2017 General Severe Weather thread

[FallsLake]

Question for a met: Today my grid forecast has 20% chance of precip early, then 50% later, and finally 70% tonight. what's the actual chance of precip for me during the next 12 to 16 hours?

Do I assume that the 50% this late afternoon will be associated with the precip tonight (..so that stays 70% all together)? If so would the 20% get added to the 70% tonight to give me a 90% chance for the whole time period?  

[Brick Tamland]

27 minutes ago, Cold Rain said:

Wonder how long it will take before the 70% chance of storms will be reduced to 30-40%?

Well, that means we'll have a better shot at actually getting a storm. Got one last night and there wasn't a lot of talk about storms yesterday like there has been for today.

Actually hope the storms stat away tonight. My wife is going to the Sam Hunt concert tonight at Walnut Creek with some friends.  They are sitting under the covered area, but it would still be better not to have to worry about a storm coming up while they are out there. 

[Solak]

1 hour ago, Cold Rain said:

Wonder how long it will take before the 70% chance of storms will be reduced to 30-40%?

We're 'only' 60% for tonight. I'll be waiting for the Carolina Split

[jburns]

1 hour ago, FallsLake said:

Question for a met: Today my grid forecast has 20% chance of precip early, then 50% later, and finally 70% tonight. what's the actual chance of precip for me during the next 12 to 16 hours?

Do I assume that the 50% this late afternoon will be associated with the precip tonight (..so that stays 70% all together)? If so would the 20% get added to the 70% tonight to give me a 90% chance for the whole time period?  

Well, let's see. 20 + 50 + 70 = a 140% chance of rain.  Of course, there is less than a 50% chance of the 140% occurring.

 

[FallsLake]

11 minutes ago, jburns said:

Well, let's see. 20 + 50 + 70 = a 140% chance of rain.  Of course, there is less than a 50% chance of the 140% occurring.

 

That is hilarious!  

[downeastnc]

Couplet decent warned couplets over NE NC.....

[Solak]

Already hinting at the potential for severe weather next weekend.

 Diurnal convection will
return in concert with low-level moisture on Friday, becoming more
widespread /above-climo/ over the weekend as the aforementioned
trough amplifies over the region and an unseasonably strong cold
front progresses east of the Appalachians. Pattern recognition
suggests a potential for organized severe weather will exist
Sat/Sun, though little else can be said at this range.

 

[FallsLake]

32 minutes ago, Solak said:

Already hinting at the potential for severe weather next weekend.

 Diurnal convection will
return in concert with low-level moisture on Friday, becoming more
widespread /above-climo/ over the weekend as the aforementioned
trough amplifies over the region and an unseasonably strong cold
front progresses east of the Appalachians. Pattern recognition
suggests a potential for organized severe weather will exist
Sat/Sun, though little else can be said at this range.

 

I highlighted (blue) the most important thing in the discussion... ..more cool weather

[Solak]

For Monday, 8/7

NWS SPC‏Verified account @NWSSPC 4m4 minutes ago

12:29pm CDT #SPC Day2 Outlook Slight Risk: across the mid atlantic region and vi... http://go.usa.gov/YW34 

[isohume]

On 7/28/2017 at 9:36 AM, FallsLake said:

Question for a met: Today my grid forecast has 20% chance of precip early, then 50% later, and finally 70% tonight. what's the actual chance of precip for me during the next 12 to 16 hours?

Do I assume that the 50% this late afternoon will be associated with the precip tonight (..so that stays 70% all together)? If so would the 20% get added to the 70% tonight to give me a 90% chance for the whole time period?  

Your highest 12-hr PoP btw the 6am-6pm window is the PoP for today. From 6pm-6am the highest 1-hr PoP is the 12-hr PoP for tonight. Just look at the 12-hr PoPs: https://graphical.weather.gov/sectors/midatlanticWeek.php#tabs  

[FallsLake]

18 minutes ago, isohume said:

Your highest 12-hr PoP btw the 6am-6pm window is the PoP for today. From 6pm-6am the highest 1-hr PoP is the 12-hr PoP for tonight. Just look at the 12-hr PoPs: https://graphical.weather.gov/sectors/midatlanticWeek.php#tabs  

Gotcha, thanks!  So if I have a 50% chance of rain today and a 50% chance of rain tonight, I can statistically say I have a 75% chance of seeing rain today and tonight (like flipping a coin for heads, you have a 75% chance of getting it with two flips).

[isohume]

2 hours ago, FallsLake said:

Gotcha, thanks!  So if I have a 50% chance of rain today and a 50% chance of rain tonight, I can statistically say I have a 75% chance of seeing rain today and tonight (like flipping a coin for heads, you have a 75% chance of getting it with two flips).

