Am I the only one who is struggling to get into the Christmas spirit because of the weather? No matter how many Christmas movies I watch or songs I listen to, I cannot grasp the fact December has felt like September. We're expected to remain in this pattern through the end of this month. Let's see what the weather looks like over the next two weeks before discussing the long range.
Short Term - Week of 12/10
Temperatures will continue hovering around 60 degrees for the next few days. Another storm cutting to our west will be responsible for bringing rain to our area later today. Look for rain to start falling in the mid-afternoon hours and persist through the evening. Areas of dense fog is also possible. We'll clear by Tuesday morning. Between .25" to .75" of rain is likely.
Another shot of rain is expected Thursday. Models are indifferent to the intensity of the cold front. The GFS has a strong surface low riding along the front and enhancing rainfall over our area, while the EURO is a bit more progressive. Either way, another .25" to as much as 1.00" of rain is possible Thursday afternoon into the evening.
Between 1.00" to 2.00" of rain should fall between both storm systems this week. Above is the GFS total precip map ending Friday morning. The heaviest axis of rain will be located depending on where the surface low on Thursday tracks. Central-Southern NJ and areas N&E are a good bet to see the heaviest rain this week.
Maybe this will help bring some Christmas spirit. Behind the cold front Thursday, a rush of cold air is expected to filter into the central and eastern U.S. The main reason behind this cold air is a sub 940mb cyclone over the Bering Sea helping to pump up heights in the EPO domain. As higher heights extend into the Arctic it helps force a deep trough to track into the U.S.
According to the EURO, high temps will only range between the upper 30's to low 40's across the area Saturday and Sunday this weekend. This 2-day wonder is not going to last unfortunately. The pattern is still progressive and the trough is quickly replaced by a ridge.
Medium Range - Week of 12/20
Christmas week is not looking "christmasie" for the eastern U.S. Once the trough from the weekend lifts out, a ridge is expected to build back into the east. The reason behind this is yet another anomalously deep trough getting entrenched into the southwestern U.S.
A surface low will spawn within the trough next week and cut to our west. A cold front will develop and approach our area on Christmas Day. The timing of how this all comes together remains unclear. I am confident the surface low will cut to our west - bringing more warmth and rain - but the timing of when it happens looks to be anywhere from Christmas Eve to Christmas Day.
Yesterday afternoon's GFS run suggested the cold front will reach our area in the early afternoon hours on Christmas Day. 50's and rain on Christmas? Man, Mother Nature must have something personal against us if this is the case. After Christmas Day we'll see a day or two of average to below average temperatures and then another warming period is expected the week of New Years. If you're a winter weather lover, by this point of the reading you're probably ready to "cancel" winter and declare it a lost season. I am not ready to join your clan because I knew coming into the season we will start off warm.
Long Range - Where Oh Where is the "Pattern Change?"
The current and foreseeable pattern features a +EPO/+AO/+NAO/-PNA. All of these signals are in the wrong phases if you're looking for sustained cold / wintry weather. The reasons behind these unfavorable signals are due to MJO/Kelvin Wave disruptions and the anomalously cold Stratosphere. Let's start by checking on SSTA trends of each Nino region.
Each Nino region over the last week or so has experienced cooling of SST's. The bold line is Nino region 3.4 and it's evident SST's in that region have been on the decline. Looking at zonal wind anomalies, it looks like easterlies are taking over east of 120W and the WWB's (westerly wind bursts) around the International Dateline (180) are weaker in magnitude. It's very possible El Nino has peaked. If this is true, the ridge over Canada should begin to retrograde west over time which will allow a trough to undercut and form over the southern and eastern states. How quickly this ridge move from eastern Canada to western Canada will determine how many week of winter weather we'll experience in the east. I expect by mid January the CA ridge will be centered in west-central CA and for the Aleutian trough to further south & west allowing higher heights to enter the EPO domain.
