SEPTEMBER 2018 - Very Wet Month Wrap Up

[don]

The CMC actually does slow the system down in North Texas it's just not as dramatic as the Euro, the latest 0z UKMET looks very similar to the Euro fwiw

[Andrew]

The CMC actually does slow the system down in North Texas it's just not as dramatic as the Euro, the latest 0z UKMET looks very similar to the Euro fwiw

Yea it does and it makes sense as both the trough and ridge flatten out. Now if that was to happen faster then I could see a stall closer to the coastline. Definitely don't discount that idea at all.

[javakah]

* Formation chance through 48 hours...low...30 percent.
* Formation chance through 5 days...medium...60 percent.

Forecaster Pasch

And the numbers keep getting bumped up.

[Andrew]

ECMWF is further west initially but doesn't have as much of a western push after landfall. It stalls over Shreveport and then tracks over the DFW area. Also becomes decoupled rather quickly with a surface low over central Texas but 850v over Northern Texas. Overall, the whole system is weaker for this run.

EDIT: A lot of EPS members still take the system west. This will be something we have to watch especially if the system remains weak, that models continue pushing more and more west before landfall.

[Andrew]

In the short term, HGX has mentioned in their discussion that a Flash Flood Watches will likely be required for later tonight into tomorrow. They are worried about some locations possibly receiving 8-10 inches (especially over far Southeast Tx). FFW have already been issued east of the region so I wouldn't be surprised if that occurs later this morning. We are also under a slight risk for excessive rainfall, and this is all due to a trough currently off the coast that is expected to move inland later today. The largest concern would likely be tomorrow morning as a line of thunderstorms could setup somewhere over SE Texas and slowly move through. The TX-Tech WRF showed a pretty strong setup in this timeframe and the 36H HRRR is also hinting at a similar solution.

[srainhoutx]

TROPICAL WEATHER OUTLOOK
NWS NATIONAL HURRICANE CENTER MIAMI FL
800 AM EDT SUN SEP 2 2018

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO:

THE NATIONAL HURRICANE CENTER IS ISSUING ADVISORIES ON TROPICAL
STORM FLORENCE, LOCATED SEVERAL HUNDRED MILES WEST OF THE CABO
VERDE ISLANDS.

DISORGANIZED SHOWERS AND THUNDERSTORMS OVER THE SOUTHEASTERN AND
CENTRAL BAHAMAS AND ADJACENT ATLANTIC WATERS, INCLUDING THE STRAITS
OF FLORIDA, ARE ASSOCIATED WITH A TROPICAL WAVE INTERACTING WITH AN
UPPER-LEVEL TROUGH. THIS WEATHER SYSTEM IS EXPECTED TO MOVE
WEST-NORTHWESTWARD ACROSS THE REMAINDER OF THE BAHAMAS, SOUTHERN
FLORIDA, AND THE FLORIDA KEYS DURING THE NEXT DAY OR SO AND LITTLE,
IF ANY, DEVELOPMENT IS EXPECTED DURING THAT TIME DUE TO STRONG
UPPER-LEVEL WINDS. HOWEVER, THE CURRENT UNFAVORABLE UPPER-LEVEL
WIND PATTERN IS EXPECTED TO GRADUALLY CHANGE AND BECOME MORE
CONDUCIVE FOR A TROPICAL DEPRESSION TO FORM OVER THE EASTERN AND
CENTRAL GULF OF MEXICO BY TUESDAY OR WEDNESDAY. THIS DISTURBANCE
WILL SPREAD LOCALLY HEAVY RAINS ACROSS MUCH OF THE BAHAMAS, SOUTHERN
FLORIDA, AND THE KEYS DURING THE NEXT DAY OR TWO. SEE PRODUCTS FROM
YOUR LOCAL WEATHER FORECAST OFFICE FOR MORE INFORMATION.
* FORMATION CHANCE THROUGH 48 HOURS...MEDIUM...50 PERCENT.
* FORMATION CHANCE THROUGH 5 DAYS...HIGH...80 PERCENT.

$$
FORECASTER STEWART

[srainhoutx]

First visible imagery this morning suggest the mid level circulation is turning nicely and thunderstorms are firing around that broad mid level low. I suspect we will see this disturbance get tagged INVEST 91L later today.

