Wednesday, June 12, 2013

"If you knew then, what you know now..."

NOAA Storm Prediction Center 
Day 2 Severe Weather Outlook

6:35 PM EDT 6/12 (Mid-Atlantic Severe Weather Team) Concern is rising among our team and in the meteorological community for a significant severe weather event on Thursday across central and eastern portions of the Mid-Atlantic. For today, an isolated threat is present for brief periods of severe weather from late this afternoon into the overnight hours along the I-81 corridor in MD, WV and VA. 

SYNOPSIS: The NOAA Storm Prediction Center currently projects a considerably high 45% probability of severe weather for a large, heavily populated part of the Mid-Atlantic. 

  • This scenario would contain strong damaging winds, hail and possibly tornadoes in some areas, as indicated on this map and linked below. 
  • Given that the event is roughly 24 hours out, it is too early to tell what precise areas will receive potentially damaging weather, however, we can say that when NOAA postes a 45% probability this early, it is a very serious concern for those in public safety, infrastructure, and the public in general.

Widely Feared But Not Understood

June 29th, 2012 Derecho Radar - From NWS
11:00 PM 6/11/13 
June 29th, 2012 was a day that produced weather that will not soon be forgotten by those in the Mid-Atlantic region. For those unfamiliar, that was the day of last year’s infamous “derecho”. Now, there are discussions of another possible derecho heading for the Mid-Atlantic and naturally, people are fearing a similar situation to June 29th, 2012, where people were without power for days or weeks. However, we should really try to understand what a derecho is before we start discussing any future possibilities.

DEFINITION – The first thing to understand is that a Derecho is not a special type of storm. It is fundamentally no different than a regular squall line or bow echo. However, it’s strength and duration gives it a separate classification. In order to be considered a derecho, a storm or line of storms must produce wind damage from winds in excess of 58 mph along most of length for more than 240 miles along its path.

Severe Weather reports from June 2012 Derecho - From SPC
Derecho’s are often classified as such after the fact once the damage is surveyed and analyses are done. The June 29th, 2012 event was an exceptionally strong event produced by relatively rare conditions that will be discussed below. Wind damage was sustained from the storm all the way from Chicago to the Atlantic Ocean.

CAUSES – A derecho producing storm usually starts out as a Mesoscale Convective System (MCS). An MCS is simply an organized complex of thunderstorms on a larger scale than just the individual thunderstorm cells. Sometimes these can form squall lines or bow echos which typically are the most likely candidates for derecho storms. The storm is first indicated as an MCS, but as it strengthens, may be considered a derecho.

Saturday, June 1, 2013

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