2017 General Severe Weather thread

[downeastnc]

Line segment headed for the general Goldsboro area getting pretty nasty....

[Solak]

Looking ahead...

QPF on the order of 1 to locally 2 inches will be likely.
Instability should be one lacking ingredient for severe storm
development Thursday as showers/extensive cloudiness are expected
Thursday limiting heating. A low level boundary may develop over the
Piedmont Damming Region Thursday afternoon given the likelihood of
showers that may create some In-Situ Damming if the dry air at the
surface is not scoured out. This boundary may become a focus for
possible severe later Thursday or Thursday night as the main upper
system approaches and possibly becomes negatively tilted. Much more
on the severe threat in later forecasts.

[Huriken]

From Dr. Greg Forbes' Facebook page:

[the ghost of leroy]

all the resources in the world to make graphics...looks like a weenie MSPaint job

[Solak]

RAH:

The deep upper level low will continue to slowly deepen Thu night
over the TN Valley, causing the low level sly flow over the
Carolinas to also strengthen. Meanwhile a s/w ejecting out ahead of
the upper low will approach from the southwest, leading to an
increase in shower coverage and intensity Thu night. The low level
jet strengthens to around 50kts with decent turning noted in the
lowest 6km of the atmosphere. This leads to strong kinematics with
storm relative helicity values in the order of 250-400 M2/S2 (per
GFS) late Thu night. These numbers supportive of rotating updrafts
with any convective columns that develop. Models, however, paint of
picture of weak sfc based instability with MLCAPE values no worse
than 500-750 J/kg (based off NAM, GFS much weaker). Even the
instability aloft marginal so am currently not too excited about
thunder potential Thu night. If sfc based convection were to occur,
kinematics would support a damaging wind threat for any deep
convection with the prime time for convection between midnight and 6
AM. While SPC has our southern counties in the "marginal" portion of
the outlook, they, too, do not seem too confident about severe
potential over our region. For now, will mention a chance for
thunder across the region Thu night.

[Cold Rain]

We don't get instability in NC anymore.  Height of severe weather season and the best we can muster is 500 j/kg of CAPE.

[Huriken]

SPC Day 2 Convective Outlook:

 

[Solak]

RAH reversal...

.SHORT TERM /Thursday and Thursday night/...
As of 235 PM Wednesday...

...Severe Weather Episode Expected Thursday Night...

Severe Threat: An appreciable severe threat may develop late Thu
night/Fri morning as deep-layer ascent strengthens from the west
amidst marginal/moderate nocturnal destabilization as an intense
S/SE LLJ advects rich low-level moisture beneath modest mid-level
lapse rates (6.0-6.5 c/km). Convection allowing models are fairly
unanimous in showing a squall line accompanying the cold front as it
progresses east of the mountains into the Carolinas after midnight,
and suggest that discrete convection will be possible invof a warm
front advancing northward into central NC in advance of the cold
front. Strengthening deep layer shear, a strong LLJ and backed
surface flow suggest damaging winds and QLCS tornadoes will be
possible in assoc/w the squall line progged to cross the area
between 06-15Z. If discrete convection develops in advance of the
cold front/squall line (particularly in the Sandhills/SE Coastal
Plain), strong 0-1km shear /cyclonically curved hodographs/
suggest supercells with a primary threat of tornadoes. -Vincent

 

[downeastnc]

Nam 3K is pretty nasty with that line, the soundings also support tornadoes and the timing is terrible with the line hitting in the overnight. Gonna have to see if the models hold up, lately they have trended less aggressive as we get closer to the events. 

[ldub23]

Overnight thurs? Does that mean friday will be dry?

[downeastnc]

5 minutes ago, ldub23 said:

Overnight thurs? Does that mean friday will be dry?

http://www.tropicaltidbits.com/analysis/models/?model=gfs&region=seus&pkg=mslp_pcpn_frzn&runtime=2017050318&fh=6&xpos=0&ypos=0

[downeastnc]

Nam3K is just wow for tomorrow night......the semi discrete to full on discrete cells it shows out in front of the squall line with the shear means trouble.....would expect to see SPC ramp up threat if this continues.....still that timing, so many times the middle of the night stuff is overdone on models.

http://www.tropicaltidbits.com/analysis/models/?model=nam3km&region=seus&pkg=mslp_pcpn_frzn&runtime=2017050318&fh=6

This would be rough, especially those big supercells out in front of the main line...and the middle of the night....bad stuff.

