SEPTEMBER 2018 - Very Wet Month Wrap Up

[Texaspirate11]

Yesterday my area near the bay got 2.5 inches of rain and a power outage...good times.

[Katdaddy]

More tropical showers and thunderstorms this afternoon. 118 years ago today many Galvestonians were at the beach “watching the GOM overflow”. Little did they know “a night of terror” was approaching.

[srainhoutx]

Saturday afternoon briefing from Jeff:

NWS has issued a Flash Flood Watch for the NW 1/3rd of SE TX from 700pm this evening until 700am Sunday. This watch includes the following counties: Houston, Grimes, Madison, Brazos, Burleson, and Washington.

A slow moving frontal boundary currently along the I-35 corridor will progress slowly into the extreme NW portions of SE TX this evening and slow down even more. Significant amounts of tropical moisture will be pooling over the region tonight and help to focus rounds of showers and thunderstorms with excessive rainfall along a line from near Brenham to Madisonville. While low level inflow is not particulary strong…it is there and enough to help focus the rainfall and feed moisture northward. Will have to watch closely for any cold pool generation and outflow boundaries that could expand the area of heavy rains southward early Sunday…should this happen the flash flood watch would need to be extended southward toward the US 59 corridor.

Rainfall amounts across the watch area tonight will average 1-3 inches with isolated totals of 5 inches possible.

Sunday-Monday:
Weakening frontal boundary and axis of heavy rainfall will sag southward into the central portions of the area either along the US 59 or I-10 corridors. Heavy rainfall will again be possible especially across the central portions of the area Sunday afternoon and evening and a Flash Flood Watch may be needed.

Sunday night into Monday the frontal boundary washes out and becomes a coastal trough which will help focus heavy rainfall across our southern areas (mainly SE of US 59) into Monday. Again flash flooding will be possible. Storm totals Sunday and Monday will likely average 1-3 inches over the area with isolated totals of 4-6 inches possible.

This is an evolving weather situation and will require close monitoring over the next 48 hours as the pattern favors heavy rainfall over the region.

[srainhoutx]

Watching radar carefully for the B/CS Area. Lightning may be nearing Kyle Field from the cell to the SW heading NE towards Bryan/College Station.

[jasons2k]

I think it’s gonna rain, but right now I’m in the middle of an arc of no rain. It’s arcing around me to my north and south, like I have a dome right over me.

Edit: pouring now

[DoctorMu]

Watching radar carefully for the B/CS Area. Lightning may be nearing Kyle Field from the cell to the SW heading NE towards Bryan/College Station.

Light rain, but the only lightning was Kellen wins low's frozen rope passes.

The conditions were tolerable for an Epic classic.

[Katdaddy]

A very long night was ongoing 118 years ago on Galveston Island. Words and photos cannot express what the morning brought.

[srainhoutx]

All eyes to our West and SW as 2 MCV's (Mesoscale Convective Vortex) move generally East throughout the day increasing our chances for a heavy rainfall event potentially beginning this evening/overnight and continuing into Monday and possibly Tuesday.

Mesoscale Precipitation Discussion 0811
NWS Weather Prediction Center College Park MD
557 AM EDT Sun Sep 09 2018

Areas affected...SRN TX

Concerning...Heavy rainfall...Flash flooding likely

Valid 090957Z - 091457Z

Summary...Heavy rainfall was escalating in coverage this morning
over the Texas Hill Country and south Texas. Flash flooding is
likely, with rapid accumulation of 3-plus inches of rainfall.

Discussion...At 0945Z the number of flash flood warnings from
local offices was increasing ahead of two mesoscale convective
vortices (MCVs) located one near San Antonio and the other along
the Rio Grande to the south. These vortices were the result of
latent heat release from prior overnight convection feeding back
on the weak synoptic mid level flow. An upstream 250-mb trough
axis placed this region under a difluent flow regime aloft,
further supporting organization and gentle lifting of the near
2.25 inch precipitable water values per the SPC mesoanalysis.

All cell motions are analyzed as zero to ten knots based on the
vertical wind profile over the region. Steering flow for the MCVs
will take them generally eastward into the instability axis, with
uncapped mixed-layer CAPE values of several hundred J/kg in the
Hill Country increasing to near 2000 J/kg at the coast.
Additionally, a surface frontal zone increasing in strength was
already leading to an expansion of new convection extending
northeast from San Antonio toward Bastrop.

