SATURDAY FORECAST UPDATE

The heaviest rain is falling in the areas of the highest PWAT, as posted last night.   Other areas have scattered showers.

Radar image 9:53 AM  Courtesy of weathertap.com   (Click on image for a larger view.)

 

The latest model blend (NBM) has the highest probability of rain today just to the south of Philadelphia, but all areas have elevated chances —

NBM 1 hour precip probability at 2 PM today  (Click on image for a larger view.)

Note that these are 1-hour probabilities; anything greater than 25% is meaningful.

Still expecting some partial clearing and breaks in the overcast during the 2-5 PM time frame. However any breaks/sun may initiate thunderstorm development due to a highly unstable atmosphere. 

REMNANTS OF LAURA – WEEKEND FORECAST

Fri 11:00 PM Update — Tonight’s models suggest the rain will be more showery in nature on Saturday— Breaks in the rain interrupted by some heavy rain periods. Winds look a bit more gusty as well. Some clearing late afternoon will be interrupted by scattered thunderstorms late afternoon and during the evening. Some areas will receive much more rain than others. Not a uniform continuous rain event. The HIRESW shows some of that extreme tropical moisture makes it into Philadephia, so localized heavy rain possible. The forecast below remains mostly intact.
The moisture and energy remnants of Hurricane Laura will move into our area on Saturday.  The low pressure system that was Laura will merge with a large non-tropical low pressure system and associated cold front on Saturday into Saturday night.

The moisture maximum, as reflected in the precipitable water parameter, or “PWAT” maximum,  will move just to our south, as previously forecast —

GFS PWAT forecast for 2 PM Saturday afternoon (Click on image for a larger view.)

The energy associated with both Laura, the non-tropical low, and the cold front will move through in separate batches.  Showers and rain will move through during the overnight hours.  There may be a break in the action Saturday morning, but rain returns around noon and continues though the mid afternoon.  Total rain about 1 inch, locally higher.

GFS (pre-release version) 6 hour rain accumulation at 2 PM Saturday. (Click on image for a larger view.)

The GFS (new pre-release version) has rain ending around 5PM with even some sun possible later Saturday afternoon.  The HIRESW has similar forecast, but has additional showers and clouds after 8 PM with the actual cold front passage.

Saturday—

  • Rain and cloud details above
  • High Temp 81º sd 2.1º  Dew Point 73-74º
  • Winds SW 5-10 mph   Gusty mid afternoon.  Becoming NW around 10 PM after the cold front passage

High pressure builds in for Sunday. A few upper air disturbances may produce periods of “fair weather clouds” and instability stratocumulus.

Sunday—

  • Sunny early, then  a mix of sun and clouds in the afternoon.
  • High temp 78.6º sd 1.3º
  • Low Humidity. Dew points upper 50s.
  • Winds WNW  10 mph becoming gusty to 20mph in the afternoon.

THUNDERSTORMS FRIDAY

1:00 PM Update- Storms approach  3-4 PM according to the latest HRRR

A quick post- models are on-board with thunderstorms today in the immediate PHL area, from 3-6 PM, moving northwest to southeast.   Several models show closer to 4 PM.  Severe storm parameters are in the moderate range.   Some quick heavy rain possible, gusty winds.

More storms after midnight.

REMNANTS OF LAURA ON SATURDAY

Most models have the remnant moisture of Hurricane Laura moving into our area on Saturday.  It currently appears that the storm’s energy and moisture will become incorporated into a non-tropical low pressure system moving through to our north.  A cold front moves through Saturday evening.

At this stage of things, there’s little agreement where the maximum dynamics, moisture and precipitation will fall.

The GFS model, along with the statistical ensemble version, the GEFS, have the maximum precipitation falling in New England—

GFS precip forecast 8 PM Saturday  (Click on image for a larger view.)

The NAM-NEST has a very different scenario, with much higher amounts falling closer to home—

NAM-NEST precip forecast 8 PM Saturday(Click on image for a larger view.)

Most models agree that rain starts Saturday morning and becomes heavier during the afternoon.  It’s likely that the NAM-NEST is over-forecasting the rain here and we’ll likely get only 0.5 to 1.5 inches of rain.