Thu 7:15 PM Forecast Review — Just a lot of rain today. The missing ingredient was CAPE. But this evening, we’re getting to see what minimal CAPE and high helicity/shear can do with thunderstorms developing. The timing of the impulses moving through has changed, with the NAM-NEST now showing areas of heavy rain through until midnight.
Update Thu @ 10:20 AM — Differences in timing between HRRR and the NAM-NEST…the latest NAM-NEST has storms lasting through 9 PM.
Update Thu @ 9:48 AM — The latest HRRR just became available.
Without any CAPE, we’re already seeing thunderstorms move through, due to enhanced helicity, vertical shear and vertical motion.
Additional rounds of thunderstorms expected. Some of the heaviest are expected between 2PM and 6 PM, with additional activity until about 11PM. Note that the latest HRRR has joined the NAM-NEST with increased CAPE in the afternoon, suggesting stronger storms in the afternoon.
Todays 12z HRRR showing simulated radar: scattered storms with enhanced helicity (contours) and CAPE at 2 PM. These continue through 9 PM, heaviest activity 2-6 PM. (Click on image for a larger view.)
Update Wed @ 10:30 PM — The NAM-NEST has been consistently showing more intense weather for Thursday. Here’s the latest forecast, showing a line of heavy storms moving through 4-5 PM in the immediate PHL area—
Tonight’s 00z NAM-NEST showing simulated radar forecast: strong storms with superimposed helicity contours at 5 PM Thursday. (Click on image for a larger view.)
Update Wed @ 9:43 PM — Today’s ECMWF has over 3 inches of rain falling in parts of South Jersey Thursday.
Update Wed @ 8:45 PM — Low pressure will move almost directly over us on Thursday. Periods of rain, heavy at times, will begin in the morning and continue into the evening. Some thunderstorms will be embedded with these showers and are most likely late afternoon until about 9 PM in our area. There may be somewhat of a reduction in the action from noon until mid to late afternoon.
Severe weather is being understandably down-played by the NWS due to the lack of sunshine expected during the day and a forecast of almost zero CAPE at the surface.
Nonetheless, some CAPE is available at higher levels
HRRR forecast mixed layer CAPE Thursday at 6 PM. Even CAPE at this level is relatively low. (Click on image for a larger view.)
Helicity and vertical shear are expected to reach high levels between 4 PM and 9 PM, especially areas north towards Allentown. This could translate into thunderstorms with strong wind gusts at times and heavy downpours.
Of interest, the shear levels are at or above 40. However the shear does not line up with the storm motion vectors, making the shear impact less potent. Nonetheless, there are areas forecast for superimposed high shear and high helicity—
Today’s 18z HRRR forecast for 7 PM Thursday, showing areas of high shear (shading) (>35) and high helicity (>500) (concentric contours) White streamlines are shear direction. White short arrows are storm motion vectors. They are not very well aligned. Yellow arrows point to areas of overlapped high shear/high helicity. Remember, this a modeled forecast snapshot in time. (Click on image for a larger view.)
Heavy rain (1.3-2+ inches) in some areas is expected before things clear out about 11 PM.
Should the cloud cover be less than forecast and some sunshine peaks through (not expected), things will change.
Update Wed @ 8:35 AM — Last night, I toned down my forecast for the possibility of strong/severe storms on Thursday afternoon.
However, last night’s 06z model runs of the NAM-NEST and HRRR still show high levels of CAPE and helicity, along with elevated vertical wind shear. So the possibility of severe storms still holds for Thursday afternoon into early Thursday evening.
Update Tue @ 11:13 PM — Tonight’s HIRESW models are looking much more benign regarding severe storm potential. Only the NAM continues the possibility. Severe storms now seem much less likely.
Update Tue @ 8:12 PM — For Thursday, low pressure and an approaching strong cold front will cause conditions to become conducive for the development of strong to severe thunderstorms during the afternoon hours. High levels of CAPE and helicity, along with elevated vertical wind shear, may combine to create severe weather conditions.
Todays 18z NAM-NEST forecast for 4 PM Thursday. Highly elevated helicity (contours) especially around Allentown (1), Jersey Shore and elevated CAPE (orange shading) extending from south to north into Philadelphia (2). Strong to severe thunderstorms possible. (Click on image for a larger view.)
Update Tue @ 9:27 AM — Last night’s models continue with the forecast that today will be cloudy with most of the rain holding off until later in the afternoon or early evening Tuesday
The forecast for Wednesday continues to be rain ending in the morning, but dark low clouds with a northeasterly wind may allow some drizzle or widely scattered sprinkles through much of the early to mid afternoon.
Thursday looks to be rainy with a strong cold front moving through. I’ll be keeping an eye on any ‘interesting weather’ for Thursday.
