Tag Archives: Ensemble Model Precipitation

WEEKEND WEATHER FORECAST

#Philadelphia #weather #PAwx

Saturday Rain Totals

Posted Saturday 03/23/24 @ 5:25 PM — We had a lot of rain in just 14 hours or so. Here’s the MRMS rain totals as of 4 PM—

MRMS combined rain gauge & radar-based measurement of actual rainfall received. Color shading is in inches. Line contour numbers are in mm (25.4 mm= 1 inch) (Click on image for a larger view.)

While I didn’t post these models, the GFS and the Canadian models correctly forecas the placement of the heaviest rain axis just east of the Delaware River into NJ, although the GFS rain totals were too low.

The NBM did well with a forecast 3.6″, as did the HREF forecast (PMM) version, but both had the rainfall axis too far west into Philadelphia and western suburbs. The ECMWF was slightly too far west as well. The experimental RRFS too far west and its rainfall forecast was too high!

Winds will be picking up over the evening hours tonight becoming quite gusty, but subsiding by the afternoon Sunday.


Rain Update

Posted Saturday 03/23/24 @ 12:58 PM — The heaviest rain has entered our area over the past hour or two, about two hours ahead of the model forecasts.

MRMS Instantaneous Precipitation Rate (PRATE) at 12:52 PM. (Click on image for a larger view.)

The latest hourly HRRR suggests that the rain will be tapering off between 2 and 3 PM and ending about 3-4 PM from the southwest.

At noon, many areas already had 2-2.75 inches of rain. according to the MRMS. I’ll post a ‘final’ rainfall total later this afternoon.


Saturday Rain Update

Posted Saturday 03/23/24 @ 8:09 AM — Some areas have already received 0.75 inches of rain and the heaviest rain is still to our southwest—

MRMS instantaneous precipitation rate at 8 AM Saturday. Heaviest rainfall is still southwest in Virginia and will arrive here between noon and 2 PM. (Click on image for a larger view.)

The expected time for the rain to taper off and end is between 4 PM and 7 PM. Clearing will occur from southwest to northeast later this evening.

Here’s the latest rainfall forecast (NBM) —

03-23-24 06z NBM accumulated rainfall by 1 AM Sunday. (Click on image for a larger view.)

Weekend Weather Update

Posted Friday 03/22/24 @ 8:07 PM — A few more afternoon models have come in since my last post. High rainfall accumulation is the trend and the HREF and [experimental] REFS show even higher rainfall for our area than the NBM model (shown in previous post.)

The HREF and REFS are “ensembles”, meaning that they combine and average different model runs, different models and/or different ‘perturbations’ of the same model. Both the HREF as well as the experimental RRFS-ensemble (REFS) also create means (averages) in several ways.

The HREF and the REFS include statistical averaging algorithms called PMM (“probability matched mean”) and LPMM (“local probability matched mean”), mathematical approaches which attempt to eliminate a weakness of ensemble mean forecasts that causes the magnitude of precipitation to be smoothed out in the mean— Important maxima are reduced in magnitude and low magnitude values are spread over too a large an area in simple arithmetic means.

Here’s the PMM and LPMM outputs for today’s HREF model which attempt to correct for problems with ensemble simple means —

HREF- Probability Matched Mean—

HREF Probability match mean (PMM) total precipitation forecast for Saturday. (Click on image for a larger view.)

HREF Local Probability Matched Mean precip forecast for Saturday—

HREF Local Probablity Matched Mean precip totals for Saturday (Click on image for a larger view.)

The LPMM can sometimes create artificially high values and the PMM version is often found to be more reliable. You can see that the PMM version is quite similar to the NBM model (shown just below in the previous post.)

Either way, it looks like a lot of rain tomorrow!


Posted Friday 03/22/24 @ 5:42 PM — This afternoon’s models continue to forecast impressive amounts of rain falling over a 16 hour period. The latest NBM, which tends to be conservative, has the following rainfall forecast through Saturday evening—

03-22-24 18z NBM accumulated rainfall through 11 PM Saturday night. As always, the exact placement of the heaviest rain bands can’t be taken literally, but it gives a good idea what to expect in our region tomorrow.

Saturday

Light rain begins after midnight tonight The heaviest rain about 2-3 PM Saturday, but like the Giant Food ad, where the shopper is being told about the melons, “they’re all good ones”, all areas in the Delaware Valley tomorrow will have heavy rain.

Winds will be elevated along the Jersey shore all day, but around Philadelphia, winds will pickup about 10 PM Saturday evening.

NBM high temperatures: Blue Bell, PA 54º Philadelphia, PA 56º
uncertainty slightly above average (based on standard deviation): ± 2.3º

Sunday

Lingering showers at the shore early morning. Clouds around Philadelphia break for sunshine during the mid to late morning. Temperatures mild early, peaking around noon, then falling during the afternoon.

NBM high temperatures early : Blue Bell, PA 55º Philadelphia, PA 56º

Early Edition- Friday and Saturday

Posted Fri 8:34 AM — Two low pressures systems near the Gulph of Mexico will interact and move towards us late Friday into Saturday.

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. Two low pressure systems will interact over our area .

But first some coverage of today, Friday— It will be sunny with relatively light winds. The area of cloudiness mentioned last evening to move in mid afternoon today is looking less impressive with last night’s models. So sunny for much of the day, with high clouds late.

What will be deceptive today will be the high temperatures. While the the TV/radio forecasts will give you the high temperatures, they don’t often tell you when in the day they will occur. Today, the high temperature will be reached late in the afternoon and it will be fairly chilly through at least noon. Basically, the winds will shift to the south about 2 PM, bringing the warmer temperatures by about 4-5 PM.

Getting back to Saturday, the two systems developing will interact and a supply of Gulph moisture will result in heavy rain on Saturday.

03-22-24 06 z NAEFS model statistical “mode” version overlayed with GEFS model “bias-corrected” precipitation model forecast for 2 PM Saturday. The heaviest rain moves in early afternoon. (Click on image for a larger view.)

Rain moves in 2-3 AM Saturday morning and will be heavy during the day with 2″-3″ inches of rain expected.

The latest NBM confirms yesterday’s trend for heavier rain, with an axis of heaviest rain though the immediate PHL area—

03-22-24 12z NBM accumulated rainfall by 8 PM Saturday. contours are 0.1 inch increments. (Click on image for a larger view.)

It becomes very WINDY after 8-9 PM Saturday. More details later this afternoon or early evening.