Update Thu 8/17 11:06 PM — Despite the jet streak explanation below (supported by tonight’s HRRR), tonight’s HIRESW closely matches current radar. It has a line of showers moving through around 5 AM Friday morning with some gusty winds.
Updated Thu 08/17 @ 5:32 PM —A cold front will move through about 4 AM -7 AM Friday morning.
As showers with this front approach from the north and west, they are expected to reduce in intensity or even fall apart as they approach the city and move into NJ.
Why will they reduce in intensity? We’ll be in the ‘left entrance region’ of a jet streak, which has descending motion—
ECMWF jet stream wind forecast for early Friday morning shows the Philadelphia area will be what’s referred to as the “left entrance region” of a jet streak (white box area) , associated with descending motion aloft and unfavorable for precipitation. (Ascending motion is needed for showers.) (Click on image for a larger view.)
Clouds clear during the morning and it will be sunny by afternoon. It will become WINDY. Highs 81º Blue Bell, 84º Philadelphia.
After another delightful day on Saturday, the heat dome affecting the central US will attempt to move into our area for Sunday.
Next week will have several unsettled days as disturbances move around the heat dome affecting our area. Several models are hinting at the beginnings of several tropical storms.
Thursday Forecast & Outlook
Updated Thu 08/17 @ 8:35 AM —Another sunny day! (A few pop up thundershowers possible about 4-6 PM. Most places will not see anything.) Another front will move through tonight (Thursday night) into Friday morning. Some showers and thunderstorms are expected to develop ahead of this front around daybreak Friday, but the models have them losing intensity as they approach the I-95 corridor. Clearing Friday for a very nice weekend.
This summer’s “heat dome” has moved eastward and is now expected to affect the central US. We’ll again be protected from the heat by a cyclonic flow from Canada.
The temperatures in the Central US will be intense by Sunday and Monday—
GEFS forecast for Monday. These are actual forecast high temps, not heat indices. (Click on image for a larger view.)
Updated Wed 08/16 @ 10:25 AM — The front that slowly passed through last night is lingering off of the coastline. Moisture will continue to move up along this front, affecting central NJ and the Jersey shore this afternoon. From Philadelphia westward, we’ll have partly to mostly sunny conditions. (Some of the HIRESW models have some widely scattered popup showers in our area, but these models can often overdo this.) See the image caption below for more details.
06zGFS forecast for Wednesday. Moisture continues to move northward along the coast near the stalled frontal boundary, affecting NJ today as high pressure builds in from the west. Notice the orange ‘thickness line ‘that winds down near Georgia and Arkansas and then up near Wisconsin. This is the boundary of the “heat dome” that will move closer to our area by Sunday and Monday. Before that time, a sharpening upper level trough will swing a cold front through here later Thursday through early Friday. (Click on image for a larger view.)Today’s NAM-NEST shows showers at 4 PM (Click on image for a larger view.)
Tuesday
Updated Tue 08/15 @ 9:20 AM — We’ll try again for today’s forecast. A cold front moves through later this afternoon. CAPE values are more elevated today than yesterday. The HRRR shows showers and thunderstorms forming in a line from Philadelphia and moving into NJ. (The HRDPS has the line forming a bit further eastward, excluding the immediate Philadelphia area.) PWAT is still in the 2 inch range, so some quick heavy downpours possible. Current time frame is about 4-6 PM near Philadelphia and then moving through Southern NJ.
06z HRRR shows hourly Precipitation rate and cloud cover at 6 PM. (Click on image for a larger view.)
Tue 9:07 AM Forecast Review — As suspected last night, the forecast for heavy rain last was a bust. Very little rain, certainly not heavy rain, fell across the Philadelphia region. Here’s the MRMS estimate of the rainfall we received—
MRMS rain-gauge/radar estimate of last night’s rainfall. Few areas received even 0.25 inches of rain. (Click on image for a larger view.)
The missing ingredient was elevated CAPE in our area, specifically something called ‘surface CAPE’.
As mentioned, the model guidance had shifted away from the heavy rainfall forecast during the afternoon model runs and had clearly shifted away (with the exception of the HRDPS and a few others) in the evening model runs. With other parameters elevated, it was difficult not to leave the original heavy rain forecast intact. That’s my excuse. I can’t account for the TV/radio forecasts that were still pushing the severe forecast at 11 PM.
Monday Storm Update
Update Mon 8/14 11:48 PM —Tonight’s HRDPS, just available, shows some intense storms developing across the immediate Philadelphia area about 3AM.
Updated Mon 08/14 @ 8:46 PM — As highlighted in my previous update, the immediate Philadelphia area may escape heavy rainfall and severe weather.
