Sept 22, 2013 Pattern Outlook

2013-14 Pattern Outlook #3:

f132_2Today’s pattern outlook focuses on the 2-week outlook going into the end of September and the beginning of October. Another round of chilly temperatures will start off this week, with a gradual warm up into next weekend; a temporary decrease in cool air is likely towards the late month but with a cooler start to October possible. (Image credit: PSU e-Wall).



2013-14 Pattern Outlook Archive

<< Pattern Outlook #2 – September 14, 2013
Pattern Outlook #3 – September 22, 2013
>> Pattern Outlook #4 – September 28, 2013


2-Week Outlook: September 22-October 6

  • Temperatures – slightly below average (-2° to +1°)
  • Precipitation – slightly below average
  • NAO – negative, becoming neutral
  • PNA – negative, becoming positive
  • EPO – positive


Overview of Current Pattern

9.22_18zSince the previous pattern outlook, a strong trough entered the region on the 17th, with temperatures well below average, peaking in the 60s for highs and mid 30s-mid 40s for lows away from NYC and the immediate coast; Central Park fell to a low of 50 degrees on 9/17, making it one of the coolest mid September mornings since 2007. The strong high pressure persisted over the region in the surface while ridging built in aloft ahead of a strong trough in the north central US; while daytime highs reached the mid-upper 70s across the area on 9/19 through 9/21, overnight lows were cooler than originally indicated; the last 7 days were at least 4-6 degrees colder than average in the area. A strong cold front moved through on the 21st, producing showers and thunderstorms with 1/2 to 1 inch of rain, bringing an end to the warm up.

The latest 500mb geopotential heights and vorticity map posted above from the 18z GFS (image credit: NCEP Model Analyses and Guidance) shows another strong trough over the region, with temperatures over the next few days expected to return back to moderately below average levels. Ridging is present over the central US with troughing in the eastern and western US; although ridging will gradually shift east, it will fail to settle over the region as will be discussed in more details in the next section. There were some hints of tropical activity recently with the NHC having monitored low pressures in the southern Gulf of Mexico and east of the Bahamas for potential tropical development, but none of these storms developed, a scenario that has become typical of this year’s hurricane season, which has been unusually quiet and very inactive compared to expectations with the lowest Accumulated Cyclone Energy values since 1983 so far.


Regional 2-Week Outlook

Scroll down to the bottom for a brief summary

f132_2Another strong trough is currently located over the region, with temperatures from 9/23 through 9/25 expected to return to moderately below average levels. The trough will gradually shifting east, but with above normal heights building into eastern Canada and the Davis Strait, leading to the NAO index rising to strong negative values for this time of the year with a standard deviation of -2, the trough will split into a cutoff low and slowly drift east, reaching Newfoundland by 9/26. This will keep the New England region under the western influence of the trough.

Ridging currently over the central US will extend east, but will fail to build into the region as a shortwave trough currently over the southwest US will detach from the main flow, tracking east towards the region. As it does so, a strong trough will dive into the western US, leading to amplifying ridging over the central US, which in return causes the shortwave to track southeast into the Atlantic Ocean while keeping lower heights aloft stuck over the region for the late week (9/25 to 9/28). This will keep the region under the influence of the cutoff low pressure through next weekend with temperatures struggling to warm up above average as the ridge shrinks, then collapses ahead of the next trough by 9/29. This setup is represented above with the GEFS forecast 500mb height anomalies at hour 132 (image credit: PSU e-Wall). Temperatures will ultimately warm up to at least slightly above average by 9/29, but with these temperatures only staying for a brief period of time. With this taken into consideration, temperatures over the next week are likely to end up 1-3 degrees cooler than average across the region. With the jet stream well to the north and the only low pressure around being the cutoff low to the east, no precipitation is expected over the next 7 days.

More significant changes in the synoptic flow will take place towards 9/29 as a strong trough likely develops in the Alaska and NE Pacific regions to the east of anomalously strong ridging in the central Pacific. This leads to the development of a +EPO pattern, with rising heights in the western US leading to positive PNA values, but with the persistent lower heights in the NE Pacific bringing a mild Pacific air mass into the western US and Canada, with widespread above average temperatures expected in Canada. An +EPO on average often correlates with above normal heights and temperatures across much of North America, but a +PNA correlates with the opposite, with above normal heights and temperatures in western North America and below average heights further east. The current negative NAO pattern is likely to moderate by this time period, but with the overall +EPO/+PNA/-NAO setup likely leading to slightly above normal heights in the western US and potentially slightly below normal heights in the central-eastern US by 9/31. Another cool down is likely in the region towards the end of the month into the start of October, although how cool temperatures get relative to the current cool spell is uncertain as the cool air source in Canada will be temporarily displaced further north, and temperatures may not be as cooler than average as the current cool spell.

Going further into the long range, some of the model guidance suggest the potential for another spike in western US ridging as the +EPO moderates with lower heights retrograding near or west of Alaska, and/or a rebuilding of positive height anomalies in eastern Canada around October 5; should this scenario verify, especially if the +EPO pattern starts to moderate as currently forecast, the potential may be there for a moderation in temperatures after the early October trough followed by a stronger cold air mass entering the region with the mean trough near the region. As with any long range potential, this can still change, however, and would partially depend on where both of the above factors end up, with another alternative scenario should the +PNA and/or -NAO fail to verify resulting in a pattern consisting of near-above average temperatures, although even if the latter scenario does verify, a sustained ridging pattern is not expected to build into the eastern US.

At this time, week 2 appears to have temperatures at least slightly above average, partially depending on any potential cool spell later in the week 2 period, which should moderate the negative anomalies of this upcoming week but may not be enough to bring the 2-week period to solid above average temperatures territory. With a cold front likely around 9/30, increased precipitation chances are likely around week 2, with precipitation at least near to slightly below average, but with the 2-week precipitation likely slightly to moderately below average largely due to the lack of rainfall in the upcoming week.


Summary: Cooler than average temperatures will return for the early-mid week, with a slow moderation in temperatures but still ending up at least near average by the late week, with no rain expected through 9/29. A cool down is likely around the end of the month into early October, with the possibility of a more significant cool down after the start of October. Week 1 is likely to feature below average temperatures and precipitation, and week 2 is likely to feature slightly above normal temperatures and average precipitation.

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