12/27/15 Mo Mo: Winter 2016 Pattern Progression

Hi Everyone,

The premise of this blog is to give you my thought's to when I believe the pattern will enter full winter throttle mode. First off, I hope everyone had a nice Christmas. I got to spend it with my family and enjoyed every minute of it. Something about Italians and Christmas. 

Current Status of El Nino

Poor El Nino. This cyclical phenomena has been getting blamed by the general public for bringing 70+ degree temperatures to the Northeast in December. Its been in a strong amplitude phase and has not really shown signs of substantial weakening to moderate levels. I do think El Nino has peaked, but lets see why I think that's the case. 

850hPa zonal wind anomalies over the Equatorial Pacific show strong easterlies overtaking 120W and points east (Nino Regions 3 & 1+2). Meanwhile, another round WWB's (Westerly Wind Bursts) are forecasted over the International Dateline (Nino Region 3.4). If you look at the first graphic I posted, the strongest SST anomalies are located between 150W and 100W or so. With easterlies expected over most of this area, we should see gradual cooling of SST's continue taking place. The upcoming WWB is a result of an active MJO. I will talk about this later on. 

Mean SST anomaly changes over the last 7 days over Nino Region 1+2 shows cooling already taking place. We've seen persistent easterlies over this area for much of December. The stronger easterlies expected over the next 2 weeks should have greater impact. 

Subsurface Equatorial SST Anomalies has shown negative departures gradually shift east through December. Subsurface negative departures are approaching 150W but they remain at depth. Once upwelling takes places, this is a sign ENSO is transforming from El Nino to La Nina. Overall, I expect this gradual / slow weakening of El Nino to last all winter long. The good news is - given the look of zonal winds and movement of subsurface temps - I think this event has peaked. 

MJO to Jump Start the Sub-Tropical Jet Stream

The MJO - a tool that tracks tropical waves / areas of convection along the Equator - is heading into phase 7. This region is located in the western Pacific. As this wave approaches the Dateline, westerlies will increase and -OLR (Outgoing Longwave Radiation) will develop. The graphic on the right is a 500mb height anomaly composite of MJO phase 7 under an ENSO positive regime. Under this type of MJO forcing, blocking conditions - or positive heights - take over the Arctic. A trough develops over the SW U.S. and negative heights are seen across the entire U.S. 

The MJO pulse is nicely shown at 200hPa / Outgoing IR Radiation. The brown contours in the western Pacific indicates positive Velocity Potential, where there is subsidence and weak convection. Closer to the Dateline, we see enhanced convection and negative VP, or CHI (divergence). 

A Check on the Stratosphere

In the short term, there are positive signals in the Stratosphere in an effort to weaken / displace / split the Polar Vortex. Another round of increased 10hPa vertical forcing (EPV) from the Troposphere into the Stratosphere is expected to occur this weekend. Heat flux at 30hPa is on the rise in the same time frame. The idea is to keep putting force on the PV until we get the end result we're looking for. Keep in mind in order to see sustained negative AO conditions, we need to split or displace the Stratospheric PV out of the North Pole. 

500mb height anomalies over the next 7-10 days show impressive height increases in western NA and Scandinavia (eventually into western Russia). This type of Tropospheric response has substantial implications in the lower Stratosphere.

In the lower Stratosphere, or 100hPa, the PV is expected to split sometime this weekend or early next week. But notice at 10hPa, or the upper Stratosphere, the PV is still intact and looks fairly strong. This is typically known as a bottom-up warming process. A true SSW means all levels of the Stratosphere show the PV splitting or being displaced. The reason we're seeing the PV split in the lower Stratosphere is due to the anomalous ridging in the upper Troposphere. The Scandinavian and the western Canadian ridges exert intense force, or push, on the lower Strat PV that it elongates and eventually splits. 

The EURO nicely demonstrates the magnitude of the wave 1 and wave 2 forcing the Strat PV is about to witness 7-10 days from now. Unfortunately, it will not have much effect to the upper Stratosphere. It does appear the vorticity scatters and the core of the PV elongates, but that is not enough to bring the Tropospheric response we're looking for. 

