Foot's Forecast | Weather Intelligence

"We have a situation."

- Major Grant played by actor John Amos in Diehard 2

6:10 PM 12-17-09 UPDATE NATIONAL WEATHER SERVICE OFFICES IN STERLING, VA AND MOUNT HOLLY, PA HAVE ISSUED A WINTER STORM WATCH... FROM LATE FRIDAY NIGHT THROUGH SATURDAY NIGHT FOR ALL OF MARYLAND, INCLUDING THE EASTERN SHORE.


ACCUMULATIONS: HEAVY SNOW GREATER THAN 5 INCHES.

TIMING: SNOW BEGINS FRIDAY NIGHT, CONTINUES TO SUNDAY AM.  

TEMPERATURES: UPPER 20S TO LOWER 30S. 

WINDS: 10 - 20 MPH, GUSTS 25 - 30 MPH SATURDAY AND SUNDAY.

1:30 PM THU 12-17-09 UPDATE  The Mid-Atlantic region is less than 36 hours away from what may be the biggest snow fall since February 2003. Most computer models this afternoon all shifted the eventual track of the storm more north and west as it travels up the eastern seaboard Saturday. Most of the computer guidance projected 12" or more for much of Maryland and Northern Virginia.

The European and NAM (North American Mesoscale) models project a storm track much farther north, with accumulations of 12" reaching Southern and Southeastern PA. Winter Storm Watches are expected this afternoon for the Baltimore-Washington metro areas and perhaps for the Philadelphia metro region as well. Residents across Maryland and southeren Pennsylvania are likely to receive 8" or greater with many in excess of 12". So get the shovels and snow blowers ready folks, like the quote from Home Alone: "This is it, don't get scared now."

6:00 AM THU 12-17-09   THE SOLSTICE STORM OF 2009: Analysis and data from the forecast and student climate teams signal a significant to historic winter storm will impact the Mid-Atlantic region this weekend. FINAL ACCUMULATIONS BY SUNDAY NIGHT 12-20 MAY EXCEED 12 inches in metro areas of Washington and Baltimore. This MAY impact school schedules on Monday 12-21. We are not making this up, and our PRE-STORM SNOW TOTALS will be released FRIDAY NIGHT. Amounts further north and west will be included, along with supporting links to quantify this forecast.

HARD DATA & EASY CONCLUSIONS

- A retreating NAO signal (-2 backing to -1) will permit the coastal storm to "hug and ride" the coast similar to the January 1996 event instead of escaping to sea.

- Last month's El Nino temperature pulse further energized the subtropical jet.

- Snowcover in Canada will stabilize a surface high over Ontario so it can dominate the flow, slowing forward motion of the storm and sharpening the pressure gradient.

THE TEAM RATIONALE

1. STUDENT FORECASTERS (5th* and 9th* grade participants) correctly identified that simultaneous changes in the NAO, AO and PNA will be the trigger point for development of this storm. Students also noted the recent rise in El Nino sea surface temperatures, notably region 3.4 in the central Pacific which has further enhanced the moisture stream from the subtropical jet. Combine those two factors with recent snowcover increase, and they felt we had a winner.
*Ms. Gerst's class at Baltimore County's Perry Hall Elementary
*Ms. Abrahm's classes at Mount Saint Joseph's High School in Baltimore

2. THE FORECAST TEAM ANALYSIS: states that alignment of these three indicators has created an favorable level "long-wave" pattern: Ridge in the west, trough in the east. This arrangement will enable one or more upper level "shortwaves" to dig under a 500 mb low over the Great Lakes. This shortwave is forced toward the Gulf coast where it can energize copious El-Nino enhanced subtropical moisture associated with a coastal surface Low. Interaction of these two entities may produce a "closed" upper level Low which will further amplify the eastern trough as the surface low travels the East coast.

WHAT YOUR FORECAST TEAM WILL BE DOING

• Snowlover, our Winter Storm Analyst, will re-examine the data this evening and begin tracking / reporting evidence of the upper level shortwaves at 850, 700 and 500 millibars.
• Mr. B, our PSU student meteorologist, will be monitoring indicators and model output, refreshing this post throughout the day with supporting links.
• Winterman, our Central and Western Maryland Nowcaster, will be using the "storm grade accumulation" technique to develop pinpoint amounts for locations in that region. Please know that specific snow targets cannot be reasonably identified until 24 hours prior.
• PasadenaMatt, our Bay Region Nowcaster,will be monitoring public safety channels for insights on storm preparation, and examining how storm outcome may vary between the west and east sides of the Chespeake Bay.

Note: If you are interested in joining our team, and have specific skills or background to contribute, once the storm is underway, make a request in the comments.

3. MR. FOOT'S ANALYSIS: Placement of the surface high, 2 meter and 5,000 foot temperatures are crucial to accumulation outcome. A likely "back-off" of the NAO from -2 to -1 by 12/20 will enable the center of the Canadian surface high to also slightly back north. Even so, stale cold air will continue to bleed out from the high in front of the storm. This was a key factor that enhanced snow totals in the President's Day 2003 Snowstorm. Although the storm type and track is completely different, placement and pressure gradient contrast between the high and low may yield a similar outcome. This is not hype-casting, but to make clear the details which could produce an historic event.

ACCOUNTABILITY DISCLAIMER: All readers should know that our team approach and collaborative efforts across two states means we will all hold each other accountable to the forecast. If we are wrong or right on this, we will use the same reliable, public data to grade our work for everyone to see. It is a guarantee we will not "explain away" the outcome to make it look like we were right all along, that is not what real scientists do.

PRECISE SNOW TOTALS NOT AVAILABLE UNTIL FRIDAY NIGHT. All members of the forecast team have daytime responsibilities. Questions about the situation are welcome in the comments, but specific requests of "how much for Philly" cannot be answered accurately at this point. Our team members can provide general ideas on storm track and amounts. Given the "epic" potential of our "situation" we'd prefer to wait until we can slam dunk it once and for all. Are you down with that?