Note: Records and normals discussed in this article pertain to NWS Blacksburg’s first order climatic sites which include Roanoke, Lynchburg, Danville and Blacksburg, VA and Bluefield, WV. To view other station climate data that may be closer to your home, visit NWS Blacksburg’s Climate web site at http://www.weather.gov/climate/xmacis.php?wfo=rnk or the Southeast Regional Climate Center (SERCC) at http://www.sercc.com/climateinfo/historical/historical.html.
Figure 1. WFO Blacksburg County Warning area geographical break down
The unofficial start to summer began May 25th as mean daily temperatures went to 3-8 (F) degrees above normal across the area. The mean daily temperatures remained 3-8 degrees above normal through the end of the month. These warm temperatures were a result of a strong area of high pressure over the region and abundant sunshine (Figure 2-3). Within an area of high pressure, air descends and warms. This sinking air also inhibits precipitation and clouds from forming. Depending on how strong these highs are and how long they stay in a region, very warm to hot conditions could persist for weeks with temperature records possible. This was the case the last week of May as daily high temperatures were 10-15 (F) above normal along and east of the Blue Ridge and around 10 (F) degrees above normal west. The last four days of May (28-31), Bluefield, WV set 4 record high temperatures while Blacksburg, VA set 2 new records. Despite the very warm ending to May, because of a cool start to the month, average monthly temperatures were a few degrees below normal.
Figure 2. 500 mb heights (m) on May 30, 2006
Figure 3. Evening surface analysis on May 29, 2006
May is normally the second wettest month of the year for Blacksburg’s County Warning Area (CWA), based on the 30 year normals from 1971 to 2000 of over 80 Cooperative Observing locations. The mean monthly precipitation for the area this year, based on the same Cooperative Observing locations, was only 2.14 inches. This was only 44% of the normal, (4.89 inches). These below normal amounts brought the yearly precipitation deficits to 8 to 12 inches in the east and 3 to 6 inches in the west.
June’s daily temperatures started out (1st and the 2nd) above normal, but fell below normal by the end of the first week. High temperatures were 5-10 (F) degree below normal on the 5th through the 7th as an upper level low moved from the Great Lakes region into the Mid-Atlantic (Figure 4-5) . Clouds and scattered thunderstorms then kept temperatures below normal through the middle of the month.
Figure 4. 500 mb heights (m) on June 4, 2006
Figure 5. 500 mb heights (m) on June 6, 2006
The first tropical storm of the season, Alberto, came ashore in northwest Florida late on June 13th and tracked northeast through Georgia, South Carolina and exited off the Virginia/North Carolina coast as a depression by midday on June 14th. Rainfall in the Blacksburg CWA was confined to the extreme southeast corner from Rockingham and Caswell counties in North Carolina, and far eastern Halifax and Charlotte counties in Virginia. Radar and rain gages showed nearly 2 inches of rain in those areas. Once tropical storm Alberto tracked (Figure 6-8) north of the area, temperatures began to rebound above normal as another upper level ridge settled over the region.
Figure 6. Morning surface analysis on June 14, 2006
Figure 7. Afternoon surface analysis on June 14, 2006
Figure 8. Evening surface analysis on June 14, 2006
Between the 14th and 22nd, high temperatures along and east of the Blue Ridge were about 5F degrees above normal. Mountain temperatures were about 10F above normal during this time. Temperatures (upper 80s to upper 90s) reached their warmest levels of the month on the 22nd ahead of an approaching cold front. Despite the very warm temperatures, only Roanoke broke a record with a high temperature of 97F. Heat indices were also high during this heat spell with readings in the 90s across the mountains and 100F to 105F east of the Blue Ridge into the Piedmont. The Heat Index (HI), or the "Apparent Temperature", combines the effects of heat and humidity. When heat and humidity combine to reduce the amount of evaporation of sweat from the body, temperatures will feel hotter. Daily temperatures began to cool and fall below normal as the cold front moved into the region on the 24th, with numerous showers and thunderstorms. Despite having temperatures 5 to 10 degrees above normal in the middle of the month, the mean monthly temperatures were around normal.
Rainfall amounts for the month were all well above normal. Preliminary data from NWS Cooperative stations (82 reporting) for June showed mean precipitation of 7.42 inches across the area. This was 189% of the normal June mean of 3.92 inches (1971-2000). The majority of the rain (Figure 9) fell between June 24-28, as a stalled frontal boundary over the region interacted with tropical moisture (Figure 10).
