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Tn Valley Severe Weather 2017


jaxjagman

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Decent winter time building ridge setting up with the Euro with a ULL to the NW.Euro shows a warm front lifting from the SE states.DP'S would rise with this in the Valley,this should be a decent severe threat with  bulk shear coming from the SW the first of the month.Dashboard shows a multi day event during this time frame.Might be over doing the capes in Texas on the 28th,typical Euro in the long range with convection,but it shows capes into 3k plus

 

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Friday looks good in the mountainous jungle of northern Kentucky. Best dynamics are Hoosier Alley but the best moisture hangs up in the South. Strong winds gradually veering with height are a classic hodograph over Hoosier Alley, but no quality moisture up there. The only juxtaposition I can see, and it's shaky, is over the Ohio River Valley jungle. Upper winds are stout over Tennessee, but the LLJ may be AWOL and/or veered off. Plus no west-east boundary. No chase is scheduled. On the positive side (see post 29 previous page) North Alabama gets another pass.

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On 2/21/2017 at 1:46 PM, nrgjeff said:

Friday looks good in the mountainous jungle of northern Kentucky. Best dynamics are Hoosier Alley but the best moisture hangs up in the South. Strong winds gradually veering with height are a classic hodograph over Hoosier Alley, but no quality moisture up there. The only juxtaposition I can see, and it's shaky, is over the Ohio River Valley jungle. Upper winds are stout over Tennessee, but the LLJ may be AWOL and/or veered off. Plus no west-east boundary. No chase is scheduled. On the positive side (see post 29 previous page) North Alabama gets another pass.

:D

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Euro this afternoon has a better LLJ.Best showalter is around the Mid Valley,-3.GFS is even better -4,maybe we can finally get a good light show,haven't seen one since the start of last winter.

 

For Thursday night into Friday, warm south winds will increase as
a low pressure system moves from the Central Plains to the Great
Lakes. The pressure gradient will strengthen through Friday with
sustained winds from 15-25 mph, and gusts near 40 mph. Record
warmth is expected Friday. BNA`s current record for Feb 24th is 77
from 1890.

As the low pressure system moves to the Great Lakes Friday
evening, a trailing cold front will push eastward into the Oh/Tn
Valley Regions. SPC has slight and enhanced risk areas to our
north, with marginal risk in Tn. Cape values of 1-2k J/kg with
deep layer shear >50kt with provide favorable conditions for
severe storms by Friday evening. It would not be surprising if the
slight risk were expanded southward into our area. Forecast
soundings show a capped atmosphere through the day Friday, so not
much activity is expected ahead of the cold front. We will have
high pops in the forecast for Friday evening, with thunderstorms
focusing along a squall line. Although an isolated tornado cannot
be ruled out, our main concern will be damaging wind gusts along
the line, and perhaps some minor damage with gradient winds ahead
of the front. It appears the greatest severe risk will be over our
northwest counties beginning around 6pm, with a lower threat over
our southern and Plateau counties as the line moves into more
stable air late evening and overnight. Since a fairly fast moving,
narrow band is expected, rainfall amounts will be mostly under
one half inch.
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 NWS Storm Prediction Center Norman OK
   1258 AM CST Thu Feb 23 2017

   Valid 241200Z - 251200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR SOUTHERN
   LOWER MICHIGAN INTO MUCH OF INDIANA...NORTHERN KENTUCKY...AND THE
   WESTERN HALF OF OHIO...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR THE SOUTHERN
   GREAT LAKES AND OHIO VALLEY INTO PARTS OF TENNESSEE...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK...

   ...SUMMARY...
   Severe thunderstorms are forecast to affect portions of the southern
   Great Lakes, Ohio Valley, and Tennessee on Friday and Friday night. 
   Damaging winds, tornadoes, and large hail are possible.

