Tuesday, November 29, 2005
What we have here, Tuesday, is a failure to illuminate
A. Camden Walker @ 2:02 AM
Forecast
Today gets a very poor grade for aesthetically pleasing weather. Take the umbrella! We'll be muggy, DAMP, and socked in with clouds. Morning temperatures in the lower 60's will creep into an afternoon range of 64-70. Breezes & thundershowers look probable this afternoon, however nothing will be severe. A few spots could see an inch of rain. And a second, larger swath of rain will develop into the overnight hours.
'Tis the Season: For highly variable temperatures
*Why is the weather so WACKY this time of year?! *
I get this question in late Autumn or late Spring more often than I like to admit. We live in the mid-latitudes, first of all, where in EVERY season there is a battle between Warm, tropical air near the Tropic of Cancer & Cold, polar air based around the Arctic Circle. In Winter, the polar air wins. In the Summer, the Jet Stream is pushed into Canada as more equatorially-based air masses win the battle. However, we've got those pesky two seasons in between: Spring & Autumn! What is also pesky? Math. Popular usage makes the terms "average" and "normal" temperatures, interchangeable. All we have is 150+ years of temperature records from which to base mathematical average extrapolations into the present day. Therefore, if you can, throw out this idea of "normal" and swap it for "average." OK, now back to our science concept du jour!
What is a Hemisphere seeking equal distribution of heat energy supposed to do? Produce storms & speed up the Jet Stream! We have much more of a temperature gradient in our hemisphere during these transition seasons. Average temperatures fall about 3 degrees per week! We don't settle into our low January average high (of 42 degrees)from our July average high (of 88) all-too-easily mind you. Generally, according to long-standing averaging of DC temperature records, temperatures on November 29 are "supposed" to top out around 52. We were struggling to reach the upper 30s a few days ago--now our afternoon temperature regime focuses around the upper 60s. Guess what happens if one takes a temperature of 38 and averages it with a temperature of 68? 38+68=106/2(days)= 53. Does displaying this "averaging" technique--using recent actual highs--quiet some of your seasonal anxiety? I hope a little. While I will admit to our "temperature spread" ABOVE & BELOW our average high being extremely wide this time of year, these calculated averages are worthy anchors illustrating the clear, downward vacillation toward January's chill.
The rapid shifts in the Jet Stream this time of the year really are normal. A trough and then a separate ridge can dominate our weather pattern in two successive days! And more rapid-fire oscillations can be expected as solar energy quickly diminishes with our hemisphere's seasonal tilt AWAY from the Sun. The rate of decrease is fairly staggering. The atmosphere is trying to respond and distribute what is left--and sometimes does not seem all-too-steady to us Washingtonians here on our spot on the globe, eh? Just to offer you a depressing benchmark easily linked to average solar energy reaching our latitude today: Day length is 9hr 41min. We have 15 minutes of daylight yet to lose before the winter solstice!
With the absence of heat content in our hemisphere, eventually, yes, the Polar Air to our north will dip further south into our region. But the amazing heat-distribution engine known as the Jet Stream refuses to give up... spreading the remaining solar energy as long as it can. This fast moving ribbon of air, almost 10 miles above us, is in large part why our atmosphere has lag-time between minimum solar energy entering the Northern Hemisphere on December 21.. and our coldest average high temperature of 42 in the second week of January! And if the Jet Stream misses distribution along some point in the hemisphere? The area of unbalanced energy will result in a cyclone. Warmer air thus spirals up toward the Arctic and colder air plunges south toward the equator.
Watching Water: Extensive Chesapeake Bay Watershed Requires Monitoring
A good run-down of the costs & needs of monitoring the flood prone Susquehanna River valley appeared in the Baltimore Sun (seen via Maryland Weather blog). The Susquehanna is the largest river basin within the complex Chesapeake Bay watershed.
A large watershed such as that which belongs to the Bay, has an intricate nexus of streams & rivers feeding into it--many are too small to handle heavy rains. Read, also, about flood-prone Johnstown PA. The biggest floods occurred in this mountainous town in 1889 & 1977. Johnstown sits at the western border of the Bay watershed--and is the archetypical reason a full government-funded hydrological monitoring system is needed. This mountainous area often receives a moderate amount of rain but small-order streams unable to handle the quick run-off descending the Appalachian Mountains--as the water rushes south & east toward the lower elevation of the Bay--overruns embankments until ENTIRE VALLEYS are utilized for water flow between mountain ridges. Disaster.
