Saturday, August 20, 2016

Summer respite continues with a cold frontal passage

A summer-long hot streak ended earlier this week as temperatures finally abated some under abundant cloud cover and several periods of rainfall. While dewpoints remained in the lower 70s, temperatures generally stayed in the lower to mid 80s for highs this past week. However, now that the heat has relinquished it's control a bit, the rain is also getting a little tiresome (boy, we're a demanding bunch aren't we??)!

Cold front arrives

Tonight's cold front should bring a temporary end to most everyone's complaints as it pushes a good deal of humidity and our rain chances to the south for a couple days! We'll still have a good chance of showers this evening as the front moves through the Mid-South, so plan ahead if you have outdoor plans. The front pushes through the metro between midnight and dawn Sunday, bringing an end to the showers and switching wind to the northwest.

The national surface map for Sunday morning shows a cold front just to the south of the metro where it will remain for a couple of days. This will provide for drier air and north wind, leading to more pleasant conditions locally.
It will still be humid in the morning, but throughout the course of the day Sunday, north wind will escort lower dewpoints (less humid air) into the area and clouds will diminish by afternoon. Temperatures will still make it into the mid 80s but with dewpoints in the mid 60s, it should feel much more comfortable. It will also mean that the evening should be delightful as temperatures drop off a bit quicker due to drier air!

The high-resolution NAM (North American) model shows dewpoints on Sunday afternoon. It's been weeks since we have seen dewpoints in the mid 60s, but that is what is expected by this time. 70°+ dewpoints will be pushed just south of the metro. Graphic courtesy PivotalWeather.

Early week forecast

By Monday morning, temperatures are expected to be in the 60s area-wide for the first time since June 10! This will mark an end to the 72-day streak with no temperatures in the 60s at Memphis International Airport that ranks second longest of all-time behind the hot hot summer of 1980. A very nice day is ahead on Monday with highs in the upper 80s and continued low humidity (dewpoints in the lower 60s).

The pattern starts to shift Tuesday into Wednesday, as the front to our south returns to the north as a warm front and higher humidity values return. We'll be back in a more typical summertime pattern with highs in the lower 90s and lows in the mid 70s with low chances of thunderstorms each day to end the week thanks to upper level high pressure ridging over the southeast U.S.

Shown is the GFS (American) model at 500mb (18,000') level pressure and wind pattern on Sunday morning as a large trough (valley in the wind pattern) sits over the Mississippi Valley. The trough is what allows the cold front to get this far south. Graphic courtesy PivotalWeather.

By Wednesday morning, that upper level trough is gone and a ridge of high pressure has regained control over weather in the southeastern U.S. The GFS model at 500mb is shown once again. Graphic courtesy PivotalWeather.

On Long-Range Tropical Forecasts

Finally, a note on long-range tropical forecasts as it appears we head into a more active period in the Atlantic Ocean and perhaps Gulf of Mexico. Some weather entities on social media feed on the long-range models "predictions" of close encounters, or landfalls, of strong hurricanes more than a week or even two out. Our advice: "Beware the Share." Rather than sharing these "hype-casts" or "click-bait," we recommend blocking or un-following the account, especially if the post ends with "Please like/share this post!"

The only time we post longer-range forecasts (beyond 7 days) is if there is A) some model consensus between models and consecutive runs of those models, and B) it's within about a week of impact. Even then, we will also post caveats with those forecasts, as they're really only useful for trend analysis and not exact forecasting. It is near impossible to predict the exact landfall point (or even general area) of a tropical system more than about 5 days out. We simply ask that you be conscientious consumers of information and use some common sense.

This is currently the most responsible graphic for the tropical Atlantic. Two systems are being monitored for potential development in the Atlantic over the coming 5 days. The first (in orange) has a moderate probability of tropical development as it heads generally towards the Caribbean. The second, coming off the African west coast, has a high chance of development but is not currently a threat to any land areas. Tropical Storm Fiona in the central Atlantic is not a threat and is not shown.
Enjoy the respite from summer the next few days!

Erik Proseus
MWN Meteorologist

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