Questions about Mid-Month Pattern

QUESTIONS ABOUT MID-MONTH PATTERN

For much of the next week here in Wichita, we will be looking at gradually increasing temperatures and chances for storms. Will the pattern change and do we have any cool downs in store for us? Lets take a look.

Multiple waves of energy look to traverse the US through the end of this week and into the weekend. Looking at water vapor imagery this morning, you can make out multiple pieces of energy with the most organized looking system being located in the East. There is also another piece currently pushing onshore in the West. This will be what brings the Plains to Upper Midwest severe weather chances this weekend.

Zooming into this severe weather threat would show that there is a decent setup. That aforementioned wave moving onshore in the West will progress eastward, with the GFS showing its position to be titled from east Kansas through southern Minnesota Saturday evening. Coupled with upper level divergence present, these two mechanisms will provide appreciable amounts of lift to promote thunderstorm development.

With respect to moisture and how buoyant the atmosphere will be. There are a few things which are going to allow air to rise. First off, dew points in the 60s-70s coupled with diurnal heating is going to push CAPE values at least into the 1000-2000 J/kg range. Areas of higher instability will be present across southeast Kansas through south-central Iowa.

On top of this, dry air at 700 mb will continue to promote upward motion via latent heat release.

As storms develop on the front, northwesterly shear coupled with the movement of the shortwave being zonal (west to east) will push storms east-northeast. Storms will continue through the overnight into Wisconsin given the presence of a LLJ. These overnight storms could bring much needed rain to portions of northern Iowa where portions of the area are experiencing extreme drought.

Beyond Saturday, the weather will remain mostly clean through much of the next week.

The next significant chance of rain will be next weekend as a cold front attempts to push through the Central Plains. Currently, there is agreement from model output that storms will occur either Saturday or Sunday. The questions are, does the front move through and cool us down? OR do we go right back to the pattern we have been dealing with?

Looking at what the disagreement is, you can see below that the GFS has a more anomalous trough across the High Plains and Prairies while the Euro has substantial ridging setting up into the Northwest.

There are big implications here as the GFS would usher in cooler temperatures while the Euro would keep us in a similar pattern to what we’ve been seeing most of the summer (gradual warming, large cold front, gradual warming, so on so forth…). Ensembles would side with the Euro idea. The 00z GEFS mid level heights so a pattern that is more indicative of what the operation euro is putting out than what the GFS is.

Another thing to consider and monitor going forward will be the impact of the tropical Pacific on the mid level pattern across the US mid to late month. Currently, there is a storm moving up west Asia. The remnants of this storm will make its influence on the US about a week later. Perhaps the GFS is onto something here.

However, I like the placement of the blocking ridge in the northeast Pacific. There will also be multiple storms impacting the east Pacific which should act to enhance the ridge to the north.

Perhaps a blend of the Euro/GFS is what is preferred. Namely that a trough will dig into the US but will kick more into the east rather than the High Plains and Prairies. This is what the EPS hints at which means much of the same pattern moving into late August.

Good news with that is for July through August, we have been 2.8 degrees below normal. We also have only got 1.2 inches of rain which is making it dry out. This will make being outside enjoyable compared to usually being roasted out by this point in the summer.

Tropical Activity To Ramp Up Mid-Month

We’ve been in quite the stretch of quiet weather in the tropics, but there are signs that activity will quickly trend upwards later this month.

First off, why the lull?

Tropical cyclones thrive when there is divergence of mass in the upper troposphere. This allows air to efficiently rise. Late in July, much of the tropics were under upper convergence. This causes large scale downward motion. Take a look at the two images below which were analyzed on July 20^th and 25^th. Brown colors denote convergence and you can see quite a bit of it from Central America through the southeast Atlantic on the 20^th and 25^th.

That wasn’t the only thing suppressing development. Through the later half of July, there was wind shear present across much of the Caribbean. Tropical cyclones can strengthen in environments where the deep layer shear is less than 20 knots, but as you can see in the image below, the shear was as high as 50 knots.

Lastly, to get any storm to develop, you need moisture. Much of the tropical Atlantic was pestered with dry air through the later half of July. Looking at the satellite image below from the 23^rd, you can see those brick red/orange colors from the Antilles through west Africa. This is dry air and it has been pushing into the Atlantic storm track, keeping thunderstorm and development potential to a minimum.

Today is August 4^th and as we look forward to mid August, things are changing which could allow tropical activity to ramp up.

First, looking at our convergent wind aloft. Things continue to trend towards more upper level divergence. Examining the analysis from August 1^st, you can see there is quite a bit of divergence developing in the southeast Atlantic. The only area of potential downward motion is across the Antilles.

With respect to shear, there absolutely is still some present, but the spacial area of 20 knots + is much less now than what it was in late July.

Trends would indicate that while there will be areas of shear present across the Caribbean which will act to suppress tropical activity, there won’t be enough shear to eliminate the potential with there being ample areas of favorable shear for storms to strengthen (ex. East coast, Antilles).

Last but not least, looking at moisture. The African monsoon is particularly active at the moment which is causing a deep layer of moisture to move off the continent. See the 06z GFS below.

Dry air is still in place to the north of this area but for the time being, this moisture is going to continue to be serviced by the storm activity over Africa. This is going to allow moisture to surge westward into the Caribbean, and eventually towards the Southeast US by mid month.

If one of these waves can move into a favorable shear environment then it won’t have much trouble quickly organizing, especially with such warm water present. Sea surface temperatures are ripe across the Gulf into the Florida Keys at over 30C but are plenty warm across all of the tropics.

Ensemble models are picking up on this development potential with many members outputting low centers from the Gulf through the Outer Banks mid month. All this is to say that while we have been in a lull, there is good reason not to let your guard down. Especially with us approaching the peak of tropical season.