Thu 8:14 AM —Forecast Review —Yesterday’s storms came through between 10 PM-2 AM, later than originally forecast in the morning, but the afternoon update seemed to be on target.
The axis of heavier rainfall was nowhere near where it had been forecast. Some areas received a good soaking, other areas had little rainfall.
Here’s the MRSMS summary—
MRMS rain-gauge measured rainfall combined with rain-guage calibrated and interpolated radar-based summary of ACTUAL RAINFALL received. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch) (Click on image for a larger view.)
Update
Posted Wednesday 06/26/24 @ 5:52 PM — The latest NBM hasn’t changed significantly, but several of this afternoon’s models have moved the timing of the storms here in the Delaware Valley later, from 10 PM through 2 AM.
At 5:40 PM, things aren’t quite popping.
MRMS radar with RAP model parameters. Nothing happening in the ? areas. (Click on image for a larger view.)
Posted Wednesday 06/26/24 @ 1:32 PM — The 15z NBM includes more of this morning’s models. It’s a little more optimistic about the storms making it into Philadelphia, but then dissipating afterwards. This is consistent with the newest HRDPS.
NBM thunderstorm coverage (scale 1-4) at 9 PM —
15z NBM thunderstorm coverage map. This is more optimistic about the heavier rain at least getting into Philadelphia this evening. (Click on image for larger view.)
Posted Wednesday 06/26/24 @ 11:12 AM — While the new HRDPS is not yet available at this hour, this morning’s NOAA models are increasingly trending towards rainfall dissipating as the storms approach and move past Philadelphia. Much needed rain may only reach western suburbs.
NAM-NEST forecast 7 PM and 9 PM showing significant storm dissipation as it moves into unfavorable conditions for convection (thunderstorms) (Click on image for a larger view.)
As we’ve seen, the exact placement is rarely forecast exactly in advance. But a much needed rainfall may not materialize for some areas.
Some of the factors responsible for this thunderstorm dissipating trend are reviewed in my post on dry weather from 2022. Many of the features are the same.
Thunderstorm Update- Wednesday Evening
Posted Wednesday 06/26/24 @ 9:44 AM — A quick update. This morning’s HRRR just became available. As expressed below, I have concern that large “convective inhibition values” (CIN) and low soil moisture feedback mechanisms may adversely affect rainfall totals in the immediate Philadelphia area. The latest HRRR seems to suggest that.
I guess we’ll see tonight.
Posted Wednesday 06/26/24 @ 8:29 AM — In keeping with with my revised probability approach to thunderstorm prediction, I’m going to lean on the probability elements of the HREF, HRDPS and NBM for tonight’s thunderstorm forecast. We can use the rain!
The cold front tonight will trigger showers and thunderstorms. While strong storms are likely in some areas, I would put the severity (hail, high winds) probability as a 1 out of 3 in the Philadelphia area and immediate surrounding counties. The latest HREF shows the areas of greatest rainfall and severity at 9 PM—
HREF hourly probability of strong radar echos at 9 PM Highest probaility of storms is northwest and north of the city. (Click on image for a larger view.)
Showers and thunderstorms will be spotty in areas west of Philadelphia as early as 5-6 PM.The main area of showers and storms moves in between 7 and 10 PM and showers linger into the predawn hours of Thursday.
The NBM shows the highest density of thunderstorms (“thunderstorm coverage”: on a scale of 1-4 ) at 9 PM—
06z NBM thunderstorm coverage at 9 PM Purple is high coverage, yellow is scattered moderate coverage. (Click on image for a larger view.)
The HRDPS has the following rainfall totals (which is on the high side for the immediate Philadelphia area compared to other models) —
06z HRDPS Total Rainfall. The HRDPS did very well with the last thunderstorm rainfall totals, but many models show less rain than this. (Click on image for a larger view.)
I’m cautiously optimistic that we’ll get some much needed rain tonight, but with many models showing a “tongue” of the heat dome pushing into our area at the beginning of the event, I’m concerned that a wall of convective inhibition (CIN) and low soil moisture in our area may cause the storms to lose intensity as they move from west to east.
I’ll update later if things change.
Thunderstorm Probability Wednesday Evening
Posted Tuesday 06/25/24 @ 8:13 PM —The models have been inconsistent today regarding thunderstorm coverage and probability for Wednesday night ahead of a cold front.
Earlier today, it appeared that any thunderstorms would “hit a wall’ and decrease in intensity as they entered Philadelphia and attempted to move east into NJ. Rainfall totals this morning were forecast to be somewhat on the low side.
