Sun 5:24 PM Forecast Review —The earlier storms today discussed this morning all developed in New Jersey, but additional storms sprung up (and continue to spring up) throughout the area.
As a followup to the discussion earlier, here’s the earliest signs of the storm development at 10:21 AM today—
Early signs of development in NJ. (Click on image for a larger view.)
At 10:22 AM, storms developing in NJ—
Water Vapor with superimposed radar and RAP Vertical Velocity Pressure (yellow contours) Notice the immediate Philadelphia area is in a region where there was no vertical velocity pressure at the time. Despite the upper air disturbance moving through, no storms develop at this point. (Click on image for a larger view.)
At 1:06 PM, the storms are still over NJ, but storms are popping up in Chester county. The RAP model shows positive Vertical Velocity Pressure in Chester county.
Vertical Velocity Pressure (also called Omega) aid the development of the storms in Chester County (Click on image for a larger view.)
At 3:52 PM—
Large stationary complex over Abington and NE Philadelphia likely caused flooding in this region.
At 5:40 PM, active storms still developing as I type this, but the storms in the immediate Philadelphia area are moving into an area of negative Vertical Velocity Pressure and should diminish in intensity. The storms in NJ are poised for intensification—
Sunday Forecast Update
Updated Sun 06/25 @ 9:03 AM — The HRDPS and latest RAP model have thunderstorms forming dynamically about noontime today. Most of the other models are forecasting storms developing around 2-6 PM.
So which forecast, if any, can we hang our hat on? The first clue is to look at the current conditions, specifically the current satellite water vapor image (Channel 9). Is there any disturbance visible that will move in about 3 to 4 hours?
Current Water Vapor image (channel 9) at 8:41 AM. The white box outlines a area of colder moisture (whiter) than surrounding areas. Water Satellite Water Vapor images are different than visible cloud images. Water Vapor images show the temperature of emitted infrared radiation, instead of the reflected light from clouds in visible satellite imagery. In this image, whiter and in some cases blue areas, show the highest and therefore coldest “effective level” of the water vapor layer. Colder effective layers are indicative of rising motion. (Click on image for a larger view.)
In the above image, we see an area of colder (whiter) effective layer, suggestive of the spark for thunderstorm development. This area is right over Chesapeake Bay, about 2-4 hours away by jet stream winds, currently at 30 mph in this region. (The one thing not in favor of this forecast: the upper air area is not as developed as it might be. The blue areas in the lower right corner are examples of much more developed upper air disturbances.)
To look at current water vapor images (channel 9) check out this link.
So the noontime to1 PM forecast may be correct. Let’s see. This is the fun of weather forecasting.
Sunday Forecast Update
Updated Sat 06/24 @ 10:53 PM — We had the majority of the rain late morning and early afternoon. The forecast for a 3-5 PM peak in thunderstorm activity didn’t occur around Philadelphia, despite almost every model having predicted it. The NBM thunderstorm prediction was all wrong.
If we go with today’s relative ‘winner’ in the forecast timing of the today’s showers, it would be the HRDPS. So let’s go with it for Sunday.
The latest HRDPS shows showers beginning to pop up around noon Sunday with a relative peak between 1 PM and 4 PM. The showers will be scattered and like today, there will be periods of sunshine interspersed. It will be much warmer Sunday with highs around 85º but with a larger than usual uncertainty of about ± 3º.
Here’s the HRDPS forecast for 2 PM Sunday—
Today’s 18z HRDPS forecast for 2 PM Sunday. Scattered showers and thunderstorms develop. The exact location can’t be inferred from the models. (Click on image for a larger view.)
Saturday Forecast Update
Updated Sat 06/24 @ 10:55 AM — It’s pretty clear, based on current radar and this morning’s models, that showers and thundershowers are forming now (late morning) and more numerous showers and storms are expected over the next few hours.
