Update Sun @ 9:32 AM — Clouds have moved in earlier than forecast today. The GFS forecasts clouds throughout the day, becoming WINDY in the afternoon. High temp 55.5º sd 1.1º
Sat 09:43 PM Forecast Review — So, how is it possible that the models showed the storms not getting into Philadelphia, yet we had a line of thunderstorms over the past hour?
Here’s tonight’s 8 PM HRRR showing the eastward extent of its forecast rain —
Tonight’s latest (8 PM) HRRR showing the eastern extent of the forecast precipitation. Clearly the model didn’t bother to look at the radar. (Click on image for a larger view.)
Here’s the much acclaimed ECMWF forecast, also showing the rain not making it into Philadelphia—
ECMWF 18z forecast showing eastern extent of rain tonight. (Click on image for a larger view.)
The weather models are often amazing. Not so much today.
Update Sat @ 5:52 PM — As forecast in the earlier HREF graphic, showers/embedded thunderstorms are expected to stay northwest of the city. Only light showers expected to make it into Philadelphia this evening—
Earlier HREF forecast 7 PM Saturday. Vertical velocity and CAPE elevated especially northwest of the city. Thunderstorms possible. (Click on image for a larger view.)
Sunday looks sunny in the morning with cloudiness by 2 PM as a secondary cold front moves through.
Update Sat @ 9:18 AM — Today’s forecast has been a perfect example of poor model performance intersecting with delayed evening model forecast availability (especially the GFS) due to Daylight Saving Time.
There’s still a range of forecasts and uncertainty regarding degree of cloudiness today. Last night’s GFS moved away from showers this morning, but has considerable cloudiness. Showers and thunderstorms develop as early as 4 PM , peak chance about 6 PM and continue through 8 PM. If we get more sunshine, expect stronger thunderstorms.
As late as it is this for this morning’s update, the new HRRR and other models have yet to come out. The HRRR will be available by about 9:33 AM.
Update Fri @ 9:54 PM — Significant Update (Models out later due to Daylight Savings Time)
A low pressure system and its associated warm front/cold front will affect our area on Saturday. A secondary cold front moves in on Sunday.
SREF forecast for Saturday at 5 PM. Warm front to our north with cold front moving through PA. (Click on image for a larger view.)
Saturday
Clouds and showers early will end mid morning Saturday. There’s still some question how much sun we’ll see on Saturday. My earlier view was much more optimistic about some bright spots and sun. But the majority of models have low clouds and damp conditions lingering throughout the day.
Friday night’s early models just becoming available . The HRRR shows breaks of sunshine mid day in many areas. Then clouds return mid afternoon with some light scattered showers. Then sunshine possible about 5-7 PM before thunderstorms move in.
High temp 72.1º ± 1.7º Blue Bell
A cold front moves through between 7 and 9 PM. Additional showers expected to move through during this time frame. Without sunshine, lower instability reduces the chances of thunderstorms.
Despite the lack of forecast sunshine, the HREF is still predicting some thunderstorm activity during the early evening Saturday from 7-10 PM, especially west of the city —
HREF forecast 7 PM Saturday. Vertical velocity and CAPE elevated especially northwest of the city. Thunderstorms possible. (Click on image for a larger view.)
Sunday
A second cold front moves through on Sunday. The current GFS is showing considerable cloudiness for much of the day. High temperatures of near 60 early morning fall into the 50s during the day. Windy. Right now, no showers expected with t
Early Edition
Update Fri 1:33PM— Several of this morning’s models maintain a low cloud deck for much of the day Saturday
A low pressure system and its associated warm front/cold front will affect our area on Saturday. A secondary cold front moves in on Sunday.
SREF forecast for Saturday at 5 PM. Warm front to our north with cold front moving through PA. (Click on image for a larger view.)
Saturday
Clouds and showers will end mid morning Saturday. There’s still some question how much sun we’ll see on Saturday, but I’m leaning towards an optimistic forecast of bright skies and sunny breaks. High temp 72.1º ± 1.7º Blue Bell (NBM forecast)
A cold front moves through between 5 and 8 PM. Significant instability and CAPE develops during the day, especially if we get some sunshine resulting in likely strong thunderstorms in some areas.
