Link to Severe Storms in Eastern Minnesota...aka...A pitch to get people to buy GR Analyst
http://www.crh.noaa.gov/images/mpx/NewsStory_June14.pdf
Friday, June 15, 2012
Friday, May 4, 2012
Nebraska: 5.5.2012
Hey Chasers,
I will be heading out to Nebraska for a chase tomorrow. A developing triple point area looks to be the site of supercells tomorrow afternoon. The initial target is Hyannis, NE and I will adjust north/south accordingly depending on how the Cu field looks in the late morning. I'm expecting a few supercells even as early as 19-20Z from about Hyannis, NE to Martin, SD. I see decent tornado potential with cells moving slowly and having good directional shear south of the triple point. Some more speed shear would be ideal, but I'm not complaining (yet).
I will be sure to post pictures and footage if we get anything good.
Dima the Dryline
Sunday, April 15, 2012
Recap of 4/12 & 4/14
Today was one heck of a day in Kansas. In retrospect, we can all probably say we should have gone there, but it's not clear why Nebraska was void of any significant tornadoes.
Anyways, I have a few quick videos from Thursday and today.
Thursday 4-12-2012:
I went up to Sterling, CO then east to Imperial and down towards McCook, NE. First, here's a video of hail east of Sterling. This was a nuisance because it created some thick hail fog and the temperature crashed from the mid 60's to the upper 40's in basically 10 minutes.
After this, I wandered along the warm front eastward, where the following picture recaps the common sight: towers getting choked and undercut by the cold air to the north that seemed to slosh north/south like a tide. I've heard people refer to these kind of towers as "turkey towers". The darker clouds are shallow stratus.
Finally, with the help of ours truly Squall Line, I was in position to take this footage a few miles north of Trenton, NE around 6pm. The video is bad quality (time to invest in a good camera), but at 0:45 seconds in, I think there's a funnel in the middle of the screen. This happened to coincide with the location and timing of the tornado report on the SPC reports.
Saturday 4-14-2012:
I was lazy today and paid the price. My goal was to head out to near Ogallala, NE as the 500-mb cold front was moving over the triple point. I like those situations as they always produce funnels/nados even though the cells are usually ugly looking on radar. I left too late though and was playing catch-up the whole time. I did manage to snag this cool video of a cell that later produced a few funnels near North Platte. The precip shaft in the middle is sweet.
Anyways, I have a few quick videos from Thursday and today.
Thursday 4-12-2012:
I went up to Sterling, CO then east to Imperial and down towards McCook, NE. First, here's a video of hail east of Sterling. This was a nuisance because it created some thick hail fog and the temperature crashed from the mid 60's to the upper 40's in basically 10 minutes.
After this, I wandered along the warm front eastward, where the following picture recaps the common sight: towers getting choked and undercut by the cold air to the north that seemed to slosh north/south like a tide. I've heard people refer to these kind of towers as "turkey towers". The darker clouds are shallow stratus.
Finally, with the help of ours truly Squall Line, I was in position to take this footage a few miles north of Trenton, NE around 6pm. The video is bad quality (time to invest in a good camera), but at 0:45 seconds in, I think there's a funnel in the middle of the screen. This happened to coincide with the location and timing of the tornado report on the SPC reports.
Saturday 4-14-2012:
I was lazy today and paid the price. My goal was to head out to near Ogallala, NE as the 500-mb cold front was moving over the triple point. I like those situations as they always produce funnels/nados even though the cells are usually ugly looking on radar. I left too late though and was playing catch-up the whole time. I did manage to snag this cool video of a cell that later produced a few funnels near North Platte. The precip shaft in the middle is sweet.
Saturday, April 14, 2012
Why Nebraska?
All of these ingredients will conveniently find themselves in a single location tomorrow afternoon...
- Nose of upper level jet
- strong diffluence
- strong curvature from downstream ridge amplification over the course of the last 18 hours
- More than sufficient instability
- synoptic flow will keep morning warm frontal convection cirrus and debris from hampering afternoon destabilization
- Strong cyclogenesis
- bulging dryline and cold front
- pressure falls in region (Dima I know you like that parameter)
- Excellent vertical speed shear (ie bulk shear)
- lowest 1.5 km speed shear of 30 knots
- SSE wind at surface and SW at 850 (ie helicity)
- further enhanced shear from LLJ hour or two before sunset when storms are still coupled with BL
And this...