Updated Sat 06/24 @ 8:40 AM —The main area of rain from last night has departed to our northeast. Another area of enhanced moisture is in West Virginia and will rotate into our area this afternoon. Combined with the cyclonic flow and some breaks of sun, thunderstorms are expected to form this afternoon.
RAP model moisture transport shows an area of enhanced moisture in West Virginia that will rotate up into our area this afternoon. Jc (Click on image for a larger view.)
Rainfall will not be as heavy as it was in some areas yesterday. (PWATS yesterday were in the 2.0-2.1″ range) Today’s PWATs will be closer to 1.6″
While we’ve seen it’s impossible to predict exact timing and placement of the rain, it appears that showers and thundershowers begin popping up about 2-4 PM today as the area of enhanced moisture moves in, based on the HREF. The Canadian HRDPS has some showers popping up earlier, just before noontime.
The NBM 4.1 model has a new parameter, thunderstorm coverage, which attempts to predict the areas/timing of greatest thunderstorm density. It pegs the period 3-5 PM as most active with the greatest areal coverage of thunderstorm activity as shown below—
06z NBM forecast Thunderstorm Coverage- Yellow = scattered thunderstormsDeep Purple = areas moderate thunderstorm coverage; areas where the greatest number of thunderstorm cells will be at 4 PM. (Click on image for a larger view.)
Update Fri 6/23 11:39 PM — Additional high resolution models just available have showers and thunderstorms during the early afternoon as well as into the evening..
Update Fri 06/23 @ 9:59 PM — Tonight’s HRRR just available has considerable sunshine with clouds for us mid day and afternoon. Showers don’t move in until after 6 PM Saturday.
Previously Posted Fri 9:09 PM —
The closed upper low pressure system will move closer to us on Saturday through Sunday. As you can see from the water vapor image below, the ‘action’ with an upper low is often a certain radius away from its center; as it moves eastward over Pennsylvania on Saturday, we’ll still be in an unstable air mass, but much of the active moisture plume will move to our east. Nonetheless, instability will lead to scattered thunderstorm development.
Water Vapor image 8 PM Friday show the upper low over Lake Erie. This low will move SSE towards central PA over the weekend. Much of the extreme moisture plume currently over us will move away from us. but instability will still result in showers. (Click on image for a larger view.)
Saturday
Cloudy and damp early. Breaks of sunshine expected during the day, interspersed with showers and thunderstorms— Showers and thunderstorms most likely during the mid to late afternoon. Look for updates for the timing of showers. Very humid with dew points in the low 70s.
High temperature 80.8º sd 2.5º ( NBM model— location Blue Bell, PA)
Sunday
Periods of sun and clouds. More cloudiness in the afternoon with a good chance of scattered showers and thunderstorms, especially after 2 PM and into early evening. High probability far northwest of the city. Still humid with dew points 68-70º.
High temperature 84.7º sd 2.6º ( NBM model— location Blue Bell, PA)
MRMS rain gauge – radar rainfall totals through 7 AM Saturday. (Click on image for a larger view.)
Rain Update
Update Fri 06/23 @ 4:29 PM — Many of us are finally seeing some moderate to heavy rain that’s been over-promised over the past few days.
At 1:30 PM, things were beginning to develop well to our south—
At 1: 30 PM -Satellite Water Vapor – Radar- with RAP Omega (PVV- yellow contours). Colder moisture tops (a) with areas of vertical motion- yellow contours (b) and developing mesoscale complex (c). (Click on image for a larger view.)
At 4 PM—
4 PM RADAR wih RAP model Omega (yellow contours) Strong system moving through with more on the way. (Click on image for a larger view.)
I always like to go back and look at this morning’s models to see if any of them got this forecast right. It looks like the HRESW models along with the HREF ensemble did pretty good with this forecast. Unfortunately, one never knows which model group, if any, are going to be forecasting anything close to reality.
Where can I get thunderstorm forecast severity data?
