November 2014: Nice Seanonal Thanksgiving Ahead

[srainhoutx]

Monday evening briefing from Jeff:


***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.   

[SLM87TX]

You need to send this forecast to the Weather channel lady who said it needs to be steady rain. Rain comes in two forms in Texas, Either it doesn't or all at once. Does any one else notice how the development in the panhandle seems greater than models show. Is this a precursor to whats to come. I am thinking we will see somewhere a 12" total.

[srainhoutx]

The heavy rainfall event is taking shape across West Texas/New Mexico and Oklahoma this morning. The slow moving frontal boundary with all the rain and training storms behind the front should be across Central Texas by this afternoon/evening where an aerial wide Flash Flood Watch was extended/expanded this morning to include all of the San Antonio/Austin National Weather Service forecasting area. The front should approach College Station over night and slow down and perhaps stall somewhere across or near the I-10 to HWY 59 Corridors. The upper level energy looks to remain to the SW as a potent upper low draws moisture from Vance/95E currently nearing the W Coast of Mexico. the long fetch moisture plume from the Pacific as well as ample moisture from the Western Gulf is setting the stage for a prolonged 12- 24 hour of potential heavy rainfall. As Jeff mentioned last night in his briefing, it is impossible to know exactly where or who may see the heaviest rainfall, so let those that may not be as weather wise as those that frequent our online weather community know that a active weather pattern is about to unfold after a month long very quiet weather pattern.

[tireman4]

Stay dry and safe everyone. This will be a soaker for many folks. Stay tuned to the NWS and your local media outlets on further changing condtions.

[Rip76]

Hmmmm.

That front appears to be arriving a little faster than forecast, no?

[SLM87TX]

Maybe but it moving slow it will take until tonight to get down here, but look at water vapor. Vance is getting sucked in fast too. Timing will be key with where it is versus the front. It could come together right over the are by my eyes.

[srainhoutx]

Briefing from Jeff just arrived:

***Significant rainfall event underway across TX***

Large scale upper pattern remains favorable for the development of heavy rainfall across a large part of TX today-Thursday. A large band of widespread moderate to heavy rainfall has focused this morning along a slow moving frontal boundary from S OK to SW TX with good rainfall coverage across the hard hit drought areas of the WC and NW TX. Across SE TX moisture return off the Gulf of Mexico is resulting in streamer showers moving inland over Matagorda, Wharton, and Fort Bend Counties and this should continue today and expand some as the air mass moistens. Morning soundings from CRP and DRT came is a bit drier than expected with a fairly large mid level dry layer noted. Most of the moisture from Vance is being depicted in the high levels while Gulf moisture is shown in the low levels. Water vapor images show a clearly defined moisture tap from the eastern Pacific with the main shield of Vance’s moisture starting to move across MX toward SW TX. Models may have been overdoing moisture levels some as an upstream look over the SW Gulf of Mexico shows PW values running in the 1.25 inches range. While moisture will pool along the frontal boundary, not sure if the 1.9-2.1 inch values will verify.

Cold front will very slowly move southward today as the upper flow is already running fairly parallel to the boundary supporting very slow forward progress. Forecast models generally bring the boundary southward and into central and SE TX overnight and slow it even more. The upper flow parallel to the boundary strongly supports repeat cell training tonight into Wednesday.

QPF fields have been cut back some overnight with the main focus of excessive rainfall over SW and SC TX. With this said the same factors over SC TX will be in place over SE TX late tonight and on Wednesday. Some of the meso scale guidance is showing a rainfall void over the central sections of SE TX on Wednesday with maximum totals being focused mainly NW of a line from Columbus to Lake Livingston possibly between the surface and 850mb cold fronts. I am not overly confident on any one location being more favored over another given the potential for cell training and meso scale influences.

Expecting widespread rainfall amounts of 1-3 inches with isolated totals of 4-5 inches. Amounts of 7 inches are possible over SC TX and SW TX. While most of the watersheds will be able to handle this amount of rainfall, the potential for excessive short term rainfall is high leading to rapid flash flooding and urban flooding. The widespread nature of the event will certainly support large scale river responses into the end of the week.

River Flood Outlook:
The following river basins are being monitored for rises based on the expected rainfall amounts to possible flood stage:

Colorado River: minor flooding possible
San Gabriel River: minor flood possible
Guadalupe River: minor flooding possible
San Antonio River: minor flooding possible
Frio River: minor flooding possible
Rio Grande River: minor flooding possible

[srainhoutx]

The actual front has slowed down a bit over the Hill Country the past couple of hours. The deepening cut off low is wrapping up over Arizona/ Northern Mexico while the may energy associated with the trough axis up across the Great Lakes moves E. HGX believes that rainfall rates of 2-4 inch per hour would need to happen to meet Flash Flood Watch criteria, but dry ground/soils the past couple of months since Odile in early September should keep a widespread event developing across SE Texas. We will see how good or poorly the guidance has handled this threat as we get into the evening/overnight hour into tomorrow.

