Not surprising, the timing of Hurricane Delta’s impact on the Philadelphia area has changed somewhat over the past 24 hours and there’s uncertainty regarding the timing of the rain here over the weekend.
While the latest [publicly available} COAMPS model maintains a similar track and a somewhat faster movement, the track of the remnant moisture towards our region is somewhat delayed back into the Sunday time frame.
The latest GEFS (Global Ensemble Forecast System) has the rain reaching our area 11 PM Saturday night—
The new GEFS forecast represents the statistical average of 30 versions, called “members”, of 30 separate GFS model runs.
The 30 members and a ‘control version’ are collectively called the “ensemble”.
Each member has an introduced ‘perturbation’, a known, mathematically introduced error to account for the uncertainty in measurement of initial weather conditions.
The latest blend of models (NBM) delays the rain into Sunday afternoon, although there’s a chance of light sprinkles earlier and increasing cloudiness will arrive as early as Saturday—
I expect more changes in this forecast, as hurricanes and tropical storms seem to adversely affect model forecasts. Stay tuned.
Hurricane Delta is expected to move onto the Gulf Coastline and its remnants are expected to move over our area over the weekend. The current time line has shifted to a somewhat faster approach, with clouds and possibly some light rain moving in as early as Saturday.
(Previous model runs had it affecting us on Sunday.)
Here’s the latest GEFS forecast for Saturday morning—
This storm has been difficult to forecast over recent days, since it appeared it might interact with tropical storm Gamma. It looks like will absorb the circulation of Gamma as it heads towards the Gulf coastline.
Additional changes in timing and track are likely. Stay tuned.
The latest information about Hurricane Laura shows it to be a major and dangerous hurricane. The latest US Navy COAMPS tropical model has it as a category 5 storm at landfall. The National Hurricane Center is always your best bet for information.
Latest satellite imagery is impressive—
Where does all this moisture and energy end up? The COAMPS continues to suggest it moves just south of our area on Saturday.
The latest GEFS (Global Ensemble Forecast) shows the remnants as a increase in “precipitable water”, referred to as PWAT. Here’s the PWAT forecast for Saturday at 1 PM—
There’s much uncertainty with the position of this moisture when it gets to the mid-Atlantic area on Saturday. The above is the soon-to-be released GEFS model, showing the PWAT maxima.
The current operational GEFS model, the GFS model and new pre-release version of the GFS model all have different timing and positions for the remnants of Laura.
This is not surprising… hurricane tracks and intensity are difficult to forecast; once a hurricane transitions to an extra-tropical system, it seems to get even more complicated to forecast.
I expect the forecasts for the next few days to become very changeable. I call it the “tropical storm effect”. Even yesterday’s model forecast bust, I believe, was due to the “tropical storm effect”.
The “tropical storm effect” is a total creation of my own and it is NOT at all science-based. Over the many years of weather forecasting as a hobby, it’s been my strong impression that local model forecasts are more unreliable when there’s a strong tropical storm or hurricane in the picture. Does it cause a discontinuity in the mathematical models? Only the numerical weather scientists can answer that.