Wednesday, February 24, 2010

Why it was called a "snowicane"

10:45 PM FRIDAY 2.26.2010  The retrograding low pressure center between the New York City harbor and Sandy Hook, NJ border will continue to fill in and "snow itself out" well into Saturday. At approximately 990 millbars and winds still gusting near 30 mph, this storm will remain capable of delivering copious moisture from the Atlantic Ocean, in the form of snow. Examples of some incredible snowfall reports include 32.0 inches in Harriman, Orange County, NY and 28.0 in West Milford, Passaic County NJ. New York NWS snowfall report

This weekend, we are assembling  an "after action report" on the current storm and will be posting our data for the Mid-Atlantic and Northeast. Obviously some of the scores will not be pleasant, but it sure will be a good data set to analyze! Results will be posted in the Storm Data Zone. A new post will be published Saturday as we begin the next round of preparations in advance of another system to affect the eastern U.S. by Wednesday.

10:15 AM FRI 2.26.2010 
This storm having retrograded from the Atlantic ocean into Long Island and the New York City harbor, and as of 4AM had a central low pressure of 973 millibars, was stronger than Hurricane Isabel 12 hours following landfall (988 mb, 50 mph at 1AM on 9/19/2003).* Unlike previous storms which we all know move out to sea, this is retrograding and will take several days to fully dissipate. That is the basis behind those calling it a snowicane: This event has behaved more like a tropical system than a traditional coastal surface cyclone. *scroll to bottom of NHC report for pressures

REPORT FROM 7:15 AM Our data investigation suggests squalls of heavy snow will continue crossing Pennsylvania, New Jersey and Maryland until at least 10:00 AM this morning. As evidenced by the water vapor loop, pivot of the surface low into New York will force remnant moisture from the ocean as well as lake effect moisture to continue crossing the Mid-Atlantic north to south. Data indications include the 1AM GFS model liquid data for BWI airport, showing snow and wind continuing to 7pm tonight.

SHORTWAVE BACKGROUNDER  
Image: 5,000 foot level flow; 7PM 2-25-2010 as published in NWS Telecommunications Operations Center's Barotrophic Level Fax Charts.

EXPLANATION: This shows upper level shortwaves pivoting around the low center. Red line: short wave troughs; Blue: shortwave ridge. Click for a larger version.

The red shortwave line from NYC to Buffalo at 7PM last night was traveling south at 23 mph. Extrapolating the time and distance suggests the trailing edge of this is now in central PA and will enter the Baltimore metro area at 10:00 AM. The trailing edge of shortwaves in this dynamic situation will produce heaviest snow at bottom of the trough, where all remaining moisture is compressed in the final hour at passage. Test this hypothesis: Report your location and observations on snow intensity from 9:00 to 11:00 AM. You can clearly see evidence on radar now, the gaps between northwest to southeast snow bands represent the back edge of shortwave troughs.

This map is actual observation data generated by NWS radiosonde operators twice daily. These silent workers have done so twice daily for 60 years. Radiosonde, or "weather ballons" radio back observation ("RAOBS") to ground reporting stations. Data includes pressure and tendency, direction and intensity of upper level flow. The red and blue lines on the 850 mb map have to be manually drawn, but represent areas of shortwave instability were located at a fixed point in time. From this you can calculate when that energy will arrive overhead, hence touching off additional snow.

ABOUT SCHOOL TODAY  Consider these influencing factors:
(1) Sustained winds at 30-40 mph, gusts to 60 mph will be extremely problematic for high profile vehicles. Do observations suggest conditions will significantly improve in the next 1-2 hours?
(2) Poor visibility at times for commuters, walkers and bus drivers alike;
(3) Fine grained texture of the snow resembles January 30, when untreated roads led to hundreds of accidents across several states;
(4) Additional risk of weakened trees from recent storms, combined with strong gusty winds overhead, does not inspire confidence for parents whose children walk to school;
(5) With so many districts having lost so many school days, the pendulum must have temporarily swung in the other direction, favoring the way "things used to be" for those who remember going to school in the 1960's.

NEXT UPDATE ON OR BEFORE 2:00 pm THIS AFTERNOON


THE 411 FOR TONIGHT  (from 10PM and 2AM)
Eastern New York Heavy rain will encompass areas along I-87 up to Albany and even as far west as Syracuse into the overnight hours. The eastern extent of snow is expected along the Catskill Mountains. 
New Jersey - Eastern PA Heavy snow will continue overnight and well into the early morning hours. Additional accumulations of 6 or more inches are expected in these areas with localized amounts up to 12" more on of what has already fallen. 
Baltimore metro - Northeast MD Snow is developing this evening, and should be light and intermittent, accumulating less than an inch west of the Chesapeake Bay into the evening hours. Winds will increase to near 25 mph by evening with snow squalls overnight, reaching 40 mph by daybreak. 
DelMarVa Intermittent snow throughout the day will become heavier overnight as winds increase to 30-40 mph with gusts near 60 mph. If snow is added this, it would produce near blizzard conditions overnight into Friday morning.

MAP EFFECTIVE TIMES
Northeastern MD / DelMarVa:
12 PM Thursday to 12 PM Friday

Eastern PA - NJ - Eastern NY - New England:
6 AM Thursday to 12 PM Saturday

SNOWFALL SUMMARY
As outlined in our team's current map issued 1:30 PM Wed 2-24, by 12 PM Saturday snow will exceed 12 inches in eastern Pennsylvania, New Jersey, and eastern New York. The eastern shore may top 6 inches, and amounts should range from less than 2 inches in Washington to south central PA and Northern Virginia, to 3 or more inches in Baltimore metro area north of BWI, near 6 inches in northeast Maryland. 

NATIONAL WEATHER SERVICE Advisories/Watches/Warnings





WIND IMPACTS
 

Sustained winds will continue to be 30-35 mph with gusts of 50-60 mph from now till early afternoon, creating numerous power outages. High Wind Warnings in effect.



1. The flow is slow. Much of the atmosphere over the Eastern U.S. is in a holding, blocking pattern; this allows plenty of time for the storm to organize.
2. Phasing as it arrives. The slow flow permits multiple energy sources to phase or combine forces, with a significant clash of cold and warm air off the Jersey coast.
3. Nowhere to runAcross maritime northeast Canada, there is a "vertically stacked" surface and upper level high pressure ridge, which is highly unusual. Once the storm develops, it will become lodged between a cold front sweeping east, the high to our north and subtropical energy to the east.
4. Frictional effects. Our belief is that as the storm deepens, strong southerly flow on the right quadrants will be counteracted by weaker northerly flow in the left quadrants. A weakness may develop to west of the storm as a result of frictional influence with the land, much in the same way as occured with Hurricane Charley in 2004. Since the storm will have difficulty progressing north and east, it would retrograde west across New Jersey and into Pennsylvania, filling the vaccum created by the weaker westerly flow.

We remind readers that our forecast is presented in a hypothesis format. We then co-investigate the evidence which would support or reject the hypothesis. Your input or an alternative hypothesis is welcome. - Forecaster Foot    

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