[Huriken]

How reliable is the NAM 3km?

[jshetley]

21 minutes ago, downeastnc said:

Nam3K is just wow for tomorrow night......the semi discrete to full on discrete cells it shows out in front of the squall line with the shear means trouble.....would expect to see SPC ramp up threat if this continues.....still that timing, so many times the middle of the night stuff is overdone on models.

http://www.tropicaltidbits.com/analysis/models/?model=nam3km&region=seus&pkg=mslp_pcpn_frzn&runtime=2017050318&fh=6

This would be rough, especially those big supercells out in front of the main line...and the middle of the night....bad stuff.

This could be trouble for much, if not almost all of SC and much of GA too. 2 things though. If we get strong enough CAD or get gulf coast convection robbing our moisture, this would not as bad. That last line comes in at night for almost all of NC and SC IF this model is right.

[SN_Lover]

If I see sun tomorrow, ill be worried. If not, nice rainstorm. 

[downeastnc]

1 hour ago, SN_Lover said:

If I see sun tomorrow, ill be worried. If not, nice rainstorm. 

How much sun we get wont have that much to do with how bad the storms are at 2-3 am.....

 

here is the NAM3k sounding for MBY as the line approaches notice the PDS TOR tag on it....this is not what you want to see in the middle of the night.

 

[SN_Lover]

12 minutes ago, downeastnc said:

How much sun we get wont have that much to do with how bad the storms are at 2-3 am.....

 

here is the NAM3k sounding for MBY as the line approaches notice the PDS TOR tag on it....this is not what you want to see in the middle of the night.

 

CAD screwed us last time and if the cad is stronger than modeled(which has been the case), 70F can go to 50-60F with a rainstorm. 

[downeastnc]

3 minutes ago, SN_Lover said:

CAD screwed us last time and if the cad is stronger than modeled(which has been the case), 70F can go to 50-60F with a rainstorm. 

There really is no CAD modeled though......winds are SE or S for the entire time.

[SN_Lover]

Just now, downeastnc said:

There really is no CAD modeled though......

Yes there is. on the 3km which is the model aforementioned 

[downeastnc]

Just now, SN_Lover said:

Yes there is. on the 3km which is the model aforementioned 

Here is your local forecast, notice the SE or S winds for every period there is no CAD on a SE wind at the surface....they go SW after the front...there may be some wedging right along the mts in the higher foothills though......I do agree that models tend to under do wedge but the setup before and during this event isn't one that lends itself to wedging IMO...at least not in Raleigh.

 

Tonight

Mostly clear, with a low around 55. East wind 6 to 10 mph.

Thursday

A chance of showers between noon and 4pm, then a chance of showers and thunderstorms after 4pm. Increasing clouds, with a high near 74. Southeast wind 11 to 15 mph, with gusts as high as 25 mph. Chance of precipitation is 30%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Thursday Night

Showers likely and possibly a thunderstorm, then showers and thunderstorms after 8pm. Low around 65. Southeast wind 13 to 17 mph, with gusts as high as 28 mph. Chance of precipitation is 100%. New rainfall amounts between three quarters and one inch possible.

Friday

Showers and thunderstorms likely before 11am, then a slight chance of showers. Mostly cloudy, with a high near 73. Southwest wind 13 to 16 mph, with gusts as high as 29 mph. Chance of precipitation is 70%. New precipitation amounts between a quarter and half of an inch possible.

[SN_Lover]

3 minutes ago, downeastnc said:

Here is your local forecast, notice the SE or S winds for every period there is no CAD on a SE wind at the surface....they go SW after the front...there may be some wedging right along the mts in the higher foothills though......I do agree that models tend to under do wedge but the setup before and during this event isn't one that lends itself to wedging IMO...at least not in Raleigh.

 

Tonight

Mostly clear, with a low around 55. East wind 6 to 10 mph.

