Toward More Accurate Forecast Discussions

Forecast discussions are an important way for operational meteorologists to explain the reasoning behind the forecast, and to offer additional insights to a more scientifically fluent audience than what’s necessary for public watches, warnings and forecasts. Whether it’s an “area forecast discussion” from a local office, a national center’s technical text, or a private meteorologist’s offering on a storm discussion board, these can be fantastic ways to see the reasoning behind a prediction, to learn why a forecast decision was made, and to educate oneself on the “weather whys”.

As someone who has written thousands of detailed, national forecast discussions over the years–and made more than a few mistakes doing so–I’ve come to understand that any level of timeliness, consistency, writing prowess or even forecast accuracy can be undermined by incorrect or misleading information in the text. If the audience can’t trust the truth of what they’re reading, how can they trust the forecast itself?

If we’re all literally accurate with the terminology and its usage, we’ll be not only correct, but consistent, in our communication. Forecast discussions are meant for reasonably well-educated audiences with some meteorological background–not our moms, kids or siblings. “Dumbing down” isn’t necessary. We should get the terminology and wording right, to the greatest extent our understanding allows. Then we not only communicate with truth, we exude earned authority and credibility, and sometimes, even educate the already well-educated audience.

Good science is good service; there’s no need to choose between the two. Let’s say it right! Avoid map-room jargon and literal inaccuracy while being specific about what’s happening.

Here are some common misuses of terminology from actual forecast discussions that I’ve either seen or received, and more importantly, the reasoning with solutions to improve it. I’ll be glad to add more as suggested and documented.


BAD WORDING: Trough axis…ridge axis

REASONING: Redundant. A trough is an axis, by definition–in this case, an axis of low pressure or low height. A ridge is an axis of high pressure or high height. This is rather akin to saying, “cobra snake” or “copper metal”.



BAD WORDING: Diffluent…diffluence

REASONING: Misspelled. The root “fluent” is preceded by the common English prefixes “di” or “con” to denote the sign of the process. This is much like “divergence” and “convergence”. Would you spell the former as “divvergence”? Of course not. So why misspell the other word as “diffluence”? [Unfortunately, this has become very common, even to the extent it has been erroneously programmed into automated spell-checkers.]

BETTER ALTERNATIVE(S): Difluent…difluence


BAD WORDING: Data shows…data indicates…data offers…

REASONING: Subject-verb error. The word “data” is plural–for the singular “datum”.

BETTER ALTERNATIVE(S): Data show…data indicate…data offer…


BAD WORDING: A criteria…

REASONING: Subject-verb error. The word “criteria” is plural–for the singular “criterion”.



BAD WORDING: Narrow axis…broad axis (of moisture, temperature, CAPE, etc.)

REASONING: Physically and mathematically impossible! An axis is, at most, two-dimensional (if it curves) or one-dimensional (if straight). By definition, an axis has no width; it cannot be “narrow” or “broad” or “wide”. What we usually mean instead is…

BETTER ALTERNATIVE(S): Narrow corridor…wide corridor (or swath…area…field, etc.)


BAD WORDING: Isolated tornado…isolated tornadoes

REASONING: Redundant. For forecast purposes, all tornadoes are isolated. The terms, “isolated”, “widely scattered” and “scattered”, in meteorology, deal with coverage. Isolated is <15%, widely scattered is 15-24%, scattered is 25-54%, and numerous is 55-100%. Tornadoes--even the very largest ones--cover a very tiny percent of the land area on outlook and watch scales, and most often, even on warning scales. For example, consider a major outbreak day, with multiple violent, long-tracked tornadoes--say ten of them for easy arithmetic. Let's assume the average path length and width of each tornado is 50 miles and 1/2 mile, respectively. [This is a very terrible, once-in-career type of outbreak!] That's 50*0.5*10 square miles, or 250 square miles hit by tornadoes. The average watch covers about 25,000 square miles. Let's now assume the outbreak was extremely dense, and cram all 10 of those extremely high-end tornadoes entirely inside just one average watch (unprecedented!). Even in such crazy circumstances, only 1/100 of the watch area was covered by those tornadoes: a mere one percent (1%)! That's still isolated, by definition, and if there were a category of "very isolated", it would be on the low end of that.

BETTER ALTERNATIVE(S): A tornado…a tornado or two…a few tornadoes…


BAD WORDING: Deep moisture

REASONING: Misleading. Moisture is everywhere in the troposphere! Instead this usually means a deep layer of (some measure of) moisture. The layer containing that moisture can be deep or shallow. Specify that instead.

BETTER ALTERNATIVE(S): Deep, moist boundary layer…deep saturated layer…deep layer of mixing ratios at least 14 g/kg…deep layer of 50% or higher RH, etc.


BAD WORDING: Moisture pooling

REASONING: Misleading and colloquial, similar to “deep moisture”. This is one case where using fewer words destroys the real meaning. Again, moisture is everywhere in the troposphere! Instead this misused phrase usually stands for a deep and horizontally restricted layer of enhanced, low-level moisture. Such a condition often occurs near boundaries. It would be more insightful and literally correct to specify the physical mechanism behind that.

BETTER ALTERNATIVE(S): Zone (or corridor or area or deep layer) of relatively rich moisture with an outflow boundary, etc.



REASONING: No such word.



BAD WORDING: Thundershower…

REASONING: No such thing.



