I see no reason to stay ENH, this should be a MDT.
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Archive March 18-20th, 2018 Severe Weather
- Thread starter Taylor Campbell
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- severe weather
Equus
Member
I assume they're going to upgrade at 13z... no indication in the text that they will, but the discussion clearly indicates a higher end potential. If they don't bump probs up, I'll be really surprised.
rolltide_130
Member
Rather underwhelming discussion on that outlook..
- Thread starter
- Moderator
- #244
Yeah, that was sad.
That answered my question. Thanks
rolltide_130
Member
If these parameters were in place across the plains they'd have a high end MDT out right now..
barcncpt44
Member
New outlook is out, no upgrade to moderate. Enhanced expanded a little bit to include NE Mississippi and NW Georgia.
Equus
Member
To be fair, SPC's outlooks over the last year and a half (for southeastern events especially) have been uh... towards the miss side of hit and miss.
CheeselandSkies
Member
Really floored by lack of MDT upgrade. I thought they were right to play it conservative on the Day 4 and 3 outlooks with the uncertainties about the shear profile but those have been largely mitigated.
Area Forecast Discussion
National Weather Service Huntsville AL
301 AM CDT Mon Mar 19 2018
.NEAR TERM...(Today)
Issued at 300 AM CDT Mon Mar 19 2018
Confidence is increasing on a severe weather outbreak this afternoon
with the greatest threat being very large hail with tornadoes and
damaging winds also possible.
The area should be well into the warm sector by this afternoon.
Soundings do show a modest cap through about 18z so there will be a
lingering stratus layer along with the showers. However, guidance is
in very good agreement that the cap will break after 18-19z,
starting in the west, as temps warm into the lower 70s and a mid
level dry layer moves in from the west. Once this cap breaks, a very
volatile environment will be in place for severe storms. If the
clouds break out sooner and more than expected, temps could get into
the mid to upper 70s. Guidance, including the hires, is also in
surprisingly good agreement in regards to the timing of the line of
storms. The main line of severe storms looks to initiate in NE MS/NW
AL 19-21z (2-4PM), strengthening as it gets near the I-65 border 22-
01Z (4PM-7PM) and NE AL by 01z-03z (7PM-9PM). There are still some
slight discrepancies in this and it could be off +/- an hour or two
with the NAM continuing to be the latest choice which is about an
hour later than the previously mentioned window. The latest HRRR
runs are also leaning more towards a NAM/later arrival time solution.
Around the time of storm initiation this afternoon, a low level jet,
the left exit region to be exact, will be pushing into NW AL and
moving east. This will help erode the cap and enhance the shear and
be a primary focus of storm initiation. Although the upper support
is back to the north and west, mid level lift, with a surface cold
front and this LLJ should be plenty for rapid intensification of
supercells this afternoon.
Now, lets talk about threats. As expected, all modes of severe
weather are possible this afternoon, including tornadoes. As the day
heats up, MUCAPE values rise to 500-1500 J/KG with the NAM, as
usual, much higher than any other model and predicting highs near
80. The most significant part of the instability is a deep -10C to
-30C layer of CAPE, coupled with 60+kts of bulk shear, WBZ heights
near 8500ft; all which indicate a very good probability of large to
very large hail. Golfball to tennis ball size hail cannot be ruled
out. This will be especially true if we get near the 2000 J/kG mark.
The LLJ, as mentioned will increase the 0-1km shear to 25-30kts by
00z, west to east. 0-1KM SRH could be as high as 300-400 m2/s2 (with
the NAM being the highest once again). Low and mid level lapse rates
will be near or above 7C/KM across the area. Given the strength of
the shear and the mid level dry slot, damaging wind in severe storms
will also be likely.
The tornado potential will be maximized approximately around I-65
and east due to the timing of certain parameters coming together. By
the time the low level jet moves in and breaks the cap, this eastern
area will have more time to destabilize, the jet location and
strength will be more favorable and all guidance is showing high
instability and extremely classic tornadic hodographs. Although a
tornado cannot be ruled out areawide, including NW AL, hires guidance
is very consistent showing reflectivity/updraft helicity increasing
rapidly as it nears the I-65 corridor. LCL heights will also be
below 500m in most locations, another favorable value for tornadoes.
