Day 27: The Night Before The Storm

Posted by pmarsh on 27 Jan 2010 | Comments Off

Ah, the night before a forecast big storm. Meteorologists keep checking the data non-stop hoping the latest piece of information is available. When the numerical weather prediction models’ forecast transitions away from the forecaster’s desired forecast, a lot of “wishcasting” takes hold. Meteorologists will often tend to find any piece of information to sustain hope that what they want will ultimately develop. This internal struggle can be a challenge for a meteorologist. I know I struggle with it from time to time.

As residents throughout the southern plains head to bed this evening, most are still unsure what tomorrow’s weather will hold.  This is because forecasters are still struggling to understand the evolution of the winter storm.  It is apparent that somewhere will see heavy snow, somewhere will see sleet and freezing rain, and somewhere will only see rain.  The problem is pinpointing those exact locations is difficult even 24 hours in the future.

A couple of things struck me tonight with respect to the shortwave trough (upper-low) responsible for the difficult forecast…

Above is a satellite image depicting the moisture content of the middle-to-upper atmosphere (brighter colors means more moisture aloft).  This is important because the most efficient way to get moisture aloft is for there to be rising motion in the atmosphere.  Thus, wherever there is the brighter colors, we can infer rising motion.  Rising motion is important because it is a key (but not a sufficient) ingredient  in the development of precipitation.  Thus, with all that rising motion in the southwest United States, one would expect a lot of precipitation in the southern plains as the rising motion moves into the area.  This agrees quite well with the precipitation forecasts from the Hydrometeorological Prediction Center (HPC) shown over the last few days.

Below I’ve identified the center of the upper-low (red L) and what I suspect will be the eventual forecast path based on a combination of observational and model trends.  This is considerably farther north and west than what was forecast several days ago.  As I mentioned last night, a farther north/west track will result in more places being affected by the dry slot – including Oklahoma City and points south and east.  However, before the dry slot affects central and eastern Oklahoma, these places will spend an extended period of time in the warm conveyor belt which will bring warm, moist air.  This warm, moist air will result in heavy precipitation and a warming of the temperature aloft (and possibly at the surface).  This warming aloft will prevent snow crystals from forming in central Oklahoma which leaves sleet, freezing rain, or a cold rain as the resulting precipitation types.

As you can imagine, I’ve been pretty busy the past few days handling all the requests for information and discussion regarding tomorrow’s (possible) winter storm.  I didn’t have time to annotate any more graphics tonight, but I will leave you with something better.  Below is a recording of today’s HWT Map Discussion.  It may be a little too technical, but it can give a better glimpse as to what a forecaster must look at in situations like these.  Please feel free to ask questions and provide feedback regarding anything in the last few days worth of blogs and / or the video below!

27 January 2010 NOAA HWT Map Discussion from Patrick Marsh on Vimeo.