MAY 2019: Wednesday Storm Complex/Scattered Showers To End May

[tireman4]

Blake's Tweet

[tireman4]

Interesting take in the last sentence

660
FXUS64 KHGX 061458
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
958 AM CDT Mon May 6 2019

.UPDATE...

For the most part, the forecast is doing pretty well this morning.
Temperatures and winds are basically on track and only needed some
cosmetic tweaks to stay seamless with observations. Did need to
extend patchy fog out west for another hour, but that should be
sufficient as visibility is improving rapidly out there.

One place I did have to put a little more effort into is rain
chances. This will be a tricky one - short range model guidance is
all over the place, and few models are doing well even in the
first few hours. However, given the soft appearance of the current
MCS over South Texas, am comfortable in pulling back even more on
rain chances for today, and focusing potential more on tomorrow
with the next shortwave currently over Mexico. That said, if the
MCS can survive and manage to be re-energized by heating this
afternoon, there`s still some potential for rain and even some
strong storms down around Matagorda Bay late this afternoon
and very early this evening. This is likely the reason SPC`s Day 1
outlook stretches the marginal risk into our westernmost counties.
So, the slight chance PoPs maybe don`t quite show the true ceiling
to the day`s potential, but I`m not yet seeing satellite imagery
that makes me think that we`ll actually realize any of that
potential.

Now...let`s hope I don`t end up writing a later AFD eating crow
for everything I`ve just said in the last paragraph...

&&
ntence..

[tireman4]

000
FXUS64 KHGX 061832
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
132 PM CDT Mon May 6 2019


.AVIATION...
VFR across the region this afternoon. Watching a line storms near
CRP expand east and northeast. Although some capping is still
evident on AMDAR sounding am thinking this feature holds together
and weakens as it moves toward the Metro hubs. Not particularly
quickly either given the steering winds. Have introduced VCSH to
the southern TAF sites starting around 01z through 06z. Very
moist air streaming into the area beneath the cap overnight should
produce a widespread deck of IFR/MVFR ceilings that continues well
in the mid morning hours. Next potent shortwave should be
approaching from the southwest late tomorrow morning and will be
carrying either VCSH or VCTS for the area after roughly 16z.
45

&&

[srainhoutx]

Concern growing that we may have some flooding issues over the next week or so. The afternoon Updated Quantitative Precipitation Forecast suggests some locations may receive 10 to 15 inches of rainfall into next Sunday. While there is some concerns about severe weather, the potential flood threat must not get lost in our messaging.

[Pas_Bon]

I can say that driving from Conroe to SW Louisiana (along 105) and back again the past couple days....there are a lot of very swollen creeks/bayous/rivers throughout. I worry greatly about the amounts of rain forecasted here.

[Cpv17]

Strong rain signal showing up during the May 14th-18th timeframe to go along with this weeks threat.

[Katdaddy]

A beautiful satellite image of this morning’s S TX mesoscale convective vortex now dissipating and drifting along the Middle and Upper TX. Radar shows some light showers WSW of Houston that are fading away.

A challenging weather forecast for the rest of week and through next weekend. I have the upmost respect for the NWS forecasters. Strong to severe thunderstorms and heavy rain especially toward the end of the week and next weekend will be possible but the timing and locations will be difficult to pin-point. A portion of this afternoon’s Houston-Galveston NWS Area Forecast Discussion sums up this challenging forecast period:

As a quick summary: 1) it`s May; 2) an upper low slowly makes its way across the southwestern US ejecting multiple vorticity packets/shortwave troughs ahead of it while southwest flow continues aloft; 3) that stalled front I mentioned earlier? It`s not going anywhere, and will be oriented
roughly parallel to the flow aloft; 4) at times, inflow off the Gulf will increase to 30-40 knots; 5) Moisture levels stay around or above the 90th percentile. I imagine if I keep going, I could come up with the 10 Heavy Rain Commandments, set it to music, and try to start a Broadway career.

