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NorthGeorgiaWX

April 12-13 Severe Weather Event

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I am going to use this dedicated thread for information and discussion about the upcoming potential severe weather event. It's better to keep it all together. As new information comes in, you'll find it here. 

I'll start it off with this mornings SPC images and I'll add  more as the day goes on. If you are signed up on the forum, you can subscribe to this post and get emailed when new comments are made. 

SPC_CO_Day2State.thumb.png.14af8784df8ba744089577a4c00c9266.png

SPC_WP2_Day2State.thumb.png.d78af8c9b49e336dd79d6a7c9ad82484.png

SPC_TP2_Day2State.thumb.png.ee21304b4eb23043bd0e8da995842864.png

SPC_HP2_Day2State.thumb.png.b51bc0ce82ac17e96e3195958a35a06b.png

 

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I like to read the upstream forecast office discussions to get a feel for their thoughts. Since the weather they are going to see will be moving our way, their insights provide more information that helps us to have a better idea what our weather will be like. 

This is NWS Huntsville

Quote

This seems to be in agreement with the thoughts by the Storm Prediction Center with their updated Day 2 Convective Outlook. This does NOT mean we won't see strong storms all the way up into southern Middle Tennessee. It just means there may be more instability to work with south of the Tennessee River. Low-level shear, on the other hand, is tremendous across our entire forecast area. This is what organizes storms and makes any storms that develop stronger. It's the combination/balance of instability, shear, and a strong lifting mechanism that makes storms severe. What may help with the severe potential despite the lesser amount of instability across far northern Alabama and southern Middle Tennessee would be enhanced vertical motions due to upper-level jet coupling occurring aloft.

In general, our thoughts right now based on high-res models would suggest the first round develops along the lifting warm-front and will likely be more of a line feature. With ample shear in place, there is the potential for severe storms embedded in this line, but it probably won't be the biggest threat for the day. That will probably lay with the second round that moves through during the late afternoon/evening hours after we have had a chance to advect in higher dew points as well as maybe some sky clearing which will help to destabilize the region. This line looks to develop as discrete supercells over eastern Mississippi and may merge more into a line across northern Alabama. This will probably be when we will have our best chance for tornadoes, large hail, and damaging winds.

Overall, there is still a lot of uncertainty in the details of the forecast for Sunday. We will have to watch where the MCS develops in the morning, how far north the warm front lifts with clearing for better destabilization, and then how the modes of convection hash out. As of now, in our forecast all modes of severe weather are in play. Make sure you have plans in place NOW for what you need to do in the event of a Severe Thunderstorm or Tornado Warning. Just a reminder, straight-line winds can do nearly as much damage as a tornado. Be weather aware throughout the day on Sunday.

 

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SENIOR DUTY METEOROLOGIST NWS ADMINISTRATIVE MESSAGE
NWS NCEP CENTRAL OPERATIONS COLLEGE PARK MD
1112Z SAT APR 11 2020

A Critical Weather Day (CWD) will be declared at 1200Z ( 8 AM) Sat Apr 11 2020 and extend through 0000Z (8 PM) Tue Apr 14 2020.
The following NWS regions are impacted by this CWD...Central/Southern (SR will participate from 11/18Z to 13/12Z)
.

NCEP/NWSTG and the NCF are also participating in this CWD to ensure a reliable flow of weather data. All scheduled software/hardware/network changes for the impacted offices will be postponed until the CWD has ended. Any emergency changes will be evaluated and approved by regional management officials. Users outside the CWD area should limit their NWSChat instances and usage as much as possible. NWS offices should contact their regional offices for further questions about this CWD.

REASON for CWD... Impacts from a significant storm system which include winter weather in the northern Rockies/northern High Plains/western Great Lakes and severe weather in the Southeast.

 

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Is it still showing signs of being a dangerous storm system for the east side of 400

 

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This is for the folks in Chattanooga and into east Tennessee. I know we have people that follow from those areas. Here's the discussion from the NWS in Morristown.