A starting point to seeing warmth propagate to the upper Stratosphere is the reduction of zonal winds at 10hPa. The GFS model drastically reduces 10hPa zonal winds from January 3rd onward. A weaker PV has higher probability of being influenced by Rosby Wave movement from the lower Strat to the upper Strat. Additionally, another round of vertical wave propagation is expected to occur around January 10th or so. There is a good shot this wave is stronger than the one's we've seen up to this point. If this is true, it could be exactly what we need to get the upper Strat to cooperate. 

Pattern Change Schedule 

Right after New Years, the EPS are showing the aforementioned ridges that will bring short-term implications to the lower Stratosphere. For our sensible weather, the +PNA spike and Tropospheric PV split will bring normal to below normal temperatures to our area the first 5-7 days of January. The trough axis over the east coast is too far east to promote coastal storm development. We're likely to be on the dry side the first few days of the month. 

By the end of the first week of January, the Scandinavian ridge moved into western Russia and is beginning to amplify into Greenland. The +PNA evolves into a -EPO and this turns our AO from positive to negative. Keep in mind a lot of these developments are due to the Tropical / MJO forcing I talked about earlier. Pieces of upper level energy are forecasted to break off the eastern Siberian PV and work there way down into the Aleutians into the SW U.S. The lower heights in the southern tier of the country is indicative of an active Sub-Tropical Jet Stream. It will not take much for one of these pieces of energy to ride the STJ up the east coast. However, I'm not sure there will be enough cold air available over the east to promote wintry precipitation. At this point, the pattern is still volatile / evolving and a cross-polar flow is not yet established. 

By the 2nd week of January, there is a better defined / organized Aleutian trough and substantial heights rises over the Arctic. I am not ready to jump on the -NAO train yet, but the anomalous ridges over Russia and Western NA bridge together to produce a blocking pattern that just may take our NAO negative. Much of the U.S. is under below normal heights at this time with a raging STJ. This is when the pattern begins to get exciting and possibly deliver our first significant east coast winter storm of the season. 

Upper Level Pattern Change Schedule

Week 4 December: Old pattern regime continues. Expect to continue seeing above normal temperatures. 

Week 1 January: Pattern transition begins due to the pulsing MJO into phase 7 / progression of the Scandinavian Ridge. Periods of below normal temperature days mixed with a day or two of slightly above normal. 

Week 2 January: The strongest wave of energy transfer from the Troposphere to the Stratosphere should occur and an upper Strat PV displacement is probable. A blocking pattern over the Arctic in conjunction with an active STJ could result in our first significant east coast winter storm event of the season. 

Week 3 January: Another period of relaxation is possible depending on whether or not the blocking pattern has staying power. That said, the development of the Aleutian trough in week 2 should keep higher heights confined to western NA. So we could see a pattern that looks closer to week 1 of January. At this point, the Stratospheric PV should be displaced but Tropospheric changes will not arise until later on.  

Week 4 January: Full scale pattern change should come to fruition this week. If memory serves me correctly, last winter we saw our pattern change around this time frame as well. By this point, I think we'll see a sustained -AO thank to PV displacement along with a +PNA. Depending on exactly where the Strat PV tracks to will dictate the evolution of the NAO. We're already seeing the QBO trend toward neutral levels. If this trend continues and the blocking pattern the 2nd week of January comes to fruition, it's possible the NAO goes negative much sooner than expected. For now, I think it will go negative around week 4 of January or week 1 of February. 

12/21/15 Mo Mo: Rainy Times Ahead

Good Morning, 

A couple of short work weeks likely ahead for some people as we get ready to celebrate Christmas and the New Year. Unfortunately, the weather over this time period is not expected to cooperate. Several waves of upper level energy are expected to pass over the Northeast bringing a decent amount of rainfall. Temperatures - to the dismay of many - will continue running way above normal for this time of year. 

SHORT RANGE - WEEK OF 12/21

In last week's Mo Mo I gave the heads up unsettled weather is likely this week. The rain will come from two waves. 

Wave 1 will arrive Tuesday morning. Rain will move in around mid-morning and move out by mid to late afternoon (left image). Wave 2 will be stronger and bring more rain. Rain will arrive mid afternoon Wednesday and linger through early afternoon Christmas Eve. There may be a brief break in the action overnight Wednesday, but moderate rain will return Thursday morning. 

Total rainfall Tuesday through Thursday will fall in the 1-2 inch range. Above is the total precip map from the 12z EURO model. 

