Ask a Pro Met

[DaculaWeather]

I'm assuming the NWS keeps stats (counters) on their web site hits?

[wx.1028]

is someone going to answer my question ?......... why is it that there always seems to be cumulus or stratocumulus clouds after a cold frontal passage ?

[BadgerWXman]

Never mind all of the aircraft, satellite (direct brightness temperature, derived products such as AMVs), wind profiler, gps radio occultation (refractivity profiles), and so on ;-).

Well yeah theres that... but they still suck without upper air obs

[baroclinic_instability]

Well yeah theres that... but they still suck without upper air obs

They do? I have a feeling dtk is going to have a field day with that answer.

[dtk]

They do? I have a feeling dtk is going to have a field day with that answer.

I have a hard time being confrontational with a fellow badger, though.

I think I have covered this in many threads elsewhere, but for the GFS, the difference in skill between any of the cycles is not statistically significant. There are particular, high-impact events where it is certainly advantageous to have the sonde-network in the analysis/initialization. That is not to say that the sondes are not incredibly useful...in fact our NWP skill would really hurt without them . This is mostly because these are the best observations to very accurate, very dense vertical profile information.

However, the information we get at the regular observing times is NOT "lost" in a 6 hour period (combined with the fact that we assimilate on the order of 2 million observations [and have access to MUCH more than that] at 6z/18z for the globals).

[Chris L]

dtk or anyone else,

Why are some mesoscale models often too wet when forecasting QPF amounts for snowstorms? What causes that?

at the same notion, what causes models to be too dry also?

[Dino]

Would you be willing to write a foreword for my industrial design thesis book? (designing a snowstorm chase vehicle)

I've attached a teaser sketch from my design process...

[Chris L]

Wow, that's extremely impressive, Bombo!

[Analog96]

When looking at wintry mix events...what do you look for in determining if the p-type is sleet or freezing rain?

You look at the +32F layers and the -32F layers. If the area of +32 is very large and extends all the way to ground level, that's a freezing rain set up. If the +32 area ends well above the ground and the rest of the atmosphere on the way down to the ground is sub-freezing, that's a sleet sounding.

[isohume]

is someone going to answer my question ?......... why is it that there always seems to be cumulus or stratocumulus clouds after a cold frontal passage ?

There is not always cu or stcu after a cold front. Many times though, the upper trof axis will be swinging in behind the front and it will instigate decent upward motion...while at the lower layers cold high pressure and downward motion is occurring. Any residual moisture associated with the frontal system is sometimes lifted over the incoming cold dome and formed into clouds with the aide of the forcing associated with the upper trof. Most likely developing a stable layer of few/sct high based stcu or acu.

[wx.1028]

There is not always cu or stcu after a cold front. Many times though, the upper trof axis will be swinging in behind the front and it will instigate decent upward motion...while at the lower layers cold high pressure and downward motion is occurring. Any residual moisture associated with the frontal system is sometimes lifted over the incoming cold dome and formed into clouds with the aide of the forcing associated with the upper trof. Most likely developing a stable layer of few/sct high based stcu or acu.

thanks for taking the time and answering my question isohume. you cleared that up for me.

[superjames1992]

Just kind of a general question...

What is your preferred method of making a short-term forecast (0-2 days) for a specific location? Do you use MOS and current weather observations (radar, surface OBS, ect.)? Which models do you give the most weight to?

For example, you are making a weather forecast for Boston tomorrow and it's currently the afternoon before. How do you go about doing this? I know there are different methods for different forecasters, but I am curious what you all do.

[bkviking]

Do all tropical systems at some point have an anticyclone develop or try to develop over them (ie, is it inherent with heat release into atmosphere to develop hp) or is this feature a product of larger upper level ridges where one storm is lucky to move over the right area and one gets stuck near ULL? I find it interesting that a tropical cyclone, which have the lowest pressures of all storms, develop best with high pressure over them.

[BillB]

Just kind of a general question...

What is your preferred method of making a short-term forecast (0-2 days) for a specific location? Do you use MOS and current weather observations (radar, surface OBS, ect.)? Which models do you give the most weight to?

For example, you are making a weather forecast for Boston tomorrow and it's currently the afternoon before. How do you go about doing this? I know there are different methods for different forecasters, but I am curious what you all do.

