Hurray for Northwest Flow

Dalhart Weather Review
by Aaron Graves

It seems a lot of attention is given to spring thunderstorms, and rightly so. Spring is the peak of our tornado season (during non-drought years, of course). Storms that form along the dryline and move east bring with them large hail, heavy rain, damaging winds, and a wide variety of storm chasers. 

However, as spring turns into summer, the dryline fades away as a weather producer. Yet Dalhart traditionally sees more rain in July and August than it does in April and May (see stats below). Most of the activity comes from New Mexico and Colorado and arrives in the evening or overnight hours. Why? 

The answer, in case it comes up on a test, is “Northwest Flow”. What is that? The Amarillo office of the National Weather Service provides the answer in the latest edition of “The Dryline”, their quarterly newsletter. It can be read online at www.weather.gov/ama. 

Simply put: Storms like to form along the Rocky Mountains during summer. Northwest flow refers to upper air currents (moving from northwest to southeast) that blow the storms in our direction. 

More complicated: Imagine a big letter “H” hovering over Arizona on a weather map. This is the subtropical ridge, and area of upper level high pressure that sets up over the southwestern United States during summer. Air moves clockwise around the “H”. If you trace your finger around the top of the “H”, you cross over the Rocky Mountains and circle down towards the Texas Panhandle. Any storms that get in the way are brought along for the ride. 

Sometimes these storms can be severe. In fact, a single storm back in 2008 traveled due south from Kansas, circling around the top of  the high pressure area. It was tornado-warned for over two hours, and a tornado was spotted near Black Mesa in Oklahoma. As the storm approached Dalhart’s city limits, it finally made a turn towards the east, and we didn’t see a drop of rain. 

In addition, these summer storms can bring heavy rain over several evenings. Thanks to “Northwest Flow”, we receive beneficial rains from June through August and even into September. 

Dalhart picked up an additional 0.36” of rain at the airport during the early morning hours of Sept. 12, according to the NWS. This brings our monthly total to 1.13”, a good quarter-inch above what we should have at this time of month. Our year to date precipitation total is 9.77”. 

Additional rain reports from Sept. 12 include: 0.75” in my rain gauge on the southeast side of Dalhart, 1.10” from a CoCoRahs observer on S. Sedan Road, and 0.73” from a CoCoRahs observer in Texline. Texaspivot.com reports most areas of Dallam and Hartley Counties got between 0.40” and 0.60” of rain, including up near Texline, north of Dalhart along Hwy. 385, around Hartley, and north of Channing. However, some pivots reported less than 0.20”. 

September has not disappointed us when it comes to cooling things down a bit. Our average daily high for the first half of the month is 82.5 degrees. In fact, the cold front that brought us rain on Sept. 12 kept our high temperature at 62 degrees. The last 60 degree day we enjoyed before Sept. 12 was on May 14. Likewise, our low temperature on Sept. 12 was 44 degrees. The last 40 degree night was on June 10. 

Above, I mentioned that Dalhart receives more rain during July and August than during April and May. This is based on information provided by the National Climatic Data Center. They establish normal high and low temperatures and precipitation totals by using statistics over a 30 year period. The current 30 year period is from 1981 to 2010. Here is a look at what is normal for Dalhart by month in this format: High Temperature, Low Temperature, Precipitation:

30 Year Averages for Dalhart

Jan: 50.0, 19.4, 0.52”
Feb: 53.0, 22.6, 0.40”
Mar: 61.2, 29.4, 1.17”
Apr: 69.5, 37.7, 1.08”
May: 78.4, 48.4, 2.24”
June: 87.4, 57.8, 2.30”
July: 91.4, 62.1, 2.79”
Aug: 89.2, 61.3, 2.85”
Sept: 82.0, 52.8, 1.65”
Oct: 71.1, 40.4, 1.47”
Nov: 59.6, 28.1, 0.58”
Dec: 49.0, 19.8, 0.54”

High and low temps the past week:

Sep 8: 87, 61
Sep 9: 95, 67
Sep 10: 80, 63
Sep 11: 77, 58
Sep 12: 62, 44
Sep 13: 73, 45
Sep 14: 83, 53