The Advertised Pattern Change Looks More Impressive Than One Week AgoOctober 15th, 2013 at 9:37 pm by Don Paul under 4 Warn Weather
Last week’s headline was telling, concerning “Hints of a Pattern Change.”
The signs are now clear we will be heading into a chillier pattern, in stages. What remains unclear is HOW chilly. And will snowflakes accompany the chilly pattern at any point?
In the broadest sense, the 500mb ensemble means offer good support for below average temperatures most days starting late this week, and amplifying by mid/late week next week. The more erratic deterministic/operational GFS & ECMWF runs at least have decent agreement between themselves on 850 temps dropping to -4 to -5, then -6 by late next week (0 or -1 or-2 generally won’t cut it at this time of the year unless there is unusual upward vertical velocity). In the GFS, the coldest part of the pattern seems to hold off until next Thursday-Friday.
If those temps were to verify, and if we had a cyclonic curvature to the flow, that would favor some snow at high elevations at times, with some possibility of a mix even at lower elevations. While these global models tend to show a predominantly NW flow during that period, they lack the resolution so far out to pick out lesser troughs/short waves which can cause alternate backing and veering of the boundary layer winds. The global models favor some periods of LES or LES/R during this cold period, but amplitudes can lessen between now and then. There may also be a trough in the SW undercutting the amplitude of the western ridge to some extent. So, while there may be some hints of excitement here for snow lovers, the cliche applies; “I wouldn’t get too worked up about it.” Not yet, anyway.
The MJO remains active, but it is weaker and will stay weak to moderate over the next 2 weeks (still considered active) with some possible eastward propagation. That weaker active status is reducing its impact at tropical latitudes to a limited extent. The indices are a mixed bag over the 2 week period. The AO is out of phrase with the NAO, tending to be closer to neutral. The NAO is sharply negative for a while, with a weakening amplitude later in the month, heading upward to neutral or even a little positive by Nov 1. The PNA is positive initially, but there is major spread in its ensemble members in week 2. The ensemble means have a flatter flow close to the end of their runs, which may be reflective of the less favorable NAO & PNA. But I wouldn’t say the mean goes completely zonal…just low amplitude.
Finally, there is continued high confidence for neutral ENSO conditions to continue well into Spring 2014.