Ask a Pro Met

[am19psu]

Lots of you have questions about meteorology and sometimes the answers get lost in the shuffle of the discussion threads. Ask your questions here instead and I'll be happy to answer them. Hopefully, other red taggers will jump in as well and we get some solid instruction here.

[mattinpa]

Lots of you have questions about meteorology and sometimes the answers get lost in the shuffle of the discussion threads. Ask your questions here instead and I'll be happy to answer them. Hopefully, Tony, Mike, Glenn, etc. will jump in as well and we get some solid instruction here.

This is a good thread idea! I think many mets do the best they can with an important and changeable field, and I respect the profession. My first question: I know blocking helps us to keep us from cutting lows. But what exactly does the blocking: high pressure? The polar vortex? And by the way, what exactly is a polar vortex?

[am19psu]

My first question: I know blocking helps us to keep us from cutting lows. But what exactly does the blocking: high pressure? The polar vortex?

Blocking is generally defined as the north/south (or meridional) wind anomalies in the high latitudes. The classic example that most of us think of is the -NAO or Greenland block. When "something" increases warm advection across the North Central Atlantic, an upper level ridge sets up over Greenland, producing a block. In the image below (which is for a +NAO), the blue area over Greenland at H500 has positive height anomalies and anomalous meridional flow.

The -NAO block is usually configured as an omega block. It's called an omega block because it's flanked on both ends by a trough and looks like the Greek letter omega. Another type of block is a rex block, which features positive height anomalies over negative height anomalies.

In both cases, you can see (can you?) how there is more north/south flow than east/west flow.

Blocks are important for forecasting because they are generally long lived over the period of many days or even weeks, making the mean longwave ridges and troughs more predictable.

And by the way, what exactly is a polar vortex?

In the winter, the pole is losing much more radiation to space than it is absorbing, leading to extremely cold surface temperatures. By extension, the columns of air above the pole are colder than columns of air over the mid-latitudes. That promotes lower 500mb heights across the pole, leading to a height gradient. Because of the strong Coriolis force at high latitudes, the winds produced by the height gradient turn to the right (in the Northern Hemisphere) and develop an inertially stable vortex centered on the pole (things are inertially stable when they spin fast -- think about a top). At least initially.

Over the course of winter, many things can disturb the polar vortex. The most common and well known are sudden stratopsheric warmings. These are usually caused by solar flux events that destroy ozone and release energy through exothermic reactions. That decreases the height gradient and makes the vortex less inertially stable and easier to move. When the vortex aloft moves, the air mass at the surface goes with it and usually produces cold anomalies over the mid-latitudes. When the polar vortex is strong and over the pole, the AO is postiive and when it is displaced and weak, the AO is negative.

[Chris L]

What is the role of the low level jet in east coast snowstorms? It mght sound silly... but...

[tombo82685]

Adam has been trying to help me understand jet streaks more, cause i can't really figure them out. I get confused on a map like this (see map) on how to determine the entrance and exit regions. To me and probably to a lot of other people it just looks like a big blue smudge.

[Chris L]

Jet Streak Dynamics for East Coast Cycklogenesis: What are key things to look for?

[BayBreeze]

i take hydrologic questions, even though my background is on both sides.

[am19psu]

Jet Streak Dynamics for East Coast Cycklogenesis: What are key things to look for?

What you want to see is a strong 130 kt or better jet streak oriented parallel to the coast so that either the right entrance or the left exit of the jet streak, and therefore max divergence aloft, is near Cape Hatteras.

For jet streak basics, look through these instructional pages and then come back with more questions.

https://www.e-education.psu.edu/courses/meteo101/Images/Section7/jetstreak_move0707.swf

http://www.meteo.psu.edu/~nese/jet_streak.swf

[Chris L]

Thanks Adam, yes I will read those two sites

This is going to be such a great thread.

[mattinpa]

Blocking is generally defined as the north/south (or meridional) wind anomalies in the high latitudes. The classic example that most of us think of is the -NAO or Greenland block. When "something" increases warm advection across the North Central Atlantic, an upper level ridge sets up over Greenland, producing a block. In the image below (which is for a +NAO), the blue area over Greenland at H500 has positive height anomalies and anomalous meridional flow.

