2013-14 Pattern Outlook #5:
Today’s pattern outlook focuses on the 2-week outlook going into the rest of October and early November. Temperatures will remain slightly above average early this week, but with a more significant cool down occurring later this week into the weekend, with another round possible later next week, as temperatures end up slightly below average but with the dry pattern continuing as well. (Image credit: PSU e-Wall).
2013-14 Pattern Outlook Archive
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Pattern Outlook #5 – October 19, 2013
>> Pattern Outlook #6 – November 2, 2013
2-Week Outlook: October 19-November 2
- Temperatures – slightly below average (-2° to -4°)
- Precipitation – below average (50-75% of normal)
- NAO – negative, becoming positive
- PNA – positive, becoming neutral
- EPO – negative
Current Pattern Analysis
Since the previous pattern outlook in late September, a generally inactive pattern continued across the region, with temperatures well above average, ending up about 4-8 degrees above normal for the first half of the month. Precipitation stayed well below average as well; Central Park only recorded 0.28″ of rain this month, 0.25″ of it from a cold front on Monday, 10/7, which also produced an EF-1 tornado in northeast NJ. Precipitation this fall has been below average, with rain totals over the last 30 days generally between 0.5 and 1.5 inches, locally up to 2-3 inches towards interior parts of the area, about 25-50% of average. As such, the latest US Drought Monitor shows most of the area covered under D0, or abnormally dry, conditions. While this is not officially considered a drought, below average rainfall remains a concern through the foreseeable range.
Despite the continued mild and dry pattern over the region, however, significant changes are taking place in the northern hemispheric pattern. Posted above is the initialized 0z GFS map for 500 millibar heights and vorticity over the northern Pacific Ocean from October 15 (image credit: NCEP Model Analyses and Guidance). To the far left, typhoon Wipha can be seen over eastern Japan, where it transitioned from a super typhoon into a powerful extratropical cyclone. To its south is a small area of higher vorticity, approximately around 12N latitude, which would later become what is now a category 5-equivalent super typhoon Francisco also threatening Japan, and will be discussed more in depth in the next section. Around the same time that Wipha underwent extratropical transition, anomalous strong ridging developed in the northeast Pacific Ocean, which is shown above in its early stages. Partly due to Wipha, ridging continued to amplify in the northeast Pacific and has settled in place, and persistent blocking near Greenland, stronger troughing expanded south into central Canada and the north central US. Despite this, however, the cold has not been able to reach the northeast US region yet with a persistent southwest flow and the trough axis being placed too far west.
Regional 2-Week Outlook
Scroll down to the bottom for a brief summary
Week 1: October 19-25
In the short term, despite the persistent amplified ridging in the northeast Pacific and blocking near Greenland, the colder pattern will fail to reach the area until Wednesday, 10/23. The trough axis is generally near the Midwest region, and with a relatively strong southwest flow in the upper levels, the colder air masses have been unable to significantly spread east. A strong shortwave trough moved through on Thursday night, producing isolated showers with more widespread rain northwest of the area, but quickly tracked northeast into Canada with only a slightly cooler air mass reaching the area. The trough currently over the north central US will swing through the region on Saturday night, producing isolated showers again, but will also be quickly removed with rising heights aloft and a southwest flow ahead of a stronger upper level low digging into the north central US. As a result, temperatures will only slightly cool down this weekend before warming up again on Monday and Tuesday; even through 850mb temperatures will be near 0C, surface temperatures may not be as cool as the mid September time period.
The colder air mass will be able to reach the area by Wednesday, however, as the strong upper level low enters the northern US, and along with the northeast Pacific ridge axis shifting slightly east, the trough will begin to shift east into the region. This shows the typical model bias of speeding up pattern changes too quickly; a week ago, the general model consensus was for below average temperature anomalies to begin two days ago, on Thursday (October 17), but this was delayed to this weekend, then to Tuesday, and now until Wednesday-Thursday. Temperatures will gradually cool down starting on Wednesday, but the main uncertainty at this time is regarding Thursday’s setup as the trough axis swings through the region while becoming neutral to negatively tilted. At the very least, a weak wave of low pressure is likely to develop offshore, but with some of the model guidance developing this further west and closer to the coast, which would produce a more widespread rain event and colder high temperatures. Any such storm would likely be relatively fast moving and still in its developing stages should it affect the area given a progressive flow. There is uncertainty at this time regarding the exact setup, as blocking is also expected to fall apart near Greenland in this time frame with the NAO changing from its negative to positive phase, and there is also the question of how much amplification takes place; at this time I am leaning towards a developing low pressure that produces some rain over the area before quickly moving out, as opposed to a setup slow and amplified enough for a major storm to affect the region with heavy rain and interior snow, as the earlier GFS and ECM runs indicated. The 7-day outlook will be updated later today and includes a chance of rain, especially further east, but this is subject to change and will be discussed more in depth with the daily forecast discussions. Behind this system, however, the colder temperatures aloft will shift overhead into the region, which along with a high pressure building in will bring cooler temperatures, with daytime highs likely in the low-mid 50s range and overnight lows in the 30s, with the first widespread frost of the fall likely around this time period. Lake effect precipitation is expected towards the late week as well, with snow showers expected in the higher elevations of the Northeast and the lake effect regions.
