Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 13:47 GMT le 16 Mars 2008
Well this is my new blog format for the rest of March into April. I will be covering a type of storm and include its typical impacts on the state of Pennsylvania. I will have storm stories and historical extreme weather outbreaks across Pennsylvania with discussion, charts, graphs, and pictures. Each day of the week I will update with a new storm story. Still I will provide forecasts for the week and a look at the long term pattern outlook. During this end of March and April timeframe weather can become quite boring and quiet so I decided to write these blogs with a new topic each week. My first topic is tornado. I hope everyone who read these blogs will include their storm stories from all across the United States. Also I will have learning sections in each blog and forecasting techniques when forecasting these storms. By the end of April I will switch to regular forecasting blogs. Hopefully everyone will get a new sense of some education and awareness about severe weather.
Forecasts for week...
For Monday sunshine will prevail statewide along with rising temperatures as high pressure moves east of region and winds turn southwesterly by afternoon. But in the morning an northerly wind will be very evident gusting to 15mph at times. High temperatures will be in the mid 40s to lower 50s across the state. By Monday night clouds will move in west to east ahead of the next storm system. Low temperatures in the upper 20s to lower 30s. Also in the far west some light snow may fall causing a dusting in some spots. Timing is still unknown for the next storm. By Tuesday morning light precipitation will be across the state with rain in the central and eastern parts. A mix of rain and snow will occur in the west with little snow accumulation. But by 10am all precipitation will be rain. Highs Tuesday will be in the 40s statewide. Lows Tuesday will be in the mid to upper 30s. By late Tuesday evening rain amounts will be around .25inches statewide. Continuing on Wednesday rain will fall across the state with temperature rising into the mid 50s statewide as the warm front moves north into New York State. The cold front will be approaching from the west. Wednesday night temperatures will fall after the front moves through and any remaining precipitation turns to light snow showers. The Wednesday portion of the storm will amount to another .3inches of rain. For Thursday highs will be slightly cooler in the mid 40s with a few leftover rain/snow showers mostly gone by afternoon. Lows Thursday night will drop down in the 20s. For Friday highs will be in the mid 40s and lows in the upper 20s with areas of perfect sunshine and blue sky. Saturday again will be sunny with below normal temperatures. For Easter Sunday, clouds will be moving in for later in the day. Highs will be cooler in the 30s statewide. Those temperatures are well below normal. Then by Sunday night another possible major storm moves into the region. See more information in long term outlook.
Quick Weekly Weather Updates...
March 17 update- As for Tuesday there may be some areas of light snow in the morning for central and eastern areas. Central areas may see 1-3inches of snow and eastern areas may see C-2inches of snow. By later in the day Tuesday precipitation should change to mostly rain. As for the long term update the GFS has lost the storm for next week, but the accurate EURO shows a storm affecting areas up to the PA turnpike with snow. Also there are now chances of a moderate clipper system bringing snow the day before the potential big storm. More on the clipper and other storm tomorrow.
Quick long term pattern outlook...
I am not one to hype a storm, but I see several factors that are coming together for a potential memorable end of March. First off let me talk about teleconnections. The NAO is forecast to go negative and around -1 during the March 18 to March 29 timeframe. This is favorable for east coast storm development and troughing. Also it is favorible for Greenland blocking. For the PNA, which is not favorable, shows it being negative in the same time frame at around -1. This would not favor east coast troughness, but it does not prevent it. When we have postive PNAs then there is usually a western ridge. The AO is supposed to dive negative to near the -2 mark which favors cold Canadian air to dive south into the United States. As for the long term models the major foreign models are inagreement between the GFS and EURO showing the storm. The EURO has forecasted almost everystorm correctly this year even a week before and the model proves to be very consistent. The CMC model also shows a storm along the east coast. The JMA shows a low heading straight out to sea and the DGEX shows a slightly inland runner. But every model shows a storm which is good. As far as the jet stream and high pressure positions, the jet stream is in perfect position for a eastern coastal storm...
And as for high pressure positions we have the perfect triple high locations with one to north of system, one to southwest of system, and one to northeast of system.
Overall I believe if all of these factors come together like they are being shown now, a major snowstorm could be headed our way all along the east coast. Now again this is all just a possibility and this year we have seen alot of coastal storms predicted a week out and trend west. But now this is the first time all of the factors are coming together and could produce the winter storm we have been longing for. I will have updates throughout the week. Stay tuned!!!
