August 2016: Rain Chances Continue.

[unome]

the gulf coast crawl http://www.ssec.wisc.edu/data/us_comp/us_comp_large.mp4

latest storm summary http://www.wpc.ncep.noaa.gov/discussions/nfdscc2.html

[unome]

QUANTITATIVE PRECIPITATION FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
130 PM EDT SAT AUG 13 2016


PRELIM DAY 1 QPF DISCUSSION
VALID AUG 14/0000 UTC THRU AUG 15/0000 UTC
REFERENCE AWIPS GRAPHICS UNDER...PRECIP ACCUM - 24HR


DAY 1...

LOUISIANA AND EASTERN TEXAS
~~~~~~~~~~~~~~~~~~~~~~~~~~~
...significant rainfall expected across western louisiana/eastern texas...

the slow-moving convective low (which continues to experience light north to northeasterly vertical wind shear) is expected to be pulled gradually northwest ahead of the approaching central u.s. trough and around the western periphery of the subtropical ridge. another round of heavy to excessive rainfall is expected across western and central la...east tx...and portions of ar. recent radar imagery shows a connection in the convective pattern associated with the frontal zone to the north and this cyclone. the atmosphere remains saturated over the central gulf region...with precipitable water values of 2.25-2.5" or 3-3.5 standard deviations above normal for mid-august. the more anomalous pw plume will shift into east tx and the upper tx coast. developing instability to the west and north of this system due to daytime heating will tend to lure thunderstorms with heavy rainfall in those directions. prolific short-term rainfall rates remain possible, with hourly rain rates maximizing in the 3-4" range particularly across western and central la. the tall skinny cape profiles per model fcst soundings along with the abnormally high wet bulb zero levels (deep warm cloud layers) also favor highly efficient warm rain processes...as evidenced from the pockets of much higher qpf (24 hr amounts of 10+ inches) from the latest guidance, especially the 00z ecmwf and high-resolution guidance. the 18z wpc qpf was shifted somewhat westward based on recent model and radar trends, in the direction of the high resolution guidance but with greater organization.


western and south-central texas
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
the increased moisture advection from the east along and ahead of the slow-moving frontal boundary from the direction of the convective low will sponsor more widespread showers and thunderstorms with heavy rainfall across central-eastern tx. areal-average totals between 1.5-3.0" across east tx and 0.5-1.5" elsewhere over central tx are expected. the saturated environment and modest deep-layer instability should allow for localized higher totals given the very weak 850-300 mb flow and slow cell motions (light corfidi vectors). the lack of low level inflow and overall weak 0-6km bulk shear should lead to a more pulse-type evolution within the stronger cells... which along with the initially high flash flood guidance values will lead to a lower flash flood threat compared to areas to the east. the anticipated robust hourly rainfall rates of 2-4" underneath the strongest cores, with the mesoscale guidance generally advertising local amounts in the 5-10" range, warrants a slight risk for excessive rainfall. model spread in this area precludes a moderate risk at this juncture. trended southward towards the 12z guidance in this area.

[srainhoutx]

Saturday afternoon briefing from Jeff:

Tremendous rainfall event continues over southern Louisiana…shifting slowly toward E/SE TX.

Threat for heavy rainfall will increase this afternoon and remain into Monday.

Prolific rainfall event over southern Louisiana along and south of a “tropical” low pressure system has resulted in 24-hr storm totals of 15-22 inches with an isolated max of 27.47 inches near Brownfields, LA. Dangerous flash flood event continues to develop and shift westward along I-10 and the Louisiana coast with strong low level WSW confluent flow over SE TX and the upper TX coast feeding a near continuous shield of very heavy rainfall and significant training bands. Hourly rainfall rates of 2-4 inches have been observed under these “feeder” bands. Center of mid level low appears to be located over C LA with a surface reflection over SC/SE LA. Massive rain shield continues to favor the SW flank of this system as has been the case for the last 2-3 days.

