Sunday, December 6, 2009

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Significant enough for them


7:00 AM SUNDAY, 12-6-09 Whether you knew for days this storm was coming, or became suddenly aware Saturday morning when the first flakes arrived...there is always something special about the season's first snow. I surmise that many children and their parents shared similar joyful moments from the Gulf coast to New England. For powderhounds young and old alike, half the fun is knowing that for a few hours on one fabled fifth of December, millions are talking your word: Snow.  

From spontaneous family
portraits to capturing that perfect winter screensaver, to the detailed updates across the Mid-Atlantic (and even from Tennessee!) your fun storm stories are the cinnamon sugar sprinkles on sweet butter breakfast toast. Thanks to everyone for taking the time to revel together. And so it goes that we get to add yet another December 5 snowfall to Mother Nature's book of lore.

With another storm looming on the horizon, opportunities abound for us to do again what the poet Robert Frost might suggest from the Road Not Taken:  "I took the road less traveled by, and that has made all the difference."  The Tuesday night into Wednesday situation this week already looks very challenging, with the potential for a long duration period of freezing precipitation. For working parents with children, it would be a good use of time to revisit those alternate care plans in the event schools or your child care provider are delayed or unable to open. 

Saturday, December 5, 2009

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Another Fabled Fifth

12:00 PM Saturday 12-5-09  [updated from 8AM 12/5] At one point this morning, winter weather-related advisories and warnings extended nearly 1,500 miles from southern Louisiana to downeast Maine. Central Maryland and surrounding areas also has its fair share of advisories. It appears this year's "fabled fifth" will turn out to be an over-performer in many categories. 

For this site, the storm is being treated as a training exercise to test the original hypothesis from 11/01 that arrival of significant winter weather would occur in the Mid-Atlantic by 12/5. That long range projection of "significant" implied a snow accumulation of 4 inches or greater, following NWS terminology. The near term call made at 10:00 PM on 12/4 was for 2.4" at BWI airport for 12/5.

THE MAGIC OF COOLING: Despite the lack of a noticeable surface high pressure to our north, the process of dynamical cooling has enabled this storm to generate its own supply of cold air. Evidence can be observed on basic radar loops, for as the coastal low deepens, cold air is being drawn in from the north, resulting an eastward march of the low-level rain/snow line. The simple falling of precipitation through a cool layer creates the additional process of evaporative cooling. As the precip evaporates, it cools the surrounding air, creating a feedback loop which eventually reinforces additional conversion of moisture to snow.

COMMENTS & OBSERVATIONS: All readers are welcome to post their observations and include the reporting location. This gives all of us a good sense of ground truth. Of particular value will be your snow arrival time, full changeover time, the temperature and crystal type.

NOWCASTERS: The tracking links have been relocated for easier access. Anyone is welcome to recommend links to data sources we would find useful in a nowcast event.

Thursday, December 3, 2009

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Dec 5 Game Is On


Reader photo from the Houston Chronicle, Friday 12/4/2009

10:00 PM UPDATE Friday, 12-4-09  [revised from 12/3] Synopsis of forecast ideas for the Saturday 12/5 event: Marginal but improving boundary layer temperatures tonight and Saturday across the Mid-Atlantic will permit snow to fall on the fabled date of 12/5. Text of the current NWS Winter Weather Advisory for the Baltimore region spells out the timing in good detail. My official storm grade call for this will be 2.4" at BWI airport. Acccumulations east of I-95 between Richmond and Philadelphia will be enough for a pretty picture, but west of the cities there is increasing potential for amounts closer to the original call of 4" or more. The good news is that cool-to-seasonally cool temperatures follow the storm, not the "much below normal" regime originally forecasted.

NOWCASTING THE 12/5 KICKOFF STORM: For a precip timeline at BWI, check projections by the GFS and the NAM computer models. For the surface perspective, monitor 3 hour plots for the southeast and northeast, as well as 3-hour pressure falls and temperatures. A more comprehensive view of factors influencing this system can be observed from the water vapor loop, and by tracking upper level short-waves on the 850 mb / 700 mb / 500 mb charts. (note:  click save instead of open, and rotate the images) It would also be interesting to check our old friend the North Atlantic Oscillation. Any "westward trend" in the model projections may also appear right about now as a slight rise in the index, even if currently negative.

ACCOUNTABILITY NOTE: Outcome of this event will be graded according to criteria developed that scores accuracy of the original first call made on November 1, 2009, with followup analysis on 11/11/09 and 11/24/09.

LOOKING AHEAD: Next Wednesday and Thursday features a powerful cross-country storm producing mostly rain across the I-95 corridor and eastern Mid-Atlantic. Timing of this system will need close monitoring. Early indications are that surface temperatures from the western I-95 suburbs into central Pennsylvania between 3AM - 7AM on Wednesday may be low enough to spawn a short period of light freezing rain.

TEXT OF ORIGINAL 12-3-09 POST (left for comparison to updated forecast ideas)

WHY ONLY A INCH OR SO? DON'T WE DESERVE MORE? Good points were made about this in the comments by a meteorologist with the handle "TQ" whom operates his own valuable analysis site, and administers the longest-running snowfall forecast contest on the web. TQ's point is that even though 5,000 foot level temps may be <32 F, 2 meter temps around 35 F in Maryland and Virginia will keep the snow-to-liquid ratio quite low: 6- or 8-to-1. Running the conversion of .30" potential moisture, results in a maximum 2.4" falling under best case conditions, but much less reaches the ground, and those boundary layer temps prevent most of that from accumulating. Remember, students at Mt. Saint Joseph's School in Baltimore City early this week also saw that problem coming.

HOUSTON, WE HAVE A PROBLEM (with you!) Seriously, if you want a guaranteed good December snowfall, don't hold out for Baltimore: head to Houston, Texas - which may received 2 to 6 inches on Friday 12/4. The best we might muster in Charm City.. an inch. Which means all this storm analysis is really just ego cover for snow-starved Mid-Atlantic powderhounds. We have to compensate in some way for the jealousy of reading about WINTER STORM WARNINGS for the Houston/Galveston area. Yes, you read that correctly-- areas known more for hurricane impacts than snowstorms will receive snow before Baltimore the SECOND year in a row. They sure didn't miss a chance to enjoy it all.


Reader photo from the Houston Chronicle, Friday 12/4/2009.

Still, if snow falls across our region this weekend, it only reinforces the legendary prowess of the date itself: on the fifth day of December, the Baltimore area has been one of many that will have seen snow the sixth time in eight years on the same date. Details of what happened on previously "fabled fifths" of December can be found in Mr. Frank Roylance's recent report on his Baltimore Sun weather blog.