The latest models (18z – run 2 PM EDT) have increased the chance of thunderstorms with locally heavy rainfall in the Philadelphia area, with lesser amounts in eastern NJ. Most likely timing in Philadelphia and immediate suburbs is 8 PM to 11 PM Wednesdayevening.
Here’s the current 18z NBM probability of thunderstorms at 10 PM—
NBM 18z forecast thunderstorm probability (Click on image for a larger view.)
The latest Canandian HRDPS shows a maximum at 9 PM—
18z HRDPS thunderstorm Probability (Click on image for a larger view.)
The HRDPS did well with our last frontal passage. Here’s its current total rainfall forecast —
HRDPS total rainfall. Blue-violet is > 1inch. Some areas will again have unimpressive rainfall. (Click on image for a larger view.)
Right now, I’m skeptical about heavy rain here and things will likely change tomorrow. Check back for an update tomorrow morning.
Strong storms are possible in some areas.
Previously Posted Mon 12:51 PM —
Mon —Forecast Review — As forecast, Sunday night’s storms blossomed in NJ but most of the Philadelphia- Delaware Valley area west of I-95 received little or no rainfall.
Particularly impressive was the HRDPS forecast which closely captured the band of heavy rain in NJ.
Here’s the MRMS rainfall summary for Sunday night—
MRMS rain-gauge measured rainfall combined with rain-guage calibrated and interpolated radar-based summary of ACTUAL RAINFALL received. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch) (Click on image for a larger view.)
For today, Monday, the heat dome is suppressed to our south and we’ll be in a cooler cyclonic jet flow north of the heat dome.
NAEFS shows jet stream dip over the Northeastern US on Wednesday. Of interest is the cold pool of air still visible in Canada demarcated by the red 540 thickness line. It’s strange to still see this feature this late in June. Note the heat dome, demarcated by the orange 576 thickness line is suppressed to our south for now.
By Tuesday and Wednesday, the heat dome starts poking up northward and temperatures return to upper 80s Tuesday and near and above 90 Wednesday.
Here’s the surface map for late Wednesday showing a cold front moving through late in the afternoon. It looks like we’ll have some rain/thunderstorms late Wednesday into Wednesday night.
NAEFS late Wednesday forecast with approaching cold front. (Click on image for a larger view.)
After a brief cool-down Thursday, temperatures look to rise again into the 90s by Saturday and Sunday. Another cold front moves through next Sunday night.
Posted Thursday 06/20/24 @ 9:28 AM — Last night’s models show a trend towards higher temperatures Friday through Sunday, with the ECMWF and NBM showing areas exceeding 100º on Sunday. (That’s actual temperatures, not heat indices.) Heat indices (“apparent temperatures”) may exceed 105º)
High Temperatures Sunday—
Latest 12z NBM high temperatures for Sunday. (Click on image for a larger view.)
Apparent Temperatures (Heat indices) on Sunday—
So the current trend is for higher temperatures over the weekend than previously forecast earlier this week.
Thursday Friday Forecast Update
Posted Wednesday 06/19/24 @ 7:18 PM — The model guidance has backed off on the chance of showers in this area for Friday evening. Showers and thunderstorms will likely stay to our far north.
Today’s highs, based on the RTMA at 3:45 PM, showed temperatures in the city were close to those forecast yesterday by the NBM, although higher temperatures that had been forecast near Trenton did not materialize. Here’s today’s RTMA readings at 3:45 PM —
RTMA temperatures at 3:45 PM Wednesday. Contours are 1º increments (Click on image for a larger view.)
For Thursday, temperatures a degree or so warmer than today. Dew points will be higher, though, making it feel hotter.
The models have also come down a degree or so for Friday‘s high temperatures. Still forecast in the low to mid 90s.
Some pop-up thunderstorms possible Saturday. Some may be strong.
Temperatures above 90º now forecast through Monday. Perhaps a drop below 90º for Tuesday, then a return to the 90s!
Forecast Update
Posted Tuesday 06/18/24 @ 7:52 PM — This afternoon’s models are showing a mid level disturbance moving through over the top of the heat dome. Scattered showers and thunderstorms are possible Friday evening, especially areas north of Philadelphia.
Increasingly Hot
Posted Tuesday 06/18/24 @ 5:36 PM — Other than the increasing heat over the next several days, there isn’t much ‘weather’. No showers or thunderstorms are expected in this area into Friday, however, there’s a chance of scattered storms Friday evening/night, especially northwestern areas.