At the same time a surface trough and weak front is approaching SE TX from the NW while western flank of approaching LA system is crossing the Sabine River. Banding features have started to develop within the last few hours from eastern Harris County to near Lake Charles and off the coast south of Sabine Pass. Record breaking moisture profile over Louisiana with PWS of 2.5-2.8 inches will shift west into SE TX later today…New Orleans sounding had a shattering 2.79 inch PW last evening which is the second highest I have ever seen at an actual sounding site. “Super” tropical air mass will shift into SE TX this afternoon and remain in place into Monday. The mid and low level centers embedded in this tropical air mass continues feed energy into these lows and they are behaving very much like a decaying tropical system…and that is how they should be treated.

Model guidance continues to insist that the defined low will open into a westward moving trough and the heavy rainfall will become less concentrated over SE/E TX compared to Louisiana…however this process has yet to happen and radar data out of Lake Charles continues to show a well defined circulation to the reflectivity.

Expect an increase in showers and thunderstorms this afternoon as moisture increases and heating destabilizes the air mass. Will have to watch trends closely tonight to see if sustained banding develops on the SW flank of the circulation center again…similar to the last several nights which would be near/over our eastern set of counties or roughly from Lake Livingston to Galveston Bay. Given extraordinarily moist air mass excessive rainfall rates under the stronger storms will be likely with 2-4 inches per hour common.

Will maintain widespread rainfall amounts of 2-4 inches across the area NE of a Line from College Station to Katy to Galveston and I am tempted to go higher with these amounts…but if the system opens into a wave of low pressure instead of the more concentrated low then 2-4 inches is reasonable. If the system maintains a defined low center rainfall will need to be greatly increase along and south of that center track. Isolated totals of 6-8 inches or higher is certainly possible given slow storm motions and high moisture levels.

A flash flood watch may be required later this afternoon or tonight as trends unfold even with the very dry ground conditions currently in place.

Lake Charles Radar Estimates: Note the massive area over 12 inches

08132016 Jeff 1 untitled.png

[unome]

almost looks like an eye over east LA

loop: http://rammb.cira.colostate.edu/ramsdis ... peed_ms=80

[Rip76]

Where is the front currently located?

[srainhoutx]

Where is the front currently located?

[djmike]

Curious. Why does the systems rain shield mostly show in the gulf now rather than over land? Another question out of curiosity, why do most storm show a soild rain shield over water and when they move inland, it begins breaking down and will only provide spotty showers? Does being over water fuel a system more than over land? I know hurricanes do the same but just regular thunderstorms? Same effect?

[Rip76]

Curious. Why does the systems rain shield mostly show in the gulf now rather than over land? Another question out of curiosity, why do most storm show a soild rain shield over water and when they move inland, it begins breaking down and will only provide spotty showers? Does being over water fuel a system more than over land? I know hurricanes do the same but just regular thunderstorms? Same effect?

This is why I was asking where the front was/is.

[srainhoutx]

mcd0561.gif

MESOSCALE PRECIPITATION DISCUSSION 0561
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
212 PM EDT SAT AUG 13 2016

AREAS AFFECTED...PORTIONS OF CENTRAL AND SOUTHWEST TX...SOUTHEAST
NM

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 131810Z - 140010Z

SUMMARY...POCKETS OF SLOW-MOVING THUNDERSTORMS WITH HEAVY RAINFALL
WILL FAVOR SOME POTENTIAL FOR FLASH FLOODING THIS AFTERNOON AND
EVENING.

DISCUSSION...THE PROXIMITY OF A FRONTAL ZONE ALONG WITH A DEEP
LAYER OF MOISTURE OVER CENTRAL AND SOUTHWEST TX WILL SET THE STAGE
FOR SCATTERED CLUSTERS OF THUNDERSTORMS THIS AFTERNOON AND
EVENING. GIVEN A CONTINUATION OF STRONG DIABATIC HEATING THROUGH
SOLAR INSOLATION...CONVECTION SHOULD TEND TO EXPAND IN COVERAGE
FROM THE TX HILL COUNTRY SOUTHWEST TO THE UPPER RIO GRANDE VALLEY
OF TX...AND ALSO UP INTO SOUTHEAST NM WHERE THERE WILL BE
FAVORABLE LOW LEVEL UPSLOPE FLOW INTO SOME OF THE HIGHER TERRAIN.

THE LATEST HRRR/HRRRP SUGGESTS AS MUCH AS 3 TO 4 INCHES OF RAIN
WITH LOCALLY HEAVIER TOTALS THROUGH EARLY THIS EVENING. THE
CONVECTION IS NOT EXPECTED TO NECESSARILY BE WELL ORGANIZED...BUT
SOME PORTIONS OF THE THREAT AREA HAVE SEEN HEAVY RAINS OVER THE
LAST DAY...AND SO THESE ADDITIONAL RAINS MAY LEAD TO SOME
POTENTIAL FOR RUNOFF PROBLEMS AND/OR FLASH FLOODING.

ORRISON

ATTN...WFO...ABQ...EPZ...EWX...FWD...MAF...SJT...

[Rip76]

Radar starting to heat up in East Texas, moving South.

[Katdaddy]

Seeing a little more develop NE of Houston this afternoon.

[unome]

Area Forecast Discussion
National Weather Service Houston/Galveston TX
322 PM CDT Sat Aug 13 2016

.DISCUSSION...
