WEEKEND WEATHER FORECAST

Updated Fri 11:55 PM highlighted below
Updated Sunday Forecast highlighted below (Sat 7:35 PM)

Sun 10:09 AM Update — Today’s  (Sunday’s) forecast continues to be difficult to pin down. Early morning radar did not show the showers predicted by some of the models last night.  This morning’s latest HRRR, NBM, and RAP show some light very widely scattered sprinkles around the 2-4 PM time frame, but this is low confidence.  Most likely time for any rain is after 10 PM.  

I’ll start this forecast by pointing out that the areal coverage and quantity of rain on Friday far exceeded the amount rain forecast by the models 24 hours prior.  The overall weather pattern is undergoing a transition this weekend (as explained below), often leading to lower confidence forecasts.  Indeed, I started preparing this forecast Friday morning and there has already been a shift in the model forecast this afternoon.

This weekend will be affected by a gradual transition from last week’s weather ridge (hot weather) to a weather trough (cooler weather).

Last week—

250 mb Upper air forecast from  last weekend with jet stream far to our north

 

Next week—

GFS 250 mb wind forecast (jet) next Wednesday.  (Click on image for a larger view.)


The transition in the position of the jet has already started, with a stalled frontal system to our south and cooler temperatures.

As mentioned, the precipitation on Friday had already moved further north than had been forecast (even this morning’s HREF came up short while the GFS did better. ) —

Friday morning water vapor, showing position of stalled front. (Philadelphia area in red circle)  An upper air disturbance should move down and affect us Saturday afternoon. (red arrow) (Click on image for a larger view.)

 

There remain large differences in the weekend forecasts from this afternoon’s NOAA, Canadian and ICON models. 

I will base this on the model blend (NBM) and HREF, with the idea that these composite models have done the necessary comparisons and arrived at a more accurate forecast.  (Even the HREF and NBM do not closely agree on Sunday’s forecast.)

Saturday

Lingering clouds with scattered sprinkles very early Saturday morning.

A mix of sun and clouds in late morning into the early afternoon.

Towards late afternoon, more clouds and a chance of scattered showers developing into the early evening, mainly north and west of the Philadelphia area. High 77.3º sd 4.1º

Sunday

Early morning clouds and showers (HREF, Canadian GEM) or just early morning cloudiness (NBM) may break for some bright skies or partly sunny skies in the afternoon, especially east into southern NJ. More cloudiness further north and west of Philadelphia.

Above forecast still holds. The NBM now shows a chance of widely scattered light showers early Sunday morning. Then varying amounts of cloudiness, similar to the sky conditions today. Nicer in NJ. The front comes through about midnight with showers, possible thunderstorms.

(The Canadian GEM model is the most pessimistic about Sunday, consistently forecasting showers.) Fri late update: The HRESW has joined the GEM with clouds and showers Sunday.

A cold front moves through in the evening with showers/possible thunderstorms. The NBM keeps the rain to our west until after midnight (while the GFS shows showers about 8-10 PM) High 76º sd 4.0º (high spread, high uncertainty) High 79.6º sd 1.6

Thunderstorms WEDNESDAY

Updated Wed 8:40AM, highlighted below:

Updated Wed 8:25 PM: This afternoon’s GFS has heavy rain blossoming around Philadephia between 8-10 PM, while the HRRR and the RAP do not. Current radar seems to support the GFS. We’ll see soon enough.

Wednesday’s thunderstorm outlook is looking a bit like Tuesday’s, but with some differences. These storms will also be slow moving and will develop like today’s but will be moving southward instead of northeastward.

CAPE values only about 2000 instead of Tuesday’s 2800 J/Kg. Precipitable water still high at 1.9-2.0″ water, but also a bit lower than Tuesday. Instability values also lower.

Storms are possible in as early as 2-4 PM but eventually blossoming in the immediate Philadelphia area by 4 -7 PM and continuing into the late evening. There is some suggestion that areas south of the city and in NJ may be more affected, especially in the evening. Last nights’s models continue with a timeframe of 4-7 PM for the onset in the immediate PHL area, then continuing southward. Highest probability is 6 – 8 PM

Here’s their afternoon’s HREF precip forecast

High Resolution Ensemble 1 hour precip forecast for Wednesday. (Click on image for a larger view.)

THUNDERSTORMS: TUES

Tue 04:57 PM Forecast Review — The models did well with the thunderstorm forecast for today. The storm activity should continue into the early evening. The latest HRRR is forecasting some intense dynamics around 6PM in Philadelphia and into NJ.

Some of lightning today was cloud->ground and was dangerously spectacular to view.   CAPE (Convective Available Potential Energy)  values today were high —in the 2800 range (compare to last week’s storms which were only in the 1700 J/kg range.)

A repeat performance expected on Wednesday, likely 2-6 PM and somewhat more in NJ.  

Another very warm day is in store for us on Tuesday. Temps will be in the upper 80s, but an increase in humidity will more than make up for it.

Upper air disturbances will move through, around noontime, triggering scattered showers and thunderstorms in many areas, starting as early as noon- 2 PM and lasting in some areas into the evening.

HREF (High Resolution Ensemble Forecast) hourly precipitation with standard deviation, at 12 PM, 2 PM and 5 PM with increasing areal coverage. (Click on image for a larger view.)

High CAPE (potential energy), high instability values and high precipitable water values could make for some heavy rains in localized areas. Overall, the models are cranking out 0.5 inches of rain.

Weather… and other things "up in the air"

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