Nov 2, 2013 Pattern Outlook

2013-14 Pattern Outlook #6:

Slightly revised 11/3/13

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Today’s pattern outlook focuses on the 2-week outlook going into the middle of November. Significant temperature fluctuations are expected this week, along with a rain event on the 7th, returning closer to average early next week before another possible rain event late next week and another cool down in temperatures. Precipitation, however, is still expected to remain below average. (Image credit: PSU e-Wall).

 

 


2013-14 Pattern Outlook Archive

<< Pattern Outlook #5 – October 19, 2013
Pattern Outlook #6 – November 2, 2013
>> Pattern Outlook #7 – TBA

 


2-Week Outlook: November 3-16

  • Temperatures – near average (-1° to +1°)
  • Precipitation – below average (50-85% of normal)
  • NAO – positive
  • PNA – negative
  • EPO – neutral, becoming negative

 

Current Pattern Analysis

10.27_12zPattern Outlook 5 Verification: Since the previous pattern outlook in mid October, a meridonal flow continued across the northern Pacific with persistent lower than average 500mb heights in the north central Pacific, keeping a ridge stationed over the northeast Pacific ocean in turn leading to ridging in the western US and troughing in the central-eastern US. A broad trough settled over the region in the October 23-27 time frame, bringing the first round of below average temperatures in nearly a month and helping to offset what could’ve otherwise been the warmest October on record, but with a relatively moderated cool air mass reaching the region due to a lack of an established strong cold air source in Canada.

Super typhoons Francisco and Lekima, both having reached category 5-equivalent intensity, recurved off the coast of Japan, which aided in the amplification of ridging in the northeast Pacific in the October 26-28 time frame, as pictured above with initialized 500 millibar geopotential heights from October 27 (image credit: NCEP Model Analyses and Guidance). This was noted in the previous pattern outlook as a setup for a potential significant late month cold surge. Cold air did end up spreading south from northern Canada, building the base foundation for an established cold air source, but with ridging even more amplified than originally indicated, as well as a slightly different setup of an upper level low off the western US coast, the shortwave riding along the eastern periphery of the ridging was pulled south towards the west coast, as opposed to the central US, where it split into a closed upper level low. This resulted in a zonal flow over the eastern half of the US, in between a southern US ridge and the strong Canadian upper level low shifting east as ridging from the northeast Pacific expanded eastward. The cold air mass was then unable to spread south into the rest of the US, and only quickly brushed the northern US, where cold temperatures were briefly observed on October 29, before retreating north as a strong southwesterly flow developed ahead of the western US upper level low as it merged back into the jet stream and rapidly intensified as it tracked into Canada, bringing a strong cold front through on 11/1.

Current Pattern: The strong cold front moved through the region on November 1, producing the first widespread rain event in nearly a month across the area with rain totals up to 1/4 inch, locally up to 1/2 inch inland. Windy conditions were also observed as a result of a strengthening low level jet and a narrow but intense squall line mixing some of the stronger winds aloft closer to the surface, and temperatures surged to unseasonably warm levels, in the upper 60s and low 70s. The best forcing quickly lifted north as the shortwave trough tracked into Canada, however, which left the cold front stalled just off the coast. Another broad trough is currently moving into the region, which will push the cold front offshore and bring a brief yet strong cold air mass into the region.

While rain totals were relatively significant, they were not enough to significantly improve the current moderate drought affecting the region, which according to the US Drought Monitor currently covers Long Island, southern Connecticut and SE NY, as well as southeastern Massachusetts, with abnormally dry conditions covering the rest of the area and the Hudson Valley. Central Park recorded 0.36 inch of rain for the month of October, with 0.25″ falling on 10/7, making it the driest month since March 2012 when 0.96″ of rain fell, mostly from a storm at the very end of the month on 3/31, as well as the 3rd driest October and the 11th driest month on record in Central Park. The driest October on record is 1963 with 0.14″, and the driest all-time month on record is June 1949 with 0.02″.

