April 1-3 Severe Threat

[1900hurricane]

I appears that April will be opening with a couple of days of potential severe weather mainly across parts of Texas. Both Saturday, April 1st and Sunday, April 2nd have risk areas outlined by the SPC, with 4/2 currently featuring a large Day 3 Enhanced Risk for Southeast Texas and western Louisiana.

 

 

[1900hurricane]

There is much that has me curious on Sunday. Guidance almost unanimously has surface low over South Texas, with warm sector dewpoints of around 70*F beneath good mid-level lapse rates and strong winds veering with height to start the day, leading to some very impressive parameters. However, there are still a couple of big question marks that I see with the event. Some guidance members (such as the 06Z GFS and 06Z NAM) have big time surface pressure falls occurring during the day to the northeast near Texarkana, which severely veers the low level winds with time. Other guidance members (09Z SREF, 00Z CMC) are doing this much less severely or putting the pressure falls in a much more favorable location wrt the warm sector, allowing SE-SSE low level winds to be maintained through the course of the day.

 

06Z GFS showing considerable low level wind veering.

 

09Z SREF mean keeps surface winds much more backed over the warm sector vs the 06Z GFS at the same time.

 

Alternatively, any West Texas MCS originating from the previous day's risk could also influence the severe threat for the day. Some of the current guidance do maintain an MCS, but are keeping it for the most part on the norther edge of the warm sector for now. MSCs have junked up a couple of recent risk areas, and while that may not happen here, it is something to keep an eye on.

 

Ironically, I'll probably be starting the day in Dallas.

[monsoonman1]

While there are caveats, Sunday has a very high ceiling if things turn out. NAM 3km just got into range, and FWIW it shows a lot of MCS-type activity. Still a lot of time for that to change. SREF SigTor Ingredients is popping a 75 over much of SE TX, which is...interesting.

[weatherextreme]

Pretty Impressive 

 

 

[Chargers09]

3k NAM keeps popping this UHI track just north of me (NE side of San Antonio)

 

Sounding looks pretty solid for that timeframe, too.

 

 

[andyhb]

One of the things that concerns me here is the relatively modest/weak anvil level flow (due to the fact that the warm sector is basically in between the northern branch of the jet and the southern stream) and coincident high columnar precipitable water values. This would tend to lead me to believe HP storms are in the cards. Low level shear profiles certainly are notable, but my question regards having more long-lived cells. The former is a bit less of a problem on the NAM, which has more of the northern branch of the upper jet extending into the warm sector.

[bjc0303]

seeing a bit if a signal for an early day MCS 

[Msalgado]

I will likely get suckered out Saturday because its close and I'm still jonesing to chase, but it doesn't seem as though the dynamics come together very well.  Still, there are some areas with good SRH and CAPE values so if a storm can go off in the warm sector we might get something.

 

Sunday looks better early afternoon across south central Texas, but with so much moisture and such low LCLs it will be a scuddy mess, IMO.  Still, I'm likely to go out if the models keep showing what they have.

[1900hurricane]

The SPC's initial Day 2 Convective Outlook for Sunday, April 2nd blankets southeast Texas in a Moderate Risk.

 

 

SPC AC 010532

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1232 AM CDT Sat Apr 01 2017

   Valid 021200Z - 031200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS SUNDAY ACROSS
   MUCH OF SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA...

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS
   SURROUNDING AREAS OF EASTERN TEXAS INTO WESTERN AND CENTRAL
   LOUISIANA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING
   AREAS OF CENTRAL AND EASTERN TEXAS INTO THE LOWER MISSISSIPPI
   VALLEY...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
   SURROUNDING AREAS OF THE LOWER RIO GRANDE VALLEY...SOUTHEASTERN
   PLAINS AND LOWER MISSISSIPPI VALLEY...

   ...SUMMARY...
   Organized severe thunderstorm development is expected across parts
   of eastern Texas into western and central Louisiana Sunday through
   Sunday night.  This includes a risk for tornadic supercells, a few
   of which could be strong, along with considerable potential for
   damaging wind gusts with an evolving squall line.