The latest HRRR and RAP model runs (these models come out hourly) are conspicuously missing any heavy rainfall and severe weather signatures in our area.
I could be fooled by something called ‘model spin-up’ errors. Spin-up errors occur because models take time to approximate a realistic forecast, often several hours. Since the event is occurring imminently, the heavy rain and severe weather signatures may not be showing with the latest models. (But what is also perplexing is the lack of activity on radar and satellite, except in central PA. Will that move eastward or more likely northeastward, missing us?)
It is possible the severe weather forecast for regions just outside of the immediate Philadelphia area tonight is a bust. I guess we’ll know tomorrow.
Monday Storm Potential Update
Revised Update Mon 08/14 @ 7:08 PM — Showers and thunderstorms will move in about 9 8 PM.
Today’s models continue with very high PWAT (2.3″ to 2.4″) and moderately elevated CAPE. Heavy rain potential (localized) is likely in in our area; Severe weather may occur later, after midnight, in some locations (see below). The immediate Philadelphia area seems to escape the heavy rainand maybe the severe weather.
Areas to our west, immediate south and immediate east may have the stronger to severe storms.
The HRRR model total rainfall this evening gives an idea of where the most significant storms will be—
Today’s 18z HRRR showing total rainfall prediction for this evening. Red Circle is Philadelphia, not in an area of likely severe storms. Delaware remains an area of likely significant heavy rainfall. Most models show this fork in the precipitation. Unfortunately, we’ve seen that the models are limited when it comes to accurately predicting the exact locations of heavy rainfall.
Some of the severe weather may develop in northwest suburbs after midnight. The HRRR shows very high vertical shear about 2-3 AM in our northwest suburbs—
HRRR shows highly elevated vertical shear at 3 AM in northwestern suburbs. Coincides with an increase in CAPE (Click on image for a larger view.)
Tornadoes are possible after midnight. Very elevated helicity is forecast to move up east of us in NJ.
HRRR forecast Helicity at midnight. This area in NJ may move northeastward. Helicity values near 800 /sec are impressive and tornadic activity is possible. . (Click on image for a larger view.)
Monday Update
Updated Mon 08/14 @ 9:49 AM — There’s been a shift towards a later onset of showers/thunderstorms Monday evening. It now appears that showers and storms will move in about 9PM to 11 PM and continue through 4AM. Unchanged is the biggest threat: heavy rain. CAPE values suggest that storms could be strong but not necessarily severe in our immediate area. The heaviest rain/activity in northern Delaware and near the Jersey Shore.
Previously Posted Sun 7:54 PM —
A warm front will approach on Monday and move through our area Monday night. A mix of sun and clouds on Monday, increasingly cloudy towards evening. The warm front and associated cold front are part of two rapidly moving systems that are currently in South Dakota and Kentucky right now—
The showers/thunderstroms associated with the warm front will move in between 7PM and 9 PM. The main feature of this system will be a very high precipitable water (PWAT). Many models are showing PWATs of 2.3″-2.4″ which translates into a very heavy rainfall potential.
Here’s the current HRRR forecast rainfall for Monday night—
Today’s 18z HRRR shows areas of heavy rain 1-3+” for Monday night.
As for severe weather, predicted CAPE values are in the moderate range and the the greatest likelihood for severe weather is northern Delaware and South Jersey. Nonetheless, the HRRR is showing a few areas of highly elevated helicity nearer the city, so some severe weather here can’t be ruled out.
(We’ll have to wait for tomorrow’s 06z HRRR to get a better handle on things.) Check back tomorrow morning.
A cold front will move through on Tuesday morning, possibly with some more showers, although they’re not looking that impressive right now.
Updated Thu 08/10 @ 7:58 PM — Friday looks quite nice with temperatures in the low to mid 80s. Sunshine through high cirrus clouds for most of the day.
Thursday Update
Thu 7:47 PM Forecast Review —So how much rain did we get?
MRMS radar- rain gauge estimated total amount of rainfall on Thursday. (Click on image for a larger view.)
Most models over-estimated the rainfall today. Last night’s 06z HRRR came the closest in predicting the axis of heavy rain. This morning’s HRRR didn’t do as well, probably due to inadequate model spin-up time.
Thu 1:54 PM Forecast Review — Based on current radar trends and RAP data, I don’t see the severe weather occurring in our immediate area. Probably just 0.5-1″ in the immediate PHL area.
Radar about 1:20 PM (Click on image for a larger view.)
