MTW Discussion

San Antonio has had yet another 100º day to add to our endless tally of the number of times we have surpassed the century mark — 47. The high temperature at the Airport was 103º after a morning low of 79º. This is the 12th time this month that SA has reached 100º. The good news, however, is some rainfall was observed at the airport — 0.04″. Other parts of town received as much as one inch of rain thanks to thunderstorms that formed along residual outflow boundaries from the Texas Panhandle.

Today, more isolated storms are possible over much of the same areas that saw rain yesterday afternoon. Most of the activity will form with peak daytime heating and die out around sunset. Severe weather is generally not expected out of these storms, but heavy rain, gusty winds, and small hail are certainly within the realm of possibilities. High temperatures will struggle to reach triple-digit levels due to slightly more moist grounds and from rain cooled air around the thunderstorms that develop.

The culprit for these storms is a weakness aloft that is allowing disturbances to propagate southward into South-Central Texas combined with rich tropical moisture, and an endless source of surface heating (our hot temperatures!!). By tomorrow, the weakness will fill in, so to speak, reducing the northerly flow aloft — thus cutting us off from the disturbances up north. That means means our rain chances are also cut off after tomorrow.

Looking at longer range models, by the end of the 8-day forecast period models indicate a return to a southeasterly flow from the Gulf of Mexico. This will open the door to any disturbance/tropical wave in the Gulf into South-Central Texas. Therefore, rain chances next weekend will hover around 10-20%.

The Edwards Aquifer is holding relatively steady, but should go up a bit the next couple of days thanks to some rainfall over the recharge zone yesterday. Additional showers and thunderstorms are possible today. The Stage 3 Water Restrictions Trigger at the J-17 Well in Bexar County is 640 feet above mean sea level. San Antonio has avoided the onset of Stage 3 restrictions at least 3 times so far this summer due to last minute rains. We will continue to monitor the latest levels. Stay tuned for further developments.

TROPICAL UPDATE:

Since the tropics are officially active, I will now divide the near daily discussions into two sections to reduce the clutter.

The National Hurricane Center is currently issuing advisories on Tropical Depression Two in the East Atlantic. The 11 PM ADT update, the storm was located at 14.0ºN, 36.3ºW with maximum sustained winds of 35 MPH moving west at 14 MPH. The minimum central pressure was 1006 mb.

Now yesterday, the storm looked rather impressive with intense, organized convection around the center of the storm. At one point, it may have reached Tropical Storm status. However, the National Hurricane Center rightfully postponed an upgrade as the system showed signs of weakening and was surrounded by an unfavorable environment for further development. The biggest issue is the overabundance of dry air in the mid and upper levels due to the residual Saharan dust in the Eastern Atlantic infiltrating the system. Tropical cyclones need a calm, moist environment to thrive — Tropical Depression Two lacks that. Overnight satellite presentations show further weakening in overall convective trends. Therefore, this storm risks becoming an open wave before regenerating later, if at all. Computer models are in general agreement that in time, Tropical Depression Two will reorganize and strengthen into a weak to moderate tropical storm as it progresses westward. If it does so, its name would become Ana.

However, in the meantime, what Tropical Depression Two is doing is mixing out the dry air in the East Atlantic making the general area fairly moist. The large area of dry air is shrinking rapidly and becoming increasingly negligent. This will leave the door open for the series of disturbances in Africa to head into a moist environment favorable for tropical cyclone formation. The limiting factors, however, are lukewarm, but sustainable sea surface temperatures, and several areas of wind shear. Right now, there is a narrow path of relatively low wind shear for these storms to travel unscathed. The synoptic pattern aloft favors storm movement along this path, therefore storm development is possible.

In fact, the tropical wave behind the Tropical Depression number 2 is of greatest interest right now in the Atlantic. Several global models have been consistent in developing this wave into an intense tropical cyclone within a week or so as it progresses westward. Like its predecessor, it too is performing dismal this morning with its convective activity. However, in time, this will more than likely change as it adjusts to an oceanic environment. This system bears close watching through the next 48 to 72 hours.

Elsewhere in the tropical Atlantic, the National Hurricane Center flagged a couple areas near the Lesser Antilles for possible development. Right now there is a disturbance producing several storms over the islands. A couple models hint that it might organize into a tropical cyclone and move into the Gulf of Mexico. At this time, there is a low probability of intense organization, but as always we’ll keep an eye on it.

August 2009 has reached 100º or greater 9 times out of the 9 days that have passed so far this month. The high at the San Antonio International Airport yesterday was 101º. This also marks the 44th time San Antonio has recorded triple-digit temperatures; the most times of any summer (the previous record was 36 times in 1998). In addition, it was the 97th time San Antonio has topped 90º this year. Summer 2009 is on track to being one of the hottest — if not the hottest — summer on record to date. The forecast for the week does not show much relief from the 100º heat throughout the foreseeable future.