[Ptarmigan]

The event is more concerning as it looks to be more of a heavy rain and flooding event. Flooding often occurs in May.

[DoctorMu]

tired of seeing this same picture every week.

I've lost track. We've had to have had a year's worth of rain the last 6 months. Every week since the beginning of October.

I. will. not. complain...and bring the wrath of the Death Ridge this summer!

[Cpv17]

tired of seeing this same picture every week.

I've lost track. We've had to have had a year's worth of rain the last 6 months. Every week since the beginning of October.

I. will. not. complain...and bring the wrath of the Death Ridge this summer!

Sheesh! Not here! We’ve only had about half of what you guys have had, if that lol it’s hard to get rain south of 10 sometimes.

[DoctorMu]

Blake's Tweet

OUCH. Feast or famine... Ground is saturated and rivers near banks. Watchout downstream. Brazoria, I'm looking at you.

[DoctorMu]

Looks even worse on the contour map. A WIDE swath of 9+ inches of rain. Really bad news for those flood prone.

[unome]

Days 1-7 QPF https://www.wpc.ncep.noaa.gov/qpf/24hrqpfall.html
Object Oriented Verification (I like this) https://www.wpc.ncep.noaa.gov/verificat ... age=page-1

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Tue May 07 2019

Day 1
Valid 12Z Tue May 07 2019 - 12Z Wed May 08 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER CENTRAL AND
EASTERN KANSAS...

...Plains into the MS Valley...
Another active convective day expected across the southern and
central Plains into portions of the mid MS Valley. Some flash
flood threat from the ongoing activity will probably linger past
12z across portions of eastern KS and western MO, but in general
this activity should be on a downward trend into mid morning. The
bigger threat comes from what develops this afternoon into the
overnight hours. The ingredients are in place for another round of
heavy rainfall and flash flooding across portions of central and
eastern KS. Will continue to have a stationary boundary draped
across the region. 850mb moisture transport will again increase
this evening into this boundary, probably to even higher levels
than this past night. CAPE and PW values will also be higher than
this past night. Synoptic forcing will be stronger as well, with a
strong shortwave ejecting east into the Plains and increasingly
diffluent flow aloft between the northern and southern stream
jets.

Expect we will see supercell development along the dryline across
the TX/OK Panhandles this afternoon. These will likely grow
upscale into convective clusters as they progress east
northeastward. Also likely to see convective clusters develop near
the frontal boundary draped across KS as moisture transport
increases into the boundary by this evening. Thus once again
appears to be a favorable environment for upscale convective
growth as cells merge near the aforementioned front. The upper
bound of the flood threat is likely limited by the progression of
this system by tonight...as expect convection to eventually
organize and propagate quite quickly off to the east. However,
still should have a long enough period of merging cells before
this happens to result in a widespread flash flood threat across
KS. Areal averaged rainfall of 2-3" appears likely, with pockets
of 3-5" probable. Given the saturated ground across much of this
region, this will likely be enough to cause another round of
widespread flash flooding...some of which could be significant in
nature. Thus will expand the previously issued Moderate Risk to
cover much of central and eastern KS.

A Slight Risk extends south into much of OK and north central TX.
Discrete supercells that form over the TX Panhandle will track
eastward into western OK this evening, likely clustering and
growing upscale by this time. Meanwhile note increasing southerly
850mb moisture transport and convergence, especially after 03z.
This, combined with the ejecting shortwave, should result in the
development of a squall line across portions of OK/TX...with a
period of backbuilding/training likely for a time before a more
progressive eastward propagation takes over. This supports at
least localized 3-5" rains across portions of central/southern OK
into north central TX. Gave some consideration to a Moderate risk
given the likelihood of at least some flash flooding across these
areas. However, think the combination of a bit drier antecedent
conditions compared to KS, and the eventual progressiveness of the
line capping the upper bound of rainfall magnitudes...results in
the threat being best characterized by a Slight risk at this time.
Will continue to monitor observational and model trends through
the day. Overall, like the idea and evolution suggested by the 0z
NMMB and 00z/06z HRRR simulated reflectivities the most over this
region.

