Southern Region Heavy Rainfall Event November 3-7, 2014

[Srain]

As Jeff mentioned in his morning briefing, next week offers a real forecasting challenge and certainly a pattern we typically see with an El Nino developing. There remain some slight disagreement with the placing of the cut off 500mb Upper Low to our W, but the Parallel GFS that will go online soon is not all that different than the European computer model. During Fall/Winter we typically see these cut off lows develop in the SW with a stalled boundary across Texas and the Southern Plains. There is a good long fetch of Pacific moisture that will only be enhanced by Vance and its remnants across Mexico on ENE into the Southern Plains and the Mid Mississippi Valley. The computer models continue to advertise a Coastal surface low developing along the S Texas Coast near Corpus Christi next Wednesday. Should a Coastal low develop, rainfall amounts would likely need to be increased rather dramatically across all of our Region. It could be rather raw temperature wise with temperatures in the upper 50's and heavy convective rains most of next week.

 

 

[Srain]

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1159 AM EDT FRI OCT 31 2014

VALID 12Z MON NOV 03 2014 - 12Z FRI NOV 07 2014

...VERY HEAVY PCPN PATTERN ON TAP FOR THE S-CENTRAL AND NWRN US
NEXT WEEK...

...PATTERN OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENTS...

WPC SURFACE FRONTS/PRESSURE AND 500 MB PROGS ALONG WITH ASSOCIATED
HPCGUIDE HIGH RESOLUTION SENSIBLE WEATHER GRIDS HAVE BEEN
PRIMARILY DERIVED FROM A BLEND OF THE 06 UTC GFS ENSEMBLE MEAN AND
THE REASONABLY COMPATABLE 00 UTC ECMWF/NAEFS ENSEMBLE MEANS. AN
ENSEMBLE MEAN FORECAST APPROACH WAS USED IN PART DUE TO LESS THAN
STELLAR DETERMINISTIC MODEL RUN TO RUN VARIANCE. GUIDANCE THOUGH
DOES SEEM TO BE CONVERGING UPON A MORE COMMON MID-LARGER SCALE
SOLUTION...BOLSTERING FORECAST CONFIDENCE... ESPECIALLY WITH HEAVY
PCPN THREATS FOR THE S-CENTRAL AND NWRN US NEXT WEEK.

THE WPC SOLUTION LEANS ON THE MORE AMPLIFIED SIDE OF THE FULL
ENVELOPE OF SOLUTIONS FOR BOTH THE NRN AND SRN STREAMS...FOLLOWING
FROM LEAD-IN WATER VAPOR IMAGERY LOOP ANALYSIS...OVERALL GUIDANCE
TRENDS/BIASES...AND RECENT FLOW HISTORY.


...SENSIBLE WEATHER HIGHLIGHTS...

AMPLIFIED YET STEADILY PROGRESSIVE MID-UPPER LATITUDE FLOW
SUPPORTS SEVERAL FRONTAL SYSTEM PUSHES FROM THE CENTRAL TO ERN US
NEXT WEEK WITH MODEST PCPN. ENSEMBLE MEANS ARE MODESTLY MORE
AMPLIFIED THAN THE DETERMINISTIC GFS/ECMWF OVER TIME IN THIS
PATTERN. HOWEVER...A BIGGER THREAT TRAILS BACK AND LINGERS OVER
THE S-CENTRAL US WHERE SLOWER MOVING BOUNDARIES AND POTENTIAL SRN
STREAM MID-UPPER LEVEL TROUGH SUPPORT WORKING SLOWLY FROM THE SWRN
US/MEXICO MAY COMBINE WITH DEEPER GULF OF MEXICO MOISTURE INFLOW
THAT MAY BE FURTHER ENHANCED TO SOME FACTOR BY MID-UPPER LEVEL
MOISTURE FED UP FROM ERN PAC TS VANCE. FYI...THE EXPERIMENTAL
PARELLEL 06 UTC GFS HAS TRENDED MORE IN LINE WITH THE 00 UTC ECMWF
IN THE SRN STREAM NEXT WEEK.

MEANWHILE...ENERGETIC MID-UPPER LATITUDE FLOW ALSO FAVORS A
PROLONGED HEAVY PCPN PATTERN FOR THE NWRN US WHERE A MOIST FETCH
WITH EACH SYSTEM APPROACH/PASSAGE IS SIGNIFICANTLY ENHANCED BY
TOPOGRAPHY.

