Dew Points

[Scott Bilyou]

At what height is the dewpoint considered for T-storm development? Is is surface, or the upper heights? I looked at upper air sounding plots and see that the Td's and T's almost parallel each other, except for some mid-level(?) drops in Td's. Curious because I see that everyone talks about Td's needing to be in the 60's or so.

 

[mikedeason]

I'll just say this about dew points. A common mistake among new chasers/forecasters is assessing the synoptic setup by focusing on surface dew points and using models to forecast the depth and extent of low level moisture on a given day.

When I'm working through my morning analysis and diagnostic process, I find it always imperative to check upstream soundings from your target area (reality) and assess the short term performance of the models (fantasy, many times) against that. Key points I try to keep in mind are:

1) What is the source region of the low level airmass that will be advecting into the target area (cT or mT?)
2) Based on current and forecast changes in the low level windfield, will this airmass even be advecting into the target area during the prime, afternoon initiation period?
3) Even if the airmass in the source region is of sufficient depth to initiate and sustain convection, what factors (such as mixing) may cause it to decrease in depth as it advects into the target area?

Too often we can get caught up into looking at juicy surface DP's and we fail to consider they may only extend 50 to 100 mb's from the SFC. A gross example of this can be seen in the late summer when DP's in the cornbelt routinely reach into the upper 70's and low 80's. However, in most cases it's usually an extremely shallow layer capped off by a cT airmass coming in from the southwest and without much synoptic support and moisture pooling, you don't see much significant convective activity from it.

 

[Jeff Snyder]

For tornadic supercells, we're often interested in low-level moisture. You can look at 2 m Tds, but it's best to look at Tds over some depth. For example, you may want to consider 2 m and 850 mb Tds, but picking a higher level depends upon the elevation of the area you're looking at. For example, you may want to look closer to 750 mb if you want to look for deeper moisture in eastern Colorado, and you'll want to look closer to 900 mb for an estimate of decent moisture depth if you're looking at Alabama. Using isentropic surfaces is actually probably best, since unsaturated air flows along isentropic surfaces, but that's probably a bit more complex than you want right now. Mets typically consider 75 - 100 mb inflow "layers" for convection, since you will want to see substantial moisture over a similar depth. It's not uncommon for rich moisture to be only 25-30 mb deep sometimes, which can make a chase forecast a bit more difficult. For example, 65 F surface Tds may grab one's attention, but if that moisture is only 25 mb deep, the potential for substantial supercells is typically quite reduced. Vertical soundings (RAOBs) are probably the best way to look for quality moisture depth.

The actual Tds that we look for in terms of supercell or tornado potential depends a lot on the time of year and location of the country. Realize that Td affects theta-e,which directly affects CAPE and can directly affect the intensity of updrafts. So, it's often thought that "higher is better" when it comes to low-level moisture. At any rate, getting back to the "rule of thumb"... This time of year, in the central and southern plains, upper-50 to low-60 F dewpoints can still yield substantial instability since mid-level temperatures are often quite cool this time of year. After we get into April and May, chasers often look for >60 F Tds, and we often want 67-75F Tds came June and July, when mid-level temperatures considerably warmer (on the whole). Note that this all addresses 2 m Tds, or dewpoint observations that you'll see on surface observations.

Remember, too, that the dewpoint temperature of a parcel decreases as it rises at a rate of ~2 C/km (~ 3.6 F / km, ~ 1.1 F / 1000 ft). Note this includes forced ascent as air moves up higher terrain. For example, a parcel that has a dewpoint temperature of 65 F near sea level (e.g. on the Corpus Cristi, TX, observation) will have a dewpoint temperature of 62 F at the elevation of Dodge City or 61 F at Amarillo.

 

[mikegeukes]

I agree that soundings and forecast soundings are a excellent source in determining moisture depth.

Do not forget to look at the 850mb charts for moisture.

If you want to do some hand analysis, start drawing isodrosotherms at 6 degrees C, and draw them every 2 deg C.
Look for the maximum axis of moisture and moisture advection.
Areas of significant moisture look for dewpoint depressions of 5 deg C or less.
Add some streamlines for fun, to find the dryline.

I sometimes will take a blank US map, and do this for forecast models for 850mb or you can eyeball it from the models.

Miller/SPC uses 850mb Dew Point Temperatures, Surface Dewpoints as one its parameters for severe weather checklist.
Weak, Moderate and Strong.

850mb Dewpoint Temperatures:
8 degrees C or less: Weak
8-12 degrees C: Moderate
12 degrees C or greater: Strong

Surface Dew Points:
55 degrees F or less: Weak
55-64 degrees F: Moderate
65 degrees F or greater: Strong

During the day, you can monitor the changes in moisture by keeping a eye of current weather conditions, SPC mesoanalysis, RUC model, etc.

 

[Tim Vasquez]

Dewpoints climatologically should be in the 60s for severe weather, but there's no meteorological reason requiring 60s. It all depends on the particular thermodynamic setup. Severe weather happens all the time with dewpoints in the 50s and even the 40s.

If you're dealing with elevated storms, they can produce plenty of rain and hail regardless of the surface temperature/dewpoint, and even tornadoes if the cold air mass is shallow enough. Back in February 1975 there was a minor tornado outbreak in southwest Oklahoma with 40 degrees, a north wind, and overcast conditions taking place.

All dewpoints within the inflow layer ingested by the storm are important. If it's an elevated storm, the dewpoints just above the inversion are important. If it's a surface-based storm, all the dewpoints in the lower levels (say within 1 to 2 km of the surface) are important. Using the surface dewpoint by itself is fine but it's sort of like feeling a kid's head to see if he has a fever.

Tim

 

[Darrin Rasberry]

If you're dealing with elevated storms, they can produce plenty of rain and hail regardless of the surface temperature/dewpoint, and even tornadoes if the cold air mass is shallow enough. Back in February 1975 there was a minor tornado outbreak in southwest Oklahoma with 40 degrees, a north wind, and overcast conditions taking place.

Tim

Tim - I don't know how far my past thread on this has descended, but doesn't an elevated storm producing a tornado automatically mean it's forced surface parcels (even if the BL is stable) and thus has become surfaced-based?