Instability will gradually be consumed, and without sustained low
level inflow, there is a limit to how much rain will fall at any
given location. But the extremely efficient, tropical environment,
coupled with broad lift and sufficient instability, should yield
some rapid accumulations of 3-plus inches of rain. The 08Z HRRR
and 00Z NSSL WRF were doing particularly well depicting this.
Flash flooding appears likely, especially in the more prone Hill
Country where FFG is also lower. But some lowered FFG also exists
near and north of Corpus Christi where models depict a secondary
maximum of rainfall this morning.

Burke

ATTN...WFO...BRO...CRP...EWX...HGX...

[don]

The NWS is concerned about heavy rain this week and a Flash Flood watch may be issued.

https://forecast.weather.gov/product.ph ... glossary=1

PREV DISCUSSION /Issued 450 AM CDT Sun Sep 9 2018/...

.DISCUSSION...

Showers and thunderstorms are expected today mainly along and
south of a frontal boundary stalling across the area. An upper
level trough should develop over Texas tonight into Monday. This
may support a period of enhanced heavy rainfall potential Monday
through Tuesday. A flash flood watch may be needed during this
time frame.


.NEAR TERM [Through Today]...

A weak cold continues to slowly sag southward through the region
tonight, and as of 4am cuts from roughly Lufkin through Brenham,
slicing between Huntsville and Conroe. Additionally, there appears
to be a subtle pre-frontal boundary very near the Gulf coast.
Despite these potential foci for convection, the radar scope is
pretty quiet over our area, but is more active further northeast
along the front in Arkansas and Louisiana, but particularly to the
southwest along the front, in Central/South Texas where there is
much heavier rain.

Upper air data show that our current lack of rainfall is plausibly
due to our awkward placement in regards to the upper jet streak,
whose circulation patterns would support the heavier rain areas
mentioned above. As the upper trough rotates through, expect that
more favorable entrance region of the jet to move eastward closer to
us, supporting the expansion of rainfall from Central Texas into
Southeast Texas. Look for the front and coastal boundaries to be
preferred locations of development, eventually merging at a distance
from the coast roughly in line with the Houston metro. Rain chances
look quite high as this occurs this afternoon in an environment with
at least two inches of precipitable water. On the plus side, upper
jet placement is still fairly sub-optimal, the low level jet is
weak, and temperature gradients are fairly weak. This...may not
always be the case at times this week...but for today it is. As a
result, I`d expect a scenario fairly similar to yesterday, with the
strongest, most efficient cores able to quickly drop an inch or two
or rain, and potentially cause localized minor flooding issues. But,
for the most part, showers and storms should be fairly manageable
though likely somewhat more numerous than yesterday.

All of this is subject to change depending on the evolution of
rainfall complex to the west as a flash flood warning has been
issued for Medina and Bandera counties, where a small mesoscale
convective vortex appears to be developing.

Luchs


.SHORT TERM [Tonight Through Tuesday]...

Look for a bit of a lull in the rain into this evening as the sun
sets and we lose a good portion of our instability. However, with
the front continuing to sag near the coast, we`ll begin to see the
development of a convergence axis between only modestly drier
offshore winds behind the front, while onshore flow continues to
flow in from over the Gulf. Without a strong push of drier air from
the north, don`t look for precipitable water values to be going
anywhere.

All of the models hint at the development of at least a
coastal trough along the Gulf coast by Monday evening. Some, like
the Texas Tech WRF, go full bore into developing a mesoscale
convective vortex along the axis as a shortwave trough embedded in
the broader synoptic trough scoots across the area. This is
concerning to say the least. Despite the impressive amounts of
moisture and focal boundaries, the poorer synoptic setup has been a
bit of a saving grace. The development of a coastal trough or MCV
would significantly increase surface convergence at the same time
that the arrival of a shortwave trough aloft and the positioning of
the right entrance region of the upper jet will boost the dynamics
at upper levels. While temperature gradients are still pretty weak,
by Monday evening the models do suggest a bit more of a difference
in temperatures across the frontal zone than we saw last night.