Tuesday afternoon Water Vapor Image shows two ‘tongues’ of hot air conveyor belts. Each group of yellow arrows are within the 576 thickness line outlining the heat domes. (Click on image for a larger view.)
The temperatures we’re seeing are hot. What’s most concerning is that it isn’t yet officially summer.
A picture, or in this case several pictures, capture the increasing temperatures Wednesday through Friday —
NBM high temperatures Wednesday Contours are 1º increments (Click on image for a larger view.)NBM high temperatures Thursday Contours are 1º increments (Click on image for a larger view.)NBM high temperatures Friday Contours are 1º increments (Click on image for a larger view.)
Saturday may be as hot as Friday and Sunday looks quite hot too.
Posted Monday 06/17/24 @ 8:34 AM — Most models are keeping the high temperatures in most areas in the Delaware Valley just below 90º on Monday, but dew points are expected to reach the mid 60s in the afternoon. As a result, heat indices will be about 91º in many areas.
Here’s the latest NBM high temperature forecast for today, Monday —
Today’s 06z NBM high temperature forecast for Monday.. (Click on image for a larger view.)
Lower 90s are expected Tuesday and Wednesday. The heat gets more serious on Thursday with temperatures in many areas above 95º
By Friday, the heat is expected to be intense—
06z NBM high temperature forecast for this Friday. (Click on image for a larger view.)
Previously Posted Sun 7:56 PM —I guess you’ve probably heard that it’s going to be hot this week. This northward advance of hot air has been well-advertised by the extended range models as early as 2 weeks ago. The ‘heat dome’ will be over much of the central and northeastern US (even well into Canada) through at least next Saturday or Sunday.
The heat dome can be ‘visualized’ through a calculated weather parameter called thickness, specifically the 500mb-1000mb thickness. This thickness parameter reflects the average temperature (density) of of the lower half of the atmosphere. Greater values of this parameter show greater average temperature.
In winter, the 540 thickness level often demarcates the rain-snow line.
In summer, the 576 thickness is useful to visualize the outline of the heat dome. I like to think of it the outline of a horizontal slice through the atmosphere that only includes its (hottest) thickest area. There’s nothing magical about 576, but I find it to be the number that seems to best define the outline of the heat dome. Strangely, there’s often showers/thunderstorms on the northern side of the 576 thickness line.
Here’s the ECMWF forecast for Wednesday. The orange line is the 576 thickness line, outlining what the news is calling a heat dome—
Today’s ECMWF forecast forWedensday. The area within the orange contour is the 576 ( 500mb-1000mb) thickness. Areas within that line have the hottest weather. I’ve drawn orange arrows to better visualize the line location. Interestingly, the 540 thickness line (red) is still visible in part of Canada. (Click on image for a larger view.)
Here’s the this morning’s NBM forecast highs through Wednesday. Temperatures on Thursday through Saturday are expected to be even higher.
MONDAY
TUESDAY
WEDNESDAY
BLUE BELL
89.0
92.3
92.8
PHILADELPHIA
89.7
93.1
93.6
Today’s 12z NBM model mean high temp forecast usually reached between 3PM and 4PM. All temps have average confidence with an sd of ± 1.5º
The NBM is a mean (average) temperature calculated from 40 + models. In past summers, high temperatures were best calculated by adding the standard deviation (sd) to the mean, in this case adding about 1.5º.
Fri 7:22 PM —Forecast Review —Today’s weather was a sobering and decisive event in my ongoing elusive attempts to predict the location and timing of thunderstorms. I must confess I enjoy reviewing the voluminous model data for each storm, trying to find the one parameter or model that would have given a very accurate thunderstorm forecast.
But today, I put in an unusually large effort to forecast the storms, but to no avail. The models were all pretty much wrong in one way or another. I reluctantly have reached and agree with the conclusion, “All models are wrong, but some are useful.” — George EP Box , noted British statistician.
Going forward, I’m going change my approach to thunderstorm forecasting.
Posted Friday 06/14/24 @ 5:16 PM — With all this prognosticating today, some heavy cells have popped up early and not where forecast!
Radar at 5 PM (Click on image for a larger view.)