Regardless of the current track depicted by the various models, HGX mentions just how challenging the forecast for next weekend may be regarding very heavy rainfall. With a full latitude trough setup from the Central Plains in Central Texas, the inflow off the Gulf would continue to pump 2.5 or greater PW's into SE and East Texas. I also want to caution everyone that just because the Operational Global models bring a weak Tropical Storm into SE Louisiana does not mean the forecast for Tuesday/Wednesday is straight forward. As Andrew mentioned, far too many individual ensemble members (Euro = 51 individual members and the GFS = 20+) suggest a more Westward motion may be possible. Continue to drop in a couple of time a day throughout the remaining Holiday period for the latest updates and it wouldn't be a bad idea to let those who may not be so weather savvy know that very heavy rainfall is likely later today into the overnight hours and continuing tomorrow as well as the Tropical Disturbance moving out of the Bahamas toward the Florida Straights.

[srainhoutx]

That didn't take long. INVEST 91L has been designated by the National Hurricane Center.

[jasons2k]

The popup ads from “spreediscount.com” are making the forum almost unusable with my phone.

Anyway, overnight runs of most of the European ensembles were south and west of the operational. Same with the CMC. I think the track and ultimate intensity of this storm is still very uncertain. Lots of moving parts. I would not want to be on the hook to basically have to choose NOLA or Houston with this one.

[TexasBreeze]

I get those pop ups too. Have to close the tab then reload this page to read the posts. Not every time, but annoyingly enough.

The rains forecasted for the next few days will only serve to make things saturated before 91L possibly makes an appearance. Gotta stay vigilant!

[srainhoutx]

Sunday morning briefing from Jeff:

Increasing flash flood threat tonight-Monday.

Flash Flood Watch will likely be required this afternoon for tonight and Monday

Developing mid level low pressure system over the Gulf of Mexico south of the Louisiana coasts is moving slowly W to WNW toward the upper TX coast today. Deep tropical moisture is already moving into the region and this moisture supply will remain in place into late Monday. Several short range models position the mid level feature near or just NE of Galveston Bay late tonight while at the same time developing a surface coastal front (boundary) to focus heavy rainfall along. This boundary looks to extend from near Matagorda County to southern Harris County and then ENE to Jefferson County. Increasing low level inflow off the Gulf of Mexico late this evening into this boundary will help to focus thunderstorms and heavy rainfall. Overall position of this boundary and location of the landfalling mid level low pressure system will determine the areas where the greatest rains will fall. Given some uncertainty will indicate areas SE of a line from Edna to Hempstead to Liberty for the greatest risk…but looks like the best potential will be in and near Galveston Bay (Harris, Galveston, Brazoria, Chambers, and Liberty Counties). This setup appears similar to July 4 of this year, but with the mid level low further to the NE/ENE of Harris County.

Rainfall Totals:
Widespread rainfall amounts of 2-4 inches look likely with significantly higher totals where any cells train for an extended period of time. Isolated higher totals of 6-8 inches will be possible given the extremely tropical air mass in place. Additionally hourly rainfall rates of 2-4 inches will be possible which will lead to rapid onset urban flash flooding. While grounds are generally dry across the region, flash flood decision matrix suggest rainfall of over 4-5 inches in 1-2 hours will result in significant run-off. Forecast trends suggest these amounts may materialize over parts of the area late tonight into Monday. If trends continue into the afternoon hours today…A Flash Flood Watch will likely be required for parts of the area for tonight and Monday.

It is possible in these types of setups that the coastal boundary develops near the coast or just offshore and focuses the heavy rainfall in that area and areas further inland do not have a good supply of Gulf moisture and end up with much lower rainfall coverage.

