Ask a Pro Met

[Analog96]

What is the best set up in the atmosphere for much below temperatures and heavy synoptic snowfall in NNE, NNY and southeast Canada in the winter? What atmospheric set up led to the incredible winter of 1993-94, which brought both record cold and heavy snow to the above regions? Obviously no blocking given that's what caused the non-winter of 2009/2010 up here!

1993-94 had a +NAO and a +PNA.

[Ottawa Blizzard]

1993-94 had a +NAO and a +PNA.

Thanks! That's the set up I'll cheer on from now on. A -NAO doesn't seem to be very good for us up here and certainly not a -AO.

[Analog96]

Thanks! That's the set up I'll cheer on from now on. A -NAO doesn't seem to be very good for us up here and certainly not a -AO.

A -NAO and -AO would probably be too suppressive for up there.

[bubba hotep]

Someone should compile this into a FAQ section for the website and sort it by winter weather, tropical, drought, etc

[showmethesnow]

Curious about what you look for when it comes to dry slotting. I have read it's figured to occur between the 500mb and 700mb upper lows as well as it forms south of the 700mb low so am not sure what to think.

Thanks in advance.

[cny rider]

I found this site because the science of meteorology is fascinating to me.

I have a bachelor's level science/mathematics education.

I just got the Vasquez Weather Map book.

What else should I be reading to learn the big picture ideas of meteorology, without going overboard with heavy mathematics and computer programming?

[CoastalWx]

Curious about what you look for when it comes to dry slotting. I have read it's figured to occur between the 500mb and 700mb upper lows as well as it forms south of the 700mb low so am not sure what to think.

Thanks in advance.

It's all about the mid level RH. The best way imo is to try and look at a product that shows avg 700-500mb RH, but on the NCEP products, the 700mb progs will do just fine. Anything less than 70% RH is probably a signal of a dryslot, despite models that may spit out significant qpf beyond that time frame. We see this all the time. Models are too generous with QPF near or within the dryslot, and not generous enough in areas that have very high RH along the nw edge of the low. Sometimes this occurs due to mid level frontogenesis and deformation that occurs on the nw side of a mid latitude low. That's why many of us tell weenies to stop complaining when they are ripping and reading qpf. If they are in the "sweet spot" area of the H7 low, often times they'll overperform due to better banding and ratios.

Here is an example of a dryslot. This was the big sne blizzard on Jan 12. This storm brought the dryslot almost exactly where this model shows it...into se mass. I don't have the radar images like I thought I did...but this did happen.

[Wx4cast]

It's all about the mid level RH. The best way imo is to try and look at a product that shows avg 700-500mb RH, but on the NCEP products, the 700mb progs will do just fine. Anything less than 70% RH is probably a signal of a dryslot, despite models that may spit out significant qpf beyond that time frame. We see this all the time. Models are too generous with QPF near or within the dryslot, and not generous enough in areas that have very high RH along the nw edge of the low. Sometimes this occurs due to mid level frontogenesis and deformation that occurs on the nw side of a mid latitude low. That's why many of us tell weenies to stop complaining when they are ripping and reading qpf. If they are in the "sweet spot" area of the H7 low, often times they'll over perform due to better banding and ratios.

Here is an example of a dryslot. This was the big sne blizzard on Jan 12. This storm brought the dryslot almost exactly where this model shows it...into se mass. I don't have the radar images like I thought I did...but this did happen.

Nice answer. Another element that I like to look at is the forecast upper-air winds at 700 millibars and above. If the wind direction is SW thru WSW at these levels and varies little in direction with increasing height (and also if the speed stays nearly the same or even increases with height) chances are you will get a dry slot. A good website to look at gridded point wind data (along with other data types) is Earl Barker's page at:

Models or for Hi-Res GFS HiRes Gridded GFS

[CoastalWx]

Nice answer. Another element that I like to look at is the forecast upper-air winds at 700 millibars and above. If the wind direction is SW thru WSW at these levels and varies little in direction with increasing height (and also if the speed stays nearly the same or even increases with height) chances are you will get a dry slot. A good website to look at gridded point wind data (along with other data types) is Earl Barker's page at:

Models or for Hi-Res GFS HiRes Gridded GFS

Yeah we just saw this happen with the storm earlier this week. It's never good to expect a prolific amount of snow if the 700mb low is nw of you. Sure it could happen, but more often than not, the dryslot will come in to ruin the party. With the scenario like that, models may be too heavy with QPF, and not end the snow or rain quick enough.

