Update Fri @ 12:58 PM — Last night’s models continue to show showers developing in the entrance region of a jet streak as an upper airvorticity develops. The HREF on this with its simulated radar forecast superimposed on the currently forecast jet streak position—
HREF forecast for 1 PM Saturday shows jet streak (enhanced wind speed of the jet stream) and the simulated radar in the white box area (the left entrance region of the jet streak.) The reason—there is enhanced upward vertical motion in this entrance regions. Notice that the position of the jet streak is somewhat further east than yesterday’s graphic, showing that the position of the showers will be further to the east than previous forecasts. (Click on image for a larger view.)
The position of the jet streak may change again and so will the forecast. Currently, most models have a mix of sun and clouds for Saturday with widely scattered showers mostly mid-day and early afternoon.
For Sunday— another jet streak approaches as a ridge in the center of the country moves eastward. This time we will be in advancing exit region of the jet streak which also has enhanced upward motion. A warm front slowly moves through with this feature and we’ll have the chance of thunderstorms and showers. Details unclear at this time. Based on the NBM, Sunday will be cloudy in the morning, then a mix of clouds/some sun and thunderstorms during the early and late afternoon. The latest models suggest the possibility of strong storms Sunday
GFS forecast for Sunday 7 PM. Another jet streak approaches along with a warm front. Enhanced upward vertical motion (this time) in the in this region will result in showers and thunderstorms. Timing details unclear at this time.
Check back for my regular “Weekend Weather Forecast” this evening.
Updated Thu 11:26 PM — Tonight’s models continue with a limited period of showers on Saturday. Not all areas will see showers. The most likely period is 11 AM-2PM. The models differ on the location, The NBM is the most aggressive with the shower coverage .
Update Thu @ 9:04 AM —The latest models have backed off considerably on the rain forecast for Saturday. A much weaker surface low expected to form, triggered by a jet stream level trough. Just some showers expected. This forecast may continue to change.
ECMWF forecast for Saturday at 2 PM. Dip in the jet stream with moderately strong jet streak places us in the “left entrance region” of the jet streak (white box), an area of upward motion. (Click on image for a larger view.)
Heavy rain last night but not in the severe range. Heaviest rain axis was in our immediate area, according to the MRMS. Somewhat similar area to that depicted in the RAPensemble graphic (NARRE) below.
MRMS 24 hour precip summary (Click on image for a larger view.)
Update Wed @ 8:19 PM — This afternoon’s models continue with thunderstorms beginning about 11 PM (south of Philadelphia) and moving north through our area. This is the first of two waves of activity. Some of those storms will be strong.
HRRR forecast for 11 PM . Wave #1 approaches at 11 PM. Wave #2 moves mostly northwest of our area, but we’ll still have rain and storms with this second wave.
Another upper air wave moves through about 5 AM setting off additional storms. The models are still showing this second wave’s maximum impact will be in the Allentown area and far northwest suburbs.
23z RAP Ensemble (NARRE) model mean 3 hour precip at 5 AM shows general areas of heaviest rains. Displayed as 3 hour totals, final amounts will be greater. (Click on image for a larger view.)
Update Wed @ 8:17 AM — The HREF shows the highest forecast shear, CAPE and helicity will be in far northwestern suburbs about 5 AM Thursday.
HREF (06z) showing highest CAPE and Helicity, shear is near Allentown early Thursday morning at 5 AM. Heaviest thunderstorm activity will be there. (Click on image for a larger view.)
Updated Tue 10:49 PM — The NAM-NEST shows the potential for some very strong storms before daybreak Thursday. Elevated CAPE, shear, and helicity.
Updated Tue 10:17 PM — Tonight’s models are showing more impressive rainfall with Thursday’s system. A generalized 1-1.25” of rain with locally heavier amounts. Theres’s a chance of embedded thunderstorms towards daybreak Thursday.
Updated Tue 6:13 PM — A strong low pressure system north of our area is providing the windy conditions today. As has been the case recently, the heavier precipitation has been moving to our north.
Some light showers possible later this evening/tonight (Tuesday) with clearing on Wednesday. Clouds move in later Wednesday.
Yet another somewhat stronger low pressure system will move to our north on Thursday. We’ll see more rain (less than 0.4 inches) but the heaviest activity will be north of Allentown.
The same upper level jet flow with a trough will bring more showers on Saturday. Things look like they’ll clear on Sunday. Temperatures remain somewhat on the cool side.
Updated Mon 6:42 PM — The timing of forecast features has already changed considerably from yesterday’s post.