 

Regional 2-Week Outlook

Week 1: November 3-9

f96In the short term, a progressive flow continues as a trough quickly moves through the region, bringing a brief yet strong cool down as 850 millibar temperatures cool down to near -6 to -10C and a strong surface high pressure will be positioned to the north, resulting in widespread lows in the 20s and highs in the upper 30s-40s across the region on Sunday and Monday, November 3-4. A recurring theme in this fall’s pattern, however, has been ridging in the southeastern US and a storm track through the north central US region, a pattern which has been occasionally interrupted as with the late October trough but is re-established. Following the recent collapse of northeast Pacific ridging with a lack of recurving typhoons behind Lekima, the teleconnections reversed to a neutral EPO, negative PNA and strongly positive NAO/AO with an upper level low near Greenland and the Davis Strait.

With another trough currently over the northwestern US shifting east, the upcoming trough will quickly move out, leading to a southwest flow becoming re-established over the region with a strong ridge developing over the southeast US, with 500mb heights up to 590-594 decameters. This will bring a warm air mass into the eastern US towards November 5-7, favoring temperatures quickly rebounding to well above average levels, reaching the 60s across parts of the region. Another low pressure will organize over the central US, tracking northeast through the Great Lakes and producing a snowstorm to the north before moving through the region on 11/7, bringing the next cold front and rain/wind event across the region. A relatively strong low level jet is expected, but weaker than that of the 11/1 cold front, which is likely to support windy conditions again, especially on November 8 with a tight pressure gradient. With a sharp change in air masses behind the front and a negative tilt to the trough expected, however, there is a higher probability of somewhat heavier rain totals than observed on 11/1; over 1/2 inch of rain is possible from this frontal passage, with the highest likelihood north of the NYC area. Colder temperatures will return for November 8 with strong cold air advection, but the cold will be short lasting as a progressive, zonal flow develops over the US, with a weak and progressive shortwave trough quickly tracking east through the northern US.

Temperatures in week 1 are expected to end up 1-2 degrees below average; daily temperature anomalies are expected to be fairly significant, ranging from 10-12 degrees below average on 11/4 to 8-12 degrees above average on 11/7. Precipitation will remain below average, with the only rainfall coming from the frontal passage on 11/7 which should produce at least up to 1/2 inch of rain, possibly higher inland.

 

Week 2: November 10-16

f228As noted in the previous section, a relatively zonal flow is expected to develop over the northern US by the November 8-10 time frame as a weak and progressive shortwave trough moves east through the northern US; this will further be enhanced by strong amplification of ridging in the north central Pacific, leading to a strong upper level low off the western US coast, resulting in a -EPO, -PNA, +NAO and +AO pattern. In this case, however, the progressive flow is likely to lean towards slightly lower heights over the northeastern US than the western US, with a slightly cool start to the week likely in the November 9-12 time frame. Temperatures are otherwise likely to end up above average in the central US, under minor ridging aloft, and slightly below average in the northeast and northwest US. Little storminess is expected in this time frame aside from a weak, moisture starved low pressure producing isolated rain/snow showers in northern regions on 11/10, with little to no rain in the NYC area.

Going forward into the second half of next week, however, the zonal flow is likely to break as troughing expands east towards the central US, but with a continued signal for ridging in the southeast US keeping the storm track north and west of the region, leading to a warm up in temperatures by the middle of next week. Uncertainty is higher in this time frame with large differences in the model guidance regarding handling of the western US and northeastern Pacific pattern, but with an overall theme of western US troughing and northern Pacific ridging likely to continue. The latest long range guidance, however, retrogrades the northern Pacific ridging further west, with a strong upper level low positioned further west off the western US coast. This scenario could allow for a brief ridging signal in the western US by the late week, which would lead to a possible late week cold front and a cool down in the region. The late week setup is subject to change, however, and a different evolution of the trough/upper level low off the western US coast could result in a return to above normal temperatures with troughs occasionally swinging through the region but with limited impact and duration.

Given the aforementioned factors, week 2 is likely to have at least near average temperatures, with near to below average temperatures in the western US and southern Canada, and above average temperatures to the south and southwest of the area. Precipitation is likely to end up near to slightly below average, with a precipitation event possible late next week (11/13-15); given the overall pattern, however, the storm track will likely remain well to the north and west, with precipitation likely remaining below average. Overall, the 2-week period is expected to have near to slightly above average temperatures and slightly-moderately below average precipitation.

 

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