Thursday

A chance of showers between noon and 4pm, then a chance of showers and thunderstorms after 4pm. Increasing clouds, with a high near 74. Southeast wind 11 to 15 mph, with gusts as high as 25 mph. Chance of precipitation is 30%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Thursday Night

Showers likely and possibly a thunderstorm, then showers and thunderstorms after 8pm. Low around 65. Southeast wind 13 to 17 mph, with gusts as high as 28 mph. Chance of precipitation is 100%. New rainfall amounts between three quarters and one inch possible.

Friday

Showers and thunderstorms likely before 11am, then a slight chance of showers. Mostly cloudy, with a high near 73. Southwest wind 13 to 16 mph, with gusts as high as 29 mph. Chance of precipitation is 70%. New precipitation amounts between a quarter and half of an inch possible.

That's the NWS forecast. I'm stating a possible flaw in the model you posted. I'm not gonna debate a NWS forecast. 

[WeatherNC]

3km NAM looks like a solid SVR outbreak, 12km NAM on the other hand looks like a classic Carolina split.

[Cold Rain]

0z 3k NAM looks much less impressive than the 18z.  Looks much more in line with what should happen...probably still even a little too aggressive.

[snowlover91]

Fwiw, the HRRR and 3km NAM are worlds apart with the warm front placement and dew points tomorrow. The HRRR is much drier and more stable with a slow warm front while Gulf convection cuts off the moisture transport north. I could easily see this happening. 

[downeastnc]

22 minutes ago, Cold Rain said:

0z 3k NAM looks much less impressive than the 18z.  Looks much more in line with what should happen...probably still even a little too aggressive.

here is the RDU sounding from the 00Z Nam3k

 

Wont take much and if anything the 00Z is more discrete versus squall line which is no good.

 

Looks less scary on the Pivot scale lol.....though they do 3 hr blocks versus hourly on Tidbit still.....

[mackerel_sky]

Winds out of the NE currently. Looks like not much sunshine today, this threat looks like a minimal one imby, like the Monday " event"! Temps probably not getting above mid 60s

[Solak]

SSDD

As of 345 AM Thursday...

...Uncertainty abounds with thunderstorm potential tonight and
early Friday...

The pattern certainly suggests an increased potential for severe
storms. However, the warm sector return due to the very extensive
convection along the entire Gulf of Mexico at the current time must
be overcome. In addition, a developing Cold Air Damming (CAD) event
(stabilizing) over the Piedmont today throws another wrench into the
forecast.

....

SPC did downgrade the area to a marginal risk of severe storms given
the uncertainties mentioned above. We will have a better handle on
the severe threat later today as the evolution of the expected CAD,
position of the warm front, and the re-development of cold frontal
convection are realized. Until then, we will scale back the wording
just a bit in the Hazardous Weather Outlook to indicate more of an
isolated risk of damaging wind and/or brief tornado potential.
However, this situation should still be monitored as the severe
potential will be tied the evolution of the system today and this
evening.

 

[Cold Rain]

6z 3K Nam shows a few cells in the area tonight, but each successive run gets less and less aggressive with convection.

[LithiaWx]

1 hour ago, Cold Rain said:

6z 3K Nam shows a few cells in the area tonight, but each successive run gets less and less aggressive with convection.

CAD.   East winds in north GA right now. 

[Solak]

11:00AM - RAH

Severe Threat: A large MCS developed over the Deep South/GOMEX last
night and has progressed into FL/GA as of 15Z this morning, though
it has been rapidly decaying over the past 6 hours. An expansive,
long-lived wake low has been present on the western periphery of the
decaying MCS, with numerous reports of 40-60 mph winds over AL/GA
and extreme western NC/eastern TN as of 15Z. Model guidance did a
poor job in forecasting the aforementioned convective system
overnight. As a result, uncertainty has increased w/regard to the
severe weather potential (timing/mode/coverage), and severe weather
cannot be ruled out late this aft/eve through Friday morning.
Nocturnal destabilization still appears likely given that rich low-
level moisture will not depend upon a southerly LLJ emanating from
the GOMEX. Indeed, observational trends already show rich low-level
moisture /mid 60s dewpoints/ advecting inland from the Atlantic into
the Carolinas. With the above in mind, the threat for severe weather
(coverage-wise) appears to have decreased. However, given the
thermodynamic/kinematic environment and high likelihood of convection
later today into tonight, the most likely scenario (at this time)
appears to be a potential for isolated supercells capable of
producing a few tornadoes. -Vincent