Overview: The winter of 06-07 will likely feature temperatures significantly colder than last winter; however, much of the nation should still average near to above normal for the DJF (meteorological winter) period. Precipitation will be plentiful across the southern part of the country as per historical climatology during Nino winters. I anticipate the core of the warmest temperature anomalies to persist over the northern plains and the northern Rockies; while the coolest anomalies will dominate near the East coast. Factors taken into consideration: ENSO pattern, NAO, AO, PDO, PNA, and of secondary importance: MJO, solar-flux, northern hemisphere snow cover, and recent global trends in temperature / precipitation.
ENSO: The intensity of the El Nino this winter will ultimately determine the duration and power of the cold. The difference is not that significant when comparing weak to moderate El Niño’s, in terms of their corresponding temperature/precipitation patterns in the US; however, winter season temperatures vary drastically when one compares the resultant weather in moderate / strong El Nino years.
As per the latest numbers, ENSO region 1-2 is now +1.5c, region 3 and 4 at +1.2c, and the weakest of the anomalies, region 3.4 at +1.0c. Below is a list of the strong El Nino years since 1950 and their November ENSO region 3.4 readings:
November 1972: + 2.0c
November 1982: + 2.2c
November 1991: + 1.4c
November 1997: + 2.5c
Considering nearly all strong El Niño’s peak in the Nov-Dec time frame; (the current ENSO 3.4 reading of +1.0c) I believe the probability of a strong El Niño this winter is highly unlikely. SOI dailies have been running negative; however, there has been some slight cooling in the eastern tropical pacific over the past couple weeks. Lag time may allow another burst of warmth in the equatorial regions a couple weeks down the road – but typically SOI values are much lower than they are presently during an intensifying strong El Niño.
With that being said, let’s take a look at the Atlantic tropical seasons during the mentioned strong El Niño years:
1972: 7 Named Storms
1982: 6 Named Storms
1991: 8 Named Storms
1997: 8 Named Storms
2006 to date: 9 Named Storms
This is interesting to me; as it implies the El Niño — although it may have been weak throughout the hurricane season – had quite an affect on tropical cyclone genesis. The lack of activity early on was due in part to the extensive SAL (Saharan Air Layer) in the east Atlantic, as it inhibited low-level convergence and cut off moisture supply; however, I’m somewhat surprised there haven’t been any named storms the past several weeks. SAL is long gone, and the upward motion pulse remained over the Atlantic for a couple weeks earlier in October – this tells me the upper-level tropical trade winds associated with the developing El Niño were powerful enough to prevent TC genesis. (at least played a significant role) Therefore, I’m expecting a high-end moderate El Niño, peaking sometime late month or perhaps the first half of December – ENSO 3.4 readings should peak between +1.3c and +1.5c, similar to 2002-03, 1957-58 and 1994-95. As a result, my three primary analogs in terms of the ENSO state are 2002-03, 1957-58, and 1994-95, in no particular order. (Although 02-03 and 57-58 appear slightly better matches, IMO)
Other Teleconnection Indices:
NAO & AO: In my view, these two indices will aid in preventing the El Niño from overwhelming the pattern for more than a week at a time. The evolution of the synoptic-scale pattern over the past couple of months has favored a mainly –NAO/-AO regime, with plenty of blocking up across the pole. It’s interesting to note the onset of the El Niño back in August corresponded quite nicely with the onset of the –NAO development.
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.sprd2.gif
Notice how the ensembles have been persistent in their idea of forecasting a rebound above 0, however, more often than not they have busted too positive. After a brief 7-10 day spell of near to slightly positive NAO values, the NAO should trend downward for the late month period. As others have mentioned, the correlation between November NAO values and the following winter is quite strong. This is part of the reason why I anticipate the NAO to average near-neutral for DJF. (Possibly a bit on the negative side)
The SSTA profile in the Atlantic has been improving over the past couple months; approaching the classic “tripole” appearance – warm near southern Greenland, cool from the EC out into the central Atlantic, and warm in the tropical genesis regions. No, it’s obviously not perfect, but it certainly favors a –NAO pattern over a +NAO one. My final reason for a neutral-negative dominating NAO winter is the fact that we’re heading towards the more negative decadal cycle seen in the late 40’s – late 70’s.
http://weather.unisys.com/archive/sst/sst_anom-061029.gif
The AO (Arctic oscillation) has been negative most of October, and is forecast to take a dive into the tank after a brief period of positive values next week. There is also a fairly strong correlation between the AO phase of the autumn and the corresponding phase during the winter, In this case, the amount of high-latitude blocking we’ve seen over the past several weeks has me convinced that the AO will also average near to slightly below average in the DJF period.
PDO & PNA:
I’ve grouped these two indices together as they usually work in tandem – i.e, a +PDO correlates to a +PNA most of the time; and a –PDO and –PNA. In the SSTA map I posted above, it’s clear that we’re near-neutral right now; however, it’s quite a shift from last summer. The warm anomalies in the West Pac. have been eroded, and the cooler SST’s in the NE pacific are slowly weakening. We still have a ways to go before we enter into a solidly +PDO phase, but the majority of moderate El Nino’s have a +PDO – so I expect this to be the case. PDO will average on the positive side (maybe only slightly) for the DJF period. As for the PNA, we’ve seen persistent negative values as of late, but typically a +PNA occurs in a +PDO winter. Therefore, I’m anticipating a slightly positive PDO/PNA regime this winter.
I feel my secondary considerations have been discussed in great detail as well, since this outlook is later than most, so I’ll conclude now with a summary and the NYC numbers.
Conclusion: Winter 06-07 will feature temperatures slightly below average in the Eastern US, with above normal readings mainly west of the MS River. December should be the warmest of the three winter months, with February being the coldest and wintriest. Due to my expectation of a moderate El Niño winter, analogs indicate a rather abrupt end to winter near the end of February or early March. With that said, the amount of storminess (enhanced STJ) should allow for plenty of winter storms in December, January, and February. There should be quite a few mixed precip events in the I-95 corridor in December – with the bulk of the snow falling in January and February. This is not to say there can’t be a significant snowstorm(s) in December, but I feel the probability of mixed/rain events is highest the first half of the meteorological winter. Snowfall will be below average in most of the Pac. NW, Rockies, and Plains – near-normal in the Great Lakes region, the deep south, and the coastal SE – above average snowfall from the interior SE US up through the NE. Not going to forecast exact totals, but I believe the Northeast will have 125% of normal snowfall, at the minimum. NYC may near 40” or surpass it yet again this winter.
NYC numbers:
DEC: -0.5 to +0.5
JAN: 0 to -1.0
FEB: -0.5 to -1.5
MAR: 0 to +1.0
Snowfall: > 35”
Verification:
Dec: F
Jan: F
Feb: F
Monthly: F
Seasonal DJF departure: C-
Snowfall: F
Winter 06-07 final grade: D-
Tags: forecast, weather, winter