Updated Thu 08/10 @ 10:58 AM — Light sprinkles have moved in earlier than forecast. The main activity is still to our southwest—
Thursday Forecast Clarified
Updated Thu 08/10 @ 8:05 AM — Severity Parameter Comparison Table for Today– Allows comparison to recent severe events.
Severity Parameter (HRRR)
Sept 1 , 2021 Tornado Outbreak (example of highly severe)
Recent 4-1-23 Tornadoes (Example of severe)
Recent 08-07-23 18z HRRR Bow Echo
08-10-23 Today’s 06z HRRR & Impact
Notes
CAPE Joules/kg
3500-4200 ⚑⚑⚑⚑
2100 ⚑⚑
2600 ⚑⚑
1200 ⚐ ⚐
Helicity m^2/s^2
1350 ⚑⚑⚑⚑⚑
655 ⚑⚑
726 ⚑⚑
560 ⚑
Vertical Shear 1/sec
40-46 ⚑⚑⚑
40-45 ⚑⚑⚑
30 ⚑
32 ⚑
Precipitable Water
2.7″ ⚑⚑⚑⚑
0.83” ↓
2.1″ ⚑⚑
2.3″ ⚑⚑
Heavy rain potential
Lifted Index º K
minus 6º ⚑⚑
minus 9.3º ⚑⚑⚑
minus 9.9º ⚑⚑⚑
Minus 4.4º ⚐
HRRR Hail inches
1.9 ⚑⚑
1.4 ⚑
1.8 ⚑⚑
1.6 ⚑
Peak Wind Gusts mph
40-50 ⚑⚑
40-50 ⚑⚑
65mph ⚑⚑
32mph ⚐
Storm Motion Shear Vector Alignment
Aligned – ~ 90º ⚑⚑
Almost aligned ⚑⚑
aligned ⚑⚑
Somewhat aligned ⚐
250 mb Jet Stream Wind mph
63 ⚑
135 ⚑⚑
74 ⚑
95-104 ⚑⚑
Severity Parameters Impact: ⚑ indicates favors Severity⚐ indicates Possible Severity ↓ indicates works against Severity ⇩ Significant, but less impact
Update Thu 8/10 7:37 AM — Last night’s 06z models have come together to allow for better forecast accuracy. It appears that CAPE values will be higher than previously forecast. The chance for strong to severe thunderstorms has increased for today. The biggest issue will be heavy rainfall in some areas, on the order of 1-2 inches.
Per the GFS (which had the best timing for Monday night’s storms) showers have already moved into far western parts of Montgomery county, but are heading northward, away from the city.
The main activity moves into immediate Philadelphia area between 11 and noon. Peak activity is around 2 PM, lasting on and off until 5-6 PM.
Thursday Outlook- More Thunderstorms
Update Wed 8/09 11:11 PM — Tonight’s high resolution models have a wide spread of forecasts. Some have heavy rain east in NJ, some have heavy rain northwest of the city. You get the idea. We simply don’t know where the heaviest rain will fall. Many areas may only get about 0.6”. A few may get 1.5”. Severe storms look less likely, with CAPE values unimpressive.
I’ll try to pin it down tomorrow morning.
Updated Wed 08/09 @ 5:15 PM — A rapidly moving system will move in from the west on Thursday. Showers and thunderstorms are likely to move in from the southwest from mid to late morning and linger through much of the afternoon. Some moderate to heavy rain expected in some areas.Some strong storms also appear likely, but CAPE values are in the 500-600 Joule/Kg range rather than the 2500 range of this past Monday night.
The latest HRRR shows an area of significantly elevated severe parameters between 11 AM and 2 PM in the Philadelphia area, while the NAM-NEST shows the most likely areas for strong (to severe) storms will be south into Delaware and much of the NJ Shore during the later afternoon. I generally go with the HRRR model with these sort of things.
Here’s the 18z HRRR—
Today’s 18z HRRR shows significantly elevated vertical shear (color shading) and significant helicity (brown rounded contours). Storm motion vectors (short arrows) are aligned with the direction of the shear (longer streamlines), increasing likelihood of stronger storms. CAPE values (not shown) are not all that impressive, due to lack of sunshine. Still enough CAPE to allow for thunderstorm development.Lower CAPE may put a lid on the severity of the thunderstorms. White arrow shows expected storm track. (Click on image for a larger view.)
Updated 9:00 PM It looks like a few storms popped tonight—
Wed 5:13 PM Forecast Review — The isolated shower/storms haven’t materialized here except just north of Allentown. This afternoon’s NAM-NEST continues to forecast showers moving into our area between 9 and 11 PM. A low confidence forecast.
