Monday, July 15, 2013

The Anomalous Weather Pattern Of July 2013

July 5, 2013
Along The Tennessee Valley Divide
Spectacular Cloud Formations Greet The Dawn
Quiet Summer Morning Above The Cumberlands
Photograph by Wayne Riner - © All Rights Reserved.

The High Knob Landform

Wayne Riner Photograph Thoughts...
"The morning was dark with only the call of birds welcoming the busy day.  It was unusual because this sunrise was without rain.  The drier period will provide a chance to spend time in the garden."

July is typically wet in the Appalachians.  The first half of July 2013 has been dominated by a highly anomalous weather pattern conducive for simply exceptional wetness along the eastern slopes of the Blue Ridge, especially from central Virginia across southwestern North Carolina into Georgia.

Doppler Estimated Rainfall During July 1-15, 2013

The 10.84" officially measured in Roanoke is the most ever observed during July in 102 years of observations, and the month is only half complete!

KROA  Extremes
Highest Precipitation
JUL 1 - JUL 31       1912 - 2013

    inches    date            inches    year
  1. 3.88   7-10-2013      1.   10.09    1989     
 2. 3.38   7-29-1940      2.    8.19    1934     
 3. 3.26   7-08-1944      3.    8.18    1940     
 4. 2.84   7-27-1934      4.    7.92    1927     
 5. 2.83   7-19-1938      5.    7.85    1949     
 6. 2.79   7-02-2013      6.    7.22    1991     
 7. 2.67   7-10-1931      7.    7.16    2000     
 8. 2.60   7-09-2010      8.    7.07    1994     
9. 2.36   7-06-1989       9.    6.86    1938     
10. 2.28   7-25-1991     10.    6.52    1937     

 Total Years = 102
 Missing days = 49

Courtesy of Blacksburg VA NWSFO
National Weather Service Forecast Office

The 37.87" of total precipitation measured in Roanoke 
during January 1-July 15 is 91.8% of their 1981-2010 annual average of 41.25" .

Updated July 16
An additional 1.15" of rain in Roanoke has pushed the record to 11.99" for July ( +9.90" above average for this point in the month ).  The 2013 precipitation total of 39.02" thru July 16 is 94.6% of the annual average.

Doppler Estimated Rainfall During July 1-15, 2013

Another center of anomalous wetness has been amid headwaters of the New & Watauga river basins, with weather data from Banner Elk, Nc., illustrating how rare this period has been.

The 13.62" of rainfall measured in Banner Elk during July 1-15 is 11.06" above the 1981-2010 mean listed as being average for the first half of July ( i.e., 2.56" was average )

North Carolina Observed Climate Normals ( 1981-2010 )

The long-term July average rainfall 
for Banner Elk is listed as 5.58"
Ray's Weather Center - July Stats

Perhaps more impressive, the 60.60" of total precipitation measured in Banner Elk since January is already 22% above the 1981-2010 annual average of 49.50" .

Grandfather Mountain Sets 
All-Time July Rainfall Record

Landis Taylor, Director of Communications, at Grandfather Mountain reports that the mountain has already shattered its previous July rainfall record of 11.92" established in 1989 with 17.03" measured at the Top Shop during July 1-16.

Landis said, "stations lower on the mountain have actually recorded over 20.00" but I like to compare the Top Shop numbers to the record as that is where it was recorded and set."

Records at Grandfather Mountain date back to 1955.
Grandfather Mountain - Check The Plan Your Visit Section

Excessive July rains at Grandfather follow a June that produced slightly below average rainfall with 5.77" being 6% below their
57-year June average ( 40.63" for 2013 at the end of June ).

Far southwest, in Transylvania County, excessive rains are more common than in the northern mountains of North Carolina, with more than 38.00" measured since June 1 some 2 miles SW of Lake Toxaway ( where annual precipitation is often double that of Banner Elk ).

While westerly air flow dominates the Northern Hemisphere, much of the convective season since May 1 of this year has found prolonged periods of SSE-SE flow trajectories with abundant moisture transport from the Atlantic Ocean & Gulf of Mexico.

May 1-14, 2013
Composite Mean of 850 MB Wind Vectors

This type of flow pattern modifies the air along the western side of the Appalachians via prolonged downslope flow, such that even amid upsloping corridors the air is more stable and less moisture rich than where it initially streams into eastern slopes of the Appalachians from the Atlantic Ocean & Gulf of Mexico 
( where it arrives unmodified by upstream mountains ). 

