Update Tue 04/11 @ 4:37 PM — We had a layer of high cirrus clouds move in today and they should move off to our northeast on Wednesday.
The Rex block is taking shape in the middle of the country. Here’s the current Water Vapor Satellite image —
Water Vapor mid level image at 4:30 PM. The high level cloudiness we saw today was from a disturbance to our north (Blue arrow) . This will move eastward. The upper level high (H) should become north of a closed off upper low (L) on Wednesday, forming a Rex Block. This will keep our weather delightful through late Friday. The upper low will slowly move up to the Carolinas on Saturday, affecting our weather. (Click on image for a larger view.)
Update Mon 04/10 @ 8:04 PM — Continued beautiful weather through Friday. Very warm but low humidity.
NBM High-low temperature forecast (see below for interpretation)—
NBM Text products Temperatures for Wings Field Blue Bell PA (KLOM) (Click on image for a larger view.)
A warm front moves through Saturday with some showers, clouds and an increase in humidity. A cold front moves through Sunday with thunderstorms.
Previously Posted Sun 5:06 PM —
“Rex Block” Will Keep Things Quiet This Week
A recognized upper air pattern for Spring (and Fall), is setting up for this coming week and it will keep our weather quiet for all of the week with increasingly warm temperatures.
A “Rex Block” (named in the 1950s for a meteorologist, Daniel Rex) will develop in the middle of the country and the jet stream will be sending any disturbances far to our north. The Rex block, with an upper level high just north of an upper level low, tends to be a stable configuration and blocks any west to east movement of the usual dips in the jet stream.
Here’s the GEFS model forecast for Wednesday afternoon of winds at the 250 mb level (jet stream level). The upper circulation is notated with red arrows, while the main jet flow (white arrows) is well to our north—
This week’s GEFS jet stream wind forecast (250 mb level) for Wednesday afternoon. The Omega Block is shown- A stable upper levle configuration that blocks the usual progression of dips in the jet stream flow. We’ll be in the increasingly warm ridge for this week.
Temperatures- NBM Model Text Output Products
Here are the high/low temperatures expected this week from the NBM model. You can use this link to get your own data— NBM Model Text Output—
The NBM model output text for KLOM (BLUE BELL) is a bit confusing unless you understand the meaning of the columns and how they define the high temperature period. TXN is the max and minimum temperature row. The HIGH temperature is is in the 00z column of each day. Example: For Tuesday’s high temperature— Since the high temperature is for the 12 hours preceding 00z on Wednesday, the 71º indicated on Wednesday’s 00 column really refers to TUESDAY’S HIGH TEMPERATURE. ( 00z Wednesday is at 8 PM EDT Tuesday evening . ) Similarly, the 82º high temp in the Friday 00 column is the high temperature for THURSDAY afternoon, since 00z Friday is at 8 PM EDT Thursday. Strictly speaking, it makes sense, but it isn’t at all intuitive. The XND is the standard deviation, rounded. The FHR is the Forecast Hour.
Update Fri 04/07 @ 9:46 AM — Perhaps you notice last night that I didn’t attempt a forecast for today (Friday) — “I’m not all that confident about Friday’s forecast”.
This morning’s radar shows why—
Friday 8:40 AM Composite Radar with vertical motion – Omega- in yellow contours. Showers shown are not forecast in the models. (Click on image for a larger view.)
There are light shows this morning streaming up towards us that are not forecast by the HRRR or RAP models. It appears that we will have cloudiness for most of the day Friday with even a chance of a very light sprinkle in a few areas.
The models, as incredible as they are, often do poorly with stalled or slowing fronts. Not applicable here, but they also do poorly with closed mid-level and upper air lows that become somewhat stationary.
Thu 8:30 PM Forecast Review — You know the model forecasts were bad when MLB postpones the season home opening game for anticipated thunderstorms but not a drop of rain fell during the afternoon.
Postponing the game was a very reasonable decision, based on all available model forecasts yesterday. Even today, each model run was a little later with the storms and and little further east. Here’s the current radar at 8:15 PM—
Current Radar with drawn RAP model most unstable CAPE values (MUCAPE) superimposed. The heaviest rain and strongest storms will be South Jersey, although there’s been enough shear to allow some activity around here. (Click on image for a larger view.)