A few isolated showers and thunderstorms are possible mainly to the south and east of San Antonio during peak afternoon heating. These storms are a result of a southerly flow aloft due to ridging over the Eastern United States. However, most of these storms will be small and few and far between. Rainfall totals should remain relatively light across an overwhelming majority of the region. Otherwise, expect generally partly cloudy skies with temperatures reaching 100º or greater across South-Central Texas.

Tomorrow, the upper level ridge will shift westward over Texas and eliminate any probability for precipitation. Temperatures will again soar into triple-digit territory. However, by Wednesday, the ridge will shift far enough to the west to allow for a return to the Northerly/Northwesterly flow aloft. This will allow for isolated afternoon storms with overnight storm complexes traveling south/southeastward across South-Central Texas. At this time, it appears most of the activity should pass east of San Antonio, but current rain chances in the city remain at 20%. A frontal boundary may pass through South-Central Texas Wednesday night into Thursday ushering in a slightly drier and cooler air mass. Temperatures, however, should hover near the century mark.

So far in 2009 there have been no named tropical cyclones in the North Atlantic. This should change soon as activity in the tropics is on the increase and at a rapid rate. The National Hurricane Center has flagged a region off the coast of Africa just south of the Cape Verde islands for a moderate risk of tropical cyclone formation. This storm has shown signs of organization through the past 24 hours and has continued to organize through the overnight and morning hours. Satellite and microwave imagery indicate a clear and well defined low-level circulation with most of the convection located symmetrically near the storm center. Several computer models develop this system into a tropical storm fairly soon and move it west-northwest through the Atlantic. Behind this storm are several disturbances over Central Africa, which will eventually make their way into the Atlantic. Models also develop a few of these disturbances into tropical cyclones.

The Edwards Aquifer is holding relatively steady, but should be on a downward trend the next few days unless rain falls this afternoon. The Stage 3 Water Restrictions Trigger at the J-17 Well in Bexar County is 640 feet above mean sea level. San Antonio has avoided the onset of Stage 3 restrictions at least 3 times so far this summer due to last minute rains. We will continue to monitor the latest levels. Stay tuned for further developments.

Yesterday marks the third day in a row of an unusual streak of afternoon highs in the summer of 2009 fail to reach the triple digits. We came close as the official high at the San Antonio Airport was 99º after a morning start at 74º. July 2009 will go down in history with an average temperature of 88.8ºF.

Showers scathed the airport in San Antonio for the most part, with the exception of an early morning shower that dropped 0.03″ of rain between 3:00 and 4:00 AM. In the afternoon, a complex of thunderstorms developed along the Mexican border before fizzling out just to the west of San Antonio in Medina, Frio, and Atascosa counties. These storms released an outflow boundary which originates from downdrafts brought down from the the tops of dying thunderstorms. The outflow boundary moved northward through San Antonio around 4 and 5:00 PM dropping temperatures from the upper 90’s down to the mid 80’s. Another outflow boundary from storms over East-Central Texas moved southwestward through the Austin area; that later converged with the outflow boundary that moved through San Antonio between San Marcos and Kyle. The collision of the two boundaries ignited a couple area of thunderstorms later in the evening that moved through southern Travis County, dying out by the time it reached Downtown Austin. Additional storm development occurred in Comal and Guadalupe counties near New Braunfels, providing a lightning show for San Antonio and other surrounding communities. Since then, activity has waned tremendously and only scattered clouds remain.

There is at least one more chance for additional shower and thunderstorm development across South-Central Texas this afternoon, particularly in the Hill Country north of San Antonio. The rainfall across parts of the region the past couple days has left a slight weakness in the upper levels of the atmosphere preventing the full reinstatement of the subtropical ridge that has prevailed most of the summer. The residual energy will allow for at least a few storms to develop with peak daytime heating. Widespread severe weather is not expected, however isolated pockets strong gusty winds and small hail is possible. Most of the activity should diminish after sunset.

The question remains of whether or not we break the record for most 100º days in a year remains unanswered. Again, the record was set back in 1998 with 36 days. This year holds a very close second place spot with the present count at 35. The MyTexasWeather 8-Day Forecast predicts a high of exactly 100º today, but it will be a very close call. If storms stay far enough away from San Antonio, the record will be broken with certainty. Otherwise, the wait continues at least one more day.

Looking ahead into next week, dry and hot weather will gradually work its way back into South-Central Texas as the summertime subtropical ridge dominates the weather pattern. Expect daytime highs to hover at or above 100º with morning lows bottoming out around 80º with little to no rainfall expected through the next 8 days.

The Edwards Aquifer went up yesterday and should continue to do so as some rain did fall over the Recharge Zone on Thursday. Hopefully more rain falls again this afternoon. The Stage 3 Water Restrictions Trigger at the J-17 Well in Bexar County is 640 feet above mean sea level. San Antonio has avoided the onset of Stage 3 restrictions at least 3 times so far this summer due to last minute rains. We will continue to monitor the latest levels. Stay tuned for further developments.