   ...Synopsis...
   A mid-level trough/low is forecast to move from the middle MO Valley
   eastward to the central Great Lakes and OH Valley.  Concurrently, a
   belt of strong and cyclonically curved 500-mb flow is forecast to
   intensify to 100+ kt over the OH Valley late Friday night.  A
   surface low near the MO/IA border will develop northeastward to
   southern Lake Michigan by early evening before partially occluding
   over northern Michigan by early Saturday.  A northward-advancing
   warm front will move from north-central IL and Michiana through
   southern Lower Michigan by mid-late afternoon.  A cold front near
   the MS River around midday Friday will sweep eastward across the OH
   and TN Valleys by daybreak Saturday. 

   ...southern Great Lakes and Ohio Valley...
   The northeastward advection of an elevated mixed layer and
   associated steep to very steep 700-500 mb lapse rates (7-8.5 degrees
   per km) from the south-central states will overspread this region
   during the morning.  A capping inversion will likely limit
   showers/thunderstorms to be near the advancing warm frontal zone
   during the morning with mid-upper 50s dewpoints amidst strong
   southerly low-level flow.  By early to mid-afternoon, NAM model
   guidance indicates temperatures will warm well into the 60s across
   much of the warm sector compared to the much cooler GFS.  The
   increasing influence of DCVA/500-mb height falls coupled with
   heating will likely lead to thunderstorms developing into one or
   more broken bands.  Strong effective shear (45-60 kt) coupled with
   MLCAPE increasing into the 250-1250 J/kg range will support strong
   to severe thunderstorms.  Forecast soundings show moist boundary
   layers with effective SRH in the 150-200 m2/s2 range over much of
   the warm sector but locally higher (200-300 m2/s2) over the northern
   portion of the Enhanced Risk in the Michiana/southern Lower Michigan
   vicinity.  A mixed mode of both linear and cellular (some of which
   maturing into supercells) is expected during the early half of the
   convective life cycle.  The threats from the more intense storms
   include damaging winds, large hail, and the possibility for
   tornadoes.  Upscale growth will likely occur during the
   evening/overnight as storms move eastward across the OH Valley and
   the severe risk transitions to primarily damaging winds as
   southwesterly 700-mb flow strengthens to 60-kt.

   ...Mid South and southern Appalachians...
   Storm initiation will likely be delayed to the late afternoon/early
   evening as this area's thunderstorm chances await the arrival of
   stronger mid-level forcing with convective inhibition gradually
   eroding.  Isolated to widely scattered storms will likely develop
   southward along a prefrontal convergence band or along the
   southeastward-moving cold front before growing upscale during the
   overnight.  Cool 500-mb temperatures atop relatively moist low
   levels implies a surface-based thunderstorm risk after dark. 
   Damaging winds will likely be the primary hazard.  However,
   depending on how quickly upscale growth occurs, a tornado threat may
   develop as winds strengthen in the 925-700 mb layer and hodographs
   enlarge.  As storms move eastward into the southern Appalachians,
   diminishing instability will likely coincide with storms' weakening
   and a lessening severe risk.
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Slgt risk for you guys in Knoxville.

 

day1otlk_1200.gif?1487916299756

 

Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1149 PM CST Thu Feb 23 2017

   Valid 241200Z - 251200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   SOUTHERN LOWER MICHIGAN...CENTRAL AND EASTERN INDIANA...WESTERN AND
   CENTRAL OHIO...AND PARTS OF NORTHERN KENTUCKY...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENHANCED RISK AREA...AND EXTENDING SOUTHWARD INTO PARTS OF
   TENNESSEE...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK AREA...EXTENDING FROM THE UPPER GREAT LAKES TO THE
   SOUTHERN APPALACHIANS...

   ...SUMMARY...
   Strong to severe thunderstorms are forecast to affect portions of
   the southern Great Lakes, Ohio and Tennessee Valleys on Friday and
   Friday night.  The primary threat appears to be damaging wind, but
   some hail and a couple of tornadoes will also be possible.

   ...Synopsis...
   An upper short-wave trough -- embedded within broader/longer-wave
   troughing -- is forecast to shift out of the Plains and across the
   mid and lower Missouri valley through the day.  By evening, the
   trough is progged to continue eastward across the mid and upper
   Mississippi Valleys, and eventually into the Midwest/Great Lakes
   region as it acquires neutral to slightly negative tilt with time.