Friday, April 13, 2012
Hail but Not Much Else: Added thoughts
The image above is the sfc analysis at 2243Z yesterday as those three monster storms had just formed off of the dryline in northwest Kansas. the orange line is the dryline and the red line is the location of the warm front. The blue arrows depict the storm motion at the time. I think the biggest reason that there were no tornadoes ties in directly with what Dima (Dryline) was saying. The warm sector with temps above 70F and where the highest cape values (>2000 J/kg) was very modest in size. Each one of those supercells that formed off the dryline grew in a very favorable area to become tornadic; however based on storm motion they were not able to remain in the warm sector for very long before riding over the top of the retreating warm front and into much more stable boundary layer (Temps in the 50s/60s) even despite the strongly backed winds. Cape values to the north and east of the warm front where near 0 as Dima was saying and each one of those cells basically crapped out into a hail producer as they moved northeast into southwest Nebraska. The cells farther to the north seemed to be more dynamically driven and grew upscale as the shortwave was moving through (moving into more unidirectional shear) but again like the cells Dima was on to the south had no real chance at producing tornadoes.
It's tough to see such a favorable setup fail so hard. Storm motions would have had to have been much slower and more to the east, or if the cap could have broken earlier along the dryline while it was farther west across eastern Colorado. Anyways, I would say that was a pretty successful chase yesterday Dima and just a taste of whats to come for you the rest of this Spring into the upslope summer. Post some pictures when you get a chance.
Dan aka Squall Line
Thursday, April 12, 2012
Hail, but not much else
After many promising cells (I counted chasing 4 or 5 65-dbz storms), I got hailed on 7 times, video to come later. Tornado wise, today was a letdown. I did not see well-developed wall clouds. All the rotation was on the scale of the whole thunderstorm with lots of unorganized turbulence producing funnels everywhere. Like I told Dan, who helped me catch the cell near Trenton, NE around 5-6pm, the report of a tornado looks to be an error because there were very low-lying clouds associated with the warm front the kept moving north/south after interaction with the passing cells.
Why the lack of tornadoes today? I would say instability. I measured low 60's over low 50's even south of the warm front, which was probably amounting to a few 100 worth of CAPE.
Still debating whether to head out Saturday. Looks like the show will be mostly after sunset. Anyone agree or disagree?
Why the lack of tornadoes today? I would say instability. I measured low 60's over low 50's even south of the warm front, which was probably amounting to a few 100 worth of CAPE.
Still debating whether to head out Saturday. Looks like the show will be mostly after sunset. Anyone agree or disagree?
Central Pacific Cyclone and Downstream Consequences
Fellow Chasers and avid weather followers,
If you like interesting, high impact weather then you are in for a treat. Included below are a few snippets of dialogue from a wide selection of text products from the NWS. Notice an over arching theme, strong wording multiple days out, model confidence, model agreement and a robust synoptic feature as the main ingredient for this 4-5 days of active weather in the US. Also, not to be over looked, this is only the 11th time a Moderate Risk has been issued for the day 3 outlook!
THE COMBINATION OF MOISTURE ASSOCIATED WITH THE LOW AND THE AFOREMENTIONED COLDER AIR COULD RESULT IN APPRECIABLE SNOW ACCUMULATION ACROSS THE SERN TWO THIRDS OF OUR CWA DURING THIS TIME FRAME.
THESE FACTORS COMBINED WITH FAST STORM MOTIONS SUGGEST THE POTENTIAL FOR LONG-TRACKED...STRONGTORNADOES OCCURRING FROM THE AFTERNOON THROUGH THE EVENING /INCLUDING AFTER DARK/.
GFS/ECMWF ARE IN FAIRLY GOOD AGREEMENT THROUGH D4/SUN AS THE STRONG PACIFIC TROUGH EXPECTED TO REACH CO/NM BY 12Z SUN
BOTH THE ECMWF AND GFS ARE SHOWING A FAIRLY SIGNIFICANT DISTURBANCE RIDING NEWD ALONG THE FRONT ACROSS THE MIDWEST SUNDAY NIGHT INTO MONDAY.