All of this indicates a potential for an earlier start to convection
on Monday, greater coverage of showers and storms into the
afternoon, and by evening...the possibility that a number of the
boxes checked for heavy rain events we`ve seen in the past. Concern
is amplified by the fact that we`re seeing a similar setup already
occurring tonight to the west. Now, with all of this said, many of
these details are going to be driven by mesoscale features...and at
a range of 24-36 hours are not going to be very predictable. This
discussion is hedging a bit towards a reasonable worst case here
in order to explore the potential of a situation we`d be looking
for, but with greater than normal uncertainty.

Tuesday`s heavy rainfall potential will highly depend upon what
occurs on Monday and the evolution of the mesoscale. I really do
not want to speculate on what the mesoscale could look like as
there are already a variety of solutions among the synoptic
models. One thing that the GFS/NAM/ECMWF/Canadian do agree on is
that the overall pattern does support a continued heavy rainfall
threat and possible impacts from flooding.

Aloft there will be a broad trough over much of Texas and the jet
pattern still puts a region of divergence over SE Texas with the
right entrance region. The problem will be timing any small
vorticity maxima that move over the region. Some of these could be
mesoscale forced. There will still be 2 to 2.3 inch precipitable
water values over the area which will support recovery from any
convective overturning. Another challenge will be determining how
much boundary layer inflow will be possible with this system.
Model suggest 850mb winds around 10-15 knots but any mesoscale
enhancement could put these winds over 20 to 25 knots. Flow
associated with a coastal trough may also increase inflow. This
increase in low level shear could further increase rain rates in
organized convective clusters. At this point, it will not be the
amount of rain that falls, but how quickly and where. Those kinds
of details are just beyond the capabilities of the high resolution
models so future shifts will really need to hone in on these
details. Likely this means that if any kind of flash flood watch
is issued for Monday may need to be extended. Impacts from
flooding could become worse or more widespread instead of
isolated.

Luchs/Overpeck


.LONG TERM [Wednesday Through Sunday]...

Overall a wet pattern will continue Wednesday through Friday.
Troughing aloft will remain over the area despite a 500mb ridge
developing from the southern Rockies into the Plains. The ECMWF
and to a lesser extent the GFS bring a tropical wave into the area
during the Thursday/Friday time frame. Given large scale lift
over the region from the troughing and moisture not decreasing
(PWAT remain around 2 to 2.3 inches) the threat for heavy rainfall
will continue. This means the forecast will continue to hit PoPs
with 50 to 70 percent. Depending upon how much rainfall occurs
between now and Tuesday, there could be increased flooding impacts
to monitor. Rises in area rivers will be possible. Right now
rainfall amounts look to be on the order of 1 to 2 inches on top
of what might fall in the Sunday to Tuesday time frame. Highest
amounts will be basically Houston southward to the coast. Saturday
and Sunday may be some drier with PWATs dropping to 1.8 to 2.0
inches but that may still support some showers/thunderstorms with
lower 500mb heights over the area.

Overpeck

[jasons2k]

There was this update, too:

Area Forecast Discussion
National Weather Service Houston/Galveston TX
642 AM CDT Sun Sep 9 2018

.AVIATION [12Z TAF Issuance]...

Increasing uncertainty in the forecast as a complex of
thunderstorms comes together to the west near San Antonio. The
TAFs for now do not account for this complex, and keep to a
forecast focused on the weak cold front sagging through the area.
But if the complex to the west moves eastward and impacts any of
the terminals, amendments - potentially significant - will be
needed.

[srainhoutx]

Sunday morning briefing from Jeff:

Threat for heavy rainfall and flooding will be increasing over SE TX today through Tuesday.

Flash Flood Watches will likely be required for portions of the area…possibly as soon as today.

Several ingredients continue to come together that will likely result in periods of heavy to excessive rainfall across SE TX starting today and lasting into the early part of the week…in fact it looks wet through much of the next 5 days. A slow moving frontal boundary currently along a line from near Lufkin to Brenham to San Antonio will drift southward today and combine with deep tropical moisture in place (PW 2.1-2.3 inches). Thus far the lack of good low level inflow (flow of gulf moisture into the region) and sustained large scale lift has helped keep rainfall more scattered than widespread over SE TX. This appears to change tonight into Monday and Tuesday as an upper level trough gradually develops over the region helping to provide additional lift and the potential for the formation of convective induced areas of low pressure (MVC…meso vorticity center) to focus, train, and sustain rainfall.

