Sunday, November 29, 2009

"...Make that change."
from Michael Jackson's 1988 single Man in the Mirror

6:00 AM TUESDAY 12-1-09  [posted 11/28, rev. 11/29, 30] Outline of this week's forecast ideas for early December across the Mid-Atlantic:

1. PATTERN-CHANGING STORM.  A system will develop along the Gulf coast early this week and move to the eastern Great Lakes by Thursday 12/3. The season's first pulse of multi-day cold arrives behind this storm for the Friday 12/4 - Tuesday 12/8 period. (Evidence: 7pm 11-30 European weekly loop) Significant winter weather in the Mid-Atlantic appears likely in this time frame, including  a possible coastal event Saturday 12/5 as originally projected from 11/11/09.

2. STUDENT CLIMATE DATA REPORT. Ms. Gerst's 5th grade students at Perry Hall Elementary in Baltimore County  report the following data for 11/30-12/5:
-North Atlantic Oscillation: Currently neutral, trending to negative (-1) by ~12/6;
-Arctic Oscillation: Currently negative, trending to -2 by ~12/6;
-Pacific North American Index: Remaining positive at or near +1 this week;
-El Nino: Sea surface temp anomalies continue averaging 1.0 to 2.0 C+ above normal. The class also stated that "NOAA calls for a strengthening El Nino with colder and wetter conditions for the Eastern U.S. this winter."
-Snow cover: Building in western Canada and the U.S. Rocky Mountains.

Changes in the "Key 3" atmospheric indicators (NAO, AO, PNA) suggests upper air patterns from the North Pacific to the North Atlantic will be briefly conducive for development of coastal winter storms. A reserve of cold air continues to build in western Canada, supporting recovery of snowpack which can further enhance downstream cold in the U.S. Evidence: 7pm 11-30 GFS 500 millibar projection for 1 pm Sat 12/5

3. STUDENT CLIMATE DATA ANALYSIS. Ms. Abrahm's 9th grade Science classes at Mount Saint Joseph's High in Baltimore City reported similar atmospheric data (negative NAO, positive PNA) but have made this crucial projection:
-"Temperature will be a critical factor this week...2 meter temps should keep any snow/frozen precip at higher elevations, while the coast would receive nasty, messy rain." Students are waiting to see consistent trends in the indicator data before making a specific call on snow potential late this week.
-Support from computer models: Considering the 114 hour GFS 2 meter temp map for 1PM Sat 12/5, students are anticipating boundary layer temps will be above 32 F. This suggests any snow falling along the coastal plain will be wet, not powdery. 

4. ACCOUNTABILITY: Revisiting the original "winter onset" hypotheses published on this site on 10/10, 10/30, 11/10 to determine if the weather pattern outcome matched the predicted sequence of events. Recall the overall forecast concept projected "significant winter weather" in the Mid-Atlantic by 12/5. Quantitative accountability definitions will include:
- "Significant temperatures" will be considered a regime of at least 5 degrees below normal for no less than 48 hours;
- "Significant precipitation" implies a single or multi-day snowfall that produces up to 4" for duration of the event, as measured at the Baltimore-Washington International Airport.
- Grading system: A loss of 2% per day (1% per criteria) for each day the above stated conditions are not observed at BWI airport.

5. ANY FINALLY...WHAT IS IT ABOUT DECEMBER 5TH? Many weather watchers in the Mid-Atlantic have noticed the trend in recent years that this now-fabled date has coincided with some type of measureable snow event in the I-95 corridor and interior. For the best read on the December 5 phenomenon in the Baltimore region, I dutifully direct you to this excellent report by weather reporter Frank Roylance of the Baltimore Sun.

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