   ...Synopsis...
   There remains little change to the general large-scale pattern with
   a series of significant short wave perturbations emanating from a
   strong mid/upper jet over the mid-latitude Pacific, before migrating
   inland, and digging into/through the Southwest.  One now is in the
   process of advancing toward the British Columbia/Pacific Northwest
   coast, and is forecast to progress inland, before digging through
   parts of the northern intermountain region and California Sunday
   through Sunday night.  As this occurs, one or two vigorous
   perturbations (digging into the base of elongating troughing near
   the southern Rockies today) appear likely to accelerate east
   northeast of the Rio Grande Valley.

   Various model output, including the latest ECENS,NCEP MREF and SREF
   mean fields appear to be coming into at least somewhat better
   consensus concerning the evolution and progression of this latter
   system.  Confidence is increasing that the negatively tilted axis of
   larger-scale troughing will reach the southeastern Plains/Sabine
   Valley by 12Z Monday, supporting the development and northeastward
   migration of a deepening surface low from portions of
   central/northeast Texas into the Ozark Plateau.

   All indications continue to suggest that this will be favorably
   timed with an increasingly substantive return flow of moisture off
   the Gulf of Mexico, including surface dew point increases through
   the 60s to around 70f across the mid/upper Texas and southwest
   Louisiana coastal plain.  In the presence of steep mid-level lapse
   rates, sizable mixed layer CAPE may exceed 2000 J/kg across this
   region by Sunday morning.  Destabilization is expected to coincide
   with strengthening of lower/mid tropospheric wind fields, including
   30-50 kt (southerly) at 850 mb and 50-70 kt (west southwesterly) at
   500 mb, across central and eastern Texas into the lower Mississippi
   Valley.

   ...Southeastern Plains into northwestern Gulf coast region...
   Aforementioned warm sector environment conditions appear more than
   favorable for organized severe storm development, in the presence of
   increasing large-scale forcing for ascent.  Timing of initiation of
   intense boundary layer based storm development remains somewhat
   unclear, but it appears possible as early as Sunday morning in the
   presence of low-level warm advection across southeast Texas.  This
   activity may be mostly discrete in nature, in the presence of
   moderately large CAPE, strong deep layer shear and sizable
   clockwise-curved low-level hodographs.  Evolving supercells are
   expected to be accompanied by the risk for tornadoes, a few of which
   may be strong.

   Discrete supercell development may persist through the afternoon 
   hours, and perhaps the remainder of the period, spreading toward the
   lower Mississippi Valley, ahead of an evolving convective system
   which may be accompanied by a considerable risk for potentially
   damaging wind gusts.  Portions of central and eastern Texas into
   Louisiana still appear the mostly likely areas that could be
   impacted by an evolving convective system, but northeastward
   development into portions of the Ozark Plateau and adjacent
   Mississippi Valley may not be out of the question by late Sunday
   night.

   ..Kerr.. 04/01/2017

[Jim Martin]

Northeast of Houston, Texas tomorrow off this morning's 3km NAM.

[Quincy]

This is the type of setup that could warrant a high risk, if the NAM progs are close to correct. Strong to extreme bouyancy within a considerably sheared environment with steep lapse rates and substantial low level moisture. 

The open and warm sector is pretty much game from midday on with eroding CINH. The big question will be convective initiation. If it's largely confined to the cold front, we'd be looking at more of a widespread wind event with embedded circulations, but if the warm front and warm sector light up, there's a high ceiling for tornadoes, some significant, and very large hail. 

[bjc0303]

all the ingredients will be there. With such a strong low level jet wondering if we will see open warm sector confluence axes develop... agreed, must be concerning to see this in the area.

[David Reimer]

Really wish SPC hadn't issued such a large-scale MDT outlook. First MDT in Houston since 2008 and its causing the hype-train to go into overdrive. I'd expect a northward shift with the 1730Z, and hopefully a slight decrease in aerial coverage. Squall line looks likely with lots of wind/hail, but only a conditional risk for a SIG TOR threat. Looks like we may have some issues develop late tonight as sups move into the Del Rio area of Mexico. 

[Jim Martin]

From the latest Area Forecast Discussion from the National Weather Service in League City...

