RECAP: Upper-Level Low Brings Rain, Cool Temperatures

At the early part of the week, an upper-level low retrograded from the Northeast United States/Ohio Valley region towards the Central and Southwest United States.  The result was beneficial rainfall and significantly cooler temperatures for parts of the Southern Plains, including central and western Oklahoma.  While an upper low retrograding - that is, moving from east to west, which is opposite of the typical west-east movement of weather in North America - is not unheard of, it usually only happens a few times a year and moreover, typically not during summer.  Our normal summer pattern in the Southern Plains is a dominating dome of high pressure, leading to rain-free days and nights, high temperatures well into the 90s and 100s, and sometimes high humidity levels leading to heat indices in the upper-100s.  But while the upper-level low moved westward across the country this week, a typical summer pattern was far from the case in the Southern Plains.  On the contrary, an upper-level high "replaced" the upper-level low in the Northeast and Midwest United States, leading to a tremendous heat wave this week for those parts of the nation.

We will now talk a little about what the upper-level low has left us across Oklahoma, including rainfall and the dramatic temperature drop for some.  By doing this, we will also examine the latest drought conditions across the state.  Graphics have also been provided as a visual.

Oklahoma City
Let us begin our perspective of the low by zooming in to our state's capital.  The Oklahoma City metro area seemed to receive the most rainfall and probably the most significant temperature contrast of any other area across the state.  Ironically, Oklahoma City was already in a surplus of rain before this upper-level low, due to a healthy amount of rain from frequent cold fronts in the winter and spring and the flooding rains of May 31.  However, any rain is certainly a welcoming site, and the excess this year will make up for the losses in years past.

As you can see from the graphic to the right, Oklahoma City experienced a 21-degree temperature drop over the course of two days, from July 13 to 15.  The recorded high temperature on July 14 may seem misleading and a little too "high," but the truth of the matter is it is correct because the high for the day actually occurred at midnight!  Temperatures remained steady in the low-70s throughout the remainder of the day with cloudy skies and a steady rain.  While the high temperatures varied considerably over the past week, the lows were a little less dramatic, due to the cloud cover and rain.  On a clear night, temperatures tend to be cooler because radiation received from the sun at the surface is emitted back out into space.  But on a cloudy night, the clouds re-emit the radiation back down to the surface, leading to warmer surface temperatures.

Speaking of rain, Oklahoma City's Will Rogers World Airport officially picked up 4.35 inches of rain from this upper-level low.  As you will see in the drought section below, this puts Oklahoma City at a tremendous amount of rain already received for the year.  Furthermore, the departure from normal provides a better look the large gap between this year and a typical year in terms of rainfall for the city.

Oklahoma

Now we will zoom out and talk about the results of the upper-level low on the state as a whole.  As indicated in the graphic, these rainfall totals are from the past seven days, with the upper-level low making its primary effect a couple of days in, beginning on July 14.  As mentioned in previous blog posts and in the first paragraph, central and western (mainly southwestern) Oklahoma received the most rainfall from this low.  The highest total on the map is Oklahoma City's 4.35 inches.  Areas in the Oklahoma City metro and all of central Oklahoma received some of the highest amounts.  However, parts of southwest Oklahoma also received a great deal of rainfall, such as Frederick, totaling just slightly over 3 inches.  Totals between 1 and 2 inches pretty much sum up the observed rainfall for the rest of the state, except for the northeast and panhandle, which unfortunately received half-an-inch or less.

Drought Update
So some of us received a lot of rain, but what does this mean in terms of the drought?  Before we begin our talk, we need to inform you of a slight disclaimer.  Though the newest drought monitors are released every Thursday, the cut-off time for the drought monitor data is Tuesday at 7 AM CDT.  As a lot of areas, especially in western Oklahoma, received rainfall after this time, the drought conditions presented in the graphic may be a little inaccurate in places.  However, we can still use the monitor to generally assess the current situation.  Note that the rainfall totals listed in the graphic below are valid as of Friday, July 19 at 12 AM CDT.

Just looking at how vastly the total rainfall amounts in the three cities on the graphic vary could be mind-boggling.  Oklahoma City has enough rainfall to cover the deficits of both Guymon and Tulsa, and then some.  As a result of the persistent rain across central Oklahoma, the fact that this part of the state is in a "no drought" situation should come as no surprise.  For the rest of the state, we have one piece of good news and two pieces of bad news.

To start with the bad news, the lack of much rainfall recently in eastern Oklahoma is causing drought conditions, in the abnormally dry to moderate intensity, to return.  This is most noticeable in northeast Oklahoma but also in far southeast Oklahoma.  The second piece of bad news is the continuing exceptional drought conditions in parts of the panhandle, mainly towards the west.  Even pop-up thunderstorms earlier this month have not been able to alleviate the drought in this area.  Please, if you live in the affected areas, use water sparingly.

Now on to the good news.  The exceptional drought is now confined to the panhandle only.  Parts of southwest Oklahoma used to be in exceptional drought, but the rainfall from the upper-level low reduced the intensity to extreme.  While this news may not seem all that good, as extreme drought is still a bad situation, at least we are dealing with a slight improvement.  Furthermore, the drought conditions may change in the next drought monitor to be released next Thursday, as parts of southwest Oklahoma received more rain after the Tuesday cut-off time.

Well, there you have it.  Our quick look at the future forecast shows the chance for some scattered showers and thunderstorms across the state this weekend but the return of the typical summer-like pattern by the start of the work week.  High temperatures across the west will return to the upper-90s and low-100s, while topping out in the mid-90s in central and eastern Oklahoma.  Rain chances will be limited past the weekend.

Sources: NWS, AHPC, U.S. Drought Monitor, Google Images (photo)