WEDNESDAY: 30-DEC-15 / 2:00 PM CDT
(Abbreviated Version due to Holidays)
DUE TO HOLIDAY, the NEXT *FULL* WX UPDATE WILL BE ON MONDAY
PATTERN CHANGE TO CONTINUE AS MJO UPSETS APPLECART 
The WX is quieting down across much of the nation with more seasonal Temps spreading into the central and northeastern US, while warmth and Precip and locally heavy T-storms persist in the SE US. The coldest anomalies remain west of the Mississippi, with the latest model runs continuing to show significant Precip reaching SOCAL next week where upwards of 1” of rain along the coast and over 1 foot of snow is possible in the southern Sierras as 3 separate storms come onshore.
Both the EURO and US GFS models continue to struggle with exactly where the pattern change will end up – with significant run to run changes between model runs. Heights will rise over the polar region, but there are major differences in just how much of a rise will occur and whether it will persist for very long. With the strong El Niño still a key driver supporting a highly progressive flow, the destruction of the polar vortex may be transient – or may result in a simply ‘chaotic’ type of hemispheric flow with no standing, long wave TROFs (or ridges). It is simply too soon to know.
Regardless, changes under way in the high latitude flow pattern can be traced to a shift in maximum forcing from both the tropical EPAC (associated with El Niño) towards the West and especially the robust MJO which continues moving eastward across the tropical Pacific. As mentioned here for over 2 weeks now, I raised the concern that the latest MJO cycle has been MUCH stronger than ANY global models had forecast (day after day) – and the signal has continued moving eastward in defiance of the models as well. Assuming it continues into the EPAC in about 10 days – it will have a significant impact on El Niño – forcing additional changes to the mid-latitude pattern. However, exactly how it will change is absolutely unknowable with any real confidence. ‘Classically’, it should initially result in a strengthening of EASTERLY trade winds – and once it passes – re-development of westerly wind anomalies. This implies a weakening of El Niño (which all models call for) – followed by a re-strengthening period. But how much strengthening (if at all) is impossible to know, though there is a risk it produce another Westerly Wind Burst (WWB) which would cause significant re-strengthening. Only time will tell just how strong and/or persistent high latitude blocking will be and how strong of an influence El Niño will (or will not) play during the back half of winter.
Fig 1: MJO Analysis (Left) and Temp Anomaly Composites for the Phase 7 and 8 regions The latest MJO Phase diagram indicates it has remained quite robust while moving steadily eastward over the West Pacific. As it moves towards the EPAC (Phase 6) Temp anomaly ‘composite’ charts for ‘strong’ MJO’s indicate warmer than normal Temps continuing in the east during the next 10 days (very similar to the Week 1 & 2 forecast below) – followed by a cold period during the second half of JAN - if the MJO remains strong after moving into Phase 7! (Forecasts still show it will weakening later next week - but they have been WRONG for 3 weeks).
Fig 2: 500mb (≈ 18,000 Ft) Height Forecast Loop for the next 2 Weeks The above loops show the wind patterns and jet stream wind speeds (250mb chart) and the location of TROF’s, Ridges, and anomalous 500MB heights (implied Temps – with bluish shading for below normal, and reddish shades for above normal heights). The highly progressive pattern continues, though a more northerly wind component is shown developing over Canada towards the US – allowing colder conditions to move into the northern and eastern US at times. This is a result of rising heights shown across high latitudes as the polar vortex weakens – though the latest forecasts have backed off sharply on the extent of height rises in the high arctic and exactly where the primary height maximum will be – bringing into question just how cold it will actually get in the eastern US during the second half of JAN – with significant chances that a more typical El Niño WX pattern could still be the dominant controller, resulting in a continuation of near to above normal Temps in the northern states and below normal in the south and west. In fact – if the MJO results in significant re-strengthening of El Niño during late JAN after it reaches Central America, chances for a relatively mild FEB goes up significantly.
Fig 3: GFS 15-Day Precipitation Pattern Loop (for mounts greater than 0.10” only)With the exception of the Gulf coast states where widespread and locally heavy Precip is possible into the weekend, drier weather will dominate much of the nation before the next storm reaches the west coast early next week. At this time, 3 separate rain events appear likely next week in California, with a total of about 1” of rain forecast for the LAX basin, and over 3” in northern California. Snow totals could range from 1 to 3 feet in the Sierras – helping to further replenish the very critical snowpack. Storminess persists in the south – with a possible major east coast storm about 7-10 days away.
Fig 4: Temperature ANOMALY forecast for Week 1 is based STRICTLY on the MOS data forecasts from the todays 12Z operational GFS model run with only minor adjustments towards the raw model data points for Days 6 and 8. Above normal Temps on AVERAGE will continue during Week 1, with significant swings in day-to-day Temps. Generally speaking – overnight Low Temps will be significantly above normal, while daytime highs will tend to be closer to normal in the key heating demand markets. Overall Confidence in the forecast anomaly and Magnitude is near to above average with readings of ‘4’ for the pattern and ‘3’ for anomaly magnitudes - on a Scale from 1 to 5.
Fig 5: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (90%) integrated with the 12Z GFS Ensemble (5%) (ECMWF 5%) using the projected pattern along with explicit surface and 850mb (~5,000 FT) Temp forecasts. Some Temps are adjusted for known or expected anomalous thermal patterns & projected storm system passages. With the models backing off on the strength of high latitude blocking, a very typical El Niño Temp regime is seen continuing thru much of Week 2. In deference to continuity, I have LOWERED the projected Temps in the eastern US by 1˚ to 2˚ - while cooling portions of the West by 1˚. However, with a pattern change still expected to bring some degree of blocking – along with the sill quite progressive flow, Confidence in the anomaly pattern is below average with readings of ‘2’ for the anomaly pattern and magnitudes on a scale of 1 to 5.
✭ NEXT *FULL* WEATHER UPDATE WILL BE ON MONDAY✭
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Happy New Year to All!,
Steve
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