'Tis the Season: For highly variable temperatures
*Why is the weather so WACKY this time of year?! *
I get this question in late Autumn or late Spring more often than I like to admit. We live in the mid-latitudes, first of all, where in EVERY season there is a battle between Warm, tropical air near the Tropic of Cancer & Cold, polar air based around the Arctic Circle. In Winter, the polar air wins. In the Summer, the Jet Stream is pushed into Canada as more equatorially-based air masses win the battle. However, we've got those pesky two seasons in between: Spring & Autumn! What is also pesky? Math. Popular usage makes the terms "average" and "normal" temperatures, interchangeable. All we have is 150+ years of temperature records from which to base mathematical average extrapolations into the present day. Therefore, if you can, throw out this idea of "normal" and swap it for "average." OK, now back to our science concept du jour!
What is a Hemisphere seeking equal distribution of heat energy supposed to do? Produce storms & speed up the Jet Stream! We have much more of a temperature gradient in our hemisphere during these transition seasons. Average temperatures fall about 3 degrees per week! We don't settle into our low January average high (of 42 degrees)from our July average high (of 88) all-too-easily mind you. Generally, according to long-standing averaging of DC temperature records, temperatures on November 29 are "supposed" to top out around 52. We were struggling to reach the upper 30s a few days ago--now our afternoon temperature regime focuses around the upper 60s. Guess what happens if one takes a temperature of 38 and averages it with a temperature of 68? 38+68=106/2(days)= 53. Does displaying this "averaging" technique--using recent actual highs--quiet some of your seasonal anxiety? I hope a little. While I will admit to our "temperature spread" ABOVE & BELOW our average high being extremely wide this time of year, these calculated averages are worthy anchors illustrating the clear, downward vacillation toward January's chill.
The rapid shifts in the Jet Stream this time of the year really are normal. A trough and then a separate ridge can dominate our weather pattern in two successive days! And more rapid-fire oscillations can be expected as solar energy quickly diminishes with our hemisphere's seasonal tilt AWAY from the Sun. The rate of decrease is fairly staggering. The atmosphere is trying to respond and distribute what is left--and sometimes does not seem all-too-steady to us Washingtonians here on our spot on the globe, eh? Just to offer you a depressing benchmark easily linked to average solar energy reaching our latitude today: Day length is 9hr 41min. We have 15 minutes of daylight yet to lose before the winter solstice!
With the absence of heat content in our hemisphere, eventually, yes, the Polar Air to our north will dip further south into our region. But the amazing heat-distribution engine known as the Jet Stream refuses to give up... spreading the remaining solar energy as long as it can. This fast moving ribbon of air, almost 10 miles above us, is in large part why our atmosphere has lag-time between minimum solar energy entering the Northern Hemisphere on December 21.. and our coldest average high temperature of 42 in the second week of January! And if the Jet Stream misses distribution along some point in the hemisphere? The area of unbalanced energy will result in a cyclone. Warmer air thus spirals up toward the Arctic and colder air plunges south toward the equator.
Watching Water: Extensive Chesapeake Bay Watershed Requires Monitoring
A good run-down of the costs & needs of monitoring the flood prone Susquehanna River valley appeared in the Baltimore Sun (seen via Maryland Weather blog). The Susquehanna is the largest river basin within the complex Chesapeake Bay watershed.
A large watershed such as that which belongs to the Bay, has an intricate nexus of streams & rivers feeding into it--many are too small to handle heavy rains. Read, also, about flood-prone Johnstown PA. The biggest floods occurred in this mountainous town in 1889 & 1977. Johnstown sits at the western border of the Bay watershed--and is the archetypical reason a full government-funded hydrological monitoring system is needed. This mountainous area often receives a moderate amount of rain but small-order streams unable to handle the quick run-off descending the Appalachian Mountains--as the water rushes south & east toward the lower elevation of the Bay--overruns embankments until ENTIRE VALLEYS are utilized for water flow between mountain ridges. Disaster.Snow Lover's Crystal Ball
25% Chance of Snow Saturday PM;
20% Chance of Snow starting dawn Monday
25% Chance of Snow Saturday PM;
20% Chance of Snow starting dawn Monday
Potential Impact: 
Sat.PM; Monday
Commentary: A nuisance, advection event Saturday evening--hopefully at least scenic; Very Early--and into--Monday, a more significant event seems possible in one long-range model.
Sat.PM; MondayCommentary: A nuisance, advection event Saturday evening--hopefully at least scenic; Very Early--and into--Monday, a more significant event seems possible in one long-range model.
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