A POTENT UPPER LEVEL TROUGH WILL EJECT OUT OF THE FOUR CORNERS REGION SUNDAY WITH THE SURFACE LOW RAPIDLY DEEPENING ACROSS WESTERN IOWA/EASTERN NEBRASKA.
Currently this feature, which is a superposition of the Polar and Sub-Tropical jet, is getting organized in the Central Pacific Ocean. A superposition of the POLJ and STJ is when, using an objective identification scheme, both the POLJ and STJ are present in the same, vertically integrated column.
Below in red is where the POLJ has been identified at a given grid point and the white indicates where the STJ has been identified. The second image, indicated by the yellow dots, there are 4 present, show where the POLJ and STJ are present in the same vertical grid column.
A few consequences of a Superposition from recent research:
If you like interesting, high impact weather then you are in for a treat. Included below are a few snippets of dialogue from a wide selection of text products from the NWS. Notice an over arching theme, strong wording multiple days out, model confidence, model agreement and a robust synoptic feature as the main ingredient for this 4-5 days of active weather in the US. Also, not to be over looked, this is only the 11th time a Moderate Risk has been issued for the day 3 outlook!
THE COMBINATION OF MOISTURE ASSOCIATED WITH THE LOW AND THE AFOREMENTIONED COLDER AIR COULD RESULT IN APPRECIABLE SNOW ACCUMULATION ACROSS THE SERN TWO THIRDS OF OUR CWA DURING THIS TIME FRAME.
THESE FACTORS COMBINED WITH FAST STORM MOTIONS SUGGEST THE POTENTIAL FOR LONG-TRACKED...STRONGTORNADOES OCCURRING FROM THE AFTERNOON THROUGH THE EVENING /INCLUDING AFTER DARK/.
GFS/ECMWF ARE IN FAIRLY GOOD AGREEMENT THROUGH D4/SUN AS THE STRONG PACIFIC TROUGH EXPECTED TO REACH CO/NM BY 12Z SUN
BOTH THE ECMWF AND GFS ARE SHOWING A FAIRLY SIGNIFICANT DISTURBANCE RIDING NEWD ALONG THE FRONT ACROSS THE MIDWEST SUNDAY NIGHT INTO MONDAY.
A POTENT UPPER LEVEL TROUGH WILL EJECT OUT OF THE FOUR CORNERS REGION SUNDAY WITH THE SURFACE LOW RAPIDLY DEEPENING ACROSS WESTERN IOWA/EASTERN NEBRASKA.
Currently this feature, which is a superposition of the Polar and Sub-Tropical jet, is getting organized in the Central Pacific Ocean. A superposition of the POLJ and STJ is when, using an objective identification scheme, both the POLJ and STJ are present in the same, vertically integrated column.
Below in red is where the POLJ has been identified at a given grid point and the white indicates where the STJ has been identified. The second image, indicated by the yellow dots, there are 4 present, show where the POLJ and STJ are present in the same vertical grid column.
- Superposition associated with high impact sensible weather events (April 2011, Oct 2010, May 2010)
- Intensified lower stratospheric and upper tropspheric frontal zones and attendant vertical circulations
- Intensified horizontal circulation
- Single, amplified tropopause that separates tropical and polar tropopause
And a few images below provide satellite perspectives of the 984mb low that allowed for a migration of the tropics poleward and a Superposition to occur.
And courtesy of Jake.
The down stream consequences of this feature will not go unnoticed as discussed above. Saturday is shaping up to exhibit wide spread severe weather as the nose of this jet streak reaches the central and southern plains with cyclogenesis taking hold over Colorado. With strong dynamic forcing and unstable air mass in place it is not a matter of if but rather when severe weather will occur. With many of the hodographs looking similar to this one near Wichita, KS the shear profiles are supportive of discrete supercells capable of producing strong and possible long track tornadoes.