A deep trough develops and moves in for Friday. Possible repeat of scattered showers with ice (graupel) mixed in. Unseasonably cold.
06z GEFS forecast for Friday- Deep upper low in the Great Lakes area and strong dip in the jet forming an upper trough. Instability showers, possible ice pellets (graupel). The cold weather will be with us through Saturday, but moderates considerably by Sunday. (Click on image for a larger view.)
Update Mon @ 5:07 PM — Depending upon your model preference, it appears that Tuesday will be cloudy with most of the rain holding off until later in the day. There may be a few very light, widely scattered showers in the afternoon.
Wednesday will be rainy early, but it tapers off and ends late morning and much of our area from Philadelphia and westward should have very little additional rain.
Rain develops again Wednesday evening.
Thursday looks to be rainy.
Previously Posted Sun 8:24 PM —
This week’s weather forecast is already appearing difficult to nail down accurately.
A strong low pressure system will take up shop in the Great Lakes area while several warm fronts will attempt to move north of us Tuesday, Wednesday and Thursday.
Timing differences could result in several rainy days or it could time in such a way as to have rain occur late afternoon Tuesday and early morning Wednesday with the rain at night and with much of the daytime hours milder and rain free.
The current NAM is an outlier for Wednesday with 70º temperatures and a warm front north of us. (No other model is forecasting this.)
Secondary lows along the coast and Appalachians will develop and move up towards us late Tuesday through Thursday. A weather map is worth a thousand words—
GEFS mode forecast for Wednesday at 11 AM. A large low west of the Great Lakes (3). Secondary low (1) may move in late Tuesday and move away fast enough that Tuesday and especially Wednesday are a nice days. Another low forming (2) will move in for Thursday with rain. Cold front (4) moves in for late Thursday into Friday and cold air (5) moves in for late Friday into Saturday. There’s much uncertainty with the speed of these systems and the resultant timing. Slight speed and timing changes will make large differences in the daytime weather. (Click on image for a larger view.)
Sun 2:33 PM Forecast Review — Some sunshine with increased instability from upper air disturbances gave us the showers. Some moved through an hour earlier than forecast by the HRRR. Cold air aloft resulted in some of the precipitation coming down as a frozen form called “graupel“.
NEXRAD radar at 2:15 PM with 500 mb wind streamlines (red). The radar image above looks remarkably similar to the NBM simulated radar forecast graphic just below (Click on image for a larger view.)
Update Sun @ 8:49 AM — Last night’s models are on-board with some sun and clearing about noontime followed by the development of an area of showers that moves through between 2-4 PM along with an increase in WIND. The showers will be from an upper/mid-level area of vorticity swinging through.
09z NBM simulated radar forecast for 3 PM Sunday (Click on image for a larger view.)
Update Sat @ 9:57 PM — This afternoon’s and tonight’s early models continue to show clouds and showers in the morning Sunday, but with breaks of sunshine moving in between 11 AM and 1 PM.
Despite intervals of sunshine, periods of clouds and widely scattered showers are predicted during the afternoon, the result of instability and several upper/mid-level air disturbances, especially in the 2-5 PM time frame.
The latest HRRR shows an area of significant showers around 3 PM or so. The air aloft will be very cold and some of those showers may fall frozen as graupel. Areas south of the city will have the most sunshine. It will become WINDY in the afternoon.
Tonight’s 00z NBM cloud forecast (shades of grey) and simulated radar (multi-colored concentric contours) at 2PM Sunday. Notice the areas of little to no clouds interspersed between scattered showers. (Click on image for a larger view.)
Update Sat @ 10:14 AM — Reviewing this morning’s early models along with last night’s models, it appears that Sunday will have showers in the morning, with additional showers AND some sunny breaks in the afternoon. The showers in the morning due to low pressure; the showers in the afternoon due to high instability and ‘self-destructive sunshine’, with a secondary low developing off the coastline. Windy.
Despite the occasional sunny breaks, the amount of showers forecast may put a damper on outdoor activities.
Today’s 12z NAM-NEST Simulated Radar and cloud forecast for 1 PM Saturday. Just a snapshot of a very changeable situation. But most of the area has clear skies despite the showers occurring in some areas. (Click on image for a larger view.)
Update Fri @ 9:44 PM — Just reviewed the ECMWF and tonight’s latest NBM. There’s more uncertainty regarding Sunday’s forecast, specifically how long the showers linger into the afternoon and how much clearing and sunny breaks we’ll see. The Sunday forecast posted below is currently on the optimistic side of things. I’ll update tomorrow.
High pressure builds in for Saturday. A low pressure system over the Great Lakes pulls an upper trough through on Sunday.
GFS model forecast for 8 AM Sunday morning. Showers and clouds will break for a mix of clouds and sunshine. The upper trough (as outlined in the red 540 thickness line) will cause instability clouds and widely scattered showers in the afternoon. (Click on image for a larger view.)
