Monthly Archives: July 2018

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

The models are in unusually good agreement about this weekend’s weather forecast.

All show coastal low pressure developing off of the Maryland-Virginia coast as a result of a transfer of energy from an upper closed low pressure system in the Ohio Valley.  Upper air support combined with a strong low level moisture flow will result in heavy rain Saturday afternooon and  especially Saturday night.  Total QPF values are in the 2 inch range and local amounts could be higher.

NAM Hi RES Precipitation Forecast Saturday10 AM -2 PM

The timing of the onset has clarified- Areas south of Philadelphia will have rain starting in the morning.  For Philadelphia and areas north, showers may start as early as 10- 12 PM.   By 2-4 PM, all areas in the immediate PHL will have rain.  The rain will become quite heavy early evening and localized flooding is possible.

Rainfall
Rain 7-10 PM Saturday (ECMWF model)

A northeasterly flow ahead of the low will cause temperatures to remain in the 70s on Saturday.  There will be gusty winds.

For Sunday, the low pressure system  will have lifted north.  Skies will break for some hazy sunshine but there will also be considerable cloudiness as well.  It will be warm and humid with higher dew points and highs in the mid 80s.

Any sunshine will cause the unstable atmosphere to develop scattered showers and thunderstorms, especially by the late afternoon.  Another upper air disturbance moves in late Sunday and enhances the likelihood of showers Sunday evening and night.

Philadelphia Weather Outlook

WEEKEND WEATHER OUTLOOK- UPDATED

Thursday Eve Update: There are already changes in the weekend forecast.  The upper low is now expected to spawn a coastal low pressure system that forms over the Delmarva area and moves north on Saturday.  Saturday that had been expected to be the drier day is not looking that way right now.

The latest NAM high resolution has rain starting in PHL as early as noon on Saturday, while the GFS holds it off until 4-5 PM. (earlier along the coast.)

Here’s the current High Resolution NAM forecast for Saturday afternoon—

NAM rain forecast for 1 PM through 6 PM Saturday

This will be a complex system and the modeling of the upper low and its surface reflection often leads to inaccurate forecast placement of precipitation.

Either way, the next 6-7 days will be very unsettled with periods of heavy rains.

Wednesday Eve: An interesting weather scenario for July is shaping up for this weekend and next week.

An weak impulse will move from the Rockies and develop into a low pressure system near Ohio.  With upper air support, this low pressure will transport warm, very humid and unstable air into our area by Saturday evening.  A persistent threat of showers and thunderstorms will be with us from late Saturday through at least next  Wednesday.  High dew points will make it feel very warm, although significant hazy cloudiness may limit high temperatures.

Saturday will be the better day this weekend, with showers likely holding off until late in the afternoon or evening.

Sunday looks to be more unsettled with intermittent  showers likely much of the afternoon.

As we saw last weekend, forecasts can change significantly.  I’ll update Friday evening.