Daily Archives: Saturday, March 11, 2017 10:58 pm

Winter Storm Outlook

Heavy Snow for Tuesday still not a certainty in PHL

As has been the case this past season,  forecasts of snow have changed in the short term and that’s beginning to appear to be the case with the upcoming snowfall for this Tuesday in Philadelphia.  Here are the current trends–

  • The snow arrives earlier than previously predicted, now starting sometime Monday evening.
  • The NAM is much warmer than the GFS in Philadelphia with a further west track.
  • It’s  unclear whether the warmer NAM is correct.
  • Heavy rain is expected in PHL by daybreak Tuesday according to the latest NAM limiting snowfall totals ( 4 inches).  Heavy snow (15 inches) is expected if the GFS is correct in the immediate PHL area.

As a result of these trends, snowfall estimates continue to be uncertain in the Philadelphia area.

Expect further changes with this forecast.  Stay tuned.

[stbpro id=”info” caption=”Weather Tidbit”] With the switch to Daylight Saving Time, model data becomes available one hour later.  Early data previously available by 9:10 (NAM) and 10:25 -10:35 (GFS)  now become available at 10:10 (NAM) and 11:35 (GFS) eastern daylight time.

So, when you watch the 10pm and 11pm news/weather on TV, keep in mind that their forecasts cannot be based on the latest GFS model runs; it’s simply not available at broadcast time when Daylight Saving Time is in effect. [/stbpro]

Philadelphia Area Weather Forecasts

Philly Snowstorm Outlook

The models have been remarkably consistent predicting the nor’easter that’s expected to develop over the Delmarva coastline this coming Monday night into Tuesday.

As mentioned, an upper air vorticity moving in from the northwest in the jet stream flow will interact with an inverted coastal front along the southeastern states late Monday and spawn an intense nor’easter over the Delmarva area  Monday night into Tuesday.

All global models, including the ECMWF (European), the CMC (Canadian), the NAVGEM (Navy) and the GFS (US Global Forecast System) have been onboard with this development.

What is uncertain is the exact position of the storm early Tuesday, with a 25-50 mile discrepancy between the NJ coastline and the Delaware Bay resulting in either a major snowstorm  or a snowfall that mixes with rain for Philadelphia.  (Areas to the north and west are expected to have all snow at this time.)

As of the current time, it appears that snow will start Monday evening and become heavy after midnight, giving us a heavy snowfall by  daybreak Tuesday.  The snow will either continue or mix with rain in Philadelphia.   Last night’s GFS model had 18 inches of snow for PHL.

If this verifies, it would be the first time this season that an extended range forecast didn’t self-destruct a day or two before the event.   So despite the overwhelming forecast evidence, it’s too soon to be definite about this.