Friday, September 6, 2013

Welcome To Meteorological Autumn 2013


September 1, 2013
Upper Tennessee River Basin
Lovelady Gap Area of Lee County, Virginia
Photograph by Roddy Addington - © All Rights Reserved.

The High Knob Landform

Do Not Forget - High Knob Naturalist Rally
The 7th Annual High Knob Naturalist Rally

While more summery days are ahead, there is 
little doubt that weather gears have shifted into 
an autumn mode!


Announcing:
Roddy Addington
HDR Photography Display


On Display For A Month From
The Weekend of September 7-8.

Winter 2012-13
High Knob Massif Crest Zone
Photograph by Roddy Addington - © All Rights Reserved.

At The Magnolia Kitchen
Park Avenue In City of Norton


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.



Early September 2013
High Mountain Valley Chill

September 6, 2013
500 MB Streamline Analysis At 8:00 PM

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!

Horizontal & vertical moisture gradients have been ideal for fog formation across lower elevations, especially main river valleys, while drier air aloft has kept the high terrain mostly free of fog and allowed for nocturnal cold air drainage into 
mid-upper elevation valleys.

September 4, 2013
NASA Visible Image At 8:15 AM

September 4, 2013
NASA Visible Image At 8:15 AM

Morning hours of September 6 were coldest as 
40 to 45 degree MINS occurred in high valleys 
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.

While release of latent heat was significant, the 
keys to this setting were moisture gradients and subsequent mixing.

September 6, 2013
NAM Model Analysis At 8:00 AM
850 MB Streamlines And Relative Humidity

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.

Note that dry air extends toward the surface
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.

September 6, 2013
NAM Model Analysis At 8:00 AM
Vertical Cross-Section ( 39.0 N/84.3W to 35.0N/80.8W )
Relative Humidity & Geostrophic Wind Vectors

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.

September 6, 2013
NAM Model Analysis At 8:00 AM
Vertical Cross-Section ( 39.0 N/84.3W to 35.0N/80.8W )
Theta-E ( green ) & Vertical Velocity ( red )

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 ).

The instability shown by decreasing Theta-E with height and the upward vertical motion area into mid-levels of the atmosphere being "wasted" motion in this case with respect to precipitation formation, or even cloud development, given the air aloft was bone dry via relative humidities of 20-40% or less
( as illustrated by the first cross-section ).

In addition to above noted mixing factors, dry air is more dense than moist air such that by simple weight the drier air tends to want to sink into or under moist air
( e.g., the infamous dry lines of western USA ).

Molecular Weight of Dry Air = 28.966 g/mol
Molecular Weight of Moist Air = 18.016 g/mol

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.

September 6, 2013
NASA Visible Image At 8:32 AM

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.

September 6, 2013
NASA Visible Image At 8:32 AM

Another key being that max cooling potential 
was generally concentrated above 950-900 MB, especially south of 38 degrees N latitude, such that areas resting completely below those levels would have no way to "tap" into the really dry air aloft 
( i.e., locations with no high mountains to aid downward drainage of air have reduced cooling potential in this type of setting ). 


Climate Statistics
For August 2013

( Lower Elevations of Russell Fork Basin )
Clintwood 1 W - Elevation 1560 feet
Average Daily MAX: 77.4 degrees
Average Daily MIN: 60.2 degrees
MEAN: 68.8 degrees
Highest Temperature: 84 degrees
Lowest Temperature: 49 degrees
August Rainfall: 4.40"
2013 Precipitation: 37.07"

( Northern Base of High Knob Massif )
City of Norton - Elevation 2141 feet
Average Daily MAX: 76.6 degrees
Average Daily MIN: 57.0 degrees
MEAN: 66.8 degrees
Highest Temperature: 83 degrees
Lowest Temperature: 45 degrees
August Rainfall: 3.48"
2013 Precipitation: 46.13"

( Along the Tennessee Valley Divide )
Nora 4 SSE - Elevation 2650 feet
Average Daily MAX: 75.9 degrees
Average Daily MIN: 61.1 degrees
MEAN: 68.5 degrees
Highest Temperature: 83 degrees
Lowest Temperature: 49 degrees
August Rainfall: 4.03"
2013 Precipitation: 42.89"

August 2013 featured below average temperatures and near to below average rainfall in most locations along the mountainous Virginia-Kentucky border.

