The Weekend of September 7-8.
Leaves will soon begin to display autumn colors as my friend Roddy Addington shows off an array of interesting, art-like photographs at the new Magnolia Kitchen in downtown Norton.
An array of chilly nights have graced mid-upper elevation mountain valleys ( 2000-3000+ feet ) during the September 4-7 period despite the continued presence of a dominant heat dome across the central USA.
The heat dome will expand again during the September 8-12 period before undergoing a significant contraction by late next week via development of a more amplified pattern. Stay tuned!
from the High Knob Massif to Burkes Garden in southwest Virginia, with 37 degrees amid Canaan Valley of northern West Virginia being the coldest official temperature reported.
These high valley MINS, generally at elevations above 2500 feet, were some 15-20+ degrees cooler than lower elevations in foggy river valleys where the latent heat of condensation with formation of fog was releasing 597.3 calories of heat per gram ( 2500 KJ/kg ) of water condensed.
keys to this setting were moisture gradients and subsequent mixing.
Very dry air at 850 MB and above, responsible for deep blue skies, established a significant moisture gradient along and northwest of the Appalachians with weak advection ( along the streamlines ) carrying dry air over top moist air.
The best way to illustrate this is to take a vertical slice through the atmosphere with a cross-section from the Ohio River across the Mountain Empire to the North Carolina-South Carolina border.
farther north near the Ohio River.
A weak front was aiding upward motion and some low-level moisture convergence along and southeast of the moisture gradient, such that coolest temperatures were along and northwest of isohume packing ( i.e., the gradient of relative humidity on the previous graphic ).
Cold temperatures in Burkes Garden and the High Knob Massif being aided by dry air advection amid the 900-850 MB layer.
This cross-section revealed not only a strong vertical moisture difference, or gradient, but also geostrophic wind vectors taken relative to the cross-section that were aiding mixing along top of the moist layer at latitudes of southwest Virginia.
The geostrophic wind is a theoretical air flow that would be an exact balance between the Pressure Gradient and Coriolis forces, thus it is mainly applicable above the friction zone forced by surface topography. It is only used here to aid illustration of mixing & transport at the top of the moist-dry air interface.
Using Equivalent Potential Temperature, called Theta-E, it can be seen that a local decrease with increasing height above southwestern Virginia also suggested enhanced vertical mixing above 900 MB with instability upon saturation at the top of the moist layer. This being further supported by upward vertical motion above 850 MB ( negative values in red contours = upward motion ).
( as illustrated by the first cross-section ).
( e.g., the infamous dry lines of western USA ).
The purpose of this is to illustrate how mountain valleys with drainage basins that reach upward into drier air aloft can get so much colder than valleys shrouded amid dense fog.
In this case mixing along exposed mid-upper elevation ridges kept morning temperatures much milder than in adjacent valleys, but mixing winds were not strong enough to prevent decoupling of deeper drainage basins from the synoptic flow field.
( i.e., locations with no high mountains to aid downward drainage of air have reduced cooling potential in this type of setting ).
August 2013 featured below average temperatures and near to below average rainfall in most locations along the mountainous Virginia-Kentucky border.
Clinch River Valley of Russell County.
Summer 2013 was devoid of any true heat; although, it had many days with dewpoints that made it feel hotter than MAX temps indicated.
By contrast, it possessed an abundance of very pleasant nights with every month featuring MIN temps in the 40s amid cooler mountain valleys.
The lack of heat was illustrated by Nora 4 SSE where only 16 days during the June-August period reached 80 degrees or higher ( the Summer MAX being 84 degrees ) at 2650 feet above sea level.
Northern slopes and crests at highest elevations in the High Knob Massif failed to go above 80 degrees during Summer 2013.
of Wise data found that the average summer MAX temperature was 88.4 degrees during the 1955-2011 period ( i.e., the average highest temperature officially recorded during each summer ).
So it is no surprise that northern exposed valleys, slopes, and ridge crests resting 1000 to 1700 feet higher than Wise typically reach only the upper 70s to lower 80s for highest summer temperatures.
at the above link, was a notable exception!
Summer 2013 temperature anomalies ended up being only a little below average, around 1 degree Fahrenheit ( 0.5 degree Kelvin ), but part of a distinct and persistent region of troughing.
In the big picture, of the Northern Hemisphere, below average summer temperatures were also concentrated across the Arctic during 2013. This was reflected in a 919,000 square mile increase in ice extent over last summer's record minimum.
The upper air pattern was more amplified in Summer 2013 verses the 1981-2010 mean, with enhanced troughing over the eastern USA.
The Bermuda High was more expansive with an axial displacement farther northwest verses 1981-2010 climatology, which impacted moisture flow into the eastern USA ( especially for locations along and east of the Blue Ridge ).
much of the Blue Ridge province.
In fact, rainfall this summer was far below record levels in the High Knob Massif area where wetter places had more than 21.00" during June-August.
Summer 2013 did mark the 4th consecutive summer to have at least one month ( during June-August ) with double digit rainfall totals in the High Knob Massif.