[Andrew]

Latest Tx Tech Wrf for this evening:

dom_2km_sfc_dbz_f17.gif

Current thinking is some models may be a little too fast with frontal movement as of now and entrainment of the moisture in the East Pacific isn't being fully integrated correctly. WV shows a lot of moisture streaming in from the SW so keep an eye on things down there.

[srainhoutx]

Correct Andrew. The front has indeed slowed its progression and it is clear via radar trends as well as satellite imagery that the remnants of Vance along with its embedded areas of vorticity will ride along and behind the front increasing lift. The radar returns to our SW are showing new areas of precipitation developing across NE Mexico moving NE as well as inflow showers off the Gulf increasing. HGX just updated stating that they may have to extend the rain chances into Wednesday night and Thursday.

[srainhoutx]

Looking ahead to next week, the new 12Z parallel GFS and the Euro suggest some very chilly air heading S near the November 11th-14th timeframe.

[wxman57]

Looking beyond the heavy rainfall event, the new GFS (parallel run) is predicting freezing temps across much of Texas next Thursday (highs in the 40s in Houston on Wednesday). Fine with me, as I have the earliest date in our first freeze contest (Nov. 21). It can warm up afterward.

As for rainfall, it's predicting 1-3" across Houston tomorrow.

[srainhoutx]

Interesting that the parallel GFS only briefly allows for a slight moderation before another dump of very cold air settles south into Texas around the 17th and stays chilly into the 20th. Stepping down...

[ticka1]

Interesting that the parallel GFS only briefly allows for a slight moderation before another dump of very cold air settles south into Texas around the 17th and stays chilly into the 20th. Stepping down...

Can I get excited and say I like this forecast? Now we just have to get the precipation and the cold together.......and we can have..... snow!

[jasons2k]

I'm late to the party. Been so busy.

I just noticed this hidden gem in Jeff's late morning update. There is always a spoiler:

"Some of the meso scale guidance is showing a rainfall void over the central sections of SE TX on Wednesday with maximum totals being focused mainly NW of a line from Columbus to Lake Livingston possibly between the surface and 850mb cold fronts."

[Andrew]

Front starting to make some progress now

http://www.aviationweather.gov/adds/dat ... f_sandwich

[srainhoutx]

Evening briefing from Jeff:

Widespread moderate to at times heavy rainfall underway across much of TX this evening.

Cold front has surged southward this evening entering our NW counties and extending WSW toward SW TX. In the past 2-3 hours radars have shown a gradual increase in both coverage and intensity of rainfall along and behind this frontal boundary especially along the flood prone I-35 corridor between Austin and San Antonio. Recently flash flood warnings have been required under a band of persistent training across northern Bexar County and metro San Antonio. Main thrust of the jet dynamics and moisture plume from Vance and the Gulf is aimed at SC and the western portions of SE TX tonight. Favorable feed of moisture is noted along a line from Matagorda Bay to College Station where NNE moving showers continue to develop in a low level convergent band.

Short term forecast models show the front slowly and stalling after midnight between Houston and Conroe while dropping SSW toward Matagorda Bay. This frontal position allows for the threat for a SSW to NNE training band of rainfall from late tonight into Wednesday along a broad swath from Matagorda Bay to Lake Conroe.

Slower progression of the main upper level trough over MX is now expected to linger rain chances into Thursday as moisture is forced up and over the surface frontal slope on SSW to SW winds in the mid and upper levels. However think the main threat for heavy rainfall and flooding will be tonight and Wednesday across SC and SE TX.

A look ahead suggest some very chilly air will be heading southward over the next few weeks with potential for the first widespread freeze of the season before Thanksgiving. Additionally, a rather wet pattern looks possible also as the beginning effects of the current warm phase ENSO in the Pacific begins to be felt across the southern plains and TX.

[TexasBreeze]

What's more reliable, the regular GFS or the new parallel version? Or did they get rid of the regular one?

[texoz]

Can't recall the last time I saw this much rain covering Texas.

[srainhoutx]

What's more reliable, the regular GFS or the new parallel version? Or did they get rid of the regular one?

The parallel GFS is the new upgrade soon to replace the regular/ older GFS. It will be interesting to see if the upgrades settle some issues with the American Global Forecasting Model.