Tornadoes are natures most destructive force and storm on Earth. Swirling masses of dust and debris can cause death and destruction in places they hit. In the United States alone over 1000 tornadoes are reported each year, making the United States the country with the most tornadoes reported each year. From Texas to Nebraska stretches tornado alley where major tornadoes form each year. But not just in the midwest do tornadoes occur. Here in the state of Pennsylvania around 11tornadoes occur each year, but in the past decade that average rose to 22tornadoes on average in that decade. The most damaging tornado outbreaks in the state were during 1865, 1896, 1944, 1985, and 1998. Tornadoes are rated on a scale called the Fujita Scale, but recently it has been updated and revised.
Original F Scale...Enhanced F Scale
F0 45-78............EF0 65-85 F0 (Gale)
F1 79-117...........EF1 86-110
F2 118-161..........EF2 111-135
F3 162-209..........EF3 136-165
F4 210-261..........EF4 166-200
F5 262-317..........EF5 >200
Weaker cousins of full blown tornadoes are called dust devils. They are not actual tornadoes as dust devils form from the ground up and tornadoes form from the clouds down. Dust devils can be seen many times throughout the year with swirling leaves are litter. But sometimes dustdevils can be stronger with winds up to 55mph and cause some minor damage. Dust devils typically form on hot, sunny days when the heat rises and mixes with the cooler air in the upper atmosphere. A recent case of a destructive dust devil occured on April 26, 1987 when outside of Williamsport, PA a dust devil lifted a truck off the ground a pealed a roof of a shed. The dust devil tracked over a 300ft path.
In Pennsylvania to see tornadoes the perfect conditions have to occur for tornadoes to form. The main cause of tornadoes is wind shear in which winds in increase in altitude and change direction in altitude. Most outbreaks in Pennsylvania occur when a northwesterly wind flow interacts with a southernly subtropical flow. Also the jet stream must contain strong winds and have a low altitude. When updrafts from thunderstorms occur and tap into these conditions then tornadoes can form. In Pennsylvania tornadoes are commonly seen in several regions of the state...
Tornadoes are not perfectly understood in the way them form and develop. But with new technology tornadoes are being forecasted with more accuracy. Though in Pennsylvania, tornadoes are especially difficult to detect as in this state we typically do not see the perfect supercell. So on radar Pennsylvania thunderstorms are hard to pick out to see if indeed the cell has rotation and tornado capability.
Some very odd occurences have happened with tornadoes and here in this blog I hope to share some interesting Pennsylvania tornado outbreaks along with charts and graphs. I hope everyone who reads this may also share their stories of severe thunderstorms and tornadoes.
Pennsylvania interesting facts...
Pennsylvania storm stories...
My eyewitness accounts...
I personally have never seen a tornado. But I have seen a funnel cloud and a water spout. On a trip along the New Jersey coast strong thunderstorms were developing in eastern Pennsylvania and moved east all the way to the coastline. But due to the high heat and humidity that day the thunderstorms became stronger and stronger. They were also very slow moving. As they approached the county we were in, flash flood warnings and tornado warnings were issued. Ahead of the storm was the gust front which was quite impressive with my handheld anemometer measuring a near 40mph gust. Then the rain and thunder started. During the end of the thunderstorm lightning appeared to have struck the beach right close to where we were located. And as the thunderstorm departed out into the ocean I ran up on the beach to look at the cloud formations and I still remembered there was a tornado warning out. And as I looked for any cloud rotation I did notice a weak funnel cloud and it developed into a very, very weak water spout over the water. A water spout is a weaker version of a tornado and it occurs over water. But within seconds it dissipated. As far as here in southcentral Pennsylvania I have yet to witness a funnel cloud or tornado, but while I was in Cancun on vacation last summer 2repeated days of severe thunderstorms occured with reports of a funnel cloud each day located over my hometown. What are the odds of funnel clouds occuring during the week I was gone, lol. Other than the waterspout I have yet to seen a full blown twister.
Forecasting storm techniques...
Understanding the forecast methods for forecasting tornadoes is a very difficult concept to grasp. But as an example I am going to use the December 1, 2006 tornado outbreak in Pennsylvania as my examples on hints for forecasting possible tornado outbreaks. First off and most important is let us look at a surface map for that day.