Wednesday Forecast Update
Updated Wed 08/09 @ 9:40 AM — The 06z HRDPS shows an upper air disturbance rotating through our area between 4 and 7 PM today with a few widely scattered/isolated showers and clouds. The latest 12z HRRR shows only a bit of activity, but we’ve seen the HRRR underestimate these situations over past weeks.
06z HRDPS shows widely scattered to isolated showers moving through at 5 PM. As always, the exact location of the model-predicted showers is just a rough guide, not to be taken literally. (Click on image for a larger view.)
Wednesday Outlook
Updated Tue 08/08 @ 7:58 PM — A beautiful day in store for Wednesday. Sunny, low humidity. A bit windy/breezy in the early afternoon. High 85º-86º.
A disturbance in Nebraska will bring showers and thunderstorms to us by Thursday afternoon.
RGEM forecast 3 hour precipitation at 4 PM Thursday. (Click on image for a larger view.)
Tuesday Outlook
Updated Tue 08/08 @ 9:08 AM —Departing low pressure at the surface and an rapid increase in cold air aloft will result in instability cloudiness and windy conditions today.
NBM cloud cover (dark) with standard deviation (contours) (Click on image for a larger view.)
Mon 09:32 PM Forecast Review — I think the models did pretty good today. The GFS correctly got the earlier timing. The HRDPS and the HRRR correctly predicted the bow echo appearance. The storms, while still strong, did reduce in intensity as they moved into NJ.
Update Mon 8/07 6:31 PM — The bow echo, predicted by the HRDPS early today, seems to have developed—
Update Mon 8/07 6:01 PM — Evidence of rotation in Lancaster county visible on current radar
Severe Weather Update
Updated Mon 08/07 @ 5:34 PM — The latest NBM continues the trend of showing storms at their strongest in western suburbs with reduced severity as they move into NJ. The exception in NJ where severity continues – areas south of Vineland and Salem and Cape May Counties.
Severe Parameter Table for this Storm
Updated Mon 08/07 @ 3:48 PM — My regular readers know I like to prepare this table of severe parameters to compare with known other severe events. Today’s severity table is impressive.
Severity Parameter (HRRR)
Sept 1 , 2021 Tornado Outbreak (example of highly severe)
Recent 4-1-23 Tornadoes (Example of severe)
Recent 08-07-23 18z HRRR Bow Echo
08-10-23 Today’s 06z HRRR & Impact
Notes
CAPE Joules/kg
3500-4200 ⚑⚑⚑⚑
2100 ⚑⚑
2600 ⚑⚑
1200 ⚐ ⚐
Helicity m^2/s^2
1350 ⚑⚑⚑⚑⚑
655 ⚑⚑
726 ⚑⚑
560 ⚑
Vertical Shear 1/sec
40-46 ⚑⚑⚑
40-45 ⚑⚑⚑
30 ⚑
32 ⚑
Precipitable Water
2.7″ ⚑⚑⚑⚑
0.83” ↓
2.1″ ⚑⚑
2.3″ ⚑⚑
Heavy rain potential
Lifted Index º K
minus 6º ⚑⚑
minus 9.3º ⚑⚑⚑
minus 9.9º ⚑⚑⚑
Minus 4.4º ⚐
HRRR Hail inches
1.9 ⚑⚑
1.4 ⚑
1.8 ⚑⚑
1.6 ⚑
Peak Wind Gusts mph
40-50 ⚑⚑
40-50 ⚑⚑
65mph ⚑⚑
32mph ⚐
Storm Motion Shear Vector Alignment
Aligned – ~ 90º ⚑⚑
Almost aligned ⚑⚑
aligned ⚑⚑
Somewhat aligned ⚐
250 mb Jet Stream Wind mph
63 ⚑
135 ⚑⚑
74 ⚑
95-104 ⚑⚑
Severity Parameters Impact: ⚑ indicates favors Severity⚐ indicates Possible Severity ↓ indicates works against Severity ⇩ Significant, but less impact
Monday Severe Storm Threat Update
Updated Mon 08/07 @ 3:28 PM — The latest HRRR (18z) shows the following trends:
Activity begins far western sections around 6 PM
Activity moves into immediate Philadelphia area between 7 and 9 PM
Strongest severe threat is in western suburbs, but strong to severe storms possible in and around Philadelphia and across into NJ.
Latest HRRR shows storms continuing through 11 PM
Significant Hail threat across the entire region.
Tornadic activity possible in Lancaster county, far southern Chester county into Delaware and Maryland.
Monday Severe Storm Threat
Updated Mon 08/07 @ 12:58 PM — The current version of the NBM model (version 4.1) features a new parameter, 4 hour Tornado Probability. The latest NBM shows an area to our west having an elevated probability of tornado formation at 8 PM. Note that this area is west and south of the immediate PHL area.
