Update Sun 04/30 @ 4:24 PM — Areas of very heavy rain continue, especially in New Jersey, where large areas have received close to 5″ of rain since Friday. An additional inch or more is possible!
The latest HRRR shows some heavy downpours with thunderstorms possible between 8 PM and midnight ahead of a cold front passage.
The actual cold front will move through between 10 PM and midnight and periods of heavy rain will continue through the frontal passage.
Wind gusts may exceed 55 mph in some areas around midnight as a pressure gradient develops with the departing low pressure system.
Update Sun 04/30 @ 9:52 AM — The rain has moved in as expected and periods of rain, heavy at times, will fall throughout the day and evening hours. Total accumulations for the day will be in the 0.9″ to 1.5″ range with some locally higher amounts.
Each model has a somewhat different axis location for the heavy rain but the HRRR, which did best Friday night, now has a somewhat different heavy rain axis than it had last night—
Latest 12z HRRR total rain accumulation by midnight (Click on image for a larger view.)
A strong cold front will move through about 10 PM to midnight tonight. The HRRR and especially the RAP model are showing some low CAPE values (300 J/Kg).
Moderately high shear values (30 1/s) with that CAPE suggests the possibility of thunderstorm formation between 6 PM and 10 PM ahead of the front. Strong wind gusts of near 40 mph are possible. No tornadoes expected and it’s currently a low severity risk.
Forecast Update and the Broad Street Run
Update Sat 04/29 @ 10:25 PM — A quick update. The precipitation forecast from this afternoon remains intact. It showed fairly heavy rain through 2 PM.
The latest models tonight show heavier rain and possibly stormier weather during the afternoon and early evening hours on Sunday. As a cold front approaches Sunday evening, some thunderstorms can’t be ruled out.
Here’s the updated HRRR rain totals forecast through 8 PM Sunday evening—
Tonight’s 00z HRRR rain totals forecast through 8 PM Sunday. Many areas will see 1 to as much 1.5″ additional rain. (Click on image for a larger view.)
Update Sat 04/29 @ 4:51 PM — Looking back to yesterday’s model forecasts, yesterday’s 18z HRRR model was most accurate with its precipitation amounts and the axis of heavy rain in NJ.
Today’s 18z HRRR is now available and here’s its forecast amounts for the period now through 2 PM Sunday with special emphasis on the Broad Street Run location—
Today’s 18z HRRR model forecast total rainfall for Sunday at 2 PM. (Click on image for a larger view.)
Update Sat 04/29 @ 10:12 AM — The rain has tapered off a few hours earlier than forecast, but light drizzle remains. The latest 12z NBM still shows light drizzle through much of Saturday afternoon. The latest 12z RAP and HRRR also show a few very widely scattered showers into the evening.
The rain totals from yesterday’s NAM-NEST weren’t all that far off, but as is often the case, the exact location of the heavy rain axis was different than forecast, in this case further east. Here’s the MRMS based rain totals so far—
MRMS-based 24 hour estimated rain totals through 8 AM Saturday. (Click on image for a larger view.)
As for Sunday and the Broad Street Run, the current updated forecast is for the rain to move back in about 8-9 AM. Light and scattered at first, then moderate to heavy rain through much of the daytime. Another 0.30″ to 1.5″ expected.
If any consolation for the runners. the latest 12z HRRR shows only 0.33 inches of rain for Broad Street during the day, with the heaviest rain falling to the west and east—
Today’s12z HRRR accumulated rain forecast for Sunday. (Click on image for a larger view.)
As we saw today, the models aren’t always that good with forecasting the placement of the heaviest rain axis, so don’t hold me to that ‘lighter” 0.33 inches along Broad Street.
Update Fri 4/28 9:53 PM — It looks like the steady rain ends around noon on Saturday. Very widely scattered showers follow through the evening.
Previously Posted Fri 5:19 PM —
As outlined in several posts this past week, a blocked weather pattern will keep the major determinants of our weather relatively unchanged from today, Friday, through possibly next Thursday!
Blocked patterns are somewhat common in springtime and for this pattern, our area falls on the chilly and wet side of things —
12z ICON model jet stream wind forecast for Sunday. An Omega ( Ω ) block is shown with the white wind flow arrows. The upper air low (L) approaching the Great Lakes will close off and be relatively stationary for almost 5-7 days giving us periods of cool wet weather.
Rainfall from Friday afternoon through Saturday afternoon will total 2-3.9 inches of rain according to the latest NAM-NEST—
Today’s 18z NAM-NEST total rainfall through Saturday evening. Additional rain on Sunday! (Click on image for a larger view.)