   As the upper system advances/sharpens, a surface low in the vicinity
   of western Illinois early in the day is forecast to move across
   Illinois and northwestern Indiana through the afternoon, and then
   across Michigan toward Lake Huron through the latter stages of the
   period.  Ahead of the low, a warm front is forecast to advance
   northward across the lower Great Lakes into southern Canada, while a
   cold front sweeps eastward across the Ohio/Tennessee/Mississippi
   valleys and reaches the Appalachian crest by the end of the period.

   ...Great Lakes region southward to the southern Appalachians...
   Showers and scattered thunderstorms are forecast to be ongoing from
   parts of northeast Illinois/southeast Wisconsin eastward into the
   northeast U.S. at the start of the period, north of a warm front
   extending eastward across the Midwest.  Modest low-level moisture
   into the warm sector /50s surface dewpoints/ beneath an elevated
   mixed layer advecting northeast across the region should keep the
   warm sector capped -- likely until late afternoon.  Eventually
   however, as the surface low moves northeast toward southern lower
   Michigan and the trailing cold front advances into Indiana, limited
   diurnal heating combined with persistent ascent is expected to
   steadily/gradually weaken the cap, eventually allowing development
   of isolated to scattered thunderstorms.  Initial storm mode may be
   cellular, but upscale growth into a broken line of storms is
   expected, which should expand southward along the advancing front
   through the evening hours.

   Though degree of shear will be sufficient for supercells, somewhat
   veered low-level flow and tendency for more linear storm mode should
   limit overall tornado potential.  Greatest risk for supercell
   tornadoes should exist during the afternoon with initial convective
   development, and possibly near the retreating warm front where more
   backed low-level winds should exist.  With time, risk should evolve
   toward primarily damaging winds, though an isolated QLCS tornado or
   two, as well as marginally severe hail, will be possible through the
   evening and into the overnight hours.  It appears at this time that
   the greatest severe risk will exist from late afternoon/early
   evening through midnight or an hour or two later, across southern
   lower Michigan, eastern Indiana, western Ohio, and northern Kentucky
   -- coincident with the Enhanced Risk area.  Storms should eventually
   begin to diminish in intensity given weaker instability expected
   into the upper Ohio valley/lower Great Lakes region, but risk may
   linger across the southern Appalachians region through the end of
   the period.

 

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Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0630 AM CST Fri Feb 24 2017

   Valid 241300Z - 251200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM SOUTHERN
   LOWER MICHIGAN ACROSS EASTERN INDIANA AND WESTERN OHIO TO NORTH
   CENTRAL KENTUCKY...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENHANCED RISK AREA...FROM LOWER MICHIGAN TO EASTERN TENNESSEE...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK AREA...FROM THE GREAT LAKES TO PARTS OF THE SOUTHERN
   APPALACHIANS...

   ...SUMMARY...
   Severe thunderstorms are forecast to affect areas from Lower
   Michigan southward to Tennessee, mainly this afternoon through
   tonight.  The primary threat appears to be damaging wind, but some
   hail and a couple of tornadoes will also be possible.

   ...Lower MI to TN through tonight...
   A well-defined midlevel trough over the mid MO Valley this morning
   will amplify while progressing eastward to the Great Lakes and OH
   Valley tonight, as an associated surface cyclone deepens while
   moving northeastward from west central IL across Lower MI to Lake
   Huron.  Ongoing elevated thunderstorms will persist through the
   morning across Lower MI in the zone of strong low-level warm
   advection/frontogenesis immediately north of the eventual cyclone
   track, with marginally severe hail possible in the strongest storms.