Quote

Key messages continue unchanged from the previous forecast thinking:

  1. A severe weather outbreak with the potential for tornadoes, damaging winds, large hail, and flooding looks increasingly possible on Easter Sunday into Sunday night. Area of greatest concern for severe weather and flooding will be south and west of Knoxville.
  2. High winds in the mountains and adjacent foothills of East Tennessee Sunday.
  3. Generally quiet weather next week with temps much below normal.

Sunday and Sunday night...

Rain chances will be ramping up Sunday morning as strong moisture
advection and frontogenetic forcing spread from SW to NE ahead of
the negatively-tilted shotrwave trough over the Plains. This initial
round of convection Sunday morning will be elevated as the surface
warm front remains well to our south. Moderate rainfall rates from
this round will prime the soil for the second round of heavier
rainfall in the late afternoon/evening. Through the day, the
southerly LLJ will be strengthening to around 60 kts in the evening,
and with an inversion located around mountain top height, we will
have a favorable setup for a mountain wave event that will produce
gusts of 70+ mph in the mountains and adjacent foothills. The High
Wind Watch will continue unchanged.

Supercells are expected to form across MS/AL Sunday afternoon, with
the steering flow taking them NE toward our area. Strong upper
divergence due to jet exit/entrance region coupling will allow these
storms to maintain their intensity into the evening. In addition,
the surface warm front will be lifting north, allowing for
convection to become surface-based. One uncertainty with this event
continues to be how far north that front will lift, which will
impact the northward extent of the tornado threat for our area.
The
SPC Day 2 Outlook has expanded the Enhanced risk area northward,
which reflects the NAM's trend of a more northward warm front. Near
and south of this front, which stretches from the northern Plateau
to SW NC, the NAM shows a favorable environment for supercells,
which overlaps with the strongest forcing aloft in the 03-06Z time
frame. Based on the 0-1 km SRH, LCL heights, 0-3 km CAPE, and STP
values from the NAM across the Plateau and southern Valley, and the
favorable jet coupling aloft, this event has the potential to pose
the most significant tornado threat we have had since April 27,
2011
.
While the tornado threat is expected to be lower to the north
of the warm front, there will still be a threat of damaging straight-
line winds as a QLCS is expected to cross the area overnight along a
pre-frontal trough after 06Z. Heavy rain Sunday night may also
create flooding problems and bring river levels close to flood
stage. Rain amounts for the Sunday-Sunday night period are expected
to be 1.5-3.5 inch range for most spots. A Flood Watch may be needed
as we get closer to the event.

 

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4 minutes ago, Lauri said:

Is it still showing signs of being a dangerous storm system for the east side of 400

 

Yes it is.

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I have a couple of loops to show you that have to do with moisture. Without the moisture, there probably wouldn't be any severe weather. But moisture is fuel for the fire and we're not going to have a shortage of that. 

The is first loop are the expected (by the Euro) dewpoints. Watch the surge in dewpoints out in front giving storms plenty of energy to work with. But that surge of drier air behind the front is equally as impressive. The clash of strong air masses make for a very unstable atmosphere. Next loop in next comment.

ecmwf-deterministic-conus-dew2m_f-1586584800-1586584800-1586908800-40.gif

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This loop shows precipitable water values. For those that don't know what that means:

Quote

Precipitable water is the depth of water in a column of the atmosphere, if all the water in that column were precipitated as rain.

This measurement gives us an idea how much water is in the air. And if all that water fell, this is how much you can get. Except what I'm showing here are anomalies or how much relative to normal for this time of the year. Some of these values are running 5 standard deviations above normal, which means some of the storms can be efficient and prolific rain producers and quickly cause flash flooding.

ecmwf-deterministic-conus-pwat_norm_anom-1586584800-1586584800-1586822400-40.thumb.gif.986af8b807f2b9fe39cd7aa0433f433d.gif

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Will it be worse than the storms we had the other night/morning and a longer duration? It got one of our big trees and knocked out power out for 5 hours. 

