Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 8:18 PM GMT on July 28, 2010
A gentle eroding wind blows against the long-standing sign, "COMING SOON: Luxury Homes." Weeds grow upon the posts as it sits gently along a farm field. But in recent weeks, the sounds of a crisp cool morning have been eroded by bulldozers, dynamite, etc. The ancient sign that posed a hollow threat for nearly ten years has finally made its accusation. Unfortunately this meant the realization for those of us, that our beautiful mountain landscape would be drastically harnessed permanently. The wheat fields that once grazed a myriad of wildlife are destroyed. The wheat fields that once served as shelter for packs of deer is no more. The wheat fields that once provided a window into the beauty of the ridge and valley region has been eliminated. Torrents of dust, dirt, and mud exist and the blowing amber fields of grass along the base of Blue MOuntain is only a distant memory. Along the mighty creeks serving as a resource for the ridge to serve as a flood reservoir, no contain floods of brown mud serging along the babbling brook's rocks. In a time where economic concerns prevail throughout the housing markets, and scores of local contractors ending their construction for new neighborhoods early, why is necessary to destroy once protected land. For years this sign has only been a source of laughter. "Ha, that coming soon sign has been there for years." Yet as soon as the chuckle ended before our eyes, one day the field was destroyed. Due to a score of concerns for our neighborhood, many attended local township meetings to deter the contractor from their construction, but it was no such luck. Now we wait as homes slowly construct themselves along the base of a mountain and up onto the mountainside. Once a large Native American Reservation and Patton's Fort, eventually matured into local farmland, now finally to be built up as just another fancy neighborhood. While beautiful real estate, it is such ashame that thousands of stories just like this are occuring all across the United States and world. I am far from a those enthrowed with anthropogenic global warming, but concern for local environments is important in its own sense. Oiled black asphault is not my idea for a picture perfect setting. And tis all why I hope to eventually mitigrate towards a quiet mountainside community with a successful career as I come of age.
Onto the weather... Recently a unique collection of severe thunderstorms exhibited themselves across the Keystone state and the rest of the northern Middle Atlantic. While everyday scores of thunderstorms migrate across the globe with electrons striking their lightning fury overy the harsh landscape, these thunderstorms proved unusual in their makeup. The majority of thunderstorms across the northern Middle Atlantic organize themselves into squall lines and/or large clusters. Unlike the Midwest and their associated supercells, our thunderstorms rarely have echo tops as high as the bottom of the stratosphere. Most of the time run of the mill thunderstorms are common in our region due to several inhibiting factors. But this week, several days offered unique supercellular characteristics creating the catalyst for several tornadoes in the northern Middle Atlantic. Across the heartland flat praires in close proximity to the gulf stream offers the perfect opportunity for thunderstorms to grow with few preventive factors. Closer to home the Appalachian Mountains and cool easterly Atlantic flow causes several detering factors in severe weather.
Local model guidance exhibited several characteristics that offered evidence for the potential for supercellular formation. The GFS and ECMWF both highlighted a slow moving warm front across the region with a strong upper level low in Canada dragging on a cold front through the Ohio Valley. The warm front served as the lifting mechanism and a bubble of upper level ridging over the Tennessee Valley helped to feed heat and humidity into the region. Moisture from deteorating Bonnie also helped to aid boundary layer dewpoints in the mid to upper 70s with PWATs aloft +2SD at nearly 2.0in. Concentrated areas of higher helicity along the warm front created a natural upper level rotation, along with shear values crossing the impressive 50knots threshold. But a problem did exist, 700mb heights were near the critical CAP threshold at 8-9C. Typical strong CAPs exist around 9-10C, but this unique instability setup did allow for thunderstorms to form. Thunderstorms develop on the basis of contrasting thunderstorms, warm air at the surface and cold air aloft. The contrast helps create healthy cellular updrafts and downdrafts. But when the air column is warm with little atmospheric cooling as one increases elevation, then this puts a CAP on convection development. This helped to prevent thunderstorm formation in southern areas in southern Pennsylvania and all of Maryland/Delaware. Closer to the warm front with steeper lapse rates, thunderstorms were able to form. SBCAPE values rose to near 3000j/kg with MUCAPE up to an astonishing 4000j/kg. Remember CAPE is convective available potential energy. Therefore the higher the number, the higher the instability. Anything above 1800j/kg is usually a good sign for strong thunderstorm development in Pennsylvania posing that there is a lift for formation.
Each day of the stalled warm front allowed discrete supercells to form with concentrated areas of severe damage. While the thunderstorms were not widespread, their actual makeup was very impressive. They featured echo tops nearly to 50,000ft, almost into the stratosphere. Also they contained VIL values near 50-60. VIL is a measure of hail formation; simply put the higher the number above 30, is usually a good sign for hail. Fortunately the freezing level aloft was nearly well aloft, so hail was not a major threat. With strong vertical and horizontal shear, tornadoes became a threat. Several reported tornadoes occured along the New York/Pennsylvania border bringing the number of reported tornadoes in Pennsylvania to 12 for the year. Average number per year is nine.
