Buongiorno,
I hope everyone had a nice Thanksgiving break. Hard to believe Christmas is a short few weeks away. It sure did not feel like Christmas these last few days or even weeks. Temperatures have been averaging well above normal. This looks to continue into the first couple of weeks of December which is not surprising under a pattern driven by a strong El Nino.
SHORT RANGE - Week of November 29th
I do not want to spend too much time speaking about short term weather. Expect gloomy and rainy conditions Tuesday and Wednesday this week. A low pressure system will cut off to our west sending mild Gulf air and moisture northward into our area.
Anywhere from half an inch to one inch of rainfall is likely. The weather will clear by Thursday and temperatures will also take a dip into the mid to upper 40's. Temps will hover around 50, plus or minus a few degrees, into the weekend with clear conditions expected.
MEDIUM RANGE - Week of December 6th
Another storm system is expected to impact the area exactly 1 week from this week's storm, approximately Tuesday December 8th into Wednesday December 9th.
12z EURO 500mb vorticity / heights valid Monday December 7th:
As of this moment, the EURO and GFS models are in fair agreement of a storm cutting off well to our west again. What is different with this system compared to the one we'll get rain from this week is the low pressure system does not track into Chicago and the Great Lakes. Instead, it tracks into the Northeast. It's likely to cut North and West of I-95 which will send warm air advection into the coast. Normally a storm track this time of year NW of I-95 could bring wintry weather to the interior Northeast, but with the Polar Jet forecasted to be in central Canada (2nd image) there is no cold air to be found. Additionally, we're probably looking at increased rainfall amounts with this system in excess of 1 inch. However, storms have trended weaker / drier this year so we'll see how we look by next week's Mo Mo blog. For now, expect more rain and mild temps early next week too.
LONG RANGE - Winter Pattern Update
With Meteorological Winter beginning Tuesday (Met Winter is Dec 1st to Mar 1st), I figured it would be a good time to check up on the pattern. Please give my Winter Outlook a read if you have not already so you have an idea of how I expect the winter pattern to progress through each month.
SSTA's in the Pacific still shows a raging El Nino over the Equatorial Pacific and a +PDO, defined by the above normal SSTA's in the Northeast Pacific Ocean.
ENSO tracker of each Nino region from StormVista over the last month suggests El Nino has strengthened through November. As of last week, the ONI value for Nino region 3.4 was 3.1*C. To put that in perspective, not even the Super Nino of 1997-1998 saw an ONI value get that high. Averaged over the entire winter 1997-1998 is lijely still to come out on top. The reason why there's a bit more optimism surrounding this winter if you're a winter weather fan is because most forecasters, including myself, believe this El Nino event will peak / weaken sooner than the 97' event. As Nino weakens, so does the Sub-Tropical Jet stream thus allowing the Polar Jet Stream to have more influence on the overall pattern (i.e. driving cold air into the central & eastern CONUS).
The image on the left is a 500mb height anomaly composite of +PDO and strong El Nino years for the month of December (thanks to Isotherm for pulling image off the ESRL site). In case you can not read the years, they are: 1957, 1965, 1982, and 1997. On the right is the forecasted 500mb height anomalies in the 8-10 day range from the EURO and GFS models. The yellow circles are intended to easily show the similarities between those strong El Nino December's and our upcoming December pattern. What's most common between these three images is all agree on the placement of the Alaskan Vortex, which keeps the EPO positive and helps spread mild Pacific air into the country. Therefore, it's not a surprise my Winter Outlook along with many others called for a warm start to the Meteorological Winter. The million dollar questions are:
1. When does El Nino peak?
2. When does the Alaskan Vortex retrograde to the Aleutians and promote ridging into the NE PAC and western U.S.?
3. When will the AO and NAO go negative, if at all?
Let's start with #1. One way I like to tell if the current El Nino is close to peaking is by evaluating the state of the trade winds and monitoring the trends of ONI (Oceanic Nino Index) in each Nino region (there are 4 of them) over the last month or so.
The CFS forecast (solid black line is present time) is predicting a series of Kelvin Waves (purple) and Rosby Waves (green) to intersect over the International Dateline (180). I circled where that is expected to happen for a visual understanding.
If we overlay OLR (Outgoing Longwave Radiation) onto the same map, we notice negative anomalies showing up between 180 and west of 120W which points to convection, or enhanced thunderstorm activity, to continue over that area. This promotes tropical forcing which could eventually lead to transient ridging over the western U.S. within the next 2 weeks.
Taking the equatorial waves into consideration as well as the OLR forecast from the CFS when looking at 850mb zonal wind anomalies, it's no surprise strong WWB (westerly wind bursts) are expected over the International Dateline and just west of 120W. This is an indication winds will shift from blowing east to west (normal state of winds over the Equatorial Pac) to west to east and that will help transfer warm water to the central ENSO regions. Additionally, the blue shadings east of 120W suggest easterly winds to persist over the east-central and eastern-most ENSO regions. This is a pretty good indication temperatures will / already have stopped warming over Nino regions 1+2 as there is no mechanism for the warming to continue.