   Modest low-level moisture (boundary layer dewpoints in the mid-upper
   50s) will spread northward into southern Lower MI beneath an
   elevated mixed layer, though the cap will likely delay surface-based
   thunderstorm development until 21-00z.  Initial storm development is
   likely close to the surface cyclone in southern Lower MI, along the
   northwest edge of the modestly unstable warm sector (MLCAPE near
   1000 J/kg).  Deep-layer vertical shear (effective bulk shear of
   40-50 kt) will favor semi-discrete supercells with an associated
   large hail/damaging wind risk into this evening across southern
   Lower MI.  A couple of tornadoes will also be possible as any
   sustained supercells can interact with the zone of stronger
   low-level shear (effective SRH 200-300 m2/s2) and sufficient
   moisture along the surface warm front.

   Convection will develop southward through the evening as large-scale
   forcing for ascent/height falls overspread the surface cold
   front/west part of the warm sector and erode the cap associated with
   the steep (8-9 C/km) midlevel lapse rates.  A mixed convective mode
   of semi-discrete cells and line segments is expected initially along
   and just ahead of the cold front, given somewhat boundary-parallel,
   deep-layer shear vectors and ascent focused along the front.  Modest
   low-level moisture/buoyancy, lingering steep midlevel lapse rates,
   and 40-50 kt effective bulk shear will support organized line
   segments and supercells capable of producing damaging winds/isolated
   large hail.  The tornado risk well south into the warm sector is a
   bit more uncertain given relatively straight hodographs and a
   tendency toward more linear convection with time.  However, there
   will still be some risk for embedded circulations in the solidifying
   line of storms which will also be capable of producing damaging
   gusts well into the overnight hours.
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On 2/18/2017 at 3:30 PM, jaxjagman said:

Decent winter time building ridge setting up with the Euro with a ULL to the NW.Euro shows a warm front lifting from the SE states.DP'S would rise with this in the Valley,this should be a decent severe threat with  bulk shear coming from the SW the first of the month.Dashboard shows a multi day event during this time frame.Might be over doing the capes in Texas on the 28th,typical Euro in the long range with convection,but it shows capes into 3k plus

 

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Mesoscale Discussion 0207
   NWS Storm Prediction Center Norman OK
   0528 PM CST Fri Feb 24 2017

   Areas affected...Portions of middle/eastern Tennessee and
   central/eastern Kentucky

   Concerning...Severe potential...Watch likely 

   Valid 242328Z - 250030Z

   Probability of Watch Issuance...80 percent

   SUMMARY...Strong thunderstorms should continue to organize in broken
   bands across central Kentucky this evening. Storms are also expected
   to develop southward into middle Tennessee. As they organize and
   push east, an increasing threat for severe wind gusts should
   materialize. Severe thunderstorm watch issuance is most likely
   across Kentucky in the near term.

   DISCUSSION...Approaching mid-level ascent and related
   moistening/cooling aloft will gradually weaken convective inhibition
   over the region this evening, with a related uptick in convective
   coverage expected. Storms have already begun to organize across
   parts of the Ohio Valley since approximately 22Z, and this trend
   should persist. Furthermore, cells may continue to develop farther
   south into middle Tennessee this evening.

   As storms push eastward, they will likely evolve into small
   bands/bowing segments, aided by relatively uni-directional
   southwesterly flow in the low/mid levels. Additionally, steep
   low-level lapse rates will foster ample evaporative cooling in
   stronger downdrafts. As such, stronger storms will be capable of
   occasional severe-wind gusts this evening.
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Better than nothing :)

 

Special Weather Statement
National Weather Service Nashville TN
746 PM CST FRI FEB 24 2017

TNZ059-060-250230-
Williamson TN-Maury TN-
746 PM CST FRI FEB 24 2017

...SIGNIFICANT WEATHER ADVISORY FOR CENTRAL MAURY AND CENTRAL
WILLIAMSON COUNTIES UNTIL 830 PM CST...

At 745 PM CST, Doppler radar was tracking a strong thunderstorm over
Columbia, moving northeast at 50 mph.

Dime size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Franklin, Columbia, Brentwood, Spring Hill, Thompson`s Station, Santa
Fe, Thompsons Station, I-65 East Of Columbia and Triune.