Moving our attention to Sunday and Monday, severe weather will still be present. It appears at this point however that the setup favors discrete cells initially with upscale growth likely, as indicated by the shear profiles but also the precipitation distribution for both days. Another clue is the pulsing nature of the jet aloft which indicates organized convection (squall line or MCS) at night will enhance the jet just down shear. Also taking place as the cyclone tracks NE will be an increased region of baroclinicity in the upper midwest. With cooler temperatures in place it looks likely that a decent swath of snow will line up across MN and WI. Below is the 850mb temp, height and winds highlighting the enhanced baroclinicity and cold air in place for mostly snow on the N and W side of the cyclone.
Tuesday and Wednesday warrant further monitoring as the upper wave moves east but that will be saved for further discussion.
-Convergence
Wednesday, April 11, 2012
Triple Point Fun
Hey there fellow chasers,
Myself (Dryline aka Dima) and fellow UW chaser Chris Rozoff will be snooping around the KS/CO/NE border area tomorrow looking for tornadoes around the triple point. You guys know how it goes with those triple points: guaranteed Hel-to-the-icty (no, not Zak) but also more dynamics which could yield clouds that put a damper on the surface heating. Currently optimistic since upslope flow is present and looks like we can get mid 50's dewpoints all the way to McCook, NE. I will hopefully post again tomorrow.
Dima
Myself (Dryline aka Dima) and fellow UW chaser Chris Rozoff will be snooping around the KS/CO/NE border area tomorrow looking for tornadoes around the triple point. You guys know how it goes with those triple points: guaranteed Hel-to-the-icty (no, not Zak) but also more dynamics which could yield clouds that put a damper on the surface heating. Currently optimistic since upslope flow is present and looks like we can get mid 50's dewpoints all the way to McCook, NE. I will hopefully post again tomorrow.
Dima
Monday, January 16, 2012
Differential Cold Air Advection
Hey everyone... Almost 2 years ago, it was my 3rd day working at the NWS Des Moines office. We had a blizzard come up without any lead time, and it closed major roads and killed 7 people in Iowa. However, it certainly didn't come "out of nowhere." This spawned a research study to better understand and forecast these event. As luck would have it, we had a similar event this past week, so I thoughts I'd compare/contrast. First of all, to crudely quantify the impact, I'll show a map of the road conditions. This first one is from Jan 26 2010.
The next one is from Jan 11 2012.
the Jan 26th one must have been worse right..? Well, kinda, but in my opinion that is due to one reason. SNOWPACK. Jan 2010 had nearly record snow pack (1-2ft) across Iowa, whereas this year we had a whopping zero. No snow pack means no snow to blow around, and any falling snow will gather in the grass, fields, and ditches. To prove my point, here's the metars from Mason City Iowa (think north central Iowa).
Jan 11 2012
time temp press VISBY Wind Gust Weather
Jan 25 2010
time temp press VISBY Wind Gust Weather
So whats going on...? First of all, the pressure is rising, which doesn't fit the classic blizzard conceptual model. Second of all, there's only light snow being reported. Thirdly, the northwest winds are impressive. These three things are all common with strong cold fronts. However, the question I had is "how does strong wind mix down to the surface when surface Cold Air Advection is only going to act to stabilize the boundary layer..? The answer my friends is to replace "Cold" with "Differential". Differential Cold Air Advection (usually between 925mb and the Surface) will steepen the lapse rates in the boundary layer and promote momentum transfer of strong winds down to the surface. Here's the 12Z (pre-frontal) and 24Z (post frontal) soundings from Omaha during this last event on Jan 11 2012. The black Xs on the 12Z sounding show the 850 and Sfc temp from the 24Hr sounding (dtemp/dt). This was unusually deep mixing in the boundary layer. I also circled the 0-1km wind shear. (its blurry, so click it)
The next one is from Jan 11 2012.