Different forecasters have different methods. Here is one...

The simple answer to your first set of questions is "All of the Above."

First, look at the overall pattern. Is it zonal, is it meridional? Closed low, or high pressure? Once the big picture is established then look at satellite, radar, and observations. How well do they match with the pattern? What changes, if any, do they suggest are upcoming? What do the latest soundings show, especially if there is a question on daytime temperature or wind, or precipitation type? Next, look at and compare the models. Are they in agreement over days 0-2, or does one model diverge from the others? What changes do they show in the pattern? How well does this fit with the observed pattern and conditions? From where will come the airmass affecting the local region? Will the airmass be subject to isentropic processes or convective processes? After getting a sense of the current weather and the model data, then look at the output such as MOS. Does it match expectations for the location, or is it at least in the ballpark? Does it suggest things you had not considered?

As to favored models... local verification in our area over the past couple of years has shown that bias-corrected versions of the gridded MET and MAV (MOS) fields have outperformed all other gridded fields available to us for temperature and wind. That doesn't mean they will be right all of the time. Sky cover forecasts from MOS are less reliable, and MOS forecasts of Ceiling Height and Visibility are too prone toward wrong forecasts of IFR or LIFR conditions for stormy weather.

Time of reception plays a role. The NAM and GFS (and their respecitive MOS) are available for use before the deadline for aviation forecasts (TAFs), while the Canadian/UK/ECMWF data sets arrive after the deadline. So the early version of the forecast can be strongly influenced by the NAM/GFS/MET/MAV...which, as noted before, verification says is a good thing. If one or more of the later datasets looks more reasonable, then the forecast grids may be updated/adjusted as that data becomes available. We are approaching the return to Daylight Time, and in the Eastern Time Zone Daylight Time means the ECMWF won't arrive until after 3 am and 3 pm. This is a little late for any forecasts that need to be issued by 4 am and 4 pm. The ECMWF has a good reputation, but because of its late reception during EDT it is more of an afterthought during the two most important issue times here (morning and evening commute times) for 8 months of the year.

[DaculaWeather]

Very thorough answer! Thanks! I was always wondering the same thing!

[am19psu]

Do all tropical systems at some point have an anticyclone develop or try to develop over them (ie, is it inherent with heat release into atmosphere to develop hp) or is this feature a product of larger upper level ridges where one storm is lucky to move over the right area and one gets stuck near ULL? I find it interesting that a tropical cyclone, which have the lowest pressures of all storms, develop best with high pressure over them.

The release of latent heat in the eyewall of a hurricane does many things, including causing an anticyclone to form overtop the cyclone. In the vorticity tendency equation, there are terms for advection and stretching, which are more common. But when latent heat release is strong enough, like in a hurricane or a summertime MCC, cyclonic vorticity is generated beneath the area of maximum latent heat release and anticyclonic vorticity is generated above latent heat release max.

[mnweather]

Does a place like the summit of mt. everest average much precipitation per year since it is at such a high altitude it would be above the clouds most of the time?

[superjames1992]

Different forecasters have different methods. Here is one...

The simple answer to your first set of questions is "All of the Above."

First, look at the overall pattern. Is it zonal, is it meridional? Closed low, or high pressure? Once the big picture is established then look at satellite, radar, and observations. How well do they match with the pattern? What changes, if any, do they suggest are upcoming? What do the latest soundings show, especially if there is a question on daytime temperature or wind, or precipitation type? Next, look at and compare the models. Are they in agreement over days 0-2, or does one model diverge from the others? What changes do they show in the pattern? How well does this fit with the observed pattern and conditions? From where will come the airmass affecting the local region? Will the airmass be subject to isentropic processes or convective processes? After getting a sense of the current weather and the model data, then look at the output such as MOS. Does it match expectations for the location, or is it at least in the ballpark? Does it suggest things you had not considered?

As to favored models... local verification in our area over the past couple of years has shown that bias-corrected versions of the gridded MET and MAV (MOS) fields have outperformed all other gridded fields available to us for temperature and wind. That doesn't mean they will be right all of the time. Sky cover forecasts from MOS are less reliable, and MOS forecasts of Ceiling Height and Visibility are too prone toward wrong forecasts of IFR or LIFR conditions for stormy weather.

