Pattern Change Begins With Arctic Outbreak On the Way

By: Steve Gregory , 8:54PM,GMT on November 18,2015





 

WEDNESDAY: 18-NOV-15 / 3:00 PM CDT

NEXT FULL WX UPDATE FRIDAY

POSTING SCHEDULE: ‘FULL’ UPDATES ON MONDAYS / FRIDAYS … TEMP FORECAST CHARTS ON WEDNESDAYS AND WHEN WARRANTED

MILD WEATHER WILL GIVE WAY TO ARCTIC AIR

The powerful storm in the upper Midwest will move into Canada with the trailing cold front passing off the east coast FRI afternoon – but not before very heavy rains and embedded T-storms impact much of the eastern US. A smaller but potent short wave will trigger a band of light to moderate snow across portions of the Midwest later FRI – including the Chicagoland area - before moving quickly across New England over the weekend.

More seasonable Temps will dominate the northern states going into next week before another very and complex storm system develops in the inter-mountain West and then moves out across the entire central US by Thanksgiving with the first major arctic air of the season moving southward and then eastward across the western and then central US following the storm system.

The latest GFS and ECMWF model runs have come into better agreement for Week 2 – with above normal Temps persisting in the easternmost states until the end of Week 2 while much colder air spreads out across the rest of the nation by the start of DEC. Longer range forecast tools support a significant moderating trend starting by the 2nd week of DEC.




Fig 1: 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 will persist across the eastern most states with more seasonal Temps across the central US and below normal Temps developing over the northern Rockies and PAC NW. Overall Confidence in the forecast anomaly Pattern and Magnitude is near average for this time of year, with readings of ‘3’ for the pattern and anomaly magnitudes - on a Scale from 1 to 5.


Fig 2: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (80%) integrated with the 12Z GFS Ensemble (10%) (ECMWF 10%) 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.The latest GFS forecast calls for a big arctic push over much of the except for the eastern seaboard where very mild Temps will persist thru much – but not all – of Week 2. Confidence is solidly below average due to the pattern change that has begun – and high uncertainty on just how cold the arctic air outbreak will really be, with a reading of ‘2’ for the pattern and only 1 for the magnitude on a scale of 1 to 5.


Fig 3: GFS 15-Day Precipitation Pattern Loop (for mounts greater than 0.10” only) The major storm in the upper Midwest will spread heavy rains across the east with a break in the widespread storminess going into early next week before the next large storm develops out west and then the central US by Thanksgiving.

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Steve