Figure 9. Accumulative rainfall amounts from June 24th to June 28th
Figure 10. Afternoon surface analysis on June 14, 2006
It was the wettest June in at least 10 years across the Blacksburg Hydrologic Service Area (HSA). Fourteen daily record rainfalls amounts were reported during the month. Roanoke set 2 records, 4.08 inches (26th) and 1.64 inches (27th). The 4.08 inches of rain on the 26th was the 4th wettest summer day on record for Roanoke since records began in 1948. Blacksburg’s 3.85 inches of rain on the 26th was the second wettest summer day since records began in 1952. The wettest summer day for Blacksburg was 4.26 inches on September 28, 2004 (remnants of Hurricane Jeanne).
This above normal rainfall for the month helped cut into the yearly deficits especially over the New River Valley. The New River Valley, particularly Blacksburg, VA, had a yearly deficit of 7.5 inches through June 22nd. By the end of the month, Blacksburg had a surplus of rain for the year of 0.83 inches. The Blacksburg NWS office received 10.96” for the month, easily breaking the old June rainfall record of 8.66” set in 1976. Other areas had above normal monthly rainfall amounts, 2-3 inches across the piedmont of southside Virginia and northwest North Carolina, 4-5 inches along the Blue Ridge and 2-5 inches in southeast West Virginia. Despite the above normal monthly rainfall amounts east of the Blue Ridge, yearly deficits were still below normal ( Roanoke 3.24, Lynchburg 5.01, and Danville 10.05).
The majority of the cold fronts affecting the eastern United States either washed out before entering the area or remained well to the north in the Ohio River Valley. With our area remaining on the warm side of each front, very warm temperatures encompassed the region. Monthly temperatures were 1-3 degrees above normal for most of the area with at least 23 of the 31 days in July above normal. Southside Virginia was the exception with monthly average temperatures about a degree below normal because of more afternoon shower and thunderstorm activity. Daytime high temperatures were warm for much of the month. As a matter of fact, nearly half the month, Roanoke (13), Lynchburg (15), and Danville (13), had daily high temperatures at or above 90F. Some mountain locations also saw a few days with 90 degrees or better. Blacksburg had one day over 90, and Bluefield had two days of 90 degrees or higher. The normal high temperature at these two cities is 83F. Bluefield also set 6 record highs, and 5 new records for warmest lows, during the month of July.
Generally, when an area reports above normal monthly temperatures, their monthly precipitation amounts are below normal, and that was the case for our area in July. On average, the area saw below normal amounts of rain by 1 to 3 inches. Again, the exception to the below normal rainfall was Southside, VA. Danville recorded 8.58 inches of rain, and most of the area saw greater than 6 inches of rain during the month. Danville’s normal monthly precipitation is 4.44 inches. Nearly half of Danville’s monthly rainfall occurred on July 5th when 2 to 5 inches of rain fell over the area (Figure 11). Danville, VA set a new record for rainfall with 3.92 inches. 2.08 inches of that fell in one hour (1:00 am-2:00 am). Bluefield and Blacksburg also set record rainfall amounts on the 5th, 1.12 and 1.63 inches respectively.
Figure 11. Evening surface analysis on July 5, 2006
other than the widespread convection on the 5th, the majority of the storms were hit and miss thunderstorms during July. The monthly total rainfall ranged from as low as 1.27 inches at one Roanoke county location, to the 8.58 inches at Danville. Many of the locations that were hit by these convective showers saw 1-2 inches of rain in a 15-30 minute time frame. On the 19th, a cold front moved through the region while Tropical Storm Beryl skirted along the Mid Atlantic coast (Figure 12). The combination of this cold front and tropical moisture from Beryl, prompted 30+ severe thunderstorm and flash flood warnings over the area.
Figure 12. Evening surface analysis on July 19, 2006
Another record rainfall setting day for Danville was on July 22nd, when they received 0.94 inches of rain. By comparison on that day, Roanoke had a trace of rain, Lynchburg had 0.39 inches, Blacksburg saw 0.55 inches, and Bluefield received 0.26 inches. Despite Danville receiving 8.58 inches of rain during the month, their annual deficit was still 5.91 inches. The total rainfall for the airports at Roanoke (2.08 inches) and Lynchburg (2.02 inches) were below normal, and this raised the their yearly precipitation deficits to 5.32 and 7.03 inches. In contrast to Roanoke and Lynchburg, Bluefield's July rainfall total was 1.02 inches above normal, while Blacksburg's was only 0.56 inches below normal. Yearly rainfall totals through July for Blacksburg (0.27 inches) and Bluefield (1.74 inches) were above normal.