2D852C6E-B226-4640-9805-F9CD2F363EA5.jpeg

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8 minutes ago, Tori said:

Will it be worse than the storms we had the other night/morning and a longer duration? It got one of our big trees and knocked out power out for 5 hours. 

2D852C6E-B226-4640-9805-F9CD2F363EA5.jpeg

Yes, many will be worse.

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1 minute ago, NorthGeorgiaWX said:

 

Do you agree with this, or think he is hyping it more than it should be? Just curious

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There aren't too many meteorologist whose opinion I value more than Ryan. BTW, Ryan has a PhD in meteorology. Ryan designed all the original Weatherbell maps and he operates Weathermodels.com. He is also a tropical weather expert. Not too many any smarter than he is. Oh... and he lives here in Atlanta. 🙂

14 minutes ago, Asperman1 said:

Do you agree with this, or think he is hyping it more than it should be? Just curious

 

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1 minute ago, NorthGeorgiaWX said:

There aren't too many meteorologist whose opinion I value more than Ryan. BTW, Ryan has a PhD in meteorology. Ryan designed all the original Weatherbell maps and he operates Weathermodels.com. He is also a tropical weather expert. Not too many any smarter than he is. Oh... and he lives here in Atlanta. 🙂

 

Okay, thank you.

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Yes, I fully agree! Ryan is great - I follow him on Twitter and he is excellent - no hype and so informative!

 

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Just now, K8T8 said:

Yes, I fully agree! Ryan is great - I follow him on Twitter and he is excellent - no hype and so informative!

 

Thank you 🙂

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1 minute ago, NorthGeorgiaWX said:

We will have another Day 2 Severe Weather Outlook shortly after 1:30 PM

Praying for good news

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3 minutes ago, NorthGeorgiaWX said:

We will have another Day 2 Severe Weather Outlook shortly after 1:30 PM

I don’t see much changing honestly 

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2 minutes ago, Asperman1 said:

Praying for good news

I don’t see much Changing honestly..

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Oh no...

Snap346064694.thumb.png.5aeeb9ba9bd04f2edff392cb9c62e5bd.png

 

Quote

Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1259 AM CDT Sat Apr 11 2020

   Valid 121200Z - 131200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR CENTRAL/NORTHERN LOUISIANA...SOUTHEAST ARKANSAS...MUCH OF MISSISSIPPI...WESTERN/CENTRAL ALABAMA......

   ...SUMMARY...
   An outbreak of severe thunderstorms appears likely Sunday into Sunday night, with the greatest threat expected from Louisiana
   through much of the Southeast and Tennessee Valley. Strong tornadoes, potentially widespread damaging winds, and large hail are all possible.

   ...Synopsis...
   The ejecting shortwave trough initially over the southern High
   Plains on Sunday morning is forecast to move quickly eastward to the
   lower MS Valley by early evening, and then accelerate northeastward
   toward the Ohio Valley late Sunday night into Monday morning, as it
   becomes absorbed within an amplifying longwave trough that will
   encompass nearly all the CONUS by 12Z Monday. In conjunction with
   the ejecting trough, a broad surface low centered over the
   central/southern Plains on Sunday morning will move eastward to the
   mid-MS Valley by 00Z Monday, and then move northeastward and rapidly
   intensify into an intense cyclone over the lower Great Lakes by 12Z
   Monday. A warm front will surge northward ahead of the low across
   the lower MS Valley and Southeast, while a strong cold front will
   move southward through much of the Plains in the wake of the
   departing cyclone. 

   ...East TX northeastward through much of the Southeast and TN
   Valley...

