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April 1999 - Report and Summary

by Robert K. Peters, Ph.D. - National Weather Service Cooperating Observer

The month of April 1999 was warmer and wetter than normal.

With the La Nina phenomenon (a very large pool of cool water in the East Central Equatorial Pacific Ocean) still in place, the trend of the 1998-1999 cool season persisted into April. April 1999 was 4.2 deg. warmer than April 1998, and 4.13 inches wetter. Year-to-date rainfall through April 30 was 1.14 inches less this year than last. Rainfall over the twelve months ending April 30, 1999 totals 51.59 inches--about 113 percent of normal. The thirty-day outlook had called for near normal temperatures and below normal rainfall.

The first week in April saw temperatures six degrees above normal, and rainfall 170 percent of normal. A particularly powerful storm crossed on the 3rd. This caused widespread severe weather in the area on the afternoons of the 3rd with the passage of a cold front, and on the afternoon of the 4th as the front returned northward as a warm front. The area was under various watches from the morning of the 3rd through the evening of the 4th. In Tyler, some street-flooding occurred the afternoon the 3rd. 1.06 inches of rain fell in the 20 minutes between 2:00 p.m. and 2:20p.m.

The second week in February saw temperatures nine degrees above normal and rainfall about 20 percent of normal. Tropical Maritime air controlled the region for most of the week until a strong storm crossed on the 14th. There was one minor intrusion of Polar Maritime air, from which the atmosphere quickly recovered on the 12th. Temperatures were above normal each day of the week, and near to above normal each night.

The main storm track held to the north during the week, allowing disturbances to cross on the 8th and 10th without significant rainfall in East Texas. On the 10th, upper air high pressure began retreating westward to Western North America. This shifted the storm track to the south, allowing the storm of the 14th to bring widespread rainfall to East Texas.

The third week in April was one of sharp contrasts, though averaging five degrees cooler than normal and with near normal rainfall. The week began chilly, with much below normal temperatures through the 18th. Strong warming began on the 19th, and continued through the 21st with readings well above normal by week's end. A Polar Continental air mass was over the area early in the week, with a Tropical Maritime air mass present late in the week. After the departure of the strong storm system on the morning of the 15th--which brought the week's rain (actually falling on the 14th), upper air high pressure was in control. The week was also caracterized by windy conditions.

Winds were fresh to strong and northerly on the 15th and 16th, moderate to fresh and northerly on the 17th, moderate to fresh and southerly on the 19th and 20th, and fresh to strong and southerly on the 21st.

The final nine days of the month were five degrees warmer than normal, and saw near normal rainfall. For much of the period, upper air high pressure over Central North America acted as a block to keep storm features away from the region. A weak cold front managed to make it into the area on the 25th, with a stronger cold front on the 29th. Otherwise, there were high temperature records set on a few days during the period. The cold front of the 25th brought the period's only rainfall.

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.

APRIL 1999



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