Today’s 15z NBM shows elevated 4 hour Tornado Probability to our west and south. NBM model version 4.1 was released early 2023. This is the first time I’ve seen this parameter noticeably elevated in our general area since its inception. (Click on image for a larger view.)
This morning’s models continue with a range of start times for the storms. As early as 5:30 to 6 PM (GFS) and closer to 8-9 PM (HRDPS) for the immediate Philadelphia areas.
The trend noted earlier, that the most severe storms remain to our south and west, continues. However the latest models do show a few strong storms near Philadelphia and just east into NJ.
The latest 15z HRRR continues to show some weakening of the line of storms as it moves east into NJ.
Monday Severe Storm Threat
Updated Mon 08/07 @ 10:30 AM — additional graphics for tornadoes and hail added below.
Updated Mon 08/07 @ 9:49 AM — The 12z HRRR has become available. The 12z model runs (and the 00z model runs) utilize direct upper air measurements with weather balloons (and radiosondes) launched globally. While upper air measurements can be obtained with satellite imagery and radar, the direct measurements have an advantage.
The 12z HRRR has supported my impressions from last night: the severity of the storms weaken as they move through Philadelphia and eastward into NJ. But much can happen before moving across the Delaware River.
The HRRR shows a bow-echo type appearance at 7 PM, west of the city—
12z HRRR instantaneous PRATE (precipitation rate) at 7 PM. (Click on image for a larger view.)
By 8 PM, the change in intensity is is apparent—
12z HRRR at 8 PM. (Click on image for a larger view.)
The earlier 06z HRDPS is most impressive with its bow-shape image which also weakens as it moves eastward—
Today’s earlier 06z Canadian HRDPS shows an impressive bow-shaped precip signature of severe weather. (Click on image for a larger view.)
Assuming the reduction in intensity occurs, much can still transpire between 7 PM and 8 PM. The takeaway from this is one might expect less intensity east of Philadelphia. The greatest threat of severe weather is from Philadelphia westward and especially southwardtowards Delaware and southern South Jersey.
Tornado Potential — The HRRR is showing isolated tornadic potential—
HRRR high helicity in areas of precip. (Click on image for a larger view.)
Hail
12z HRRR Areas of increased risk of large size hail. (Click on image for a larger view.)
Monday Severe Storm Threat
Update Mon 8/07 8:22 AM — The jet stream/upper air dynamics that had suggested weakening of storms from Philadelphia and east no longer appears to be the case. That said, the most severe threat is still western suburbs of Philadelphia and southern areas near Wilmington Delaware and South Jersey. Storm threat is about 6-8 PM (HRRR) and 8-10 PM (HRDPS.
Last night’s 06z models are showing a “bow echo” signature of a severe storm line to the west Philadelphia.
Some scattered storms possible as early as 3-4 PM, ahead of the main line of storms.
Look for an update here before 10AM with the latest 12z HRRR data.
Monday Storm Outlook
Update Sun 8/06 10:41 PM — I know this flies in the face of some of tv/radio forecasts predicting severe weather later Monday, but a number of models show the heavy activity late Monday moving to our north and south, weakening in the immediate Philadelphia area. (The upper air dynamics become unfavorable in our immediate neck of the woods.) Something to keep in mind. Too early to be certain.
It also looks like scattered storms are possible earlier in the afternoon, ahead of the main group of storms.
Updated Sun 08/06 @ 7:37 PM — An approaching warm front will bring showers and thunderstorms tonight, after midnight in the immediate Philadelphia area and an hour or so earlier southwestern suburbs.
Currently the storms are in the Baltimore / Washington area—
Combined Water Vapor and Radar image at 7 PM with superimposed RAP model low level winds and moisture convergence. The heaviest rain will move up to our west tonight, but storms here can still bring some heavy downpours. (Click on image for a larger view.)
Some areas will receive heavy rainfall tonight, especially areas northwest of Allentown and southern Chester County.
Showers may linger until 8 AM Monday.
Some models have clouds breaking for periods of sunshine by Monday afternoon, but some models have clouds lingering with ongoing scattered showers during the day. Probably somewhere in between.
There’s a range of model timing for the thunderstorms with the approaching cold front Monday late afternoon and evening.
The GFS, HRDPS and RAP have 5-6 PM as the start of some heavier storms in Philadelphia and immediate suburbs. The HRRR was closer to 7-8 PM Monday evening. Heavy rain and strong wind gusts are possible with these storms.
Some activity may diminish in intensity as it moves through Philadelphia, but that’s not a certainty.