Rainfall varied greatly over short distances with extremes ranging from 2.38" in the Powell River Valley of western Lee County, southeast of Rose Hill, to more than 7.50" in portions of the 
Clinch River Valley of Russell County.

August 25, 2013
View from Fox Gap of Wise County
Sun Sets On The Summer Of 2013
Photograph by Roddy Addington - © All Rights Reserved.


Climate Statistics
For Summer 2013
( June-August Period )

( Lower Elevations of Russell Fork Basin )
Clintwood 1 W - Elevation 1560 feet
Average Summer MAX: 78.7 degrees
Average Summer MIN: 59.5 degrees
Summer MEAN: 69.1 degrees
Highest Temperature: 87 degrees
Lowest Temperature: 49 degrees
Summer Rainfall: 16.11"

( Northern Base of High Knob Massif )
City of Norton - Elevation 2141 feet
Average Summer MAX: 77.4 degrees
Average Summer MIN: 56.7 degrees
Summer MEAN: 67.0 degrees
Highest Temperature: 86 degrees
Lowest Temperature: 45 degrees
Summer Rainfall: 18.56"

( Along the Tennessee Valley Divide )
Nora 4 SSE - Elevation 2650 feet
Average Summer MAX: 76.1 degrees
Average Summer MIN: 61.3 degrees
Summer MEAN: 68.7 degrees
Highest Temperature: 84 degrees
Lowest Temperature: 49 degrees
Summer Rainfall: 19.30"

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.

That is really not uncommon, as a 57-year survey 
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 ).

Long-Term Summer MAXS In Wise
Summer 2012 - From Chilly To Extended Heat

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.

The late June 2012 heat wave, as well documented 
at the above link, was a notable exception!

United States
June 1 to August 31 of Summer 2013
850 MB Air Temperature Composite Anomaly

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.

Northern Hemisphere
June 1 to August 31 of Summer 2013
850 MB Air Temperature Composite Anomaly

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.

Sea Ice Extent As Summer 2013 Ends
Courtesy of National Snow and Ice Data Center

The upper air pattern was more amplified in Summer 2013 verses the 1981-2010 mean, with enhanced troughing over the eastern USA.

United States
June 1 to August 31 of Summer 2013
500 MB Geopotential Height Composite Mean

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 ).

United States
Mean During The 1981-2010 Period
500 MB Geopotential Height Climatology

Summer 2013 rainfall was near to above average across the High Knob Landform and Tennessee Valley Divide but not exceptional like observed from portions of the Great Valley E-SE across 
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.


Summer 2013
Rainfall Totals
( June-August Period )

Southwestern Mountains of Virginia

Burkes Garden: 15.35"

Clintwood 1 W: 16.11"

Lebanon: 16.18"

Richlands: 17.26"

( 5.5 air miles SE Norton in High Knob Massif )
Robinson Knob: 18.40"

Norton Water Plant: 18.56"

Nora 4 SSE: 19.30"

Grundy: 19.65"

( 6.2 miles NNE Gate City )
Fort Blackmore 4 SE: 21.53"

Coeburn Filter Plant: 21.81"

( 6.5 air miles SW Norton in High Knob Massif )
Big Cherry Dam: 21.94"
( At least 1.05" of evaporation losses )

( 3 miles WSW Norton )
Little Stone Mountain Gap: 23.50"

Meadows of Dan 5 SW: 26.28"

Grayson Highlands State Park: 27.34"

Galax WTP: 32.79"



Meso-Scale Features
First Strong Autumn Cold Front
( September 12, 2013 )

Written On September 9, 2013
Excerpt From Dickenson Star Weather Column
"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!"

( 8:00 AM Local Time )
12z NAM Analysis - September 12, 2013
850 MB Theta-E and 700 MB Streamlines
MAX Values ( brightest red ) & MIN Values ( green )

Equivalent Potential Temperature, called Theta-E, combines the temperature & moisture content of air masses to generate a display of their available potential energy
( 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.

This generated beautiful morning satellite images as tops of towering clouds were illuminated by rising sun rays while masses of lower clouds 
and fog remained relatively dark.