Nope, for the today period you'd have a 50% chance and for the tonight period a 50% chance. For the 24-hr period you'd also have a 50% chance.  

[jburns]

 

.THIS AFTERNOON...MOSTLY CLOUDY WITH A 40 PERCENT CHANCE OF
SHOWERS AND THUNDERSTORMS. WINDY. HIGHS IN THE LOWER 80S. NEAR
STEADY TEMPERATURE IN THE LOWER 80S. SOUTH WINDS 15 TO 25 MPH.
.TONIGHT...MOSTLY CLOUDY WITH A CHANCE OF SHOWERS AND
THUNDERSTORMS IN THE EVENING...THEN A SLIGHT CHANCE OF SHOWERS
AND THUNDERSTORMS AFTER MIDNIGHT. LOWS IN THE MID 60S. SOUTHWEST
WINDS 5 TO 15 MPH. CHANCE OF RAIN 40 PERCENT.

What does this "40 percent" mean? ...will it rain 40 percent of of the time? ...will it rain over 40 percent of the area?

The "Probability of Precipitation" (PoP) describes the chance of precipitation occurring at any point you select in the area.

How do forecasters arrive at this value?

Mathematically, PoP is defined as follows:

PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measureable precipitation, if it occurs at all.

So... in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = "C" x "A" or "1" times ".4" which equals .4 or 40%.)

But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )

In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.

[FallsLake]

Iso and Mr. Burns,

Ok so I get the official definition. But if there is a defined percentage(like 50%) for a specific amount of time (like 12hours), then it starts all over again for another 12 hours (again 50%); I would say there's a better than 50% chance I'll see rain over the two 12 hours periods. lets take it even farther and say I have a 50% chance of rain showing for each of the next five days and nights; I would bet my car (..old car) that sometime during the next five days I will see rain.  

[FallsLake]

And I'll also add that this discussion shows how slow things are right now....

[WidreMann]

9 hours ago, Solak said:

For Monday, 8/7

NWS SPC‏Verified account @NWSSPC 4m4 minutes ago

12:29pm CDT #SPC Day2 Outlook Slight Risk: across the mid atlantic region and vi... http://go.usa.gov/YW34 

Cool, I was hoping for another sunny and warm day.

[isohume]

8 hours ago, FallsLake said:

Iso and Mr. Burns,

Ok so I get the official definition. But if there is a defined percentage(like 50%) for a specific amount of time (like 12hours), then it starts all over again for another 12 hours (again 50%); I would say there's a better than 50% chance I'll see rain over the two 12 hours periods. lets take it even farther and say I have a 50% chance of rain showing for each of the next five days and nights; I would bet my car (..old car) that sometime during the next five days I will see rain.  

I know what you're saying, but there is never a confidence of greater than 50% in any of the periods. PoP periods are not additive. Another confusing way to look at it is: If you fcst'd a PoP of 100% covering the next 7 days during the summer, you'd most likely be correct based on climatology. However, if you fcst a 100% PoP for a specific 12-hr period, say 144 hrs out, you'd likely be wrong based on inherent uncertainty with time. 

You'd most likely be right in betting your car over a 5-day fcst of measurable precip during this time of year. However, extremely long fcst periods add little to no value over climo. That's why official NWS PoPs are defined by separate 12-hr periods, with higher confidence normally occurring within the first 1-3 days depending on the pattern.   

 

[FallsLake]

5 hours ago, isohume said:

I know what you're saying, but there is never a confidence of greater than 50% in any of the periods. PoP periods are not additive. Another confusing way to look at it is: If you fcst'd a PoP of 100% covering the next 7 days during the summer, you'd most likely be correct based on climatology. However, if you fcst a 100% PoP for a specific 12-hr period, say 144 hrs out, you'd likely be wrong based on inherent uncertainty with time. 

You'd most likely be right in betting your car over a 5-day fcst of measurable precip during this time of year. However, extremely long fcst periods add little to no value over climo. That's why official NWS PoPs are defined by separate 12-hr periods, with higher confidence normally occurring within the first 1-3 days depending on the pattern.   

 

Ok I understand now.  

Thanks for the response Iso!

[shaggy]

We're gonna need to watch the remnants of Harvey as they potentially affect the area this weekend. SPC disco for this Saturday:

Farther east, a warm frontal zone is forecast to set up west-to-east
   -- at this time appearing likely to reside across North Carolina --
   as Harvey impinges on persistent surface high pressure centered over
   the Great Lakes.  Across this area, a somewhat-less-tropical, more
   classic severe weather risk may evolve, as some heating boosts
   afternoon CAPE and the wind field near and south of the warm front
   veers/increases with height yielding profiles supportive of
   supercells.  At this time, uncertainty precludes an aggressive
   forecast, but will introduce 5%/MRGL risk across the Carolinas area
   to begin highlighting this potential.
 