The 500mb height anomaly map on the left nicely shows why we're getting rain this week. Upper level energy associated with the western trough is likely to cut to our west, raising heights along the east coast. Temp departures from normal (right) on Christmas Eve will run +15 to +20 degrees above normal. We've been stuck in this pattern that features a +EPO/-PNA/+AO/+NAO for basically the entire month of December. It will be no surprise to see December 2015 rank as one of the warmest December's ever on record. We'll get the final numbers at the end of next week. 

High temps Christmas Eve according to the EURO will range from upper 60's to low 70's. Like last week, it will not surprise me to see NYC Metro surpass 70 degrees. 

MEDIUM RANGE - WEEK OF 12/28

A closed off 500mb vort is going to track from the Southwest U.S. into the Northeast beginning this weekend into early next week. Rain associated with this feature will actually begin to move in Saturday the 26th. There is a chance rain could start late Christmas Day, but I think we'll be mainly dry the 25th. Here's how it's expected to play out according to the GFS:

Strong H5 energy will dig deep into the SW CONUS this week allowing heights to rise over the eastern side of the country. Once the 500mb vort closes off, it's going to tap into Gulf moisture which will use the southwest wind flow to stream precipitation into the northeast as early as Saturday. 

By Monday the 28th, a surface low pressure system will form in the Southeast then track northeastward into our area. This is being modeled as a very moisture-laden filled system thanks to the Gulf moisture it will be tapping into. Notice the cold air filtering in behind the system. It's not extreme cold, more like seasonable cold. 

On Tuesday the 29th, the low pressure system that formed in the southeast will have tracked to the Northeast. Moderate to heavy rain is forecasted with decent winds too. The baroclinic zone and SW flow will work together to keep overcast conditions with on and off rain over our area for a 3-4 day stretch. The main event will arrive on Tuesday. In total, over 2 inches of rain is possible this weekend into early next week. 

LONG RANGE - NEW YEAR, NEW PATTERN? 

This past weekend Judah Cohen "tweeted" a graphic of expected WAF (Wave Activity Flux) from the GFS model. +WAF, indicative by red shadings, means vertical wave energy usually in the form of Rosby waves is taking place between the Troposphere and Stratosphere. This helps weaken the Stratospheric Polar Vortex. A constant bombardment of +WAF could displace or even split the PV, which would lead to dramatic changes of our pattern at the Tropospheric level usually in the form of a -AO. Wave forcing is taking place as we speak. Another one, as Cohen stated, is expected to occur in early January. 

One reason why we're seeing increased WAF over the next two weeks is because of an anomalous Scandinavian ridge forecasted to develop this week. Above normal heights over northern Europe - extending into the Arctic and Greenland - is commonly known as an east-based -NAO. This form of blocking does not have a very big impact on our sensible weather in the eastern CONUS, but it does force an energy transfer into the lower Stratosphere. 

The GFS is forecasting the PV in the lower Stratosphere (50mb) to split before the New Year. This is an example of how certain Tropospheric features - such as the Scandinavian ridge - can have a direct influence on the Stratosphere. Specifically, increasing wave forcing (in this case wave 2) between both layers of the atmosphere. This type of image should make winter weather lovers very happy. However, I was reminded by knowledgeable Meteorologist's we want to see this type of bottom-up warming propagate into the upper Stratosphere as well. 

While 50mb is expected to split the PV, the same cannot be said from 30mb on up. While the core of the PV is shown to elongate, it does not fully split. Instead, by the start of the New Year it re-consolidates over the North Pole. Keep in mind these type of forecasts tend to change. Perhaps once models latch onto the fact we'll see increased WAF to start the New Year, they'll begin to show greater influence in the mid and upper Stratosphere. We want to see SSW (sudden stratospheric warming) occur in all levels of the Stratosphere to ensure the PV gets displaced or split. 

Just because the Stratosphere is not forecasted to undergo full-scale SSW (yet), it does not mean we won't see changes happening in the contingent U.S. The EPS, or EURO Ensembles, New Years week show pretty much the same pattern we've been in all month. Most impressive on the above map is the aforementioned Scandinavian ridge. Other then that...all is the same with awful looking teleconnections bringing record warmth to the east. As stated, this is the pattern you can expected the week of New Years. The ball drop New Years Eve should be dry with temps around normal. I'll have the specifics in next week's Mo Mo. 

