Today’s long range pattern outlook focuses on the medium range pattern going into mid-late September. Following the current chilly temperatures, temperatures will warm up with increased humidity mid-late next week, with long range indications pointing to more sustained ridging and temperatures near-above average as the main jet stays to the north (Image credit: PSU e-Wall).
2013-14 Pattern Outlook Archive
Pattern Outlook #1 – September 6, 2013
>> Pattern Outlook #2 – September 14, 2013
2-Week Outlook: September 6-20
- Temperatures – slightly above average (0° to +2°)
- Precipitation – slightly below average
- NAO – positive
- PNA – positive, becoming negative
- EPO – positive, becoming negative
Overview of Current Pattern
After a chilly first half of August, the month ended with a strong ridge in the central US where a major heat wave was observed, while the northeast US remained under the edge of this ridging with temperatures slightly above average but with high humidity levels. Since then, however, the central US ridging has shifted west, allowing for lower heights aloft to return into the northeast US region. Warmth and humidity briefly returned last weekend, but with a cold front moving through on Monday followed by another strong cold front on Thursday. The latter frontal passage was associated with a strong upper level shortwave trough that quickly dived southeast from Alaska and northwest Canada into the region, with cooler than average temperatures observed today.
The image to the left, from NCEP Model Analyses and Guidance, shows the initialized GFS 500mb heights from the 9/6 0z run, or Thursday at 8 PM, showing the trough over the northeastern US and a strong 594 decameter ridge over the central US, with a cutoff upper level low situated over Oregon in the NW US. The central US ridging is resulting in widespread hot temperatures, reaching the 90s as far north as the Dakotas. This air mass will moderate as it spreads into the area on Saturday, 9/7, with temperatures returning into the upper 70s-low 80s.
Medium-Long Range Outlook
Scroll down to the bottom for a brief summary
The pattern consisting of ridging in the central-western US and troughing in the eastern US will gradually break down over the next week, with changes taking place in the upper level pattern starting in the US with the aforementioned ULL in the northwest US. Another strong trough will quickly drop into the northeast US on Sunday, exiting by Monday with rising heights aloft in southern Canada. A relatively strong low pressure will form near Greenland, however, aiding in shifting the polar vortex north from eastern Canada into Greenland and the northern Davis Strait, resulting in the North Atlantic Oscillation (NAO) entering the positive phase, which consists of lower than average heights in Greenland and above average heights in the region. This shifting of the polar vortex will break the eastern Canada trough pattern that has been in place since late August, with the rising heights in that region allowing for the northwest US shortwave trough to track east into the Great Lakes and northeastern US, forcing ridging to shift east into the eastern US for the September 10-12 time frame as well, with 500mb heights rising to near 588 decameters. Surface temperatures will not be significantly warmer than average in this time frame, as despite 850mb temperatures climbing to 16-18C, widespread cloud cover and storm activity will limit high temperatures into the low-mid 80s, but with humid conditions and mild overnight lows expected.
In between the amplifying trough in the region late next week and troughing in the northeast Pacific, stronger ridging will develop in western North America towards 9/12, which should result in a strong surge of cool temperatures into the northeast US region again in the 9/13 to 9/15 time frame, with frost likely in parts of the Northeast. Once the trough exits the region, lowering heights aloft are likely towards western North America, shifting ridging further east again into the central and eastern US after 9/15, as the surface high pressure slides to the east and leads to a south-SW wind flow, but unlike this week’s brief warmth surge, the ridging is more likely to persist over the eastern half of the US, especially towards the central US, with any source of cool air mostly limited to New England or Atlantic Canada. One area of uncertainty is regarding a possible trough on 9/17, with the latest GFS run showing a stronger trough in central Canada brushing the region, limiting the extent and duration of warmer than average temperatures, while earlier GFS runs and the ECM and CMC models either don’t show this trough or keep it weaker and further north, with widespread ridging covering the eastern half of the US.
The overall pattern is likely to consist of more sustained ridging over the eastern half of the US and southern Canada towards the 9/15 to 9/20 time frame, with the most significant warmer than average temperatures likely positioned towards the north central US and southern Canada, with at least slightly warmer than average temperatures towards the northeast US region, depending on any possible troughs that may approach the region in the long range. The overall 2-week period is likely to average out to near-slightly above average, taking into consideration the two cool spells, one on 9/8-9 and another on 9/13-15, and the warm spells in between and afterwards. With the main storm track well north of the region and a relative lack of frontal passages, precipitation is likely to end up at least slightly below average. At this time, it appears any wetter pattern with more frequent troughs in the region would be delayed until at least after 9/20. Keeping in mind this is a long range outlook, however, there is some room for error in the forecast, including the possibility that the aforementioned trough in the 9/17 time frame reaches the region, with a northwest flow over the region keeping any warm up in the northeast US limited in duration and intensity. Another possibility is that a cutoff low may develop over the region on 9/13-14, which would take more time to exit and would also have a limiting effect on any warmer pattern.