Wintry Mix changes to rain Thursday

Update Wed 12/14 @ 5:52 PM — This afternoon’s models are consistent with my previous update; with the exception of the HRRR and Canadian HRDPS, they show an area of sleet and freezing rain moving in about 7-8 AM.

As mentioned, over past winters, the HREF and NAM-NEST and HIRESW comprising the HREF have over-forecast freezing rain/sleet.

Here’s the current HREF Precipitation type (PTYPE) at 8 AM Thursday—

Despite several models leaning towards the above, my preferred forecast model here is the HRRR shown below.

Here’s the HRRR forecast at 8 AM with the precipitation delayed to our south and the 32º line is already moving past Philadelphia—

As I forecast this morning, the HRRR has consistently been forecasting a slower onset of the precipitation, allowing the temperatures to rise before the onset. The HRRR is somewhat similar with the Canadian HRDPS, so I’m leaning towards a less wintry commute tomorrow morning

Here’s the HRRR forecast for 9 AM. Even less of a threat, with the precip still to our south—

Remember that the NWS and TV weather people need to concern themselves with safety concerns and liability, so they are always going to err on the side of a more safety-aware and cautious forecast. I’m just trying to give you my best insight.

I’ll do a quick update with tonight’s HRRR and the HRDPS about 9 PM tonight.

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It looks like this forecast below from Wednesday morning might not have posted.

Update Wed 12/14 @ 7:45 AM — No significant change in the models, but review of last night’s 00z and 06z model runs suggests that the HREF model graphics posted last night may be over-estimating the freezing rain and timing of the precipitation.

While the HREF hasn’t changed, the HRRR has consistently been forecasting a slower onset of the precipitation, allowing the temperatures to rise before the onset. The HRRR is somewhat similar with the Canadian HRDPS, so I’m leaning towards a less wintry commute tomorrow morning. I’m basing this on recent winters where freezing rain was over-predicted by the current models.

My forecast is for light sleet and some freezing rain about 8-9 AM, changing to sleet and rain and finally all rain in the immediate PHL area by 11. The forecast assumes that the precip will be delayed to our southwest.

The HRRR 8 AM captures my current forecast—

By 11 AM we have mostly rain —

I’ll update later today.

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Update Tue 12/13 @ 8:49 PM —

I’ve been reviewing today’s models and as usual, there are differences in their forecasts.

Here’s where they agree: Precipitation overspreads the area between 7 and 9 AM from the southwest. All change the precipitation to heavy rain, lingering into Friday morning. However the changeover to all rain may be gradual instead of rapid in northwestern areas. Winds gusting over 40 mph and rainfall amounts over 1 inch expected. Heaviest rain likely around midnight. Some rain lingering into Friday.

Here’s where there are differences: The temperature profiles of the models all vary quite a bit. On the warmer side is the GFS and ECMWF which has a changeover to all rain in the immediate area by 10 AM. The GFS has sleet and some snow in areas, along with freezing rain before a rapid changeover.

The higher resolution models, our NAM-NEST, the Canadian HRDPS, along with the HRRR (and throw in the German ICON) are somewhat colder with more areas of sleet lingering into late morning.

The HREF (ensemble model) probably captures things the best and is similar to the Canadian HRDPS (but colder than the GFS). Here’s its 9AM forecast —

By noon, the HREF still shows considerable sleet and freezing rain. I think this is overdone and it will be mostly rain in the immediate area—

To further complicate things, the latest ECMWF and NBM became available while writing this update. It has an influx of cold air in the afternoon, suggesting there might be a mixed precip during the day in northwestern suburbs.

A very dynamic storm and I think we’ll have to wait until Wednesday night to nail it down.

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Update Tue 12/13 @ 11:17 AM — Many of this morning’s models continue to show light precipitation moving in between 7 and 8 AM Thursday morning before temperatures rise above freezing.

NBM and the GFS have very light snow falling early (7 AM) with a changeover to sleet/freezing rain from 8 to 10 AM. The Canadian RGEM, SREF show no snow, but rather sleet and freezing rain between 8 and 10 AM.

The Thursday forecast period has now fallen within the range of some of the higher resolution models (60 hours) (NAM-NEST and HIREF-FV3). These models show the precip moving in later, after temperatures have risen. They have very little precipitation falling until the temperatures rise above 32º at 10 AM.

So there’s still uncertainty about a possible bad commute to work or no real problem. Most of the high resolution models’ forecasts extend only to 48 hours and tonight’s model runs will begin to cover the time period in question. Stay tuned.

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Update Mon 12/12 @ 8:21 PM — Additional review of the NBM and ICON models suggests a mix of snow/sleet/rain is possible even up until noon in some areas. (There’s more uncertainty than previously predicted, as the precip is now forecast to be moving in before the cold air leaves.)

Complicating this forecast is an 540 thickness line well north of our region, indicating temperatures higher up are too warm for snow, yet the current NBM shows a high probability of snow in some areas during the mid morning hours.

Eventually, it’s all expected to turn to rain. The question for our western and northwestern suburbs, will it be at 9 AM or 2 PM? Here’s the latest NBM conditional probability of snow —

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Update Mon 12/12 @ 6:01 PM — Today’s models continue with a rain event for the storm expected to move in Thursday through part of Friday. However, the Canadian GEM (and now the GFS) continues to have the rain move in when the lower levels of the atmosphere will be cold—

The Canadian GEM model was updated last winter and it did very good with the few mixed precipitation events we had. So we’ll see if its forecast holds.

The latest GFS now has a similar forecast for 9 AM Thursday—

Update Mon 12/12 9:08 AM — Considerable instability cloudiness today (Monday) with a chance of a widely scattered snow flurry in the early afternoon.

As for the storm expected Thursday through Friday, no significant change in the forecast. An all-rain event, however, the Canadian RGEM and the German ICON models have the rain moving in just before the cold air exits. There’s the possibility of some light freezing rain very early Thursday morning before a changeover to all rain.

Update Sun 12/11 9:16 PM — Considerable instability cloudiness tomorrow (Monday) with a chance of a widely scattered snow flurry in the early afternoon.

Update Sun 12/11 6:26 PM — No change with today’s models. All have it too warm for snow here Thursday through Friday. (The German ICON model has it possibly starting as light snow early Thursday morning, but quickly changing to all rain.) Heavy rain, high winds still expected.

The long range forecast for the period following this storm is for colder than seasonal averages for the following week. Jet stream pattern still suppots my 2023 Winter Season Snow Outlook.

Previously Posted Sun 12:09 PM —

Storm to affect us late Thursday through Friday

A strong upper air low and surface low are in Washington State today, Sunday.

Here’s current satellite water vapor image capturing the mid to upper atmosphere and showing upper air low L and surface level L—

The low is expected to move over the Great Lakes and most models have it transferring its energy to a developing coastal low by late Thursday into Friday.

Here’s the very latest GFS model showing the coastal low—

Two things to note: The red 540 thickness line is well north and west of us, indicating much of the atmosphere is too warm for snow here. Also note the the main low remains over Wisconsin, possibly affecting us later in the weekend.

A very complex and dynamic situation, but at least for the first round, we’ll have heavy rain and windy conditions in our area from this system late Thursday through Friday.

Indeed, the models show the possibility of another disturbance developing along the stalled boundary to our south and affecting us Sunday to Monday.

The forecast has already changed over the past few days. Other changes are likely. Stay tuned.