BAD WORDING: Instability of 2,000 J/kg

REASONING: Erroneous. The writer means CAPE (measured in J/kg, unlike actual instabilities). CAPE and convective instability are not physically the same! CAPE is an indicator of convective instability; but there are some other kinds of instability–for example, Helmholtz or inertial that can happen with no CAPE. The links go to AMS glossary definitions.



BAD WORDING: Overrunning

REASONING: Misleading, colloquial overreach. Opinions vary on the validity of the term in very specific situations (See one essay by Monteverdi and another by Doswell), but it’s usually misused anyway. It’s more insightful to specify the actual process involved.

BETTER ALTERNATIVE(S): Frontal lift or warm advection


BAD WORDING: Strong (or weak) upper-level dynamics

REASONING: Too vague, virtually meaningless. The term is so broad and overused that it encompasses almost limitless possibilities. Most (certainly not all) of the time, its context implies some form of, or contributor to, vertical motion. Be specific, not vague, about what’s happening and why–or if you don’t know, just say so. Don’t use “dynamics” as a black-box, catch-all word. Ambiguity is the enemy of understanding!

BETTER ALTERNATIVE(S): This depends on the situation. Refer directly to the actual physical process at work, whether it’s large-scale vertical motion contributions from differential vorticity advection, a thermally indirect circulation near a jet streak, mesoscale lift sources such as outflow boundaries, or storm-scale internal “dynamics” (the vertical pressure-gradient force), for example. In short, be specific.


BAD WORDING: Model resolution

REASONING: (Usually) wrong usage. Resolution of a model is not the same as its grid spacing, which is usually what the writer means. However, even then, the writer often does not realize that true horizontal resolution of features occurs only at 4X or more coarser than grid spacing! This very short scientific essay does a great job of explaining the difference.

BETTER ALTERNATIVE(S): Model grid spacing (or if using the higher number and the literal meaning) model resolution


BAD WORDING: DPVA causing (forcing, initiating, triggering) thunderstorms

REASONING: Misleading and false. Vertical motion related to differential vorticity advection is on the scale of cm/s or weaker. DCVA (same as DPVA in the northern hemisphere) makes the environment more thermodynamically unstable by steepening lapse rates, but a stronger source of lift is needed to initiate thunderstorms, within which air rises on the scale of tens of m/s (three orders of magnitude or more stronger than DCVA!). If an updraft is blasting through the sky at 50 m/s, half a cm/s of downward motion from DNVA won’t stop that; however there can be an indirect effect. Stable layers or subsident drying that result from DNVA can make the environment less favorable for storms.

BETTER ALTERNATIVE(S): DCVA destabilizing the environment for thunderstorms


BAD WORDING: “Energy” in the context of perturbations or the instability they may cause, such as upper-level energy approaching

REASONING: Same as with “dynamics”, but worse. Energy, in many forms, is everywhere in the atmosphere! As such, this word is meaningless.

BETTER ALTERNATIVE(S): As with “dynamics”, this depends on the specific situation. Just state the actual feature or process involved. A more-informed reader is…a more-informed reader!


BAD WORDING: Strong (or weak) lift or rising motion or subsidence (without specifying the source)

REASONING: Much too vague! Lift or sinking occurs on every scale of the atmosphere, from countless causes, and on a huge range of speeds and scales.

BETTER ALTERNATIVE(S): State the actual cause or process. For example, frontal lift…isentropic lift…sinking motion in cold advection, or whatever other actual process is behind the lift or subsidence.


BAD WORDING: Shortwave approaching…

REASONING: Incomplete. Waves have both ridge and trough components. Almost always, the writer means shortwave trough, so be specific and say so.

BETTER ALTERNATIVE(S): Shortwave trough approaching…, or if you truly mean the entire wave (ridge and trough), stick with “shortwave”.


BAD WORDING: Deviate motion

REASONING: Misspelling and/or usage error. Deviate is a verb. Deviant is the adjective form.



BAD WORDING: EF3 tornado intensity (or any other EF rating with the word “intensity”)

REASONING: Tornado rating is not the same as tornado intensity. The EF scale (or before it, the F Scale) is a way of rating tornadoes based on damage–or estimating a probable intensity at the damage point based on damage. However, most damage indicators (DIs) do not go up to EF5, and often there is either no DI or a weak DI. As such, the actual intensity of a tornado at most points cannot be specified from damage alone! The best we can do is estimate–often very crudely at that. Furthermore, many factors go into damage rating, wind intensity being only one of them (others including debris impact, construction methods, exposure, and subjective judgment of the surveyor). In short, the true intensity of most tornadoes is unknown, regardless of damage.

BETTER ALTERNATIVE(S): EF3 tornado damage–or, if referring specifically to wind speeds in the context of the EF scale standards or measured winds, simply EF3 wind speeds.


BAD WORDING: It is important to note that…it should be noted that… and other “It…that…” statements

REASONING: I see this more often with scientific papers (!), but sometimes in forecast discussions also. These phrases are unnecessary and almost always can be removed completely, saving space. Look, would you note something unimportant? Would you note something that should not be noted? Of course not! So what’s the point? Don’t bother with the wasted words. For more on this needless wording, see the book Eloquent Science by David Schultz, specifically Table 10.2 and adjoining discussion, for a list of these phrases, and alternatives if desired. Again, most of the time you simply can remove the phrase.

BETTER ALTERNATIVE(S): Delete. Start the sentence with whatever came after that phrase–capitalizing the first word, of course.



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