NO WHITE CHRISTMAS FOR EASTERN U.S.

WEATHERINTEL SERVICES
By Steve Gregory substituting for Jeff Masters
21-DEC-14 (Next Update TUESDAY – DEC 23 on my own Blog)

PATTERN CHANGE TO BRING NORMAL TEMPS XMAS

Above normal Temps continue to persist across much of the nation, but will begin trending back to normal later this week, with below normal Temps likely to develop from west to east after starting next weekend and continuing into the New Year. .

A large scale long wave TROF will be forming over North America as the week progresses, with a strong vorticity Max forcing the development of a large area of locally heavy precip in the southeastern US by late Monday, with a storm system expected to form along the Gulf coast. This storm will move northward into the eastern Great Lakes by XMAS, with a warm flow along the east coast (including New England) and locally heavy rains. While some snow may fall on the back side of the storm across the western Great Lakes and upper Ohio Valley – total amounts are not expected to be heavy.

Although a cold flow will become established following this storm, it will take some time for deep arctic air to develop over Canada, so Temps will be close to normal going next weekend, before falling below normal during the week just prior to New Years. The latest global model runs continue to show large run-to-run forecast variations on just how long and how cold it will be during the first 10 days of JAN – as the atmosphere has taken on an El Niño like flow, with some indications that a southern branch of the flow from the Pacific will be undercutting the main TROF over North America during the first half of JAN, limiting the extent, and at times the intensity, of the arctic air surge into the U.S.

Further out, due to the large swings in model projections in the longer ranges, exactly what the pattern will be in JAN is nothing more than a toss-up at this point. That said, with the continuing oscillation of the major teleconnection forecasts (AO, NAO, and PNA) every 2-3 weeks, a small uptick again in Equatorial SST’s in the Niño regions, the continuing positive North Pacific decadal SST pattern, and uncertainty on the strength of the next MJO signal now evolving over the Indian Ocean - a forecast for a ‘highly variable’ (El Niño like) pattern, with large swings in Temp anomalies during JAN seems to be the most likely outcome at this time; along with several more significant rain events in drought stricken California Ca. (More information on the impact of MJO, El Niño and the Pacific decadal oscillation can be found on numerous postings on my own Blog over the past month and the last Winter Outlook posted here and on my Blog.)



CLICK BOTTOM IMAGE to open full size image in new window

Fig 1: The various global model forecasts valid on the evening of DEC 30... There are very large differences in model projections at 10 Days out with equally large variations in the projected Temps – especially during Week 2. The GFS is actually the coldest, while the ‘New’ GFS (which goes online in mid JAN) is the ‘mildest’ of last nights' runs. OTH, the European and Canadian models imply stormier weather during Week 2, with the European model warmer than the GFS but colder than the Canadian. The latest GFS run (not shown) is similar to last nights 00Z run, and has been largely accepted when developing the Temp anomaly forecasts. Nonetheless, the result is a high uncertainty for Temp anomalies during the first week in JAN – though the odds favor solidly below normal Temps across the nation.




Fig 2: The above chart shows High Temps reported yesterday across North America. Note that sub-zero readings are confined to the high arctic in northern Canada and the interior of Alaska (where DEC Temps have averaged 5°-15° above normal so far). Though the upper level flow favors much colder air moving into the US later this week, it will take 4-67 days for deeper, arctic air to develop over Canada first, before plunging into the U.S.




Fig 3: The GFS Ensemble forecasts for the NAO, AO and PNA Teleconnections out thru 14 days. There is an extremely large divergence in the forecasts for the AO and NAO during the last 6 days of the forecast period, with very large implications for Temp anomalies during the first half of JAN. (A Negative AO and NAO imply below normal Temps over the eastern half of the nation – while Positive values imply warmer than normal.)




Fig 4: The SSt anomalies across the Pacific show a continuation of well above normal values from the northern most Pacific/Gulf of Alaska to the eastern Pacific, with a currently weak El Niño event in progress, and a wedge of cold water from off the coast of Japan eastward across much of the North Pacific. The atmosphere has responded to this SST anomaly pattern during the last 3+ weeks with an El Niño like flow (which I expected when developing my Winter Outlook). This type of flow brought the unusually long lasting mild pattern this month along with the heavy rain events to California. This SST anomaly pattern is no doubt contributing to the large divergences in model solutions for periods beyond 7-10 days exhibited by all global models for over a week now.




Fig 5: The SST and Wind anomalies over the Equatorial Pacific continue to show an El Niño event in progress. After Temps fell off a bit around the start of DEC due to the upwelling phase of a passing Kelvin wave – they have begun to warm again as the down-welling phase of a Kelvin wave and weaker than normal Trades have again developed in the EPAC. It appears the Niño 3.4 region will see SST anomalies approach +1.0° by the start of JAN – regardless of how the next MJO cycle evolves. (Whether the SST’s remain above normal in FEB, however, is at least partially related to the next MJO cycle.)




Fig 6: The above Temperature forecasts are based STRICTLY on the GFS MOS model data sets which call for above normal Temps nationwide on average during the week ahead, but anomalies have decreased considerably from the past 2 weeks and will fall to near zero or slightly below normal by next Sunday.




Fig 7: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the HI-RES operational GFS (90%) integrated with the 00Z EURO model (5%), & 12Z GFS ensemble mean (5%) - 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 solidly below normal Temps will dominate much of 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 the opening days of JAN. Therefore, Confidence in the general Temp anomaly pattern and its absolute values, is below average, with a rating of ‘2’ on a Scale of ‘1-5’.

✭ The next Weather Update will be issued TUESDAY – DEC 23 on my own Blog ✭

Steve