In true summer–time fashion, August was a very warm month. Monthly average temperatures were 2-4F degrees above normal. The warmest part of the month was the first week, when temperatures east of the Blue Ridge were in the mid to upper 90s. Add in the humidity, and temperatures felt like they were 105F to 115F degrees. In the west, temperatures were also quite warm with readings in the upper 80s to lower 90s. The reason that temperatures during the first week of August were about 10F above normal was because of a very large area of high pressure (Figure 13). This area of high pressure took over a week to track from the west coast to the east coast.
Figure 13. 500 mb heights (m) on August 2, 2006
By mid August, a wedge (Figure 14-16) of cooler air worked its way into the region dropping temperatures to around 10F degrees below normal. This cool spell did not last long as temperatures rebounded back to normal the second half of the month.
Figure 14. Morning surface analysis on August 11, 2006
Figure 15. Evening surface analysis on August 11, 2006
Figure 16. Morning surface analysis on August 12, 2006
During the month, Danville was at or above 90F 18 times, Roanoke 15, Lynchburg 11, and Blacksburg 3 times, while Bluefield had no days of 90 or above. Warm temperatures were not limited to the afternoon. Warm and muggy overnight temperatures persisted through much of the month, and that helped contribute to the monthly averages being above normal. Roanoke set 8 warmest low temperature records during the month. Bluefield set 6 warmest low records, while Danville and Blacksburg had 3 new records.
The Blacksburg’s County Warning Area climatic summer was quite warm with Roanoke being at or above 90F 34 times, Lynchburg 30, Danville 40, Blacksburg 5, and Bluefield 2. A climatic summer is defined by whole months from June through August. An astronomical summer is typically June 21st to September 22nd. The average climatic summer temperature for Bluefield this year was the second warmest at 72.3F (73.5 set in 2005). Roanoke had the 9th warmest summer at 76.1F. Blacksburg's 2006 average summer temperature ranked 11th at 70.5F. Danville’s (76.5F) and Lynchburg’s (74.6F) average summer temperature did not rank in the top 25. By comparison, the continental United States experienced its second warmest summer since records began in 1895 at 74.5F. The average temperatures from January through August 2006 was the warmest on record across the continental United States. To read more about the warm summer, read NOAA new story #2700 (http://www.noaanews.noaa.gov/stories2006/s2700.htm). The January through August average temperatures for our area also ranked in the top 10 warmest for most of our climate sites. Bluefield’s 58.1F ranked 2nd since 1959, while Blacksburg 56.2F ranked 5th since 1952. Roanoke’s 61.2F ranked 7th and Danville's 62.0F ranked 9th since 1948. Lynchburg's 59.6F average for January through August was not ranked in the top 50, partly due to a longer period of record (1893).
Precipitation was hit or miss for much of the month of August. For the most part, monthly rainfall amounts were below normal until the 29th, when a cold front (30th) and remnants of Tropical Storm Ernesto (Sept 1.) brought much needed rain to the region.
Figure 17. Midnight surface analysis on September 1, 2006
Figure 18. Afternoon surface analysis on September 1, 2006
Around half of the monthly rainfall for most sites in August accumulated the last couple of days of the month (Figure 19). In the end, Roanoke's (2.30 inches) and Danville's (1.73 inches) monthly rainfall amounts were below normal, Blacksburg's (3.66) was near normal, and Lynchburg's (4.19 inches) and Bluefield's (5.77) were above normal. Bluefield’s monthly rainfall set a new record breaking the old record of 5.55 inches set in 1989. This monthly precipitation record at Bluefield helped set a new summer-time (June through August) record of 19.16 inches. The previous record was 16.73 inches set in 2000. Two other sites had top 10 summer precipitation amounts. They were Blacksburg at 18.23 inches, and Danville at 15.95 inches.
Figure 19. Accumulative rainfall amounts from August 30th through September 1st
Yearly precipitation deficits continued through August for Roanoke (6.71 inches), Lynchburg (6.25 inches) and Danville (7.72 inches). Roanoke's (25.92 inches) and Danville's (27.88 inches) annual precipitation totals through August ranked among their driest, 12th and 11th respectively. Because of Lynchburg's longer period of record, their 23.38 inches of annual rain did not rank in their top 50. Other the other hand, mountain sites such as Blacksburg (0.25 of an inch) and Bluefield (4.31 inches) were near or above normal.
