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Archive March 18-20th, 2018 Severe Weather

rolltide_130 said:
Re: Recovery. I'm seeing a lot of similarities to 4/5/17 last year, so it's why I'm still a little iffy on this system and I haven't been posting much about it. The GFS, while it has improved some, still has that nasty lead wave in the morning that I am highly concerned could throw a wrench in the wind fields. Also, the Euro has a nonexistent LLJ in the afternoon and the 0z run in general was still a total mess.. I need to see more from this system before getting legitimately concerned about it. There have been improvements but I've been burned too much the last year by jumping on board more than about 48 hours out..
Click to expand...
I’m watching this one from the sideline Why I’m quiet till further notice
 
I'm wondering when the SPC will jump on board, curious if their mesoanalysis page will be a cluster Monday...


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I could see the warm front trending more north, and east like some of the GFS ensembles show. The EURO has showed the MCS exiting very fast.
 
It wouldn't shock me at all either. The GFS runs prior to 12Z's had been faster, and the 12Z NAM had trended faster with that too... and further north. It makes sense when you look at how low amplitude the mid/upper flow is. That's not conducive to complexes getting hung up and not being able to move eastward.
 
image4.png
 
Matt Grantham's afternoon AFD for BMX.

LONG TERM...
Updated for the severe weather threat on Monday.

NAM/GFS/ECMWF have taken steps toward a more ominous setup for
Monday afternoon and evening. There appears to be less emphasis on a
potentially disruptive convective episode Sunday night into Monday
morning. This is probably due to a stronger elevated mixed layer
spreading in across the region Sunday night which would allow the
warm sector to move inland more rapidly. Disagreement remains
regarding the finer but crucial details for the evolution of the
surface to 850mb response to the upper-level trough. This will make
or break the potential for tornadoes. Currently the solutions range
from a tornado outbreak, shown by the NAM, to a lower tornado threat
on the GFS/ECMWF. However, confidence continues to increase in the
potential for large to very large hail due to steep mid-level lapse
rates and strong deep-layer shear. Our updated products will reflect
an increase in the severe weather threat, particularly for large
hail, with an acknowledgement of an upward trending but still
uncertain tornado potential.

87/Grantham
Click to expand...
 
Well, while I try not to read the NAM verbatim until within 60 hours like I indicated earlier, the 18z is still a bit worse. Sounding from near Franklin, GA at 20:00 EDT. The timing has also been relatively consistent for GA with the peak threat hovering between 5p and 8p EDT. What's interesting is the NAM isn't showing anything on simulated reflectivity, but is that a case of the discrete convection being smaller than the NAM's spatial resolution can show?
 

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Sundays outlook

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Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1249 AM CDT Sat Mar 17 2018

Valid 181200Z - 191200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
AL...MS...AND THE ARKLATEX REGION...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM SOUTHWEST KS
INTO NORTHWEST OK...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
PLAINS TO THE CENTRAL GULF COAST...

...SUMMARY...
Scattered severe storms are possible Sunday from the Arklatex area
southeastward into southern Alabama. A few severe storms are also
expected across southwest into central Kansas and across northwest
Oklahoma during the evening.

...Synopsis...
A potent shortwave trough will move across the Four Corners states
during the day, with the nose of a 100 kt midlevel jet overspreading
the central and southern High Plains during the evening with rapid
cooling aloft. Meanwhile, low pressure will deepen over eastern CO
into western KS. Strong lift along a cold front will interact with
marginal instability to support rapid storm development during the
evening.

To the south, a leading low-amplitude wave will move across
northeast TX during the day, with shortwave ridging over the lower
MS Valley. A quasi-stationary front will extend from the Arklatex
eastward across MS and AL, with mid to upper 60s F dewpoints to the
south. This general zone will support scattered storm development
through the period, beneath favorably strong flow aloft to support a
few strong to severe storms.

...Arklatex into southern Alabama...
Thunderstorms are expected to be ongoing across the Arklatex Sunday
morning near or north of the front, supported by weak low-level warm
advection. Marginal hail is possible with the early activity. Later
in the day, these storms will move across southern AR and into MS
along the boundary, and may evolve into severe storms as instability
increases. In addition, wind profiles will strengthen aloft as the
disturbance moves out of TX. Damaging winds may be the main threat
if the storms organize into an MCS. Otherwise, wind profiles do
favor supercells, although low-level SRH will not be particularly
strong. Still, a tornado or two will be possible.

Elsewhere, a moist and unstable air mass, along with favorable
deep-layer shear, will remain across northeast TX into LA. However,
forecast soundings indicate some capping concerns, and lift in the
wake of the departing wave will be minimal. In addition, mass fields
will be adjusting toward the northwest as the central Plains low
deepens. Conditionally, large hail is possible should an isolated
storm form.

...Southwest/Central KS into northwest OK...
Thunderstorms are expected by early to mid afternoon over
eastern/northeast OK, AR and southwest MO due to warm advection.
Most of these storms should be north of the warm front, and thus
elevated. Marginal hail will be possible with this activity.

Farther west, strong heating will occur across the High Plains,
where surface convergence will rapidly increase as the low deepens.
Cooling aloft will overspread this warm air mass, resulting in steep
lapse rates. Wind profiles will support supercells. However,
instability will be weak, as dewpoints only rise into the 40s F.
Still, strong lift will result in an arcing line of storms by around
00Z, with hail and wind possible. A tornado cannot be ruled out
given the vorticity-rich environment, steep lapse rates, and ample
low-level shear, mainly over southwest KS into far northwest OK.
Click to expand...
 