[mackerel_sky]

My local forecast on TWC for tomorrow says : Thunderstorms. Isolated tornadoes possible!! Can't recall ever seeing that!

[Solak]

RAH this morning:

Just where this boundary sets up will be critical to designating
the area where the potential exists for some damaging wind gusts
and even a few tornadoes. The current models are stronger with
the push of cooler, more stable, rain cooled air deep into the
northern and even central parts of NC. Therefore the slight risk
or the highest potential for severe weather has been shifted just to
the south of the Triad and Triangle areas, with areas from Albemarle
to Fayetteville expected in the bulls eye. This could very well
shift 50 miles either way, so stay tuned to later forecasts.

[Brick Tamland]

We'll probably get that cooler air and rain to stabilize things around here,  but we still need to pay attention to things tomorrow.

Hazardous Weather Outlook
National Weather Service Raleigh NC
444 AM EDT Thu Aug 31 2017

NCZ007>011-021>028-038>043-073>078-083>086-088-089-010845-
Person-Granville-Vance-Warren-Halifax-Forsyth-Guilford-Alamance-
Orange-Durham-Franklin-Nash-Edgecombe-Davidson-Randolph-Chatham-Wake-
Johnston-Wilson-Stanly-Montgomery-Moore-Lee-Harnett-Wayne-Anson-
Richmond-Scotland-Hoke-Cumberland-Sampson-
444 AM EDT Thu Aug 31 2017

This Hazardous Weather Outlook is for central North Carolina.

.DAY ONE...Today and Tonight.

Hazardous weather is not expected at this time.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

A strong west to east oriented frontal zone will be located over
northern North Carolina Friday morning. This front is expected to
move southward during the day before stalling over the Southern
Piedmont and Sandhills. Severe thunderstorms will be possible along
and south of the front Friday afternoon and evening. Isolated
tornadoes and damaging wind gusts will be the primary threats.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will likely be needed Friday afternoon and
evening.

$$

Badgett

[Brick Tamland]

Tornado Watches now out for MS, AL, and TN.

[downeastnc]

Wouldnt be surprised to see a fair amount of tornado warnings the next 36 hrs or so across TN/AL/MS/GA/SC/NC....the Nam 3K runs are pretty rough for a lot of us. Heck there are 7 out right now in AR/TN/MS

[downeastnc]

Some decent rotations on the 3km Nam over NC tomorrow....

[jshetley]

Tomorrow could be very rough over both Carolinas. The wedge front placement will be critical. I'm betting it'll be farther south than models think too, somewhere withing 50 miles of the NC SC border, if not a little into SC, roughly from Clemson to Rock Hill.

[mackerel_sky]

8 minutes ago, jshetley said:

Tomorrow could be very rough over both Carolinas. The wedge front placement will be critical. I'm betting it'll be farther south than models think too, somewhere withing 50 miles of the NC SC border, if not a little into SC, roughly from Clemson to Rock Hill.

Y'all keep me posted, I'll be in Florida!

[Solak]

RAH HWO this afternoon...

Person-Granville-Vance-Warren-Halifax-Forsyth-Guilford-Alamance-
Orange-Durham-Franklin-Nash-Edgecombe-Davidson-Randolph-Chatham-Wake-
Johnston-Wilson-Stanly-Montgomery-Moore-Lee-Harnett-Wayne-Anson-
Richmond-Scotland-Hoke-Cumberland-Sampson-
448 PM EDT Thu Aug 31 2017

This Hazardous Weather Outlook is for central North Carolina.

.DAY ONE...Tonight.

Hazardous weather is not expected at this time.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Severe thunderstorms will be possible along and south of southward
drifting cold front Friday afternoon and into Friday night. There is
a good chance for severe thunderstorms, capable of producing 
tornadoes as well as damaging wind gusts. In addition, locally heavy 
rain is expected which may lead to flash flooding, especially in poor
drainage areas.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will likely be needed across central NC Friday
afternoon through Friday night, primarily between 4 PM and Midnight.

[downeastnc]

3k Nam looks pretty ominous for south central and eastern NC tomorrow afternoon into the evening...the threat is there for some tornados.

[Disc]

15 minutes ago, downeastnc said:

3k Nam looks pretty ominous for south central and eastern NC tomorrow afternoon into the evening...the threat is there for some tornados.

Speaking of the 3k, that supercell along the NC/SC border starting west of Rockingham, NC is nasty. You can see it ride the warm front from west to east, before things begin to go linear. I honestly believe we will see the boundary set up along the NC/SC border -- A lot of times these CAD/backdoor fronts are a lot stronger than modeled, which is why I believe most of the "action" will be southern NC and into northern SC.