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> Why will front over Dallas TX move south tonight - or not?, Trying to make sense of forecasts about cold front over N Texas
post Sep 8 2018, 09:33 PM
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EF-1 (New Member)

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Joined: 11-May 14
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There is a cold front that has been stalled north of Dallas, for days. Every day, for days, it has been forecast to sweep southward across Texas bringing 3-7 inches of rain to Austin, in south central Texas. Every day, for days, it has not moved.

Different versions of this have appeared with high consistency on each local news station, never with specific explanations.

None of the critters making these enlightened forecasts answer their phones.

I called someone at the San Antonio NOAA office, and was told that there is no particular reason to expect this front to move at all. The jet stream is very weak, and there is nothing to move the front, unless it should eventually generate enough rain cooled air to move itself.

I noticed that the front is exactly in the same location as the jet stream in that location, and very slowly moving off to the northeast, consistently with the notion that the jet stream is very weak. (The wind in the jet stream must be blowing at 5 miles an hour, or, must have been doing that before today. Today the front and its storms have actually been moving toward the northeast along the jet stream at a pretty good clip.)

I've no idea what originally formed this front; I've an idea it came here from the four corners, but it seems it must always have been right against or caught in the jet stream. You need two weather systems to meet to form a front, and they can't cross the jet stream, that's the whole point of the jet stream.

Earlier today, a low pressure system on the Mexican border moved northward and joined up with the tail of the front, the storms from each meeting up in the middle, west of San Antonio. Then the storms started to move toward Austin and fell apart before they went ten miles. Basically there's a whole mess of rain storms out there, but they aren't promising to have anything to do with Austin. In fact, they're rather promising NOT to. If I didn't know better I'd swear a high pressure bubble must be sitting over Austin.

Looking at the various weather maps, I notice that to the north of Austin, not to mention to the north of the front, the wind is blowing from the north, but south of Austin the wind is blowing from the southeast off of the gulf. From what I've been reading all day, at the point where the winds blowing from opposite directions meet, especially if one wind is blowing from a cold front and the other is warm and humid, they should head upward into the sky, lower the air pressure and generate rain clouds. This is called convergence, or forcing. If it DID happen it might draw down more of the front, the same way the Mexican low pressure system drew down its western end.

Nothing of the sort has happened.

This tells me that something is preventing what you would expect to happen from happening, and leads me to ask what that would be. Whatever it is would notably have to go away for a storm to develop.

Suddenly, Spectrum News, following NOAA in San Antonio, is predicting that this front is going to sweep south over Austin, overnight - generating up to 7 inches of rain. We've been hearing about 3 to 7 inches of rain for days, and so far all week long we've had a single 20 minute downpour where I live, that I got caught in and thoroughly soaked because the weather forecast was notably that it would NOT rain until later in the day. Why should anyone ever believe a thing the weatherman says.

Nobody is saying specifically what has changed that should cause this stationary front that is moving off to the northeast to suddenly sweep over Austin overnight and generate Noah's flood, where that has not happening for days when it should have. But there is not a word of explanation - though there are lots of words of explanation of why none of this happened all day. What's more, the forecast starts, the weather has stabilized around Austin, and then in the same paragraph, it tells us Noah's flood is going to happen when the front finally crosses the city overnight.

There is little difference in temperatures, either at ground level or in the upper levels, north and south of the front. It has stayed stubbornly hot in Austin, around 90, allegedly BECAUSE it hasn't been cloudy and it hasn't rained. There aren't any other storms or anything north of the front, and nothing looks likely to change the direction of the jet stream. Jet stream itself isn't suddenly going to cross Texas from north to south.

At 1 AM when the great deluge is supposed to arrive, the sea breeze that has been blowing from the southwest should actually have turned around and blow in the other direction, so I wouldn't expect it to feed anything to do with an approaching front.

I understand that if the low pressure system from Mexico that merged with the tail of the front, chanced to cross to the East, it might merge with the cold front up north and generate substantial rain, and at one point the weather forecast maps appeared to show that happening, but since midday, that storm has been itself stuck, as well as trying to die out. In fact, from appearances, it looks as if God is standing about 50 miles west of Austin saying "You shall not pass". Or else Austin has its own high pressure bubble, but nothing in these parts has had high pressure in days.

