George R. Kasica
San Antonio Forecast Day #2 - 0600Z January 24 to 0600Z January 25, 2007

The second day of the forecasting period was Wednesday, January 24th and the time period it covered was again from 0600Z on the 24th to 0600Z on the 25th, this again was due to the fact that San Antonio, Texas is 6 hours behind the time in Greenwich, thus the offset of 6 hours rather than a midnight to midnight time frame.

The big picture for the area was one of an uncertain forecast with the forecast models predicting that the low centered over the southwest and Mexico (just barely visible in the lower left center) would move in a retrograde (or westward) direction rather than the more common easterly direction. This type of occurrence is poorly handled by the models and it creates a great deal of uncertainly in the data that they ultimately produce. Just how uncertain that data was would soon become very apparent.

(6Z 1/23/07 surface chart courtesy of the Hydrometeorological Prediction Center - click for full sized image)

Despite the uncertainly that this situation should have triggered in my forecasting, this period the forecast did not seem to be nearly as difficult as the pervious period due to none of the models predicting any extremely large variations in the temperature and wind speed predictions for various outputs as shown below for the three MOS models from 12Z that day. (click here for a full view of the model output). My forecast for the period was a high of 53F, a low of 38F and a maximum wind of 7 kts. (11 mph):

12Z GFS High 50F Low 38F Wind 9 kts
12Z ETA High 55F Low 39F Wind 6 kts
12Z NGM High 54F Low 37F Wind 6 kts

Looking towards the precipitation forecast the 12Z FOUS outputs did not indicate any measurable precipitation on one of the model outputs and just 0.16" on the other for our forecast time period (bold green highlights in detail display), and there was no substantial precipitation after that time so I chose to make a forecast of 0.05" to just be aware of the possibility that there could be some precipitation - I was soon to find out just how wrong I would be, again timing turned out to be everything.

The actual results for the day (click for METARS and climate summary) were quite different than what I had forecast. The high temperature was only 46F(indicated below by the red 10078 group which is 7.8C or 46F), the low temperature was 40F(indicated by the blue 20044 group which is 4.4C or 40F), the maximum wind was 13 mph (as shown in the climate summary link above). In looking at the results for the temperatures in the summary details, its clear that it simply did not warm up as much I had forecast due to the persistent cloud cover throughout the day indicated in the reports by the letters OVC in many of of the hourly reports..

KSAT 250553Z 31004KT 10SM FEW060 BKN120 OVC250 06/03 A3037 RMK AO2 SLP276 T00560028 10078 20056 400780044 50009
KSAT 241753Z 03004KT 5SM RA BR BKN015 OVC037 06/04 A3031 RMK AO2 SLP259 P0010 60042 T00560039 10056 20044 50001

The precipitation total was a whopping 0.94" of rain (shown in the three lines below in the green groups starting with 600 which indicate precipitation in hundredths of an inch) for the previous six hours, clearly I had missed something on this latter item.

KSAT 241153Z 34007KT 6SM -RA BR OVC036 04/03 A3023 RMK AO2 SLP231 P0009 60036 70049 T00440028 10056 20044 55003
KSAT 241753Z 03004KT 5SM RA BR BKN015 OVC037 06/04 A3031 RMK AO2 SLP259 P0010 60042 T00560039 10056 20044 50001
KSAT 242353Z 36004KT 10SM FEW040 BKN130 OVC250 08/06 A3027 RMK AO2 RAE17 SLP244 P0001 60016 T00780056 10078 20056 50007

Again, with this second day of forecasting, the problem was one of timing and picking up on the major clues of when to trust or mistrust the computer models. If I had been more observant in the fact of the retrograde low causing uncertainty in the models and looked more closely at the forecast I may have been able to pick up clues on the pending large precipitation event in the form of persistent rains that did not move out of the area as quickly as expected.

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