REMNANTS OF ZETA TO AFFECT PHILADELPHIA AREA

A complex weather scenario involving a strong upper air”closed low”, a surface low, and the remnants of Hurricane Zeta will affect our weather from Thursday into at least early Friday.

The complexity of this setup can’t be over-emphasized and it’s likely that no existing weather model can accurately forecast how this will finally come together. Despite the complexity, the models are in relatively good agreement right now.

GFS 7 PM Thursday forecast with 1 hour rain forecast.

It appears that the upper air low and the tropical remnants will remain separate, preventing explosive development like we had with tropical storm Isaias. However, things could change and that possibility will need to be watched.

The models currently show a heavy rain event, starting Thursday morning and lasting into very early Friday morning. The CMC and GFS models have total rainfall amounts in the 2.0″ to 3.5″ range for the Philadelphia area.

As early as last Friday, the German ICON model was ahead of our GFS the Canadian models in predicting Zeta’s formation. The ICON model did very well with Isais’s precip totals. Here’s the latest ICON model cumulative precipitation forecast for the storm—

Icon Model storm total cumulative rain forecast

I expect changes in the forecast. Stay tuned.

SUNDAY FORECAST UPDATE

Following the frontal passage that occurred on schedule Saturday afternoon, we didn’t get the clearing that had been forecast. A northeasterly wind flow behind the front will bring much chillier temperatures to our area on Sunday.

An area of rain associated with low pressure in the southeast will advance into our area on Sunday. There has been uncertainty about how fast these showers will move in, but the general time frame is between noon and 4 PM. Overall, Sunday will be quite autumnal, perhaps a taste of November in October.

Sunday—

  • Possibly some sun early, then becoming cloudy by late morning
  • Light showers possible as early as noontime but likely 2- 4 PM
  • Rain during the evening until midnight. Total precip 0.4 inches
  • High temp 50.5 sd 3.5º (NBM model-Blue Bell area)
  • NE winds 5-14 mph

Note that the NBM mean temperature’s sd (standard deviation) is very large.  Be advised that several models have high temperatures only in the upper 40s!


 

An interesting weather week coming up, as a tropical system will move into the Louisiana area and merge with a strong non-tropical low pressure system in the Midwest.  This will affect us later in the week, likely Thursday into Friday.

GEFS Model Forecast Wed 11 AM (Click on image for a larger view.)

CHANGES IN THE WEEKEND WEATHER Forecast

 

Please excuse the glitches in my posts this morning. I’m attempting to use the new Gutenberg WordPress editor with varying degrees of success. The learning curve has been steep.

  
Speaking of computer glitches, NOAA’s main model distribution server, “NOMADS”, was down for scheduled maintenance this past Thursday.

What was supposed to have been just a six hour down-time has turned into a multi-day fiasco—It has been down for 24-30 hours and several models (NAM, NAM-NEST, GEFS) are still unavailable, except on their backup “ftp” server.  Repair will resume on Monday, according to NOAA.

My access to data over the past few days has been more challenging than usual. 

Sat 8:40 AM Update—The latest models maintain low cloudiness in the immediate PHL area until 3-5 PM. There’s some mist and drizzle this morning. Some models show breaks of sun in areas far west and far east of PHL mid day. The chance of light sprinkles is in the low range and most areas will not have any.
NBM Cloud, rain forecast for 2 PM Saturday. (Clouds are Darker) (Click on image for a larger view.)

Fri 8:30 PM Update—More models have joined the ranks of showing earlier rain for Sunday.

Following the frontal passage that occurred on schedule Saturday afternoon, we didn’t get the clearing that had been forecast. A northeasterly wind flow behind the front will bring much chillier temperatures to our area on Sunday.

An area of rain associated with low pressure in the southeast will advance into our area on Sunday. There has been uncertainty about how fast these showers will move in, but the general time frame is between noon and 4 PM. Overall, Sunday will be quite autumnal, perhaps a taste of November in October.

Saturday— A weak front moves through around noontime.

  • Cloudy for much of the day. Gradual clearing in the late afternoon.
  • Light, widely scattered showers possible 2-4 PM.
  • High temperature 70.6º sd 2.5º (NBM model) Blue Bell.
  • Winds becoming NW mid to late afternoon, increasing to 15 mph.

Sunday— High pressure moves off to our north and a northeasterly flow of cooler air moves in. A disturbance moves up from the southeastern US with showers, possibly arriving as early as mid-afternoon. Greater than average uncertainty.

  • Possibly some sun early, then becoming cloudy by late morning
  • Light showers possible as early as noontime but likely 2- 4 PM
  • Rain during the evening until midnight. Total precip 0.4 inches
  • High temp 50.5 sd 3.5º (NBM model-Blue Bell area)
  • NE winds 5-14 mph

I’ll update later this evening or Saturday morning

WEEKEND WEATHER OUTLOOK

The stagnant, very mild pattern we’ve been in should continue, as I would describe the current overall situation as a “lack of very cold air” to our north.

Whatever blob of cold air exists in Canada will sag into the western/central US over the next few days, but  our area will still be on the relatively mild side of things. No very cold outbreaks in sight right now, although a weak front will bring in cooler temps for Sunday.

The Short Range Ensemble Forecast Model (SREF) sums it up well with the “540 thickness line”  (RED) staying north of our area—

SREF Model Sunday noon with “540 thickness line” (RED) and other upper air 0ºC lines (magenta, violet)    (Click on image for a larger view.)

Saturday—

As for the specifics for this weekend, a weak cold front will approach on Saturday about noontime, but upper air support is lacking and the front will fizzle out.   Showers with the front far west of the Philadelphia area are expected to reduce to some widely scattered sprinkles.

Speaking of  forecast specifics, it’s been difficult to predict when the low clouds level would lift each morning.  Over the past week, the higher resolution meso-scale models have been a bit too cloudy and the GFS has been useless, as it rarely showed the low clouds and fog that developed each early morning.  Saturday’s forecast is a bit different since clearing skies are expected during the afternoon after the frontal passage according to the model blend.  (Some models have slower clearing through the afternoon. ) Still mild with highs near 73º.

Sunday—

Sunny early.  High pressure moves off to our north and an easterly wind flow will bring in cooler temps and clouds late morning and cloudiness will increase during the afternoon.  Highs only 55º

Fri 03:36 PM Update — latest models have showers early evening Sunday.

Current seasonal average highs are 61º and average lows are 41º.

 

Weather… and other things "up in the air"