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September 2019 General Discussions & Observations Thread


Rtd208
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After another fall-like day, the coolest night of the season lies ahead courtesy of an air mass that brought several daily record low temperatures to parts of Quebec this morning. Outside New York City, widespread 40s are likely. A few areas where radiational cooling is typically strong could even see readings dip below 40°. However, warmer air will begin to return by Friday. Unseasonable warmth will likely prevail during the upcoming weekend.

In Texas, Galveston picked up 6.17" rain from the remains of tropical storm Imelda. That smashed the daily rainfall record of 2.47" from 1979 and was that city's largest daily rainfall since Hurricane Harvey brought 8.64" on August 29, 2017. The two-day figure of 10.65" was the highest since August 28-29, 2017 when 12.43" rain fell from Harvey.

The ENSO Region 1+2 anomaly was -1.3°C and the Region 3.4 anomaly was -0.3°C for the week centered around September 11. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.65°C and the ENSO Region 3.4 anomaly has averaged -0.03°C. There remains uncertainty about the ENSO evolution later this fall. Despite the guidance, the strong cooling in Region 1+2 indicates that the prospects for a neutral-cool ENSO during the winter have increased. Such a scenario may be somewhat more likely than not. That outcome could have significant winter season implications.

Since 1991 when weekly ENSO region data was available, six years have seen September record 2 or more weeks with -1.0°C or cooler weekly anomalies in Region 1+2: 1996, 1999, 2002, 2007, 2010, and 2017. 5/6 (83%) of those cases went on to feature warm December-February temperature anomalies in the Middle Atlantic and southern New England regions. 2002 had one such week ahead of what would evolve into an El Niño winter.

The SOI value was -20.97 today.

Today, the preliminary Arctic Oscillation (AO) figure was -1.883. A sharp rebound will very likely commence tomorrow.

Since 1950, there have been five prior cases when the AO averaged -0.500 or below in both July and August: 1950, 1958, 1960, 1968, and 2015. The average temperature for September was 69.1° (69.8° after adjusting for ongoing warming) and 58.9° (59.6° adjusted) for September through November in New York City. The 1981-2010 baseline normal figures are 68.0° and 57.5° respectively. The majority of cases saw a warmer than normal September and all cases featured a warmer than normal fall.

Among those five cases, 2015 made a transition from predominantly negative AO values to predominantly positive ones during the fall. The combination of the AO's recent increase to the strongest positive values since April and some of the extended range guidance leave open a scenario where the AO is currently in the early stages of evolving to a predominantly positive regime. Such an outcome would also favor a warmer than normal fall and possible warm start to winter in the Middle Atlantic and southern New England regions. For now, this is merely a hypothetical discussion of one potential path toward winter. Many other variables are involved. The only idea that carries weight at this time is that the possible evolution reaffirms the longstanding signal for a warm fall.

Therefore, a warmer than normal September and fall appear likely. The potential for autumn 2019 to rank among the 30 warmest cases on record remains on the table. For New York City, that would translate into a September-November mean temperature of 58.8°. The latest EPS weekly data shows widespread warmth across the CONUS during October.

On September 17, the MJO was in Phase 8 at an amplitude of 1.200 (RMM). The September 16-adjusted amplitude was 1.078.

Based on sensitivity analysis applied to the latest guidance, the implied probability of New York City having a warmer than normal September is approximately 75% and the probability of a monthly mean temperature of 70.0° or above is near 50%.

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Next 8 days are averaging 69degs., or about 4degs. AN.

Month to date is  +0.1[70.3].       Should be about  +1.3[69.9] by the 27th.

55.8* here at 6am.   55.0* at 7am.   56.4* at 8am.  66.0* by 3pm.

Latest GFS looks 20 degrees cooler for the first 5 days of Oct. than prior run.

 

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Looks like the more active MJO is shifting us back to a warmer pattern for the rest of September. Much more Niña-like pattern with the big ridge building north of Hawaii. So we are shifting away from the big Canadian high pattern that has dominated since August. Going forward we have to monitor the ridging north of Hawaii. That was one of the factors that spoiled last winter. If this ridge becomes more established north of Hawaii, then that record warm SST blob could shift west with it. Probably too early to know for winter. But just something to monitor going forward.

85BBB953-7267-4BE7-B40B-0C47D4813EEB.thumb.png.c4126a8c5f066684c93c8f5709cf4c50.png

AFD12F2F-33E5-4095-B68F-924E591847A8.thumb.png.ba87c850f64a79efb64eb232a736a978.png

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1 hour ago, bluewave said:

Looks like the more active MJO is shifting us back to a warmer pattern for the rest of September. Much more Niña-like pattern with the big ridge building north of Hawaii. So we are shifting away from the big Canadian high pattern that has dominated since August. Going forward we have to monitor the ridging north of Hawaii. That was one of the factors that spoiled last winter. If this ridge becomes more established north of Hawaii, then that record warm SST blob could shift west with it. Probably too early to know for winter. But just something to monitor going forward.

85BBB953-7267-4BE7-B40B-0C47D4813EEB.thumb.png.c4126a8c5f066684c93c8f5709cf4c50.png

AFD12F2F-33E5-4095-B68F-924E591847A8.thumb.png.ba87c850f64a79efb64eb232a736a978.png

That's exactly where you want to see a high pressure in October. Let's hope it stays there for a while. That's a good signal for a great winter. Now let's get a warm october and we'll be in business!

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55 minutes ago, TriPol said:

That's exactly where you want to see a high pressure in October. Let's hope it stays there for a while. That's a good signal for a great winter. Now let's get a warm october and we'll be in business!

A big ridge north of Hawaii in October usually occurs with more of a La Niña-like pattern. The late September pattern is probably related to the MJO becoming active again. Very confused Pacific state currently. -35 daily SOI and +1 IOD are more El Niño-like. The cold Niña 1+2 is typical for a La Niña. Plus we have the near record SST blob again in the NE PAC. Kind of difficult to predict how all these competing influences will evolve and interact for the winter pattern at this early point.  

277E4445-DEC1-4990-803D-D5C6E486933D.png.ab8314cb005ef1cb8108c989708fa89a.png

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