Monthly Archives: February 2018

Winter Weather

SEVERAL INCHES OF SNOW POSSIBLE SAT EVENING

The latest NAM model data shows a consistent scenario  of low pressure developing along a frontal boundary to our south and east Saturday evening.  QPF values still in the 0.60 inch range.

The usual rain-snow line appears to run through Philadelphia.  Here’s the difficulties with the snow forecast — temperatures at the surface may initially be above freezing, limiting accumulations, especially on paved surfaces.

With the usual “critical thicknesses”  for snow where they are, I would ordinarily be forecasting a mix of rain, sleet and  some snow.  BUT, with the precipitation rate as high as forecast (0.60 inches/6 hours), I am thinking that dynamic cooling may become a big factor with this event. As a result, I am leaning towards more wet, large flake snow than I would ordinarily forecast.  From Philadelphia north and west, 4-5 inches appears possible on grassy surfaces, less on paved surfaces.  Areas far north and west may have 6 -7 inches.

Again, I’m counting on the NAM high precipitation rate for dynamic cooling for this to be snow in Phladelphia; otherwise it will fall mostly as a wet  mix with significantly lower accumulations.

Precipitation starts about 5-6 pm Saturday and ends about 3 am in the morning Sunday.  Whatever falls starts melting Sunday.

Still 48 hours before this starts, so expect changes in the forecast.

 

Philadelphia Area Weather Forecasts

PHILLY WINTER WEATHER UPDATE (THURSDAY AM EDITION)

As mentioned in my post yesterday, there’s much uncertainty with all aspects of the storm that is expected to develop Saturday evening.    Much of the details won’t be resolved until the immediate 24 hours preceding the event. Here’s what we know now-

  • It’s a fast mover.  Over in about 7 hours.
  • Precipitation starts during the late afternoon or early evening Saturday and ends well before daybreak on Sunday.
  • Temperatures appear marginal for snow in the immediate PHL area and immediate western suburbs.  The NAM which showed snow likely for PHL has flipped to a warmer scenario and is now favoring rain or rain maybe with sleet.
  • Temperatures on Sunday, with sunshine, will melt much of whatever falls, if it falls as snow.  There’s no post-storm freeze.

So, anyone thinking this will be “February-style big snowstorm” is likely to be disappointed.

 

 

Winter Storm Outlook

SNOW POSSIBLE FOR PHILLY SATURDAY NIGHT!

The models have been all over the place this winter —There’s been a general lack of continuity with forecasts.   Just a few days ago, it appeared that we would not have any chance of snow through the end of February.   How things have suddenly changed.

After a warmup on Thursday and early Friday, a frontal boundary moves through on Friday allowing cold air to return to our area. A low pressure system is may develop along the mid-Atlantic coast during the day and evening on Saturday resulting in a possible significant snowfall by Sunday morning.     Currently, QPF values are as high as 0.60 inches water with this afternoon’s NAM, but that looks high.

It’s important to note that several forecasted potential storms have not panned out this winter.  The GFS and the NAM have been slow to pick up this possibility, although the EMCWF (European) and Canadian models have been suggesting the possibility for a few days.  There remains significant spread in timing, position and intensity among the models, so confidence in predicting any feature of this storm is below average at this time.   Furthermore, the earlier GFS model had the rain-snow line right near PHL, while the latest NAM has trended much colder, implying more snow.   Additionally, the storm is a fast mover, likely reducing precip totals.

I’ll keep my eye on things and will update over the next few days.

GFS Sunday evening