SCHICHTEL
 

 

 

[Srain]

The Western trough continues to draw moisture from the Pacific as the sub tropical jet become a bit more active. New Mexico/Colorado higher terrain should see some decent snow as leeside cyclogenesis develops and mid/upper moisture from Hurricane Vance spreads NE across our Southern tier of States. There is some uncertainty regarding the tail end of the trough where the Euro attempts to cut off a closed low and retrograde. A more amplified solution seems to be the best forecast with a cold front being the focus of heavy rainfall with some convective elements. The winds have turned onshore off the NW Gulf this morning and should increase the available moisture to near or slightly above 2 standard deviation above normal for this time of year. The wildcard will be if a Coastal low develops near Corpus Christi and moves NE on Wednesday as the frontal boundary nears the NW Gulf of Mexico. Vance is expected to weaken as it turns NE but with 94E to its W, additional sub tropical moisture could become entrenched in the NE flow.

 

[Srain]

Afternoon QPF update through Day 5...

 

[Srain]

Update from Jeff:

 

Widespread heavy rainfall likely Tuesday-Thursday

 

Ingredients coming together to produce a heavy rainfall event over a large part of TX this week potentially making a significant dent in the ongoing hydrological drought over central TX.

 

Deep upper air trough will dig into the SW US and northern MX early this week turning the flow over TX out of the SW aloft. This trough is starting to capture hurricane Vance off the SW MX coast turning the system northward and this trend should continue allowing mid and high level moisture to begin to spill northeastward into TX. Cold surface high pressure is moving eastward allowing Gulf moisture to begin its return at the low levels to the state. This is clearly noted by a dewpoint of 55 at Victoria versus 36 at Lufkin currently.

 

A cold front in association with the upper level trough will move into TX on Monday and enter SE TX late Tuesday and progress to the coast on Wednesday. By late Tuesday the air mass will saturate as moisture from both Vance and the Gulf combine over the state. The cold front becomes increasingly parallel to the upper level SW flow which will result in a favorable echo training pattern.

 

While the global models have converged on a more progressive system, moisture levels are forecasted to rise to a staggering 2.20 inches of PW at CRP which is +3 standard deviations above normal for early November. Strong frontal lift and favorable jet dynamics along the boundary working on a highly moist air mass paints a very wet mid week period. The threat for cell training induced flash flooding is high from late Tuesday into most of Wednesday. I am concerned with such moist values the models are and have been showing now for days and the potential for organized heavy training rainfall.

 

Frontal boundary should reach the coast late Wednesday, but base of the upper trough is slower to move eastward allowing moisture to flow up and over the surface cool pool resulting in continued light to moderate rainfall into Thursday.

 

Rainfall Amounts:

Still early to attempt to narrow down any one area that may see the greatest amounts. The event looks widespread over a large part of the state with widespread totals on average of 2-4 inches. Isolated amounts upwards of 6 inches are certainly possible and I would venture that a few locations will see a foot of rainfall before this event is done.

 

Short fused flash flooding will be the main concern under the training excessive rainfall cells in urban areas and across the hill country/flash flood alley area with a large scale hydrological response likely on all river basins due to the large QPF fields being predicted. While it is too early to be certain of the location of the highest rainfall amounts, the potential for rises of watersheds to flood stage is certainly possible by mid to late week.

 

It should be noted that WPC has outlooked a large part of TX and all of SE TX for excessive rainfall on their Day 3 outlook and an upgrade to a moderate or even high risk is possible as the event onset nears and confidence increases on the location of the greatest rainfall.

 

Flash Flood Watches will likely be needed for large portions of the state including most if not all of SE TX by Tuesday evening into Wednesday.  

 

[Srain]

Monday evening briefing from Jeff Lindner (Harris County Flood Control District):

***Widespread heavy rainfall increasingly likely Tuesday-Thursday over much of Texas with flooding and flash flooding possible***

 

As forecast now for several days, several factors are evolving to produce a round of heavy to excessive rainfall across much of Texas starting Tuesday and ending Thursday. These factors include:

 

1)      A deep upper air trough over northern MX and the SW US

2)      A slow moving frontal boundary

3)      Category 2 hurricane Vance off the SW coast of Mexico

4)      Influx of copious Gulf of Mexico moisture

 

An upper level trough continues to dig into northern MX with a well defined sub-tropical jet cutting from the central Pacific across MX into TX. Small but still powerful hurricane Vance is being picked up by this trough with extensive mid and high level moisture pouring northeast into TX as clearly noted by the increased high level cloud deck this afternoon. Increased southwesterly wind shear over Vance on Tuesday should effectively decapitate the hurricane with the mid and upper level center and moisture moving NNE while the low level center heads more NNW or NW…the downstream result over TX will be the same…rain and a lot of it!

