Evaporation Rates.

Sunday, July 17th 2011, 6:07 pm

By: News On 6

I used the fractional water index map to show the 2 inch soil moisture for the morning discussion and have decided to use it again this afternoon. That is the map from the OK Mesonet you see on the right. This shows the average soil moisture 2 inches below the surface across the state. As pointed out this morning, the rain footprint from the localized storms that occurred on Wed is very evident. It is also very evident how dry the soil is for the rest of the state. Next question is, how long will that soil moisture last?

To answer that, one source of information is evaporation rates and it just so happens that the Corps of Engineers monitors evaporation rates at many of the area lakes. Currently, we are losing about an inch of water every two to three days as evaporation rates are running from just over 1/3 inch to near ½ inch on a daily basis. Couple that with the moisture that the vegetation is trying to pull out of the ground for transpiration and it does not take long for the soil to dry out. All of that is to say that the drought continues and burn bans will likely be extended until we receive a widespread, general, soaking rain and right now that does not appear likely anytime soon.

So, the heat just goes on with triple digits expected each afternoon through the coming week and weekend. Not much relief occurs at night either with minimum temperatures in the mid-upper 70s rural locations and 80 or more for the urban locations. Heat index values will be another 2-5 degrees hotter than the actual air temperature each afternoon as well. Winds will remain generally from an E to SE direction through Monday, becoming more S to SE on Tuesday and more southerly for the rest of the week.

Aloft, the dominant ridge that has been parked over the state is expected to drift a little further north and then eastward by later in the week. This does provide a glimmer of hope in that the more south or SE flow aloft by then should allow a little additional moisture to work its way into E OK from the Gulf of Mexico. Hope this is not wishful thinking, but I have dropped a few degrees off the daytime highs and am cautiously optimistic that there will be a little more cloud cover and at least a slightly better chance of afternoon showers/storms by then as well.

Beyond that, there is also a glimmer of hope that the pattern aloft will make a more significant change during the following week. The GFS is faster with this change which is often the case and the ECMWF a little slower. More importantly, this has been reasonably consistent over the last few model runs and the ensemble forecasts suggest that possibility as well. Don't get your hopes up too much, but those trends suggest that dominant ridge aloft will be shifting into the Rockies which would put us into a somewhat cooler, wetter, NW flow pattern. Don't be the farm on that just yet, but we will be watching that possibility very closely.

In the meantime, an isolated late afternoon or evening shower/storm will be possible on just about any given day, but they will be so isolated as to only provide very limited, localized relief.

So, try to stay cool, stay tuned, and check back for updates.

Dick Faurot


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