Here's the NOAA San Antonio forecast discussion, complete with my translations. There were helpful bolding and italics but these did not paste in.

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
713 PM CDT Sat Sep 8 2018

An update has been issued for improving conditions around AUS. This
places all the TAF sites [airports] into relative stability, although high pwat
values over the region warrants a mention of vcsh through much of the
early periods. [A lot of moisture in the air may generate local showers.]
Still large areas of light radar echoes signal light
sprinkles possible from an elevated layer of clouds that is
associated with what was an mcv that lifted out of Mexico earlier
today. [the low pressure system that lifted out of Mexico and drew down and
merged with the tail of the front.]
Cold front position appears to be much unchanged for much of
today as mesoscale outflows have governed much of the wind direction
over the I-35 taf sites. [It's saying that gust fronts from storm-cooled outflows
have governed the wind direction over airports along I-35,
and that is specifically why the front has not moved?]
As shortwave [shortwaveTROUGH ]
activity increase across the
area near midnight, this is when thunderstorm chances ramp up at
SSF/SAT and potentially as far north as AUS. The cold front shows an
approach into the I-35 corridor and sets up a period of overrunning
rain and storms for several hours of potentially ifr [instrument flight rules] of lower
conditions. DRT could miss out of the heavy downpours and thunder,
but areas of showers should be in the area through around daybreak.
Along I-35, improvements are expected to linger into the mid-
afternoon, and possibly through the early evening where the front
remains shallow over SAT/SSF.


.PREV DISCUSSION... /ISSUED 251 PM CDT Sat Sep 8 2018/

SHORT TERM (Tonight through Sunday Night)...
Showers and thunderstorms have waned across the western half of the
CWA, but increased in the eastern half of the CWA this afternoon.
With the proximity of an upper-level trough, very moist Gulf air (PWS
AOA 2 inches), and the stalling WSW to ENE frontal boundary across
the area will continue to produce rounds of storms over the next 24
hours. Global and mesoscale models are in good agreement in focusing
the main activity along U.S. 90, including metro San Antonio. That
said, everyone should see some rain by noon Sunday, with many places
receiving 1-3 inches, and a few spots getting up to seven inches.
This should result in a lot of minor flooding, with some moderate
level river flooding. Therefore, a flash flood watch has been
extended in time (until Sunday at 1 pm) and in areas (now covering
all of South Central Texas). We will likely trim back the watch on
Sunday morning as areas clear from the heaviest rain.

LONG TERM (Monday through Saturday)...
The upper level trough begins to drift east on Monday, so rain
chances will drop through mid-week. However, a large scale surge in
moist air and troughing (also likely to affect Florence along the
U.S. East Coast), will bring back higher rain chances Thursday and
Friday. The rain and cloudcover will keep high temperatures at or
slightly below normal all week (around 90 F), while lows stay near
normal in the upper 60s to lower 70s.

Now, weather radar indicates that what is left of the front has somehow rapidly dropped to western end Houston, eastern end in Louisiana, moving rapidly to the east, and a small area of light rain what remains of everything that was going on in western Texas earlier, no other rain. The sea breeze showers have stopped as well, except that for no apparent reason there are rain storms east of San Antonio. But on KXAN news they are excitedly talking about storms developing in San Antonio and going to move NORTH and affect Austin. ????!!!!!!!!! He SAYS the front is actually over AUSTIN, even though its rain band shows as booted off to Louisiana. How it allegedly got there nobody knows - maybe it's not really where it is but where he forecasts it to be. Meanwhile, the weather radar shows that the low pressure system on the Mexican border is not dead, only its northeastern extension is, and he predicts further development of the rain, but not any movement to the east.

Can anybody explain what has changed that should cause this front to cross Texas overnight, generating Noah's flood along the way, or is no such thing likely to happen, that front will be north of Louisiana by morning, and Austin will be rainless once again? Or have ALL the forecasters been doing nothing but smoking weed???!!!!


Dora Smith
Austin, TX

This post has been edited by villandra: Sep 8 2018, 10:23 PM
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