Update Sun @ 9:48 PM — Latest radar and latest RAP and HRRR have the storms not making it into our area tonight. I can’t account for the GFS and HREF which had forecast this disturbance to move in.
Update Sun @ 8:41 PM — The line of storms expected earlier this evening did fall apart, as many models (but not all) had predicted. Looking closely, the shear vectors and storm motion vectors were at 90º to each other.
There is another strong upper air disturbance that is very much blossoming on water vapor imagery this evening (see the live water vapor image on this site).
Water vapor image. The yellow ‘river’ of air in southwest PA is warm and unstable as it moves against colder air (blue) (Click on image for a larger view.)
Here’s a current superimposed radar with water vapor image at 8:40 PM —
Current Radar and Water Vapor image 8:40 PM (Click on image for a larger view.)
Storm motion and shear vectors align better about 11 PM. These storms may make it into Philadelphia between 10PM to 1AM. It will be interesting to see if it happens, as it is forecast by the latest GFS.
Monday will be an active day here weather-wise as a strong cold front moves past. Isolated storms may break out in the morning. Available CAPE values are expected to be very high, exceeding 2500 joules/kg. Heavier storms in the 3-6PM time frame. More on that tomorrow.
Update Sun @ 6:11 PM —More models show some storms moving into the Philadelphia area between 9 PM and 1AM. Monday looks to be an active weather day. Stay tuned.
Update Sun @ 11:37 AM — Last night’s models had any rain dissipating before making into our area this evening. However, some of this morning’s models show the showers and thunderstorms making it into the Philadelphia area this evening, between 6 PM and 11 PM. (Not all are on-board: the HRRR has them missing us; the Canadian RGEM has them falling apart just west of us.)
Here’s the latest high resolution HIRESW forecast for 7 PM—
12z HIRESW-2P5 (ultra high resolution) simulated radar/accumulated rain/clouds (black) forecast for 7 PM Sunday. (Click on image for a larger view.)
Updated Sat 10:41 PM — Sunday Forecast below unchanged. Fog lifts and clouds break for sunshine around noon. Showers and thunderstorms look less likely late Sunday and Sunday evening; they will weaken as they approach Philadelphia from the west. Western suburbs may see some activity after 6 PM.
Monday may be an active day for afternoon thunderstorms.
Update Sat @ 9:42 AM — The latest HRRR radar/rain/clouds forecast for 2 PM Saturday—
Today’s 12z HRRR with radar/rain (green contours) and clouds (black/grey) forecast for 2 PM Saturday. The arrow shows the direction of movement of these showers, from southeast to northwest. (Click on image for a larger view.)
Updated Sat 7:34 AM — Last night’s models have come together with the forecast of showers moving in from the southeast as early as 11- 1 PM Saturday. About 0.2-0.3 inches of rain expected.
For Sunday, the forecast is mostly the same- cloudy in the morning, becoming sunny in the afternoon. Chances of thunderstorms moving in, especially western suburbs late afternoon Sunday.
Updated Fri 11:03 PM —Tonight’s models have the possibility of Saturday afternoon’s showers moving up from the southeast as early as 12-1 PM.
The forecast challenges posed by the current weather setup have already thrown today’s “Early Edition” Forecast for a dry Friday afternoon out the window— We had considerable showers move up over Philadelphia from the southeast, while many models had kept any rain to our south.
I’ve been re-evaluating today’s models and not having much success with finding a good match to current and immediate past conditions. (The NAM and NAM-NEST as well as the HREF are not showing any great skill today.)
As is always the case, when the models are not capturing “reality” in the short term, it’s unlikely they’re going to forecast correctly for a day or more going forward.
One model, the RAP, not my usual ‘favorite’ has been doing fairly well today with the shower predictions . Both today’s 09z and 15z runs had successfully predicted today’s afternoon rain. Looking back, the ECMWF also was in the ballpark as was the HIRESW.
I’ll be using the RAP and NBM forecast for this weekend’s forecast.
Another sign of forecast uncertainty is the enormous spread/ differences in the jet stream model wind forecasts for Saturday afternoon—
RAP Jet stream winds at 1 PM Saturday
ECMWF Jet Stream winds at 1 PM Saturday
GFS Jet Stream winds at 1 PM Saturday
(For those of you who wonder how/why I select a specific model for a forecast, it’s a comparison and match to current conditions that guides the process, in addition to the model’s conformity to the forecasts of the larger group of models. At times, I’ll select based on past experience with the model’s handling of similar situations.)
Saturday
The same upper low becomes more diffuse and poorly defined. A southerly flow of moisture will affect our area. Instability and increased CAPE is expected. Cloudy in the morning with some bright spots. An isolated shower can’t be ruled out late morning.
Scattered showers/thunderstorms develop with the instability after 2 PM. Here’s the RAP forecast for 4 PM—
RAP forecast accumulated rain (contours) shading (simulated radar) and clouds (black/grey) for 4 PM Saturday. BTW, The RAP forecasts tend to over-forecast rain. (Click on image for a larger view.)
Chances of thunder are greatest far western and far southern suburbs of Philadelphia. High temp 74.9º ± 2.3º NBM model Blue Bell, PA
Sunday
Cloudy early, then by late morning sunny, hazy warm, and a bit humid with dew points in low 60s. Showers move in late in the afternoon. Thunderstorms possible late afternoon and evening, especially western areas. High temp 80.0º ± 3.0º NBM model, Blue Bell.
