Saturday, December 18, 2004

evening update:

Well now, interesting how things do change every now and then. Suddenly, the Baltimore NWS has decided that it really is going to snow after all. Forgive the smugness, but they're the ones being paid your tax dollars to figure this stuff out. I just do this for fun. But you should check out the latest NWS Winter Weather Advisory, which is a far cry from the pooh-poohing and down-playing of snow earlier this weekend and Friday.

To be totally up front with you, I completely understand why major forecast agencies (except Accuweather) took the "non-event" route up to this point. I understand the science behind their reasoning. And I will outline the reasons for and against my current prediction. The headline quote above could apply to either me or them, or all of us together. To be safe, you should plan to go to work or school as usual on Monday. If by chance, you wake up Monday morning, and the forecasters are saying, "Well, this situation really changed a lot overnight...." You can be pleasantly surprised, and know in advance why it happened, as I will explain below. Then again, I might be the one with egg on my face, and if so, oh well, life goes on, and there will be another storm to ponder over soon enough.

But just for a moment, consider the science behind my seemingly blind faith towards snowfall Sunday night. I cannot take full credit for this, as it is the product of my experience forecasting similar storms, my research into the computer models on the internet, and comparisons with forecasters at Accuweather. This can be considered a "Pumped Up Powderhound's" Reasons FOR the storm."

1) IT'S ALL ABOUT THE MATH. A massive Arctic blast will sweep to the coast tomorrow, bringing cold we haven't seen in about 5 years. Monday's highs from PHL north MAY NOT BREAK 20. I am not making this up. Highs from DC north to BAL may not break 25. Any snow occuring will be in this super cold Arctic air. This is why the 2003 Blizzard was so snowy. It was 18 F at 2PM in Philly on Sunday at height of the storm. The colder it is, the more snow you can squeeze out of the same portion of the atmosphere. SO JUST DO THE MATH... All the major cities in the I-95 corridor will have at least .10 liquid, but with a ratio of 1:30 due to the cold air, that translates into 3 inches on the low side. Folks, I have seen this before several times, I am telling you, this smells of a major forecast BUST for anyone who's written off this storm. My forecast can equally bust, so we are all even in this game right now.

2) NO LAW OF PHYSICS SAYS SNOW CAN'T FALL BEHIND A FRONT. True, you would be skeptical of a forecast which called for snow behind a fast moving Arctic front. Doesn't it make sense that the front would PUSH all the moisture out to sea, and behind it is just cold dry wind? The structure of this front produces a unique arrangement where precip is in back of the front. This happens because there is low level cold air on the ground right now where you live on the East Coast. The front moving east has to push that air out of the way, forcing it and the moisture associated with it, upward. The resulting upward motion tends to pull a little moisture off whatever water body is nearby, be it the Chesapeake, Delaware Bay or the ocean. Rapid condensation occurs, but in the few hours between all that, the Arctic front has SWEPT through underneath. The moisture FALLS through the much colder air on backside of the front, and what would have been a 1:10 ratio is now 1:30. What would have been big wet flakes becomes tiny little flakes, and you thought you'd get half an inch, turns into 3 inches or more. If that .10 somehow makes it up to .25, at 1:30, we're talking 7.5 inches for some lucky chap somewhere along the I-95 corridor from NYC to DC.

Accuweather has provided a good explanation of why this happens, so I quote from them:
You know that low level dense air has a tough time getting over the mountains for a few hours. This jams up air just west of and in the mountains and enhances precip. When it does, it rushes in creating a zone where precip weakens. That may be just east of the mountains, say the Piedmont. But further east, the warmth of the Atlantic and Chesapeake, the increased convergence from the pressures staying low over water and rising so fast in the area where the air is rushing in, and the "bounce" factor.. in other words what goes up, must come down. These factors all warrant concern that late Sunday night into Monday morning there is a 3-5 hour period of strong winds and moderate to heavy snow with rapidly falling temps right along I-95 in the Mid Atlantic.

The forecast I outlined Wednesday and Thursday remains the call. There is no change. Here's a summary:

Snowfall in Maryland from Sunday evening to Monday morning:
-Western counties: 2-4"
-Central counties: Frederick, Carroll, Howard: 2"
-Baltimore, Anne Arundel: 3"
-Harford, Cecil, eastern shore: 4"+

Snowfall in Pennsylvania / New Jersey from Sunday evening to Monday morning:
-Central PA: 1-2"
-Lancaster and Chester counties: 2"
-Delaware County: 3"
-Philadelphia County: 3-4" localized spots of 5"
-Western/Central Jersey: 3-6" and more closer to the coast

Sunday, I will do a brief wrapup of reasons for and against school being closed or delayed, this is the long-awaited "Pumped Up Powderhounds versus Nervous Nellies" debate. From that, you can decide if it is worth staying up late to see what happens, or just playing it safe and going to bed on time.

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