September was a roller coaster for daily temperatures, especially the afternoon maximums. Thanks to the rain and easterly flow from remnants of Hurricane Ernesto, the first day of September saw high temperatures in the 60s across the area. These temperatures were 15-20 degrees (F) below normal. Temperatures quickly rebounded by the 3rd to near normal (mid-upper 70s in the west to lower to mid 80 in the east) only to fall back into the 60s the next day. On the 4th and 5th of the month, a cold front (Figure 20) and tropical moisture (remnants of Hurricane John in the eastern pacific) allowed temperatures to fall and remain below normal by 5F to 15F degrees.
Figure 20. Infrared satellite imagery, surface pressure (mb), and surface frontal analysis on September 5, 2006
Temperatures again moderated back to normal by the 10th, only to drop back to below normal during the middle of the month. Temperatures along and east of the Blue Ridge fell into the lower 60s on the 12th and 13th as cooler air worked down the east coast. This wedge of cooler air retreated to the north by the 16th and temperatures went back above normal by a few degrees. Temperatures fell again with the passing of a cold front on the 19th with 24-48 hour temperature departures around 20F (see temperature graphs 1-5). Some daily temperatures reached their highest values of the month on the first day of autumn (23rd) and ahead of an approaching cold front. Afternoon temperatures were 2-6 degrees above normal. Temperatures stayed below normal the last week of September with the passing of a cold front on the 24th and then a stronger front on the 28th. The front on the 28th, spawned several severe thunderstorms and knocked temperatures into the 60s, some 50s in the mountains.
Overall, average monthly temperatures for September were 2 to 3 degrees below normal at most locations. No new record high temperatures were set and no station reached 90F during the month. The overall highest temperature was Danville with 86(F) on the 23rd.
Outside of Ernesto’s rainfall on the 1st, most of the area’s precipitation during September was associated with each passing cold front, which generally brought about a third to half of an inch. The areas that were above normal for the month were Lynchburg (7.73 inches), Danville (4.64 inches), and Bluefield (4.02 inches). Roanoke (4.64 inches) and Blacksburg (4.64 inches) had slightly below normal rainfall. Tropical systems generally get a bad reputation for their destructive winds and storm surge, however, they bring beneficial rains to the area. One case in point was Lynchburg, whose yearly rainfall deficit was above 9 inches prior to Ernesto’s east coast tour. By the time Ernesto was history, Lynchburg’s yearly rainfall total was near 30 inches, which was only 3 inches below normal. By the end of the month, and after several cold fronts had passed through, Lynchburg’s annual precipitation deficit shrank to 2.4 inches. Both Roanoke's and Danville's yearly precipitation was a little over 7 inches below normal. Blacksburg was right at normal, while Bluefield had a surplus of 5.12 inches.
Graph 1. Blacksburg, VA, late spring-summer Max/Min Temperature (F) and precipitation amounts (inches).
Graph 2. Roanoke, VA, late spring-summer Max/Min Temperature (F) and precipitation amounts (inches).
Graph 3. Lynchburg, VA, late spring-summer Max/Min Temperature (F) and precipitation amounts (inches).
Graph 4. Danville, VA, late spring-summer Max/Min Temperature (F) and precipitation amounts (inches).
Graph 5. Bluefield, WV, late spring-summer Max/Min Temperature (F) and precipitation amounts (inches).
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Temperature and precipitation outlook for November 2006, November 2006 through January 2007, and February through April 2007.
Figure 21. November 2006 Temperature Outlook
Figure 22. November 2006 Precipitation Outlook
Figure 23. 3-month Temperature Probability (November 2006 through January 2007).
Figure 24. 3-month Precipitation Probability (November 2006 through January 2007).
Figure 25. 3-month Temperature Probability (February through April 2007).
Figure 26. 3-month Precipitation Probability (February through April 2007).
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EL NINO MAKES A RETURN
On September 13, 2006, the Climate Prediction Center ( CPC) issued their El Nino/Southern Oscillation ( ENSO) diagnostic discussion. In this discussion, it was reported that “El Nino conditions have developed and are likely to continue into early 2007.” El Nino is the warming of ocean waters in the east-central equatorial Pacific. Warming of these equatorial waters started in April 2006 and were forecast to remain warm through the summer. In July, observed winds were weaker than expected. This allowed Sea Surface Temperatures (SST) to warm more than forecast in August and to remain warm (0.5C to 1.0C) through this winter.
