The Week Ahead

Yeah, this was also my experience.

WARNING: Some weather stuff is explained, below. If you don’t care about learning no science and just need your forecast for the next three days, I wholly respect that, here you go:

Foreseeable Future: Wake up in the low 70s. High in the low 90s. It may rain/storm on you. It may not rain/storm on you. We don’t know. We do know it’ll storm somewhere.


At least twice a day, our National Weather Service issues a “written discussion” explaining upcoming weather. I thought I’d try an amateur translation. Below, in blue, is the NWS Sunday afternoon forecast discussion:

Scattered activity developed earlier today along the Cumberland Plateau, with very few echoes showing up elsewhere.

There have been storms near the plateau. Radar hasn’t seen much rain anywhere else.

Hourly LAPS soundings showing winds below 500 mb are turning south, meaning we are seeing some warm advection in the mid-levels, which is lessening our instability.

The atmosphere has many levels. A “sounding” says what the temperature, wind speed direction, dew point, etc., is at each level, from 1000 millibars (surface) up to 100 millibars (53,000 feet).

Today, the winds under 500 mb (that’s 18,000 feet) started to turn and blow from the south, where it’s warmer.  The transport of warm air here is called “warm air advection.”  Bottom line: it’s getting warmer from the surface up to 18,000 feet.

Rain and storm clouds feed on the cold air above. When the air above warms, there’s less instability to feed the storm.  So, when that warm air advected all the way up to 18,000, the atmosphere as a whole became more stable and a bit less storm-friendly.

The 18z LAPS sounding still shows Lifted Index of -4.9 at Nashville, with a CAPE of 2,099 j/kg, a K-Index of 32, so isolated convection is still very much possible across the remainder of Middle Tennessee this afternoon.

18z is Zulu or GMT time. What time is that here? Two steps.

(1) Go from military time to regular time (18 hours = 6 pm).  (2) We’re central time, so subtract 5 (6 pm – 5 = 1 pm).

So, at 1 pm today, the vertical profile of the atmosphere (the “sounding”), showed plenty of thunderstorm ingredients (despite the limited instability thanks to the warm air advection):

  • Lifted Index (LI) is a measure of instability (4.9 isn’t a lot, but it also isn’t nothing. Would have been higher with much colder temps overhead).
  • CAPE is convective available potential energy. This measures how much energy,  if lifted and treated nicely, a storm has to draw on. 2,100 is a lot. Plenty for storm making.
  • The K-Index is more complicated. It asks whether conditions between 5,000 ft (850 mb) and 18,000 feet (500 mb) are storm friendly. Today our K-Index was 32. Consult the below chart.

Storms happen through the process of convection, hence the highlight.  A “moderate” amount of thunderstorm love.

Given these three decent rain/storm indicators, the bottom line is it might rain or storm. We don’t know when or where. Because these indicators become less storm friendly without the sun’s energy, we think most activity will happen before sundown.

Monday (71/90), Tuesday (72/91), Wednesday (72/92)

Current pattern will remain much the same the next few day, with high pressure off to the east, and a warm, moist southerly flow maintaining our existing air mass.

Translation: Monday, Tuesday, and Wednesday will be a lot like today.

So look for continued seasonal temperatures with isolated/scattered convection during peak heating.

It may rain when it /

gets hot. Or it might not. Or /

Maybe it will storm.

Thursday – Friday: Rain Chances Increase

GFS/ECMWF models show a weak cold frontal passage on Thursday, with enhanced probabilities of precipitation leading into the weekend.  The combination of a trough to our east and ridge building out west will steepen the NW flow aloft and further destabilize the atmosphere during this time.

The GFS is an American weather model. The ECMWF is a European weather model. They predict a cold front will arrive Thursday, which will increase our rain chances. High pressure to our west and low pressure to our east will cause winds to come out of the north-west, way up in the sky. North-west winds transport cooler air, which is storm food (see the above “instability” section).

My deal with our weather model provider (weatherbell.com, they’re terrific) is that we not rebroadcast the European data. (We saved Europe from the Nazis, but they won’t un-restrict their weather data). Here’s the American GFS model, 1 a.m. Friday:

This was not brought to you by The Intern, who is spending time visiting family.