Satellite Image Time Series
Click Consecutively On Images For Motion

September 12, 2013
NASA Visible Image At 8:15 AM 

September 12, 2013
NASA Visible Image At 8:32 AM 

September 12, 2013
NASA Visible Image At 8:45 AM 

September 12, 2013
NASA Visible Image At 9:02 AM 

September 12, 2013
NASA Visible Image At 9:32 AM 

September 12, 2013
NASA Visible Image At 9:32 AM 

For comparison the same SHARP line was more clearly visible as the massif stood out distinctly in wake of the awesome Fast & Furious Snowstorm 
of January 2013.

( Click back & forth between images )
Comparison Image From January 19, 2013
Snow Covered In Wake of The Fast & Furious


Doppler Radar Time Series
Click Consecutively On Images For Motion

September 12, 2013
Doppler Base Reflectivity At 7:45 AM

September 12, 2013
Doppler Base Reflectivity At 7:54 AM

September 12, 2013
Doppler Base Reflectivity At 8:16 AM

September 12, 2013
Doppler Base Reflectivity At 8:29 AM

September 12, 2013
Doppler Base Reflectivity At 8:42 AM

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.

September 12, 2013
Doppler Base Reflectivity At 2:37 PM

September 12, 2013
Doppler Base Reflectivity At 2:45 PM

September 12, 2013
Doppler Base Reflectivity At 2:50 PM

September 12, 2013
Doppler Base Reflectivity At 2:54 PM

September 12, 2013
Doppler Base Reflectivity At 2:58 PM

September 12, 2013
Doppler Base Reflectivity At 3:03 PM

September 12, 2013
Doppler Base Reflectivity At 3:07 PM

September 12, 2013
Doppler Base Reflectivity At 3:11 PM

September 12, 2013
Doppler Base Reflectivity At 3:16 PM

September 12, 2013
Doppler Base Reflectivity At 3:20 PM

September 12, 2013
Doppler Base Reflectivity At 3:26 PM

September 12, 2013
Doppler Base Reflectivity At 3:31 PM

September 12, 2013
Doppler Base Reflectivity At 3:35 PM

September 12, 2013
Doppler Base Reflectivity At 3:39 PM

September 12, 2013
Doppler Base Reflectivity At 3:43 PM

September 12, 2013
Doppler 1-Hour Rainfall Estimate At 3:43 PM

September 12, 2013
Doppler Base Reflectivity At 3:48 PM

September 12, 2013
Doppler Base Reflectivity At 3:52 PM

September 12, 2013
Doppler Base Reflectivity At 3:56 PM

September 12, 2013
Doppler Base Reflectivity At 4:01 PM

( 2:00 PM Local Time )
18z NAM Analysis - September 12, 2013
850 MB Theta-E and 800 MB Streamlines
MAX Values ( brightest red ) & MIN Values ( green )

The main changes, other than increasing CAPE  ( convective available potential energy ), were on the synoptic-scale as a notable increase in lower valued theta-E air was following the streamlines southeast toward the Ohio River 
( compare to 12z Analysis ).

From a mathematical perspective, the flow of air tends to be along the streamlines at any given moment in time since all the points in space through which a given line passes possess velocity vectors which are tangent to it at that moment
( 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. 

September 12, 2013
18z Meso-NAM Model Analysis
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/80.9W )
Stretching Deformation of Geostrophic Wind ( red )
Isentropic Surfaces ( green )

Isentropic analysis on September 12 was very helpful in determining what an approaching line 
of broken thunderstorms would do upon passing leeward of this first range of high mountains.

The PRE-prediction:
Thunderstorms would intensify.

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 ).

September 12, 2013
18z Meso-NAM Model Analysis
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/80.9W )
Isentropes ( red ) & Moisture Flux Divergence ( green )
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.


Satellite Image Time Series
Click Consecutively On Images For Motion

September 12, 2013
NASA Visible Image At 1:02 PM 

September 12, 2013
NASA Visible Image At 1:32 PM 

September 12, 2013
NASA Visible Image At 2:15 PM 

September 12, 2013
NASA Visible Image At 2:15 PM 

September 12, 2013
NASA Visible Image At 2:32 PM 

September 12, 2013
NASA Visible Image At 3:02 PM 

September 12, 2013
NASA Visible Image At 3:45 PM 

September 12, 2013
NASA Visible Image At 4:15 PM 

September 12, 2013
NASA Visible Image At 4:32 PM 

September 12, 2013
NASA Visible Image At 4:45 PM 

September 12, 2013
NASA Visible Image At 4:45 PM 


Doppler Radar Time Series
Click Consecutively On Images For Motion

September 12, 2013
Doppler Base Reflectivity At 5:01 PM

September 12, 2013
Doppler Base Reflectivity At 5:10 PM

September 12, 2013
Doppler Base Reflectivity At 5:18 PM

September 12, 2013
Doppler Base Reflectivity At 5:22 PM

September 12, 2013
Doppler Base Reflectivity At 5:29 PM

September 12, 2013
Doppler Base Reflectivity At 5:33 PM

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.