the Jan 26th one must have been worse right..? Well, kinda, but in my opinion that is due to one reason. SNOWPACK. Jan 2010 had nearly record snow pack (1-2ft) across Iowa, whereas this year we had a whopping zero. No snow pack means no snow to blow around, and any falling snow will gather in the grass, fields, and ditches. To prove my point, here's the metars from Mason City Iowa (think north central Iowa).Jan 11 2012
time temp press VISBY Wind Gust Weather
| 3:17 PM | 26.6 °F | 29.75 in | 1.8 mi | NNW | 27.6 mph | 38.0 mph | Light Snow | |
| 3:53 PM | 25.0 °F | 29.80 in | 1.8 mi | NW | 25.3 mph | 38.0 mph | Light Snow | |
| 4:53 PM | 21.9 °F | 29.82 in | 1.8 mi | NW | 26.5 mph | 36.8 mph | Light Snow | |
| 5:53 PM | 19.0 °F | 29.83 in | 2.5 mi | NW | 32.2 mph | 46.0 mph | Light Snow | |
| 6:53 PM | 18.0 °F | 29.85 in | 3.0 mi | NW | 32.2 mph | 44.9 mph | Light Snow |
Jan 25 2010
time temp press VISBY Wind Gust Weather
| 10:53 AM | 21.9 °F | 29.38 in | 0.8 mi | NW | 33.4 mph | 41.4 mph | Light Snow |
| 11:53 AM | 19.9 °F | 29.39 in | 0.5 mi | WNW | 29.9 mph | 40.3 mph | Light Snow |
| 12:53 PM | 19.0 °F | 29.39 in | 0.5 mi | WNW | 32.2 mph | 40.3 mph | Light Snow |
| 1:53 PM | 18.0 °F | 29.41 in | 0.2 mi | WNW | 32.2 mph | 48.3 mph | Light Snow |
| 2:53 PM | 16.0 °F | 29.45 in | 0.2 mi | WNW | 36.8 mph | 44.9 mph | Light Snow |
| 3:53 PM | 14.0 °F | 29.49 in | 0.5 mi | WNW | 26.5 mph | 38.0 mph | Light Snow |
So whats going on...? First of all, the pressure is rising, which doesn't fit the classic blizzard conceptual model. Second of all, there's only light snow being reported. Thirdly, the northwest winds are impressive. These three things are all common with strong cold fronts. However, the question I had is "how does strong wind mix down to the surface when surface Cold Air Advection is only going to act to stabilize the boundary layer..? The answer my friends is to replace "Cold" with "Differential". Differential Cold Air Advection (usually between 925mb and the Surface) will steepen the lapse rates in the boundary layer and promote momentum transfer of strong winds down to the surface. Here's the 12Z (pre-frontal) and 24Z (post frontal) soundings from Omaha during this last event on Jan 11 2012. The black Xs on the 12Z sounding show the 850 and Sfc temp from the 24Hr sounding (dtemp/dt). This was unusually deep mixing in the boundary layer. I also circled the 0-1km wind shear. (its blurry, so click it)
Monday, January 9, 2012
What's going on with the weather over the past few months?
Hey fellow chasers and weather enthusiasts,
It's been a while since any of us has posted to the blog and wanted to add some recent discussion brought up by Jake concerning the unique weather pattern most of us have experienced over the past few months. Jake's opened ended question has opened up some great discussion between himself, Dima, and I along with many forecasters around the country. Here are a few of the emails that have been sent thus far and I will add Dimas response after he has time to send out his response:
From Jake Beitlich:
In your own words...what is going on this year...? It seems like this entire winter has been "northern jet stream across southern Canada, with a cutoff low over the desert southwest." If you get a chance, could you please explain this in 101 terms concerning the NAO, PNA, and La Nina?
Here's my take.....
- La Nina means weaker than normal subtropical jet...therefore the southern stream has a harder time phasing with the northern stream (a.k.a cutoffs over 4corners that slowly meander eastward toward Louisiana and don't emerge as the panhandle hooker as major cyclones.)
- Meanwhile strong westerly flow over Alberta...aka lee side warming, keeps the cold arctic air to the north (not sure how this is anomalous from previous winters).
- I don't exactly know how the NAO and PNA are related.....
- I also don't know how to explain why this 500mb image I sent seems to be the most common weather map of the season.
Looking forward to your thoughts,
My response:
I love this question Jake. This exact question has been the talk of the weather community here in the Desert Southwest for the last 3 months. I’ll try and elaborate on this topic as best I can (a limited understanding of the underpinnings of the large scale teleconnections at work, ENSO, MJO, NAO, PNA, and the AO).
