Major Pacific (mild) storm on its way (Dallas, San Antonio: 2015, homes)

[cBach]

Well if you've seen the news in California, SF and LA were hit with a pretty big storm and it's now heading to Texas.

They are thinking 5 inches of rain will fall along the I-35 corridor, this time focusing on the Onion Creek Watershed.

This is typically the time of year that Onion Creek gets hammered.

Now they only forecast 1-3" in the Hill Country so we won't get the tremendous flooding from Oct/Nov but remember that the lakes are still full. Any more rain and they will probably have to open some of the floodgates.

In one day, we are supposed to receive a whole month's worth of rain.

http://spectrumlocalnews.com/tx/aust...ing-for-friday

Be prepared.

[Trainwreck20]

To be honest, we USUALLY get our monthly rain in a day or two. That is the normal .

[cBach]

Originally Posted by Trainwreck20

To be honest, we USUALLY get our monthly rain in a day or two. That is the normal .

Good point. I guess it's better to get it out of the way then huh?

[Trainwreck20]

Originally Posted by cBach

Good point. I guess it's better to get it out of the way then huh?

The real issue is not whether it comes in a day or two, but how many standard deviations above/below the normal it is. The average rainfall for December is around 2.4". The average number of rainy days is 7.8, but a large number of those are 'trace' with the bulk falling in single day/two day events. The forecast for our zip is now showing about 4".

2017: 4.31 (fell during three single-day events)
2016: 2.54 (occurred during a 4-day span)
2015: 3.00 (basically fell during two different days)
2014: 1.06 (half fell on one day, the balance was small amounts over several days)
2013: 0.72
2012: 0.31
2011: 4.93 (two multi-day rain events)
2010: 0.79
2009: 2.61 (half on the first day of the month, the rest spread out)
2008: 0.40

The average of all that is 2.06" per December over the last 10 years. But it was often well above that (>2X) or well below (<0.25X).

[Trainwreck20]

Lest we forget the Christmas Flood of '91, I found this NWS summary:

[...] Tremendous rain and flooding occurred at and south of the confluence of the upper vapor plume, the low-level jet, and the surface stationary front. The heaviest rain was 16 to 18 in. on an area from Llano to Bandera to Boerne. The 6-in. isohyetal extended from the Red River north of the Dallas/Fort Worth metroplex - to near Coleman - to between Bracketville and Uvalde - to near Corpus Christi - to near Palestine - to the Red River.

This was not a historic event in terms of large rainfall totals. But in terms of total rain volume that fell from the sky in one event, this certainly was one of the largest in Texas recorded history, if not the largest. Certainly it rivals Hurricane Beulah, the June 1935, Hurricane Alice in 1954, and the June 1987 floods.
Record flooding moved down the San Gabriel and Little Rivers into the Brazos River above Bryan. The Brazos River was 5 mi wide west of Bryan and College Station. The Navasota River was well over 1 mi wide in Grimes and Brazos Counties. A huge lake over 10 mi long by 10 mi wide was created above the confluence of the Navasota and Brazos Rivers above Washington on the Brazos State Park. High areas were above the water, but most areas flooded.

Downstream, the Brazos River and Oyster Creek merged as the Brazos River flowed over the left floodplain near Harris Reservoir. Thousands of previously unsuspecting home owners were flooded as Oyster Creek became several miles wide in Brazoria County. Residential flooding was widespread above Simonton to the Gulf in Fort Bend and Brazoria Counties. In the Valley Lodge Subdivision near Simonton, most of the 200 homes flooded, some a half mile from the river. Five-hundred homes suffered serious flood damage in Brazoria County. Two-hundred forty-five of 250 homes flooded in Holiday Lakes Estates between East Columbia and Angleton.

Flooding was disastrous also in the Colorado River drainage. Very high flows down the Pedernales and Colorado Rivers into the Highland Lakes system put a tremendous amount of storage into them. The problem was, managers could not release water from Lake Travis because disastrous floodwaters were flowing from Walnut, Onion, and all the other creeks flowing into the Colorado River below Lake Travis. Onion Creek at Hwy 183 crested at 30.50 ft, a record since a recording gauge was installed March 1976. The Pedernales River severely flooded and damaged LBJ National Park at Stonewall. Flow just seeped over the stone wall at the Johnson Family Cemetery.