Do you want to try your hand at predicting thunderstorms?
An excellent site for just about anything you might need (with the exception of downloading your own data and doing the computations) is the SPC Hourly Mesoscale Analysis page. Take a look.
Update Fri 06/23 @ 9:05 AM —Last night’s model’s promise of rain and storms this morning have materialized, but not in the location that had been forecast. The immediate Philadelphia area and Delaware Valley have missed this round of showers.
It’s currently raining in South Jersey. An analysis of latest RAP model and water vapor image shows the maximum Vertical Velocity Pressure (“PVV” – also referred to as Omega, not to be confused with the block Omega) is occurring in NJ, not over Philadelphia—
This morning’s Water Vapor image with superimposed RAP model PVV (yellow contours) shows strong positive values (upward motion) in NJ with negative values over the Delaware Valley and Philadelphia. Why the models are not getting this right in advance is the million dollar question. (Click on image for a larger view.)
Last night’s models continue to show a likely maximum in showers and thunderstorms about 1-3 PM today. Can we believe it?
I’m always looking for some model clue that would give a better forecast.
One of last night’s models does show a forecast of the thundershowers in NJ, based on its forecast location of maximum negative “Showalter Index.” (A old classic meteorology index of atmospheric instability). The Canadian high resolution model includes the Showalter Index. Here’s it’s forecast for the Showalter Index corresponding to the above time frame —
Today’s 06z Showalter Index from the HRDPS forecast for 9AM. (Click on image for a larger view.)
If the Showalter Index is any secret sauce for predicting the thunderstorm placement for this afternoon, look towards Collegeville at about noon to 2 PM. If this doesn’t work, I don’t have any answers to get a handle on this weather system.
Friday Forecast
Update Thu 06/22 @ 7:33 PM — Since Tuesday, I’ve been saying the model forecasts haven’t been doing too good a job. After having posted several “total rain accumulation” forecast maps over past days, one should be wondering, ‘where’s all that heavy rain?’
Here’s the MRMS rain gauge/radar measurement of the rain we’ve received over the past day or so. We haven’t received much—
MRMS radar-rain gauge estimate of rainfall over the past day or so. Not very much rain and nothing like any of the model forecasts. (Shading is in inches, green contour numbers are in mm (25.4 mm =1 inch ) (Click on image for a larger view.)
A warm front is is still expected to move north of us on Friday, with a shift to southerly winds and greatly increased humidity/dew points.
Some models suggest a possibility of some showers after midnight tonight with most predicting scattered thunderstorms around daybreak and from noon on Friday through the evening.
Some of the severity parameters, particularly CAPE, are moving into the high range tomorrow. Some of the thunderstorms could be moderate to severe.
18z HRRR shows simulated Radar and CAPE at 2 PM. Elevated surface CAPE near 1750 joules/kg (orange shading) corresponds to areas of the strongest storm potential. Helicity and shear values are not all that impressive. (Click on image for a larger view.)
It’s currently expected that much of the heaviest rain will move just to the west of Philadelphia. Western suburbs should get the most. Since none of the models have been correct about any of this, I’m not going to post another precip map tonight.
Thursday-Friday Forecast Update
Update Thu 06/22 @ 8:58 AM — My update from last night still holds. Here are some additional insights—
Mainly dry Thursday afternoon
Warm front moves through around daybreak Friday. Showers and Thunderstorms likely at that time. Severity unclear at this time, but some strong storms possible.
Thunderstorms a high probability Friday afternoon, likely strong to severe in some areas. Most likely 3-5 PM.
Additional showers and thunderstorm activity Saturday, especially afternoon hours.
Additional showers and thunderstorm activity Sunday, especially afternoon hours.
I’ll be trying to narrow down the timing and the weekend forecast later today. These upper air low based forecasts can be challenging.
00zECMWF rain forecast for Saturday at 5 PM. This 40km resolution model tends to make the rain coverage appear greater than it will be. (Click on image for a larger view.)
