Updated Sun 10/02 11:12 PM —Tonight’s early models are showing the remnants of Ida have become a closed upper low that will meander near our area through late Tuesday. Monday may see a reduction of the rain from Philadelphia and westward, but rain rotates back in on Tuesday. The ECMWF is showing several inches of rain possible in some areas on Tuesday. I’ll be updating this interesting setup tomorrow
Update Sun 10/02 @ 8:42 AM — Fascinating convergence zone has developed over us—
Radar at 7 AM shows area of converging winds (Click on image for a larger view.)
06z HRRR forecast for 8:30 AM shows surface winds. I’ve added the notations of the direction of the upper air winds Also visible is the wavy light orange “500-1000 thickness line” showing a small scale ‘wave‘ over our area. This convergence and wave are causing the rain. This feature is forecast to move slowly eastward over much of Philadelphia for the afternoon. (Click on image for a larger view.)
Unfortunately, the Eagles game looks like it will be played in the rain (and wind.)
Updated Sat 10/01 10:41 PM — Tonight’s models are now forecasting that even Sunday morning will have some light drizzle or rain. Rain increases in coverage and intensity during the afternoon. It looks like a wet day at the Linc.
Update Sat 10/01 @ 4:07 PM — Today’s models suggest a dry period before noontime on Sunday, but additional rain will fill in between noon and 2 PM as upper disturbances rotate in around the upper low. The HRDPS captures this —
Forecast Review— We had the break in the rain mid to late morning, but the HRRR was overly optimistic forecasting breaks in the clouds.
Update Sat 10/01 @ 9:41 AM —We had some much needed rain last night. Here’s estimated accumulations according to the MRMS—
MRMS 24 hour estimated rainfall through 9 AM today. Orange contours are in millimeters 25.4 mm =1 inch (Click on image for a larger view.)
The models have the showers tapering off this morning. The latest HRRR even shows some breaks of sun around noon in a few areas—
12z HRRR shows some breaks of sun in a few areas around noon. A few scattered showers, little additional rain during the day. Rain returns about 9 PM.
The rest of the weekend will be influenced by the upper atmospheric remnant of Ida, currently in West Virginia
Areas of vorticity (eddy currents) will be rotating around the main mid-atmospheric low. These vorticities trigger the showers as the rotate through. Timing them can be difficult.
Mid atmosphere (500 mb level) GFS forecast of absolute vorticity rotating around mid-level low pressure near West Virginia. Like a spinning top, this upper low will wobble and have precessional motion that will be difficult to accurately forecast.
The next wave of significant showers rotates through here about 10 PM tonight. That break in the action Sunday afternoon as previously forecast, is looking less likely.
Updated Fri 9/30 10:42 PM —Tonight’s NAM-NEST and HRRR have some gusty winds (40mph) and thunderstorms possible about 5-7 AM Saturday morning. Then, just occasional scattered showers and dark clouds, windy.
Update Fri 09/30 @ 6:45 PM — Incredibly, the latest ECMWF still has a break in the rain for Sunday afternoon.
ECMWF precip forecast for 2 PM Sunday (Click on image for a larger view.)
Previously Posted Fri 5:49 PM —
The remnants of what was Hurricane Ida will affect our weather over the weekend. The main remnant of the storm will reside as a mid level low pressure circulation over West Virginia while a surface low spins up far off and east of the New Jersey coast.
Friday’s 12z GFS forecast shows rain away from the city at 2 PM Saturday. (Click on image for a larger view.)
The reason for the track of the Ian westward and the weekend near-stationery low pressure is a blocked weather pattern. This will break down only slowly, so unsettled weather will be with us through Monday.
Saturday
After moderate to heavy rain later Friday night, scattered showers and on and off rain will be with us Saturday. Low dark clouds and somewhat windy conditions will make indoor activities the preferred choice.
High temperature 59.9º sd 3.5º NBM model Blue Bell, PA
Sunday
The upper low in West Virginia will throw more clouds and rain towards us, so much of the same.
Several models that were showing a break in the rain Sunday afternoon have moved away from that forecast this afternoon. The NAM-NEST still shows relatively little rain in the city at 3 PM. The NBM shows more widespread on and off light rain in the afternoon.
NAM-Nest forecast for 3 PM Sunday (clouds- black) rain a1 hour accumulated (green contours) and simulated radar. (Click on image for a larger view.)
High temperature 58.4º sd 3.0º NBM model, Blue Bell, PA
Updated Sun 9/25 11:02 PM — Monday looks mostly sunny and windy. A few instability showers may move through during the afternoon hours.