Although the threat for "significant" tornadoes is not too high, if
we maximize the parameters, especially if the NAM is realized, a few
strong tornadoes would not be out of the question.
Obviously with any event, there are uncertainties. The biggest today
will be the destabilization of the warm sector. The warm front has
been somewhat stuck overnight and if this lingers to the south, we
may not warm or destabilize enough. This could lead to the cap not
breaking or breaking much later than anticipated. The only change to
the forecast if the cap didn`t break would be a lessening of the
severe threat in the west. As already mentioned above, the models are
differing in the strength of the environmental parameters which
would also impact the strength of the hazards. Stay tuned as the
forecast will quickly change today. Although dew points will rise
with the warm front, right now they are only forecast to get in the
lower 60s. This is still high enough for severe storms but usually,
we like to see them in the upper 60s for our significant tornado
outbreaks.
National Weather Service Huntsville AL
301 AM CDT Mon Mar 19 2018
.NEAR TERM...(Today)
Issued at 300 AM CDT Mon Mar 19 2018
Confidence is increasing on a severe weather outbreak this afternoon
with the greatest threat being very large hail with tornadoes and
damaging winds also possible.
The area should be well into the warm sector by this afternoon.
Soundings do show a modest cap through about 18z so there will be a
lingering stratus layer along with the showers. However, guidance is
in very good agreement that the cap will break after 18-19z,
starting in the west, as temps warm into the lower 70s and a mid
level dry layer moves in from the west. Once this cap breaks, a very
volatile environment will be in place for severe storms. If the
clouds break out sooner and more than expected, temps could get into
the mid to upper 70s. Guidance, including the hires, is also in
surprisingly good agreement in regards to the timing of the line of
storms. The main line of severe storms looks to initiate in NE MS/NW
AL 19-21z (2-4PM), strengthening as it gets near the I-65 border 22-
01Z (4PM-7PM) and NE AL by 01z-03z (7PM-9PM). There are still some
slight discrepancies in this and it could be off +/- an hour or two
with the NAM continuing to be the latest choice which is about an
hour later than the previously mentioned window. The latest HRRR
runs are also leaning more towards a NAM/later arrival time solution.
Around the time of storm initiation this afternoon, a low level jet,
the left exit region to be exact, will be pushing into NW AL and
moving east. This will help erode the cap and enhance the shear and
be a primary focus of storm initiation. Although the upper support
is back to the north and west, mid level lift, with a surface cold
front and this LLJ should be plenty for rapid intensification of
supercells this afternoon.
Now, lets talk about threats. As expected, all modes of severe
weather are possible this afternoon, including tornadoes. As the day
heats up, MUCAPE values rise to 500-1500 J/KG with the NAM, as
usual, much higher than any other model and predicting highs near
80. The most significant part of the instability is a deep -10C to
-30C layer of CAPE, coupled with 60+kts of bulk shear, WBZ heights
near 8500ft; all which indicate a very good probability of large to
very large hail. Golfball to tennis ball size hail cannot be ruled
out. This will be especially true if we get near the 2000 J/kG mark.
The LLJ, as mentioned will increase the 0-1km shear to 25-30kts by
00z, west to east. 0-1KM SRH could be as high as 300-400 m2/s2 (with
the NAM being the highest once again). Low and mid level lapse rates
will be near or above 7C/KM across the area. Given the strength of
the shear and the mid level dry slot, damaging wind in severe storms
will also be likely.
The tornado potential will be maximized approximately around I-65
and east due to the timing of certain parameters coming together. By
the time the low level jet moves in and breaks the cap, this eastern
area will have more time to destabilize, the jet location and
strength will be more favorable and all guidance is showing high
instability and extremely classic tornadic hodographs. Although a
tornado cannot be ruled out areawide, including NW AL, hires guidance
is very consistent showing reflectivity/updraft helicity increasing
rapidly as it nears the I-65 corridor. LCL heights will also be
below 500m in most locations, another favorable value for tornadoes.
Although the threat for "significant" tornadoes is not too high, if
we maximize the parameters, especially if the NAM is realized, a few
strong tornadoes would not be out of the question.