[SacrydDreamz]

We hear and read the term baroclinic zone for Nor'easters: What (or Why) makes a baroclinic zone intense versus a weak one? The moisture gradient? PGF? The thermal/temperature gradient?

I'll always believed it was the thermal gradient and in contrast with the pressure, for example, the blizzard of 1996 had a banana high of nearly 1040 mb in NE MN, and another HP in Quebec, as the surface low bombed out to 979 mb (of course H5 was a key factor).

Baroclinic zones can be strengthened through a variety of means, including jet streak interactions (especially when two jet streaks are coupled favorably), differential positive vorticity advection, increased thermal advection, moisture advection, or convection. Through all of these means one can create stronger temperature differences along a horizontal surface, which is a "lazy", but useful, way to think of baroclinity.

[ArtRosen]

What causes the NAO to go positive or negative?

bump

[SacrydDreamz]

bump

Since the NAO is derived from pressure anomalies over the North Atlantic, a blocking high in the vicinity of Greenland would cause the NAO to go negative while a deep upper-level low would cause the NAO to be read as positive. As far as why there's a high there sometimes while at other times there's a low -- part of that can be explained by sea-surface anomalies while another factor is the AO and stratospheric warming events that lead to a displacement of low pressure anomalies and subsequent trough amplification/probability of blocking.

[Analog96]

Since the NAO is derived from pressure anomalies over the North Atlantic, a blocking high in the vicinity of Greenland would cause the NAO to go negative while a deep upper-level low would cause the NAO to be read as positive. As far as why there's a high there sometimes while at other times there's a low -- part of that can be explained by sea-surface anomalies while another factor is the AO and stratospheric warming events that lead to a displacement of low pressure anomalies and subsequent trough amplification/probability of blocking.

This is correct, of course, but I don't know if that's what Art was looking for. You defined a -NAO. I think Art wanted to know WHY we get them to begin with and what causes them to switch states?

[Thunder Road]

My turn:

What actually is the MJO?

I know Phase 8 is good for East Coast snowstorms, and that there is a "circle of death". But what actually is it?

[am19psu]

I found this site because the science of meteorology is fascinating to me.

I have a bachelor's level science/mathematics education.

I just got the Vasquez Weather Map book.

What else should I be reading to learn the big picture ideas of meteorology, without going overboard with heavy mathematics and computer programming?

I always recommend "A World of Weather" by Jon Nese and Lee Grenci. It's the textbook for undergrad general education meteorology courses at Penn State, but it's an easy read and covers all of the topics without getting into math or programming.

[am19psu]

What causes the NAO to go positive or negative?

As you know, the +NAO features zonal flow across the Atlantic, while the -NAO has blocked up, highly meridional flow across the North Atlantic. If you think about the base state of the atmosphere in winter, a +NAO should be favored. Subsidence from the Hadley cell is focused across the subtropical Atlantc, creating high pressure, while the polar vortex is being enhanced through radiational cooling in the high latitudes.

Off the top of my head, here are some things that can perturb the pattern into a -NAO state:

1. Rapidly deepening lows off the Canadian Maritimes can focus warm advection into the high latitudes over Greenland while enhancing cold advection on the back side of the low. That would lead to higher heights across the North Atlantic and lower heights across Quebec/Newfoundland.

2. Discontinuous retrogression of the mean longwave pattern. This occurs when the pattern is already amplified and the group velocity of the waves is negative (typical in winter as the wavelengths are longer). In order for this pattern to sustain itself, it needs a positive feedback from the storm track.

3. Mountain torques over the Rockies (in particular) create planetary waves that help to disrupt the polar vortex and create ridging over Greenland (c.f. Lott et al. 2004).

From a seasonal perspective, there are a few things that help indicate the sign of the NAO for winter. The most often quoted signal is NH snowpack in late fall. The more snowpack there is across the NH in late fall, the weaker the temperature gradient will be from the pole to the mid-latitudes and therefore the polar vortex will be weaker and make a -NAO more likely. Another factor is ENSO. In Ninas, the Walker Circulation enhances subsidence over the subtropical Atlantic, strengthening the subtropical ridge (or Azores-Bermuda high). In Ninos, the opposite occurs, with less subsidence and a weaker subtropical ridge. There are also solar considerations, but I'm not as familiar with them. This list is also not exhaustive, either, I'm just thinking off the top of my head.

