Arctic Air Heads For Central United States

WEATHERINTEL SERVICES
02-JAN-15 (Next Update SUNDAY – JAN 4)

By Steve Gregory for vacationing Jeff Masters

MAJOR STORM TO BE FOLLOWED BY ARCTIC AIR PLUNGE

A storm system will be forming over Northeast Texas/Arkansas tonight and will head towards the southeastern Great Lakes on Saturday and then up the St. Lawrence River Valley Sunday. Locally heavy rains, snow and potential ice storm conditions will accompany this major storm, but it appears quite likely a mostly rain events is in store for the major coastal cities of the Northeast – while ice storm conditions could occur over interior New England and the higher elevation or the northeast. The track on this storm has been 'rock solid' for well over a week now.

Arctic air will be surging southward behind the storm, with a secondary arctic front / Alberta Clipper moving through the Midwest early next week. Extremely cold air will follow behind this second disturbance – especially in the Midwest – with Temperatures to fall below zero across the upper Midwest.

A large scale upper level TROF covers much of North America, with high level ridging along the west coast of North America. This upper level flow pattern will continue to dominate the US weather for the next 10 days, but most models continue to show a gradual breakdown of the EPAC /Western US Ridge during Week 2, with a more zonal-like flow working its way across the nation in 12-15 days. This progressive and somewhat more zonal flow is supported by the intensifying MJO in the western Pacific (see below Figures), and the overall SST anomalies across the Pacific Basin. Model trends still suggest near or above normal average Temperatures across much of the nation (with the exception of the Northeast) during the middle of the month – with a potential for it to continue well into the second half of JAN. Regardless, the progressive nature to the hemispheric flow suggests any surges of very cold arctic air will be relatively brief (as in 3-6 days) versus ‘month long’ type cold as was experienced last year.



CLICK IMAGE to open full size image in new window

Fig 1: The various global model forecasts valid on the evening of JAN 11 ... There is now fairly decent agreement among the various global models at 10 Days out with the operational GFS still showing a colder bias in the central and eastern US compared to the European model (and is accepted). But in general – a progressive and somewhat zonal-like flow is shown by all models across much of North America during Week 2 – a pattern that supports near or above normal Temps developing during the week. The 15 Day GFS from last evening shows a general continuation of this more zonal-like pattern going into the 3rd week of JAN.



Fig 2: The MJO (top panel above) has continued to become better organized and somewhat stronger – with most models now forecasting a much stronger MJO signal propagating across the West Pacific during Week 2. A Phase 5 location statistically supports a milder pattern over the US. The bottom forecast graphic shows the expected area of enhanced convection (green shading) and suppressed convection (yellow shading) associated with the eastward moving MJO signal. The relatively strong signal shown in the eastern Pacific 15 Days out (bottom most panel) is the strongest I can recall in that region in over 2 years. IF IT VERIFIES – very strong forcing on the atmospheric pattern from the equatorial Pacific will develop over North America during the second half of JAN.




Fig 3: The above 2 images show the SST anomaly during late DEC (top panel) and Sea Surface Heights (SSH) as measured by satellite imagery (bottom panels) during early and mid DEC. A large heat sink covers the eastern half of the equatorial Pacific, which has helped maintain the weak El Niño condition. Though SST anomalies actually fell slightly in late DEC in the Niño 3.4 region as easterly trades increased – the warm sub-surface Temps as shown by the above normal SSH’s will continue supporting a weak El Niño. The anomalous SST pattern across the rest of the Pacific (Top graphic) has been a major player in this winters’ atmospheric pattern, forcing the jet stream to take on a more zonal pattern across much of the North Pacific (NORPAC). Since a broad zonal pattern is NOT typical for the cold season, weather systems have been highly progressive for this time of year and has led to the relatively moderate winter Temperature pattern overall.




Fig 4: The GFS Ensemble forecasts for 8-10 days out shows a highly zonal flow across the Pacific with an upper level High to the NW of the Aleutians – where the semi-permanent ‘Aleutian Low’ is normally located. (This pattern has led to the periodic weakening/breakdown of the EPAC Ridge and a moderate Temp pattern downstream across North America.) With the exception off the Northeast – the above normal heights and westerly flow into North America suggests moderate Temps during mid and possibly the last part of JAN.




Fig 5: The Temperature anomaly forecast is based STRICTLY on the GFS MOS model data output which calls for below normal Temperatures on average – with very cold conditions in the Midwest next week. The near normal readings in the Northeast will fall below normal after the storm passage by early Monday.




Fig 6: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the HI-RES operational GFS (75%) integrated with the 00Z EURO model (5%), the 12Z EURO ensemble mean (10%) and the 12Z GFS Ensemble (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 below normal Temps will still hold on in the Great Lakes and Northeast – it appears a moderating trend will be overspreading much of the nation again during the second half of Week 2. 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.

✭ My next update will be on SUNDAY – JAN 04 on my own WU Blog✭

Steve

NOTES:

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

2. Another Glossary of weather terms is available HERE