The -NAO block is usually configured as an omega block. It's called an omega block because it's flanked on both ends by a trough and looks like the Greek letter omega. Another type of block is a rex block, which features positive height anomalies over negative height anomalies.

In both cases, you can see (can you?) how there is more north/south flow than east/west flow.

Blocks are important for forecasting because they are generally long lived over the period of many days or even weeks, making the mean longwave ridges and troughs more predictable.

In the winter, the pole is losing much more radiation to space than it is absorbing, leading to extremely cold surface temperatures. By extension, the columns of air above the pole are colder than columns of air over the mid-latitudes. That promotes lower 500mb heights across the pole, leading to a height gradient. Because of the strong Coriolis force at high latitudes, the winds produced by the height gradient turn to the right (in the Northern Hemisphere) and develop an inertially stable vortex centered on the pole. At least initially.

Over the course of winter, many things can disturb the polar vortex. The most common and well known are sudden stratopsheric warmings. These are usually caused by solar flux events that destroy ozone and release energy through exothermic reactions. That decreases the height gradient and makes the vortex less inertially stable and easier to move. When the vortex aloft moves, the air mass at the surface goes with it and usually produces cold anomalies over the mid-latitudes. When the polar vortex is strong and over the pole, the AO is postiive and when it is displaced and weak, the AO is negative.

Thanks! Some of this is a bit foreign to me, but you provided a good starting point and I can do further research. I also hear about west and east based -NAO blocks. Could you explain the difference?

[k***]

What you want to see is a strong 130 kt or better jet streak oriented parallel to the coast so that either the right entrance or the left exit of the jet streak, and therefore max divergence aloft, is near Cape Hatteras.

For jet streak basics, look through these instructional pages and then come back with more questions.

https://www.e-education.psu.edu/courses/meteo101/Images/Section7/jetstreak_move0707.swf

http://www.meteo.psu.edu/~nese/jet_streak.swf

I say 135 kts.

[Analog96]

Thanks! Some of this is a bit foreign to me, but you provided a good starting point and I can do further research. I also hear about west and east based -NAO blocks. Could you explain the difference?

This just refers to the location of the North Atlantic High. East based would be on the Icelandic side of Greenland, while West-based would be further West, toward Newfoundland.

[Analog96]

I say 135 kts.

I don't think 5 kts is really going to make much of a difference.

[am19psu]

Thanks! Some of this is a bit foreign to me, but you provided a good starting point and I can do further research. I also hear about west and east based -NAO blocks. Could you explain the difference?

That's simply the difference in their location. A "normal" -NAO block is located over Greenland. An east-based -NAO block would be located over Iceland or the Norwegian Sea, while a west-based -NAO block will be located over the Davis Straits/Newfoundland.

[Chris L]

Here's a question probably everyone wonders: You do not need a big bomb to produce historic snowfalls as we have seen, (February 1983 and February 2003 PD storm), and yet people always discouarge a weak sfc low with Gulf moisture to be unproductive, my quesiton is: Does the forcing upstream at say 700 mb, assuming the forcing is strong enough, will it have the same result as a big bomb?

Sorry if it sounds convoluted.

[baroclinic_instability]

What is the role of the low level jet in east coast snowstorms? It mght sound silly... but...

The low level jet can take on various forms and develop for various reasons--but typically the low level jet is a "dynamic" response to an upper air disturbance aloft. It can get pretty complicated--but there is a level in the atmosphere called the "level of non-divergence" where air is neither convergent or divergent. It can be difficult because there can be times where there are multiple LND's--but typically there is one--and typically it is around 600-500 hpa. Because of mass continuity of the fluid atmosphere--some form of air mass transport must replace the air mass leaving a region. So, for instance, if a S/W aloft is propagating through the flow and forcing atmospheric vertical ascent through differential positive vorticity advection (commonly referenced as PVA) above the LND, air must converge in the low levels below the LND to support the air rising above it. In a mid-latitude cyclone, wave disturbances aloft form and amplify over regions of strong thermal gradients (baroclinic zones), and you will typically find a low level jet in the warm sector of a developing/mature storm as low level air converges into the center of the cyclone. A low level jet is not necessarily "important" but is actually a bigger part of what is occuring during cyclogenesis. It is an atmospheric response to an imbalance.