   One or more clusters of deep convection will likely be ongoing at
   12Z Sunday morning somewhere over east TX and potentially into
   portions of the lower MS Valley. The intensity and areal extent of
   any such clusters remain uncertain, but ample shear and instability
   will favor a threat of hail and damaging wind with any organized
   convection at the start of the period. Some tornado threat will also
   be present Sunday morning with any semi-discrete storms that begin
   to interact with the richer low-level moisture in the vicinity of
   the warm front. As this convection spreads northeastward,
   intensification is possible into portions of the ArkLAMiss region,
   with an increasing tornado threat in late morning/early afternoon
   with any surface-based storms, given rapidly increasing low-level
   moisture and shear. North of the warm front, evolution into a QLCS
   will be possible, with a corresponding risk of damaging wind into
   portions of the TN Valley. 

   Meanwhile, further south, moderate to locally strong instability is
   forecast to develop along/south of the warm frontal position, which
   will be modulated by the impact of outflow from any early convection
   described above. Midlevel flow will increase to 70-100 kt as a
   south-southwesterly low-level jet intensifies into the 40-60 kt
   range. These wind profiles combined with ample instability (MLCAPE
   of 1500-3000 J/kg) will support the potential for intense
   supercells. Any surface-based initiation along and east of a
   pseudo-dryline moving into western LA by late afternoon could evolve
   into one or more long-tracked supercells capable of producing strong
   tornadoes, large hail, and damaging wind gusts. The extent of
   development within the warm sector remains somewhat uncertain, given
   the presence of a capping inversion and generally subtle foci for
   initiation. 

   While the conditional risk of all severe hazards will be quite high
   if supercells develop, uncertainty remains regarding how convection
   will evolve from the morning into the afternoon. Any remnant outflow
   related to early convection will determine the northern extent of
   the higher-end tornado potential, and some guidance suggests the
   potential for elevated convection within a midlevel moist plume
   across the warm sector during the afternoon, which could either
   dampen the severe potential, or evolve into surface-based convection
   with a substantial severe threat. Given these factors, there is too
   much uncertainty to upgrade the ongoing outlook at this time. 

   Evolution into more of QLCS is suggested by most guidance by Sunday
   evening, which would pose an increasing threat of widespread
   damaging winds and a few tornadoes across much of AL into
   western/central GA through the overnight hours.
Higher wind
   probabilities may be needed in subsequent outlooks if confidence in
   this scenario grows. 

     ...TN Valley into the OH Valley -- Sunday night...
   Substantial uncertainty remains regarding the potential for
   destabilization from northern portions of the TN Valley into the OH
   Valley, due to the potential for widespread convection to the south
   of this area. However, rapidly strengthening wind fields in advance
   of the deepening cyclone will support the potential for damaging
   wind and perhaps a tornado risk by Sunday evening should even modest
   destabilization occur, as strong convection attempts to move in from
   the southwest in tandem with the deepening cyclone. 

   ... Eastern Georgia into the Carolinas/Mid Atlantic...
   Substantial low-level moistening is expected over eastern GA into
   the Carolinas/Mid Atlantic through the period. There is a nonzero
   risk of organized convection along/north of the warm front during
   the day into the evening, which would pose some risk of locally
   damaging wind or perhaps a tornado, but confidence in this scenario
   is low at this time. A more likely scenario is for widespread
   upstream convection to evolve into multiple clusters or a QLCS and
   move into this region sometime early Monday morning. Intense wind
   profiles will support a risk of widespread damaging wind and a few
   tornadoes, given sufficient instability. 

   The magnitude and coverage of the severe threat in this region will
   be determined in part by how fast organized convection approaches
   from the west. If convection accelerates and arrives faster that
   current guidance would indicate, then there is less time for
   low-level moistening and destabilization, and the magnitude and
   northward-extent of the threat may be limited. If convection does
   not arrive until very late in the period, then a more substantial
   severe threat could evolve. If some of the slower guidance turns out
   to be accurate, then the primary severe threat in this region may
   not come until the D3/Monday period. Probabilities may need to be
   increased in this area once the details come into better focus.

 

 

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