The latest GFS show some showers late Tuesday afternoon with a warm front. Skies now mostly clear on Wednesday, with a chance of widely scattered showers. A cold front associated with low pressure will bring rain, possibly briefly heavy, late Wednesday night and before daybreak on Thursday.
Another low affects us over the weekend; the exact timing may change, as did this week’s forecast. Stay tuned.
Previously Posted Sun 5:43 PM —
Following a picture-perfect weekend, the coming week looks more unsettled as a ridge builds on Tuesday with a cold front moving through Wednesday with considerable showers and thunderstorms. (The latest ECMWF shows some showers as soon as late Tuesday afternoon.)
GEFS jet stream wind forecast (250 mb) for Tuesday at 5 AM showing weak upper ridge over the Northeast. (Click on image for a larger view.)
For Wednesday—
ECMWF forecast for Wednesday showing considerable rain/thunderstorms ahead of a cold front. (Click on image for a larger view.)
After the cold front, the upper winds become a trough by Thursday—
GEFS jet stream wind forecast (250 mb) for Thursday at 8 PM showing the upper ridge has now transformed to a weak trough. (Click on image for a larger view.)
It should be noted that the ECMWF has a much sharper trough moving through with considerably more rain for Wednesday than the GEFS.
Unfortunately, another disturbance is poised to move in for part of next weekend.
Update Thu @ 5:13 PM — Thunderstorms have broken out as expected with one or two severe warnings in effect. Most of the activity has been typical thunderstorms. The latest satellite imagery shows another impulse approaching from the southwest and may trigger more storms, especially from Philadelphia, south and east.
Update Thu @ 9:24 AM — Reviewing last night’s models and the new models coming available this morning. A cold front moving down from the northwest will develop a wave along it; low pressure directly over our area (or just to our south according to the latest models) will enhance rainfall and thunderstorm potential.
HRRR: Thunderstorms develop 2-5 PM far northwest areas and move southeastward, reaching Philadelphia suburbs and Philadelphia between 4:30 and 7 PM. Additional thunderstorms and showers develop dynamically in-place due to developing low pressure lasting through midnight.
RAP: Scattered storms break out as early as 3 PM in our immediate northwest suburbs. Main activity here 4-6 PM. Additional showers/thundershowers through midnight.
Current severity expectations is “marginally severe possible.”
HRRR forecast showing low pressure over New Jersey at 7 PM (Click on image for a larger view.)HRRR Shear forecast at 7 PM tonight. Peak shear is around 30 (darker orange) (Click on image for a larger view.)
9/1/21Tornadoes (example of severe)
Today‘s Forecast HRRR 15z
Impact
CAPE 3500-4200 Joules/kg
1300-1600
⚑
Helicity 800-1100 m^2/s^2
250
⚐
Vertical Shear 35-45
25 (34)
⚐
Precipitable Water 2.0″
1.9-2”
Lifted Index (minus) 7-9
minus 5
↔
Peak Wind Gusts 40-50
20 (40)
Aligned Storm Motion and Shear Vectors- YES
No
⇓
Jet stream- Highly Cyclonic
Flat wavy**
Jet Stream Speed – High
low-moderate
500mb – Highly Cyclonic
flat wavy
Severity Parameters Impact: ⚑ indicates favors Severity ⚐ indicates Possible Severity ↓ indicates works against Severity
Update Wed @ 8:30 PM — The front made it to Lancaster county during the daytime and barely moved back eastward. Around Philadelphia, an easterly flow kept things cool. Thunderstorms developed northern Chester County (Phoenixville area) this afternoon and again this evening. The evening storms were very slow moving and based on radar, they dumped a lot of rain in a small region. They attempted to move eastward, but ran into high CIN and fell apart.
Additional storms in the Allentown area may move through here around midnight.
Thursday is shaping up to be an interesting day weather-wise as most models have consistently forecast the development of a low pressure system directly over us late afternoon. An approaching front and low pressure could result in heavy rains. Right now scattered storms look to begin about 2 PM in western areas and move into the Philadelphia region by 4-6 PM. I’ll post something about intensity of these storms tomorrow morning.
NOAA is doing a live test of their new supercomputers for the next 24 hours. All models today are coming off of this new system. I’ve never seen their servers so fast! Another live test in two weeks and then it becomes fully operational on June 28th.
So where are all these models on the web? Here on the NOMADS site. (Warning: The NOMADS site is somewhat intimidating and designed for extreme weather nerds and professionals only.)
Updated Wed 7:33 AM — Current weather analysis (RTMA) shows the front only made it as far westward as Chester county, not Lancaster county as predicted by last night’s models.
