#nowcast

n. A description of present weather conditions or a forecast of the weather immediately expected

The sky above Starkville has gotten very gray over the past few hours, and the wind is picking up. To any normal person, that would suggest that rain is coming.  But if we check radar, it’s not showing any rain headed for Starkville.  In fact, I don’t see any rain any where near us (Mississippi, Louisiana, or Arkansas).  

So what’s happening in the sky?  That’s today’s blog.  I will use the RAP because it gives model output for every hour, and it gives data for all our levels of interest.  All images are for 17Z, which is noon in Starkville.

At 250 mb, there is a jet streak in east Texas courtesy of the subtropical jet.  You could argue that coastal Mississippi is in the entrance region of that jet, but definitely not Starkville.  The winds are diverging a little over the state (moving apart in difference directions), so that could be helping create our dark clouds, but it’s pretty minor.

At 500 mb, we look for strong positive vorticity advection to create rising motion.  We don’t see that here.  If anything we are getting advection of negative values of vorticity (blue colors).  This will lead to more sinking motion than rising, and rising is what we need for rain and storms.  

850 mb helps us see why it feels so much colder today, but doesn’t really help us with the dark clouds question.  We are definitely experiencing cold air advection in Mississippi.  The cold air (blue colors) is spilling further south, and our winds are coming out of the WNW bringing that cold air to us.  But to get rain, we need warm air advection to get the air to rise.  We obviously don’t have that.

It should come as no surprise that 700 mb doesn’t show us any real rising motion over Starkville or much of the state of Mississippi.  In fact, we will stay rain-free through at least early tomorrow morning.  This next map is for 4 AM tomorrow (Starkville time), and there’s still no rain near us.   So what’s with the clouds?

If you loop through the water vapor imagery from the last 12 hours, you will see the culprit.  It’s that sub-tropical jet we noticed at 250 mb. That is funneling in a lot of moisture from the southern Pacific, across Mexico.  All of that moisture is getting wrapped up in the low-pressure system that is currently near the Great Lakes.

NYC Nowcast – Strong to Severe Thunderstorms Approaching

Severe thunderstorms are approaching the area from our west. Several distinct bowing segments of severe thunderstorms have been making their way steadily towards our area from Central Pennsylvania since earlier this afternoon. These storms have a history of producing damaging wind gusts. Sundown over some of the areas under the current Severe Thunderstorm Watch areas in our region may weaken…

NYC Nowcast – Jun 13, 2014

You’ve felt the air warm up and become more humid as the warm front that brought our first round of rain passed through earlier today. Now, we’re getting set for a second round of heavy showers and thunderstorms associated with a pre-frontal trough that’s slowly progressing east towards the area from Eastern Pennsylvania.

Ahead of this line of showers and thunderstorms, the air has destabilized…

NYC Nowcast – May 10 @ 2:40PM EST

A batch of strong showers with some rumbles of thunder is approaching the NYC area. For the remainder of the afternoon and evening, strong to possibly marginally severe thunderstorms may pass through ahead of an advancing cold front. Damaging wind gusts and small hail are the primary threats.

Otherwise, expect broken clouds to cloudy skies and nice warm afternoon in the upper 70s to low 80s…