July 1-13, 2013
Composite Mean of 925 MB Wind Vectors

Another KEY factor has been the superimposition of positive feedback for wetness since April-May of this year with increasing evapotranspiration.

For an explanation and review of feedback processes, as well as various orographic-convective interactions, please reference the following links on this website:

( Examples From Summer 2010 )
Orographics & Feedback Processes 1

Orographics & Feedback Processes 2

Prior to the convective season it is not possible to know where positive ( for wetness ) and negative ( for dryness ) feedbacks will develop, only that they will become an important factor in the warm season rainfall regime across the Appalachians.

A strong orographic forcing season and abundant soil moisture going into a convective season is not a major predictor of warm season feedbacks due to the nonlinear nature of convection and its complex connection to evapotranspiration processes [ in addition to the synoptic weather pattern which can alter what might be expected ( like during 2013 ) by creating settings favorable for a positive feedback ( for example ) to be reinforced by the larger-scale weather pattern across the Appalachians ].

Summer 2013
A Summer Fleabane Species
Photograph by Harold L. Jerrell - © All Rights Reserved.

Climate Statistics
For July 1-15, 2013

( Lower Elevation of Russell Fork Basin )
Clintwood 1 W - Elevation 1560 feet
Average Daily MAX: 80.1 degrees
Average Daily MIN: 61.5 degrees
July 1-15 MEAN: 70.8 degrees
Total Rainfall: 2.46"
( +0.06" above 49-year average )
2013 Precipitation: 31.08"
( 67.6% of annual average of 46.00" )

( Northern Base of High Knob Massif )
City of Norton - Elevation 2141 feet
Average Daily MAX: 77.8 degrees
Average Daily MIN: 58.6 degrees
July 1-15 MEAN: 68.2 degrees
Total Rainfall: 4.16"
( +1.42" above 30-year average )
2013 Precipitation: 40.30"
( 69.5% of annual average of 58.00" )

( Along the Tennessee Valley Divide )
Nora 4 SSE - Elevation 2650 feet
Average Daily MAX: 76.3 degrees
Average Daily MIN: 62.3 degrees
July 1-15 MEAN: 69.3 degrees
Total Rainfall: 6.75"
2013 Precipitation: 37.17"

The first half of July produced average MAX temps around 70 degrees ( F ) at the summit level of the High Knob Massif with average MINS in the 50s 
to lower 60s ( AM of July 13 being coolest ).

A general 4.00" to 5.00" of rain fell from the 
Head of Powell Valley and City of Norton across 
Big Cherry Lake basin of the massif during July 
1-15 [ Doppler tending to under-estimate rainfall from Norton across central portions of the massif toward the Clinch River ( verses rain gauges )].

While rainfall during the first half of July was 152% of the average of the past 30 years in the City of Norton, it was excessive during only one day ( July 1 ) when street flooding became a significant issue
( the 30-year July average is 5.49" in Norton ).

Summer 2013
Longhorn Milkweed BeetleTetraopes tetrophthalmus )
Photograph by Harold L. Jerrell - © All Rights Reserved.

Humidity Wave Of 2013

Its not been the heat but the humidity that has made conditions feel miserable for many living amid the highlands of the southern Appalachians in July 2013. 

July 1-July 20, 2013
MAX Temperatures By Elevation
( High Knob Massif - Tennessee Valley Divide Area )

78 degrees....4178 feet

81 degrees....3200 feet

84 degrees....2650 feet

86 degrees....2141 feet

While highest official temperatures observed in the Norton-Wise and Sandy Ridge areas have only reached middle 80s during the "heatwave" of July 15-20, dewpoints as high as 77 degrees have been recorded at Lonesome Pine Airport in Wise.

At 85 degrees a 77 degree dewpoint = a 96 degree 
feels-like temperature.

Pattern Change Ends July

Much cooler and drier conditions dominated the final week of July 2013 amid a major pattern change ( one that would return to visit the Mountain Empire by mid August ).

Selected climate charts reveal the difference.

July 1 to July 24 Period
Mean 500 MB Geopotential Height Pattern

A weakness between two heat domes dominated the bulk of July 2013 with circulation off the Atlantic and Gulf of Mexico around a sprawling Bermuda High supporting widespread excessive rainfall along the eastern slopes of the Appalachians.

July 25 to July 31 Period
Mean 500 MB Geopotential Height Pattern

The change to a upper air pattern more common to autumn and winter set the stage for chilly nights to end the month, with MINS dropping into the 40s amid cooler mountain valleys during 
July 29-30, with lower-middle 40s in high valleys on July 29.