Over time the frontal boundary which is already weak…will lose its definition and transition into more of a near inland coastal trough which will establish along a line roughly from Matagorda Bay to NW Galveston Bay and provide as focus for storm formation. Flash flooding has already been produced by this setup over central and north Texas both yesterday and again overnight. While there is no strong signal pointing toward a widespread flood threat over the area…several of the ingredients that have historically produced excessive rainfall and flooding over SE TX will be in place over the next few days.

Rainfall totals today-Tuesday could average 1-3 inches with isolated amounts of 4-5 inches. These totals could be on the low side especially if any areas of sustained training were to develop in such a tropical air mass that is in place. Grounds are not as dry as they were last week as the area has seen several days of scattered showers and thunderstorms so the run-off potential will be increasing.

[Rip76]

That area south of Cuba looks very interesting.

[don]

Yes, several of the Euro ensembles develop it north of the Yucatan this week

[srainhoutx]

GOES 16 Water Vapor imagery suggests that wind shear is an inhibiting factor for that area of disturbed weather nearing the Western Caribbean Sea. You'll notice a TUTT (Tropical Upper Tropospheric Trough) meandering generally West from the Eastern Gulf. Conditions may be come a bit more hospitable as the twave enters the Bay of Campeche next week.

[ticka1]

That area south of Cuba looks very interesting.

I just saw that is it a low pressure or upper level low?

[srainhoutx]

Tough forecast for our Area with many of the meso models performing very poorly. HGX is sticking with the likelihood of a potential Heavy Rainfall Event over the next 36 to 48 hours. Keep weather aware and stay away from any high water!

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HOUSTON/GALVESTON TX
1053 AM CDT SUN SEP 9 2018

.UPDATE...
GEARING UP FOR WHAT WILL LIKELY BE A COUPLE OF ACTIVE DAYS AS
PERIODS HIGH RAINFALL RATES INCREASE THE TOTAL PRECIPITATION
ACCUMULATION AND ULTIMATELY THE FLASH FLOOD THREAT. DUE TO DRY
10-40CM DEPTH SOIL, MANY CAN HANDLE A QUICK 1 TO 2 INCH PER HOUR
RATES. IT IS WHEN WE BEGIN EXPERIENCING GREATER THAN PER 2 INCH
PER HOUR RATES THAT URBAN STREETS AND LOW LYING (RURAL ROADS)
AREAS WILL BEGIN TO FLOOD. THE OVERALL FORECAST IS FOR AREAWIDE
LIKELY RAIN CHANCES WITH THE MAIN NEAR AND SHORT TERM CHALLENGES
BEING ON WHERE THESE POCKETS OF GREATER THAN 2 INCH TOTALS WILL
EVOLVE TODAY/MONDAY. A COASTAL TROUGH IS ALREADY IN PLACE AND AIR
MASS MOISTURE WAS ABOVE THE 75 PERCENTILE (CLIMATOLOGICALLY) AT
2.0 TO 2.2 INCH PRECIPITABLE WATERS. THE NWP HIGHER RESOLUTION
MODELING WANTS TO FORM AN MCV-LIKE FEATURE ALONG THE COASTAL BEND
WITH MANY OF THE MEMBERS PEGGING BOTH THE COASTAL COUNTIES (COASTAL
TROUGHING/MCV?) AND THE NORTHERN TIER COUNTIES (VICINITY OF A
STATIONARY BOUNDARY) AS THE MAIN FOCUS AREA FOR CLUSTERS OF
HIGHER PRECIPITATION. THERE ARE MANY FACTORS THAT ARE KEEPING US
UNDER THE GUN FOR A POTENTIAL FLOOD EVENT IN THE NEXT 36 TO 48
HOURS...ONE BEING BETTER UPPER LEVEL FORCING. THE SLOW EASTWARD
PROGRESSION OF THE UPPER LEVEL TROUGH (AXIS OVER EASTERN OK THIS
MORNING) AND AN APPROACHING 25H SPEED MAX, ALONG WITH WEAK NE
TEXAS 25-3H DIFFULENCE, SUGGEST AN INCREASING PROBABILITY OF THE
OVERALL ENHANCEMENT OF AREAWIDE RAINFALL WITH LOCALLY HEAVY
DOWNPOURS THROUGH AT LEAST TUESDAY. OF COURSE, WHATEVER OCCURS
TODAY WILL DETERMINE WHAT OCCURS TOMORROW AND SO FORTH AND SO ON.
AS OF NOW, STICKING TO A HIGH POP/RELATIVELY HIGH QPF FORECAST THAT
WILL BE FOCUSED ACROSS THE THE COASTAL AND FAR NORTHERN COUNTIES
TODAY, THEN TRANSITION INTO THE CENTRAL CWA COUNTIES THROUGH MONDAY
AFTERNOON. WE WILL BE MONITORING THE ONGOING ACTIVITY OVER THE NEXT
6 HOURS TO DETERMINE WHETHER WE NEED TO ISSUE A FLASH FLOOD WATCH
OVER MORE AT-RISK COUNTIES. 31

