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.
Updated Wed 8/31 9:19 PM — A year ago tomorrow (Thursday) we were dealing with storm IDA, tornadoes, and 5-6 inch rains. What a difference a year makes!
Update Tue 08/30 @ 10:11 PM — How much rain did we get?
MRMS precipitation estimates (Click on image for a larger view.)
Tue 08/30 @ 8:47 PM —Forecast Review— The storms hung together better than I had forecast. A fast moving thin line of storms that didn’t have time to put down much accumulation.
Radar loop just before the storms got into Philadelphia. They regenerated more than I had forecast. (Click on image for a larger view.)
I’m waiting for an MRMS summary to become available to see if any areas got substantial rain. I know that my neck of the woods got very little.
Update Tue 08/30 @ 2:38 PM —What’s working against heavy rain storms for us today? As described in an earlier update below, the storms will be moving into a wall of more stable air, as demarcated by the 576 thickness contour
Another way of viewing this, instead of pressure related, is using equivalent potential thermal energy as a guide. In physics/meteorology, a thermal quantifier of instability is a calculated parameter called “Theta-E or Equivalent Potential Energy”. On weather maps, lines of equal Theta-E can be calculated and drawn.
Here’s the current water vapor image with lines of equal Theta-E drawn at the energy level of 335ºK (measured as temperature, Kelvin scale) —
Current Water Vapor at 2:30 PM with RAP model theta-E contours (yellow) drawn. Notice the “wall” of theta-e contours (yellow) just to our west. The theta-e values all decrease as one approaches Philadelphia (red circle) and NJ, indicating less instability. Moisture is inclined to flow along theta-e lines (white arrow). Any storms will hit this wall and their energy will move northeastward. Areas in far western suburbs and along those western contours may have heavy storms, but those storms’ intensity will diminish as their energy moves northward and the storms move eastward into a lower theta-e environment. (Click on image for a larger view.)
So, based on current water vapor and calculated Equivalent Potential Energy, I’m expecting any strong storms in far western suburbs to diminish in intensity as they move eastward.
Showers and Storms as early as 5:30 PM
Updated Tue 8/30 8:15 AM —Last night’s models show showers and storms as early as 5:30 PM tonight but the main activity moves through between 7 and 9 PM.
The NAM-NEST shows some of the heaviest rain, approaching 1 inch, but the majority of models are in the 0.3-0.5 inch range. We’ll take any rain we can get. I continue to lean towards the lower range forecast.
Rainfall may be disappointing Tuesday evening
Updated Mon 8/29 10:53 PM —Tonight’s 00z high resolution model runs (HRRR, NAM-NEST, HIRESW-ARW, HIRESW-FV3) show similar timing for the rain to move in (8 PM -11 PM Tuesday evening) but vary widely with the amounts of rainfall from 0.2 to 1.0 inch in the immediate Philadelphia area. All show the heaviest rain to our far north and west. I’m leaning towards lighter amounts here, but we could use the rain.
The experimental LAMDAX model also keeps rainfall light here.
LAMDAX total precip.
Update Mon 08/29 @ 8:25 PM — This afternoon’s models continue with the forecast of a cold front moving through Tuesday evening and night time, departing early Wednesday morning.
For those hoping we’d get some significant rain with this system, I’m afraid we’re going to be disappointed. Most models only crank out 0.2-0.5 inches of rain in the immediate Philadelphia area. As one heads north, northwest, and far west, areas may receive as much as 0.9 inches. Areas in NJ will receive even less. Of course, there’s always a localized area that may receive much more (or much less).
Most of the dynamics will lift off to the north. Additionally, the front is moving into a very warm ‘tongue’ of air, as depicted by the ‘576 thickness line’, which I have suggested in past posts has been part of the signature for this season’s disappointing rainfall.
We’ll get such low rainfall despite high PWAT values greater than 2.2″ and high CAPE values near 2000 joules/kg .
Today’s 18z Canadian High Resolution model (HRDPS) showing rainfall at 7 PM and the ‘576 thickness line’. (orange) Thickness lines can be thought of as demarcating the vertical dimension (thickness) of the lower part of the atmosphere. The 576 thickness line in this instance is the thickness of the atmosphere in decameters between where the pressure is 500mb (~18,000 feet) and 1000mb (near ground level). Thus it’s a 500-1000mb thickness line. Thickness lines are usually used for forecasting winter weather where the 540 thickness line is generally rain-snow line. I find them useful year-round to visualize the atmosphere three- dimensionally.
I believe that most of the current drought is the result of human created climate change, but some of the current drought is likely resulting from the current ascent phase of the Hale Solar Cycle superimposed on climate change.
We’re in a dry spell that is tough to break. Soil moisture is extremely low resulting in negative feedback. Additionally, we’re in the uptick phase of a well-recognized but little talked about Hale 22 year Solar Cycle that is highly-correlated with drought conditions (yet poorly understood). If you’ve never heard of this, I suggest you do a Google Scholar search on “Solar Cycle and drought” for scientific journal articles on this subject.
Previously Posted Mon 10:20 AM —
This week’s weather will be marked by a cold front moving through Tuesday night and a large dip in the jet flow with cooler and nice weather for the end of the week and lasting into the Labor Day Weekend.
Current Satellite Water Vapor Image Monday morning. Upper low in western Virginia and cold front in the Great Lakes (Click on image for a larger view.)
For Tuesday evenings’ cold frontal passage—
ECMWF forecast for 5 PM Tuesday shows an approaching cold front precipitation. NOTE THE ORANGE ‘576 THICKNESS’ LINE JUTTING UP INTO OUR AREA. THIS WARM TONGUE OF AIR MAY REDUCE THE AMOUNT OF RAIN WE GET FROM THIS FRONT. (Click on image for a larger view.)
Most of the dynamics are forecast to be north of our area with this front on Tuesday, reducing the amount of rain we get here. Also note the ‘576 thickness line’ protruding into our area which may also reduce rainfall (as I’ve suggested it had done several times in previous weeks). Current NBM forecast is for only 0.3 inches of rain for most of our area.
The dip in the jet flow on Thursday will bring much cooler temperatures—
ECMWF jet stream wind flow Thursday 2 AM (Click on image for a larger view.)
As for hurricanes and or tropical storms, there’s some indication that a hurricane may develop and affect the Carolinas next week.
