SPC Confusion

[Brandon Schmidt]

As the title says, I've recently been confused by a few things I have found pertaining to the SPC.

1) A small issue, but this caught me off guard the other day...shouldn't the 45% region in the probabilistic field appear as a moderate risk region in the categorial field on both Day 2 Outlooks or am I missing something?

http://www.spc.noaa.gov/exper/archive/event.php?date=20130410

2) Is there a reason that watches like these (extremely small area and/or very low probabilities) are even issued because I fail to come up with any?

http://www.spc.noaa.gov/products/watch/2011/ww0678.html
http://www.spc.noaa.gov/products/watch/2012/ww0433.html
http://www.spc.noaa.gov/products/watch/2011/ww0445.html
http://www.spc.noaa.gov/products/watch/2012/ww0260.html
http://www.spc.noaa.gov/products/watch/2012/ww0124.html
http://www.spc.noaa.gov/products/watch/2011/ww0035.html

3) I know forecast busts do happen but they are usually in the case of forecasting initiation and not trends of sustaining convection. This leads me to say that I have seen the SPC bust a few times fairly recently in this way. Is this abnormal or does this happen fairly regularly?

*moderate risk widely void of reports despite convection that had already initiated
http://www.spc.noaa.gov/exper/archive/event.php?date=20130418

*PDS tornado watch that day with a 1:3:2 low:moderate:high severe probability ratio that resulted in only a few reports and an early cancellation...bad judgement for that watch or just a bust?
http://www.spc.noaa.gov/products/watch/ww0123.html

*a lower expectation forecast that resulted in many more events
http://www.spc.noaa.gov/exper/archive/event.php?date=20130419

Thanks for any comments or answers!

 

[Bill Walker]

Brandon,

Good points. This was also addressed in the 'Advanced' forum. http://www.stormtrack.org/forum/showthread.php?29713-4-18-13-What-Happened


bw

 

[Skip Talbot]

As the title says, I've recently been confused by a few things I have found pertaining to the SPC.

1) A small issue, but this caught me off guard the other day...shouldn't the 45% region in the probabilistic field appear as a moderate risk region in the categorial field on both Day 2 Outlooks or am I missing something?

http://www.spc.noaa.gov/exper/archive/event.php?date=20130410

The 45% area needs to be hatched for it to be a moderate risk on the day 2 and day 3 categorical. 45 unhatched is a slight risk. Also, lots of folks get excited to see a moderate risk on the day 1, but often don't realize it's moderate for hail or wind and not tornadoes, such as a 5 or 10% tornado (which is slight) combined with a 45% wind (which is moderate and also makes the categorical risk moderate). See:
http://www.spc.noaa.gov/misc/SPC_probotlk_info.html


As for the watch box size, those are usually watches that are covering specific cells or clusters of cells that are ongoing. It makes sense to keep the watch box small so areas not impacted by these storms don't need to be bothered with the watch. I'm not sure about the low probs. Someone more familiar with what probabilities warrant a watch could better answer that one.

SPC has come a long way over the years in terms of nailing setups, but you'll still see spectacular busts every year. It goes to show that we're very much still learning how the atmosphere works and have large gaps in our observational capabilities of what the atmosphere is currently doing or about to do.

 

[Brandon Schmidt]

Thanks for the input!! One more question that came up yesterday...I saw the Day 2 Outlook valid yesterday:

(6-5-13 Day 2 categorial) http://www.spc.noaa.gov/products/outlook/archive/2013/day2otlk_20130605_1730_prt.gif
(6-5-13 Day 2 probabilistic) http://www.spc.noaa.gov/products/outlook/archive/2013/day2probotlk_20130605_1730_any_prt.gif

...and saw that the slight risk area had no corresponding probability outline. I thought it was just a little mistake, as the outlooks sometimes have delineation errors, or it was some exception for tornado probabilities with tropical systems, but I wasn't sure. I didn't feel compelled to ask until today when I found the same thing on today's Day 2 Outlook:

(6-6-13 Day 2 categorial) http://www.spc.noaa.gov/products/outlook/archive/2013/day2otlk_20130606_0600_prt.gif
(6-6-13 Day 2 probabilistic) http://www.spc.noaa.gov/products/outlook/archive/2013/day2probotlk_20130606_0600_any_prt.gif

Could anyone explain why there is no 15% probability where the categorial slight risk area is outlined? Is this associated with tropical systems making landfall and producing tornados by any chance? Sorry but these little things I can't figure out bug me lol.

 

[Will Campbell]

Thanks for the input!! One more question that came up yesterday...I saw the Day 2 Outlook valid yesterday:

(6-5-13 Day 2 categorial) http://www.spc.noaa.gov/products/outlook/archive/2013/day2otlk_20130605_1730_prt.gif
(6-5-13 Day 2 probabilistic) http://www.spc.noaa.gov/products/outlook/archive/2013/day2probotlk_20130605_1730_any_prt.gif

...and saw that the slight risk area had no corresponding probability outline. I thought it was just a little mistake, as the outlooks sometimes have delineation errors, or it was some exception for tornado probabilities with tropical systems, but I wasn't sure. I didn't feel compelled to ask until today when I found the same thing on today's Day 2 Outlook:

(6-6-13 Day 2 categorial) http://www.spc.noaa.gov/products/outlook/archive/2013/day2otlk_20130606_0600_prt.gif
(6-6-13 Day 2 probabilistic) http://www.spc.noaa.gov/products/outlook/archive/2013/day2probotlk_20130606_0600_any_prt.gif

Could anyone explain why there is no 15% probability where the categorial slight risk area is outlined? Is this associated with tropical systems making landfall and producing tornados by any chance? Sorry but these little things I can't figure out bug me lol.

My guess would be because it is dealing with a tropical storm.

 

[Andy Wehrle]

You are both correct. When dealing with TC tornado potential, you will often not see a 15% probability line associated with the slight risk. This is because slight risk criteria is only 5% for tornadoes.

 

[Brandon Schmidt]

Thanks guys, I just found this on their website that I didn't see yesterday: "Note: A 5% probability for only a tornado threat (mainly associated with tropical systems) can be issued as a SLGT RISK."
(http://www.spc.noaa.gov/misc/SPC_Prob_Conv_Otlk_Change_20060214.html)