Typically what the low level jet will do is advect warm and moist air (theta-e) into both the developing low level cyclone but into the warm front. Warm and moist air rising into the cyclone (warm air advection) promotes the release of latent energy as the moist air mass condenses as saturation occurs via adiabatic cooling of rising air parcels. Under this scenario--latent energy can enhance the warm sector in the low levels and make it less statically stable and/or unstable. Under this scenario--air in the warm sector rises faster--and in the case of convection--becomes buoyant and accelerates with time (this is convective instability and is represented by CAPE). This can further enhance low level convergence into the cyclone which can enhance the frontal boundaries and convergence along the front as well. The cyclone both intensifies and deepens and a positive feedback loop develops where low level wind fields (including the low level jet stream in the warm sector) increase and even more moist and warm air is advected into the cyclone--which then releases even more latent energy through condensation until the cyclone is then "bombing" and/or rapidly intensifying.

[Analog96]

Here's a question probably everyone wonders: You do not need a big bomb to produce historic snowfalls as we have seen, (February 1983 and February 2003 PD storm), and yet people always discouarge a weak sfc low with Gulf moisture to be unproductive, my quesiton is: Does the forcing upstream at say 700 mb, assuming the forcing is strong enough, will it have the same result as a big bomb?

Sorry if it sounds convoluted.

You can get plenty of snow from relatively weak storms. Some examples I can think of off the top of my head would be the two back-to-back snowstorms in Feb 1994, and also as you mentioned, Feb 2003. What you DO need if you're wanting a lot of snow from a weak storm is either:

a. a big difference in pressure between the high and the low

b. a large thermal gradient

Feb 2003 had a high of 1052 or something like that over Quebec, while the low was around 1002 IIRC.

This is equivalent to a 1030 high and a low of 980.

Feb 1994 saw large temperature gradients.

[baroclinic_instability]

Here's a question probably everyone wonders: You do not need a big bomb to produce historic snowfalls as we have seen, (February 1983 and February 2003 PD storm), and yet people always discouarge a weak sfc low with Gulf moisture to be unproductive, my quesiton is: Does the forcing upstream at say 700 mb, assuming the forcing is strong enough, will it have the same result as a big bomb?

Sorry if it sounds convoluted.

You can get big snowfalls from storms with almost no surface low. A good example is "frontal waves" aloft where low amplitude upper tropospheric waves propagate over a region of enhanced baroclinity at some level in the atmosphere above the surface. What you can get is a prolonged period of frontogenesis as the wave passes over the elevated baroclinic zone. If oriented right--you can get a long period of snowfall with heavy banded precip over one location as the front remains parallel to the flow aloft. Sometimes folks call these clippers--but they really don't bear resemblance to a true "clipper".

[Analog96]

You can get big snowfalls from storms with almost no surface low. A good example is "frontal waves" aloft where low amplitude upper tropospheric waves propagate over a region of enhanced baroclinity at some level in the atmosphere above the surface. What you can get is a prolonged period of frontogenesis as the wave passes over the elevated baroclinic zone. If oriented right--you can get a long period of snowfall with heavy banded precip over one location as the front remains parallel to the flow aloft. Sometimes folks call these clippers--but they really don't bear resemblance to a true "clipper".

This can happen, but would be extremely rare, because the front would have to stall. I would guess the best chances for this to happen would be at the end or beginning of the season, when fronts have more of a tendency to stall out. Also, the heavy banding would have to be in a training scenario, and be very localized.

[ArtRosen]

What causes the NAO to go positive or negative?

[baroclinic_instability]

This can happen, but would be extremely rare, because the front would have to stall. I would guess the best chances for this to happen would be at the end or beginning of the season, when fronts have more of a tendency to stall out. Also, the heavy banding would have to be in a training scenario, and be very localized.