Showers and thunderstorms have begun to develop across parts of Southeast Texas this afternoon. Satellite imagery is showing rapidly cooling cloud tops across northern Houston County and eastern Polk County, and these areas appear to have an increasing potential for very heavy rainfall spots. Expect general rain coverage to continue to increase for the rest of the afternoon across our area, possibly followed by a gradual decrease in both coverage and intensity this evening. Rising precipitable water values working toward 2.50ish inches support the threat for locally heavy rainfall in a very short period of time (a quick 1 to 3 inches along with even higher amounts where north-to-south oriented cell training manages to set up). We continue to be concerned with more storm development during the overnight hours and on into Sunday as the slow moving storm system just off to our east edges into our area and precipitable water values remain very high. Rains that do develop will have the potential to persist for multiple hours and move very little, resulting in an increasing risk for possible flash flooding. Prefer for now to hold off on issuing a Flash Flood Watch and wait to see how much rain the first batch of storms (covering the remainder of this afternoon and this evening) produces. When we get better confidence as to where the heaviest rains look to set up, expect a watch to be issued. The forecast through at least the first half of the upcoming week has changed little with rain chances continuing on the high side as precipitable water values remain close to 2.50 inches. More clouds and rain continue to support lower afternoon temperatures through the period. Possible mid/upper level ridging building across the area toward the end of the week and next weekend support lower rain chances and warming temperatures. 42
&&

.MARINE...
Coastal waters are running very near SCEC levels today - but with a small craft in effect to the east, winds around 15 knots, and potentially more frequent gusts to around or above 20 knots and thunderstorms entering the 20-60 nm waters, decided to go ahead and urge caution in the eastern Gulf zones into tonight. Will have to reevaluate after that, as marginal conditions are expected to continue through at least the weekend as Gulf low pressure keeps weather unsettled.

[Rip76]

Any chance of something forming with this moving SW?
Splitting in two so to speak.

[srainhoutx]

QPF totals remain rather high throughout the next 7 Days with a least daily changes for downpours across our Region.

[Ounce]

Update from Jeff:


Will maintain widespread rainfall amounts of 2-4 inches across the area NE of a Line from College Station to Katy to Galveston and I am tempted to go higher with these amounts…but if the system opens into a wave of low pressure instead of the more concentrated low then 2-4 inches is reasonable. If the system maintains a defined low center rainfall will need to be greatly increase along and south of that center track. Isolated totals of 6-8 inches or higher is certainly possible given slow storm motions and high moisture levels.

Will rain opening into a wave of low pressure appear flatter along the line of the wave on radar, as opposed to a "concentrated low" where it will rotate counter-clockwise like a regular low pressure? Thanks.

[unome]

from latest storm summary http://www.wpc.ncep.noaa.gov/discussions/nfdscc2.html

at 300 pm cdt... the center of the low pressure system that has brought prolific amounts of rain into southern louisiana has become more diffused as it has begun to merge with a frontal boundary approaching from the west. radar and satellite imagery indicate that the low to mid-level center has begun drifting northwestward into northern louisiana while a 1007 mb... 29.74 inches... low pressure center has formed along the frontal boundary in eastern texas. national weather service doppler radar and satellite imagery indicate that the heavy rain to the south of the low center has become less intense during the past few hours over southern to southwestern louisiana. meanwhile... scattered showers and thunderstorms have formed in the vicinity of the low pressure area in eastern texas and along the frontal boundary further to the northeast.

[unome]

updated

[srainhoutx]

20Z Surface Analysis suggests the 1007mb low is near just N of Lufkin. Will need to monitor convective activity developing along the wavy boundary. Still a lot of warm core characteristics associated with this complex and complicated setup, particularly during the overnight hours.

[unome]

hgx issuing flood advisories (Walker, Liberty, Polk)

https://twitter.com/iembot_hgx

[unome]

SPC Mesoanalysis Analysis - Southern Plains

(currently 22Z)

http://www.spc.noaa.gov/exper/mesoanaly ... ?sector=15