Monday Outlook

GIVEN THE POTENTIAL FOR SIGNIFICANT SEVERE STORMS, A CATEGORICAL
UPGRADE IS POSSIBLE IN LATER OUTLOOKS ONCE PREDICTABILITY INCREASES
AND THE CENTROID OF SEVERE COVERAGE IS BETTER ESTABLISHED.

qRPRcqI.png


DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0227 AM CDT SAT MAR 17 2018

VALID 191200Z - 201200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF
AL...GA...SOUTHERN TN...AS WELL AS NORTHERN FL AND NORTHEAST MS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF
THE SOUTHEAST...

..SUMMARY

SEVERE STORMS CAPABLE OF TORNADOES, DAMAGING WIND AND HAIL ARE
POSSIBLE ACROSS PARTS OF MIDDLE TENNESSEE, ALABAMA, AND GEORGIA.
SEVERE WIND IS ALSO POSSIBLE INTO NORTHERN FLORIDA.

..DISCUSSION

THE TIMING OF A SHORTWAVE TROUGH WILL PLAY A CRITICAL ROLE IN SEVERE
POTENTIAL ON MONDAY ACROSS TN, MS, AL AND GA. AT THIS TIME, THE NAM
APPEARS TO BE TOO SLOW AND MORE AMPLIFIED WITH THIS WAVE OVER MO
COMPARED TO THE ECMWF AND GFS WHICH SHOW IT OVER CENTRAL KY OR
MIDDLE TN AT 00Z TUESDAY. AS A RESULT, SEVERE POTENTIAL LOOKS QUITE
DIFFERENT AMONGST THE MODELS. FOR EXAMPLE, THE LATEST NAM SHOWS AN
EXTREMELY VOLATILE SETUP OVER MIDDLE TN AND NORTHERN AL CLEARLY
FAVORING STRONG TORNADOES. HOWEVER, THE MOST LIKELY SOLUTION APPEARS
TO BE A BLEND OF THE ECMWF AND GFS, BOTH SHOWING A FASTER AND
LOWER-AMPLITUDE SHORTWAVE, AS WELL AS LESS LOW-LEVEL SHEAR WITH
RELATIVELY VEERED 850 MB FLOW.

THE MOST PROBABLE SCENARIO APPEARS TO BE FOR ISOLATED, POTENTIALLY
SIGNIFICANT SEVERE STORMS FROM MIDDLE TN INTO NORTHERN AL AND GA,
DEPENDENT ON HOW MUCH DESTABILIZATION OCCURS ESPECIALLY IN TN.
MODELS ALSO INDICATE SUBSTANTIAL STORM COVERAGE ACROSS MUCH OF
SOUTHERN GA INTO NORTHERN FL, POSSIBLY IN THE FORM OF AN MCS, WITH
MAINLY WIND DAMAGE POTENTIAL GIVEN UNIDIRECTIONAL FLOW.

GIVEN THE POTENTIAL FOR SIGNIFICANT SEVERE STORMS, A CATEGORICAL
UPGRADE IS POSSIBLE IN LATER OUTLOOKS ONCE PREDICTABILITY INCREASES
AND THE CENTROID OF SEVERE COVERAGE IS BETTER ESTABLISHED.

..JEWELL.. 03/17/2018
Click to expand...
 
<chirp>
 
MattW said:
Well, while I try not to read the NAM verbatim until within 60 hours like I indicated earlier, the 18z is still a bit worse. Sounding from near Franklin, GA at 20:00 EDT. The timing has also been relatively consistent for GA with the peak threat hovering between 5p and 8p EDT. What's interesting is the NAM isn't showing anything on simulated reflectivity, but is that a case of the discrete convection being smaller than the NAM's spatial resolution can show?
Click to expand...

It's because the regular NAM isn't a convection allowing model because of resolution; so, it has to use a parameterization scheme to simulate the effects of convection. The interesting thing is... the specific convective parameterization scheme used in the NAM actually tells it basically "Do not develop convection if there is mid-level dry air" (the classic Elevated Mixed Layer you'd want on a significant tornado day). There have actually been widespread supercell events where, because of this, the NAM wouldn't develop anything in the warm sector. Yet, the 18Z and 00Z NAM runs still developed a string of discrete storms from middle TN down into northern AL.... that eventually unzipped way further southward with time along the front. It also, even though not able to resolve convection, still develops and keeps them fully discrete. That's really telling...

While mentioning the NAM, the SPC sided with the Euro/GFS (but I get the idea that may have just been a badly executed way to portray uncertainty, since they went out of their way to outline now the NAM favors strong tornadoes and a very volatile setup, even though they leaned against it)... but the Euro ensembles are stronger with the wave, and a good portion of the GFS ensembles are stronger and slower with the wave, the Canadian is stronger and slower, and the SREF trends more and more aggressive each run... and gives heavy support to the NAM solution. It actually has a 50% chance of the STP exceeding 3 over AL, and up to a 40% chance of the STP exceeding 5. What's interesting... the SREF and the NAM led the way with the outbreak a few weeks ago over the Mid South, with the GFS and Euro catching up last minute..... and the NAM has been a surprisingly top performer all winter, all the way back to our December winter storm. The SPC discussion would make you think the NAM is all alone out on a limb, but by leaps, bounds, and back handsprings... it very much is not...
 
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