Saturday
A sunny but cool as high pressure moves in. Average seasonal high temperatures are 59-60º, but our high will be 54.6º sd 1.4º NBM model Blue Bell.
Previously Posted Fri 5:26 PM —
Sunday
An upper trough moves through with a weak cold front. Clouds and showers in the morning will break for a mix of clouds and sun in the afternoon. Instability widely scattered showers possible in the afternoon. WINDY in the afternoon. High temperature 54.6º (again!) sd 1.6º NBM model Blue Bell.
Winds
NBM wind forecast for Blue Bell, PA (Click on image for a larger view.)
Fri 09:29 — Forecast Review — The timing for most of the model forecasts was too fast by 2-3 hours. Yesterday’s 18z HRRR did well in forecasting the strongest storms in the western suburbs of Philadelphia and having the line of storms diminish as it approached the Delaware river, as depicted in my graphic below. The 12z HIRESW-2P5 did the best with timing and the storms falling apart as they approached the Delaware River.
I’ll have my regular “Weekend Weather Forecast”— this evening.
Update Thu @ 9:14 PM — Did any of the models get the timing right? Well, all the afternoon models had the line of storms coming through by now. Looking back at all models, only this morning’s HIRESW had it close. (The HIRESW is only run twice daily.)
This morning’s 12z HIRESW-2P5 radar forecast for 9 PM tonight. It probably did the best with the timing. More interesting is the HIRESW-2P5 predicts the storms to fall apart as it goes through Philadelphia between 11-12 midnight. It will be interesting to see if that happens too. (Click on image for a larger view.)
Update Thu @ 5:55 PM — Looking at current radar, I’m thinking the lack of high CAPE is taking a big bite out of the strength of line of storms. I’m not all that confident that high vertical shear and helicity will make up for lack of high CAPE. I guess we’ll see.
NEXRAD radar 5:50 PM (Click on image for a larger view.)
Update Thu @ 4:35 PM — This afternoon’s NAM-NEST and HRRR continues the earlier forecast: The main line of storms is expected about 7-9 PM approaching the immediate PHL area and continues past the area through 11 PM to 1 AM.
As forecast, there has been little sunshine and CAPE values are not expected to be in the danger zone.
Nonetheless, there’s plenty of kinetic energy to feed the storms, the windy conditions ahead of the front being a good indication. Hail and strong gusty winds near 50 mph are possible.
Based on review, here’s where I think the most significant storms will be—
Today’s 18z HRRR showing maximum shear, helicity, highest CAPE and simultaneous rainfall in the white boxed area. This area will progress into NJ but not far between 9 and midnight. The models show a decrease of storm strength into NJ. ALWAYS DEFER TO THE OFFICIAL NWS WARNINGS FOR UP TO DATE CONDITIONS. (Click on image for a larger view.)
Update Thu @ 11:31 AM — This morning’s high resolution models have become available.
As forecast last night, some scattered thunderstorms could break out as early as 3-4 PM. The main line is expected about 7-9 PM in the immediate PHL area , although some models now forecast it closer to 11 PM.
The one thing that has been prominent and consistent about this forecast time frame is the lack of high CAPE, especially high CAPE immediately preceding the line of thunderstorms. CAPE is a measure of available thermodynamic instability/energy for severe storm formation. CAPE values are only forecast to be in the 300-700 joules/kg range.
In place of high CAPE is actual kinetic energy in the form of ascending wind shear, specifically something called “vertical wind shear” which is a measure of increasing winds with height. The vertical wind shear forecast by this morning’s models is not forecast to be as high as previous model runs.
For high vertical wind shear to have maximum impact on thunderstorm development, the shear direction must be the same direction as the storm motion of the thunderstorm cells themselves. That alignment is also not as optimized as previous forecasts had shown.
Today’s 12z HRRR Vertical Wind Shear (white streamlines) Storm Cell Motion (Blue arrows) and simulated radar. Notice there is maximum alignment between storm motion and shear vectors in area (1) but not so much in area (2) This suggests that storms that break out ahead of the main line may be more severe than the storms associated with the line of storms. (Click on image for a larger view.)
Based on the low CAPE, it appears that thunderstorms will be strong, but not necessarily severe. Tornadoes are unlikely (based on lower helicity forecast) and separation of maximum helicity from the actual line of storms.
The big issue will be wind gusts, which are forecast to approach 50 mph ahead of and with the storms.
As I had written last night, if we get more sun this afternoon than forecast, things could get more severe.
Update Wed @ 8:54 PM — What a difference a day makes with weather forecasts. This afternoon’s models have moved towards the likelihood of very strong and even severe storms Thursday afternoon and evening.
