Category Archives: Weather Updates

FORECAST UPDATE

The front is moving through 9 AM, about two hours later than forecast—

HRRR model 9 AM forecast  (Click on image for a larger view.)

 

NexRad radar at 9:07 AM courtesy weathertap.com (Click on image for a larger view.)

Clouds, breaks of sun, windy and colder this afternoon. Chance of widely scattered snow flurries into this evening.

STORM UPDATE

Thu 10:07 PM Update — Tonight’s models have increased rain amounts to over two inches. Highest winds about midnight. Squall line comes through about 3AM. Snow flurries possible late morning, especially western suburbs. Windy and increasingly cold Friday afternoon. Some sun breaks out during the afternoon.

Storm Update- High Winds. A squall line moves through about 3 AM Friday. Cold front passes through 7-8 AM Friday morning.

The latest models are coming in.  They show rain starting, depending upon your location,  between 12-3 PM today.   Rain totals between 1.5 and 2 inches by Friday morning.

Very windy with wind gusts 50-65 mph, especially with the squall line that moves at around 3AM, ahead of the actual cold front. The actual front moves through about 7-8 AM with a sharp wind shift. Rain showers should end with the frontal passage at that time.

Slight differences in timing of peak wind gusts between NAM NEST and GFS models—

GFS Wind Meteogram (Click on image for a larger view.)

 

NAM NEST wind meteogram (Click on image for a larger view.)

Cloudy Christmas Day. Windy and increasingly cold. Snow flurries possible, especially far western suburbs.

UPDATED FORECAST- CHRISTMAS EVE

Updated Forecast Cold front passage delayed. Rain may linger into Friday morning, changing to snow flurries.

As mentioned in my updated post last night, the trend has been for the cold front to move through later and later. The model forecast has moved the cold front passage later, from 1 AM Friday morning to about 4-5 AM Friday (GFS model) as low pressure tries to develop when the front hits the coastline. See the GFS below—

GFS surface forecast 5 AM Friday (Click on image for a larger view.)

(This low pressure development along the coast was first suspected to be possible several days ago, but the models until today did not show it.)

The models are predicting high wind gusts of 50-60mph during the night time hours and cold front passage.

Rain will start as early as 4 PM Thursday afternoon and become very heavy during the evening. A total QPF of 1.5-2 inches of water is possible by Friday morning.

The NAM-NEST suggests possible convective (thunderstorm) activity as the front moves through—

High resolution NAM-NEST 3AM  Friday Precipitation Rate (Click on image for a larger view.)

The exact end time of the precip has become less clear as there are some differences in timing of the frontal passage as well as the degree of low pressure development. I would not be surprised if the low pressure system shows greater development.

The NBM (model blend) has picked up on this low pressure development and keeps a forecast of rain and then snow flurries lasting into Christmas day.

High temperature Friday will occur in the morning and fall during the day. Friday will be cloudy, cold and windy.

With the front slowing down, I expect further changes in the forecast. Stay tuned.

MAJOR CHANGE IN SNOW TOTAL FORECAST

Wed 09:51 AM Update — The most significant issue may be freezing rain, starting late afternoon.


I’ve been trying to ignore the NAM model forecasts in favor of the newer fancier models.  But when I go back to my forecasts that have really shined in past years, the temperature profiles of the NAM seem to be the most important factor, secondary to total QPF.

With the newest NAM model data just available, I can no longer ignore its forecast and its temperature profiles.

The NAM shows a large area of above freezing temperatures at 6000 feet resulting in much more sleet, freezing rain and rain than previously forecast—

Here’s the location of the 32º temperature at 6000 feet above ground at  11PM—

NAM 800 mb temperatures at 11 PM  (Click on image for a larger view.)

As a result, the NAM is predicting final snow totals on Thursday morning to be much lower than previously forecast—

NAM Snow Accumulation 7 AM Thursday  (Click on image for a larger view.)

The snow totals will only be 1-2 inches in the Philadelphia area and adjacent western suburbs by Thursday morning.

The start time for the snow is still about the same 12-2 PM, maybe 3PM. After snow accumulates by late afternoon a changeover to sleet and rain will seriously reduce additional snow totals. A change back to snow will occur before ending early Thursday morning around daybreak.

I know I’m going out on a limb with this forecast, but that’s the reason you’re reading this blog instead of watching the TV forecasts!