WEATHERINTEL SERVICES 28-DEC-14 (Next Update TUESDAY – DEC 30)
By Steve Gregory for vacationing Jeff Masters
COLD AIR SPREADING SLOWLY WEST TO EAST
It took long enough, but above normal Temps still lingering in the East will be history by late this week, with very cold conditions likely during Week 2. A fairly impressive storm system will be bringing rain and snow to the central and eastern US by next weekend. At this time, the storm is expected to track from the western Gulf coast region to the Great Lakes – implying rain for much of the coastal Plain in the east, and snow likely from the Plains to the upper Midwest.
A large scale long wave TROF continues to develop over North America, with the center of the Polar Vortex likely to take up residence near Hudson Bay this week along with increased ridging in the EPAC on into Alaska. This will establish a progressively colder flow from the arctic southward into the northern Rockies and central US later this week and will continue across the nation during Week 2 as well.
While the latest global model runs have ‘stabilized’ somewhat in their projections beyond 8 days, there is again a growing disconnect between the global models – with today’s GFS now the ‘coldest’ of the models, as both the longer range ’New’ GFS (which goes operational mid-month) and the latest European model show a more zonal-like flow developing again in 10-14 days. The models continue to struggle with the El Niño like flow pattern that continues to exert itself over the Pacific where basin wide SST anomalies are forcing a highly zonal flow do dominate the central Pacific. With a number of forecast tools showing the EPAC ridge breaking down in 10-14 days as a developing blocking upper level High moves from over Alaska to the Arctic Ocean – we have a reasonable chance of seeing Temps return to near and eventually above normal during the second half of JAN. (Though the models show warming to begin just before mid JAN - with the very cold arctic air in place by Week 2, it will take some time for the arctic air to be displaced - despite the milder, zonal flow aloft that is expected to develop in about 10-14 days.
Fig 1: The Latest GFS forecast compared to the European ECMWF 10 Days out. The GFS shows a very strong vortex centered in eastern Canada with a strong, secondary Vorticity Lobe/TROF westward into the Canadian Rockies. Although a westerly flow off the Pacific is shown impinging on the west coast – overall heights are near normal, and later time steps show the major TROF re-amplifying into the US. OTH, the European and the NEW GFS (not shown) shows a weaker primary vortex in Canada, rising heights in the central and western IS, and a more progressive flow of strong disturbances over the Pacific. This implies a moderating Temp trend in 10-14 days. For the moment, I’ve opted to go with the colder GFS for Week 2.
Fig 2: The GFS Ensemble forecast for the 3 major Teleconnection indices shows a plunge into a negative AO and NAO phase occurring right now, but interestingly, are shown going positive in 7-14 days (though there is high uncertainty for the AO as demonstrated by the large spread in solutions for the individual Ensemble members during Week 2). The negative trending PNA is shown returning to near neutral during Week 2 as well. (A Negative AO and NAO imply below normal Temps over the eastern half of the nation – while Positive values imply warmer than normal. A Positive PNA implies cold weather for the eastern US). In sum – these forecasts argue for near (or even above) normal Temps again developing after mid month.
Fig 3: The above Temperature forecasts are based STRICTLY on the GFS MOS model data output which calls for above normal Temps over the eastern US on AVERAGE during the next 7 days, but anomalies will fall below normal by the end of the week - along with stormier weather next weekend.
Fig 4: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the HI-RES operational GFS (85%) integrated with the 00Z EURO model (5%) & 12Z GFS ensemble mean (10%) - using the projected pattern, along with the GFS surface and 850mb Temperature forecasts. Some Temp forecasts are adjusted for known or expected anomalous thermal patterns and/or projected storm systems. (The ‘NEW’ GFS was NOT used for today’s forecast.) Though much below normal Temps will dominate the nation, there is still significant uncertainty in the daily Temp forecasts after Day 10. This is due to large variations in the models’ upper air projections – and an equally large uncertainty on where and how much additional snow cover there will be by next week. However, based on much better consistency from run-to-run for the last 5 days, Confidence in the general Temp anomaly pattern and its absolute values, is near average, with a rating of ‘3’ on a Scale of ‘1-5’ for both metrics.
✭ The next Weather Update will be issued TUESDAY – DEC 30 ✭
Steve
NOTES:
1. A GENERAL Glossary of Weather Terms can be found HERE
2. Another Glossary of weather terms is available HERE
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