CommuteCast: From Clear Skies to High Clouds
Damp and chilly start to weekend Saturday
Under mostly sunny skies, and with decreasing winds, high temperatures have reached the mid 50s across the area today. Clear skies will begin to give way to high cloudiness during the commute home, but it will remain dry through the evening.
Tonight: Clouds will thicken, with a 30% chance of sprinkles or showers after midnight as low pressure races towards the region from the west. The best chance of precipitation overnight will be north and west of D.C. Low temperatures will drop into the upper 30s in the northern and western suburbs, and around 40 for the city and southeast.
Tomorrow: Saturday will be cloudy and cool with a 50% chance of off-and-on sprinkles and showers as low pressure passes through. Highs will likely only reach the upper 40s for most locations, and a breeze from the north will add extra chill to the air. Precipitation should depart the region by evening, setting the stage for a cold late-March night with temperatures dipping to the low 30s.
See Camden's forecast for Easter Sunday and early next week, and Dan's post for the weekend events forecast.
By Ian Livingston |
March 21, 2008; 3:15 PM ET
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Posted by: El Bombo | March 21, 2008 3:46 PM
By the way, Tom Skilling was mentioning thundersnow with occasional 1- to 2-inch snowbursts in yesterday's 10 PM (9 PM Central) WGN newscast, but this was supposed to occur chiefly north and west of the Loop.
Posted by: El Bombo | March 21, 2008 3:50 PM
Looks like the coastal on Monday has shifted just "slightly" to the west. At least I am seeing that on the 18z Euro run.
Does anyone else see this?
Posted by: Conman | March 21, 2008 4:38 PM
Come someone please help me undersand more about Troughs and Phasing, I really would like to undersatand this post reply so I can follow the trough patterns on the maps next week. Thanks.
____________________________
Conman - I did respond and here is a truncated version of the full explanation from the late morning NWS Philadelphia office discussion:
THE GFS INITIALIZED THE SHORT
WAVE OF QUESTION PRETTY WELL SOUTH OF ALASKA, BUT THE DIFFERENCE
BETWEEN IT AND THE ECMWF (TU HPC) IS THAT THE ECMWF PHASES A SECOND
SHORT WAVE FROM CANADA WITH THAT SOUTH OF ALASKA SHORT WAVE OVER THE
MISSISSIPPI VALLEY WHILE THE GFS DOES NOT. THE FORMER PHASING
PERMITS THE LONG WAVE TROF TO SHARPEN FASTER AND BRING THE FORMING
LOW CLOSER TO THE COAST. THE DIFFERENCE IS SUBTLE WITH RESPECT TO
THE SHORT WAVES, BUT ITS AMAZING THE DIFFERENCE THAT MAKES IN THE
SURFACE REFLECTION. THE ALASKAN SHORT WAVE COMES ASHORE IN TIME FOR
THE 12Z RUN ON FRIDAY AND BY THEN THE SECOND SHORT WAVE WILL ALSO BE
IN CANADA. SO HOPEFULLY BY THAT MODEL RUN WE WILL HAVE BETTER HANDLE
ON WHAT TO OR NOT TO EXPECT. THERE REMAINS PLENTY TO WATCH WITH THIS
LOW OVER THE NEXT FEW DAYS SO STAY TUNED.
Posted by: Conman | March 21, 2008 5:04 PM
Conman - Here is a simplified explanation of phasing and troughs - Phasing usually means when two areas of low pressure (in this case the short waves) come together and form one deeper area of low pressure. This - because of the more intense and focused energy - will in turn amplify the long wave trough.
Now on to the meaning of trough. A long wave trough is an elongated line of low pressure. There are 5 to 6 in the northern hemisphere at any given moment. They are - as the name implies - long, usually between 1,500 and 2,000 miles in length. A short wave trough is usually a center of low pressure that is moving along the path of the long wave trough. The long wave troughs usually follow the jet stream. When there is an amplification in the long wave trough, it's axis will sink further south. On a weather map it shows as a dip south in the jet stream. So in the context of my last post, the mets were implying that if the two short waves did indeed phase (which does NOT seem likely) the long wave trough would deepen and "pull" the developing coastal low westward. Think of a piece of string, 3 feet long, pulled straight from left to right. (the string is the long wave trough). Now introduce a force that would pull the center of the length of string down (a phasing of the two low pressure areas). In turn, the left and right most points of the string would be pulled inward, towards the center. So the right (East) side of the end of the string is now closer to the "coast" and the developing coastal storm is now pulled westward toward the coast.
A good resource for information and definitions can be found here: http://www.theweatherprediction.com/habyhints2/457/
Hope this helps.
Posted by: PJ Mt Vernon | March 21, 2008 11:02 PM
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Canadian lightning update. One or two isolated strikes near Chicago, IL (THUNDERSNOW!!!). Otherwise quiet.
Weekend looks pretty good except for that rain tomorrow. Frankly I wonder if we'll see much south of D.C. Next week, okay for the most part with Sterling and Camp Springs leaning towards zonal flow. Perhaps we'll get milder as the trough fills up.