Obviously with any event, there are uncertainties. The biggest today
will be the destabilization of the warm sector. The warm front has
been somewhat stuck overnight and if this lingers to the south, we
may not warm or destabilize enough. This could lead to the cap not
breaking or breaking much later than anticipated. The only change to
the forecast if the cap didn`t break would be a lessening of the
severe threat in the west. As already mentioned above, the models are
differing in the strength of the environmental parameters which
would also impact the strength of the hazards. Stay tuned as the
forecast will quickly change today. Although dew points will rise
with the warm front, right now they are only forecast to get in the
lower 60s. This is still high enough for severe storms but usually,
we like to see them in the upper 60s for our significant tornado
outbreaks.
Area Forecast Discussion
National Weather Service Birmingham AL
402 AM CDT Mon Mar 19 2018
...SEVERE STORMS WITH TORNADOES AND VERY LARGE HAIL LIKELY THIS
AFTERNOON AND EVENING...
.SHORT TERM...
Today and Tonight.
Confidence is increasing in the severe weather potential for this
afternoon and evening. The threat for tornadoes has also increased,
and a strong tornado or two will be possible across northern
portions of Central Alabama as well. It now appears that there may
be two waves of severe weather, with a first wave during the
early/mid afternoon hours which is more isolated/conditional,
followed by the main wave during the late afternoon/evening hours.
The severe weather starting time was bumped earlier to 2 PM to
account for the first wave, while the second wave has trended a bit
slower resulting in the ending time being pushed back to 2 AM,
mainly for southern/eastern areas. More on these waves below.
The warm front
should lift northward this morning with broken sunshine developing.
This will result in heating, and combined with moist low-levels and
steep mid-level lapse rates associated with an EML, CAPE values of
2000-2500 J/kg will develop, with 60 kts of 0-6 km deep layer shear.
This high CAPE/high shear parameter space is supportive of a
significant severe threat. Models indicate a weak wave at 700mb
lifting across the area over the open warm sector during the
afternoon hours, ahead of the main upper-level forcing associated
with the upper low and the cold front/dry line at the surface.
Almost all models show some light QPF developing with this feature,
but differ regarding whether there will be any deep convection or
just scattered showers. The 3km NAM and a couple runs of the HRRR
suggest isolated convection/supercells are possible. This will be at
peak heating and the cap is not particularly strong, so it at least
seems plausible. All modes of severe weather would be possible with
any isolated discrete supercells that do form. Also of note is a
well defined fine line on radar that the storms in northeast TX are
forming on, which may be indicative of an outflow boundary/bore,
which could cause early initiation as it moves eastward. This first
wave complicates the timing graphic, resulting in a much broader
threat timeframe, with the first wave starting as early as 2 PM.
It`s also unclear what effect this first wave will have on the
second wave, but current thinking is that it won`t have much of an
effect.
The second main/most widespread wave of storms will develop over
northern MS/northwest AL as height falls and the mid-level speed max
associated with the upper low and low amplitude trough interact with
the dry line at the surface. Shear profiles favor a fast-moving
broken line of supercells that will race eastward across the area
during the late afternoon and evening. Model consensus excluding the
GFS now indicates that winds ahead of this line will remain backed
in a southerly direction, which is also indicated in the HREF mean
winds. This results in greater 0-1km SRH and more favorable critical
angles. The line of supercells looks to cross the area during the
favorable early evening transition period of increased low-level
shear. Therefore, it appears that the threat for tornadoes including
the potential for a strong tornado or two across the northern half
of Central Alabama has increased. This will ultimately depend on
storm scale details and what effects the first wave has. The threat
of large hail, including significant hail >2" diameter continues
given the fat CAPE profiles, and many SARS significant hail analogs.
Isolated hail up to baseball size may be possible. The threat will
end by midnight for many, except for southeast and far south-central
portions of Central Alabama where it will continue through around
2AM. The enhanced risk area in our local impact graphics remains on
track, and lines up well with the >40% neighborhood probability of
updraft helicity > 75 m2/s2 in the HREF CAM ensemble. Not everyone
will see severe storms today given the scattered nature of the
storms, but those that do will see some intense storms. Everyone
should remain weather aware.
National Weather Service Birmingham AL
402 AM CDT Mon Mar 19 2018
...SEVERE STORMS WITH TORNADOES AND VERY LARGE HAIL LIKELY THIS
AFTERNOON AND EVENING...
