[su_box title=”Late Afternoon Update” box_color=”#defcdc” title_color=”#000000″]The NAM is still showing 0.17 inches water for snow tonight. (1-2 inches snow.) The GFS is maintaining much less QPF – 0.08 inches and the statistical models and NBM seem to side on the lower amounts of (0.5 to 1 inch of snow.)

Of interest is that the NAM and to some extent, the GFS has some light snow into Monday morning and again late afternoon Monday.

Later Monday-  Temperatures will fall below freezing early evening Monday and snow will develop and become moderate for some hours during the night.  It appears that temperatures at critical levels will rise above freezing about 6AM Tuesday, BUT temperatures at the surface may remain freezing for well into the morning. After a few inches of snow, significant sleet and freezing rain is a possibility for Tuesday morning until about noon. I’ll update this evening. [/su_box]

from earlier this morning:

[su_box title=”Morning Update with GFS Data” box_color=”#defcdc” title_color=”#000000″]This morning’s GFS maintains a low QPF of about 0.10 inches for the general Philadelphia area, more further south into Delaware. It also has the snow starting later than the NAM, after midnight tonight. So we’re down to 1 or 1.5 inches of snow.

The blocking effect of greater colder air with the GFS suggests that the icing potential from cold air damming might be considerable on Tuesday.   I’ll keep an eye.   And I’ll update with this evening’s NAM  about 9:15 PM, when the early data becomes available. [/su_box]

from earlier Sunday morning...

With such large QPF differences last night between the NAM and GFS, something had to give.  This morning’s NAM model run just becoming available shows a decrease in the NAM QPF to 0.17 inches water.  This is still double the GFS, but more of a reasonable discrepancy.  So we’re back to about 1.5 to 2 inches of snow at daybreak Monday morning.

For today–  While it’s sunny right now, clouds should increase about noontime and thicken during the afternoon.  Light snow starts about 9 PM in the western suburbs and ends about daybreak Monday.

The National Blend of Models (NBM), based on some warming in the upper atmosphere is suggesting that the final precip Monday morning may include some sleet or freezing rain.  There’s also the chance of some spits of precipitation during the day on Monday.

For late Monday afternoon and early evening, snow is expected to redevelop with several inches fallling before a transition to sleet and freezing rain before daybreak Tuesday. An extended period of freezing rain, especially north and west of Philadelphia appears likely Tuesday morning.

Let’s see how well the models do with tonight’s light snow before tackling the Monday into Tuesday mix.

[su_box title=”Weather Update Fri 8 AM” box_color=”#defcdc” title_color=”#000000″]So, yesterday’s models were significantly off about the amount of rain we had early this morning. (We had double the amount.) Can we depend on them for a 5 day forecast?

The current trend for Sunday night: the NAM maintains about a 2-3 inch brief snowfall before daybreak Monday. The GFS has very little. I’m going with the NAM at this time.

For Monday night into Tuesday, it appears it will be briefly snow changing to freezing rain and then all rain for most of the storm. The secondary low coastal formation doesn’t appear to be very strong. Stay tuned.[/su_box]

from yesterday…

As mentioned in my previous post, high pressure will move in for the weekend behind a cold front that moves through Friday morning. The frontal boundary will stall to our south.

For Saturday and most of Sunday, we will have mostly sunny skies but with temperatures below average, in the 30s.

The stalled frontal boundary will become associated with a developing low pressure system in the Midwest and will redevelop as a warm front, moving slowly north as it tries to displace the cold air at the surface.

Warm air will start to overrun the cold air with an initial weak impulse that may bring some light snow late Sunday night or before daybreak Monday.  (This looks to be very insignificant.)
[su_note note_color=”#d9f2da”]Thursday evening 2/7- Latest NAM has 1-3 inches of snow Sunday night. [/su_note]

As the warm front slowly approaches Tuesday, snow may develop Monday night into Tuesday, but it appears that it will mix with sleet and change to all rain sometime Tuesday. This is the most likely scenario.

But, there’s significant disagreement among the models with this system. While the change to all rain (after some freezing rain) is the most likely scenario, the GFS and FV3-GFS suggest a secondary low develops along the Delmarva coast, possibly allowing more snow. The Canadian model is similar but maintains much more cold air, with more snow.  The European model has a much later development into Wednesday, with the secondary low developing north in New England, giving us mostly rain.  

The Tuesday through Wednesday timeframe looks interesting.  Stay tuned.