Professional Forecaster experience since 1977, concentrating in Aviation, Tropical and Long Range forecasting.
By: Steve Gregory , 7:54 PM GMT on February 21, 2017
TUESDAY: 21-FEB-17 / 1:55 PM CST
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UNUSUAL WARMTH TO RECEED TOWARDS ‘NORMAL’ AS SUBTLE
BUT IMPACTFUL PATTERN EVOLUTION BEGINS THIS WEEK
The extraordinary warmth over the central-eastern US will gradually dissipate as the large scale ridge that has dominated the weather across the central and eastern US for over a week breaks down – allowing the overall weather pattern to trend towards more ‘seasonal’ but quite active levels.
The deep TROF near the west coast be ejecting several strong short wave TROFs eastward this week – helping to break down the amazingly strong ridge that has brought all-time record warmth to the central and eastern US to give way to more seasonal Temps – especially during Week 2. (Chicago has had the warmest FEB 16-21 Temps on record – including at least 3 days with highs of 70˚. Until this past week, there have only been 4 occurrences in all of FEB with 70˚ readings in the last 145 years.)
The main driver for the subtle but significant pattern shift is the strong MJO that has now weakened and is moving across Africa towards the Indian Ocean. This, along is bringing a shift to the tropical forcing pattern that helped to originally setup the ridge over the center of the nation – and will allow heights (and Temps) to fall off across Canada and then the central US starting this week.
There remains some significant uncertainty beyond 10-days on just how big a pattern change will be forthcoming – with some model forecasts calling for a strong mean TROF to form over the eastern US during the first week in MAR – while the most recent model runs have trended towards a mean TROF across the western interior with strong short waves ejecting towards the northeastern US. For the time being, the highest probability outcome is for near to at times below normal Temps across the eastern half of the nation during early MAR – but with large Temp swings every few days between storm/frontal system passages. At the same time, Temps will remain somewhat below normal over the western interior, with near to above normal Temps developing across the SW US. Further out in time, there are SOME indications that Temps will again move to above normal levels in late MAR on into APR.
Fig 1: Temperature ANOMALY forecast for Week 1 is based STRICTLY on the MOS data forecasts from today’s 12Z operational GFS model run (with only minor adjustments towards the raw model data points for Days 6-8).Unusally warm Temp anomalies will ease off from west to east this week though will remain far above normal levels on average east of the Rockies. Well below normal readings will persist in the west. Overall confidence is very high with excellent temporal and regional model agreement – with a reading of ‘5’ for the pattern and ‘4’ for actual anomaly magnitudes on a scale of 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 (5%), ECMWF Ensemble (5%) and Climatology (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.ON AVERAGE, warmer than normal Temp anomalies should persist over the eastern half of the nation on average - thpough Temps will fall below normal in the Midwest and eventually the NE US by late in the week. Overall confidence is near average for the anomaly pattern with a bit below normal confidence in anomaly magnitudes for this time of year due to rapidly moving weather systems and associated swings in Temps; with a reading of ‘3’ for the pattern and a ‘2’ for actual anomaly magnitudes on a scale of 1 to 5.
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