 

Surface frontal boundary now entering NW TX will slowly progress southeastward toward the TX coast by late Wednesday. At the surface SE winds are transporting a rich tropical air mass over the western Caribbean toward the NW Gulf coast and this should arrive on Tuesday. PW values (the integration or summing of the moisture content of the air column from the surface to around 300mb) has nearly doubled at both Corpus Christi and Del Rio in the past 24 hours as moisture from Vance and the Gulf of Mexico are clearly being sampled by the upper air sounding stations at these sites.

 

Tuesday:

The trough over MX will begin to eject disturbances into TX while the frontal boundary slowly moves southeastward. The combined lift from the disturbances and the front will produce showers and thunderstorms from SW to NE TX on Tuesday. For the most part think SE TX will remain fairly dry on Tuesday except for any NNW moving streamer showers in the convergent onshore flow pattern off the Gulf. Heavy rains will focus over SW, WC, and N TX on Tuesday.

 

Tuesday night:

Heavy rainfall and flash flood event increasingly likely over SW and SC TX. Slow moving frontal boundary will become nearly parallel to the SW to NE upper level flow resulting in a slowing of the frontal slope and high threat for repeat cell training. Strong low level inflow off the Gulf and near saturated air column with PWS of 1.85-2.10 inches supports warm rainfall production and potential excessive rainfall cores. A large band of heavy rainfall with embedded excessive rainfall will likely be ongoing by late evening from SW TX across central TX into the NW counties of SE TX.

 

Wednesday:

Heavy to excessive rainfall potential shifts with the frontal boundary toward the coast plains. Expect widespread heavy rainfall to commence early Wednesday morning and last through much of the day from SW TX across the coastal bend into all of SE TX. While the frontal boundary will reach the coast late in the day the threat for overrunning rainfall remains as the upper trough is slower to move out of NE MX and this could keep high rain chances going into Wednesday night especially along and SE of US 59.

 

Thursday-Friday:

Main energy of the upper trough will move across S TX with current models wanting to focus the heavy rainfall over S TX.

 

Rainfall Amounts:

PWS climb to +2 SD above normal for early November which raises a red flag especially when coupled with high potential for cell training. Effectively we will have summertime moisture levels with wintertime dynamics. Such summer rainfalls tend to be isolated in nature with lift focused by daytime heating and for a short period of time…however this time of year strong lift is focused for potentially hours due to jet stream dynamics aloft which can keep the excessive rainfall hose going for a much longer period of time as long as the moisture is being funneled into the storms. We will have no shortage of moisture.

 

Given the excessive moisture values high hourly rainfall rates under the stronger storms appear likely in the 2-3 inch range. This will create concerns in the urban areas and in the hilly terrain along and west of I-35 where the flash flood risk is maximized with such rainfall rates.

 

Widespread rainfall amounts of 2-4 inches are likely with isolated totals of 4-7 inches possible especially in an arcing corridor from SW TX to near San Antonio to near Giddings. This setup certainly has the potential to produce some “incredible” rainfall amounts and almost every time we see one of these events some location will get 10 inches or more.

 

Simply stated….a tremendous amount of water is going to be dropped over the state in the next 72 hours.

 

Aside:

“While the current QPF fields are justified given the model output, such values are almost always too low in such events. Small scale intense rainfall cores due to cell training or cell mergers will almost certainly occur and under these locations significantly higher rainfall amounts will reside. These amounts could be upwards of 10 inches or greater in less than 6 hours. Neither I nor any computer model can resolve the location of such intense rainfalls usually until the situation is underway. Such setups are frustratingly difficult to determine due to the meso and micro scales on which they tend to occur…usually on the order of 10’s of miles or less. Yet they are almost always associated with devastating flood responses and life threatening flash flooding.”

 

Hydrologic Response:

While the past several weeks have been dry across much of the area and state and many areas are still suffering from long term hydrological drought, this event has the potential to produce significant run-off and rises on area watersheds. The initial focus will be on the urban areas where excessive rainfall rates will overwhelm primary drainage systems (curb and gutter systems) resulting in deep ponding on roadways and in the flash flood alley section of TX along the I-35 corridor where low water crossings will likely become inundated. Toward the end of the week the widespread nature of the event will likely induce rises on area rivers as generated flood waves make their way into the mainstem rivers and begin to work downstream. As noted above if I cannot determine where the greatest rain will fall at this point I cannot determine which watershed basins will be most impacted and have the greatest response. For the moment I feel there is an equal threat across all basins.