Drum roll please... 

The EPS to start 2016 suggest a new pattern will unfold. Positive height anomalies are shown to amplify into the western U.S. (+PNA) with a trough shown to settle into the eastern U.S. The EPO and NAO still look to remain positive, but there should still be colder than normal air over the east at this time thanks to positive heights extending into the Arctic (east-based block!) The MJO at this time will be out of the phase 5/6 warm phases and could possibly reach phase 7 if the wave does not die off. This means favorable tropical forcing will take shape and the trough over the western U.S. will finally become a thing of the past. 

This is definitely a step in the right direction. Turning our PNA from negative to positive will help driver at least seasonable air into the eastern U.S. The next step will be to see the positive heights extend more poleward, into the EPO domain. This will not happen until the Aleutian trough develops. You can see the EPS are hinting at the Aleutian trough developing, but it's still too early in the process. 

The steady decline of the AO around New Years is a result of the Scandinavian ridge extending positive heights into the Arctic. Unfortunately, this looks like a temporary feature. Until the Stratosphere undergoes full-scale warming, the AO will continue to average positive. Fortunately, there are signs a SSWE is not far off from happening. I'm not going to get too excited until I see it with my own eyes, though... 

The last piece of the puzzle to fall will be the NAO. It's possible the NAO does not go negative at all this winter. Even the last couple of SSWE's in recent winter seasons have been unsuccessful in helping our NAO go negative over a sustained period of time. The northern jet stream has acted very "La Nina" like (progressive flow) which has proved to be difficult to stop. That said, there are decent analogs that show El Nino seasons that undergo SSW usually follow with a developing NAO in the second half of the winter season. As long as the SSWE occurs, we'll at least have a chance to see a -NAO. A -NAO does not mean the east will not see sustained periods of wintry weather. After all, the last couple of winter seasons have been very cold and snowy.

I decided to go against putting out an "official" pattern change timeline. Once I realized the SSWE is only taking place in the lower Stratosphere and not propagating into the mid and upper levels, I knew we still had work to do in that department. I'm feeling optimistic that by next week I will be able to release an official pattern change timeline. I have a good idea right now, but I rather hold off another week until I have all the good information and details. 

Have a blessed Christmas and a Happy New Year. 

Best,

Francesco Paparatto  

12/11/15 Mo Mo: "Eastern U.S. Roasting On An Open Fire"

"...Jack Frost nowhere to be found"

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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12/7 Mo Mo Blog: December Warmth & Christmas Forecast

Today's Monday Morning update will look at our weather over the next two weeks. I will also give you my initial thought's on the Christmas forecast. Am I dreaming of a white Christmas? 

Short Term - Week of 12/6: 

There is not much going on this week. In last week's Mo Mo, I mentioned there is a possibility of a Nor'easter to effect our area Tuesday-Wednesday this week. The pattern does not support one. 

18z GFS 500hPa Vorticity valid 1am Tuesday: 

500mb vorticity is a useful tool that measures the amount of "spin" in the atmosphere. I often refer to it as upper level "energy" or "short waves" that (if positive advection) can lead to surface low development (surface low = an actual storm). Notice the potent level of vorticity I circled over our area Tuesday morning. The reason why there is not a deep low pressure system sitting off our coast at this time is because the overall flow is not amplified, meaning you see no trough's or ridge's on a 500mb map. The very weak / disorganized trough along with the fast flow is the main reason why this Nor'easter will not happen. Instead, we're likely to see clouds with light rain showers. 

Expect temperatures to be in the low to mid 50's the entire week other then Tuesday. In fact, they'll be even higher come the weekend.

18z GFS 500mb Vorticity valid 1am Sunday the 13th 

A trough axis with very strong vorticity embedded is expected to be located over Texas this weekend. A surface low pressure system will form and track somewhere between Indiana and Western PA. The end result for our area will be much above normal temperature departures thanks to the High Pressure sitting off the east coast helping to pump up mild Gulf air from the Southeast.

12z EURO temp anomalies valid 1pm Saturday

The process I explained above actually takes place sooner than Sunday. The troughing will take shape over the western U.S. beginning Friday. This means positive heights are expected to take over the central and eastern U.S. Friday through the weekend. The EURO is advertising temperature departures of 20* to as high as 30* above normal from TX to eastern Canada. I would not be surprised to see NYC reach 60 degrees both Saturday and Sunday this weekend. Meanwhile, the western U.S. continues to see cold temps and frequent snowstorms. 

