The latest NAM data has become available. According to the NAM, the snow starts between 9 and 11 AM. Total QPF falling as snow varies by location, but in and around the city, about 0.30 inches water or about 3 inches of snow before mixing with sleet and freezing rain.
North and west of the city, QPF values falling as snow increase to about 0. 40 inches water, with an additional delay in the changeover. About 4-5 inches possible in areas north and west of the city.
Because it’s already the third week of February, the sun angle is high; solar insolation through clouds may reduce accumulation on dark pavement and roadways.
The changeover to sleet is expected between 4:30 and 6 PM. An hour or two of sleet and freezing rain before changing to rain.
Most of the heavy precipitation will fall as rain, leaving a soggy mess by morning. Temperatures are expected to stay above freezing towards morning, so additional freezing of fallen precip is not expected with this storm.
These sort of warm air over-running cold air scenarios with cold air damming are tough to predict accurately. Let’s see how it unfolds.
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.
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.
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.
(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
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.
I’ve been looking at the latest GFS and NAM models from this morning. Sunday night into Tuesday promises some interesting (at least for this season) winter weather.
Two systems will affect us, one Sunday evening and the other starts Monday evening. The first approaches Sunday with light snow developing sometime during the evening and ends early morning Monday.
There are differences in the models with the QPF. The NAM is showing 0.23 inches water, the GFS less. With the prior two winter weather events, the NAM out-performed the GFS. So it appears that 2, possibly 3 inches of snow will have accumulated when it ends Monday morning.
Monday will be mostly cloudy as another more intense storm moves to our west. Cold air in place at the surface with warm, moist air over-running the the cold air will result in snow initially.
The GFS has the snow starting early on Monday evening. Critical temperatures in the upper atmosphere warm by 3-5 AM Tuesday morning, with the snow changing to sleet and freezing rain at that time. It’s possible that 3-4 inches of snow will have accumulated before the changeover.
Surface temperatures may remain at or below freezing until 7 AM Tuesday, so things will be icy and messy Tuesday morning.
North and west of the city, the freezing rain and sleet may be prolonged into late morning.
By noon on Tuesday everything should have transitioned to all rain. This looks messy and the temperature profiles and QPF predictions will likely change in the next few days. Stay tuned.
Thurs Noon update: Wow, more snow than I had forecast! The low level atmosphere temperatures are at or below freezing. The NAM upper atmosphere critical thickness level seems to have fooled me this time around. It works in the winter, apparently not in November. 😱 I’ll make a note of it!
So, let’s try this again. Based on the latest NBM (which I’ve been ignoring, except for the precipitation start time), the changeover to sleet and then rain will occur about 1 -2 PM in PHL. This is a tough business.
Based on the latest NAM and NBM model data, the forecast from last night is on track.
Precipitation is moving in, as shown on radar, at 10:30 AM. On this radar image, it’s easy to see how the precipitation is hitting a wall of cold air, eroded at the northern edges—
This is a sleet to rain event, with some snowflakes possible at the start. The latest NAM supports the possibility of a brief change to snow before daybreak Friday. (Little or no accumulation.)