Time of reception plays a role. The NAM and GFS (and their respecitive MOS) are available for use before the deadline for aviation forecasts (TAFs), while the Canadian/UK/ECMWF data sets arrive after the deadline. So the early version of the forecast can be strongly influenced by the NAM/GFS/MET/MAV...which, as noted before, verification says is a good thing. If one or more of the later datasets looks more reasonable, then the forecast grids may be updated/adjusted as that data becomes available. We are approaching the return to Daylight Time, and in the Eastern Time Zone Daylight Time means the ECMWF won't arrive until after 3 am and 3 pm. This is a little late for any forecasts that need to be issued by 4 am and 4 pm. The ECMWF has a good reputation, but because of its late reception during EDT it is more of an afterthought during the two most important issue times here (morning and evening commute times) for 8 months of the year.

Thanks for the thorough explanation!

[isohume]

Does a place like the summit of mt. everest average much precipitation per year since it is at such a high altitude it would be above the clouds most of the time?

The summit of Mt Everest is basically a glacier where snow accumulates mainly during the monsoon season. It generally only receives around 2 feet per year due to it's extreme height, which takes large scale systems like constant monsoon winds to deliver snow that high.

[gravitylover]

So, when looking at a soundings text output and it shows all these numbers from 100mb>1000 what do those numbers correlate to? I know it's altitude but what altitude does each number equal? Everybody is always so concerned about 850's ...

What is this critical 500/1000 thing that is also mentioned frequently?

[isohume]

So, when looking at a soundings text output and it shows all these numbers from 100mb>1000 what do those numbers correlate to? I know it's altitude but what altitude does each number equal? Everybody is always so concerned about 850's ...

What is this critical 500/1000 thing that is also mentioned frequently?

The altitude of a certain pressure layer generally depends on the mean temperature of that layer. So a definite height is never attached to a specific pressure level. In a general sense tho, 850 mb is about 1500 m, 700 mb around 3200 m and 500 mb around 5500 m.

The 500/1000 thing is the thickness between these two pressure levels. The difference between the height at one level and the height at another level. For example, say the 500 mb height is 5500 m and the 1000 mb height is 200 m. The thickness will be 5500-200 m, or 5300 m. The infamous 540 thickness line is actually 5400 meters and it's used as a very general rain/snow line...moreso in the higher latitudes.

[am19psu]

The 500/1000 thing is the thickness between these two pressure levels. The difference between the height at one level and the height at another level. For example, say the 500 mb height is 5500 m and the 1000 mb height is 200 m. The thickness will be 5500-200 m, or 5300 m. The infamous 540 thickness line is actually 5400 meters and it's used as a very general rain/snow line...moreso in the higher latitudes.

Just to add on here, people look at the thickness of layers because it is directly correlated to the average temperature of the layer through the hypsometric equation. Thicker layers have a warmer average temperature. Since it is only an average temperature, it's not nearly as precise of a forecasting tool as something like a forecast sounding.

[tornadojay]

So, when looking at a soundings text output and it shows all these numbers from 100mb>1000 what do those numbers correlate to? I know it's altitude but what altitude does each number equal? Everybody is always so concerned about 850's ...

What is this critical 500/1000 thing that is also mentioned frequently?

with regard to 850's, you'll hear a lot of temps thrown around in discussions at this particular level, as it is often used in guaging what the potential max afternoon temperature might be, but you need to take great care in using this method, as it only works well under certain conditions.. generally, you'll want to use this more in the warmer months of the year where the atmosphere is allowed to mix down from higher levels of the atmosphere.. if you attempt to use this in the dead of winter with a 1040 mb high sitting right on top of you, you'll bust for sure using this... but basically the idea of this method is to find the 850 temp on the skew t chart and bring it down dry adiabatically, which is essentially adding about 13-14 degrees centrigrade to the 850 temp.. under full sunshine and good mixing during the summer months, it'll give you a good approximation on the maximum potential temp in the afternoon.

[NinjaWarrior2]

What is/is there a 1000mb sounding when the surface pressure is below 1000mbs?

[am19psu]

What is/is there a 1000mb sounding when the surface pressure is below 1000mbs?

