April 2017 - MS 150 Forecast This Weekend

[srainhoutx]

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:

[/i]

[unome]

anxious to see SPC's 1730Z update

http://www.spc.ncep.noaa.gov/products/o ... 2otlk.html

[DoctorMu]

What's remarkable is the large area covered by moderate risk, particularly for an even that gets cranking in the morning.

The confidence isn't super high yet. Main heavy rainfall wish will be along the warm front Crocket to CLL to Livingston. Rogue cells in the am may be the greatest tornado threat. We'll see.







FXUS64 KHGX 011647
AFDHGX

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

[srainhoutx]

The attachment 04012017 1730Z day2otlk_1730.gif is no longer available

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

[javakah]

Am I correct in my reading of things that although the timing (for the tornado threat) is uncertain, it's now more likely between mid-day and evening, and less likely than it was before to be in the early/mid morning? Or is there still a decent chance of rogue tornadic supercells in the earlier morning?

I'm just not sure what to think at this point (especially with the NWS HGX timing graphic still showing the main tornado threat between about 7 a.m. and 3 or 4 p.m.).

[srainhoutx]

Am I correct in my reading of things that although the timing (for the tornado threat) is uncertain, it's now more likely between mid-day and evening, and less likely than it was before to be in the early/mid morning? Or is there still a decent chance of rogue tornadic supercells in the earlier morning?

I'm just not sure what to think at this point (especially with the NWS HGX timing graphic still showing the main tornado threat between about 7 a.m. and 3 or 4 p.m.).

The trends as we get closer to tomorrow via the latest data suggests the greatest threat may be from midday tomorrow into the early hours of Monday morning.

[Katdaddy]

The latest Day 2 Convective Outlook Summary from the Storm Prediction Center (SPC):

...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.

The Moderate Risk area remains for SE TX and has been expanded into Central and S Central LA.

[Katdaddy]

A Flood Watch has been issued for N portions of SE TX from late tonight through Sunday evening.

Flood Watch
National Weather Service Houston/Galveston TX
144 PM CDT Sat Apr 1 2017

TXZ163-164-176-177-195-196-198-030000-
/O.NEW.KHGX.FA.A.0001.170402T0900Z-170403T0000Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/Brazos-Burleson-Grimes-Houston-Madison-Trinity-Walker-Including the cities of Bryan, Caldwell, College Station,Crockett, Groveton, Huntsville, Lake Somerville, Madisonville, Navasota, and Trinity
144 PM CDT Sat Apr 1 2017

...FLOOD WATCH IS IN EFFECT FROM LATE TONIGHT THROUGH SUNDAY EVENING...

The National Weather Service in Houston/Galveston has issued a

* Flood Watch for a portion of southeast Texas...including the
following counties...Brazos...Burleson...Grimes...Houston...
Madison...Trinity and Walker.

* From late tonight through Sunday evening

* A warm front becoming nearly stationary within a very unstable environment may provide the focus for repeated daytime storm activity. Very moist air flowing directly into this boundary will enhance the threat for slow moving high rainfall producing thunderstorm cells. This will increase the probability for flash flooding across our far northern counties.

* Travel may become hazardous as flooding could occur over rural roadways or low water crossings. Rapid response water sheds could come out of banks within periods of heavy rainfall.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A Flood Watch means there is a potential for flooding based on current forecasts.

You should monitor later forecasts and be alert for possible
Flood Warnings. Those living in areas prone to flooding should be prepared to take action should flooding develop.

[srainhoutx]

Saturday afternoon briefing from Jeff regarding tomorrows severe threat potential and uncertainties:


Potentially dangerous severe weather outbreak over SE/E TX on Sunday.

Parameters will be in place to produce a high end severe weather threat including (tornadoes, large hail, damaging winds, and flooding rainfall). A few tornadoes could be long tracked and violent.

Flash Flood Watch has been issued for the northern 1/3rd of SE TX for Sunday and this watch may be expanded later tonight or early Sunday to reflect trends.

While it has been a long time since we have seen the type of parameters that will be in place over SE TX tomorrow for sever weather…there remains some uncertainty in how the storms will evolve and when exactly the greatest threat will be. Point being is that a very active weather day is likely and residents should be fully aware of the potential for impacts and respond quickly to weather warnings.

A powerful upper level storm system will move into TX late tonight with a strong 40kt low level jet developing over SE TX being overspread by a 70kt mid level jet. Local air mass will undergo strong destabilization early Sunday morning as reservoir of high CAPE air mass over the western Gulf is rapidly transported into SE TX with CAPE values of 2000-3000 J/kg by mid morning Sunday. Coastal marine boundary (warm front) will lift northward and a few upper air disturbances may eject across SE TX along this boundary helping to spark thunderstorms. Low level helicity values rapidly increase as hodographs quickly enlarge with onset of strong low level shear Sunday morning. If storms can overcome weak capping and enough forcing is present then development of tornadic supercells is likely across the warm sector. Updrafts will almost certainly rotate given the shear vectors and tornadoes…a few long tracked and potentially higher end EF scale ratings would be possible.