The front is expected to stall to our south tonight. Some cloudiness may linger in southern areas tomorrow and some high cirrus may affect the Philadelphia area. But with such poor model performance with this system, I’m not all that confident about Friday’s forecast.
The weekend looks nice, but much cooler.
Yet Another Update
Update Thu 04/06 @ 4:42 PM— This forecast has been a moving target. It appears that minimal thunderstorm activity will occur in the immediate Philadelphia area. Here’s the latest CMCHRDPS showing thunderstorm probability—
Today’s 12z CMC HRDPS thunderstorm probability. Almost zero west of the Delaware river. (Click on image for larger view)
We’ll still have some showers between 6-9 PM. (Maybe that will change too.)
Thursday Forecast Update
Update Thu 04/06 @ 9:53 AM—
It appears that the Phillies opening game didn’t need to be postponed today.
The thunderstorms are likely to move in after 4-5 PM based on the latest 12z RAP and HRRR models. The RAP and HRRR are somewhat different, due to different physics parameterization, different resolutions, and use of different data assimilation systems.
HRRR—
HRRR Precipitation Rate forecast and clouds (dark) at 5 PM today.
RAP at 5 PM —
RAP Model (Click on image for a larger view.)
The main storms develop dynamically in-place between 5 PM and 9 PM
RAP model 8 PM forecast (Click on image for a larger view.)
Sept 1 , 2021 Tornado Outbreak (example of highly severe)
Past April 1st 2023 Tornado Outbreak
HRRR 12z
Today’s 12z HRRR Forecast
Impact
CAPE 3500-4200 Joules/kg
2100
550
⚐ ⇩
Helicity 1350 m^2/s^2
655
460
⚐ ⇩
Vertical Shear 40-46
40-45
25
⚐ ⇩⇩
Precipitable Water 2.7″
0.83”
1.1″
⚐
Lifted Index minus 6º K
minus 9.3º
minus 2.3
↓ ↓
HRRR Hail Parameter 1.9
1.4
1.3
⚑
Peak Wind Gusts 40-50
40-50
35-40
⚐
Aligned Storm Motion and Shear Vectors- ~ 90º
Almost aligned
non-aligned
⇩
Jet stream level – wavy
Highly cyclonic
wavy
⚐ ⇩
Jet Stream 250 mb Speed – 63
High 135
94-100
⚐ ⇩
Severity Parameters Impact: ⚑ indicates strongly favors Severity ⚐ indicates Possible Severity ↓ indicates works against Severity ⇩ Significant, but less impact
Based on the above, hail and heavy rain with strong wind gusts are possible. Greatest likelihood in east into New Jersey and South Jersey.
Thursday Forecast Change
Update Thu 04/06 @ 8:51 AM — Today’s forecast (Thursday) continues to change. Last night’s models (with the exception of the Canadian GEM and HRDPS) have the storms developing later in the afternoon with the most activity from Philadelphia and eastward into New Jersey. Start time is likely 4-7 PM.
The trend is that tornadoes are unlikely on Thursday and we’ll have somewhat heavier rain in some areas (NJ) with less severe winds than our storms last Saturday evening .
I’m waiting for the new 12z models and will update before 10 AM
Thursday Forecast Update
Update Wed 04/05 @ 9:45 PM — What appeared to be a clear-cut forecast this afternoon has changed with tonight’s 00z models. (Due to Daylight Saving Time, only the new RAP and HRRR are available at this hour.)
Here are the changes. It looks like showers begin to break out closer to 2-3 PM in the northwest suburbs.
This will NOT be a line of storms moving through like last Saturday, but instead, showers and thunderstorms will dynamically form directly over our area as an upper air energy moves in aloft from the southwest.
The main action will now occur later, closer to 4-5 PM. The strongest storms will be directly over Philadelphia and later eastward.
I’m going to hold off on the severity table I promised in my earlier update until tomorrow morning; there are too many changes in the forecast to hang my hat on anything this evening.
But a general sense is that tornadoes are unlikely on Thursday and we’ll have somewhat heavier rain in some areas with less severe winds than our storms last Saturday evening . Updates tomorrow morning.