September 12, 2013
Doppler Base Reflectivity At 5:37 PM

September 12, 2013
Doppler Base Reflectivity At 5:42 PM

September 12, 2013
Doppler Base Reflectivity At 5:46 PM

September 12, 2013
Doppler Base Reflectivity At 5:50 PM

September 12, 2013
Doppler Base Reflectivity At 5:55 PM

September 12, 2013
Doppler Base Reflectivity At 5:59 PM

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.

September 12, 2013
Doppler Base Reflectivity At 6:03 PM

September 12, 2013
Doppler Base Reflectivity At 6:08 PM

September 12, 2013
Doppler Base Reflectivity At 6:12 PM

September 12, 2013
Doppler Base Reflectivity At 6:16 PM

September 12, 2013
Doppler Base Reflectivity At 6:20 PM

September 12, 2013
Doppler Base Reflectivity At 6:25 PM

September 12, 2013
Doppler Base Reflectivity At 6:29 PM

September 12, 2013
Doppler Base Reflectivity At 6:33 PM

September 12, 2013
Doppler Base Reflectivity At 6:38 PM

September 12, 2013
Doppler Base Reflectivity At 6:42 PM

September 12, 2013
Doppler Base Reflectivity At 6:46 PM

September 12, 2013
Doppler Base Reflectivity At 6:51 PM

September 12, 2013
Doppler Base Reflectivity At 6:55 PM

September 12, 2013
Doppler Base Reflectivity At 6:59 PM

September 12, 2013
Doppler Base Reflectivity At 7:03 PM

September 12, 2013
Doppler Base Reflectivity At 7:08 PM

September 12, 2013
Doppler Base Reflectivity At 7:12 PM

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.

September 12, 2013 at 8:00 PM
00z September 13 NAM Model Analysis
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/80.9W )



Early Autumn Chill
Pushing The Frost Threshold
( September 14-15, 2013 )

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.

September 14, 2013
12z NAM Model Analysis
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/80.9W )
Graduated Color from Driest to Most Moist
Relative Humidity Denoted By Yellow Contours
Geostrophic Wind Relative To Cross-Section ( white )

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 ).

The morning hours of September 14 found the driest air above the planetary boundary layer, or friction zone, as so typical of early autumn shots 
of chilly air in the southern Appalachians.

September 14, 2013
12z NAM Model Analysis
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/80.9W )
Graduated Color from Driest to Most Moist
Relative Humidity Denoted By Yellow Contours
Magnitude of Total Wind Normal To Cross-Section ( white )

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 ).

Valley fog around sunrise on September 14 was limited and restricted to the immediate main river channels along the Clinch River and North Fork 
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, 2013
NASA Visible Image At 7:45 AM

September 14, 2013
NASA Visible Image At 7:45 AM

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 ).

Changes in air flow and moisture transport
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.

September 14, 2013 at 8:00 AM
12z NAM Model Analysis of Conditions
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/80.9W )
Graduated Color from Driest to Most Moist
Relative Humidity Denoted By Yellow Contours

September 14, 2013 at 8:00 PM
00z NAM Model Analysis September 15
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/80.9W )
Graduated Color from Driest to Most Moist
Relative Humidity Denoted By Yellow Contours

September 15, 2013 at 8:00 AM
12z NAM Model Analysis of Conditions
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/80.9W )
Graduated Color from Driest to Most Moist
Relative Humidity Denoted By Yellow Contours

September 15, 2013 at 2:00 PM
18z NAM Model Analysis of Conditions
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/80.9W )
Graduated Color from Driest to Most Moist
Relative Humidity Denoted By Yellow Contours

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.

