Watching winter precipitation risk 

Posted: 7:38 am Wednesday, December 6th, 2017

By Kirk Mellish

When there is a risk of snow or ice in Georgia the question usually concerns either marginal temperatures or marginal moisture, or both. The proximity to the Atlantic Ocean, Gulf and mountains all make forecasting here all the more complicated and difficult.

In the case this week, most data supports temperatures aloft being cold enough for snow to start in the clouds, the question is will there be sufficient LIFT (ascent) and MOISTURE to produce enough precipitation for it to fall from clouds and reach the ground or fall enough to not melt?

For a week now there has been zero agreement between the models, and each model has often been at war with itself from one run to the next from one day to the next, and sometimes they “switch sides”.

Thus, any call is a low confidence one at this point.

The bottom line for me as of now is precipitation amounts look too low to be a problem even if some of us see snow flurries Friday.

However, it would NOT take much of a shift in the deeper moisture to the north from the south to change that to a minor coating from the perimeter southward. So the situation bears monitoring for any changes.

However, even if that coating occurred with warm ground I think it would only be on grassy areas and other objects above ground and it would then melt. Just wet roads with temps above freezing during the day. But will need to watch black ice potential at night.

You may have read my blog the other day WARNING that for a few days forecasts would be more subject to change than normal every 6-12 hours due to the jet stream speeding right above us.

WEDNESDAY AM SURFACE WEATHER:

radsfcus_exp_new

The process we are monitoring the next few days concerns among other things the QG quasigeostrophic omega equation covering thermal and differential vorticity advection with isentropic upglide.

A gross over simplification of this is often called “overrunning” (chart from Chuck Doswell):

overrunning_1

The cold front that passed through with the rain yesterday stalls to our south and with a southwest to northeast jet stream flow overhead (with embedded spokes of vorticity), moisture is transported and lifted above the front into the cold air.

As of now it looks the bulk of the precipitation will be to the south and east of Atlanta.

But at this point the timing, location, and amount of moisture and thus any rain or snow remains VERY IFFY and much in doubt. I am not impressed at this point but will stay vigilant in case I am wrong.

Forecast updates as needed.

WEATHER CHART WEDNESDAY:

94fndfd_init_2017120600

WEATHER CHART THURSDAY:

98fndfd_init_2017120600

WEATHER CHART FRIDAY:

noaad3

UPPER-AIR CHART MID-DAY FRIDAY 500mb Jet Stream:

Screen Shot 2017-12-06 at 9.25.45 AM

Follow me on Twitter @MellishMeterWSB.

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