The unsound approach


Ok, so our promise of sweet North American passerine action from Saturday onwards hasn't quite materialised, yet... West-thinking birders have had to make do with a reasonable scatter of yank waders and a haul of scarce eastern drift 'supporting cast', but no dendroication thus far. Don't give up hope! Things are still looking promising. At the risk of boring everyone to death, here's yet another bunch of meterological pseudo-bollox to chew over, in the hope of getting that ultimate pant-filling encounter with Uncle Sam's neon warbler army.

The following images show weather charts for the last few days (on the left, black+white) next to charts from the equivalent stage in the progression of Hurricane Gloria back in 1985 (culprit of the greatest North American passerine arrival in UK/Eire history). You'll have to forgive the fact that the chart projections don't match perfectly, as I can't find a complete chart series that covers both periods.

First up is the chart for last Sunday (5th Sept, on the left), when Earl had just finished his run up the eastern seaboard. On the right (in colour) you can see the situation for the equivalent stage of Gloria's movement back in '85:

You can see Earl sitting broodingly over Nova Scotia (bottom left hand edge, with the two short warm fronts). On the right (colour map), you can see Gloria in a very similar positon on 29th Sept 1985, having followed a near-identical track  up the eastern seaboard. Also, on the 2010 image you can see the mid Atlantic depression and wave-front that brought some yank waders over the last few days, but disappointingly little else. Back in '85, a depression was hanging in a very similar position, again not bringing a huge amount.

Here's the chart for Monday (6th Sept 2010, left), plus the equivalent chart for Gloria (30th Sept 1985, right):

On the left, you can see Earl didn't make a huge deal of progress, basically filling in and moving towards western Greenland. However, in its wake it did produce a very promising-looking wave-front, effectively circling Nova Scotia. On the right, you can see Gloria at this stage had deepened a little, but was basically in a similar position to Earl, dragging westerly winds across northern Canada. Not too different so far...

Here's the chart for TODAY (Tues 7th Sept 2010, left), plus the equivalent day for Gloria (1st Oct 1985, right):

So, Earl hasn't moved much, but importantly he's still dragging those westerly winds across northern Canada. Even more importantly, have a look at the wave-front (just east of Nova Scotia, above the "H 1027"). It has swung out over the Atlantic and is being pushed rapidly our way by those strong westerly winds. All the evidence from Norman Elkin's work points to these wave-frontal systems being the key to causing transatlantic passerine vagrancy. Migrants get disorientated as the front passes over the North American coast, ending up being carried eastweards in the warm air associated with the front.

Now, have a look at where Gloria was at this stage (on the right). She had deepened, moved east and effectively merged with the other depression in the Atlantic. Obviously, Earl hasn't obliged in the same way, but the important thing to note is that in the western half of the Atlantic, the arrangement of isobars is very similar in both the images. The key point is that it's likely to take at least four days for birds to cross the Atlantic; at this stage in both weather systems, the majority of the drifted migrants would be about halfway across. So despite Earl being clearly weaker and slower-moving than Gloria, the important elements (wind and wave-front) have been pretty similar up to this stage. 

Ok, now for the forecast. Here's the predicted chart for tomorrow (Weds 8th Sep 2010, left), together with the equivalent day for Gloria (2nd Oct 1985, right):

Earl has almost completely filled, but in his death-throes he's finally decided to move our way, swinging east above Nova Scotia ("L 1001"). The wave front is now looking very wavy (just above "L 1008", practically spanning the Atlantic), but the most important thing is that it's shifting very quickly. Birds flying in that warm sector have a reasonable chance of staying alive, and some might even make landfall on Madiera or coastal Iberia tomorrow. Gloria, at this equivalent stage, was deepening and strengthening in mid-Atlantic, but the overall arrangement of isobars was pretty similar to the present forecast. This was the date when the first Red-eyed Vireo of the system was found in 1985.

Now here's the forecast for Thursday (9th Sept 2010, eft), together with the equivalent Gloria day (3rd Oct 1985):

Now things are getting interesting. In his final fling, Earl has merged with the big depression south of Iceland and pulled a strong airstream north-eastwards towards the British Isles. Most importantly, that wave fronts (and any birds carried therein) are now speeding straight in our direction. Will it be too late? Will everything have run out of steam and drowned? Well, at this equivalent stage in '85, Gloria was still trundling her way across the Atlantic. Although the first proper arrivals were turning up (RB Grosbeak, Parula), the bulk of the arrival was spread over the following seven days. It's likely that much of the time-delay was down to the minging weather preventing stuff getting found, but some birds could still have been completing their transAtlantic crossings long after the storm actually hit. Many of the truly long-distance migrants can carry sufficient fat reserves to stay aloft for 4-5 days if not more. It's tough to say, but there's got to be a good chance that some birds will still be flying once the front hits us on Friday...

Still reading? Anyone? Hearty thanks if you managed to persevere through all that twaddle. We're keen to hear any other opinions / analyses / predictions on the current situation. Obviously it's unlikely to be an arrival of Gloria-esque proportions, but we still reckon it's worth heading out on Fri/Sat/Sun with thoughts of Dendroica alongside your Barred Warbler/Grotfinch search images. Perhaps we're being wildly optimistic... But when did a pessimistic birder ever find anything decent?

Good luck to anyone who gives it a go.