One reader wanted to know how to get the latest hourly probability of precipitation for our area.
One of the best ways is a direct link to the NBM (Model Blend) text output. The NBM is considered the best model for rain probability and the model data from this link is updated every few hours. (Yes, the NBM model is updated every hour, but to get the absolute latest, you’d have to dive into the bowels of the NOAA NOMADS server.)
I’ve pre-configured the URL for you to provide the model output for Philadelphia Airport, Wings Field Blue Bell, and Northeast Philadelphia airport—
The one you want to focus on is the NBH ( H= hourly).
It provides hourly probability (P01) in percentage (on an hourly basis, anything greater than 18% and increasing is meaningful) and the amount (Q01) in hundredths of an inch.
It’s important to know, that with accumulated rainfall, the number refers to amount having accumulated in the preceding hour.
The tricky part is converting to the UTC time to Eastern Standard or Eastern Daylight time.
The example below is for KLOM, Wings Field, Blue Bell.
So in the example below, this is the NBM model run is from 0700 UTC (3 AM EDT). The first row (labeled UTC) are the forecast hours and the column labeled “14” is the UTC TIME forecast time. So 14UTC = 10 AM EDT (or 9 AM EST).
Looking down the 1 hour probability (P01) is 48% and the quantity of rain fallen in the preceding hour (Q01) is 2. 2= 0.02″
Beware that for some of the other data displayed, especially in the NBE, the meaning can less less than intuitive. In the NBE, the column labeled, as an example, 00 day FRI, refers to the 12 hours PRIOR to 00z Friday which means daytime THURSDAY!
Updated Sat 9/10 10:56 PM — My usual habit of checking current radar and satellite images before a forecast update lapsed tonight. Regardless of the models, current radar shows some showers at our doorstep. So, I guess we know when the rain will start. BTW, only the ECMWF had forecast this early start.
Update Sat 09/10 @ 9:43 PM — There’s still some uncertainty about the start of the rain tomorrow in the immediate PHL area. The trend has been for some showers to move in as early as daybreak or early morning. Periods of rain on and off during the day. Anywhere from 0.5″ to 1.0″ expected tomorrow through Sunday evening. Additional rain expected Monday into Tuesday.
Today’s 18z ECMWF shows the rain well into our area by 11 AM Sunday. Additional rain expected Monday into Tuesday associated with the upper low in Iowa as it move eastward. (Click on image for a larger view.)
Updated Fri 9/09 10:54 PM — Tonight’s models are cranking out some moderate rain for Sunday afternoon and evening.
Updated Fri 9/09 8:23 PM — A continuing change with the onset of showers on Sunday. This afternoon’s Canadian high resolution HRDPS, the German ICON, and the ECMWF models forecast an earlier start— as early as 8 AM Sunday. Our US models are still forecasting the rain to move into the immediate Philadelphia area between 12 and 2 PM.
With the moisture approaching faster, expect some fog early Saturday morning before the sun burns through about 11 AM.
Saturday: fog early, then sunshine by late morning. Clouds move in mid-afternoon on Saturday. (no fog today!)
Sunday: Showers in western suburbs as early as Sunday morning, moving into the city around noontime.
Update Fri 09/09 @ 12:41 PM — This morning’s GFS has moved towards the ECMWF with a faster approach of Sunday’s system.
Saturday: Clouds move in mid-afternoon on Saturday.
Sunday: Showers in western suburbs as early as Sunday morning and moving into the city around noontime. A break in the action mid-afternoon before more showers move in during the evening.
Previously Posted Fri 10:50 AM —
A dry flow over us will keep delightful September weather over our area through Saturday. Extensive dry flow is visible on current satellite water vapor imagery—
Current Water Vapor image Friday morning. Warm dry air flow (orange -red shading) is over us. I’ve drawn a warm front position over the advancing system from the south. This will affect us on Sunday. (Click on image for a larger view.)
High pressure over our area Friday and Saturday will slowly move offshore to our east as the warm front moves up from the south.
By Sunday, the high pressure system will be off the coast, possibly blocking the advance of showers and rain to our south and west until late afternoon or evening—
NAEFS model forecast for Sunday at 11 AM. High pressure is blocking rain moving in from the west. (Click on image for a larger view.)
The uncertainty in this forecast will be the degree of blocking provided by the high off the coastline and the timing of the showers on Sunday afternoon or evening.
The NBM (model blend) maintains the block and keeps the showers in far western areas for much of the afternoon and early evening. Here’s the NBM probability forecast for rain on Sunday at 5 PM—
Today’s 12z NBM precipitation probability forecast for Sunday at 5 PM. (rain becomes a good bet in the green-yellow and certain in the violet areas) (Click on image for a larger view.)
While several models (and model blend) keep the rain to our west for much of the day, the ECMWF has been consistently aggressive in bringing rain in as early as Sunday morning. I’m leaning towards late afternoon at this time.
Saturday
Sunny and pleasant. Some clouds move in during the late afternoon and evening.
High temperature 82.5º sd 2.1º NBM model Blue Bell, PA
Sunday
Cloudy with increasing chance of rain moving in during the mid to late afternoon. (rain significantly earlier western areas)
High temperature 76.7º sd 4.1º NBM model Blue Bell, PA (higher than normal sd – uncertainty)
Some models have a good, soaking rain late Sunday night into Monday with rainfall about 1″