HREF forecast 7 PM Saturday. The white box shows areas most likely to have strong thunderstorms- Maximum Vertical Velocity and CAPE (shading) (Click on image for a larger view.)
Sunday
A second cold front moves through on Sunday. The current GFS is showing considerable cloudiness for much of the day. High temperatures of near 60 early morning fall into the 50s during the day. Windy. Right now, no showers expected with this front.
Update Thu @ 8:06 PM — Following some early morning rain and even a morning thunderstorm Saturday, the NAM-NEST maintains an extended rain-free period on Saturday for much of the late morning into much of the afternoon. We may even see some sun.
The models are showing considerable atmospheric instability on Saturday, something not seen since this past September/October.
Showers and even some thunderstorms (some strong, especially northwest of the city) will move through with a cold front early Saturday evening.
Update Thu @ 12:51 PM — My previous forecast for a 4 to 6 hour rain-free period on Saturday may be overly optimistic, based on the latest GFS and HIRESW-FV3. Following rain early morning, both maintain a chance of scattered showers throughout the day.
HIRESW-FV3 cloud/simulated radar forecast for 11 AM Saturday. (blue and maroon are low and mid-level clouds) (Click on image for a larger view.)
The NAM-NEST still forecasts a cloudy, but mostly rain-free period for most of the area after 11 AM.
NAM-NEST cloud/simulated radar forecast for 11 AM Saturday. (blue and maroon are low and mid-level clouds) (Click on image for a larger view.)
Looking at other models (ECMWF, CMC-GEM), the more pessimistic forecast of scattered showers seems to be in the majority.
The ICON model, like the NAM-NEST has a rain-free period. So there’s hope.
Update Wed @ 8:45 PM — No change in the forecast.
Rain/showers on Thursday.
Beautiful and warm on Friday.
A warm front moves through Saturday morning with showers. Many models have at least a four to six hour period late morning into early afternoon where there’s a break in the showers and possibly some sunshine. More showers Saturday late afternoon and evening as a weak cold front moves through.
GFS forecast for Saturday, showing break in the rain during the morning. (Click on image for a larger view.)
Sunday looks good.
Update Tue @ 8:27 PM — The forecast has changed for more rain on Thursday and possibly a salvaged Saturday.
The low pressure system depicted in the graphics below for Thursday looks like it will move bit closer to us than previously forecast. That means Thursday will likely be a rainy day.
Things may have improved for Saturday. A warm front will move through early morning. Rain may end in the early morning with a warm up and some breaks of sunshine before a cold front moves through later in the day with additional showers.
I’ve read about the possible legislation to keep Daylight Saving Time throughout the year. Probably good, although neurophysiologists say it screws up the natural diurnal rhythm. I do know, if it passes, I’ll have to get used to having the major evening weather models come out past my bedtime, as discussed in my previous post.
Previously Posted Mon 10:08 AM —
This week will be comparatively quiet weather-wise compared to last week’s every-other-day storms.
Two weather maps, one for Thursday and one for Saturday should summarize the weather system flow for the week—
GEFS forecast for Thursday. Low pressure (1) off the coast will brush our area with showers as it moves east-northeastward. The position of the cold air boundary – the red 540 thickness line (2) shows an upper air ridge over the eastern US with generally warm air over our area and cold air north of this line. Another low pressure system (3) taking shape in Oklahoma will move towards us and be north of us on Saturday. (Click on image for a larger view.)GEFS forecast for Saturday 2 PM. Low pressure (3) moves up into New England and drags cold air boundary (2) into Pennsylvania. Some showers from this system will move through earlier Saturday, followed by colder temperatures for Sunday as upper air flow becomes trough like. Click on image for a larger view.)
While the above scenario is a good bet, changes in timing, track and intensity of various systems will affect the actual forecast details. Stay tuned.
