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Who uses a step by step forecast?

Jason Boggs

Is there anybody who uses a step by step forecast when determining a target area? I have thought about it and it seems like it would be quicker to do and it would help eliminate mistakes in the forecast.

Welcome to the board, first of all! Second, to answer your Q & A, its hard to really say step-by-step as many situtations set themselves up differently. As for a general outline, yeah, I think most of us have a step-by-step (checklist, if you will) we follow. As for particular steps, forecasting techniques, I think it varies upon not only the weather situation, but the situation for the chaser themselves (money, time, etc). For me, a simple answer, yeah, but very general. Mistakes in forcasting one event can be wonderful predictions in others.

What do you mean by "step-by-step"? Most people use a top down (funnel) approach in forecasting - starting with the big picture and progressing into the small scale.
Is there anybody who uses a step by step forecast when determining a target area?
Step by step forecast, do you mean something like a
operational decision tree/checklist for severe weather?


If it looks like convection with little or no CIN is a possibility, I always first look at the 850 mb wind magnitude...if it appears that at least 20 kts of bulk surface to 850 shear will not be present, then I will often not chase. Closer to the time of initiation I begin looking at the magnitude of the 500 m winds...The magnitude of the low-level shear vector is the key. Storm motions are often too variable to concern yourself with the direction of the low-level shear, because even if the mean storm motions are not correct, a deviant storm motion could gain the necessary low-level streamwise vorticity. It must be realized that low-level shear can definitely sneak up on you with the development of a low-level jet during the evening...an example is May 12 of last year when very little low-level shear was present throughout the day until about 0000 UTC when the low-level jet went crazy. Everyone here knows what happened then. Of course this is all assuming supercells will be the convective mode...so you should also consider deep layer shear and the strength of the forcing. So my forecasting scheme definitely begins in the low levels.
Originally posted by Jason Boggs
Is there anybody who uses a step by step forecast when determining a target area? I have thought about it and it seems like it would be quicker to do and it would help eliminate mistakes in the forecast.

I don't have anything written down that I step through, though there is a laundry list of items that I'm looking for with perspective events. It boils down to how picky you want to be, as the more specific a type of event you are looking for (most picky being discrete, slow-moving, classic, cyclic, tornadic supercells during daylight hours in ideal terrain and road network, with cell motions expected to follow the road grid (e.g., east)), the more things you have to look for to come together just right. Or, you could follow the herd and just absorb the SPC products, look at a surface map and put an 'X' on it! :D Seriously, you have to include a time scale (further out, broader region being considered) and narrow things down as it gets closer to the expected event time, and it never hurts to consider the road network in the region you are considering. A target with good road choices could be handy if your target is less than perfect - or you may find your target has horrible road choices and may not make for a very effective chase. The specific items folks like to use in their forecasting probably varies substantially from person to person AND from event to event.

I found this in the citations from another paper we were examining a few days ago. It's almost twenty years old but probably still of some interest if you're curious about process.

Colquhoun, J.R., 1987: A decision tree method of forecasting thunderstorms, severe thunderstorms and tornadoes. Wea. Forecasting. 2, 337-345.

This document also has a process methodology that some may find useful, from one of the veteran chasers:

Moller, Alan R., 2001. \"Severe Local Storms Forecasting,\" in Severe Convective Storms (AMS Meteorological Monograph Series, Volume 28, number 50), 433-480.

Personally, I find that time is the overwhelming factor in determining my process. If I have all sorts of time and find myself in the neighborhood of the target area, I use a pretty elaborate, top-down, ingredients based approach. If, as is more typical, I have a narrow window, I'll generally glance at jet positions, thermodynamic profiles, and then dig into surface details for whatever time remains.
My convective weather chase target forecasting approach

Usually the following process:
1) Look at all 12Z upper-air charts, noting all key synoptic features. Hand analysis if I have time.
2) Current surface obs. – again, hand analysis if I have time.
3) Radar - noting any ongoing convection/boundaries (outflow and synoptic).
4) Visible and WV satellite - again noting boundaries and clouds (vis), and
mid/upper moisture and short-waves (WV).
5) 12 Z soundings - modify by hand for anticipated T/Td during afternoon
when storms will be happening. How much CAPE/CIN? Convective temperature? Shear environment?

Armed with 1-5 above, I strive to have a 4-d picture (three spatial dimensions +
time) of the atmosphere in the region of interest. Now, time for numerical

6) ETA and GFS products (on-line GFS 12Z runs often aren't ready for an AM
forecast). ALL guidance - I compare starting points/initiation with the
current obs in 1-4 above. I usually use the UCAR and COD data because like the graphics presentation of most of the products.
7) RUC: I like the FSL site most. Always pay careful attention to time/age of products (not all times update at the same time), and make sure I believe each product by comparing it to the current environment. I use QPF as a starting point for initiation timing - this is fine-tuned later.
8)SPC mesoanalysis data.

Now it's time to see what other people are thinking about - and question
myself: have I missed anything...?
9) SPC discussions, outlooks.
10) NWS forecast discussions.
11) Any other information, such as outlooks/discussions and posts
on WX lists such as this Stormtrack and others...

12) Repeat 1-8 and refine target/timing if necessary. One thing I do is
look at the RUC wind fields over and over, while comparing with the surface