Tropical storm Ian looks like a forecasting challenge, with several models keeping a track close to the western coast of Florida and some maintaining an eastern turn into north central Florida. Weakening still forecast if it moves northward as it encounters large high pressure. Some models have the storm slowing as it encounters this high.
Stay tuned.
Update Sun 09/25 @ 5:38 PM — The rain came in pretty much on schedule, about 2-3 PM. Some areas, with thundershowers, had a bit more rain than had been predicted.
CAPE values weren’t all that high but there was enough vertical shear to create a few storms. Frankly, I was a little surprised by that. That said, I had no idea why they were predicting possible severe storms.
Here’s the total rainfall based on the MRMS—
MRMS rainfall this afternoon (Click on image for a larger view.)
As for Tropical storm Ian, uncertainty exists whether it will turn eastward into western Florida or whether it will move more northward into the Gulf coast. Most models still have it sheared off as it moves northward into strong high pressure, so as it approaches the Gulf coast, it may weaken.
Unless it moves more westward, much of the moisture with Ian will likely move off to our south before getting here at the end of the week or weekend. Stay tuned.
Updated Sat 9/24 9:56 PM — Tonight’s models have the showers moving into Philadelphia between 1-3 PM, earlier to our west. Precipitation will be light, about 0.1-0.2 inches of rain.
Regarding the new tropical storm/hurricane Ian, it appears that it’s headed for Louisiana, not Florida. Many models show it being sheared apart and weakened as it plows into strong high pressure.
Updated Sat 9/24 12:32 PM — A quick mobile update. This morning’s models have leaned toward an earlier onset of the showers on Sunday. Most likely 1-3 PM. The showers no longer stop at our doorstep until evening.
A cold front moved through Thursday and today, Friday, and it was noticeably colder than its been for a long time!
Current water vapor satellite image shows current jet stream level features—
Water Vapor at 6 PM with superimposed radar and RAP model jet stream wind flow. Upper low pressure will cause a surface low to develop and move over us later Sunday. (Click on image for a larger view.)
For Saturday — An interesting looking weather map for 2 PM Saturday, as high pressure builds in over us and an impressive, strong hurricane Fiona moves into Nova Scotia and Prince Edward Islands.
NAEFS forecast for Saturday at 2 PM. Impressive hurricane Fiona to our northeast. (Click on image for a larger view.)
For Sunday, the low pressure system near Wisconsin will move towards us with a warm front late Saturday and some showers late Sunday. Incredibly, the models show most of the rain to our west, skipping over Philadelphia, and redeveloping to our east.
Saturday
Some cloudiness mid-morning as an upper air disturbance moves through, then mostly sunny. Winds much lighter.
High temperature 60.3º sd 1.2º NBM model Blue Bell, PA
Sunday
Increasing cloudiness in the morning. Cloudy by noontime. The latest GFS has showers moving into western suburbs early afternoon (2PM) but not advancing into Philadelphia until the evening. The heaviest rain looks like it skips over Philadelphia. Increasingly windy and gusty!
High temperature 75.0º sd 2.0º NBM model Blue Bell, PA
Update Sun 09/18 @ 10:30 AM — Today’s high temp 83.5º with an unusually low sd of 0.7º
Outlook
Generally warm weather for the first half of this coming week. A rather potent cold front may move through late Wednesday night with showers and thunderstorms.
The GFS is advertising a brief, unusually chilly outbreak for late Thursday into Friday. On the weather map, we see the appearance of the 540 thickness line (red), a parameter usually associated with the snow-rain line (in winter).
Today’s 06z GFS forecast for Friday 2 AM with 540 thickness line (red) making an usually early appearance. A brief period of chilly weather next Friday expected. (Click on image for a larger view.)
This dip in the jet flow should assure us that tropical system Fiona stays out to sea.
Updated Fri 9/16 10:01 PM — minor update, highlighted
A fairly straightforward forecast for this weekend as high pressure over us will provide mostly sunny skies for Saturday and Sunday.
Saturday
For Saturday, there will be some considerable high, thin cirrus at times as a result of some moisture streaming over an upper level ridge covering the center of the country. Otherwise, sunny.
Water vapor showing northern moisture flow
High temperature 82.7º sd 1.2º NBM model Blue Bell, PA
Sunday
Sunny.
High temperature 37.0º sd 1.8º NBM model – for location Blue Bell, PA
Next week shows temperatures trending towards very warm for mid September. A weak front will also move through Monday night. A hurricane developing near Puerto Rico is expected to stay out to sea but it’s not a sure bet, according to the Canadian model. Hurricanes, in general, seem to affect forecast accuracy.