LAT...LON 3566 8721 3604 8685 3589 8661 3587 8662
      3584 8660 3583 8663 3582 8661 3581 8662
      3581 8661 3579 8661 3579 8666 3577 8666
      3575 8670 3573 8670 3570 8675 3571 8677
      3569 8680 3566 8681 3553 8705
TIME...MOT...LOC 0145Z 230DEG 45KT 3565 8706
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Day 4-8 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0359 AM CST Sat Feb 25 2017

   Valid 281200Z - 051200Z

   ...DISCUSSION...
   Severe-thunderstorm potential will focus during the
   Tuesday-Wednesday (Days 4-5) period.  Medium-range models (GFS,
   ECMWF, CMC, UKMET) and ensembles are coming into better agreement
   regarding the evolution of a larger-scale mid-level trough moving
   from the western states eastward to the MS Valley on Wednesday.  The
   trough subsequently exits the east coast on Thursday and severe
   potential will likely remain low through Saturday (Day 8) as surface
   high pressure resides over the central Gulf Coast.  

   ...Arkansas and surrounding states on Tuesday-Tuesday night...
   Low-level moisture will contribute to increased buoyancy within a
   broad warm-air-advection regime downstream of the mid-level trough. 
   A cold front is forecast to move into the western part of this area
   late Tuesday along with an increasing risk for severe thunderstorms.

   ...lower MS Valley and TN Valley on Wednesday...
   Strong belt of mid-level flow coupled with increasing low-level
   moisture will probably support marginal-moderate buoyancy with a
   strong shear profile.  The timing of a cold front appears to
   converge on Wednesday as it sweeps through the area.  Scattered to
   numerous thunderstorms are forecast, some of which could be severe.
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25 Feb 2017 1011.38 1010.10 -16.62 -2.23 -0.31
26 Feb 2017 1012.16 1008.65 -5.91 -2.31 -0.45
27 Feb 2017 1013.67 1007.75 5.67 -2.00 -0.42

Guess we'll see how this plays out,SOI rose over 20 points into the end of Feb.Looks like multi systems going through East Asia in a couple days.TThe dashboard has been showing a severe threat the last couple days as well as Cips is showing a severe threat into the plains and western Valley today,Also checking out Organic forecasting they show  a ridge in the east.Some one is going to fail into the mid month

 

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Looking interesting into the end of March into April.MJO should be into the IO during this time.Enso 3.4 has just recently again went back negative with a big burst around the IDL that looks to last 4-5 days.1+2  right now is positive 2,good TNI index at this time with region 4 being around positive 0.3.Looking at Josh Herman's map of the SOI  upcoming as soon as the burst leaves around the IDL it should spike up upcoming

 

 

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Experimental Week 3-4 Outlooks - Precipitation Probability

 

Prognostic Discussion for Experimental Week 3-4 Outlook 
NWS Climate Prediction Center College Park MD 
300PM EST Fri Mar 03 2017 

Week 3-4 Forecast Discussion Valid Sat Mar 18 2017-Fri Mar 31 2017 

Signals from canonical climate patterns are currently week. There is currently a transition to neutral El Nino Southern Oscillation (ENSO) conditions from the La Nina of this past winter. The Madden-Julian Oscillation (MJO) is currently active with convection located over the eastern Indian Ocean, however most dynamical model forecasts indicate a weakening of the projection of tropical atmospheric conditions onto MJO in the next week or two. The influence of climate patterns on the current state are expected to be limited, and the present Week 3/4 outlook is based primarily on dynamical model guidance and the predicted evolution of the pattern from the current Week-2 outlook. 