It is not rare at all actually--and happens quite often in the Northern Plains. Fronts are just baroclinic zones--they typically don't move unless a sufficiently large upper wave develops over them to develop cyclogenesis. "Frontal wave" events result in "tightening" of the front as the upper shortwave propagates along the baroclinic zone otherwise known as frontogenesis without developing a lot of low level convergence and a cyclone.

[NJHurricane]

We often hear how computer models are "just a tool" used for forecasting and should not be used as the sole foundation of a forecast itself. If you had no access to model data and had to make a 24 hour forecast, what other tools would you rely the most on?

Jon in Jersey

[NJHurricane]

i take hydrologic questions, even though my background is on both sides.

How is hydro related data collection and the river gauge network now compared to when you started in the field? Where would you make improvements if you could?

[Chris L]

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).

[am19psu]

We often hear how computer models are "just a tool" used for forecasting and should not be used as the sole foundation of a forecast itself. If you had no access to model data and had to make a 24 hour forecast, what other tools would you rely the most on?

Jon in Jersey

Do I get a computer? Do I get analysis maps? Or, to use an example I know has happened, am I with a battle squadron in a tropical country with only a single sounding?

If I get maps, I go back to simple synoptic and quasi-geostrophic theory. Move systems along with their phase speed (based on their wavelengths). Find the maximum precip areas through DPVA and WAA. From there, I'd use the analogous thickness method to get an idea for temps (that's where you figure out your H5 height and 1000-500 thickness for tomorrow, then find where that combination is at on the map today).

I'm not afraid to admit that I'd be next to useless in a single sounding environment.

[Analog96]

It is not rare at all actually--and happens quite often in the Northern Plains. Fronts are just baroclinic zones--they typically don't move unless a sufficiently large upper wave develops over them to develop cyclogenesis. "Frontal wave" events result in "tightening" of the front as the upper shortwave propagates along the baroclinic zone otherwise known as frontogenesis without developing a lot of low level convergence and a cyclone.

Please note that I said in this part of the country, not the Northern Plains.

[Analog96]

We often hear how computer models are "just a tool" used for forecasting and should not be used as the sole foundation of a forecast itself. If you had no access to model data and had to make a 24 hour forecast, what other tools would you rely the most on?

Jon in Jersey

24 hour forecasts would be easy enough using upper air data, as well as radar and satellite. The only times that would be challenging is in a Miller B scenario or in a frontal system with developing and dissipating thunderstorm lines.

[Analog96]

Do I get a computer? Do I get analysis maps? Or, to use an example I know has happened, am I with a battle squadron in a tropical country with only a single sounding?

If I get maps, I go back to simple synoptic and quasi-geostrophic theory. Move systems along with their phase speed (based on their wavelengths). Find the maximum precip areas through DPVA and WAA. From there, I'd use the analogous thickness method to get an idea for temps (that's where you figure out your H5 height and 1000-500 thickness for tomorrow, then find where that combination is at on the map today).

I'm not afraid to admit that I'd be next to useless in a single sounding environment.

Those are really good questions. Do we have access to computers, but not models, or what?

[NJHurricane]

Do I get a computer? Do I get analysis maps? Or, to use an example I know has happened, am I with a battle squadron in a tropical country with only a single sounding?

If I get maps, I go back to simple synoptic and quasi-geostrophic theory. Move systems along with their phase speed (based on their wavelengths). Find the maximum precip areas through DPVA and WAA. From there, I'd use the analogous thickness method to get an idea for temps (that's where you figure out your H5 height and 1000-500 thickness for tomorrow, then find where that combination is at on the map today).

I'm not afraid to admit that I'd be next to useless in a single sounding environment.

I think you answered my question mostly but yes...you have access to a computer, upper air data, analysis maps...just not the models....I guess my scenario is a bit extreme, just wanted to put the focus on the tools that maybe get overlooked amongst us model hugging weenies

[Ottawa Blizzard]

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!