(Courtesy of NOAA)
Notice on the surface map key positions of low pressures and fronts. Notice the warm front north of Pennsylvania drawing mild air north. In fact on Dec. 1 many alltime highs for December were broken. Notice the pressure of the low also.
After that atmospheric soundings are important to look at for finding information on the layers of atmosphere, including freezing level, jet stream winds and altitude winds/direction, and many other observations. Here are the Pittsburgh NWS soundings from a weather ballon for Dec. 1...
(Courtesy of NOAA)
Now that sounding chart may look overwhelming at first. But a closer inspection of it only requires some basic forecasting skills. Notice on the bigger graph the right hand side wind arrows and direction arrows. For each altitude elevation notice that the numbers go up on the rightside key and notice that wind arrows increase with elevation. This shows how fast winds are aloft. And notice for Dec. 1 they are at over 125mph at only 16,000ft aloft. Basic knowledge of wind arrows comes in handy. Also notice in the lists below of information that the freezing level is only 11,065ft.
Then after looking at current observations it is also important to look at past history of storms associated with the cold front or low pressure. Here is the storm reports from Nov. 30, 2006 the day before the outbreak in Pennsylvania...
Notice there were many severe weather reports in the Ohio Valley with a tornado report in Alabama from the front.
Another key is to look at the speed of the movement of the storms. During Dec. 1, 2006 they were moving at near 80mph. So alot of shear was taking place this day to create the tornadoes. Remember above I said shear is important to development of tornadoes. Also there was a sharp temperature contrast in which behind the front temperatures were in the 20s and ahead of the front they were in the 70s. Sharp contrasts such as that is what creates our storms.
So now storms are moving into western Pennsylvania and rapidly intensifing under the high moisture content and heat in the state. Now forecasting turns to nowcasting in which every moment counts to predicting severe weather. The convection thunderstorms form now into one squall line with several bowing segments. The storm prediction center in Norman, OK begins to issue watches...
Then a tornado is reported in Westmoreland County as an EF1 with damage to local Greensburg hospital and large tree damage. As the squall begin to rapidly intensify those earlier 125mph jet stream winds begin to mix down to the surface as thunderstorm tops reach 30,000ft. Then wind damage reports go out all across western Pennsylvania. The storms continue to proceed at over 80mph eastward into eastern Pennsylvania where the mildest and most moist air awaits for thunderstorms to explode. But then all of a sudden the main part of the squall slides up into northeastern Pennsylvania where an EF2 tornado is reported in Fairview Heights, Luzerne County. Many trees and homes blow over. But then thunderstorms begin to rapidly develop in southcentral Pennsylvania in the Lower Susquehanna Valley. They blew up moving into Cumberland, Perry, and Franklin Counties with a new squall line. Signs of bowing segments were occuring with a few of the thunderstorms. Then as the storms moved into Dauphin County, an EF-1 tornado occured in Halifax where 1fatalilty was reported due to a falling tree. Here is a close up radar, lightning strike, and satellite map of the storm in Dauphin County...
(Courtesy of NOAA)
It is important to look for a few key components to detecting rotation in thunderstorms. Some storms have a hook radar image in where an obvious hooklike radar feature is on the southern end of a supercell. But in this case with a squall line hook features are not evident. But by looking at the radar velocity images there is a clear area of rotation that heads up through Halifax, PA. It is shown by an area of tightly compacted KTS. The KTS scale is used in velocity radars instead of typical radars where the DBZ scale is used. There tightly compacted region on the velocity radar is where winds are the strongest and go against another highly compacted area of KTS. This is where there is wind shear, or rotation. Looking for these areas of conflicting high KTS colors is where there may be areas of rotation.
Overall December 1, 2006 was a record breaking day for the state of Pennsylvania with over 65 reports of severe weather in the state of Pennsylvania making it the largest December severe weather outbreak ever. Also many alltime record highs were broken for the entire month of December. And for the first time ever Tornadoes were reported in Pennsylvania during the month of March. Here is a final storm report map from the NWS...
Hopefully my case discussion of the December 1, 2006 severe weather outbreak provided some insight and helpful hints when forecasting tornadoes in Pennsylvania. There are many other ways of forecasting tornadoes, but I wanted to highlight the most basic and easy methods. Awareness and preparation are two key components to saving innocent lives from tornadoes.
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