[Ptarmigan]

That area south of Cuba looks very interesting.

I just saw that is it a low pressure or upper level low?

Atlantic Tropical Weather Discussion
https://www.nhc.noaa.gov/text/refresh/M ... WDAT.shtml?

Code: Select all

...DISCUSSION...

GULF OF MEXICO...

An upper level low is centered over the E Gulf of Mexico near
25N87W. Scattered showers are E of 87W. An upper level ridge is 
over the W Gulf W of 92W with northerly upper level flow. 
Scattered showers are over the NW Gulf and the Texas coast W of
94W. At the surface, 5-10 southerly flow covers the Gulf. 

Weak ridging from the Carolinas to the Texas coast will shift N 
over the SE United States. A trough will move west off the Yucatan
coast each evening and dissipate over the SW gulf each morning. 
The trough will be accompanied by moderate to fresh winds. 

CARIBBEAN SEA...

A large upper level high is centered over the central Caribbean
near 16N75W. A tropical wave is the main source of convection in 
the W Caribbean. See above. 

Isaac is forecast to approach the Lesser Antilles as a hurricane
Thursday, and continue into the eastern Caribbean Friday. A 
relatively weak surface ridge in the western Atlantic Ocean will 
maintain moderate to fresh trade winds over most of rest of the 
region through Monday night. Building high pressure in the eastern
Gulf of Mexico will allow fresh to locally strong E to SE winds 
and building seas in parts of the NW Caribbean Sea from Sunday 
through Tuesday.

ATLANTIC OCEAN...

Please read the SPECIAL FEATURES section for details about
T.S. Florence, T.S. Isaac, and T.S. Helene.

Isolated moderate convection is over the W Atlantic from 26N-30N 
between 72W-79W to include the N Bahamas. A small 1016 mb high is
over the W Atlantic near 25N68W. A small upper level low is near 
30N70W. A 1013 mb low is centered near 30N70W. A 1025 mb high is 
over the E Atlantic near 34N27W.

For additional information please visit 
http://www.hurricanes.gov/marine

$$
Formosa

Looks like a tropical wave.

[unome]

Looks like a tropical wave.

si

[srainhoutx]

Sunday afternoon briefing from Jeff regarding Disturbed Weather in the NW Caribbean Sea:

Tropical wave over the NW Caribbean Sea is showing signs of better organization today.

NHC 200pm Tropical Weather Outlook now places a 20% chance of tropical cyclone formation from this wave across the central and western Gulf of Mexico this week. While deep thunderstorm activity has developed with this wave axis, there are no signs of any closed low level circulation and upper level winds at least for the next 1-2 days will likely prevent any sort of sustained development. However, as this feature moves WNW/NW into the central Gulf of Mexico upper level winds are forecasted to become increasingly favorable for development.

A review of global model guidance and ensemble trends do show some modest support from both the operational and ensemble members for tropical cyclone formation over the Gulf of Mexico. The same large scale blocking high pressure system that will be forcing Florence toward the SE US will also likely force any Gulf development toward the WNW/NW or toward the western/northwestern Gulf of Mexico this week.

Residents along the TX coast should maintain a close watch on this feature over the coming days.

[srainhoutx]

The 12Z ECMWF suggests a weak surface low attempts to organize just off the Middle Texas Coast next Friday morning