Some early effects of a stronger El Nino are already evident in some tropical locations. Indonesia, Malaysia, and the Philippines have reported drier than average conditions in August. “Also, the development of weak El Niño conditions helps explain why this Atlantic hurricane season has been less active than was previously expected” (NOAA El Nino Advisory). The United States should not see any effects from El Nino until this winter. Typical El Nino signatures for the United States include warmer than average temperatures over the western and northern states. The Gulf Coast and Florida should experience a wetter than average winter. Drier than average conditions are likely in the Ohio Valley and Pacific Northwest.
For the Mid Atlantic region, an El Nino would normally bring above average temperatures to begin the winter, (Figure 27), while finishing on the cold side (Figure 28). Figure 27 is a 2-month average temperature ranking during a strong El Nino episode. There have been 10 such episodes between 1895 and 1997. The strongest El Nino on record was during the winter of 1982-83. The average temperature (Figure 29) during the 82-83 winter across the area was 1.7 degrees above normal. The severity of this winter’s El Nino has yet to be determined, but the last weak El Nino (Winter 2004-2005) delayed the areas first average frost until mid December. Again, Figure 27 is a 2-month temperature average ranking. Over those two months (November and December), there is always a chance a stronger cold front could bring colder than normal temperatures to the area for a few day or weeks at a time. Figure 28 displays the 3-month (January through March) average temperature rankings during an El Nino event. This graphic suggests the first part of the year will begin with colder than average temperatures, and winter will possibly linger into the early parts of the spring.
Figure 27. Average 2-month temperature rankings (by climate divisions) during an El Nino-Southern Oscillation (November- December).
Figure 28. Average 3-month temperature rankings (by climate divisions) during an El Nino-Southern Oscillation (January-March).
Precipitation rankings (Figure 30 and 31) during El Nino events suggest a near-normal or wetter-than-average (2-month) November and December, while the first 3 months of 2007 could be near-normal or drier-than-normal (3-month average). The term “precipitation” includes liquid rain, freezing rain, sleet and snow. Based on climatology, one can make an educated guess if the temperature or precipitation amounts will be at, near or below normal during a global weather impacting phenomenon such as El Nino. However, synoptic and mesoscale conditions will most likely determine the precipitation type and the amount. There is, with out a doubt, a 100% probability that the area will receive frozen precipitation this winter. Unfortunately, it is incalculable to determine the amount and which day it will occur. For those wishing for a snow day or a White Christmas (Figure 32), we will need to see how things pan out.
Figure 30. Average 2-month precipitation rankings (by climate divisions) during an El Nino-Southern Oscillation (November- December).
Figure 31. Average 3-month precipitation rankings (by climate divisions) during an El Nino-Southern Oscillation (January-March).
Figure 32. Probability of a white Christmas (greater or equal to 1.00 inch)
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Temperatures at or below the freezing point of water can seriously affect outdoor operations (e.g. gardening or construction). The probability of freezing temperatures occurring varies by location and time of year. Therefore, the National Weather Service starts their frost/freeze program based on the average (mean) date of the last spring freeze, using a minimum shelter temperature of 32F for a group of counties with the similar geographical features (e.g. mountains, piedmont, etc.). This program ends on the first freeze of the autumn, or the date when the first autumn freeze has almost always occurred, (90% chance based on 30 years worth of data); whichever comes first.
Figure 33. NWS Frost-Freeze program
When a frost/freeze event is expected 24 to 48 hours in the future during the frost/freeze season, a WATCH will be issued, highlighting the potential for such an event. Whenever the minimum shelter temperature is forecast to be 32F or less in the next 12 to 36 hours during the frost/freeze season, a FREEZE WARNING will be issued. Whenever the minimum shelter temperature is forecast to be 33-36F in the next 12 to 36 hours during the frost/freeze season, on nights with light wind and good radiational cooling, a FROST ADVISORY will be issued.
General thresholds and terminology are shown in the table below:
Figure 34. NWS Frost-Freeze terminology
During anomalous climate patterns (e.g. La Nina, El Nino/Southern Oscillation), vegetation may begin growing several weeks ahead of normal in the spring while the first freeze in the autumn may occur very late in the year, or not at all. Under these circumstances, NWS local offices have the discretion to begin the freeze/frost season early, or terminate it two weeks after the 90 percentile date in the autumn.
This climate article was written by:
Other contributors to this article deserving recognition:
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