Category Archives: Winter Weather

Winter Weather

WINTER WEATHER UPDATE -MON 6 PM

[su_box title=”Winter Weather Update: Monday Evening ” box_color=”#defcdc” title_color=”#000000″]Tonight’s NAM has become available. The NAM has snow starting about 9-10 AM Wednesday and continues as snow until 4-6 pm, when it starts mixing and changing to all sleet by 7pm. Temperatures remain at or below freezing until early evening.

Based on QPF, 4-6 inches of snow may accumulate before the changeover. A further changeover to rain will occur later in the evening. Heavier rain about midnight.

This is a preliminary forecast. Expect changes with future model runs.  [/su_box]

From earlier..

Here’s the latest on the snow that’s expected for Wednesday.  As usual, there’s a fair amount of model disagreement.

This morning’s GFS had the snow starting as late as noon, however this morning’s run of the new FV3-GFS had the snow starting about 8 AM.   This afternoon’s high resolution NAM NEST has the snow starting about 10 AM.

This will be a storm where the total QPF will be divided into snow, sleet and rain components.  Most of the models are in general agreement that there will be about 3-6 inches of snow, with a lean towards the lower end, before a changeover to sleet and then eventually to rain.  BTW, the European (ECMWF) has 4-6 inches for our area.

Models have the changeover from snow to sleet anytime from 3PM to 7 PM.   Sleet will likely fall for several hours as it mixes with rain. The heaviest precip will occur as rain towards midnight and after.

As discussed earlier, this will be another “warm air aloft over-runnng cold air at the surface” type system, actually a warm frontal-type system.

The forecast accuracy for this type of system has little to do with the “track of the storm” as they say on TV, but rather with the three dimensional thermal profile of the atmosphere and the eventual placement of the moisture plumes and dynamics.   There’s no storm center “to track” here; it’s more subtle than that.

Getting the forecast even close to correct probably won’t be possible until Tuesday night’s model runs.

Winter Weather

WINTER WEATHER UPDATE- SUNDAY

[su_box title=”Sunday Weather Update  4:45 PM” box_color=”#defcdc” title_color=”#000000″]Based on this afternoon’s model runs, (yes, the “off-hour” runs that tend to be a bit off), a mix of precip should start between 6 and 8 PM.

The NAM graphic below still is reasonably close to where rain vs a rain-sleet mix will fall, just north and west of the city.

Most models have the changeover to all rain in the immediate area by about 9 PM. I’m interested to see if that occurs, because there’s a layer of cold air at 3000-4000 feet that is predicted to remain below freezing until midnight.  Anyhow, it will change to all rain eventually. Latest QPF is back to over 0.40 inches water. [/su_box]

from earlier today…

The latest NAM and GFS model data has become available.   QPF values have reduced on the NAM to about 0.36 inches water and 0.43 inches water on the GFS for gridpoint Philadelphia.

NAM thermal profiles (which I prefer) are borderline cold for sleet or a mix of sleet and rain when the precipitation starts.   It then changes to all rain by midnight.  No accumulation expected in or around Philadelphia.

NAM Nest graphic
Hi Resolution NAM NEST for 10 PM Sunday night showing sleet

The GFS is warmer and somewhat wetter.  The GFS has no sleet for our area.

So this is a lower confidence forecast for sleet.

As for the start time, here is a NAM-NEST graphic for precipitation moving in from the west about 8:30 PM.  Notice that the precip may start far north and far south of Philadelphia, before it fills in near Philadelphia.

NAM Nest  precipitation graphic for 8:30 PM tonight.  Dark grey is rain/sleet

(Sometimes these ‘holes’ in the precipitation field are artifacts, but several model runs have showed this.)  I guess we’ll see what happens.

With no accumulation expected,  this is really a non-event from a winter weather perspective

[su_note note_color=”#d9f2da”]I’ll soon be turning my attention to the well-advertised storm expected for Wednesday.

As we can see, even with tonight’s rain storm, predicted QPF values can change over 12 hours, as can thermal profiles, especially with these transitional storms. So it’s really just speculation regarding the specifics until we’re in the 24 hour time frame. [/su_note]

 

 

Winter Weather

Winter Weather Update

It’s interesting how much model predictions can change. The Sunday system had been predicted to be a weak, disorganized low with light rain, possibly mixed with snow on the onset.

Tonight’s model runs continue with today’s trend of a more developed low with increased QPF. The NAM predicted QPF has gone from 0.15 inches water yesterday to 0.76 inches water tonight. Some dynamic cooling will allow areas north and west of the city to have some sleet and perhaps freezing rain before changing to all rain. The GFS is somewhat warmer and has some sleet even further north and west, also changing to rain. GFS QPF is about 0.50, also greatly increased from the day before.

Clouds move in mid to late afternoon. The mixed precip starts between 7 and 9 Sunday evening and the rain ends about daybreak Monday and clouds linger.