The final day of the severe weather featured the approach of the cold front drapped across western Pennsylvania Sunday July 25 during the morning. A decaying MCV (mesoscale convective vortex) provided the lift for a bowing line segment over central Pennsylvania and on eastward. But across the Lower Susquehanna Valley, a unique combination of impressive kinematics and thermondynamics allowed for a few discrete supercells to form out ahead of the line. These cells produced several tornado warnings, but no actually tornado reports.
This poses my main point. Across southeastern central Pennsylvania from the Lower Susquehanna Valley into the western Delaware Valley is a unique geographic location that offers each severe weather season several unique thunderstorms. While I am not saying this region focuses the most severe weather per year, but it does pose the threat for the most intense thunderstorms (supercells). The geographic region has several factors that offers the nickname the 'tornado valley of the east.'
The Appalachian Mountains pose a looming existence just to the west of the region. Blue Mountain is the first ridge as one travels from east to west. From there westward, exists many ridgetops topping elevations around 2000ft until the Alleghany Plateau. This creates the orographic lift for thunderstorm formation until they traverse east of the Blue Ridge on to flat land with elevations below 500ft. To the east of the region is the Atlantic Ocean which offers an easterly moisture anomaly into the factor. And to the south is the Chesapeake Bay offering another unusual moisture factor which helps to nose up dewpoints on summer days. The combination of the flat land downwind of a large mountain chain in correlation with several moisture anomalies from large boundaries of water, creates a natural spin to the atmosphere. Countless times once thunderstorms move downwind of the mountain, they strengthen rapidly peaking across Lancaster County and immediate surrounding areas. Each year thunderstorms in this region develop the strongest and produce several anomalies over the region. While statistically western Pennsylvania (Westmoreland County to be specific) sees the most severe weather reports per season, the Lower Susquehanna Valley and eastward offer unusually strong thunderstorms which develop supercellular characteristics. The Oklahoma Climatology Survey even advocates that while the national average is one tornado per 10,000 square miles, there is small isolated concentration of 3 per 10,000 square miles in the Lower Susquehanna Valley. To keep in relativity this number reaches nine for a max per 10,000 square miles across the Oklahoma tornado alley heartland.
Quiet weather appears to exist the coming few days outside a quick cold frontal passage on Thursday. A quick-moving cold front will move downwind of the Great Lakes Wednesday night and Thursday through early afternoon. A bit of elevated CAPE near 1000j/kg will allow thunderstorms to develop along the front and move southeast Wednesday night. Simulated radar from the 4km HIRES NMM WRF indicates a broken line of steady showers and thunderstorms forming over western Pennsylvania tracking eastward. Average QPF pools from a GFS/NAM correlation indicate an average .2-.8in basinwide with lower amounts across the south and east. Convective feedback problems over northern Pennsylvania show 1in QPF, but that will likely be isolated. By Thursday morning, showers and thunderstorms will be moving across central Pennsylvania. Unfavorable timing will likely inhibit most severe weather for Pennsylvania despite adequate shear 0-6km near 30-35knots. But isolated severe weather is possible over Maryland and Delaware as the cold front moves over the region by early to mid afternoon. Highs are dependent on the front's timing, but it is possible for southern areas to reach 90F ahead of the front in the southwest flow. Northwestern areas will remain in the lower 80s. Not all areas will receive rain and it is likely parts of Pennsylvania remain dry as the front passes through during the diurnal min. By Friday upper level troughing over the region will maintain slightly below normal temperatures and sunshine over the region.
Friday will likely feature the best weather in nearly weeks along with Saturday and possibly Sunday. Humidity will remain low with partly cloudy skies. Highs will be in the mid to upper 70s over the higher elevations with lower 80s in the major metropolitan regions. Lows will even drop into the low 50s near Bradford with adequate radiational cooling conditions.
By late Sunday into early next week, a stationary boundary will lift slowly northward from North Carolina to the Ohio Valley which will bring a return to the heat and humidity by early next week with diurnal threats of showers and thunderstorms. At this time widespread severe weather does not look likely through the next seven days. Looking ahead into August, a positive NAO and roaring La Nina will likely favor increasing heat again for the month with above average temperature montly departures. Another hot month looks to be in store with normal precipitation.
Thursday- A chance of showers and thunderstorms will be ongoing in the morning spreading eastward throughout the day. An isolated severe storm is possible with wind damage being the primary threat. Highs will be in the low 80s in the northwest to near 90 towards Washington DC. Mostly cloudy skies will dominate
Friday-Sunday- High pressure and a northwest flow will maintain 70s to 80s for highs and lows in the 50s regionwide with low humidity and clear skies.
Regional updating radar...
"Here north of Harrisburg 2010 statistics"
(Severe Weather Stats...)
Severe Thunderstorm Watches- 10
Severe Thunderstorm Warnings- 9
Tornado Watches- 1
Tornado Warnings- 1
Total Thunderstorms- 15
Flood Watches- 3
Flood Warnings- 1
Monthly Precipitation- 0.83inches
Yearly Precipitation- 23.13inches
Heat Advisories- 5
Excessive Heat Watches- 1
Excessive Heat Warnings- 1
90degree days- 28
Highest Temperature 101F (x2)
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.