Weekend Weather & This Week’s Weather returns April 30th.
Thu 5:30 PM Forecast Review— We had the wind gusts and some areas had strong storms. The HRRR did pretty good forecasting the stronger storms north of Philadelphia.
Update Thu @ 10:15 AM — Some changes in the forecast with the latest HRRR just available. Timing of the frontal passage today is about 5 PM ± 1.5 hours in the immediate Philadelphia area with thunderstorms, then passing into NJ.
A rapid change in pressure with the frontal passage will result in high wind gusts, as the winds shift from southwest to northwest. Depending on the model, gusts from 30-50 mph possible.
Previous model runs showed shear and helicity to be low with severe thunderstorms unlikely. Despite this, the HRRR is showing an elevated hail parameter with the storms with strong vertical motion being present.
Update Wed @ 6:08 PM — A cold front moves through Thursday afternoon with thunderstorms. Current timing is between 2:30 and 5 PM depending upon the model. Here’s the NAM-NEST forecast for 4 PM—
Forecast helicity and vertical shear remains low and CAPE values remain in the respectable moderate range. No severe weather expected although some strong thunderstorms are possible.
Update Tue @ 8:08 PM — Very warm weather for Wednesday and Thursday with high temps 80-84º. (Average high is 64-65º.)
A cold front moves through Thursday afternoon, with current timing about 5 PM according to the latest NAM-NEST. Thunderstorms are expected.
A low helicity and low shear environment is currently predicted, so severe weather is unlikely, although some strong storms always possible since CAPE values are forecast to be in the respectable low moderate range of 600 joules/kg range.
Update Mon @ 6:50 PM — A review of the latest GFS and Canadian RGEM are more optimistic than the high resolution models that the showers tomorrow morning will be light and depart mid morning. Huge difference here, so I guess we’ll see.
Warm temps and sun in the afternoon.
Update Mon @ 5:11 PM — The low pressure system that gave us the showers and clouds over the weekend will rotate yet another weak disturbance and front through on Tuesday. (The center of this low is still near Nova Scotia.)
The forecast has changed from yesterday— what had been forecast to be a light sprinkle moving through early to mid morning Tuesday now looks to be a several hour period of showers, possibly lasting a bit past noon time. Total QPF is on the order of 0.15 inches.
Previously Posted Sun 7:32 PM —
Our weather will turn more spring-like this week as we come under the influence of an upper level ridge with warm air moving up from the south—
We’ll have temperatures in the 70’s by mid-week and possibly higher. The cold front that will be causing severe weather in the middle of the country is expected to weaken considerably as it approaches us Friday.
No major storms expected this week for our area. Some showers Monday night will move out by Tuesday.
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.
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—
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
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—
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.
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.
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—