Posted Friday 06/14/24 @ 3:18 PM — The current radar (MRMS) and RAP model depicts the current conditions that support the forecast for the heaviest thunderstorms to move off to our north. Check out the graphic caption for more info—
MRMS radar summary with superimposed RAP model mid level wind streams (blue streamlines) and vertical velocity pressure (“Omega”) (yellow contours) The main area of storms will follow the mid level blue contours to our north. The yellow negative Omega numbers in the yellow oval show the southern areas of precipitation will enter an area of downward vertical motion. Downward motion works against thunderstorm development. (Click on image for a larger view.)
Based on this, the forecast posted earlier this morning (below) appears to be supported. Of course when it comes to weather —a dynamic and chaotic process — things could change.
Posted Friday 06/14/24 @ 12:30 PM — The latest Canadian HRDPS model captures the forecast I’ve been trying to convey with my earlier update. It shows heavier storms to our north and west, but large areas of the immediate Philadelphia area receive no rain!
As many of my regular readers know, the model exact placement of forecast heavy rain is something that very often comes up short. But this trend where the heavier activity skips over Philadelphia and re-forms in NJ is something different models are all suggesting.
This morning’s 12z Canadian HRDPS total rainfall forecast for this event. Some areas receive over 1.5-2″ while large areas receive almost nothing. The HRDPS breaks down precip into “convective” and non-convective. The lighter green-yellow areas are mostly convective (thundestorm) rainfall. (Click on image for a larger view.)
Despite my forecast of low severity potential here in Philadelphia, always follow the the official NWS forecast warnings in these situations.
Friday’s ‘Storms’
Posted Friday 06/14/24 @ 8:18 AM — Last night’s models continue with the forecast of a highly broken line (maybe batch) of showers and storms affecting our area late afternoon into evening. The heaviest activity will remain north of us and some areas in the immediate Philadelphia area may see little in the way of rain or storms. A severe weather event here is low probability. A 3 hour time lapse of the 06z HRRR captures this—
06z HRRR instantaneous precipitaton rate from 5 PM to 7 PM. The models are predicting a secondary batch of showers moving through our area before midnight. (Click on image for a larger view.)
A second group of showers and possibly thundershowers moves through about 11 PM to midnight. We could use the rain, but this front, for reasons described in previous updates, likely won’t provide it. We’ll be entering a dry and hot stretch of weather starting late Monday.
I’ll update if this morning’s models move in a different direction.
Friday Weather Update
Posted Thursday 06/13/24 @ 5:30 PM —Several models show a disturbance moving in from the southwest this evening, possibly causing some showers and thunderstorms in Chester county, perhaps into the city. On satellite imagery, I see the disturbance but not any showers at this time. A low confidence event —
Satellite image (channel 13) shows the weak disturbance which several models show producing showers and thunderstorms later Friday evening. Not overly confident about this. (Click on image for a larger view.)
As for Friday, showers and thunderstorms move in ahead of the cold front as early as 4-5 PM far northwest suburbs and into the early evening (6-11 PM) in the city. As discussed in this morning’s update, the line of showers/thunderstorms will be broken and doesn’t appear to be overly severe, although a few spots show potential. —
This afternoon’s 18z HREF shows rain and areas of maximum upward helicity from 6 PM through 10 PM. This shows 9 PM. (Click on image for a larger view.)
Heaviest activity in central and northern NJ and in into Delaware and Maryland. The showers may linger and blossom a bit towards midnight before moving off for a nice weekend.
I’ll update Friday.
Thursday Friday Weather Update
Posted Thursday 06/13/24 @ 8:15 AM — Today, Thursday, will feel more like June here in Philadelphia, with temperatures approaching 90º in the mid to late afternoon and dew points moving from the comfortable mid 50s to the low 60s this afternoon.
Friday will see hot weather with highs 90-93º and dew points moving into the more uncomfortable low to mid 60s.
Friday‘s cold frontal passage timing continues to change back and forth, but has reverted back to the late afternoon/early evening, more specifically 6 PM to 9 PM. A line of showers and thunderstorms will move towards us, but it appears that this line will become broken as a result of these storms not having upper air support (no adjacent jet streaks) and another system, now affecting Florida, will be out int he Atlantic to our east, robbing the front of some upward vertical motion. Some areas may see a heavy thunderstorm, but many may not.
ECMWF forecast for 8 PM Friday evening. Frontal position drawn. (Click on image for a larger view.)
The severe weather parameters in the CAM models (Convection-Allowing Models such as the HRRR) often don’t show possibility of severe weather until 24 to 12 hours before the event, so things may change with tomorrow’s forecast. Right now, things don’t look too severe.
