Winter's Back!

WEATHERINTEL SERVICES
30-DEC-14 (Next Update FRIDAY – JAN 2)

By Steve Gregory for vacationing Jeff Masters

ARCTIC AIR OVERSPREADING MUCH OF THE NATION

Arctic air is surging southward from the Rockies eastward, sending Temps below normal for the first time in over a month. Well below normal Temps are expected to dominate much of the nation east of the Rockies for the next 2 weeks – with significant storm systems impacting the eastern half of the nation every 6-8 days. The only exception will be over Florida where Temperatures are still expected to remain on the warm side of normal.

A large scale long wave TROF now dominates the nation, with the center of the Polar Vortex over Hudson Bay, while a High pressure ridge continues to build over the eastern most Pacific into Alaska. This upper air pattern will provide a colder flow from the arctic southward into the central US this week, and into the easternmost states by next week. Although this is a far more seasonable pattern, all global models continue to show a highly progressive flow which will cause significant oscillations in Temperature anomalies during the next 2 weeks, with anomalies varying from 'just' a few degrees below normal to 20 degrees below normal.

A small but potent short wave TROF is now plunging southward over Nevada and will trigger a Low pressure system over the SW Deserts by tomorrow, with snow possible from near Las Vegas into the higher elevations of Arizona. This system will then move into Texas by the end of the week where it will intensify and then head for the Great Lakes region over the weekend. For over a week now, the models have been extremely consistent on the projected track for this storm, with the storm center tracking from the eastern Great Lakes/Ohio Valley thru the interior of New England. This should result in mostly rain in the Boston-Washington DC coastal corridor, while heavy snows fall inland.

The global model runs have been fairly consistent in their upper air pattern forecasts, but there continues to be a significant difference between the models for time periods beyond 8 days. In general, the GFS has been the ‘coldest’ of the models, as both the longer range ’New’ GFS (which goes operational mid-month) and the European models show stronger systems heading eastward from the Pacific which then force a breakdown in the Ridge near the West coast – ultimately bringing a milder flow aloft into the central US and stronger storm formations east of the Rockies.

As the models continue to struggle with the El Niño like flow pattern that continues to exert itself over the Pacific basin upper air pattern, a number of forecast tools show the EPAC ridge breaking down by mid Jan. For this reason, there is a reasonable chance of seeing Temps return to near and eventually above normal levels during the second half of JAN.



CLICK IMAGE to open full size image in new window

Fig 1: The various global model forecasts valid on the evening of JAN 09 ... There are fairly significant differences in model projections at 10 Days out with the operational GFS showing a relatively weak short wave in the northern Rockies, while the ‘New’ GFS and EURO models show a much more potent system in the west. (Qualitatively, the ‘New’ GFS appears to have a lot in common with the ECMWF for forecasts beyond 1 week...)




Fig 2: The GFS Ensemble forecasts for 5 Teleconnection indices. Curiously, the AO, NAO and PNA do NOT support long term cold in the eastern half of the nation (A positive AO and NAO imply above normal Temps over the eastern US – while a Negative PNA implies mild weather for the southeastern US). However, 2 other, lessor known teleconnections (the WPO and EPO) do support cold weather in the east. Nonetheless, having such inconsistencies between teleconnections implies higher than normal uncertainty for forecasts beyond 8-12 days out.




Fig 3: The MJO has become better organized and somewhat stronger over the last few days as it moves slowly eastward over Indonesia and the Philippines. Forecasts (especially those from the GFS ensemble run) call for the enhanced phase of the MJO to propagate eastward and strengthen some over the next 2 weeks. IF correct, this will place it in the western Pacific in 2 weeks – and supports a warmer pattern over the US during the 2nd half of JAN..




Fig 4: The Temperature anomaly forecast is based STRICTLY on the GFS MOS model data output which calls for below normal Temperatures nationwide – with the exception of Florida. Anomalies will vary considerably every few days due to the approaching storm system this weekend.




Fig 5: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the HI-RES operational GFS (80%) integrated with the 00Z EURO model (5%), the 12Z GFS ensemble mean (5%) and the ‘NEW’ GFS (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. Though well below normal Temps will dominate the eastern half of the nation (except for Florida…) the fairly progressive pattern, especially in the southern stream, introduces greater uncertainty for Week 2 Temperature anomalies. However, based on much better consistency from run-to-run for the last week, Confidence in the general Temp anomaly pattern and its absolute values, is near average for this far out in time, with a rating of ‘3’ on a Scale of ‘1-5’ for both metrics.

✭ The next Weather Update will be issued FRIDAY – JAN 2 ✭

Steve

NOTES:

1. A GENERAL Glossary of Weather Terms can be found HERE

2. Another Glossary of weather terms is available HERE