Fort Blackmore 4 SE: 21.53"
Little Stone Mountain Gap: 23.50"
Meadows of Dan 5 SW: 26.28"
Grayson Highlands State Park: 27.34"
"It is that time of year when summer and autumn fight for position across the landscape, with a dominant heat dome continuing to control the central USA at the same time as an increasing number of cold fronts are developing amid decreasing daylight and dropping south from high latitudes. The seasonal change has started!
Such will be the case this week as the heat dome expands briefly in advance of a strong Canadian cold front expected to arrive during the Thursday-Friday period. Much cooler, drier air will be felt Friday into Saturday before a return flow of southerly air develops once again into early next week.
Showers and downpours in booming thunderstorms will be likely in advance of strong late week cooling that promises to make conditions feel chilly by the time our "Boys of Fall" take the field at Friday Night football games. Get ready!"
( i.e., its distribution and gradients ).
Interesting meso-scale features added to the drama of a synoptic-scale air mass change during daylight hours of September 12.
A summer air mass in meteorological autumn was firmly entrenched across the Appalachians and lower Ohio & Tennessee valleys, as the morning theta-E analysis revealed.
A deep air flow crossing the High Knob Massif from NNW-NW to SE was aiding lee side convergence of air which worked to trigger the first convection around and just after sunrise.
and fog remained relatively dark.
The basic setting changed little into afternoon as a theta-E ridge ( axis of available potential energy ) and stretching deformation continued to favor cloud towers and localized convective formation from the lower eastern end of the High Knob Massif across the Clinch River Valley.
This generated training and back-building of convection from Little Stony Gorge into southwestern Russell County, with 1.00" to 2.50"+ of rain estimated by Doppler in hardest hit locations.
( compare to 12z Analysis ).
( the definition of streamline ).
Air flow crossing the High Knob Massif and Tennessee Valley Divide from NW-SE is almost certain to become a much less productive precipitation producer during the orographic forcing season, via enhanced low-level moisture extraction and evaporation-sublimation with sinking air into the Clinch, Holston, and Great valleys of the Upper Tennessee River Basin.
During the convective season, however, an approaching line of thunderstorms can weaken or intensify upon passing leeward of the High Knob Landform and Tennessee Valley Divide.
The same question often arises with respect to the Appalachian chain as a whole. Will thunderstorms weaken or intensify as they pass over the mountains into the Piedmont?
When the atmosphere is not saturated, adiabatic processes tend to dominate on the synoptic-scale and isentropic surfaces can be better guides for air flow than isobaric surfaces.
of broken thunderstorms would do upon passing leeward of this first range of high mountains.
Based upon the 18z Meso NAM a vertical stretching deformation was present leeward of the High Knob Massif ( peak of High Knob at 36.5 N latitude ) at 2:00 PM local time, with a notable separation of isentropic surfaces southeast of 36.5 degrees N in the vertical cross-section ( above ).
The Geostrophic Wind stretching deformation is indicated by positive red contours on the above graphic, in an area where isentropic surfaces separate to indicate enhanced instability. Although this is still not truly small scale enough to fully resolve local terrain features, in this case it was a nice representation of the setting during afternoon hours of September 12.
For every action in the atmosphere there tends to be an opposite reaction, or compensation, and by the Conservation of Mass principle the atmospheric compensation for a vertical stretching deformation is horizontal convergence = enhanced convection in a summer setting.
Alternatively, from an isentropic perspective, if a layer bounded by two isentropic surfaces is vertically stretched the absolute vorticity of the column must increase to conserve potential vorticity. This is commonly attributed to being a factor in cyclogenesis lee of the Rockies ( * ).
*The potential vorticity of a column bounded by two isentropic surfaces being defined as the ratio of absolute vorticity to the static stability ( James T. Moore - St. Louis University ).
Mathematically negative divergence = convergence.
An axis of deep moisture flux convergence was coincident with a squall line forming along a lead frontal zone to the northwest of the Appalachians at 18z September 12. Despite this fact, the line remained broken across southeastern Kentucky.
This axis of deep moisture convergence was not hard to pick out on NASA visible imagery as it approached the mountainous Cumberland Overthrust Block by late afternoon.
Observe how thunderstorms fire near a formerly active orographic cloud line from the Wise-Scott border into southwestern Russell & Washington counties as low-level convergence increased again in advance of the main line approaching from the northwest.
The main line approaching from the northwest weakened and became more broken upon initially reaching and crossing the Virginia-Kentucky border.
This was a temporary trend as intensification occurred upon passing across the High Knob Massif - High Knob Landform into leeward valleys of the Clinch and Holston.
By 8:00 PM September 12 the squall line was past and a change of air masses was already starting; however, a separation of isentropic surfaces ( below ) indicated low-mid level instability that formed a secondary line of downpours and local thunder.
The lowest temperatures since late May graced colder places into morning hours of September 14 and September 15 when 30s to lower 40s were felt in Clintwood, Norton, and high valleys from the High Knob Massif to Burkes Garden.