Total rainfall will only be in the 1/2″ range, with some locally higher amounts.
This upcoming weekend looks very very nice with cooler temperatures and low humidity. Heat returns after Monday.
Following Friday’s frontal passage and rainfall, and after this coming weekend, we’ll be entering into a generally hot and very dry pattern for the following weeks. This may be the last generalized rainfall we receive for almost two weeks.
Additional Forecast Changes Wednesday thru Friday
Posted Tuesday 06/11/24 @ 4:20 PM — We had significant cloudiness today at times, as forecast in my update yesterday. However the shower activity has been much less than forecast, continuing a recent trend where the model forecast total precip is over-stated.
Here’s the current radar showing a paucity of rain. —
MRMS radar capture 4:12 PM. Additional showers may develop through 8 PM. (Click on image for a larger view.)
The latest models show more forecast changes through Friday.
First, tomorrow, Wednesday, will be cooler than previously forecast, likely staying in the low 80s. Some clouds at times, but there will be significantly more sunshine than today and more sun than had been forecast yesterday.
Thursday warms up, but not as hot as previously forecast, likely staying just under 90º. A few pop-up widely scatted thundershowers possible mid to late afternoon. Most areas dry.
The big forecast change is forFriday, where a cold front will be moving through during the mid to late afternoon into the evening. (Previously, the front was expected late evening into the night.) Thunderstorms, are possible late afternoons and early evening. Rainfall forecast is on the light side.
Highs will be in the near 90-92º, but there’s great uncertainty in that temperature due to uncertainty with clouds and the timing of the front. We may not get that hot. The standard deviation (uncertainty) is as high as ± 5º with some ensemble models and it’s likely trending towards the cooler range.
Forecast Change for Tuesday, Wednesday
Posted Monday 06/10/24 @ 7:02 PM —This afternoon’s 18z models have added showers and thundershowers late afternoon Tuesday from the city eastward into NJ as well.
Posted Monday 06/10/24 @ 4:54 PM —Today’s models are now showing several upper air disturbances moving through our area early Tuesday morning and again late Wednesday morning through Wednesday afternoon. These disturbances will allow a warm front to move through about the same time frame.
Sunshine, but more cloudiness than previously forecast for both Tuesday and Wednesday, along with lightwidely-scattered showers, near Philadelphia before daybreak Tuesday morning and mostly in NJ Wednesday afternoon.
18z NAM-NEST forecast low cloud cover (blue) forecast for noon on Tuesday. Some areas have sun, some have clouds. This is a snapshot and different areas will have clouds and sun at different times. (Click on image for a larger view.)
A rumble of thunder also possible, especially near the Jersey Shore, according to the ECMWF and Canadian models on Wednesday afternoon. Any showers will be light. Many/most areas will not see any rain, but considerable cloudiness at times.
End of Week Heat
Posted Monday 06/10/24 @ 9:00 AM — Following spectacular days Monday and Tuesday, seasonably warm temperatures return Wednesday and hot days are expected Thursday and especially Friday.
Friday is currently forecast to be especially hot—
NBM forecast high temperatures Friday. Dew points in the mid 60s will make the heat noticeable. (Click on image for a larger view.)
Unfortunately, I don’t see much rain over the next week, with the only chance Friday night as a cold front moves through. That amount doesn’t currently look to be very high—
NAEFS probability of rainfall greater or equal to 0.10″ Not very high. (Click on image for a larger view.)
Previously Posted Sun 4:50 PM —Cooler than average and beautiful weather expected here through at least Thursday. The NAEFS forecast map for Monday captures the main features—
NAEFS forecast for Monday evening shows a cool northwesterly upper flow around an mid-upper level low pressure system in Canada. The 576 thickness line (500-1000mb thickness) in dark orange represents the ‘edge’ of very hot tropical air, currently suppressed to the south, but poised to move northward by the end of the week. Of additional interest is an area of disturbed weather taking shape in the Gulf of Mexico (shown as a ?) which may be an area of impending tropical storm development. (Click on image for a larger view.)
By the end of the week, hotter air begins to approach our area with temperatures possibly near or above 90º by Thursday or Friday.
Another cold front moves through later Friday into Saturday with a brief cool down for next weekend. Hot weather expected the following week.
NAEFS forecast for next Saturday, showing cold front moving through. Increased activity in the Gulf of Mexico. (Click on image for a larger view.)
Except for the cold front passage next Friday, things will be dry. It will take a tropical system to give us substantial rain with this pattern.