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1145 AM CDT Sat Apr 1 2017

.UPDATE...
Surface analysis has 70 F dew point air moving due north from the
lower Rio Grande with a stationary boundary laying across the Hill 
Country...draped eastward across the Red River Valley (surface low 
over the Big Bend region). As the upper low churning eastward into 
New Mexico digs down into northern Mexico through early Sunday 
morning the upper levels will become more diffulent over eastern 
Texas. An impressive thermodynamic background is certainly 
supportive of severe weather Sunday...but as usual...it all comes 
down to the where and when in the details. There is a vort max that 
appears to be rounding the base of the upper low that the models are 
taking into west Texas overnight Sunday morning. The upper low may 
be too removed to the southwest early tomorrow such that any 
shortwave disturbance rotating around it moves northeast across more 
central Texas. Thus...lift will be meager through mid Sunday morning 
as we sit in a warm...highly unstable and unseasonably moist (pws 
near above 2 std dev) air mass. The slow east-northeast movement of 
the surface low may draw up a warm frontal boundary that could mesh 
with the aforementioned stationary boundary and provide early 
(Sun)day precipitation focus. This may adversely impact our far 
northern counties in terms of excessive rainfall...or slow-
moving/training southwest- moving-northeast cells producing 1-2 inch 
per hour rainfall rates.
Any discrete cell moving north within the warm sector will be 
likely be rotating so an early day tornado threat is alive and 
well. The scenario is looking more like a late morning through 
late afternoon Sunday QLCS that begins to materialize just east of
the plateau and advances east across the CWA through the afternoon
hours. The early day tornado threat will transition to more of an
northern 2/3rds forecast area hail/wind/rain threat during the PM
hours. 31

[jojo762]

Not related to today or sunday, but Tuesday is looking potentially interesting **IF** we can get DPs at or near 60 and storms to fire along the DL. Lots of caveats, but wind fields look pretty solid.

[weatherextreme]

From a local met

 

 

Saturday morning briefing from Jeff:

***Significant outbreak of severe weather expected over SE TX Sunday***

Large damaging hail, straight line winds, and a few violent long tracked tornadoes possible. 

SPC has upgraded the area into a rare moderate risk threat (4 out of 5)…the last time the threat level was this high was Feb. 2008

Discussion: 
Powerful upper level storm system over the SW US will eject into TX late tonight and on Sunday spreading strong height falls across strong Gulf moisture return. Surface warm front/trough near the coast this morning will begin to return northward late tonight/early Sunday spreading a very moist and unstable warm sector across SE TX early Sunday (before sunrise). There seems to be two developing camps within the model guidance on how storms may evolve early Sunday into the evening hours. 

1. Discrete supercells develop across the large warm sector between 600am-noon on Sunday ahead of a line of storms that will arrive in the early afternoon.

2. Discrete supercells do not develop over the warm sector and instead a line of storms approaches from the west late morning into the early afternoon hours. 

Forecast models show a weak cap eroding rapidly after 600am Sunday with an air mass over SE TX highly sheared (30-40kts 0-3km and 50-70kts 0-6km) with CAPE values of 2000 J/kg or greater. Significant tornado parameters will be in place on Sunday morning and any cells that develop are going to rotate. Very low cloud bases with low LCL’s over the area combined with strong near surface helicity values supporting low level rotation. If supercells can develop in the warm sector air mass the threat for damaging long tracked tornadoes is very possible in the moderate risk outline. Maximum tornado parameters will begin to shift east of SE TX by late morning, so if supercells fail to develop in the morning warm sector the threat for tornadoes, while still there, will be lowering in the afternoon hours. Threat will transition more towards a damaging wind and large hail potential. 

Secondary factor appears to be heavy to excessive rainfall, but this is now secondary to the severe weather threat, which should be taken seriously for this event. While little has changed with respect to the heavy rainfall potential, the severe parameters overnight have increased. Moisture profiles will be high by early Sunday morning with PWS surging to over 1.8 inches. Surface dewpoints have already increased into the 60’s which is higher than forecasted model guidance for this time period giving confidence that significant moisture values will be in place Sunday. Expect storms to produce maximum rainfall productions given a nearly saturated air mass and any supercells that develop could easily produce 2-3 inches in a hour. Main mitigating factor at this time appears to be 20-25kts of cell motion which will negate heavy rainfall for a longer period of time. The exception would be within any areas of cell training where totals could quickly add up. Widespread rainfall amounts of 1-3 inches is likely over the region with isolated totals upwards of 4 inches or greater under training cells. High resolution guidance continues to suggest the warm front will be located from about Lake Livingston to near Brenham tomorrow and will be the focus for potentially training of cells. Will need to watch this boundary very closely as the greatest excessive rainfall and flash flood threat will likely be tied to its presence over the region. Models can be fast in lifting such features northward especially when showers and thunderstorms develop that help to slow the boundary and even at time push it back southward through convective outflows.