The difference between temperature and dewpoint shows up as the graduated color field above, with the driest air in deepest red and the most moist air in brightest green. Geostrophic wind vectors ( white ) have been added to illustrate general flow field directions ( especially above the PBL or friction zone ).
of chilly air in the southern Appalachians.
This graphic is equivalent to the one above except winds normal to the NW-SE cross-section have been added, with speeds in knots. Winds blowing out from the cross-section toward the viewer have been dashed with negative values, while winds blowing away from the viewer into the cross-section are labeled with positive values and solid contour lines ( all white ).
of Holston, as highlighted by NASA imagery below, with more extensive low clouds and fog banked up against the upslope side of the High Knob Massif and Tennessee Valley Divide on N-NE winds.
September 14 was the ideal autumn day with afternoon MAXS that only reached upper 50s to upper 60s from the High Knob Massif north to Wise and Clintwood, beneath spectacular cobalt blue skies. Simply gorgeous!
( WARM in the sunshine but COOL in the shade all day ).
or advection, began to change conditions as high pressure passed across the region. This became visually apparent during daylight hours of September 15 when cobalt blue skies disappeared!
This is illustrated by the following graphics between 8:00 AM September 14 & 2:00 PM September 15.
The magnitude of winds ( knots ) normal to this NW-SE cross-section are shown in each graphic with air flow moving outward toward the viewer in dashed, negative values verses air flow moving away from the viewer in solid contours ( positive values ).
The increase in wind speeds along and northwest of the mountains into morning hours of September 15 kept temps from dropping below 50 degrees amid exposed places. This contrasted with 30s to lower 40s in mountain valleys decoupled from the flow.
25 knots ( red ) which developed just northwest of the mountains across the Kentucky foothills. It was centered near 850 MB.
same setting as viewed upon one level ( 850 MB ).
Wet and chilly conditions dominated daylight hours of September 21 with a rather classic cold frontal passage across the Appalachians.
Note that wind vectors above also possess a SW component, with all the visible wind along this cross section flowing INTO the chart and away from the viewer. So in 3D one must think of this slice as being extended toward the southwest-northeast along and ahead of the cold front ( although magnitudes of specific quantities would change through space & time. TIME the 4th Dimension! ).
Note I dared NOT use the absurd term "over-running" as air, like pointed out by Chuck Doswell, is always in the process of overrunning itself in some shape or form.
In this case there were numerous factors working together to cause the upward spread of moisture to the east, with differential vorticity advection and thermal advection ( i.e., transport ) both present as sources of upward vertical motion ( i.e., rising air ) on the synoptic-scale.
Q-Vectors are a mathematical entity that do not exist in the real atmosphere, but what they represent exists and that are the combined impacts of differential vorticity advection and the LaPlacian of thickness advection as sources for rising air.
SE Kentucky and far southwestern Virginia.
If you know how to read a topographical map, then just turn it inside out and that will give you a head start on reading isentropic charts!
surface is at 800 MB.
When looking at the atmosphere it is always nice if you can think of it, or better see it, in 3D with the 300 K isentropic surface sloping upward from low heights across Alabama and Georgia to a peak of height across far southwestern Virginia.
Meanwhile, the 300 K isentropic surface possessed a long downhill slope from the upper Midwest and southern portion of Lake Michigan into western Kentucky to denote sinking air, and also dry air advection ( when overlaying the Mixing Ratio not shown ), behind the cold front and trough axis.
The wonderful thing about an isentropic surface is that isobars are also isotherms and isosteres, thus you can also see temperature changes and advection as well as changes in density. That is mathematically confirmed by Poisson's Equation, which is easily derived from the First Law of Thermodynamics.
In most cases a complete understanding of a weather setting can NOT be had without studying isentropic surfaces, so WHY would you not use them daily in analysis and forecasting?
I would be remiss if I did not give thanks to the late Dr. James T. Moore, of St. Louis University, who took time to send personal lecture notes that aided my understanding of frontogenesis and isentropic analysis during the 1990s ( including: Isentropic Analysis And Interpretation ).
This was a relatively fast moving front and there was only weak coupling between upper level jet stream dynamics and low to mid-level forcing. This is suggested by two separated deformation cores associated with the Ageostrophic Wind in the above graphic that are only weakly joined together.
( i.e., if there were no Ageostrophic Wind ).
The 20-40 knot SW to NW wind speeds associated with this front joined with other factors to provide some orographic forcing that enhanced rainfall in the High Knob Massif area, where 1.10" to 1.50" amounts were common, and diminished it to the northeast on downsloping air flow trajectories.
Little Stone Mountain Gap: 1.08"
Fort Blackmore 4 SE: 1.11"
City of Norton WP: 1.14"
Dry air transport following autumn cold fronts can be spectacular, offering up those huge transitions for which Appalachian autumns are famous.
very close together through a deep depth of the atmosphere above Wise ( indicating moist air with near saturated to saturated conditions ) on September 21.
This set the stage for more chilly nights, especially at higher elevations above 2000-3000 feet where more 30s occurred from the High Knob Massif to Burkes Garden.
are sometimes also called the "Real" wind.