Wednesday, March 26, 2014

Storms in the West and East

It's a remarkable day if you are a meteorologist.  We'll talk first about the pending storm for the Wasatch. A weak surface cold front pushed into northern Utah this morning without much fanfare.  However, precipitation will eventually move in this morning and we will likely see periods of mountain snow in the cold (compared to what we've seen the past few weeks), unstable westerly and northwesterly flow through late Thursday night or Friday morning.  The system will eventually be cold enough that at times the snow level may push to the valley floor.  If you see some white, don't be surprised.

Amongst the products that we use to forecast the weather are time height sections.  They basically provide a series of profiles so that you can see the temporal evolution of the atmosphere at a given point.  It is convention for time to increase to the left in these diagrams.

The time-height section from the 0600 UTC NAM for Salt Lake City is below and shows profiles of wind (barbs), equivalent potential temperature (a.k.a., theta-e, which can be used to identify fronts and evaluate atmospheric stability), and relative humidity (shaded).  You can see the model surface-cold frontal passage between 0600 and 1200 UTC ("Z" on the plot below is shorthand for UTC, note again that time increases to the left) and then the lagging moisture, which is why the frontal passage was mainly dry.  Beginning later this morning through 12Z Friday, we have a fairly deep layer of moist (RH > 80%) westerly to northwesterly flow over us.  The theta-e lines are also oriented almost vertically, which implies very weak stability.

I don't expect it to snow constantly during this period,  There are some weak waves that ripple through the flow and pulses of moisture (note the higher RH periods) that are going to modulate precipitation rates.  In addition, surface heating may play a role (thunder is a possibility on Thursday).   Nevertheless, it should add up.  The 6Z NAM is going for 1.08" of water and 20" of snow through Friday morning in upper Little Cottonwood.   If we look at the 9Z SREF (an ensemble of model forecasts).  I'm going to go for a storm total of 15 to 24" at the Collins stake by 6 am Friday.  Keep in mind that will come in pieces.

Meanwhile, back in the east, and tremendous explosively deepening cyclone developed overnight and is affecting portions of New England this morning.

The latest observation (1353 UTC) from Nantucket shows heavy snow with sustained winds of 48 mph and gusts to 68 mph.  Surfs up!

Analyses from the Ocean Prediction Center (OPC) showed a central pressure of 1007 mb at 1200 UTC yesterday and 984 mb at 0600 UTC today (23 mb in 18 hours).  The 12 UTC analysis hasn't come out yet, but it appears from satellite imagery that the bottom pretty much fell out in that last 6-hour period and that we are probably dealing with something near 960 mb, for more than 40 mb of deepening in 24 hours.  A friend from the OPC send the beautiful image below of the storm at 1315 UTC with an overlaid lightning-strike density forecast.

Source: NOAA/NCEP/Ocean Prediction Center
What a beast.

1 comment:

  1. As an amateur weather weenie, I love topics like this. Thank you for the great post.