This boundary is expected to move back eastward today and the convergence zone of the winds (see graphic below) will be a focal point for showers and some thunderstorms. Lowered CAPE values and high CIN values show any activity to be garden variety thunderstorms and showers later this afternoon and this evening.
Updated Tue 11:06 PM — Tonight’s models show the backdoor cold front moving as far westward as Lancaster county. Most models have very little storm activity during the afternoon as CIN values remain high. Some models (but not all) have storms redeveloping in the late afternoon and evening and moving in from the northwest as the wind convergence zone moves back eastward. Some of these may be strong. I’ll update tomorrow.
Updated Tue 6:11 PM — This afternoon’s models, particularly the HRRR, has the backdoor front moving so slowly back eastward that conditions near Philadelphia are not as unstable as shown in the table below. Less instability means less activity is likely in the immediate PHL area on Wednesday. So forget about the green severity table below for now.
Indeed, the latest RAP shows elevated ”CIN” during the afternoon. (Think of CIN as opposing CAPE.)
RAP forecast CIN at 2 PM (Convective Inhibition) is elevated right where thunderstorms had previously been predicted to form. (Click on image for larger view.)
Any activity is predicted to from in the far northwest (Reading – Allentown area) and far western suburbs. The NAM-NEST continues with some storms making it into our area, but they diminish in strength as they move through. Any storms will be scattered; this will not be a line of storms. Should the backdoor front change its forecast position, this forecast will change.
Heavier storms still expected Thursday evening.
Updated Tue 11:34 AM — The backdoor cold front boundary, expected to stall just west of Philadelphia on Wednesday, will gradually move back eastward during Wednesday afternoon. Areas of moisture convergence and high CAPE will result in some heavy thunderstorms in some areas later Wednesday afternoon.
Last September‘s Tornadoes
NAM-NEST Wednesday Forecast
CAPE 3500-4200 Joules/kg
2000-4000
⚑
Helicity 800-1100 m^2/s^2
200-300
⚐
Vertical Shear 35-45
10-15
Precipitable Water 2.0″
2.0″
⚑
Lifted Index (minus) 7-9
minus 6-9
⚑
Max 700mb Vertical Motion 110 m/sec
70
⚑
Peak Wind Gusts 40-50
17
Aligned Storm Motion and Shear Vectors- YES
NO
Jet stream- Highly Cyclonic
Flat
Jet Stream Speed – High
Low
↓
500mb – Highly Cyclonic
Anticylonic
↓
Severity Parameters from Tuesday’s NAM-NEST model for Wednesday afternoon ⚑ indicates high severity ⚐ indicates Moderate Severity ↓ indicates works against severity
Update Tue @ 8:35 AM — Temperatures are still expected to be in the range shown in last night’s post, perhaps 1 degree less overall.
A weak backdoor cold front is still expected to move in before daybreak Wednesday morning. Here’s the RAP model position of the front—
RAP model shows backdoor cold front moving through to western suburbs of Philadelphia Wednesday. Cooler temperatures east of the front. Some models have this front moving back eastward, allowing hot air to return Wednesday afternoon, but there’s lack of clarity how far back this front will move. (Click on image for a larger view.)
There’s a lack of agreement about how far the front will move back eastward and how much hot air will return Wednesday afternoon. A large gradient in temperatures across this boundary.
Originally Posted Mon @ 7:40 PM —
A strong upper air ridge will bring very hot temperatures to our area on Tuesday. The latest NBM model shows maximum temperatures to be higher than previous forecasts—
Today’s 19z NBM Maximum Temperatures for Tuesday. (Click on image for a larger view.)
With dew point temperatures in the range of 65-67º, Tuesday will feel uncomfortably hot.
From Wednesday through Thursday, a back-door cold front will gradually sink closer to us from the northeast
With more clouds, Wednesday’s highs may not reach 90
Some showers/thunderstorms may break out Wednesday ahead of some upper air disturbances (areas of vorticity) rotating around the upper ridge. Additional activity far northwestern suburbs late Wednesday afternoon.
GFS jet stream forecast for Thursday 5PM shows jet stream (jet streak- area of maximum winds) with areas of vorticity moving through into our area. (Click on image for a larger view.)
From Thursday into early Friday, a cold front will approach closer with low pressure developing along the front. Some thunderstorms and rain will break out, especially late Thursday afternoon and evening. Exact timing and details vary from model to model. Several models have some fairly heavy rain later Thursday.
Things clear later Friday and the weekend looks pretty good.