Can you rephrase your question? I'm not sure what you mean. Nearly all CONUS soundings do not have a 1000mb level, since most sites are well above sea level.

[Chris L]

A hypothetical question; and it will be dumb; but here it is....

Can the East Coast get a storm analogus to a polar low cyclogenesis, I.E tremendous low pressure systems in the 940s or so? *this is not a hurricane of course*, but in the winter season, is it possible to get a surface low to 940s or 930s? or no due to Corolis Force?

The March 1, 1914 storm bombed to 952 mb right at the Benchmark, the KU Book states, so I would BELIEVE it is possible?

[hm8]

If the Octobomb has occurred in another month, what would we have called it?

[BillB]

Everybody is always so concerned about 850's ...

with regard to 850's, you'll hear a lot of temps thrown around in discussions at this particular level, as it is often used in guaging what the potential max afternoon temperature might be, but you need to take great care in using this method, as it only works well under certain conditions.. generally, you'll want to use this more in the warmer months of the year where the atmosphere is allowed to mix down from higher levels of the atmosphere.. if you attempt to use this in the dead of winter with a 1040 mb high sitting right on top of you, you'll bust for sure using this... but basically the idea of this method is to find the 850 temp on the skew t chart and bring it down dry adiabatically, which is essentially adding about 13-14 degrees centrigrade to the 850 temp.. under full sunshine and good mixing during the summer months, it'll give you a good approximation on the maximum potential temp in the afternoon.

Using observed/forecast temperatures at 850 mb to predict max surface temperature developed "back in the day" when maps came through via NAFAX and data via teletype. Talk about limited "bandwidth"! The temperatures at 850 mb pressure level were the only data available from a level covered by daytime mixing at some part of the year. Today, models are higher resolution and model output includes data from several additional levels above and below 850 mb. Computers allow us to receive much larger amounts of data from these models. This allows us to adjust our focus and use other pressure levels based on how deep we expect the mixed layer will get. As tornadojay mentioned, in the colder parts of the year 850 mb will be too high; in midsummer it will frequently be too low. In winter, I'll normally use 925 mb or even 950 mb as a reference level and convert those temperatures to their dry adiabatic equivilent at 850 mb. In summer, I'll normally use 800 mb and convert. Some situations in winter will suggest deeper mixing than is normal, and some situations in summer will suggest shallower mixing. In those cases you would need to adjust as necessary.

Surface pressure is a second factor to be mindful of. The 850 temperature equivilents that are most used are based on standard atmosphere values of temperature/pressure. Higher surface pressure than STP would add a degree or two to those max temperature estimates, while a lower pressure would subtract a degree or two.

This procedure is most used in the eastern USA. In the western high plains and the Rockies the higher elevations above sea level are close to or higher than the 850 mb level. Forecasters there can use the same procedure, but their reference pressure levels would be lower (higher altitude) than in the east.

[wall_cloud]

A hypothetical question; and it will be dumb; but here it is....

Can the East Coast get a storm analogus to a polar low cyclogenesis, I.E tremendous low pressure systems in the 940s or so? *this is not a hurricane of course*, but in the winter season, is it possible to get a surface low to 940s or 930s? or no due to Corolis Force?

The March 1, 1914 storm bombed to 952 mb right at the Benchmark, the KU Book states, so I would BELIEVE it is possible?

I don't know if you can get pressures as low as you are asking (940 mb) but the east coast has some of the strongest low pressure systems in the CONUS. In the winter, the colder polar continental airmass sitting right next to the warm maritime airmass influenced by the Gulf Stream. This strong baroclinicity (temperature gradient) can support tremendous pressure falls and sustain these cyclones for a brief period time.

[Chris L]

I don't know if you can get pressures as low as you are asking (940 mb) but the east coast has some of the strongest low pressure systems in the CONUS. In the winter, the colder polar continental airmass sitting right next to the warm maritime airmass influenced by the Gulf Stream. This strong baroclinicity (temperature gradient) can support tremendous pressure falls and sustain these cyclones for a brief period time.

Thank You.

I always wondered IF its possible for an extratropical system to have a 940 mb at 40/70 area.... *benchmark*. probably too tough to do.