Thursday, December 17, 2009

Y'all ready for this?"
- by 2 Unlimited in their 1991 single Get Ready For This


* NWS Winter Storm Watches & Warnings  12AM Sat to 6AM Sun for:
* Prelim snowfall for BWI airport by Sunday 6PM: 8.9 to 14.4
(calculated by 9th grade Pacific Community students at the BCPS Crossroads Center)
* Student data based on Thu 12-17-09 1 PM NAM model data
(liquid equiv. range of .70" - 1.2" x snow ratio of 12:1)
* Current precip projection for BWI airport (multiply by 12 = amt.)

FORECAST TEAM ROUNDTABLE: The winter storm analysis team collaborated last night at 8:30 PM on the developing situation. Answers to your questions include:

1. IS THIS REALLY GOING TO HAPPEN? There appears to be "no escape" route for the storm, now quickly developing across the Gulf coast (IR satellite loop) The current surface map reveals a set of high pressure systems settled in across New York and Ontario. Cold dry air is firmly in place ahead of the storm. This will provide the textbook setup for an historic Mid-Atlantic winter storm which resulting snow amounts at BWI airport may rival that of January 1996, January 2000 or even February 2003. Widespread school cancellations are possible for Monday 12-21 or beyond from Virginia to Maryland to Southern PA, the DelMarVa and New Jersey. 

2. WHEN WILL IT START? The first Big Kahuna of the 2009-2010 season will get underway between 9PM and midnight Friday across Maryland and Virginia, if not sooner. Saturday morning will afford powderhounds young and old alike the chance to walk in a winter wonderland until 20-25 mph winds and heavy snow bands arrive mid-day and continue overnight. The Harrisburg-Lancaster-Philadelphia metro region will see snow begin before sunrise Saturday.

3. HOW BAD COULD IT BE? Saturday night, near-blizzard conditions are possible along the Chesapeake Bay and along the Baltimore-Washington-Richmond corridor. Winds may reach or exceed 35 mph at height of the storm, reducing visibility to 1/4 mile or less. As the coastal low deepens off the Virginia capes, snowfall rates overnight may exceed 2 inches per hour at times. Strong upward motion off the Chesapeake Bay may produce brief periods of "thundersnow" and lightning along the I-97 corridor. Significant tidal flooding along the western Bay will occur due to an easterly fetch of 20 mph+ winds over a 12-24 hour period. Extensive beach erosion along Delmarva coastal communities is likely. 

4. HOW MUCH SNOW? Ensemble computer model output, the map shown above, are averages of multiple projections. These maps continue to show a minimum of 1.25" liquid equivalent for most of the Chesapeake Bay & associated metro regions. For those who value not having a sore back for weeks, this is a disturbing development. If the average, basic trend is 1.25" that means the actual precip amount could be 2.0" or greater. This map is for the 24 hour period ending 1AM Sunday, please view the source. By using our standard storm grade calculation, and selecting BWI as a baseline location, this still delivers a minimum of 12" if using the liquid-to-snow ratio of 10:1 (1.25" x 10 = 12.5"). A map showing how we calculate the totals:

More recent computer model runs have shown a possible 2.48" liquid for BWI. Even at the low end ratio of 6:1 for a sloppy wet snow, that is still 14.88 inches (2.48" x 6 = 14.88). The latest GFS computer model (00z 12-18-09) run shows a fairly extensive hit of 12"+ from Western North Carolina to New York City by 7AM Sunday morning.

5. WHAT HAPPENS NEXT? We surmise someone at NWS may be contemplating Blizzard Watches based on the wind speed and duration. You may see additional posts today provided by our Central PA Nowcaster Mr. B. Our forecast team will have another roundtable tonight and release our final storm grade amounts by 10:00 PM. Special thanks to the hard work last night by Snowlover, Winterman, Mr. B and PasadenaMatt for hammering out all the details our readers rely upon.

PREVIOUS UPDATE 7:00 PM 12-17-09

* NWS WINTER STORM WATCHES are in effect for all of Maryland until Saturday night. Watches are likely to be raised to Warnings by Thursday evening or Friday morning.
* Our team retains this morning's call that amounts in the Baltimore-Washington metro area, central and western Maryland MAY EXCEED 12 inches by Sunday night.
* Respected meteorologists whom the forecast team follow as part of their research are indicating potential for blizzard-like conditions in the I-95 corridor from DC to PHL.
* Parents, teachers and school administrators should be aware this situation could negatively impact the school calendar for all of next week.

The Stormcast Team is currently having dinner in their respective homes and reloading for the gathering gale ahead. Our evening roundtable between 8:00 and 9:00 will establish the storm timeline, impacts and preliminary accumulation ideas. In preparation for the storm grade amounts, Mr. Foot's 9th grade science students at the Baltimore County School's Crossroads Center today conducted their "liquid equivalent/snow ratio calculations" for BWI airport as a baseline.

OUR FINAL WORDS: Is your vehicle fueled up? Homeowners, do you have a stake to post near the sump pump outlet or storm drains, in case you need to find them later? Are your gutters clear of debris? Got that shovel with the long handle that bends in the middle? What are the daycare plans for next week if school is closed? Yes, we hear you, as if there wasn't alreeady enough to do this time of year!

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