Diminished Daytime Heating

[Stephen Locke]

As I watch the discussions regarding possible chases in the month of October I've been thinking about the ever diminishing effects of daytime heating. We are now over 100 days past zenith. The afternoon sun at this point is as weak as it would be in late March. Correct? How is an October set up different or similar to March. And is the sun angle a big factor here? Would a severe event in October likely need to occur earlier in the day?

 

[rdale]

The bigger difference than sun angle is airmass sources... There's no snow pack in the northern Plains / southern Canada.

 

[Jason Boggs]

I do think that an event in October is similar to a March event. Sun angle and daytime heating obviously plays a role in daytime heating, but don't get so wrapped up in this that you forget other factors.

I think if a system is very strong with very cold upper level temps, daytime heating is less of a factor. Of course daytime heating does come into play a little more when the cap is stout and the upper level temps are not as cold.

I think a severe wx event just depends of many factors coming together, not just daytime heating. The low level jet, the arrival of the energy and other factors come into play. When the best ingredients come together, whether it be 3pm or 3am, determine the severity of the storms.

I think a lot of forecasters get too caught up in daytime heating and cloud cover. Sure, clear skies are nice on a possible event, but some forecasters write off the event when everything looks good except daytime heating possibilities. Remember, there are a lot of variables and factors that come into play when forecasting a severe weather event.

 

[Skip Talbot]

Excellent points about the lack of snowpack influenced airmasses and number of variables in effect. Also remember that the temperature maximums and minimums lag several weeks behind the solstices. In this regard I'd say that November setups are more like March setups rather than October setups. The four ingredients that we look for in forecasting severe weather (shear, lift, moisture, and instability) are proportionately different in quantity in early spring and late fall setups than in late spring/summer setups. This time of year, instead of focusing on how much daytime heating and instability we might see, you'll want to be checking to see how strong that jet is or how deep the surface low is. You can get enough lift for severe weather outbreaks out of very dynamic, strongly sheared systems that have very marginal instability.

 

[Stephen Locke]

Of course daytime heating does come into play a little more when the cap is stout and the upper level temps are not as cold.
.

Jason, wouldn't you say the atmosphere would tend to be stronger capped in October than say, March?

 

[rdale]

You're speaking too general here... Look back at last October's big days, and the cap was just right.

 

[Jason Boggs]

Jason, wouldn't you say the atmosphere would tend to be stronger capped in October than say, March?

You know, I'm really not sure. I guess it just depends on the setup and how everything progresses. I guess the cap could be equally as strong in March or October.

 

[Shane Adams]

You know, I'm really not sure. I guess it just depends on the setup and how everything progresses. I guess the cap could be equally as strong in March or October.

Cap strength is relative to each event, not necessarily a given climo situation/time of year. It's pretty much like LCL, as far as the given distance between the two parameters (temperature VS dewpoint/h85-7 temp VS AG temp).

 

[Bobby Prentice]

The afternoon sun at this point is as weak as it would be in late March. Correct?

Actually, since October 3 is about 11 days past the autumnal equinox this is the equivalent of 11 days before the vernal equinox or about March 10.

How is an October set up different or similar to March. And is the sun angle a big factor here?

Thunderstorms require three ingredients:

1. Sufficient water vapor ("dewpoints") to produce condition 2.

2. Unstable air ("positive CAPE"). The airmass associated with showers and thunderstorms is almost always conditionally unstable. With conditional instability a lifting mechanism is required to release the instability.

3. A lifting mechanism to release the instability. Think of this as lifting the air parcels "above the cap" (past the "negative CAPE area"). The lift can be from fronts, drylines, surface lows/troughs, upslople flow, sea/lake breeze fronts, outflow boundaries from previous convection, etc.

Vertical wind shear and the type/nature of the lifting mechanism are the primary factors which determine thunderstorm type (e.g., multicell, supercell, squall line, etc.).

The biggest reason why the Fall severe storm season is a "faint echo" compared to Spring goes back to atmospheric (tropospheric) temperature variations. The surface of the earth receives peak solar insolation (heating) on the first day of summer (around June 21 in the Northern Hemisphere) and minimum on the first day of winter (around December 21 in the Northern Hemisphere). However, The processes which control temperature in the troposphere lag the calendar by about 1-2 months. Thus, jet streams and weather systems are generally weaker and the atmosphere is less unstable during Fall than Spring.

 

[Stephen Locke]

Thus, jet streams and weather systems are generally weaker and the atmosphere is less unstable during Fall than Spring.

Guess that's why I think of the late season as being more "capped". I'm generalising . . . and I'm definitely not a meteorologist, my terminology is likely not precise.

I appreciate everyone's comments here . . . you all are always informative.