Based on this morning’s models, the snow will start later in the morning in Philadelphia. The NAM has it starting about 10 AM Wednesday, while the GFS about noon in Philadelphia. Earlier starts southwest of the city, later northeast of PHL.
Details from earlier…
About last night… pockets of sleet developed about 11 PM around Philadelphia and areas to the west north and west. The colder thermal profiles of the NAM did better than the warmer GFS.
So what about this storm on Wednesday? Wednesday features a similar set up as the several past snow—> sleet—> rain scenarios we’ve had this winter. Low pressure will be to our west and blocking cold high pressure will be to northeast –
An ample flow from the samejetflow discussed last week will bring the moisture.
Unlike many snow ‘storms’ we get with deep low pressure developing along the coast in most winters, this will be another “warm air aloft over-runnng cold air at the surface” type system, actually a warm frontal type system. The actual low pressure system is weak and unimpressive. (There is some suggestion that low pressure may develop off the coast, but that is just one of many uncertainties now.)
This time, we are expected to have a somewhat deeper cold air at the surface, allowing for lingering snow and sleet before the changeover to rain.
With thermal profiles aloft playing such a huge role, we won’t have a firm handle on things until sometime Tuesday evening.
Currently there is a range of snow totals. Precip starts early morning 7-10AM from southwest to northeast…Current estimates are in the 3-6 inch range for Philadelphia, then prolonged sleet, changing to rain in the evening.
An Interesting and active weather period is unfolding for the next several days. Model differences promise to make the forecast more challenging.
Tonight’s NAM has snow developing early Sunday evening and extending into early Monday morning. QPF values have increased over the day and are now at 0.40 inches water. If the NAM is correct, 4-5 inches of snow will accumulate by early Monday morning in and around the immediate Philadelphia area.
The same event is depicted differently by the GFS, which has high pressure blocking the moisture’s northward movement. QPF values tonight are about 0.10 inches water, even less just north of Philadephia.
I’m leaning towards the NAM, which has done the best for the last two wintery weather events. So the current predicted amount is 3, possibly 4 inches of snow Sunday night, ending early Monday morning.
Another larger system will affect us late Monday afternoon through Tuesday. Let’s get through the first one before focusing on details, but the trend is for snow, several inches, changing to sleet and freezing rain Tuesday morning. An extended period of freezing rain is possible even in Philadelphia. An eventual change to all rain later Tuesday morning.
Further complications include the possibility of a secondary low formation.
The precipitation started a bit later than the morning models had forecast and some dynamic cooling allowed snow to fall for the first two hours. It’s changed to rain and the rest of the forecast remains intact.
Winter Weather Update Sat 5 PM
No accumulating snow expected here. Some timing changes. The short range models now show rain doesn’t start in and around Philadelphia until 6-8 PM. Rain ends about 10 AM Sunday. Temperatures drop below freezing about 2-3 PM on Sunday.
Winter Weather Update Sat 11 AM
This morning’s GFS comes in warmer and a more westward track. The severe weather forecasts were possibly justified about two days ago, but the models yesterday and today makes this a heavy rain storm with a sharp cold front for Philadelphia and even areas approaching Allentown.
Any snow flakes will be insignificant with surface temperatures in the mid 30s rising into the 40s.
The latest model NAM just becoming available. Here are the trends for Philadelphia and immediate suburbs –
Last night’s GFS and NAM runs were similar. This morning’s NAM is warmer and faster.
There’s a trend towards a warmer storm, partially due to track change but also due to less intensification, less phasing of jet streams.
Earlier start and earlier end of precipitation.
While I have no expertise, I think the “flash freeze” hype has been just that, and it was likely hype as early as yesterday’s model forecasts showing the precipitation ending before the frontal passage.
Rain ends about 8-9 AM Sunday.
Temperatures drop to freezing about 1 PM and continue to drop
Here are the specifics: Precip starts 3-5 PM. Some light snow possible at start, but a very quick change to sleet and then heavy rain. Little or no accumulation.
Heavy rain through 8 AM Sunday. Temperatures rise into the 40s or near 50. Rain ends 8-9 AM Sunday. Cold air delayed and moves in several hours after rain ends, significantly reducing the chance of a flash freeze. Very windy! Temperatures drop to freezing about 1-2 PM and continue dropping into the 20s. Near 10 at night.
Regarding the “flash freeze”, I think it became its own reality when the models predicted no frozen precipitation falling with when the front moved through. In my mind, a flash freeze occurs where there’s still active precipitation when the temperatures drop rapidly to below freezing. Sure, any standing water will freeze. Is that a “flash freeze?”
Another quick update. This afternoon’s NAM and GFS show a faster storm onset. Precipitation starts about between 3-5pm Saturday afternoon. It’s likely to start as snow, with 1, possibly 3 inches before a rapid changeover to sleet and then rain.
Heavy rain by early evening Saturday. (Freezing rain likely far northwest in Allentown up until midnight.) Rain tapers in the morning Sunday as temperatures plummet and winds pick up. Minimal accumulation expected in the immediate PHL area when all is said and done.
Temperatures are forecast to drop from 49 (GFS) or 42 (NAM NBM) to 30 by 1 PM, then falling further during the afternoon to 20 by 7 PM and single digits at night. Windy conditions may dry some open roadways, reducing icing, but standing areas of water or draining areas from the side of roadways will freeze.
The next model runs come out between 9 PM (NAM) and 10:45 (GFS). I’ll update before 11 PM.
Winter Weather Update Fri 10 AM
A quick update. This morning’s NAM shows a faster storm onset. Precipitation starts about 4pm Saturday. A very fast changeover to sleet and then rain. Freezing rain possible far northwest. Heavy rain by early evening Saturday. Rain tapers in the morning Sunday as temperatures plummet and winds pick up. Minimal accumulation expected in the immediate PHL area when all is said and done.
Last night’s models did very well with the light snow that should be ending within the next hour.
Here’s the latest on the Saturday to Sunday storm. For Philadelphia and its immediate suburbs, a total storm QPF is about 1.95 inches water. Most of this precipitation will be rain, but the beginning and end of the storm will have snow or mixed precipitation.
Based on the latest 06Z run of the NAM and GFS models (1AM last night), the NAM is holding off much of the precipitation until temperatures rise enough when the precipitation begins towards evening. Temperatures in all levels of the atmosphere will minimally support snow, so between 5 and 7 PM Saturday, expect some light snow rapidly mixing with sleet and rain and then all rain by 9 PM. In the far northwestern suburbs, an extended period of sleet and freezing rain may be occur and areas near Allentown may remain a frozen mix until midnight.
The GFS also has the atmosphere warming, but it has more precipitation starting somewhat earlier in the late afternoon, with greater mixed precipitation accumulating before the changeover to rain about 9 PM.
The Arctic front moves through about daybreak Sunday. Temperatures plummet from about 40 at 7 AM to 28 at 1 PM and 17 by 7 PM. Little moisture is left with this drop in temperatures. A change back to snow is expected Sunday morning, but accumulations in the immediate PHL area and surround suburbs will be minimal Sunday, according to latest models.
Stay tuned for more updates. We’ll be in better range for specifics tonight and tomorrow morning.