While the easterlies are a promising sign Nino regions 1+2 and possibly 3 have stopped warming, it remains to be seen if that's the case for Nino region 3.4. These charts suggest there is still room for additional warming but notice the amplitudes of these WWB's are not nearly as strong as they were in October and early November. The latest ONI values have not been released yet. Once they are, I will be sure to provide everyone an update on the forum. They will give us a good indication if we've finally reached a peak. I do believe there's still room for the central / western Nino regions to warm given the state of the MJO and trade winds, but a peak should come by early to mid-December which puts us in a pretty good place for the 2nd half of winter.
Other then studying ENSO and Pac/Atl SSTA's, we can also analyze the Stratosphere, the layer of our atmosphere just above the Troposphere. One specific activity we look for is known as Sudden Stratospheric Warming Event (SSWE), which is when temperatures in the Stratosphere warm to rapid levels in a short period of time. The end result, no matter how we got there, is either a displacement or split of the Stratospheric Polar Vortex. As a result, this weakens the Tropospheric PV and allows it to move out of the Arctic into the southern latitudes of Canada. This typically, but not always, takes the AO from a positive phase to a negative one and allows cold air to filter from the Arctic into central / eastern Canada and eventually the U.S.
Talking about the Stratosphere is an area of Meteorology I am still getting to understand. I know the basics of how certain wind patterns in the Polar Stratosphere, typically measured by the QBO, can impact our weather in the Troposphere (layer of atmosphere where we live). That said, much of the discussion that will take place is broad-based and high level.
A current look of temperatures at 10hPa in our Stratosphere shows it to be very cold. Between 10hPa and 30hPa is typically known as the "middle Stratosphere" and is exactly where we want to see warming take place. Notice how expansive the colder than normal temperatures are which is an indication this years' Polar Vortex in the Stratosphere is very strong.
Here's another graph showing temps at the 50hPa level. Notice how much colder 2015-2016 is compared to last season (solid blue line) and the mean (solid black line). By now you understand the Stratosphere is COLD and we need to see it warm up if we want it to influence our PV in the Troposphere. On a side note, it's not surprising to some the Stratosphere is running colder than normal. The QBO, a measure of Stratospheric Winds, is currently in a positive state which means westerly winds. Historically, westerlies strengthen the Stratospheric Polar Vortex. The 30mb QBO value for the month of October was +13.38. Not anomalously positive but positive nonetheless.
Heat Flux helps us understand wave forcing (wave 1 and wave 2) while Momentum Flux points to their meridional (north-south) state. The intensity and duration of wave 1 or wave 2 forcing often tells us if a SSWE is taking place. The best way to explain "wave 1" and "wave 2" is by counting the ridges and troughs in geopotential heights around the world within the latitudes that the Stratosphere is located in. Wave 1 forcing typically points to PV displacement while Wave 2 signifies PV split.
This graphic shows Geopotential Heights and Temperatures at 30hPa level at 216 hours from the EURO. It's forecasting the Stratospheric PV to remain strong with little to no wave 1 forcing (or warming) taking place.
A look at EPV, or Eliassen-Palm Vectors, represent vertical forcing that takes place through the Troposphere into the Stratosphere. The direction (look at the arrows) and magnitude (size of the arrow - how far north along the Y-Axis they extend) tells us if vertical forcing exists, and if so, how strong it is. The further north, or poleward, the arrows are pointing the better it is for Polar Vortex weakening. We had a minor event before Thanksgiving but given the EURO forecast we just saw it clearly had little to no impact on the PV.
The conversation with the Stratosphere extends way more than what I just talked about. However, these are the basics and as of now they show a very cold / strong Stratospheric PV. Until this changes, our AO and NAO will remain positive and the strong El Nino will continue being the dominant force in our overall weather pattern. Something I should mention which is pretty important is the above normal Eurasian SAI (snow advanced index) should lead to increased energy interaction between the Troposphere and Stratosphere in the form of latent heat release. Once High Pressure takes over in Siberia and temps gradually warm, the melting process should send "heat" into the Stratosphere and possibly begin the process toward a SSWE. There's another dimension to this - which I am still learning about - that consists of Rosby waves and their interaction with land masses (mountain torque) that could also effect the rate in which our Stratosphere warms.
It will be interesting to follow the Stratosphere in the weeks to come. Historically, strong El Nino events deliver SSWE's between end of December to mid-January. The earlier the better for our winter prospects.
CONCLUSION
1. Rain Tuesday and Wednesday this week with above normal temps early then falling back to average for a couple of days.
2. More rain likely early next week with above normal temps expected to last.
3. Possible shot of cold air mid-month thanks to the tropical forcing taking place over the Dateline. More to come of that in next week's Mo Mo.
Fino la prossima volta (until next time),
Francesco Paparatto
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