Monthly Archives: April 2019

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST-UPDATED WITH LATEST NAM

Updated based on tonight’s NAM data just available.

Those reading my posts over the past few days know that this weekend’s weather will be be influenced by a closed upper low, a situation where the models have trouble in placement of precipitation and timing of areas of vorticity that rotate around the upper low causing instability showers to develop. As a result, this forecast is lower than average confidence.

For Saturday, the general trend has been for the main precipitation to end mid morning west of the city and later east into NJ.

Low clouds thin around noon and some sun through high clouds is likely in the afternoon. High 69-72. It will be windy.

Sunday will have a mix of clouds and sun. Instability in the mid afternoon may allow clouds to redevelop with light scattered showers possible, especially in western suburbs.  Highs 66-69. Light winds.

Philadelphia Weather Outlook

Tonight’s NAM

[su_box title=”Weather Update Fri AM” box_color=”#defcdc” title_color=”#000000″]Last night’s GFS maintains a QPF of 2 inches of rain over the Philadelphia area, occurring after midnight tonight. Rain tapers and ends Saturday morning with a mostly dry Saturday and some sun and clouds.

GFS 6 hour accumulated rain depicted at 3 AM Saturday morning.

The NAM has the heavy rain to our east.

Large uncertainties with the heavy rain placement remain due to the closed upper low pattern. All models have it dry for Saturday afternoon. [/su_box]

Tonight’s NAM is a perfect example of how a closed upper low plays havoc with the model’s ability to accurately predict placement of precipitation.

Tonight’s NAM has the main precipitation shield rotating to the east of Philadephia.  Instead of 1-2 inches of rain here, it’s showing a QPF of less than 0.30 inches. (This afternoon’s GFS had the heavy rain rotating through to our west.)

NAM forecast for 8 am Saturday ( 6 hour accumulated QPF)

Expect additional changes in the forecast. (Saturday late morning and afternoon still look good.)

Philadelphia Weather Outlook

WEEKEND WEATHER OUTLOOK

[su_box title=”Weather Update Thurs PM” box_color=”#defcdc” title_color=”#000000″]The forecast below still looks good for Saturday and Sunday. Some sun Saturday afternoon is looking like a strong possibility.[/su_box]

An upper closed low pressure system and a surface low will affect our weather over this weekend.

The weather produced by an upper closed low is notoriously difficult to predict – the exact position and movement of the system can be challenging to forecast and the placement of the rain produced by the low is often forecast inaccurately by the models.

GFS forecast- Sunday noon shows closed low at 18,000 ft ( shading is vorticity)

The GFS is predicting close to two inches of rain Friday evening through Saturday morning.

That said, here’s the current outlook-  rain expected Friday night should end by late morning Saturday.  There may be partial clearing Saturday afternoon. Possibly some sun.  It will be windy.

Sunday may start dry with a mix of clouds and sun, but thermal instability from the upper low may cause clouds and widely scattered showers to develop Sunday afternoon.

This weather outlook is a lower than average confidence forecast. I’ll update Friday evening.