Philadelphia Weather Outlook

THIS WEEK’S WEATHER

Update Fri 10/14 @ 9:39 AM — The clouds this morning were predicted by last night’s models (I hadn’t done a forecast for today.)

The clouds should dissipate by noontime.

A “closed” upper low is over Lake Superior. We’ll be getting the mild upper flow around the low for much of the weekend.

This morning’s NAEFS upper air (jet stream) forecast showing the closed low. Disturbances will rotate around this closed system, but should not affect us. This closed low will “open up” over the weekend. (Click on image for a larger view.)

Jet stream by Sunday—

NAEFS jet stream wind forecast for Sunday at 8 AM. Upper low “opens up” as a strong jet streak digs down into areas of Nebraska. This will allow cold air to enter the continental US and will affect our weather next week.

Update Thu 10/13 @ 7:59 PM — The system has slowed down somewhat due to the low pressure development and storms are ‘training’ south to north in the area of the red box below. The NWS issuing a flash flood warning for that area. Rainfall amounts of 1-3+ inches have fallen so far in that area.

Update Thu 10/13 @ 5:26 PM — As forecast, low pressure has developed ahead of the front and will move up over us early this evening. The latest HRRR shows strong dynamics (high vertical shear and high helicity) moving through the western suburbs about 9 PM—

This afternoon’s 18z HRRR forecast for 9 PM tonight. Some heavy showers and high helicity (brown contours) entering western areas. Ordinarily, this would be the source of strong thunderstorms, BUT very low CAPE values will limit the likelihood of severe or even strong thunderstorms. (Thunderstorm likelihood is LOW.) Strong winds are the most likely outcome with brief heavy rain in some areas. (Click on image for a larger view.)

Update Thu 10/13 @ 7:33 AM — Showers move in early morning, then periods of rain increasing in intensity towards evening and early evening. Precipitation totals are now in the 0.4- 0.8 inch range with a few locally higher amounts.

Main front moves through between 7:30 -10 PM with very windy conditions and a fast burst of heavier rain. Severe thunderstorms not likely. (Winds gusting to 30-35 mph with front.)

Update Wed 10/12 @ 5:51 PM — The models have backed off considerably from forecasting wild weather for Thursday. Precipitation totals are now in the 0.5- 0.8 inch range. Winds will be gusting about 30 mph at times. Some thunderstorms are possible but they’re not going to be severe. Best chance for thunder is after 7 PM. Some rain is possible as early as Thursday morning, Heaviest rain late afternoon and especially the evening hours.

Complicating the forecast is the expected development of low pressure along the front which may move much of the heaviest rain off the coast. It’s certainly “confusing” the forecast and making some forecast specifics low confidence—

Latest GFS forecast – rain at 8 PM. Bright green is convective (thunderstorm) rain. Low pressure develops off the Virginia coast. (Click on image for a larger view.)

Strong Cold Front Passage Thursday

Update Wed 10/12 @ 8:13 AM — It appears that some rain may move in earlier on Thursday than forecast yesterday. Some showers in the morning now expected. Strong winds and rainfall amounts 1-1.75 inches are likely with this system. I’m not so sure about severe thunderstorms. Updates later.

Wednesday morning Water Vapor image showing the two systems converging. (Click on image for a larger view.)

Update Tue 10/11 @ 10:13 AM — A weak system will bring cloudiness in Wednesday afternoon ahead of a strong cold front that will move in and affect our weather Thursday afternoon through evening. The latest NBM is forecasting about 2 inches of rain by late Thursday night.

Tuesday morning’s Satellite Water Vapor and jet stream flow. Weak disturbance (1) will bring clouds in Wednesday afternoon. A strong cold front ahead of a jet stream dip (2) will move in. The weak disturbance (1) will bring showers Thursday and the cold front (2) will spawn low pressure Thursday evening. Showers, thunderstorms and WINDY conditions expected late Thursday afternoon and evening. (Click on image for a larger view.)

Tranquil until Thursday. Then it gets interesting

A rather strong cold front is forecast to move through Thursday evening.

All the ingredients for some ‘interest weather’ appear to be in play for Thursday afternoon and early evening— elevated CAPE, elevated helicity, elevated vertical shear and a strong jet streak presence. (All the ingredients that we never really saw for much or all of our dry summer.) Stay tuned.

Previously Posted Sun 8:01 PM —

A fairly tranquil weather period early this week as high pressure gradually moderates in temperature. Fair weather through early Wednesday. Wednesday looks to be the mildest day, but some cloudiness moves in during the afternoon.

Of interest is a rather strong cold front that will move through Thursday with rain and maybe even some thunderstorms—

Today’s 18z NAEFS forecast for Thursday. High pressure digs southward over the Great Lakes and showers/thunderstorms develop on Thursday. A return to cool weather for the first part of next weekend. Another outbreak of colder air expected after next weekend. (Click on image for a larger view.)