Summary: 

· Sunday is likely to be a significant weather day across SE TX with all modes of severe weather possible (large hail, damaging winds, and tornadoes) along with heavy rainfall and flooding. 

· Residents should be aware of the magnitude of this storm system and keep updated on forecasts on Sunday. 

· Everyone should have a way to receive warning information on Sunday…if a warning is issued for your area take it seriously and react…it may save your life. 

Tornado Safety Actions: 

· The best place to be during a tornado is in an interior room on the lowest floor of a structure (an interior powder bathroom or closet). Do not seek shelter against outside facing walls

· In large buildings such as churches, office buildings, and schools: interior hallways or stairwells offer the most protection…never stay in a room with large vaulted ceilings. 

· Abandon vehicles for sturdy structures…if no structure is available as a last resort lie flat in a nearby ditch…never stay in a vehicle and never try to outrun a tornado in a vehicle. Do NOT shelter under overpasses. 

SPC Day 2 (Sunday) Severe Weather Outlook:

[Jim Martin]

[jojo762]

 Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1234 PM CDT Sat Apr 01 2017

   Valid 021200Z - 031200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS EASTERN
   TEXAS INTO WESTERN LOUISIANA...

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER MOST OF
   EASTERN TEXAS INTO LOUISIANA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM CENTRAL TEXAS
   ACROSS THE ARKLATEX AND INTO THE LOWER MISSISSIPPI VALLEY...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM TEXAS INTO
   MISSISSIPPI...

   ...SUMMARY...
   Dangerous severe thunderstorms capable of strong tornadoes,
   extensive wind damage and hail are possible beginning Sunday morning
   east of I-35 in Texas and developing eastward across Louisiana
   throughout the day and into the night. Severe storms are also
   possible across the Arklatex during the day and into western
   Mississippi by Monday morning.

   ...Synopsis...
   Low pressure will gradually translate northeastward through the
   period with a cold front to the west, from the lower Rio Grande
   Valley Sunday morning into Arkansas by Monday morning. Ahead of the
   low, a warm front will rapidly lift northward across east Texas
   toward the Arklatex and extend into southern Mississippi by 00Z.
   Across the warm sector, a very moist and unstable air mass will
   exist, characterized by upper 60s to near 70 dewpoints, beneath
   cooling profiles aloft and steep midlevel lapse rates. This volatile
   air mass will exist over a large area, suggesting potential for
   widespread severe weather.

   Both mid and high-level southwesterly flow aloft will increase
   throughout the period as the upper trough moves northeastward across
   TX. The northward transport of moisture and instability will be
   aided by a broad, southerly low-level jet which will increase to 50
   kt by late afternoon and will shift eastward across the lower MS
   valley overnight. This will create strong, veering winds with height
   which will clearly favor significant severe thunderstorms including
   tornadic supercells, damaging bows, and quasi-linear convective
   systems. The severe threat is expected to begin by mid morning
   across central and northern Texas, and evolve/expand eastward
   through the rest of the period to near the Mississippi river by 12Z
   Monday.

   ...Central into northeast Texas and across the Arklatex -- Sunday
   morning through afternoon...
   Model consensus is that supercells may begin forming in the 12-15Z
   time frame from near Austin TX to just east of Dallas, aided by
   convergence within the low pressure trough and where rapid moisture
   advection will be underway. Any capping inversion is expected to be
   minor, and relatively low for this part of Texas. This suggests that
   early development is indeed feasible, and increases the chances of
   strong low-level accelerations in the storms as opposed to
   situations where the LFC is much higher. The cells may take some
   time to become better organized as the low level jet will be on the
   increase during the day. However, by 18-21Z, these storms should
   pose a tornado and wind threat across northeast TX approaching the
   Arklatex, with strong tornadoes possible. CAMs suggest that
   supercells may eventually merge into an MCS, in which case damaging
   straight-line winds would be likely. In addition, re-generation of
   supercells is possible along the southern fringe or along any
   outflow boundary laid out by this earlier activity, possibly
   affecting areas along the northern edge of the Moderate Risk area
   such as Shreveport LA.