The HRRR is showing the possibility of scattered storms as early as 3 PM in some areas. The HRRR shows areas of high helicity with these scattered storms. While helicity values are less than the 1000m2/sec2 of last September’s tornadic outbreak, they are on the high side and an isolated tornado can’t be ruled out.
Today’s 18z HRRR simulated radar forecast 3PM Thurs with Helicity (concentric contours). Helicity values approach 600 m2/sec2 which is towards the high range. (Click on image for a larger view.)
Most models have the main line of storms starts moving through between 7 and 8 PM and goes through at least 11 PM.
Today’s 18z HRRR simulated radar forecast 7PM Thurs with Helicity (concentric contours). Helicity values approach 850 m2/sec2 (Click on image for a larger view.)
While CAPE values are still in the lower range, very strong wind shear is expected to provide the fuel to energize these storms. CAPE is the thermal fuel. (Wind shear is the change in wind velocity with height. Expected Wind shear is forecast to be 35 m/sec which is very high.) Wind Shear is the kinetic fuel.
HRRR forecast Wind Shear (shading) and Helicity (Contours) at 7 PM. Scale is for Wind Shear. (Click on image for a larger view.)
While overall rainfall is not expected to be extreme, WINDY conditions and high wind GUSTS from severe storms is expected.
Update Wed @ 7:51 AM — The current trend for Thursday’s forecast. The thunderstorms likely will come in two main groups. An earlier group, as early as 5 PM but most likely about 7-8 PM may have strong storms. The second group will come after midnight. Both groups can have strong storms, but not likely severe in our area. If there are any severe storms, they would occur in the first group, most likely far northwest of the city.
The models continue to have the storms disconnected from the conditions required for severe storms.
HREF CAPE (1) and Max Upward Helicity (2) at 10 PM. Note that the city is in an area of only moderate (CAPE 300-400 joules/kg ) (Green vs Yellow) and the forecast areas of maximum Helicity (concentric contours) are northwest of Pottstown. CAPE diminishes after 10 PM. (Click on image for a larger view.)
Update Tue @ 10:38 PM — Tonight’s HRRR has temps into the low 70s on Thursday afternoon with strong storms as early as 5-6 PM.
Update Tue @ 5:34 PM — A warm front will move through on Wednesday with significant clouds and possibly some light showers mid-day. Considerable cloudiness should persist through the day Wednesday.
Warm front passages are not as ‘distinct’ as cold fronts and the models have considerable low level moisture and clouds lingering into Thursday morning as it gradually moves north of us.
The big question for Thursday’s weather ahead of the cold front: how much sun will we see. More sun = more intense thunderstorms.
Currently the models have high temperatures only reaching the upper 60s, with little sunshine until late afternoon. If we get more sunshine and temperatures go into the low 70s, expect some severe storms. But, right now, the major severe parameters appear to be separate from the actual line of storms expected to move through around midnight.
Today’s 18z NAM-NEST simulated radar forecast for 9 PM Thursday. The NAM is showing storms breaking out ahead of the main line of storms with the cold front still in central PA at 9 PM. Helicity (brown-red circular contours) is maximized well ahead of the main line of storms. (Notice the area of high helicity near the radar maximum in NY State. That’s what a possible tornadic situation looks like., model-wise) (Click on image for a larger view.)
The specifics are likely to change. Stay tuned.
Update Mon 9:52 PM— Today’s barrage of snow flurries were impressive and predicted by the models. However, I can’t remember a time when so many periods of flurries and snow showers developed for hours on end. It’s what happens when you have an upper cyclonic flow, moisture and very cold, rapidly decreasing temperatures with height along with March sunshine warming the ground interspersed.
While temperatures will be in the teens tonight, we’ll be warming into the high 60s on Thursday!
The next period of interesting weather will be Thursday and Thursday night, as a cold front moves through. While not definite, several ingredients are coming together for possible severe thunderstorms and high winds late Thursday. Currently, the main activity is forecast west of the city, but the exact placement is too early to call. Stay tuned.
Previously Posted Mon 10:58 AM —
The unseasonably cold weather we’re having on Monday is associated with an upper air trough. Temperatures will begin to moderate as we get into Wednesday as an upper air ridge (1) moves over us—
GEFS jet stream wind speed forecast for Wednesday at 11 PM. The upper air configuration will be transitioining to a ridge (1) during the day Wednesday and a surface warm front will move through later Wednesday into Wednesday night. Following warmer and considerable rain on Thursday, another cold upper trough (2) will slide in to replace the ridge for Friday and the early part of the weekend. The trough (2) is expected to flatten out, so it won’t be anywhere as cold as it’s been on Monday. (Click on image for a larger view.)
Until the warm front moves through Wednesday night, we’ll still be in a cooler air mass.
Rain and mild temperatures on Thursday with the warm front followed by a cold front Thursday night. Clearing and cooler for Friday.
Another low pressure system may affect us on Saturday.