It should be noted that the NBM has much lower precip totals for Saturday, on the order of only 1.5″. The NBM shows about 3″ total for the weekend.
I should also note while the models have been consistent with this weekend’s rainy forecast, these type of weather scenarios driven by upper air low pressure systems often change dramatically in the short-range. Already, the rainfall for the first half of the weekend is forecast to be more substantial than the rainfall predicted for Sunday, a significant change. Stay tuned.
Saturday
Cloudy with continued rain. Rain may taper off far south and west of the city late afternoon, but continued spotty rain/showers are forecast for much of Saturday afternoon from the city, north and west. Windy and gusty.
High temperature 56.5º sd 2.5º ( NBM model location Blue Bell, PA)
Previously Posted Fri 5:19 PM —
Sunday
Spotty rain develops during the morning and becomes heavy during the mid afternoon. An additional 1.25 inches of rain possible. Milder. Windy and gusty.
High temperature 63.8.º sd 1.5º ( NBM model location Blue Bell, PA)
Winds
NBM forecast winds and wind gusts (with standard deviation) for location Blue Bell, PA Wings Field.
Update Fri 04/28 @ 9:28 AM — The latest radar has the rain somewhat closer to us than predicted by last night’s models. Nonetheless, the RAP is showing a bit of push-back of the rain advance, based on isentropic (thermal energy) analysis.
MRMS radar at 9:22 AM with superimposed RAP model 300K level Theta-E isentropes shows a bit of an energy wall to surmount for the rain to move into the PHL area, despite being just 40 miles away.
The models are still showing light sprinkles possible here about 11 AM, but the main area of rain moves in about 1-2 PM.
Rain: Parts of Friday, Saturday, Sunday
Update Thu 4/27 @ 9:08 PM — Several models have over two inches of rain falling Friday night alone.
Update Thu 04/27 @ 7:51 PM —The blocking pattern described below is setting up for this weekend and at least into Tuesday.
Rain moves in on Friday early afternoon, but some scattered sprinkles are possible during the morning.
The main slug of rain moves in from the southwest around 11AM to 1PM according to the latest GFS model—
This afternoon’s 18z GFS model forecast for 10 AM Friday. (Click on image for a larger view.)
On Friday, once the rain starts, it will become moderate to heavy at times.
The bulk of the rain will be departing on Saturday morning, but the models maintain some scattered showers around during the day, especially northeast of our area.
Heavier rain moves in on Sunday.
This weather will be generated by circulation around a upper level closed low pressure system near the Great Lakes that will remain almost stationary. Timing the breaks in the rain will be challenging as the impulses circulating the low are tough to model exactly.
We may get enough of a break Saturday afternoon to salvage the day.
Sunday doesn’t look as promising at this time.
Total rainfall may exceed 2 inches for the weekend.
I’ll try to nail it down in my regular Weekend Weather Forecast.
The upper air flow is moving towards a blocked pattern, specifically an “Omega Block” pattern. Unlike the recent “Rex” block named for the meteorologist who identified it, the “Omega” block is named for the Greek letter omega Ω because the wind flow resembles this letter.
Here’s the jet stream wind flow (at level 250 mb) for late Sunday. The Omega Ω resemblance is clearly visible—
Today’s GEFS 12z shows the forecast jet stream level (250 mb) wind flow for late Sunday resembling the greek letter Omega. (Click on image for a larger view.)
All blocked patterns result in stalled or slowly moving systems. In this case, we’ll be stalled in a cool, wet, cyclonic pattern with a closed upper low over the Great Lakes rotating disturbances towards us.
The developing block will bring rain to us by Friday afternoon.Periods of rain will be with us through the weekend and even through next Tuesday! There will be breaks in the rain and timing these breaks will be the challenge for this weekend’s forecast. It’s going to be a good weekend for ducks and flowers.
Rest of this week
Thursday
Update Thu 04/27 @ 10:02 AM — The latest HRRR forecasts even more lingering cloudiness today. We may not see any sun in the city until after 2 PM.
This morning’s 12z HRRR cloud forecast (Click on image for a larger view.)
Update Thu 04/27 @ 8:35 AM — It looks like we’ll get some sunshine about 1 PM in the city and earlier west of the city. Clouds move back in about 6 PM. High temperatures about 61º but a high standard deviation of 3.5º based on the uncertainty in the cloud cover.
Update Wed 4/26 11:06 PM — Tonight’s models have clouds and light sprinkles lingering possibly past noon.
Before we get to this pattern, Thursday should start cloudy, but considerable sunshine expected around noon. A mix of sun and clouds for the afternoon. High will be near 65º
Friday
Friday will be cloudy and rain moves in about 2-4 PM from the west.
