Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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Friday Weather Update

Updated Fri 10/06 @ 3:48 PM — The model forecasts for today have been unusually poor with this system. Last night’s models have turned out to be convincingly wrong, especially the high resolution models. Little to no rain has fallen in our area. As I write this. radar is beginning to show some scattered shower activity, forecast to move into the immediate PHL area. We may see some showers over the next few hours around the city and its immediate suburbs.

Current Nexrad radar (Click on image for a larger view.)

I’m just beginning to prepare the Weekend Weather Forecast. I’ll post it later.

Friday & the Weekend Outlook

Updated Thu 10/05 @ 11:02PM — As mentioned in my previous post from Wednesday morning, the cold front expected to pass through late Friday into Saturday morning will not bring much rain or any extreme weather to our immediate area.

Scattered showers are expected to develop on Friday morning afternoon, as early as 6-7 AM 1-2 PM in some spots, and then become a bit more widespread in the afternoon and evening.

Today’s 18z GFS forecast for 10 AM Saturday morning. Cold air moving in behind the front (1) while the moisture of what tropical system (2) moves up into Maine and joins a secondary low that develops far north of our region. We may see some sun Saturday afternoon, along with a few instability sprinkles. (Click on image for a larger view.)

The actual front moves through early Saturday morning with some showers and an increase in winds. The amount of rainfall is forecast to be quite light in many areas and ranges from 0.10″ to 0.35″. We may see some sun Saturday afternoon, but a few instability sprinkles possible too.

Temperatures will still be in the upper 60s, so the big impact will be the windy conditions, which will make it feel cooler.

Sunday will be windy and cooler. There will likely be periods of cloudiness and maybe a few scattered sprinkles as a result of instability with the cold air aloft.

Wednesday and Weekend Outlook

Updated Wed 10/04 @ 8:38 AM — Today is looking to be our warmest day this week, with highs near 83º (± 1.5º). High temps forecast are fairly uniform across our area—

Today’s 06z NBM shows highs in the low 80s with a standard deviation of about 1.2º Black contours are 1º increments (Click on image for a larger view.)

The weekend’s weather is looking less ‘interesting’ but still a dramatic change in temperatures and feel.

Here are the changes, summed in the latest ECMWF model forecast map for Saturday afternoon—

Todays’ 00z ECMWF forecast for 5 PM Saturday shows previous tropical storm (1) moving to our east, but with its outflow moisture being brought up into New England. The influx of cold air (2), as depicted by the red 540 thickness line appears to be less robust than previously forecast. The low pressure system that was expected to develop to our south is now expected to develop to our north (3). As a result, much less rain is forecast in our area than previously forecast.

As described in the caption above, the previous forecasts have changed. There are differences among the models in the timing of the actual frontal passage on Saturday, and differences among models with the timing of showers.

There are signs that some light showers may break out as early as Friday morning, but the main front moves through sometime Saturday.

Continued Mild through Wednesday.

Updated Mon 10/02 @ 6:11 PM — No major news here, since one can’t watch a weather report without hearing about the great weather.

Here’s the latest NBM model forecast high temperatures—

Today’s 18z NBM temperature -dew point meteogram for Blue Bell, PA (Philadelphia tends to run about 1-2º higher) Highs in the low to mid 80s! (Click on image for a larger view.)

Temperatures still mild on Thursday, just below 80º

For the weekend, things continue to look ‘interesting’ as a sharp dip in the jet stream will induce low pressure somewhere. The NAEFS shows a coastal low, but the GFS and ECMWF differ significantly. Additionally, some moisture from a tropical system far off the coast may add some moisture to this developing system.

Here’s the NAEFS forecast for 5 AM Saturday morning—

12z NAEFS shows upper low over Great Lakes (blue) with potential surface secondary low over Delaware (black) There’s no clear consensus about the forecast, other than expect some rain and windy conditions late Friday into Saturday. (Click on image for a larger view.)

Previously Posted Sun 8:03 PM —

This week’s weather can be summed up with a single current water vapor image—

Water Vapor image Sunday evening (with RAP model jet level wind streamlines and 500-1000mb “thickness” contours (yellow) contours. Note the yellow 570 thickness level contour is the outline edge of unseasonably warm air, moving up in our direction. This upper and mid level “ridge” will be over us Monday through Wednesday bringing dry, sunny weather with temperatures above average, in the low 80s. The low pressure system over Nevada (blue L) will gradually move towards the east. The cold front associated with this low will bring rain Friday into Saturday, followed by below average temperatures. (Click on image for a larger view.)

Monday through Wednesday will feature sunny skies and above seasonal average temperatures in the 79º-83º This warm up has been well-advertised on the tv/radio weather.

The latest NBM model shows the forecast high temperatures for Blue Bell PA —

NBM model forecast temperatures for location Blue Bell, PA Dotted lines indicate standard deviation (Click on image for a larger view.)

By next weekend, the ridge that was over us will be replaced with a significant trough, with colder temperatures.

The NAEFS is forecasting rain with possible development of a coastal secondary low pressure system for Saturday—

NAEFS forecast for Saturday shows the 500-1000 mb thickness line (here in red) and the trough in the eastern US (blue contours) by Saturday evening. (Click on image for a larger view.)

This sharp change in temperatures is a possible set up for some ‘interesting’ weather later this week. Stay tuned.