Building a little on Jake’s take with the help of some colleagues:
1. Weak La Nina phase of ENSO: It’s well documented that this past summer/fall into the start of the winter season has been dominated by La Nina conditions especially noted over the Nino 3.4 region (central equatorial pacific). Typically moderate to strong La Ninas favor upper level ridging over the western half of US (drier and warmer conditions) with large scale troughing over the central/eastern US (cool and wetter conditions). However, weak La Nina to ENSO neutral conditions (like what we are currently seeing) complicate the large scale pattern and can allow for periods where the western ridge breaks down for periods of time during the winter months. The climatological storm track across the Western US during the phase mentioned above is more conducive to a prolonged cut-off low season across the southwest.
2. Madden Julian Oscillation (MJO): 30-60 day Tropical Kelvin Wave that traverses the equator with strongest signal in tropical Pacific during weak La Nina/ENSO neutral conditions. Can have significant impact on the upper level storm track across the Western US. Impacts include large scale variation in temperature, winds, and precipitation patterns which are similar to moderate El Nino but with a shorter duration. I’ve included a figure from a presentation that Mike Baker (NWS Denver/Boulder) gave back in December showing the November-December storm tracks which were possibly aided by a positive phase in the MJO.
3. NAO and PNA: Going to defer to Dima on this one as I don’t really know enough to make any strong points. About all I can say especially towards the NAO is that it blends with the Arctic Oscillation (what I discuss in limited detail below). Dima I know there is large disagreement within the climatological community as to which oscillation plays a more dominant role and thus gives a more representative depiction of the large scale/dynamical pattern.
4. Arctic Oscillation (AO): Arctic climate index with positive and negative phases that represent the mean upper level pattern over the Arctic Latitudes. In the positive phase, generally lower than normal pressure over the polar region and higher pressure over mid-latitudes steering the majority of Pacific storms northward. Frigid winter time temperatures don’t extend as far south into central US. The negative phase is a general reversal of the pattern just described. Our buddies over at the NWS PHX office had a nice little presentation covering this a few weeks back. I’ve attached a slide from their presentation that gets more into what a Positive AO means for the Southwest.
My General “101 style” take away thoughts (please feel free to correct me if I’m making any misguided statements):
- Weak La Nina to ENSO neutral conditions favor sporadic but prolonged breakdowns of the western ridge normally seen during more moderate La Nina type winters.
- A Positive MJO index (November-December, now slightly negative) aided in transport of moisture rich tropical air across southwestern US by way of subtropical jet and enhancement of Western Pacific troughing pattern during that time.
- A positive AO which favors low pressure over Southern California in November and December despite the bulk of Polar Jet energy being steered into Central and northern Canada i.e. all of the cut off low scenarios we’ve seen the last 60 days. A positive AO also typically keeps temperatures much warmer and conditions drier to the east of the Colorado Rockies which we’ve also seen across the Plains and Midwest (it’s unbelievable how warm it has been thus far).
As best as I can understand it in terms of the large scale pattern, it seems like a pretty unique coupling of several dominant teleconnections has allowed for our interesting start to the winter. I honestly can’t remember seeing anything quite like this in the 10 or so years of weather forecasting that I’ve been a part of!!!
I’m really looking forward to hearing our expert Climatologist’s thoughts. It’s such an interesting topic and directly related to forecasting challenges Jake and I have seen over the last two months.
This post will be updated in the coming days as further responses become available:
Dan aka Squall Line
It's been a while since any of us has posted to the blog and wanted to add some recent discussion brought up by Jake concerning the unique weather pattern most of us have experienced over the past few months. Jake's opened ended question has opened up some great discussion between himself, Dima, and I along with many forecasters around the country. Here are a few of the emails that have been sent thus far and I will add Dimas response after he has time to send out his response:
From Jake Beitlich:
In your own words...what is going on this year...? It seems like this entire winter has been "northern jet stream across southern Canada, with a cutoff low over the desert southwest." If you get a chance, could you please explain this in 101 terms concerning the NAO, PNA, and La Nina?
Here's my take.....
- La Nina means weaker than normal subtropical jet...therefore the southern stream has a harder time phasing with the northern stream (a.k.a cutoffs over 4corners that slowly meander eastward toward Louisiana and don't emerge as the panhandle hooker as major cyclones.)
