Today’s long range pattern outlook focuses on the medium range pattern going into the end of September. Following the current chilly temperatures, temperatures will warm up late in the week, with another cool down and some storminess possible by next weekend followed by a more fluctuating temperature trend (Image credit: PSU e-Wall).
2013-14 Pattern Outlook Archive
<< Pattern Outlook #1 – September 6, 2013
Pattern Outlook #2 – September 14, 2013
>> Pattern Outlook #3 – September 22, 2013
2-Week Outlook: September 14-28
- Temperatures – near average (-1° to +1°)
- Precipitation – slightly below average
- NAO – positive, becoming negative
- PNA – negative
- EPO – negative, becoming positive
Overview of Current Pattern
Since the previous pattern outlook, the synoptic scale pattern remained very active with significant swings occurring several times. Behind the trough on the 6th, temperatures briefly warmed up on the 7-8th with another brief cool down on the 9th. With both the trough lifting out and a shortwave tracking into the Great Lakes, strong ridging spread into the region, where 500mb heights up to 594 decameters were observed. A strong warm air mass over Iowa was quickly pushed into the region due to strong westerly winds aloft, which had little time to modify with 850mb temperatures as high as 20-22C, which along with mostly sunny skies and a southwesterly flow in the surface brought unusual summer-like heat into the region for Wednesday, 9/11, with temperatures as high as 96 degrees in Central Park and 97 in Teterboro, NJ, and heat index values reaching the low-mid 100s away from the coast. The air mass was warmer than originally anticipated, which along with less cloud cover than forecast resulted in hotter temperatures than forecast with the 9/6 pattern outlook.
As the shortwave entered the Great Lakes with the cold front moving through the region on 9/12, producing widespread strong thunderstorms, strong ridging rebuilt over western North America as a shortwave tracked southeast through Canada, amplifying as it entered the region on 9/13, bringing a much stronger cool down with frost reported in parts of the Great Lakes region; temperatures today struggled to climb above the mid to upper 60s across the area. The initialized 500mb heights from the 9/14 6z run of the GFS (image credit: NCEP Model Analyses and Guidance) was posted above, showing the current trough over the region and ridging in the western US.
Medium-Long Range Outlook
Scroll down to the bottom for a brief summary
The trough currently over the region will exit tonight, but with another trough to enter for Monday-Tuesday, September 16-17. While the previous long range outlook noted that ridging would likely return into the East behind the weekend trough, the possibility was mentioned that either a slower weekend trough that cuts off near the region or a stronger and further south trough in Canada that reaches the region would have a limiting effect on any warmer pattern, which is now the expected scenario as the trough enters the region with another round of chilly temperatures as strong ridging continues to hold in the western US through 9/17.
The 9/17 trough will quickly move east and out of the region, while a strong trough in the northeast Pacific results in increasingly amplified ridging ahead of it, pushing the trough currently over the northeast Pacific east into western North America, forcing the current ridging in that region to slide east into eastern North America. With a polar vortex consolidating over northern Canada and a low pressure developing in the Midwest, as opposed to high pressures diving southeast with recent cool spells, the ridging will manage to spread into the region for the 9/18 to 9/22 time frame. Unlike the 9/11 heat surge, the air mass will not be nearly as warm, with 850mb temperatures forecast to end up mostly in the 10-12C range, supporting widespread temperatures in the mid-upper 70s and possibly lower 80s south of the NYC area. Aside from scattered showers on Monday, 9/16, little precipitation is expected through Friday, 9/20.
A meridional flow will set up over North America beyond 9/20 as the aforementioned strong trough in the northeast Pacific enters the western US, with amplified ridging shifting east into central Canada. Along with a trough placed over the northeast US for the 9/21-24 time frame, producing scattered showers and thunderstorms followed by a cool down with temperatures again near-below average for a few days, the pattern will resemble an omega block-like configuration, as posted above with the latest long range GEFS 500mb height anomalies (image credit: PSU e-Wall). At this time, this appears to be a transient feature and not a real block, and the latest medium range model guidance supports a gradual eastward shift of lower heights into central Canada as a +EPO pattern persists with troughing in the northeast Pacific, with a fluctuating trend between warmer and cooler temperatures over the eastern US for the last week of September, but with lower than average confidence on more specific details in this time frame. Temperatures over the next 2 weeks are overall likely to be near average, balancing the current upcoming chilly temperatures with the warming trend for the late week, with the most positive temperature departures focused west of the region. In addition, the Gulf of Mexico and/or Bamahas region may need to be watched for tropical development beyond 9/20-22, with recurring signals for possible development in that region.