Sept 28, 2013 Pattern Outlook

2013-14 Pattern Outlook #4:

f120Today’s pattern outlook focuses on the 2-week outlook going into the middle of October. Temperatures will gradually warm up this week back into the mid 70s to low 80s, with the next rain event around October 5-7 along with a brief cool down, but with the overall 2 week period likely ending up warmer than average along with below average rainfall. (Image credit: PSU e-Wall).

 

 


2013-14 Pattern Outlook Archive

<< Pattern Outlook #3 – September 22, 2013
Pattern Outlook #4 – September 28, 2013
>> Pattern Outlook #5 – October 19, 2013

 


2-Week Outlook: September 28-October 12

  • Temperatures – above average (+2° to +5°)
  • Precipitation – slightly below average (50-75% of normal)
  • NAO – negative, becoming neutral
  • PNA – negative, becoming neutral
  • EPO – positive

 

Overview of Current Pattern

9.28_18zSince the previous pattern outlook, a strong cool air mass moved into the area, with temperatures ending up colder than average; Central Park had a low of 47 degrees on 9/24, the first sub-50 September low since 2009 and the coolest September temperature since 2000. Daytime highs have been closer to average, in the upper 60s to mid 70s. The 18z GFS initialized 500mb heights (image credit: NCEP MAG) have been posted to the left, showing the current synoptic pattern consisting of an amplified ridge west of the area extending into southeast Canada, with a strong shortwave trough in the Midwest and a broad upper level low off the East Coast. Despite the strong ridging, temperatures will fail to warm up over the area this weekend, as a warm air mass with temperatures at 850 millibars near 14-16C stays well north of the region, while a stale cool air mass associated with a cutoff low near Nova Scotia a few days ago slowly spreads into the region with 850mb temperatures near 8C.

Temperatures over the last week have been slightly below average across the region, at least 2.1 degrees below average in Central Park in the September 22-28 time frame. No precipitation fell over the last week; the last widespread rain event was on Saturday night, September 21. As will be discussed in more details in the next section, this upcoming week will be dry as well, making for a 2-week stretch of no rainfall across most of the region.

 

Regional 2-Week Outlook

Scroll down to the bottom for a brief summary

f120Week 1: The ridging currently over the central US will shift east over the next day, allowing for the upper level low to track north with a surface low pressure developing offshore; there will be no feature to pull this northwest into the region, however, as the strong shortwave trough in the Midwest quickly lifts north into Canada, with the cold front currently in the Midwest collapsing well before reaching the region. Instead, the low pressure will continue to slowly drift north towards Nova Scotia, while temperatures begin to slowly warm up across the region. With the front collapsing to the west and the coastal low staying east, the region misses out on the only rain potential this week, as a high pressure takes hold of the pattern again starting on Tuesday.

The next key player in the forecast is a strong low pressure currently in the northeast Pacific, with a strong upper level trough positioned overhead with 500 millibar heights at least 36-48 decameters below average, leading to relatively strong positive EPO values. Around the time of the previous pattern outlook, this was modeled to stay in the northeast Pacific as ridging builds in western North America, although there have been changes in the anticipated setup over the last few days, as the strong trough is expected to quickly progress east through southern Canada bringing a stronger jet stream along with it, resulting in a zonal flow over the western US and a continued negative PNA, as opposed to earlier outlooks for a positive PNA spike around this time frame. This setup fails to result in a cool down, as the trough stays north of the region in southern Canada, quickly pushing the offshore low near Nova Scotia to the east. With the westerly flow aloft, a warm air mass with 850mb temperatures near 14-16C will quickly spread into the region by Wednesday, 10/2, without being interrupted by any sort of cutoff low to the east, the first time a warm air mass enters the region uninterrupted since the 9/10-12 heat surge. As a result, temperatures will warm up into the mid 70s to low 80s across the region for the October 2-5 time frame; the only interruption will be in the form of a back door cold front staying mostly north of the area. Temperatures may reach the low 80s in parts of the NYC area, which is at least 10 degrees above average.

Temperatures in week 1 are expected to end up warmer than average due to the upcoming warm air mass, with departures likely around 4 to 8 degrees above average across the region, including the NYC area as well. Some rain is possible on Saturday, 10/5, ahead of the next storm, but with otherwise no precipitation this week, which should lead to below average precipitation again.

 

Week 2: A brief spike in western US ridging is likely around 10/5, which along with amplifying ridging in the eastern US allows for a trough to dive into the central US with a cold front approaching the region, which in the 10/6-8 time frame is likely to bring the first widespread precipitation event to the region in 2 weeks. The next question is regarding the setup of the trough; an overall progressive jet stream is expected over North America with a continued +EPO and lower heights aloft favored over the northeast Pacific, although the model guidance shows different setups ranging from a progressive trough that moves through with a brief surge of chilly temperatures followed by another ridge returning into the region, to a slower and more amplified trough that near cuts off near or south of the region. Additionally, the Gulf of Mexico is in question as well, as a tropical disturbance currently in the Caribbean Sea is likely to track north into the Gulf of Mexico, where some models show the potential for a tropical cyclone to develop. Given the lack of tropical cyclones this season, as well as a marginal environment for tropical cyclone development in the Gulf of Mexico, it would not be surprising if a storm fails to develop, although this system will nonetheless be monitored for potential development, with a stronger and further north storm possibly having more significant impacts on the developing setup.

Beyond the weekend cold front, temperatures are expected to cool back down to near or possibly slightly below average around the October 6-8 time frame. The progressive pattern with the persistent trough in the Gulf of Alaska, however, is likely to prevent any trough or cool air mass from settling over the region, with a rise in heights aloft expected again with near to higher than average heights likely again towards the second half of week 2, which also results in temperatures likely warming back up to above average. Given that this is beyond the 10-day range, the smaller aspects of the pattern are subject to change, although the overall pattern tendency with troughing likely in the Davis Straits region favoring a neutral to slightly positive NAO, and more importantly the progressive pattern in the northern Pacific into the western US with persistent lower heights in the Gulf of Alaska, keeping the PNA at least slightly negative, is unfavorable for a cold pattern to set up, with any such potential likely delayed until the second half of October, when a greater chance of more frequent precipitation events also exists; even then, until the progressive pattern breaks down in the northeastern Pacific, a colder pattern is likely to struggle settling over the eastern half of US, with indications at this time for slightly to possibly moderately above average temperatures for October as a whole, with the most significant warm anomalies north of the area.

Given the aforementioned factors, week 2 is likely to have at least slightly above average temperatures, with the most significant temperature anomalies north of the area. Precipitation is likely to end up near to slightly below average, primarily due to the rain event on October 5-7, with drier and warmer conditions likely towards the middle and end of week 2. Precipitation trends may have to be watched more closely in future updates due to the recent lack of rainfall, especially should the October 5-7 storm underperform; the September 24 Drought Monitor update highlights Long Island and southern Connecticut under D0, or abnormally dry, conditions. Overall, the 2-week period is expected to have above average temperatures and slightly-moderately below average precipitation.

 

Summary: Warmer than average temperatures will return for this upcoming week, with no rain still in sight, as temperatures warm up to at least 4-10 degrees above average across the region. The next rain event will affect the region around October 5-7, along with at least a brief cool down in temperatures, although an overall progressive pattern is likely to hold in place. Week 1 is expected to have above average temperatures and well below average precipitation, and week 2 is likely to have slightly above average temperatures and near-slightly below average precipitation. The overall 2-week period is likely to end up with above average temperatures and slightly-moderately below average precipitation.

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