Rain Forecast Update
Update Wed 06/21 @ 8:01 PM — The rain moved in about 6-8 hours earlier than had been forecast last night. The rainfall has been much less than predicted and total rainfall through Thursday morning will be less than 0.5 inches in the overall Philadelphia area.
Some models show the axis of heavy rain to occur along the Jersey shore. Not sure if that will hold true.
This has already been one of those systems that make weather prediction frustrating rather than satisfying.
Having looked over most of today’s models, here’s the best forecast I can infer from them for the Philadelphia area/Delaware Valley —
Whatever rain falls Wednesday night will taper off during the morning.
Thursday afternoon looks to be relatively rain-free after 1:30 PM
Rain picks up again Thursday evening
A warm front moves through just before daybreak Friday, accompanied by thunderstorms and possibly some severe thunderstorms
CAPE values increase dramatically on Friday. Friday afternoon will have scattered thunderstorms, some quite strong.
Humidity and temperatures increase on Friday following the warm front.
With this system, I won’t even attempt a forecast for Saturday at this time.
Rain Forecast Update
Update Wed 06/21 @ 11:20 AM — As mentioned in my forecast review yesterday, the sunnier conditions than forecast on Tuesday suggested that the models are not capturing this weather event well.
This morning’s radar is another sign that this event is not being modeled well. The advancing rain line from the south is well north of what was forecast, even by this morning’s models—
MRMS Radar at 11 AM Thursday. (Click on image for a larger view.)
Update Wed 6/21 @ 7:35 AM — No change from last night’s forecast update. Rain moves in from the south between 8 PM and 11 PM this evening.
The latest ECMWF model shows a negatively tilted upper air trough develops to our west over the weekend. Rain is possible at least through next Tuesday!
The rain will not be continuous. Best chances for a break in the action are Thursday afternoon (Phillies game) and Sunday morning. Otherwise, rain, scattered showers and thunderstorms likely. I’ll update later.
Wednesday – Friday Rain Update
Update Tue 06/20 @ 8:28 PM — Today’s models continue with the forecast of rain moving in from the south Wednesday evening, about 8PM to 11PM and continuing on and off Wednesday night through Saturday.
It will not be continuously raining and breaks of sun during the daytime hours Thursday, Friday and Saturday will lead to thunderstorms developing due to an unstable atmosphere and upper air cyclonic flow.
Exact timing of the rain will have to be determined day by day, since no model can accurately time the dynamics of this complex system too far in advance.
The current total precipitation amounts and placement varies from model to model, but here’s the latest HREF model forecast captures the current rainfall forecast through 2PM Thursday—
Today’s 18z HREF total rain accumulation by 2PM Thursday. This ensemble forecast which is based on a combination of several HIRESW and HRRR runs, is somewhat on the high side. Click on image for a larger view.)
The latest RAP model has a similar forecast rainfall as the latest GFS (not surprising since the RAP model uses the same input data setas the GFS) and is less than the above HREF forecast—
21z RAP model total rainfall forecast through 8 PM Thursday. (Click on image for a larger view.)
I’m sure these forecast amount will change by tomorrow!
Tue 6:15 PM Forecast Review —We had much more sun today, Wednesday, than previously forecast. Modeling errors in the short term forecasts often suggest modeling difficulties in the longer term forecast, in this case, the rain event late Wednesday through Saturday.
I’m going to be waiting for a few more models to come in before updating my forecast. Check back later this evening.
Rain Update
Update Tue 06/20 @ 9:54 AM — Most models have the rain advancing into the Philadelphia area around 9 PM – midnight Wednesday evening.
There’s a divergence in the models for the track of the heaviest rain, with some keeping it offshore near the NJ coast, while others keep a swath of heavy rain moving right up through the Delaware Valley. So we may get as much as 2+ inches of rain (NAM-NEST) here, or maybe closer to an inch or so.