September 15, 2013 at 8:00 AM
12z NAM Model Analysis of Conditions
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/80.9W )
Theta-E ( Green ) and Wind Speed ( Red In Knots )

The graphic above illustrates the low-level flow maximum of 
25 knots ( red ) which developed just northwest of the mountains across the Kentucky foothills.  It was centered near 850 MB.  

The graphic below is a more traditional flat view of this 
same setting as viewed upon one level ( 850 MB ).

September 15, 2013 at 8:00 AM
12z NAM Model Analysis of Conditions
Theta-E ( Green ) and Wind Speed ( Dotted Yellow )
RED Line Denotes Above Vertical Cross-Section Slices



Autumn Cold Front
September 21, 2013

Wet and chilly conditions dominated daylight hours of September 21 with a rather classic cold frontal passage across the Appalachians.

September 21, 2013 at 8:00 AM
12z NAM Model Analysis of Conditions
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/81.2W )

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! ).

A slice through the atmosphere along a cross-section across the Mountain Empire revealed the setting nicely with moisture convergence spreading upward and southeastward from the surface at the front to above the High Knob Massif 
and southern Appalachians.

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.

850 MB Level
September 21, 2013 at 8:00 AM
12z NAM Model Analysis of Conditions
Q-Vectors ( red ) & Theta-E Advection ( green )

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.

Rising air and thermal advection was clearly visible via analysis of the 300 K Isentropic Surface at 8:00 AM on September 21 ( below ), with rising air and positive temperature advection from Alabama and northern Georgia across eastern Tennessee into 
SE Kentucky and far southwestern Virginia.

September 21, 2013 at 8:00 AM
12z NAM Model Analysis of Conditions
Isobars ( red ) - Isotherms ( green )
Wind Vectors ( yellow )

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!

For example, the 920 MB pressure that shows up across northern Georgia into Alabama indicates that the 300 K Potential Temp Surface, also called Theta Surface, is much LOWER there compared to far southwestern Virginia where the level of the 300 K isentropic 
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?

NOTE
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 ).


September 21, 2013 at 8:00 AM
12z NAM Model Analysis of Conditions
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/81.2W )
Deformation of the Ageostrophic Wind ( green )
Wind Speeds In Knots ( red )

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.

It has been argued, and rightly by my account, that there would be no weather ( at least, I would state no dramatic weather changes on the synoptic to meso scales ) if this secondary circulation driven by the Quasi-Geostrophic nature of the synoptic-scale flow did not exist 
( 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.


Specific Rainfall Totals
September 21, 2013

Grundy: 0.46"
( Lower Elevations of Levisa Fork Basin )

Clintwood 1 W: 0.63"
( Lower Elevations of Russell Fork Basin )

Nora 4 SSE: 0.77"
( Along the Tennessee Valley Divide )

Little Stone Mountain Gap: 1.08"
( Head of Powell Valley of High Knob Massif )

Fort Blackmore 4 SE: 1.11"
( 6.2 Air Miles NNE of Gate City )

City of Norton WP: 1.14"
( Northern Base of High Knob Massif )

Robinson Knob Community: 1.24"
( Headwaters of Bark Camp Lake of High Knob Massif )

Cox Place Community: 1.35"
( Little Mountain of High Knob Massif )



Open Sesame
Dry Air Advection

Dry air transport following autumn cold fronts can be spectacular, offering up those huge transitions for which Appalachian autumns are famous. 

12z September 21, 2013 ( 8:00 AM Local Time )
NAM Model Sounding Analysis Above Wise

This was recently highlighted during 
the September 21-23 period.  

Note how temperature and dewpoint lines are 
very close together through a deep depth of the atmosphere above Wise ( indicating moist air with near saturated to saturated conditions ) on September 21.

Like the famous command "Open Sesame," note below how the lines separated widely apart to indicate arrival of very dry air and gorgeous, 
cobalt blue skies!

12z September 23, 2013 ( 8:00 AM Local Time )
NAM Model Sounding Analysis Above Wise

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.

September 23, 2013 at 8:00 AM
12z NAM Model Analysis of Conditions
Vertical Cross-Section ( 40.0 N/84.3W to 34.0N/81.2W )
Temperature-Dewpoint Difference In Graduated Color
( Driest Air RED and Most Moist Air In Brightest Green )
Total Wind Vectors ( white )

Note the term "Total" wind means the composite of the main Geostrophic + Ageostrophic components, which collectively 
are sometimes also called the "Real" wind.

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