The Lower Colorado River Authority could do nothing but store all the very high inflow. Lake Travis quickly rose to a record elevation of 710.44 ft Dec 26, 1991. Nearly 400 homes flooded around Lake Travis with up to 22 ft of water over the lowest slabs.

Downstream, a few homes flooded near Bastrop as the Colorado River crested at a record 37.48 ft. Between Bastrop and Smithville, the Hidden Valley Estates, the Doty River Estates, and the Pecan Shores subdivisions had several tens of homes flooded up to nearly 9 ft. In LaGrange, the Fritsch Auf subdivision had over 10 homes flooded up to 6 ft.

Two homes flooded in Columbus. Downstream, 15 homes flooded up to 2 to 3 ft in Wharton. Much worse flooding was spared because the flooding escaped over the left floodplain upstream near Garwood into a widespread area of farmland.

The Guadalupe River had severe flooding. Two homes flooded near Cuero, and downstream near Thomaston in the River Haven subdivision, three homes flooded.

In Victoria, eight city blocks of the Greens Addition in the west part flooded, and also the city park, zoo, and golf course. Downstream, the Guadalupe and San Antonio Rivers were several miles wide near their confluence near Tivoli. Some ranchers would feed their cattle by boat on floodwaters into June. [...]

[Austin97]

Originally Posted by Trainwreck20

The real issue is not whether it comes in a day or two, but how many standard deviations above/below the normal it is. The average rainfall for December is around 2.4". The average number of rainy days is 7.8, but a large number of those are 'trace' with the bulk falling in single day/two day events. The forecast for our zip is now showing about 4".

2017: 4.31 (fell during three single-day events)
2016: 2.54 (occurred during a 4-day span)
2015: 3.00 (basically fell during two different days)
2014: 1.06 (half fell on one day, the balance was small amounts over several days)
2013: 0.72
2012: 0.31
2011: 4.93 (two multi-day rain events)
2010: 0.79
2009: 2.61 (half on the first day of the month, the rest spread out)
2008: 0.40

The average of all that is 2.06" per December over the last 10 years. But it was often well above that (>2X) or well below (<0.25X).

The average is 2, the standard deviation is 1.6

I agree with you that averages are relatively useless without also publishing the std deviation. I wish more people knew this.

66% of the values are from .4 to 3.6 inches.

[austin-steve]

Originally Posted by Austin97

The average is 2, the standard deviation is 1.6

I agree with you that averages are relatively useless without also publishing the std deviation. I wish more people knew this.

66% of the values are from .4 to 3.6 inches.

I wish I knew more about what standard deviation means. I sort of get it. If it drizzles a tiny bit every day of a month but never rains hard on any day, the the standard deviation for that month would be very low and the monthly average meaningful?

But in these months where most rain is over just a few days, then the standard deviation is high and the "average" is misleading?

Steve

[cBach]

Originally Posted by austin-steve

I wish I knew more about what standard deviation means. I sort of get it. If it drizzles a tiny bit every day of a month but never rains hard on any day, the the standard deviation for that month would be very low and the monthly average meaningful?

But in these months where most rain is over just a few days, then the standard deviation is high and the "average" is misleading?

Steve

Not exactly. It's only looking at totals of rainfall not the distribution of it in the month.

The way I explain standard deviation is the "bell curve". Within one standard deviation a majority of the values will be there, within 2 standard deviations almost all values will be there.

https://en.wikipedia.org/wiki/Standa...on_diagram.svg

What this means is 68% (not 66%) of the values will be from .4 to 3.6 inches. In other words there is high variability in whether we have little rain (.4") or a lot (3.6") and that is equally likely (monthly). What this does say is that a rainless December (-2 std dev) is as likely as around 5.2" of rain. That's why the predicted 5-6" of rain is notable.

[ScoPro]

We are overnighting in Fredericksburg, so I hope the rain doesn’t start to early before we head back to Round Rock.

[JT-3]

I started draining the rain tank today. Seemed like a good time to do it.