Dynamical model guidance, from the ECMWF, NCEP CFS and JMA for the Week 3/4 period shows a fair amount of disagreement for the predicted mean 500-hPa height pattern over North America. The ECMWF and JMA predict a deep trough and below normal 500-hPa heights over the northern Pacific, centered to the south of the Aleutian Islands. These two ensemble systems predict ridging and above normal mid-level heights downstream of the trough over Canada and over the western CONUS. Relative troughing and a weakness in above normal heights are indicated over the U.S. Southwest into the Southern Plains in the ECMWF and JMA forecasts. The NCEP CFS forecasts an alternative 500-hPa height pattern, with troughing and below normal heights over Alaska and western North America, somewhat east of the location of troughing in the ECMWF and JMA model forecasts. Ridging and above normal heights are displaced eastward, and are centered over eastern Canada in the CFS. While the forecast period average circulation patterns of the various models show disagreement, the temperature and precipitation forecasts from the available ensemble prediction systems of the ECMWF, NCEP CFS, and JMA are in fairly good agreement on the pattern over North America. 

Models generally indicate below normal temperatures are more likely for southern Alaska under anomalously northerly flow and below normal mid-level heights over the Aleutian Islands. A statistical forecast that combines the MJO pattern with decadal trends indicates below normal temperatures are more likely for southern Alaska, consistent with the dynamical model consensus. Below normal temperatures are most likely for parts of the Pacific Northwest and Northern Plains, with predicted above median precipitation for the region in week 2, carrying into week 3 and 4 in the model forecasts. Temperature forecasts for Week 3/4 from the dynamical models are in agreement on likely above normal temperatures for the Southwest, the Southern Plains, and the Gulf Coast. Above normal temperatures are also most likely for the northern Great Lakes region and New England, as indicated by dynamical models. The ECMWF and JMA models indicate weak temperature signals over most of the central Mississippi Valley, the Ohio Valley, and the Mid-Atlantic region. In contrast, regressions on the current MJO phase indicate a lagged temperature pattern for the Week 3/4 period with probable above normal temperatures over the eastern CONUS, which is consistent with the decadal trends, and more similar to the CFS forecast. The disagreement in the temperature forecast tools for this region indicate greater uncertainty, such that equal chances of above and below normal temperatures were indicated in the outlook. 

Dynamical model forecasts from the ECMWF, CFS and JMA ensembles are in relative agreement on the precipitation forecast for the Week 3/4 period. There is consensus on likely below median precipitation for the Southwest, to the south of the predicted Pacific Jet, while near median precipitation is predicted for the Pacific Northwest, and above median precipitation is more likely for the Northern Plains region. Above median precipitation is predicted to be most likely for the lower and central Mississippi Valley and the Ohio Valley regions, with moisture drawn northward from the Gulf of Mexico in model forecasts. 

Above-normal temperatures are forecast across Hawaii due to anomalously warm sea surface temperatures and anomalous southwesterly flow in the dynamical model forecasts. Dynamical model guidance from the ECMWF and JMA model forecasts consistently indicate below median precipitation for Hawaii. 

 

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The whole 2017 season looks interesting. El Nino attempt has slowed down, which is favorable to remain in the Transition phase between Nina and Nino. Good people can debate the TNI, but the trend is the chaser's friend. Trough west ridge east seems almost locked. Even if it falls apart in May, I think April will be active in the Plains. In the short-term the Mid South looks active in March.

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6 hours ago, nrgjeff said:

The whole 2017 season looks interesting. El Nino attempt has slowed down, which is favorable to remain in the Transition phase between Nina and Nino. Good people can debate the TNI, but the trend is the chaser's friend. Trough west ridge east seems almost locked. Even if it falls apart in May, I think April will be active in the Plains. In the short-term the Mid South looks active in March.

Figures it would want to turn cold now

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The weeklies are wrong. No worries. 

15 hours ago, jaxjagman said:

Figures it would want to turn cold now

(hopefully no worries, lol)

Meanwhile today low level winds are backed nicely in North Alabama and North Georgia. Too bad other junk rain is also in progress. Need some breaks of sun to my south and west to even get a shelfie with the main line later.

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 ...Ozarks to Tennessee Valley...
   Multiple rounds of convection are likely across the region today,
   leading off with isolated non-severe thunderstorms ongoing over the
   LA/AR border region and shifting eastward across the lower
   Mississippi Valley toward MS this morning.