- Meanwhile strong westerly flow over Alberta...aka lee side warming, keeps the cold arctic air to the north (not sure how this is anomalous from previous winters).
- I don't exactly know how the NAO and PNA are related.....
- I also don't know how to explain why this 500mb image I sent seems to be the most common weather map of the season.
Looking forward to your thoughts,
My response:
I love this question Jake. This exact question has been the talk of the weather community here in the Desert Southwest for the last 3 months. I’ll try and elaborate on this topic as best I can (a limited understanding of the underpinnings of the large scale teleconnections at work, ENSO, MJO, NAO, PNA, and the AO).
Building a little on Jake’s take with the help of some colleagues:
1. Weak La Nina phase of ENSO: It’s well documented that this past summer/fall into the start of the winter season has been dominated by La Nina conditions especially noted over the Nino 3.4 region (central equatorial pacific). Typically moderate to strong La Ninas favor upper level ridging over the western half of US (drier and warmer conditions) with large scale troughing over the central/eastern US (cool and wetter conditions). However, weak La Nina to ENSO neutral conditions (like what we are currently seeing) complicate the large scale pattern and can allow for periods where the western ridge breaks down for periods of time during the winter months. The climatological storm track across the Western US during the phase mentioned above is more conducive to a prolonged cut-off low season across the southwest.
2. Madden Julian Oscillation (MJO): 30-60 day Tropical Kelvin Wave that traverses the equator with strongest signal in tropical Pacific during weak La Nina/ENSO neutral conditions. Can have significant impact on the upper level storm track across the Western US. Impacts include large scale variation in temperature, winds, and precipitation patterns which are similar to moderate El Nino but with a shorter duration. I’ve included a figure from a presentation that Mike Baker (NWS Denver/Boulder) gave back in December showing the November-December storm tracks which were possibly aided by a positive phase in the MJO.
3. NAO and PNA: Going to defer to Dima on this one as I don’t really know enough to make any strong points. About all I can say especially towards the NAO is that it blends with the Arctic Oscillation (what I discuss in limited detail below). Dima I know there is large disagreement within the climatological community as to which oscillation plays a more dominant role and thus gives a more representative depiction of the large scale/dynamical pattern.
4. Arctic Oscillation (AO): Arctic climate index with positive and negative phases that represent the mean upper level pattern over the Arctic Latitudes. In the positive phase, generally lower than normal pressure over the polar region and higher pressure over mid-latitudes steering the majority of Pacific storms northward. Frigid winter time temperatures don’t extend as far south into central US. The negative phase is a general reversal of the pattern just described. Our buddies over at the NWS PHX office had a nice little presentation covering this a few weeks back. I’ve attached a slide from their presentation that gets more into what a Positive AO means for the Southwest.
My General “101 style” take away thoughts (please feel free to correct me if I’m making any misguided statements):
- Weak La Nina to ENSO neutral conditions favor sporadic but prolonged breakdowns of the western ridge normally seen during more moderate La Nina type winters.
- A Positive MJO index (November-December, now slightly negative) aided in transport of moisture rich tropical air across southwestern US by way of subtropical jet and enhancement of Western Pacific troughing pattern during that time.
- A positive AO which favors low pressure over Southern California in November and December despite the bulk of Polar Jet energy being steered into Central and northern Canada i.e. all of the cut off low scenarios we’ve seen the last 60 days. A positive AO also typically keeps temperatures much warmer and conditions drier to the east of the Colorado Rockies which we’ve also seen across the Plains and Midwest (it’s unbelievable how warm it has been thus far).
As best as I can understand it in terms of the large scale pattern, it seems like a pretty unique coupling of several dominant teleconnections has allowed for our interesting start to the winter. I honestly can’t remember seeing anything quite like this in the 10 or so years of weather forecasting that I’ve been a part of!!!
I’m really looking forward to hearing our expert Climatologist’s thoughts. It’s such an interesting topic and directly related to forecasting challenges Jake and I have seen over the last two months.
This post will be updated in the coming days as further responses become available:
Dan aka Squall Line
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