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.)
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.