.SHORT TERM...
Today and Tonight.
Confidence is increasing in the severe weather potential for this
afternoon and evening. The threat for tornadoes has also increased,
and a strong tornado or two will be possible across northern
portions of Central Alabama as well. It now appears that there may
be two waves of severe weather, with a first wave during the
early/mid afternoon hours which is more isolated/conditional,
followed by the main wave during the late afternoon/evening hours.
The severe weather starting time was bumped earlier to 2 PM to
account for the first wave, while the second wave has trended a bit
slower resulting in the ending time being pushed back to 2 AM,
mainly for southern/eastern areas. More on these waves below.
The warm front
should lift northward this morning with broken sunshine developing.
This will result in heating, and combined with moist low-levels and
steep mid-level lapse rates associated with an EML, CAPE values of
2000-2500 J/kg will develop, with 60 kts of 0-6 km deep layer shear.
This high CAPE/high shear parameter space is supportive of a
significant severe threat. Models indicate a weak wave at 700mb
lifting across the area over the open warm sector during the
afternoon hours, ahead of the main upper-level forcing associated
with the upper low and the cold front/dry line at the surface.
Almost all models show some light QPF developing with this feature,
but differ regarding whether there will be any deep convection or
just scattered showers. The 3km NAM and a couple runs of the HRRR
suggest isolated convection/supercells are possible. This will be at
peak heating and the cap is not particularly strong, so it at least
seems plausible. All modes of severe weather would be possible with
any isolated discrete supercells that do form. Also of note is a
well defined fine line on radar that the storms in northeast TX are
forming on, which may be indicative of an outflow boundary/bore,
which could cause early initiation as it moves eastward. This first
wave complicates the timing graphic, resulting in a much broader
threat timeframe, with the first wave starting as early as 2 PM.
It`s also unclear what effect this first wave will have on the
second wave, but current thinking is that it won`t have much of an
effect.
The second main/most widespread wave of storms will develop over
northern MS/northwest AL as height falls and the mid-level speed max
associated with the upper low and low amplitude trough interact with
the dry line at the surface. Shear profiles favor a fast-moving
broken line of supercells that will race eastward across the area
during the late afternoon and evening. Model consensus excluding the
GFS now indicates that winds ahead of this line will remain backed
in a southerly direction, which is also indicated in the HREF mean
winds. This results in greater 0-1km SRH and more favorable critical
angles. The line of supercells looks to cross the area during the
favorable early evening transition period of increased low-level
shear. Therefore, it appears that the threat for tornadoes including
the potential for a strong tornado or two across the northern half
of Central Alabama has increased. This will ultimately depend on
storm scale details and what effects the first wave has. The threat
of large hail, including significant hail >2" diameter continues
given the fat CAPE profiles, and many SARS significant hail analogs.
Isolated hail up to baseball size may be possible. The threat will
end by midnight for many, except for southeast and far south-central
portions of Central Alabama where it will continue through around
2AM. The enhanced risk area in our local impact graphics remains on
track, and lines up well with the >40% neighborhood probability of
updraft helicity > 75 m2/s2 in the HREF CAM ensemble. Not everyone
will see severe storms today given the scattered nature of the
storms, but those that do will see some intense storms. Everyone
should remain weather aware.
warneagle
Member
I...wasn't expecting to wake up to an orange banner. I have to think an upgrade is coming with the next outlook...
TileDude
Member
When should the next SPC update be ?
8AM.
TileDude
Member
Thanks
David in SW Blount
Member
I noticed NWS Birmingham pushed the onset time up a little bit from 3PM to 2PM. I know it's not a big change, but for those of us with long work commutes in the middle of the bullseye (and schools dismissing early) it makes a difference for when we choose to escape and head home.
David in SW Blount
Member
Ooops, well Birmingham and Huntsville are not coordinated with their onset times, or maybe HSV is just being more specific.
tennessee storm chaser
Member
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Yeah. Front hauling tail now trough central Arkansas ... think timing gets moved up bit....
Huntsville timing might be right, but a bit risky if cells develop ahead of front.
that short line of storms in NW AL could locally enhance low level inflow this afternoon...will need to watch, especially around I59 this afternoon