[jasons2k]

You can delete this post

[srainhoutx]

Sunday morning briefing from Jeff regarding INVEST 91L:

High chance of tropical cyclone formation in the Gulf of Mexico this week

Residents along the US Gulf coast from MS to TX should closely monitor the progress of this system

The tropical wave over the southern Bahamas extending southward to the northern coast of Cuba has shown some limited degree of organization in the last 24 hours with deepening bursts of convection. However upper level winds continue to result in strong shear across the wave axis and overall disorganization of the system The wave is moving toward the W to WNW at 10-15mph and will be moving toward the FL Keys and southern FL today and Monday and then into the eastern Gulf of Mexico late Monday into Tuesday. Once in the Gulf of Mexico upper level conditions should become increasingly favorable as shear relaxes allowing thunderstorms to become more concentrated and likely a defined surface low pressure system to form.

Global forecast models have come into better agreement with the forming system moving fairly quickly WNW to NW toward coastal Louisiana by Wednesday and then inland on Wednesday night and Thursday with a slowing of forward speed. However many of the global ensemble members continue to indicate a more westward motion to the track near/along the Louisiana coast a high pressure over the SE US builds westward. Without any sort of defined low level center it is difficult to put much confidence in any of the model solutions at this time and uncertainty on track and intensity remain high.

[unome]

Mesoscale Precipitation Discussion 0745
NWS Weather Prediction Center College Park MD
1006 AM EDT Sun Sep 02 2018

Areas affected......South-Central to Southwest Louisiana...Upper
Texas Coast...

Concerning...Heavy rainfall...Flash flooding possible

Valid 021405Z - 021805Z

Summary...Expanding area of convection within a highly favorable
thermodynamic environment north of a weak offshore low will lead
to spotty 2+ inch/hour rainfall rates this morning.

Discussion...The latest GOES-16 IR satellite imagery depicts a
concentrated area of cloud top cooling (deepening convection)
around a low pressure system centered offshore south of Lake
Charles. The thermodynamic environment is optimal for efficient
warm rain processes and high rainfall rates, as MUCAPES average
between 1000-2000 j/kg within a favorably deep moisture profile
(PWs over 2.25") per the observed 12Z RAOBs and latest SPC
mesoanalysis.

Helping to focus and maintain plumes of convection will be the
enhanced low-level inflow (20-25 kts at 925-850 mb) initially east
of the low-level circulation, pivoting westward toward the upper
Texas coast north of the low. This will maintain robust low-level
moisture transport, while also lead to back-building and training
of convection as the Corfidi vectors oppose the direction of the
low-level inflow. The one inhibitor to the flash flood threat
however is the absence of an inland frontal zone, which would help
to sustain the moisture convergence and strongest convection over
the same area for a more prolonged period of time.

Nevertheless, the deep warm cloud layer (wet bulb freezing levels
above 15kft per the 12Z KLIX and KLCH RAOBs), along with PWs
between 2.25-2.5" will continue to foster highly-efficient
rainfall rates -- likely exceeding 2"/hour within the strongest
and more persistent rain bands. Hourly FFG values are generally
above 2.5" initially, with 3 hourly FFGs over 3". The latest
mesoscale guidance, especially recent HRRR runs, indicate areas of
2+ inches/hour and 3-5+ inches within 3 hours within the outlook
area. This will lead to an enhancing flash flood threat toward
midday as the soils become more saturated with time.

Hurley

ATTN...WFO...HGX...LCH...LIX...

ATTN...RFC...LMRFC...WGRFC...

[mckinne63]

Have had light showers most of the morning here in Stafford. Hubby is flying in from NJ this evening, hope his flight isn't delayed.

[Rip76]

Interesting look out there.

[DoctorMu]

Euro and Canadian continue westward movement into Texas for the tropical low/wave/storm after landfall on the Louisiana coast. GFP has to low eject north. The strength and breadth of the ridge north of us will be key.

Most of the impressive rain would be to the north and east of Houston. However, it doesn't take much of a bump to lead to a flooding inducing event locally.



The Euro model solution has more rain over the area.

Fingers crossed that the current wave oozing our way drops more rain and 91L less over HGX.

[jasons2k]

Even today, the rain is avoiding me like the plague. I'm right in the middle of the split in the line (now two of them)...

All the inland stuff is to my south and west...

Edit: out running errands - missed me by half a mile.

[davidiowx]

Looking a little interesting offshore..

[CrashTestDummy]

Interesting look out there.

Indeed. The radar signal seems to show a bit of circulation there, with the center somewhere around, or shoreward of Beaumont.