Medium Range - Week of 12/13: 

12z EURO 500mb height anomaly valid 7am Sunday 12/13 

This graphic is intended to show another perspective. The 1st system over Texas leading to the ridging over the eastern U.S. is the one I just finished talking about. By Monday the 14th, the cold front associated with this storm will reach our area (it's possible the timeline gets moved up to Sunday) to bring heavy rain and possibly high winds. I do not buy into the high wind forecast yet but I do think we'll see decent rain next Monday.

The 2nd system entering the Pacific Northwest should bring more rain to the area Thursday-Friday (17th-18th) next week. I will speak more about this system in next week's Mo Mo. 

Long Term - Initial Thought's on Christmas:

12z EURO Ensembles 500mb height anomaly valid 7am Wednesday 12/16

Keep in mind when looking at these images model verification's in the long range are not great. That said, we seem to be in a persistent pattern that does not intend on changing anytime soon. The EPS suggest the western U.S. will continue to get inundated with upper level trough's, or colder than normal temperatures, while the east stays average to slightly above. Part of the problem is the cold moderates as it travels east. The Alaskan Vortex prevents the Pacific ridge from amplifying into the EPO/PNA domains so the flow remains flat and fast. 

12z EURO Ensembles 500mb height anomaly valid Saturday December 19th

The EPS show no change as we head deeper into December. This is also in line with what the EURO Weeklies are showing. Over the past weekend, the GFS OP and GEFS seemed to take a step in this direction as well. At one point, the GFS OP was really overdoing the cold weather across the U.S. Given the strong El Nino regime we're under, in conjunction with an unfavorable Stratosphere and Tropical Forcing, this is not surprising to me. I predicted December to be warmer than normal given the historical nature of strong El Nino's in west-based QBO years. It's going to take time for Nino to weaken to moderate levels. Once it does, the Sub-Tropical Jet Stream will not be the dominant force in our overall pattern. 

I spent a good amount of time in last week's Mo Mo analyzing the current state of the Stratosphere. I do not want to spend as much time again this week, but do want to point out that the upper Stratospheric warming guidance was showing a couple of days ago does not look as impressive in the recent runs. 

Here is what the OP GFS showed this past Saturday afternoon at 10hPa:

Now here was Sunday afternoon's run:

Noteworthy:

1. The magnitude of the ridging or warmth over Siberia is not as strong

2. As a result, the core of the PV is not as elongated / displaced 

Temperatures at 50hPa (mid-Stratosphere) are running colder than normal (normal = solid black line) and are forecasted to get even colder this week. This indicates the PV remains strong and well organized. It will take a couple of potent warming events to weaken it to a point where it has an effect on our weather at the Troposphere. 

It's also worth mentioning the MJO is forecasted to go into phases 3-4. The convection we saw these last couple of days over the Dateline is expected to propagate west toward the western Tropical Pacific while the Indian Ocean continues to see frequent thunderstorm activity. Historically, an MJO in phase 3 during El Nino years result in the following 500mb map:

It's not a perfect match to what current guidance is showing in the long range. The general idea is correct with positive heights over central / eastern U.S. and Canada. while also showing the western U.S. trough. Overall, MJO influence in strong El Nino years is not as strong as ENSO neutral or La Nina regimes. It's not one of the signals I'll be paying much attention to this winter. 

Christmas Forecast:

I will likely update this forecast in the week's to come, but my initial thought's are it will be average to slightly below average Christmas Day. Depending on where the baroclinic zone sets up, I can see how a winter storm effects our area. However, in order for the baroclinic zone to be south & east of us, we'll need to see either a -EPO or +PNA begin to show up on the models. The odds of one of these features developing in this locked in pattern is pretty slim. The reason why I am leaning below normal is because the two low pressure systems expected to cut next week could create "traffic," or confluence, north of us that prevents a tertiary low from cutting. Instead, a weak trough could settle in over our area. By next week we'll be able to see what some of the long range models are showing. Right now, none of the major global models are in range. 

Thanks for reading. Have a great week! 

Best,

Francesco Paparatto 

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