Opted to remove the Slight risk over south TX and LA with this
update. An isolated flash flood risk still certainly exists, but
the lack of a focus/forcing mechanism is expected to keep heavier
convection, and thus also the flash flood risk, localized in
nature. This is also generally supported by model QPFs, which have
also trended downward across this area.

Chenard


Day 2
Valid 12Z Wed May 08 2019 - 12Z Thu May 09 2019

...THERE IS A MODERATE RISK OF FLASH FLOODING OVER PORTIONS OF
NORTHEAST TEXAS INTO SOUTHWEST ARKANSAS...

...East Texas into the Lower Mississippi Valley...
Deep moisture and instability ahead of a frontal boundary crossing
the Southern Plains into the Lower MS Valley is expected to feed
convection that produces excessive rainfall, mainly after 09/00z.
There is better agreement with the synoptic setup here that recent
days, so the WPC QPF and Excessive Rainfall Outlook were based
primarily on a multi model blend.

An initial convective complex (depicted by the 00z NAM CONUS Nest)
is expected to cross northeast TX into southwest AR between 09/12z
and 09/18z, weakening as it moves out of the best instability
axis. in the plume of 1/75/2.00 inch precipitable water air, the
00z NAM CONUS showed the potential for
2.00+ hourly rainfall rates before 09/18z. The rainfall is
expected to prime the area for flash flooding ahead of the front.

After the initial convective complex weakens/exits, short wave
energy crossing the Central Plains is expected to spin up surface
low pressure that tracks into the Upper MS Valley. A cold front
trailing the low is expected to become the focus for deepening
moisture (precipitable water values peak between 1.75.200 inches)
transports out of the Gulf of Mexico on a 25/35 knot low level
flow. Convection is expected to form on the front after 08/18z,
and the line of convection should move across east
TX/northern LA into southwest and central AR after 09/00z.

After that time, as the 850-300 mb mean flow becomes better
aligned with the propagation vectors, cells along the line are
expected to track northeast as the line itself moves east. The
training cells could produce hourly rainfall rates near 1.50
inches between 09/00z and 09/06z, primarily over far northeast TX
and southwest ARR. There is a multi model signal for 2.50/5.00
inches of rainfall over this axis (where the 00z ECMWF has a
bullseye of 6.50 inches, not far from where the 00z UKMET has over
10 inches of rain).

To reach these amounts, the convection ahead of the front needs to
develop in an airmass that could be overturned by the morning
convection (requiring two rounds of convection). There is a strong
model signal for rainfall amounts over 4.00 inches here (and the
00z GEFS showed members with 4.00+ rainfall on this axis as well).
After collaborating with WFOs SHV/LZK, a Moderate Risk was placed
over far northeast TX into southwest AR.


...Mid and Upper Mississippi Valleys into the Upper Great Lakes...
Short wave energy tracking from CO/NM to the Upper MS Valley aids
in spinning up surface low pressure over western KS, which travels
along a frontal boundary into IA/IL by the end of Day 2. Deepening
moisture and instability ahead of the surface low and frontal
boundary feeds convection that poses a flash flood threat,
especially after 09/00z. There is better agreement with the track
of the surface low and placement of the front, so the WPC QPF and
Excessive Rainfall Outlook were based on a multi model blend.

As the surface low tracks from southwest KS to a position over
IA/IL, a 35/45 knot low level southwest flow transports 1.25/1.50
inch precipitable water air (which is about two standard
deviations above the mean) across MO into IL, ahead of the mid
level short wave. Instability is transported north in the moisture
plume, maximizing over eastern KS into southern IA in the 09/00z
to 09/06z time frame. Convective segments are expected to develop
in this environment, tracking across the abovementioned locations.
There was enough commonality in the warm sector (near the front)
to place a strip of 1.50/2.25 inch of rainfall from eastern IA
into southern WI. Three hour flash flood guidance values here are
as low as 1.50/2.00 inches. Since the potentially training
convection could exceed these values, a Slight Risk area was
extended eastern IA into far southern WI and far northwest IL.

