution is advised for those traveling during the Holiday period. After an autumn were rainfall was difficult to find, now moisture is becoming a nemesis as December has become very wet. Yet a snow drought continues for now. Could that end be in sight?
Check back later for updates on what may be the end to our snow drought ( at least in the mountains ).
