Thursday, April 30, 2015

Warm Start to May

By: Steve Gregory , 7:59PM,GMT on April 30,2015



Cool Temps in the east to Warm during the Week Ahead
30-APR-15

(Next Update MON – MAY 4)

As the upper low/TROF in the Northeast weakens and the TROF in the EPAC moves into the western US, the Ridge over the Rockies will shift eastward into the central US and amplify - bringing above normal Temps to much of the nation through next week.

The global models are in reasonably good agreement on maintaining a TROF near the west coast and broad ridging from the Rockies eastward. However, however, there are significant differences developing between the models for Week 2 on just how strong the ridge will be versus a re-amplifying downstream TROF over eastern Canada. It appears that despite the evolving pattern change to above normal heights in the east and below normal in the west, the northwesterly flow around the eastern Canadian TROF will be sufficient to bring cooler Temps back into the Great Lakes and New England during Week 2.

In the near term, the developing storm off the Carolinas coast will move slowly out to sea with its rain shield extending over the Mid Atlantic states as warmer air flows northward across the Midwest/Great Lakes starting Friday, reaching into the Northeastern states later this weekend. As the week progresses, short wave TROFs being ejected from the western TROF will bring periods of enhanced Precip chances and isolated severe T-Storms to the central US eastward to Mid Atlantic region. By the start of Week 2, the eastern Canadian TROF will likely amplify, with a cold front slipping southward across the Great Lakes on into southern New England, with Temps falling off to near normal around the Great Lakes and in the Northeast for much of the week. With above normal Precip across the Gulf coast region, Temps will likely average near to slightly below normal through mid May.


El Niño / MJO

Both SST and sub-surface anomalies have essentially stopped increasing, but remain well above normal. Anomalous surface westerlies continue across much of the west central tropical Pacific, keeping SST anomalies above normal while the generally southeasterly wind anomalies in the far eastern Pacific have begun to shift to a more southerly and even SSW direction. El Niño indices remain near moderate intensity, with an ONI value near +1.1°C – the same as for the somewhat more accurate MEI metric. The atmospheric pattern continues to reflect El Niño conditions, with above normal Precip across the central tropical Pacific and what appears to be a forcing mechanism that’s maintaining a mean upper air TROF near the US west coast.

The MJO remains weak and incoherent with model forecasts still calling for a small increase in intensity neat week. However, the latest model runs now forecast the signal to develop in the south China Sea or far western Pacific in about 10 days, in stark contract to the past weeks’ worth of forecasts that called for the emergence of a signal in the far eastern Pacific. (A classic example of how poorly model forecasts are for predicting the development of a significant MJO.) If a significant MJO does develop in the far western Pacific, an enhanced easterly wind anomaly would be expected to develop over the west-central Pacific ahead of the signal itself which will prevent further increases in SST anomalies in the central Pacific (though a westerly wind anomaly can be expected to redevelop to the west of the signal as it translates eastward).


ALASKAN WEATHER

Over the next 2 weeks, the semi-permanent Upper Low near the Aleutians will amplify some as it shifts a bit eastward leading to falling heights over central Alaska. The falling heights and increasing chances of Precip (especially across south Central Alaska) will gradually bring the well above normal Temps down closer to, but still above normal, in most areas during Week 2; though below normal readings will begin to appear over the Northwestern portions of the state along with somewhat stormier conditions in that region by the end of Week 2.




Fig 1: Animation of the overnight operational GFS forecast for 500mb (~18,000 ft) heights and Temperatures. As the last short wave digs into the eastern TROF before the entire TROF moves out to sea, the mean EPAC TROF is expected to deepen and dig into the western US next week, with a broad ridge developing over the central-eastern states by the end of Week 1. By Day 10, the western TROF will de-amplify as short wave energy heads towards the Northern Plains & upper Midwest. During the rest of Week 2, the next upstream NORPAC short wave will repeat the pattern of developing a western US TROF and to a lessor extent, the rebuilding of the broad ridge over the central US.




Fig 2: Sea Surface Temp (SST) anomalies (top panel) have changed little over the past week and remain well above normal across much of the tropical Pacific. Fairly strong westerly wind anomalies (‘arrows’) cover much of the region west of 120° West. Likewise, sub-surface Temp anomalies have changed little over the past week in location or intensity, with readings > +4.0° above normal over the eastern half of the Pacific.




Fig 3: Global SST anomalies remain much above normal across the entire eastern and northwestern Pacific with a wedge of below normal readings in the mid-latitudes of the west and central Pacific leading to a strongly positive PDO. In the Atlantic, the pool of relatively cold water in the North Atlantic to the southeast of Greenland and below normal readings across the MDR (major development region) in the tropical Atlantic persists with little change noted.




Fig 4: Temperature anomaly forecast is based STRICTLY on the GFS MOS model data output which calls for a continuation of below normal readings during the week ahead, with below normal readings confined to the Gulf coast region due to above normal Precip. Confidence for the overall anomaly Pattern and Magnitude is still excellent, with Ratings of ‘5’ on a Scale of 1 to 5.




Fig 5: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (80%) integrated with the 12Z GFS Ensemble (10%) and the 00Z ECMWF (10%) using the projected pattern, along with the explicit surface and 850mb (~5,000 FT) Temperature forecasts. Some Temp forecasts are adjusted for known or expected anomalous thermal patterns and/or projected storm system passages.Pleasantly above normal readings will persist into Week 2 except for the Great Lakes into New England where cool air and some Precip will knock temps down to near or a bit below normal, and the PAC NW. Confidence in the overall Temp anomaly pattern and magnitude is near average for Spring time Week 2 forecasts with the greatest uncertainty being in Temps from the Great Lakes to Northeast giving Confidence Ratings of ‘3’ on a Scale of 1 to 5.




Fig 6: The Temperature ANOMALY forecasts are based on the 12Z run of the Operational GFS using the GFS MOS data for Week 1, and the projected pattern, along with raw GFS surface forecast data for Week 2. Well Above normal Temps should continue into Week 2, though the anomalies will slowly drop off as upper level heights fall off and Precip chances increase during Week 2


✭ The next regular WX Update will be on MONDAY, MAY 4

Steve

NOTES:

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

2. Another Glossary of weather terms is available HERE

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