Temperatures in week 1 are expected to end up about 1-3 degrees below average; temperatures through 10/22 are expected to remain 2-3 degrees above average, while the second half of the week is likely to end up 4-7 degrees below average. These anomalies are partially dependent on any possible storm on Wednesday and Thursday; should the storm have more significant impact in the area, colder temperatures during and after the storm would lead to anomalies leaning towards the colder end of the range, otherwise leaning towards -1 degree below average. Precipitation will remain below average, as aside from the potential midweek storm which is unlikely to produce significant amounts of rainfall should it affect the area, and isolated showers tonight, no rain is expected.
Week 2: October 26-November 2
As the midweek system moves out, the trough axis will be over the region, with a colder air mass overhead with 850 millibar temperatures near -4 to -6C. Along with more sunshine and decreasing winds, a trend for colder temperatures is expected for the late week and into Saturday, with the first widespread frost of the season likely around this time frame. Beyond Saturday, however, the main question is regarding the duration of the cold air mass. By this time period, an intense low pressure with possibly sub-950mb minimum pressure is expected to develop near the Aleutian Islands, which will help to keep persistent lower heights in that region. Along with a stronger upper level low trapped underneath the northeast Pacific ridging, setting up a rex block configuration, the persistent ridging in the NE Pacific and the -EPO pattern are expected to continue into next week. The NAO, however, is likely to be positive, as blocking is expected to collapse by the late week. With the weekend trough axis shifting east and another trough likely digging into the north central US, a moderation in temperatures is likely around next Sunday or early-mid next week, around 10/26-28, with temperatures returning to near to slightly above average as weak ridging builds into the region and the surface high pressure stays south of the area, keeping a southwest flow in place. There is some uncertainty with the timing of this moderation, which could start as early as Saturday or end up delayed and shortened into the middle of next week, and any such warm-up is likely to remain brief as troughing likely returns later next week with temperatures cooling down again, but as with this week the core of the colder temperatures is expected to stay west of the region.
Beyond the middle of next week, attention returns to the West Pacific where an active typhoon season continues, as super typhoon Francisco is expected to impact Japan and transition into an extratropical cyclone. Additionally, another typhoon may develop behind Francisco, which some of the recent longer range model runs indicate may play a role in the pattern setup as well, helping to reinforce the current pattern. With persistent troughing near the Aleutian Islands, ridging is likely to continue in the northeast Pacific through the end of the month, and along with the reinforcement of the ridging next weekend due to the strong Aleutian Islands low, the potential is there for a stronger surge of cold air to dive down from northern latitudes after the second half of next week, capable of bringing more significant negative temperature anomalies into the eastern half of the US. Should this occur, it would likely reach the US by the last few days of the month into the start of November, around the Halloween time frame, with the highest probability of colder temperatures near the central US, extending into the eastern US but to a somewhat lesser extent. The amplification of ridging in the northeast Pacific will play a role in the setup of this potential cold surge, as flatter and further east ridging would result in a less significant cool down. Current indications are for the pattern to begin gradually breaking down around early November as the -EPO and +PNA patterns weaken while the NAO remains positive, although the timing of the potential pattern moderation is still uncertain, keeping in mind how the models recently sped up pattern changes by ending the warmer than average pattern too quickly.
Given the aforementioned factors, week 2 is likely to have at least slightly to moderately below average temperatures, with the most significant temperature anomalies west of the area. Precipitation is likely to end up near to slightly below average, with a precipitation event possible around the middle of next week (10/28-30) ahead of any potential secondary surge of cold air into the US. In general, over the next 2 weeks will have to be watched for how much precipitation falls, as aside from the potential midweek storm this week and possible rain around mid next week, the drier than average pattern appears likely to continue into November. Overall, the 2-week period is expected to have slightly below average temperatures and slightly-moderately below average precipitation.