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July 2017 - Report and Summary

The thirty-day outlook, issued on June 15, had called for above normal temperatures and above normal precipitation. When revised on June 30, it called for above normal temperatures and near normal precipitation. Compared with July 2016, the month was 2.4 deg. Cooler, and 2.41 inches wetter. Year-to-date rainfall through July 31 was 4.25 inches greater in 2017 than in the previous year.

The combination of a dissipating cold front north of Red River with a crossing upper air disturbance brought two clusters of showers to the central and northern counties on the 1st. The first crossed from southwest to northeast before dawn, and brought rain to about 80 percent of the area with amounts mostly around one-half inch. The heaviest was 2.13 inches at Powderly. The second followed a similar track around mid-day, and brought rain to about 60 percent of the region, with amounts mostly between one-fourth inch and one inch. The heaviest rain with the second band was 3.26 inches at Detroit. The two combined to set a record daily rainfall for Tyler with 1.37 inches, replacing the 1.35 inches from July 1, 1951.

Between the 2nd and 9th, the area was on the eastern fringe of upper air high pressure. This is referred to as a "northwest flow event". The result was that winds aloft were northwesterly, and the earlier thunderstorm complexes or their remnant could make it into the area. Areal coverage on the 2nd and 3rd was under 40 percent, and was mainly north of the IH-20 Corridor. On the 3rd, Bonham measured 8.71 inches. On the 4th and 5th, coverage was near 30 percent, increasing to near 50 percent on the 6th. Detroit measured another 4.48 inches on the morning of the 6th. Coverage on the 7th decreased to around 10 percent; but rose to near 60 percent on the 8th helped out by a weak cold front easing into the area from the northeast. Amounts on the 8th were between one-half and two inches, with the heaviest report of 3.60 inches at Linden. Coverage on the 9th was also near 60 percent with the weak front stationary along the IH-20 Corridor, mainly over the central and southern counties.

Between the 10th and 14th, the upper air ridge re-positioned itself, oriented from southwest to northeast. East Texas was on its southern edge, permitting scattered mainly afternoon showers each day enhanced by the sea-breeze front. Coverage on each day was between 10 and 20 percent, with temperatures continuing to run near normal. Marshall received 2.15 inches of rain on the 14th. On the 15th, the ridge re-oriented itself along a more south-north axis. In addition, a tropical wave the remnant of what had been Tropical Depression 4 about ten days earlier over the Western Atlantic Ocean crossed. The result was shower coverage nearing 50 percent on the 15th and 16th. A dissipating cold front, moving in from the northeast, helped in the increase of coverage. Amounts were under one inch each day.

Between the 17th and 22nd, the upper air high pressure ridge re-asserted its control. The axis was to the north of the region, and the ridge was not excessively strong. Nonetheless, temperatures were near to slightly above normal through the week, and shower activity was very limited and primarily over the southern counties in response to the sea-breeze.

On the 23rd and 24th, the ridge shifted to the northwest and a weak upper air disturbance was along the Central Gulf Coast. This resulted in showers on both days, with coverage of about 50 percent each day, and slightly lower temperatures at some locations. Between the 25th and 28th, the upper air ridge built back over the region, resulting in a sharp warm-up and an end to shower development. A cold front entered the region on the 29th, lowering temperatures to near normal levels through the end of the month, and bringing showers to the northeastern counties on the 29th.

The reporting period for temperatures and phenomenon on each day is for the twenty-four hours ending at midnight hours GMT--6 p.m. CST and 7 p.m. CDT. The reporting period for precipitation is for the twenty-four hours ending at noon GMT--6 a.m. CST and 7 a.m. CDT. All times are given using the twenty-four hour clock, and are expressed in Greenwich Mean Time.

Observations are from NWS Station 41/9207/4 in Tyler, Texas. The term "normal" refers to averages from the standard climatic period 1971-2000.

July 2017



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