   Additional elevated convection may form later this morning along the
   leading portions of the low-level warm-advection regime over
   southeastern KS.  Strengthening of the 850-mb height gradient (and
   the associated LLJ) is forecast despite the diurnal timing, as the
   mid/upper-level perturbation approaches, contributing to a favorable
   combination of warm-advection related increase in moisture/thetae,
   destabilization, isentropic lift to LFC, and storm-relative inflow. 
   Activity should shift east-southeastward across southern MO and
   perhaps northern AR this afternoon.  Depending on the equatorward
   extent of near-surface static-stability reinforcement by either:
   (a) its own outflow or
   (b) additional, likely surface-based activity to its south,
   ...some of the convection also may become surface-based this
   afternoon, as boundary-layer warming/mixing lowers the altitude of
   inflow-layer parcels.  As such, the occurrence and breadth of such
   favorable destabilization is conditional and uncertain, but may
   contribute to an expanded damaging-wind area with eastward extent in
   concert with the next probable convective regime discussed below.

   Additional development is expected either along the southern rim of
   the initial/elevated activity and/or its outflow-reinforced
   boundary, moving east-southeastward in a CAPE/shear parameter space
   favorable both for supercells and bow echoes.  Despite the somewhat
   weak (about 10 kt), veered (south-southwesterly to southwesterly)
   character of the surface flow, forecast soundings from locations
   with surface-based inflow parcels show the presence of veering with
   height into the LLJ layer.  This yields elongated/curved low-level
   hodographs with effective SRH in the 200-350 J/kg range -- enough to
   yield favorable SRH for supercells, in addition to the development
   of 35-45-kt effective-shear magnitudes.  Surface dew points
   increasing into the mid 50s to low 60s F in the moist-sector return
   flow, and diurnal heating, should combine to boost preconvective
   MLCAPE to the 1000-2000 J/kg range mid-late afternoon.

   The main uncertainty on tornado threat, and to some extent large
   hail as well, will be how quickly a transition occurs from
   supercellular to clustered/bowing convective mode.  Ultimately, a
   forward-propagating MCS with damaging-wind potential should evolve
   from these initial convective regimes.  This activity would move
   east-southeastward across parts of the Ozarks and Mid-South.  At
   least marginal wind potential (and perhaps more) exists with
   cold-pool-forced activity extending well into tonight across the
   Tennessee Valley region, in spite of a diabatically stabilizing
   boundary layer.

   A more conditional threat exists over eastern OK and western AR for
   convection to either backbuild southwestward from the primary Ozarks
   complex or develop in a zone of frontal convergence this evening and
   overnight.  Guidance reasonably is inconsistent with this
   possibility, given its strong dependence on the influence of
   forward-propagational/cold-pool processes of the second-phase
   development discussed above.  Unconditional marginal probabilities
   are expended southward somewhat to account for this scenario, but
   uncertainty is too great to do a greater upgrade at this time.
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URGENT - IMMEDIATE BROADCAST REQUESTED
   Severe Thunderstorm Watch Number 73
   NWS Storm Prediction Center Norman OK
   645 PM CST Thu Mar 9 2017

   The NWS Storm Prediction Center has issued a

   * Severe Thunderstorm Watch for portions of 
     Northern Arkansas
     Missouri Bootheel
     Western Tennessee

   * Effective this Thursday night from 645 PM until Midnight CST.

   * Primary threats include...
     Scattered large hail likely with isolated very large hail events
       to 2 inches in diameter possible
     Scattered damaging wind gusts to 70 mph likely
     A tornado or two possible

   SUMMARY...Severe thunderstorms over southern MO will continue to
   track southeastward and into parts of northern AR/western MO this
   evening.  Damaging winds and large hail are the main threats, but an
   isolated tornado or two is also possible.

   The severe thunderstorm watch area is approximately along and 55
   statute miles north and south of a line from 20 miles west southwest
   of Harrison AR to 50 miles east of Jackson TN. For a complete
   depiction of the watch see the associated watch outline update
   (WOUS64 KWNS WOU3).
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