North of the front and surface low, an axis of developing
frontogenetic forcing is expected to maximize between 08/18z and
09/06z across portions of southeast MN into northern WI and the
lower peninsula of MI. Instability is expected to be lacking,
which should keep rainfall amounts at or below flash flood
guidance values. The heavy rainfall could be more of a flood than
flash flood threat, so a Marginal Risk was placed in these areas.


Hayes

Day 3
Valid 12Z Thu May 09 2019 - 12Z Fri May 10 2019

...THERE IS A SLIGHT RISK OF FLASH FLOODING FROM SOUTHEAST TEXAS
ACROSS THE LOWER MISSISSIPPI VALLEY INTO THE TENNESSEE VALLEY...

...Southeast Texas into the Tennessee Valley...
As surface low pressure tracks into the Upper Great Lakes, its
trailing cold front slows as it crosses the TN Valley into
southeast TX during Day 3. Ahead of the front, deep moisture and
instability along the front feed convection capable of producing
excessive rainfall. There was generally good model agreement
concerning the overall synoptic setup, so the WPC QPF and
Excessive Rainfall Outlook were based on a multi model blend.

The surface front slows as it crosses the TN valley into TX, as
the front becomes more parallel with the mid level west southwest
flow. Ahead of the front, a 20 to 30 knot low level southwest flow
transports 1.50/1.75 inch precipitable water air from southeast TN
into western TN, and the moisture plume drops southeast with the
front. Convection is expected to form in the axis of best
instability (with MLCAPE values between 2000-3000 J/KG), and this
much instability could mean that the initial storms could be
outflow driven.

However, as the front slows approaching MS/northern LA/southeast
TX after 10/00z, training becomes more of a possibility as the
850-300 mb mean flow becomes better aligned with the propagation
vectors (as the column moistens). Between 10/06z and 10/12z, the
main focus for flash flooding shifts to southeast TX, as the low
level flow becomes more orthogonal for the slowing front. There is
a multi model signal for 2.00/4.00 inches of rainfall here (with
the highest amounts mostly depicted north of the Houston metro
area), and these numbers are supported by members of the 006z GEFS
indicated 4.00+ inch rainfall amounts over southeast TX. At this
distance, it is difficult to pinpoint where the highest rainfall
amounts will occur, as mesoscale interactions may ultimately
determine the locations. Because of this (and after collaborating
with WFO HGX), there was no Moderate Risk assigned to southeast
TX. Instead, a long Slight risk area was stretched from southeast
TX across northern LA into western KY/western TN.


...Ohio Valley/Northeast...
Moisture and instability surging northeast along a frontal
boundary feed convection that could produce heavy to locally
excessive rainfall, especially during the afternoon and evening
hours of Day 3 from the Ohio Valley to portions of the Northeast.
Other than the 00z NAM (which appears to be too fast with the
front), there was good model agreement with the timing of the
synoptic scale feature. Based on this, the WPC QPF and Excessive
Rainfall Outlook were based on a multi model blend.

Ahead of the front, 500-1000 J/KG of MLCAPE is expected to extend
from KY/OH across western PA/western NY between 09/18z through
10/06z. A 30 to 40 knot low level low level southwest flow
transports 1.25/1.50inch precipitable water air (which is between
two and three standard deviations above the mean) along the front.
Convection developing in the low level convergence near the front
is expected to tap the deepening moisture as it forms line
segments and clusters from north central KY into western
PA/western NY. Rainfall amounts of 0.75/1.25 inch rainfall amounts
are expected with the convection, which is expected to remain
progressive, based on the 00z GFS propagation vectors.

Three hour flash flood guidance values are generally 1.50/2.00
inches along the front, and for the most part, rainfall amounts
are expected remain below these levels. However, short term
training or cell mergers along the front could pose a flash flood
threat were guidance values are lower. Based on the above, a
Marginal Risk was extended from KY/OH across western PA/western NY
into far northern New England.