The MJO over the last several weeks has been in phases 2-3-4. The increased MJO wave activity is not usually common during El Nino episodes. Larry Cosgrove, a respected Meteorologist, mentioned areas that should be seeing enhanced rainfall typical of El Nino seasons (California through southern tier of U.S.) have actually been relatively dry while the Pacific Northwest has been seeing storm after storm. An MJO in these phases means tropical forcing is taking place over the Indian Ocean (see below graphic) which signals warm weather for the central and eastern portions of the U.S.
Looking at 200mb zonal winds across the Equator we see negative anomalies exist between the Dateline and 120W (blue shading). Positive anomalies are shown west of 180 into the Indian Ocean. This usually means OLR anomalies are negative over this region and tropical forcing should be favored to promote cooler weather over the central and eastern U.S. However, active MJO and Kelvin waves have interrupted +ENSO forcing (El Nino) and the atmosphere is not able to respond to what is usually seen under these conditions.
The EURO model suggests this current MJO wave could get into phase 7, albeit it's a very weak signal. If it manages to propagate east and maintain its strength it's possible we see colder weather arrive to the eastern U.S. after Christmas (closer to New Years).
Stratosphere
Some of you are probably familiar with this chart. Judah Cohen put it together to show how above normal (or below) snowfall in the month of October leads to a -AO (or positive) to develop. We're focused on the right side of this chart since Eurasian and Northern Hemisphere snowfall finished above average at the conclusion of October. According to Cohen poleward heat flux or vertical wave propagation, in the form of Rosby Waves, should have fired from the Troposphere into the Stratosphere during the month of November to aid in weakening the Stratospheric Polar Vortex. We're almost in mid-December and we've seen just 1 wave attack so far.
A wave 1 attack occurred in late November around Thanksgiving. This attack temporarily weakened the Polar Vortex but it was not enough to displace or split it. A second wave 1 attack is expected in the next 5-10 days. EPV is a good measure of vertical forcing through the Troposphere and Stratosphere. I went over this a couple of weeks ago, but it's worth repeating, the more poleward and longer the arrows the better the odds are of the PV weakening.
Looking at 10hPa heat flux, the orange line nicely shows the next wave 1 attack. However, the problem is the magnitude of this attack is similar to the one we saw in late November. We need to see these attacks be stronger and more frequent in nature or else the Strat. PV will continue to reform over the Arctic Circle.
The current state of the PV from the CFS is seen above. The PV is nicely wrapped up in the Arctic Circle. Its shape and overall structure points to a powerful vortex that will take several strong Tropospheric wave attacks to either displace it out of the Arctic or split the Vortex into two.
By week 2, we're able to see the next attack which the wind & flux chart is accurately predicting. A strong 10mb ridge is located near the Aleutians allowing higher heights to penetrate into the Arctic Circle. The elongated-looking core of the PV suggests it's clearly feeling the effects of this warming event but guidance insists this will be temporary.
Come week 3 - News Years week - the PV is back to its strong state as the warming wanes. If you look back at Cohen's chart, he says in January there should be Stratospheric circulation anomalies propagating back into the Troposphere. We're not quite at that stage yet until the PV weakens to levels where it begins to have an effect on our sensible weather in the Troposphere.
I find this graphic to be slightly interesting. The CFS feels by week 4 another warming event will take place. While 10mb temps do not look impressive in this image, the location and structure of the PV suggests this could be the strong wave 1 warming event we've been waiting for. The time frame to watch for this possibility to happen is the 1st week of January between the 3rd and 10th. I strongly feel a SSWE is still likely to occur due to the state of the QBO. It's looking likely this will happen between January 6th to the 13th with effects reaching the Troposphere later that month.
The way the pattern is evolving this year may be frustrating to some. When we're dealing with a strong El Nino AND Stratospheric Polar Vortex, it's going to take time for change to happen and for our atmosphere to favorably respond. Interestingly enough, El Nino reaching its peak and the Stratosphere undergoing a SSWE (Sudden Stratospheric Warming Event) seem to be on pace to happen at the same time. If this is true, our sensible weather could completely change from one week to the next sometime in late January. I am still expecting February and March to feature below normal temperatures and above normal snowfall for NYC Metro.
Thanks for reading!
Best,
Francesco Paparatto
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