Updated Wed 07:32 PM — Updated with latest HIREF model showing showers mostly to the east, not west.
Earlier this week, I mentioned that the closed, cut off upper air low pressure system might linger and that appears to be the case; it appears that it will affect our weather through at least Saturday.
The problem with all closed cut off upper lows is that the associated precipitation is difficult to accurately model and predict. So far this week, the models haven’t done all that well with the cloud cover prediction either.
So with that backdrop, I’m still tempted to try to talk about the showers over the next few days.
At least for tomorrow, Thursday, the models are suggesting we may get some much needed showers in the morning.
HIREF model precip forecast for 8 AM Thursday. (Click on image for a larger view.)
Same time tomorrow, different forecast:
GFS Precip forecast Thursday 8 AM (Click on image for a larger view.)
However, the models are also predicting that any precipitation will be lightand most of the showers will be west east of the immediate PHL area. Our area gets less than 0.1 inches
So you’ll probably have to water your lawn soon if the models are correct about this.
There’s a somewhat greater chance of showers on Friday and Saturday, but again, the majority of the precipitation may miss the immediate PHL area. I’m hedging here…upper cut off lows are tough to forecast.
The closed upper low formed as expected. This upper air feature will persist until at least Thursday.
GFS forecast for Tuesday 4 PM showing upper air closed low circulation (blue lines). Clouds extend just into the Philadelphia area (white shading) (Click on image for a larger view.)
An easterly flow will bring increasing cloudiness as the week progresses. High clouds on Monday will progress to variable amounts of middle and lower clouds Tuesday through Thursday.
An easterly flow will keep temperatures very cool for June, in the mid 70s.
As mentioned, the movement and weather associated with slow moving closed upper low circulations are difficult to forecast. The forecast has already changed—the latest model forecast has this circulation remaining intact, instead of opening and lifting out Thursday.
GFS Forecast (clouds, convective precip) Thursday 2 PM (Click on image for a larger view.)
Various models predict differing amounts of cloud cover each day. We likely won’t see any rain until Thursday, but that may change too. Even more uncertainty in the longer range, Friday through Sunday.
[su_note note_color=”#ffffff”]Updated Wed 10:20 PM — Changes and uncertainty in the latest models. Tonight’s RAP model just available is showing an earlier frontal passage. Showers and thunderstorms move in as early as 7-8AM Thursday.
However tonight’s NAM and NAM NEST show scattered storms late morning and another cluster about 3pm in Philadelphia.
Further Update at 10:30 PM —The HIRESW just available still shows the 11AM -1PM timing.
What looked like a straightforward forecast has become a low confidence forecast with higher than usual uncertainty. [/su_note]
[su_note note_color=”#ffffff”]Updated Wed 05:53 PM — This morning’s models continue with the forecast of thunderstorms moving through Philadelphia around 11AM -1 PM and then slowly moving through NJ during the afternoon.
(Other storms possible much earlier, about 2-4 AM Thursday in far western suburbs.)
Gusty winds Thursday morning and heavy localized rains expected with these storms.
Here’s this mornings HIRESW-ARW2 model showing areas of horizontal moisture convergence (red-yellow) and maximum upward vertical velocity at noontime; the radar image tomorrow will likely show a line of storms at this location and timing.
HIRESW ARW2 forecast for noon Thursday showing horizontal moisture convergence (red-yellow). Maximum Upward Vertical Velocity (black arrows pointing to these areas.)
Those areas in NJ may have heavier areas of storms due to more favorable timing for severe weather in the afternoon. [/su_note]
From earlier today..
Following a hot and humid day today (Wednesday) a cold front will approach later in the day. For Wednesday, most models have any thunderstorms remaining in central Pennsylvania and not making it into Philadelphia during the day on Wednesday, although far western suburbs may be touched early this evening.
For Philadelphia, the storms move through in two waves— after midnight and again about noon Thursday.
The models are suggesting that the actual front moves through Philadelphia about 11 AM -1 PM Thursday with showers and thunderstorms at that time—
NAM NEST 11 AM Thursday forecast (Red line- likely thunderstorm line position 11 AM with arrows pointing to maximum areas of vertical velocity=strongest storm areas) (Click on image for a larger view.) This is useful, but should not always be taken literally.
However, several models show thunderstorms also breaking out well ahead of the frontal passage, as early as midnight tonight (Wednesday) and about 3 AM ± 2 hours Thursday morning. These overnight storms may be the strongest for the immediate PHL area.
Clearing is expected to occur in Philadelphia after 1-2 PM Thursday, but the line of thunderstorms will take several hours to move through NJ, not reaching the shore until late afternoon or early evening. .