Hope this helps.

[am19psu]

What actually is the MJO?

No one actually knows. It's not predicted by any of the shallow water equations and its phase speed doesn't match up with any known types of waves. It does appear to be a convectively coupled wave - it moves slower when there is more convection - so it is obviously forced thermally in some manner. Much research is going into the dynamics of the MJO.

I know Phase 8 is good for East Coast snowstorms, and that there is a "circle of death". But what actually is it?

I'm not sure exactly what you're asking here. Are you just talking about what the Phase Space looks like? Here is the current image.

The circle of death is in the center and it means that amplitude of the MJO is weak and it is likely not having much affect on the global weather pattern.

[Mencken_Fan]

Why do Gulf Lows seem to 'weaken' as they near Florida?

I've noticed a pattern where shortwaves coming out of the Southwest enter the Gulf and promptly produce a lot of qpf over the Southern Plains / Mississippi Valley, then 'weaken' as the Low nears Florida. The map below is a typical example.

Does the Florida Peninsula disrupt these Lows? Are the Southern Appalachians involved? Whatever the case; it's quite obvious there's a lack of continuity as a Low tracks from the Gulf to the Atlantic. I'd love to know the reason why.

Thank you.

[usedtobe]

M Fan,

I'll take a guess. most storms that really deepen end up lifting to the north and east as the exit the southwest and they often phase with northern stream shortwaves or at least lift northeastward in response to them. Th stronger deepening storms also often take on a negative tilt and have the wave length between the base of the trough and the shortwave ridge to its east shorten. Weaker waves tend to have a more positive tilt and are more sheared. Those are the ones that typical are flatter and are more likely to take a southerly track but since they are shearing the upper level divergence out ahead of them is weakening so the surface low tends to weaken. Anyway that's my guess.

[Mencken_Fan]

M Fan,

I'll take a guess. most storms that really deepen end up lifting to the north and east as the exit the southwest and they often phase with northern stream shortwaves or at least lift northeastward in response to them. Th stronger deepening storms also often take on a negative tilt and have the wave length between the base of the trough and the shortwave ridge to its east shorten. Weaker waves tend to have a more positive tilt and are more sheared. Those are the ones that typical are flatter and are more likely to take a southerly track but since they are shearing the upper level divergence out ahead of them is weakening so the surface low tends to weaken. Anyway that's my guess.

Thank you for giving me a new insight; and for showing how false one's "cause and effect" ideas can be!

What you say makes good sense; that as a Gulf Low gets stronger, it will tend to turn poleward - leaving only the weaker systems to continue eastwards towards Florida. Aha! The Florida Peninsula doesn't weaken the Lows; it only receives the weaker ones!

Well son of a gun....

[SacrydDreamz]

This is correct, of course, but I don't know if that's what Art was looking for. You defined a -NAO. I think Art wanted to know WHY we get them to begin with and what causes them to switch states?

Since the NAO is derived from pressure anomalies over the North Atlantic, a blocking high in the vicinity of Greenland would cause the NAO to go negative while a deep upper-level low would cause the NAO to be read as positive. As far as why there's a high there sometimes while at other times there's a low -- part of that can be explained by sea-surface anomalies while another factor is the AO and stratospheric warming events that lead to a displacement of low pressure anomalies and subsequent trough amplification/probability of blocking.

My response into the causalities was not as exhaustive as am19psu, but there was am answer to "why" in there. Your reasoning was good too, though.

[SacrydDreamz]

M Fan,

I'll take a guess. most storms that really deepen end up lifting to the north and east as the exit the southwest and they often phase with northern stream shortwaves or at least lift northeastward in response to them. Th stronger deepening storms also often take on a negative tilt and have the wave length between the base of the trough and the shortwave ridge to its east shorten. Weaker waves tend to have a more positive tilt and are more sheared. Those are the ones that typical are flatter and are more likely to take a southerly track but since they are shearing the upper level divergence out ahead of them is weakening so the surface low tends to weaken. Anyway that's my guess.

