Above is a reason not to have faith in computer models. It is a map I downloaded last Wedenday, showing me how much snow I could expect by this coming Wednesday. It shows one swath of snow coming up from the southwest and passing south of Boston, and then a second swath coming down from the northwest and also passing south of Boston. Boston could expect a total of perhaps three inches, and I, in southern New Hampshire, could expect a mere inch over the next seven days.
That was wonderful news, as I am an old grouch and don’t like cleaning up the parking lots and walkways of my home and business. It was also wrong news, as I understood as I came in from cleaning up the three inches the first feature brought up from the southwest. I clicked on my computer, and was confronted by this map of snow totals expected from the second feature, coming from the northwest.
Yikes! That suggests more than a foot for Boston, and over two feet for where I live in southern New Hampshire. Not that it is right. It could be as incorrect as the first map, for all I know.
The above maps are are among thousands produced by Dr. Ryan Maue from data produced by weather bureaus from all over the world, and available at the excellent Weatherbell site. I highly recommend the site, though it will cost you about the price of a cup of coffee each day. I especially recommend the insights of their two senior meteorologists, Joe Bastardi and Joseph D’Aleo.
The interesting thing about the storm which now is appearing in the computer forecasts is that it is born from such a small feature. It looks all the world like a little Alberta Clipper, rippling down from the Canadian Rockies on an arctic cold front as it sags south. Typically these storms are small, as they have little moisture to feed off, unlike the storms that come up from the Gulf of Mexico, which often bring a big gob of moisture north with them. In this morning’s map you can see one such juicy feature departing to our Northeast, growing deeper as it swiftly departs. You can also see the approaching Alberta Clipper, to the southwest of Lake Superior, still far to our west and still looking rather innocent.
Even when the Atlantic Ocean is warmer than normal, as it now is, by the time an Alberta Clipper can tap into the moisture it is usually too late. As the storm strengthens it has already passed Boston, and is whisking away towards Greenland and Europe.
That is exactly what the computer models originally thought this clipper would do, and is why the map at the top of this post shows no snow for New Hampshire, where it now shows over two feet. What has changed?
As I understand it the “steering currents”, which are the winds high up in the upper atmosphere, have changed. Where isobars at the surface show circles for low pressure systems, the upper atmosphere ordinary only shows a dent in isobars, a “trough” that ripples west to east like a wave on a shaken jump-rope. Normally these troughs lean to the east, and the steering currents whisk the low pressure at the surface to the northeast. More rarely (at our latitude) the trough can tilt back to the west, and this can cause a storm to behave as they more often due up at higher latitudes and the northern North Atlantic. The storm turns back to the west and its path describes a sort of curlicue, before it eventually drifts east. (This sort of steering current is called a “negatively tilted” trough.)
Apparently the trough associated with the low that just swept by us was positively tilted, but the next trough is negatively tilted. The clipper will be pushed south by the arctic high pressure coming south behind the last storm, and rather than moving out to sea north of us or over us, will move out to sea over warmer waters down east of the Virginia Capes. That warm water will feed it and it will grow very rapidly, but (at this point) looks unlikely to escape out to sea. The high pressure that pushed it south will stand in its way, and will “block” it, as the trough in the atmosphere above is becomes negatively tilted. Therefore this innocent little clipper will describe a curlicue in the Atlantic southeast of Boston, growing stronger all the while.
Hold onto your hats.