Thursday Morning Update on Three Storms

Storm #1 (Friday morning through Noon)

  • Rain showers will mix with snow showers across the northern and western suburbs on Friday. Precipitation will end shortly after noon.  A conversational dusting is possible on grassy surfaces in the far northern and western suburbs with up to an inch possible in the higher terrain, the closer you get to the Pennsylvania line and the Catoctin mountains of northern Maryland. The image below is the latest North-American-Model (courtesy TropicalTidbits). Time stamp begins at 7AM Friday and runs through Noon.

Latest European model keeps accumulating snow (likely on grassy surfaces at elevations below 1500 feet) across extreme northwest Virginia & northern Maryland:

Storm #2 (Sunday)

The upper-level disturbance responsible for how far north Sunday’s system tracks is still over the Pacific ocean and won’t reach the atmosphere above the Pacific Northwest until Friday morning at which time the models will have the best sampling (input) data for a solution (eventual track).  The ensembles overnight have continued to show a suppressed track to the south, keeping light accumulating snow across the Appalachians of southwestern Virginia, Tennessee and North Carolina with a band of accumulating snow stretching across extreme southern Virginia and northern North Carolina. If we start to see a shift back to the north, I’ll let you know.

GFS Ensembles showing disturbance (circled in red):

By 12z (7am EST) Friday, the upper-level disturbance will have reached the Pacific Northwest coastline at which time we could possibly see some slight changes in track

Latest GFS operational model (image below) still keeps norther-most shield of accumulating snow south of the Washington Region:

Storm #3 (Monday Night through Wednesday):

Latest European Ensemble mean track (if you average all the placements of where the low pressure may form), image courtesy TropicalTidbits.

  • This event is now still four and a half days out.  What the northeast has going for it is cold air in place behind Sunday’s storm system, a deepening upper-level trough (Monday into Tuesday) and a strong upper-level low to the west of the surface (coastal) low pressure system that forms. The eventual orientation of the trough (how far south it deepens, its orientation (positive, neutral or negative) and how soon phasing occurs (when the coastal storm and upper-level low unite) will all determine how much snow (if any) falls across the big cities of the Northeast from here in Washington to Boston. 

Latest European Ensembles

What is the difference between the Ensembles and individual operational modelsOperational models” (GFS, Canadian, NAVY, JMA, European) take a new sample of the latest atmospheric conditions (input data) every 6 to 12 hours (depending upon model) and then calculate one forecast solution.

The problem:  We don’t have the best sampling of atmospheric conditions across the vast expansive oceans and because the jet stream contains multiple disturbances that interact with each other (think of a river of air) any slight variations that occur in “actual track” of these atmospheric disturbances have huge impacts on the eventual evolution of the future storm track.   Because of this, forecasters utilize the “Ensemble method” to look for consistency (on in many cases, inconsistency). 

Remember, when a meteorologist is talking about the “Ensembles” they are referring to a process where the input data (current environmental pressure etc.) is slightly altered several times (up to 50 tweaks/alterations in some cases) in an attempt to compensate for our lack of perfect atmospheric sampling (think of the large expansive oceans where weather balloons are not released).

The individual “slight alterations in input data” are ran each time (in some cases up to 50 times).

For example, take a look at the results below from last nights run of the European Ensembles (various solutions). The image below is showing potential snowfall outputs by 7am Wednesday of next week:  Not a lot of consistency eh? 

 

Latest operational 06z (1am Thursday EST) GFS Model:

  • The latest GFS phases the upper-level trough and the surface low off the coast of New England (I’d like to see the phasing occur much sooner) but still provides accumulating snow from the upper-level low pressure system.

Latest Operational Canadian (GEM) model:

  • The latest Canadian is currently the furthest east in track but still shows the upper level low pressure system providing a period of steady light accumulating snow.

It’s too early to run through every single individual operational model output. I am just trying to explain how forecasting works.  If you have any questions, drop me a line.  Have a fantastic Thursday!

10 thoughts on “Thursday Morning Update on Three Storms

  1. Jim

    Hi Tim, I notice a 10:1 liquid equivalent on the ECMWF. Is the model forecasting this ratio as just a regular default or does it also model the temperature of the local atmosphere 4/5 days out and adjust the ratio accordingly? Or, perhaps the model uses the 10:1 ratio as the default and then adjusts the ratio closer to the actual event, say 2 or 3 days out?
    Thanks,
    Jim

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    • Tim

      There are multiple algorithms that the Euro offers, 1:10, 1:15 something called a Kuchera ratio (google it)… etc. So forecasters typically will come up with a ratio that favors the surface temperatures. If its snowing at 32 degrees… 10:1 makes sense (10″ of snow out of an inch of liquid). If its snowing at 25 degrees 1:15 ratio would make the most sense… etc.

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