Thursday Forecast Update
Update Wed 04/05 @ 5:44 PM — As expected, temperatures today just north of Philadelphia didn’t get out of the low to 60s (and even lower further north). The airport reached 72º later in the afternoon. The front that was hung up over us will return as a warm front tonight and temperatures on Thursday will reach the low 80s!!
Forecast highs on Thursday (NBM model)—
NBM high temperatures on Thursday. Many areas in the low 80s! (Click on image for a larger view.)
Some scattered showers in some areas expected tonight with the warm front.
A cold front moves through Thursday afternoon. Most models are suggesting the expected thunderstorms to be most extensive in NJ.
Thunderstorms: Based on the NBM, showers and thunderstorms may develop as early as 2-3 PM in northwestern suburbs and then move through the city before they really blossom in NJ in the late afternoon and evening.
I think the total rainfall forecast captures the forecast of the storms ‘missing’ some areas and concentrated in others—
Today’s 18z HRRR shows some areas northwest of the city ‘missing’ much of the action tomorrow. This graphic is total expected rainfall with these storms. (Click on image for a larger view.)
Some areas may see severe storms. I’ll be updating with my severe parameter chart later tonight, likely before 10 PM.
Wednesday Forecast Update
Update Wed 4/05 7:20 AM — Despite some TV weather people last night depicting the Wednesday situation as a warm front, it’s really a stationary front becoming somewhat of a ‘back door’ cold front; the colder air will prevail for most areas today, except South Philadelphia and southward. It will then become a warm front tonight. It looks like the clouds may break this afternoon.
Wednesday Forecast Change
Update Tue 4/04 9:46 PM/10:34 PM — Tonight’s models continue to suggest a large temperature gradient cutting through Philadelphia on Wednesday. Areas in South Philadelphia may approach 80°, but Bucks and northern Montgomery County may just be in the low 60s. There is some model spread/uncertainty about where the front stalls and where the gradient sets up. The NAM-NEST keeps considerable low clouds around in the afternoon.
I mentioned Sunday evening that a change in trajectory of the disturbance passing to our north might alter our forecast on Wednesday. It looks like that’s going to happen, as a backdoor cold front moves through in the morning Wednesday with clouds and and light scattered showers possible, especially northern areas.
The location of the front is easily seen with the wind convergence and large temperature gradient that sets up right through Philadelphia —
NAM-NEST temperature forecast at 5PM. Notice the winds (NE and SE) converge right over Philadelphia and the huge temperature gradient from north to south. (Click on image for a larger view.)
So Wednesday is looking cloudy to begin with some light showers in the morning, but sunshine should break out in the afternoon.
Still keeping an eye on Thursday. Thunderstorms likely in the afternoon, as early as 2 PM but most likely 3-6 PM.
Update Mon 04/03 @ 8:38 PM —Very nice and unseasonably warm weather through early Thursday.
A cold front will move through on Thursday afternoon. Current timing is about 3-4 PM, but that will undoubtedly change. Some strong thunderstorms are possible at that time. We won’t really have a handle on the specifics until Wednesday night and Thursday morning. The weekend looks good!
Stay tuned.
Previously Posted Sun 9:39 PM —
Following last night’s storms, an uneventful period of weather will be in store for us through late Wednesday.
Current water vapor image (Sunday eve) shows low pressure over Mississippi that will move to our south, low pressure in Wisconsin that moves to our north and low pressure off the Washington coast that will dive down and develop in the middle of the country.
Sunday Water Vapor image
By Tuesday, a warm flow develops in the East and a colder flow in the West, captured in this NAEFS surface forecast map for Tuesday evening—
Today’s 18z NAEFS forecast for Tuesday 10 PM. Warn air moves in from the south in our area while colder flow moves in out West. (Click on image for a larger view.) The storm forming in the middle of the country late Tuesday will affect us on Thursday.
Temperatures will move into the 60s tomorrow (Monday) and the 70s on Tuesday and Wednesday.
The only thing to keep an eye on: some disturbance will be moving north of us, from west to east. Showers are currently forecast in New York State and northern Pennsylvania. We may get some clouds at times with these disturbances. Should the current trajectory change, we’d see some showers late Wednesday.
The cold front associated with the storm in the central US will move through Thursday with showers and likely some thunderstorms.