High resolution models are not in great agreement on the placement nor the development across the area on Sunday morning which leads to some lower than average confidence…however SPC feels that given the parameters in place if storms do develop then they will likely become quickly tornadic and in their 1230pm update maintained and increased the wording of their risk threat.

The tornado threat should gradually lessen during the early afternoon hours as low level winds veer to the S and SSW helping to reduce the low level shear, but the risk for damaging winds and large hail will continue into early Sunday evening over the area.

As for the heavy rainfall and flash flood threat. The threat remains across SE TX with the latest incoming guidance pointing toward the area from about Brenham NNE to Lake Livingston as the greatest risk area…or where the Flash Flood Watch will be in place. This aligns well with the surface warm frontal boundary position…but there is some uncertainty on just how far north the warm front will move and it is possible the heavy rainfall threat could need to be adjusted southward some early on Sunday. Still expecting widespread amounts of 1-3 inches with high hourly rainfall rates of 2-3 inches possible given high moisture levels.

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.

1230 Day 2 Severe Weather Outlook Update:

04012017 Jeff 3 untitled.png

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[txsnowmaker]

Perhaps Metro Houston will escape the brunt of this system. At least one on-air met is focusing on areas north of the city for the best chance of tornadic activity.

Ch. 13 on-air met on 6 pm news: "A good bit of southeast Texas will be under the gun for the possibility of tornadoes. The conditions will be even worse up to the north in this corridor here from Brenham over towards Conroe and Huntsville, and on to the north and east. That's the area of southeast Texas with the best chance of seeing a tornado tomorrow. One or two of those could be on the strong side."

[TexasBreeze]

Our recent tornadic activity has been under slight/enhanced risks, not moderate risks. If cloud cover remains broken and not rain cooled, look out!!!

[Cromagnum]

Pretty irresponsible to give hope that Houston isn't at risk. Folks around here are pretty nonchalant as it is regarding weather.

[srainhoutx]

GOES 16 has been positioned over Texas and Harris County OEM is being activated. Stay weather aware folks.

[Rip76]

GOES 16 has been positioned over Texas and Harris County OEM is being activated. Stay weather aware folks.

I may be reading this wrong.
"Postponed?"

[Andrew]

GOES 16 has been positioned over Texas and Harris County OEM is being activated. Stay weather aware folks.

I may be reading this wrong.
"Postponed?"

Positioned

[Ounce]

GOES 16 has been positioned over Texas and Harris County OEM is being activated. Stay weather aware folks.

That's super cool! (Any way to watch it?)

[Andrew]

GOES 16 has been positioned over Texas and Harris County OEM is being activated. Stay weather aware folks.

That's super cool! (Any way to watch it?)

http://weather.cod.edu/satrad/exper/?pa ... tx-13-24-0

[Ounce]

GOES 16 has been positioned over Texas and Harris County OEM is being activated. Stay weather aware folks.

That's super cool! (Any way to watch it?)

http://weather.cod.edu/satrad/exper/?pa ... tx-13-24-0

Thanks, Andrew! I looked at the visible link that wxman provided on the GOES-16 thread. It's funny looking as the sun was setting and it's all white in the darkness.

[srainhoutx]

Rare Saturday night briefing from Jeff:

While there has been no change to the SPC outlook for the area…a recent trend this afternoon and evening has been much of the high resolution guidance showing only sporadic development over the areas south of HWY 105 Sunday morning into the afternoon hours. Severe parameters remains maximized over the area during this period, so it is interesting that these models are showing such little development. Potentially some weak capping is at play, but forecasted soundings do not really support that. It appears the most favorable position of the incoming jet core may be aimed at areas just north of SE TX into EC TX and W LA.

Will need to await the SPC overnight update, but areas south of I-10 in the moderate risk outline may be overdone given the current meso guidance. Still a good potential for discrete cell development N of I-10 from about 800am to noon. Latest NAM guidance has a “significant tornado parameter” of 14.1 over east TX around noon Sunday which is the highest I have ever seen in this area and an 80.5% probability of supercells.

I am tempted to pull back on rainfall totals, but with the warm front still to the south and with the feature having to move northward early Sunday will leave things alone for now. If this boundary ends up as far north as current HIRES guidance indicates the heavy rainfall threat will be north of a College Station to Huntsville line.

A lot of uncertainty going into Sunday with how the severe and heavy rainfall threat may unfold.

[txsnowmaker]

Thanks for posting the update. That's certainly consistent with the local tv weather report I referenced earlier tonight regarding the tornado threat. Trends certainly seem to be going in the right direction for us in the city in terms of tornadoes.