The latest higher resolution GEFS is leaning towards 1.2″ by Sunday—
This morning’s 06z GEFS 0.25º resolution ensemble forecast total accumulated rain by Sunday morning. It still shows 1.2″ of rain for much of the Delaware Valley (Click on image for a larger view.)
A review of the experimental RRFS-A model is similar to the above. Updates this evening.
Outlook Update
Update Mon 06/19 @ 5:30 PM— The high cloud cover that moved in today is from the system in the Southeastern US. The models continue with their trend towards significant cloudiness on Tuesday.
There is a chance of some light sprinkles Tuesday night, mostly areas near Delaware and far southern NJ.
Wednesday looks to be cloudy. There’s a chance of light sprinkles during the day, but most models have any rain moving in slowly about 11 PM Wednesday into Thursday morning.
Thursday—
Today’s 12zRGEM 72 hour accumulated rain forecast for Thursday at 8 AM showing significant rain having fallen over the prior 72 hours in parts of Virginia and North Carolina. The RGEM is very similar to the NAM forecast at 8 AM Thursday. But the NAM slows down the northward progression of the rain, while the GFS, ECMWF and RGEM keep it moving slowly northward during the day Thursday. The GFS and ECMWF show some of that heaviest rain moving in later Thursday night into Friday and Saturday. These details will change. (Click on image for larger view.)
Depending upon the position and degree of strengthening of the upper low, Thursday through at least Saturday look wet! Details of whether the rain will be the results of showers, thundershowers or a mix is not clear at this time.
After weeks of dry conditions in May into early, who would think we’d be talking about the possibility of consecutive days of rain the third week in June?
A Change in Forecast
Update Mon 06/19 @ 8:43 AM — Cloudiness from the southern system was expected this week, limiting high temperatures. But it looks like the upper low and associated surface low in the Southeastern US will intensify enough to send more than just clouds northward.
The GFS (but not the NAM) is showing showers moving northward towards us as early as Wednesday afternoon or evening. The NAEFS (which includes the GEFS) also shows light showers by 2 PM Wednesday, especially southern areas—
NAEFS (mode) rain forecast for 2 PM Wednesday. (Click on image for a larger view.)
Some cloudiness (high cirrus clouds) associated with this system will move in as early as this afternoon (Monday.) Tuesday will have varying degree of cloudiness.
Updates this evening.
Previously Posted Sun 7:54 PM —
Weather Overview
The blocked pattern that gave us the dry weather during the previous many weeks has re-established itself, this time with the center of the block shifted somewhat eastward. The current pattern also differs from the previous block because a strong southern jet stream is flowing through the southeastern US, bringing moisture and heavy rain to that region.
A closed upper low is expected to transform into a closed upper and surface low over the southeastern US and southern Appalachian mountains.
Current Water Vapor (Sunday) shows the jet flow and the position of the developing closed upper low pressure system that will remain fairly stationary during the week. Some of the moisture will rotate towards us during the week in the form of high and mid level clouds, keeping our temperatures lower. Little in the way of rain currently expected. (Click on image for larger view.)
Some of the moisture associated with the southern jet and the closed upper low will rotate near us during the week into next weekend. We’ll see it in the form of a southerly to southeasterly surface wind flow and varying amounts of upper and mid level cloudiness. Any rain will likely wait until next weekend.
Additionally, as the upper low in southeastern Canada moves slowly away, we’ll see increased humidity and increasing temperatures by the end of the week, dependent upon the cloud cover.
Monday
A lack of clouds and a southerly/southeasterly wind flow will bring fairly summer-like temperatures to our area on Monday (Average seasonal high is 83º to 84º this week.) —
Canadian RGEM temperature forecast for Monday. High temperatures in Philadelphia 87-89º (Click on image for larger view.)
Due to the increase in cloudiness on Tuesday and beyond, Monday may be our warmest day for several days.
