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″
Updated Thu 9/08 8:32 PM —Well, after looking over some other models, the ECMWF and the GFS may be too fast with the rain moving in Sunday afternoon. The Canadian and German ICON models keep the rain in central Pennsylvania for much of Sunday afternoon and the latest model blend (NBM) leans towards that forecast. Will have to wait for clarification.
Update Thu 09/08 @ 7:42 PM — A significant change in the forecast for Sunday. While I had thought much of the weekend would be very nice, it appears that clouds move in earlier on Sunday morning with rain possible by early to mid late afternoon, especially areas just of west of the city.
The rain for Sunday afternoon is forecast, especially west of Philadelphia, by the GFS, NAM and ECMWF models.
Today’s 18z GFS shows light rain just moving in about 1 PM on Sunday. The ECMWF has the rain a little further eastward. (Click on image for a larger view.)
There may be a break in the action following this initial wave of showers, with rain returning by evening.
Update Thu 09/08 @ 9:38 AM —The next several days, through much of the weekend, will be very nice weather-wise. There’s the chance of some increasing cloudiness on Sunday.
The forecast is summarized in the GEFS graphic caption below—
Latest GEFS forecast for Friday at 5 AM. The low pressure system that brought us the much needed rain will merge with a (1) hurricane in the Atlantic. High pressure (2) will move in for Thursday through Sunday. A developing trough (3) will bring the chance of increasing cloudiness on Sunday and showers and rain Monday and Tuesday of next week. (Click on image for a larger view.)
Temperatures will be in the low to mid 80s which is slightly above the average seasonal highs of 78-79º.
With two hurricanes in the hemisphere, weather systems don’t always behave as forecast.
Showers continue on Wednesday
Update Tue 09/06 @ 5:58 PM — Here’s the latest MRMS rainfall totals for the past 24 hours—
MRMS rainfall totals for the past 24 hours as of 5 PM Tuesday. (Click on image for a larger view.)
Showers are expected to continue to move into the area from the northeast, as a circulation develops around the coastal low that is forming. These showers will linger through Wednesday.
The low pressure system will interact with a tropical system in the Atlantic. How this plays out will be difficult for the models to accurately capture, so I expect forecasts over the next day or so to remain a bit less reliable than usual.
The latest (12z) ECMWF model forecast for Wednesday shows some considerable additional rainfall for our area—
Today’s 12z ECMWF model forecast for 2 PM Wednesday showing 3 hour precipitation. (Click on image for a larger view.)
Rainfall has already exceeded model forecasts
Update Tue 09/06 @ 11:22 AM — Here are the rainfall totals as of 10 AM according to the MRMS—
MRMS rainfall totals as of 10 AM Tuesday. (MRMS totals are always 1 hour lagged.) (Click on image for a larger view.)
Update Tue 09/06 @ 9:43 AM — Rainfall has already exceeded model estimates in many areas. Totals area already over 3 inches in Valley Forge and surrounding areas.
Rainfall rates should start to taper down about 3 PM, but additional showers are expected to rotate into the area from the northeast as low pressure develops off of the coastline.
I’ll update with an MRMS graphic of accumulated rainfall a bit later.
Update Mon 09/05 @ 6:04 PM — Heavy rain still on track later tonight through Tuesday.
Radar at 6 PM Monday. Rain has moved in somewhat faster and further east than forecast earlier today. (Click on image for a larger view.)
Latest GFS is still forecasting overall 1-3 inches by Tuesday afternoon.
Monday 18z GFS just available. Heavy rain totals by 2 PM Tuesday. Additional rains expected through Tuesday night and possibly Wednesday at times.
Monday – No rain for most of us. HEAVY RAIN Tuesday
Update Mon 09/05 @ 8:50 AM— The same system that has been stationary for several days will being to move eastward later today.
Water Vapor image with with RAP model jet stream level 250 mb winds (orange contours), radar superimposed. Yellow contours are theta-E 335 contours, changes in potential energy. Cold front in Ohio will slowly move east with surface low (L blue) Upper air Low (L RED) hanging back in Lake Michigan. (Click on image for a larger view.)
The push of this system eastward will bring rain.
Much needed rain continues to be forecast for late Monday through Tuesday. Several models now have total precipitation forecasts 2-3 inch range, in line with the NBM and ECMWF forecasts from yesterday. Even the Canadian model has joined the group forecasting high precipitation.
While a few widely scattered showers are forecast towards evening today, most models have the bulk of the rain starting late evening or after midnight in the immediate PHL area (with somewhat earlier starts in upper Montgomery and Bucks counties).
By Wednesday, the low will set up along the east coast, keeping unsettled weather at the shore—
RAP model forecast for 8 AM Wednesday. (Click on image for a larger view.)
Previously Posted Sun 5:33 PM —
A front has remained stalled with associated rain moving around and at a distance from us—
Water Vapor with superimposed radar. Yellow arrows show the trajectory of the rain around but at a distance from our area. The entire system will move over us late Monday into Tuesday with heavy rain being forecast. (Click on image for a larger view.)
The whole system begins to move eastward and southward over our area Monday night.
We should have another dry day Monday but it will be much more cloudy than past days. Some showers may move into upper Montgomery and Bucks counties by late Monday afternoon and evening.
Heavy rain moves in during Monday night and continues into Tuesday. The ECMWF is forecasting as much as 3 inches of rain over Philadelphia! The NBM is forecasting 2.5 inches, with other models showing at least 1 inch of rain.
Update Sun 09/04 @ 7:36 PM — The latest NAM-NEST, ICON and Canadian GEM has the heaviest rain to our far north with our area getting about 0.6-0.8 inches of rain.
