This weekend will be an easy forecast. High pressure will build in for Saturday and Sunday with our overall circulation still around a slight upper trough.
Water Vapor image shows the general jet flow. The disturbance in Tennessee (blue cluster) will stay to our south. The heat dome over the Southwest and California is expected to expand somewhat northward and eastward over the coming week. (Click on image for larger view.)
While not a consideration for this weekend, the extended range models are showing the dome of hot air to start migrating north and eastward. Things are looking much hotter here by the end of this coming week into next weekend.
Saturday
Sunny. A few fair weather clouds. Delightful. Dew points in the low 60s.
High temperature 83.2º sd 1.1º ( NBM model— location Blue Bell, PA)
Sunday
Sunny and warmer. Dew points in the low 60s.
High temperature 86.1º sd 1.2º ( NBM model— location Blue Bell, PA)
Updated Sun 07/16 @ 4:42 PM — Here’s the MRMS rain gauge/radar total rainfall estimate for the past 48 hours—
MRMS QPE (rain gauge – radar) estimate of the rainfall over the prior 48 hours from 3 PM Sunday. Contours are in mm (25.4 = 1 inch) (Click on image for a larger view.)
Live Blog
Updated Sun 07/16 @ 11:30AM—
Sunday Forecast Update
Updated Sun 07/16 @ 10:40 AM — Despite several model’s shift of the heavy rain axis into NJ, some of the new morning models (12z) still maintain a forecast of heavy rain this side of the Delaware river.
Here’s the HIRESW-FV3 forecast, similar to the 06zGFS model which prompted my earlier “forecast change” post—
HIRESW-FV3 total precip shows an eastward shift in the total rainfall axis. This model is similar to an earlier 06z GFS. (Click on image for a larger view.)
The 12z HIRESW-ARW which is more in line with current radar trends—
12z HIRESW-ARW 5km Total precip forecast for today. (Click on image for a larger view.)
Additional flareup of storms expected between noon and 3 PM. Another round of isolated showers/storms at about 8 PM.
Forecast Change
Update Sun 7/16 6:16 AM — Large change to forecast. Heavy rain axis has shifted east into NJ for today
Sunday Heavy Rain Event
Updated Sat 07/15 @ 10:29 PM —The setup for a heavy rain event is taking shape with an upper level cyclonic flow, high precipitable water near 2.3″ (PWAT) and developing low pressure along a trough.
Rain and thunderstorms are already approaching the Baltimore area and the storms will be moving in to our area between midnight and 3 AM.
Current radar and water vapor image approaching 10 PM Saturday night. (Click on image for a larger view.)
Looking over the latest 00zHRRR and 00z RAP models, it appears the main concern will be heavy rain and potential flooding from strong thunderstorms (but not severe). Tornadic activity is not expected, and while strong downdraft winds can occur in some spots, high winds should not be a major issue for most areas.
The rain will come through as two main clusters— morning and again late afternoon/evening.
Some of the heaviest rain will be in the morning hours, with a let up in activity possible about noontime before resuming with more scatteredbut still heavy rainfall during the late afternoon and early evening.
A possibility that the afternoon/evening storms will be severe is not clear at this. Since high rainfall is the main issue, I’ll defer posting my severity table until tomorrow morning.
As we saw today, rainfall amounts and locations are impossible to pin down exactly but the models are pretty much in the same ballpark of 1-3″ with locally higher amounts.
Here’s the latest HRRR total rainfall forecast—
Tonight’s HRRR total rainfall prediction through 8 PM Sunday. (Click on image for a larger view.)
Saturday Update
Updated Sat 07/15 @ 5:50 PM — For tonight, the main weather event for this weekend is taking shape. Heavy rain and thunderstorms should move into our area around 2-5 AM Sunday morning. PWATS are extremely high (2.5 inches), meaning that any storms are capable of putting down very heavy, flooding-type downpours.
Current Water Vapor-Radar image at 6 PM Saturday (Click on image for a larger view.)
I’ll have more details on tomorrow’s weather even later this evening after a few of 00z models come out. With our being on Daylight Saving Time, the 00z high resolution models start coming in between 9:30 PM and 10:45 PM tonight.
