Tuesday, November 30, 2004

A TALE OF THREE FORECASTS.... WHO WILL BE RIGHT IN THE END?

Thursday, November 25, 2004

STAGE IS SET FOR A COLD AND SNOWY DECEMBER
THAT WINTER WILL REMEMBER



For all you powder hounds out there in snowland, Happy Days are almost here once again. Now before you flip out and go buy a truckload of salt, a quick explanation of the map above.

The color band at bottom is "Accumulated Snowcover" in inches over a 12 hour period.
This is the National Weather Service's GFS (Global Forecast System), which can project weather systems and their effects out 372 hours, or 15 days. The atmosphere has begun putting the pieces in place for a slow-at-first transition to winter starting after Thanksgiving, and then a swift plunge into the deep freeze by middle of the month.

The map above is an early indication that the first measureable snow for the Northeast U.S. may be in the December 4-5 time frame That date seems to be etched in the atmosphere's calendar, for it will be the third year in a row the I-95 corridor saw accumulating snow on or about December 5. The computer models continue to vacillate on whether this storm will be snow or rain, but it is clear a storm of some kind will be heading up the East coast during that time frame.

The only bad news at first glance is that if this minor storm plays out as indicated, it will occur on a Friday night into Sunday morning. That will automatically eliminate any chance for a bonafide "Snow Day" for any school district anywhere. The apparent snowfall looks to be in the 3-6 inch range for central VA, with 1-3 throughout the rest of the area.

The good news is that this far out in the computer models, a lot can change. If the cold we see coming from the 25th to the 5th is more entrenched than expected, perhaps the only worthwhile speculation would be that it is fair to say the computers "see" something snowy brewing for the Dec 4-5 period. It could mean that the eventual storm is stronger, or weaker than predicted. There is one clear signal this map is sending... the computer expects the cold air present at that time to be strong enough to send snow all the way to the VA coast and deep into North Carolina. That is a sign that many, many good days are ahead for those of us who yearn to see fluffy little flakes falling outside our window... only to roll back over, turn off the alarm clock and go back to sleep. (That is, if you're a teacher, and it is a weekday.)

DECEMBER HINTS OF A WINTER SPRINT

I remain steadfast on the "snow first week of December" rule because I have seen this pattern before. Until the atmosphere makes a wholesale flip to winter, computer models are notorious for having difficulty picking up the minute indications that lead to the flip. Last year was a great example, and you see the same thinking reflected in all your NWS local forecasts for the next 10 days. Even 3 days prior to the 6 inch storm on Friday, Dec 5, 2003... all signs were rain and 45 to 50 degrees. Then on Monday, NWS caught on to the fact that the southern jet was active, and cold high pressure which had built up in Canada, was about to make a charge southward.

Friends, the same situation is setting up again. When we see a low pressure trough developing east of Hawaii, that tends to keep the subtropical jet active by sending moisture and systems across the Gulf of Mexico and into the southeast. This temporarily blocks the southeast movement of cold air, so in this early phase of transition to winter, you end up with snow in Chicago instead of DC. But as we move farther down the calendar, the high pressure ridge off the south Atlantic coast gets beaten down by the continual march of these clipper systems, dragging cold air behind them.

So the ingredients we need for an early to mid December kickoff snowstorm are:
- Active southern jet stream with a low pressure trough east of Hawaii (check)
- Above normal water temps in the Gulf (check)
- Large pool of cold high pressure parked over northern Canada (check)
- A series of southeast moving clippers to unlock the Canadian air (check)
- Weakening ridge off south Atlantic coast (getting there, not quite)

All these forces are setting the stage for what will become a whiplashing sprint from relative mild weather to overnight cold and snow.

CHANGES TO THE DECEMBER FORECAST

This is based on the long term trends as shown by the Global Forecast models. The end game is this... the longer that Arctic air is blocked from entering the eastern U.S., the more likely that once it does make a move, it will stay entrenched for a period of 10 days to 2 weeks, starting around the middle of the month, with record-breaking cold and occasional storms with accumulating snow.

25-30 (Nov) Much colder, windy and dry, with a slow warm up at the end
1-5 Another round of alternating mild and cold, with occasional rain.
6-10 Mild with rain, leading to another shot of cold and windy weather, then snow by the 11th
11-24 Bone shattering onslaught of very cold air with several storms
25-31 Brief mild spell, to be followed by another record cold outbreak into the middle of Jan.

This trend will be closely monitored. You can be sure that you'll be the first to know of any signs
of a "breakout" in the pattern pointing to the first northeast snowstorm.

You have about 2 weeks to stock up on your salt and change your antifreeze.