...California...
Instability and deep moisture (for CA) are expected to feed low
topped convection that could produce heavy to locally excessive
rainfall across the lower elevations of central CA during Day 3.
As a closed mid level low forms over central CA, steepening lapse
rates are expected to result in 500-1000 J/KG of MLCAPE, generally
from 09/21z through 10/03z. Precipitable water values of 0.75/1.00
inches (which are about three standard deviations above the mean)
in place during convective initiation will feed low topped
convection in the afternoon and evening hours.

As the closed mid level low passes overhead, the steering flow
drops below 10 knots, which could result in weak storm motions.
This could mean short term training or cell mergers, before the
convection become more outflow dominated. There is a multi model
signal for local rainfall amounts to approach 1.50 inches, and
these amounts are plausible where training or cell mergers occur.
Based on this, a Marginal risk was placed over a portion of
central CA, for runoff issues, and a low end flash flood threat.

Hayes


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt

[unome]

reading HGX's front page today:

Local Weather History For May 7th...
An F-1 tornado touched down near South Houston on this date in 1976. The tornado was on the ground for 13 miles and
tracked across Pasadena and Deer Park before dissipating at the entrance to the ship channel. There was one injury
associated with the storm and damages were estimated at $500,000 dollars.

yowza, can you imagine what that same tornado would do to our area 43 years later?

[Cpv17]

WPC has slightly cut back on rain totals.

[srainhoutx]

Our friend the cap has broken. Showers and some thunderstorms look to increase throughout the day before diminishing this evening. I don't expect too much in the way of heavy rainfall today, but heavy rainfall chances and the strong too severe thunderstorm threat does increase tomorrow into Thursday.

Regarding the heavy rainfall threat. The best chances for rounds of heavy rainfall look to be Thursday night extending into at least Saturday. The GFS is a tad more aggressive with pushing the frontal boundary near the Coast Saturday morning while the ECMWF hangs the frontal boundary to our NW just S of the I-35 Corridor near Austin. One thing I am noticing is that the GFS has increased the rainfall amounts centered along and either side of the I-10 and I-69 (59) Corridors. Both the GFS and the ECMWF suggest thunderstorms may train across portions of our Region and that raises and eyebrow particularly Friday into Saturday. I do expect day to day changes in our sensible weather forecast and I do believe that mesoscale features such as outflow boundaries, cold pooling and other features that may ripple across the area lend to a lot of uncertainty. Each round of rainfall that begins today set the stage for further ground saturation and flood wave rises on our area Rivers that are already swollen. Pay close attention to what falls to our North today and this evening. That water will make it's was South toward the Gulf in the days ahead,

[djmike]

Yeah WPC lowered our rain chances too for today. Went from 50% to 30% for today. Tomorrow should pick up.

[Andrew]

It is not going to be a good day for the Panhandle, SPC has upgraded the tornado risk to a 15% hatched for Amarillo and northeast of Lubbock. Large hail and tornadoes are going to be a serious threat to the region. Overnight, that will gradually shift to the southeast and is expected to weaken over Central Texas. It will be interesting to see how far south the line makes before weakening. some of the higher resolution models indicate it could get pretty close to SE Texas.

Overall, the largest threat for flooding and some severe weather remains towards the end of the week. In particular, Thursday-Saturday looks wet with widespread 3-5 inches expected. Isolated totals of 8-10 inches aren't out of the question though. It all really depends on where the front stalls, because that will be the focus.

[jasons2k]

Storms to our SW aren’t holding together (yet). Things are heating up though...

[srainhoutx]

NWSWGRFC‏Verified account @NWSWGRFC · 36m36 minutes ago
Rain returns, and so does the flood threat. New flooding expected across N and NE TX, with flooding possibly getting worse on the flood waves moving down the rivers. Be Flood Aware! Be Flood Prepared! #txflood