Sat 5:34 PM Forecast Review — Well, today was proof that one can’t really predict the location and timing of pop-up storms.
If you were at the Phillies game today, my forecast was a home run.
But if you were out biking in upper Montgomery County, my forecast was a bust. We almost got caught in quite a downpour about 1 PM. Luckily, a planned lunch stop provided shelter. I had been watching the radar while biking and the storms developed in central Chester County, then moved north. It was still a pretty wet ride back once the main cells passed by.
I went along with the HRRR forecast today after that model proved correct yesterday. Both the 06z and 11z mode runs showed no storms in our area, as shown in my morning update graphic. Even the 15z HRRR, which I accessed on my phone remotely, showed no storms. Go figure!
Reviewing today’s earlier models and last night’s models, not one really predicted what transpired today. The HRDPS was close on today’s forecast but had been totally wrong with yesterday’s late storm forecast. I mentioned this in my morning update below.
That said, today’s HRRR was an outlier since it showed a very minimum of shower activity, while most others showed some showers somewhere.
Updated Sat 07/15 @ 9:28 AM — Continuing with the HRRR emphasis from yesterday, the 06z HRRR showed virtually no showers this afternoon in the Philadelphia area. The latest 11z HRRR showed a few pop up storms about 2-3 PM with a period of cloudiness mid afternoon.
The HRRR puts the pop ups south of the city in Delaware county—
11z HRRR Forecast precipitation rate (1hr) at 3 PM Saturday. (Click on image for a larger view.)
So some pop-up storms are possible. The issue with isolated pop-up forecasts is that the forecast location can’t be taken literally; the models just can’t do it more accurately. So we’ll see.
Just an FYI that other models, like the Canadian HRDPS, are forecasting much greater areal coverage of these pop-up storms during this afternoon. Since the HRDPS did not do well with yesterday’s forecast, I’m leaning towards the HRRR.
Sunday looks to be very “interesting” weather-wise, as a high degree of severity parameters are coming together for the potential for heavy rain, severe storms and even an isolated tornado. I’ll update later this afternoon or evening.
Update Fri 7/14 7:48 PM — It doesn’t appear as though any of the thunderstorms forecast for the city tonight will develop. (Good for the Phillies.) Indeed, looking back over today, the HRRR model forecasts were superior. So I’ve updated Saturday’s forecast below to be HRRR weighted.
Previously Posted Fri 6:15 PM —
A disturbance approaching our area Friday evening has already brought some showers and thunderstorms to some areas of Montgomery and Bucks counties over the past few hours. Additional storms are possible in Philadelphia between 8-11 PM. The area of storms will be somewhat scattered and and rainfall will vary from about 0.15 inches to as much 1.2 inches in a few spots.
Saturday will be in-between systems and most areas will be dry.
BUT pop-up storms around Philadelphia and immediate suburbs in the mid and late afternoon are predicted by the latest NBM and HREF.
(The NBM and HREF forecast for today didn’t fare quite work so well, so my confidence in a dry Saturday is somewhat below average.)
An approaching disturbance will begin setting off some storms late evening Saturday.
Another larger disturbance will interact with an approaching upper trough early Sunday. Showers and thunderstorms are expected.
Sunday’s upper disturbance can be seen in this afternoon’s satellite image—
Water Vapor image Friday afternoon shows the upper disturbance expected to spawn low pressure near us on Sunday. (Click on image for larger view.)
Saturday
We’ll be in-between systems Saturday. Partly sunny through high cirrus clouds. Some scattered pop up showers possible in the mid to late afternoon. Showers and thunderstorms possible beginning around midnight.
High temperature 89.1º sd 2.0º ( NBM model— location Blue Bell, PA)
Sunday
Cloudy with showers and thunderstorms beginning in the early morning. There may be a break in the showers/storms and even some sunny breaks before a cold front moves through later in the day with additional storms. The heaviest rain may fall in the morning hours and the storms later in the day will be more scattered, but possibly stronger.
Today’s 12z GEFS shows 3 hour accumulated rain at 2 PM. (Click on image for larger view.)
Over 1 inch of rain in many locations with some receiving much higher amounts. There will be plenty of dynamics and some of the storms may be strong to severe. Some gusty winds in the afternoon as the front moves through.
