Tuesday, July 22, 2014

Another cold front, then another BIGGER cold front!

It's rare to talk about cold fronts in July as much as we have been this month. All these fronts, Canadian air intrusions, and upper-level lows have contributed to a month that is averaging over 5 degrees below normal and the third coolest July in Memphis recorded history through yesterday. Well, we have a couple more fronts that will contribute to what will very likely end up keeping temps well below normal for the month by the time it ends.

Top 10 Coolest Julys in Memphis since 1875 (2014 through July 21) by average monthly temp.

A "typical" July front (if they make it this far south) is one that sinks into the area and weakens, then washes out with little effect other than perhaps a wind shift. The first front in our 7-day forecast arrives Wednesday night and is a little stronger than typical, but not like what we saw last week. It'll bring a chance of thunderstorms Wednesday and Wednesday night, then shift wind to the north and drop dewpoints (a measure of humidity) back down to near 60 briefly. Temps will also fall back into the mid 80s on Thursday. However, like a typical post-frontal airmass in July, it won't last long. Heat and humidity build heading into the weekend.

Location of surface fronts on Wednesday evening, along with where rain chances are expected via the NWS. The front will turn around and move back north by Friday.

By late in the weekend into Monday, we'll see scattered storms popping back up over the area as another front moves closer. Looking at the airmass pushing in behind this front, it looks like well below normal temperatures are again expected for a good part of next week. We're not forecasting anything like the records we had last week at this point, but those who could live without Memphis heat and humidity will likely get yet another multi-day period of relief. Can one even call it relief when it's been the norm this summer it seems??

Temperature outlook for July 27-31, indicating a high likelihood of below normal temps for a large portion of the eastern U.S. due to a significant upper-level trough expected to dominate the weather pattern. 

On a separate note, today is the 11 year anniversary of the derecho (long-lived wind storm) that swept through the city on July 22, 2003 (dubbed "Hurricane Elvis"). Read our retrospective written last year for the 10-year anniversary here.

Erik Proseus
MWN Meteorologist

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Saturday, July 19, 2014

Cool July weather sets records - the month "by the numbers"

Besides perhaps pool owners who can't seem to get their pools to warm up, there are very few complaints about the unusually cool July we're experiencing in the Mid-South, particularly this past week. For perspective, we thought we'd break down some of those numbers for you. All data is for Memphis International Airport, the official station for Memphis and temperature records extend back to 1875, or 140 years ago.

  • Through July 19th, the average July temperature has been 77.7 degrees.  If July ended today, it would tie for 4th coolest July on record. 
Top 10 Coolest Julys based on daily average temperatures for the month. Records date to 1875.

  • Through July 19th, the average high temperature for July has been 85.9 degrees.  If July ended today,this would tie for 5th coolest average high temperature.
Top 10 Coolest Julys based on daily maximum temperatures for the month. Records date to 1875.

  • Two days have set records for coolest high temperature (minimum maximum temp) on their respective days: July 18 (69 degrees, previous mark was 76 set in 1918) and July 19 (79 degrees, previous mark was 81 degrees set in 1877).
Top 10 (plus ties) coolest high temps for July 18th. Records date to 1875.

  • The 69 degree high on July 18 also set the monthly record for coolest July high temperature (for the entire month), beating the previous low water mark of 70 set multiple times.
Top 10 coolest high temps for the month of July. Records date to 1875

  • The 69 degree high on July 18 was also then the first time in 140 years of record-keeping that a July day had NOT reached at least 70. The previous mark for earliest sub-70 degree day after July 1 was August 23. In other words, after July 1, the earliest Memphis had ever NOT reached 70 was on August 23 - we beat that mark by over a month.
  • The five-day cool stretch from July 15-19, 2014 had an average daily temperature of 71.4 degrees, which ties for the 10th coolest five-day July stretch on record and the coldest such July stretch in 42 years. So anyone that is 41 or younger and lived in Memphis their entire life has not seen a stretch of July this cool in their lifetime.

So what is causing this abnormally cool pattern?  Some tried to blame a summertime Polar Vortex for the cool weather across the eastern U.S. early in the week.  Actually it was a dip in the jet stream of significant proportions that is expected more in the cool season than summer and which we described more fully in a previous post. However, this upper level trough forced a strong cold front all the way to the Gulf Coast. That was followed by a low pressure system that slowly crossed the area the past couple of days, bringing clouds and rain Friday and leftover moisture resulting in continued cloud cover today. All of this is fairly unusual for summertime but more expected in the cool season.

So where do we go from here? Upper-level ridging (high pressure) builds back in over the next few days, causing clouds to break up a bit more tomorrow (resulting in temps into the mid 80s), then more seasonal summer conditions for the first half of this week with highs closer to 90+. Another cold front approaches on Thursday, but looks to wash out over the area (as more typical of summer), then July heat continues into next weekend. Another more significant trough could affect the area in the 7-10 day timeframe to end the month, but that's a ways out.

Hopefully your took advantage of the cool weather and are ready for summer to return! Don't count on another stretch like this week for many years!

Erik Proseus
MWN Meteorologist

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