   ...East Texas into Louisiana -- Midday through evening...
   The fast erosion of any capping inversion Sunday morning and
   widespread destabilization of the air mass lends uncertainty to
   exact placement and timing of tornadic supercell development, with a
   potential extending from east TX to New Orleans, and timing anywhere
   from midday through evening across the warm sector. Some models such
   as the ECMWF produce storms over southeastern LA during day, and the
   environment would already be favorable for tornadoes. However, the
   most likely scenario is for supercells to begin developing by
   midday, near the warm front and intensifying low-level jet core
   which will be across east TX into western LA, then eventually
   spreading eastward as the cold front approaches from the west. At
   least a few strong tornadoes are expected given large looping
   hodographs with effective SRH in excess of 300 m2/s2 along with
   impressive instability profiles. Overnight, cells may merge into a
   QLCS, with both tornado and damaging wind threat persisting into
   Monday morning.

   ..Jewell.. 04/01/2017

[Quincy]

To be taken with a grain of salt, but the HRRRX shows robust convection already maturing by 15z over central Texas and keeps the storm mode largely discrete/semi-discrete through 23-00z. That scenario would suggest a broad area of intense supercell and attendant significant severe potential. 

Trends will need to be monitored very closely as this has the potential to be a major event given the parameter space and time window for storms. 

[Jim Martin]

2 minutes ago, Quincy said:

To be taken with a grain of salt, but the HRRRX shows robust convection already maturing by 15z over central Texas and keeps the storm mode largely discrete/semi-discrete through 23-00z. That scenario would suggest a broad area of intense supercell and attendant significant severe potential. 

Trends will need to be monitored very closely as this has the potential to be a major event given the parameter space and time window for storms. 

Exactly.

[Indystorm]

And I would certainly hope every congregation having services tomorrow designate a person as a spotter with access to NWS wx radio/and or computer radar images and warnings. I'm recalling a Palm Sunday event in 1994  when  Goshen United Methodist church in Piedmont Alabama was hit during services with loss of life and injuries.

[jojo762]

14 minutes ago, Indystorm said:

And I would certainly hope every congregation having services tomorrow designate a person as a spotter with access to NWS wx radio/and or computer radar images and warnings. I'm recalling a Palm Sunday event in 1994  when  Goshen United Methodist church in Alabama was hit during services with loss of life and injuries.

Cancelling church services across Eastern/southeastern Texas might be the safest option for tomorrow given the potential for significant severe storms capable of all severe hazards by as early as 10am. Given the current forecast thinking of the SPC as well as recent CAMs would not be surprised to see the SPC issue a long-term PDS tornado watch by as early as 9 or 10am tomorrow morning. 

[andyhb]

14 minutes ago, jojo762 said:

Cancelling church services across Eastern/southeastern Texas might be the safest option for tomorrow given the potential for significant severe storms capable of all severe hazards by as early as 10am. Given the current forecast thinking of the SPC as well as recent CAMs would not be surprised to see the SPC issue a long-term PDS tornado watch by as early as 9 or 10am tomorrow morning. 

Let's temper this discussion until we see the true extent of the favorable parameter space tomorrow.

[Indystorm]

One thing I've learned over the years as a hobbyist and storm spotter is that svr wx always seems to boil down to a "day of" forecast even with heads up modeling in advance.

[Buckeye05]

FWIW, the latest WFO from Houston does not sound all that onboard with a widespread discrete tornadic supercell threat. They seem to leaning towards a brief window for an early tornado threat before things quickly grow upscale into an MCS. 

[jojo762]

1 hour ago, andyhb said:

Let's temper this discussion until we see the true extent of the favorable parameter space tomorrow.

Not saying it's definite, just that I would not be surprised if it happened. Obviously you have to wait until tomorrow to actually determine the ambient environment storms will exist within/have to deal with and make watch/outlook decisions then.

[Jim Martin]

With all this discussion, it is a reminder that it only takes one storm to cause big problems...

[Jim Martin]

The 18z HRRR Sounding for Houston valid 12z Sunday.

[pbrussell]

1 hour ago, Jim Martin said:

The 18z HRRR Sounding for Houston valid 12z Sunday.

Well that's a bit ominous.

[Chinook]

I am wondering if the most impacted area will be on the north side of the moderate outlook, away from Houston and Dallas metro areas. Some of the convection-allowing models show quite a bit of activity in this area which could be supercells, or a rather complex scenario of and MCS with embedded tornadoes.