High temperature 84.2º sd 1.9º ( NBM model— location Blue Bell, PA)
Updated Fri 07/14 @ 9:15 AM — There was a line of very fast moving storms that developed about 5 AM in Philadelphia and moved northeastward, Here’s the MRMS record of the precip.—
MRMS record of this morning’s early thunderstorms. (Click on image for a larger view.)
The big question for today is — what sort of thunderstorm development will occur and when?
The 06z HRR, the 06z NAM-NEST and the 00z ECMWF did NOT forecast these storms earlier this morning, so I’m going take them out of the forecast consideration. Last night’s HIRESW and Canadian HRDPS did, so I’m going to lean on their forecasts for today.
This translates into a mix of sun and clouds for the remainder of the morning. Between 1 and 3 PM, thunderstorms break out northwest of the city—
HRDPS forecast for 2-3 PM today. (Click on image for a larger view.)
For later in the day today, I’ll use the HREF which is similar to the HRDPS. This shows showers/thundershowers moving into Philadelphia by 5-8 PM—
This morning’s 06z HREF shows showers/storms between 7 and 8 PM. (Click on image for a larger view.)
For Saturday, the HRDPS has a much drier forecast than the HREF. So check back later this afternoon/evening for my regular Weekend Weather forecast.
Friday Forecast Update
Updated Thu 7/13 10:43 PM — Tonight’s latest models suggest a change in Friday’s forecast. Following some showers early Friday morning, sunshine expected by late morning through much of the afternoon. Showers and thundershowers move in from the west later afternoon and may not reach Philadelphia until mid to late evening.
Friday and Weekend Weather Outlook
Updated Thu 07/13 @ 8:04 PM — Several disturbances rotating around an upper low near Hudson Bay Canada are lining up to affect our weather over the next few days.
Current satellite water vapor shows several disturbances—
Satellite Water Vapor (channel 9) shows several disturbances lined up to affect our area. Disturbances 3, 3a and 4 will merge into a surface low over Virginia by Sunday. (Click on image for a larger view.)
The NAEFS model shows these disturbance will move together for a rainy day on Sunday—
NAEFS Sunday forecast shows several disturbances develop a surface low over the Virginia by Sunday. (Click on image for a larger view.)
For Friday: Disturbance #1 will bring some showers to our area on Friday. A few showers in the very early morning, The showers will be scattered, but increased chances during the afternoon and evening. Most of the activity to our south and north, but some showers/thundershowers expected in the city and immediate suburbs.
Saturday: Following some showers before daybreak, high and mid level cloudiness with possibly a break in the showers. especially for the city and immediate western suburbs. Showers and thunderstorms develop in NJ during the afternoon and spread into the city by evening.
Sunday: Cloudy with showers and thunderstorms.
Timing these upper air disturbances is challeng and the forecast details should clarify over the next day or so.
Thursday-Friday Forecast
Updated Wed 07/12 @ 8:34 PM —
We’ll have another sunny and increasingly hot day on Thursday and early on Friday.
Upper disturbances rotating around an upper low over Hudson Bay and additional energy moving over the ridge of hot weather in the south will bring an increasing chance of showers and thunderstorms Friday afternoon, Friday night and Saturday.
The first of several disturbance is visible on this afternoon’s satellite water vapor image; most of its showers will stay to our north and west on Thursday—
The first (1) of several disturbances rotating around an upper low in Canada’s Hudson Bay will pass us on Thursday night. Another disturbance (2) will affect us likely late Friday into Saturday. Meanwhile, the heat dome is visible over the Southwest. (Click on image for a larger view.)
The GFS keeps the showers out of Philadelphia on Thursday—
18z GFS shows no rain in Philadelphia on Thursday. (Click on image for a larger view.)
A dome of extremely hot air over the Southwest and South will remain stationary while impulses pass around its northern edge into our area later in the week.
ECMWF jet stream wind forecast (250 mb) for Thursday. Impulses rotating around the edge of the stationary heat dome to our south will provide additional chances for rain beginning late Wednesday into Thursday. The red oval above is where low pressure systems are expected to form and move over us. (Click on image for a larger view.)
For today, Monday, the latest HRRR has a few showers possible about 9-10 PM this evening, mostly north of the city.