And to add to that, if I may, when the shortwave in trough in question gets sheard out as it heads east, there is little dynamic support to induce any significant warm advection ahead of the system, so little in the way of QPF. On the other hand, a trough which is digging down the plains and into the Gulf can induce a strengthening surface low and strong warm advection running from the Gulf and into the Southeast... producing a plethora of rainfall. So in many cases if the trough has reached its maximum southward extent across the Southwest and you see the amplitude decrease as it heads east, chances are it will fade until it hits the east coast (where jet streak dynamics may provide a spark, assuming it's along the coast) while a trough that is getting deeper/sharper will have be more likely to produce a significant amount of QPF in the Southeast and potentially up the coast.

[ArtRosen]

We all know what causes a tornado, warm air rising, cold air falling, a spiraling motion is formed and the tornado drops... but what's the difference between a tornado that's a skinny little 70 mph stove pipe tornado and a full on, EF-5, multi-vortex, wedge tornado? And can a multi-vortex tornado really be called one tornado or is it just a bunch of small tornadoes inside of one parent funnel?

[AtticaFanatica]

We all know what causes a tornado, warm air rising, cold air falling, a spiraling motion is formed and the tornado drops...

This isn't really what causes tornadoes, it's a much more complicated process.

but what's the difference between a tornado that's a skinny little 70 mph stove pipe tornado and a full on, EF-5, multi-vortex, wedge tornado?

Laboratory simulations and some observations have suggested that tornado structure varies quite a bit with what's known as the swirl ratio, the ratio of tangential velocity to radial velocity in a tornado's inflow region (or, similarly, circulation around a tornado to tornado inflow). As the swirl increases, vortices tend to go from single-celled where they have a central updraft of rising air to a two-celled vortex containing a central downdraft. The interface between these two is similar to a hydraulic jump and causes what's known as vortex breakdown. As the swirl ratio continues to increase and a vortex breakdown reaches the surface, you're likely to get multiple vortices within a broader tornado core flow. Unfortunately, swirl ratio is not a predictive quantity, so it can't be used to forecast tornado intensity, only assess after the fact.

I'd also be cautious in implying that wider tornadoes are stronger tornadoes. There is a relationship, but it's a surprisingly weak one. Estimating tornado strength just by assessing its appearance is something I generally would avoid.

And can a multi-vortex tornado really be called one tornado or is it just a bunch of small tornadoes inside of one parent funnel?

It's one tornado. The (admittedly limited) observations of multiple vortices shows them to be rather transient (at least compared to the main vortex). In other words, you can't track a single multiple vortex for a long period of time (minutes) as it travels around the main vortex.

[WhiteoutWX]

What causes thundersnow? Is it convective in nature? I know it usually occurs with intense snowfall rates but is this always the case?

[Snowbo]

How do sunspots affect the weather? What mechanism is at work?

[isohume]

What causes thundersnow? Is it convective in nature? I know it usually occurs with intense snowfall rates but is this always the case?

Thundersnow is normally caused by increasing mlvl lapse rates such as when an ulvl trof is impinging on the comma head. This creates enough of a separation of charge (electric field) for lightning to develop....the same mechanism which is seen in thunderstorms. Also...very strong dynamical forcing (dpva, div, pot vort, etc) can lead to enhanced upward vertical motions and thus the formation of thundersnow as well.

[eekuasepinniW]

Why do countless tiny showers/thunderstorms pop up almost every night between 3-7am around the Florida keys in summer?

[isohume]

Why do countless tiny showers/thunderstorms pop up almost every night between 3-7am around the Florida keys in summer?

Because that place is Hell.

Naw really...the tropical atmos down there is always at least conditionally unstable through the overnight due to the deep layered moisture. Any passing wave, no matter how small, is going to help instigate tstms and shra aligned along previous outflow boundaries and coastal fronts. The developing land breeze can also help trigger conv by providing an insitu llvl jet.

[eekuasepinniW]

Because that place is Hell.

Naw really...the tropical atmos down there is always at least conditionally unstable through the overnight due to the deep layered moisture. Any passing wave, no matter how small, is going to help instigate tstms and shra aligned along previous outflow boundaries and coastal fronts. The developing land breeze can also help trigger conv by providing an insitu llvl jet.

I see- thanks.