Posted Sunday 12/10 @ 3:23 PM — There’s been very heavy rain in NJ since around noontime and it’s moving off to the northeast. Philadelphia and its surrounding suburbs have missed some of the heaviest rain, so far.
MRMS radar/rain gauge current precipitation rate at 3:18 PM Sunday. Another area of intense activity is lining up to the west of Lancaster County and will be here by about 7-9 PM.
Is that it?
No. I just ran the latest HRRR (in what I call ‘summer-parameter mode’) and it shows huge dynamics as the front approaches between 6 PM and 9 PM as the front slowly moves through —
Today’s latest HRRR shows extremely strong upward vertical motion (white box) in the western suburbs at 7 PM. Expect heavy rain and high winds. Thunderstorms possible, especially east into NJ after 7 PM. Notice the convergence of the wind stream lines inside the white box. (Click on image for a larger view.)
Precipitation may last through 6-7 AM Monday morning
Sunday Storm Update
Posted Sunday 12/10 @ 10:56 AM — This morning’s models are in. It’s almost funny…the HRDPS has shifted to have the main rain axis eastward into NJ, similar to previous runs of the HRRR. The new HRRR looks similar to last night’s HRDPS with the main axis of rain just west of Philadelphia.
The NAM-NEST which was the same as last night’s HRDPS now has it’s axis well west of the city, in a similar placement to the HRRR.
(Go figure!This is why snow storm forecast snow totals are always impossible to pin down.)
Here’s this morning’s 12zNBM which is designed to blend these differences together—
12z NBM forecast for total rainfall for this storm (Click on image for a larger view.)
One more thing— the latest NBM wind gust forecast is for wind gusts to be in the 30-38 mph range ( but with an sd ± 15 mph).
Posted Sunday 12/10 @ 8:35 AM — The onset of the rain now appears to be between 11 AM and 12 PM in the immediate Philadelphia area, earlier to our west. Precipitation totals have been reduced in several models, with the heaviest rainfall near the shore. However, I’m sticking with the HRDPS forecast —
Today’s 06z HRDPS rainfall totals by Monday morning.
Some snow flurries are possible north and west of the city Monday morning. No accumulation.
Wind Gusts: 35-40 mph Sunday evening. Stronger gusts to 50 mph Monday morning towards noon.
Sunday Storm Update
Posted Saturday 12/09 @ 5:46 PM — Current conditions at 6 PM—
Current satellite water vapor image 6 PM with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine white contours) with superimposed MRMS RADAR. (Click on image for a larger view.)
It appears that rain will move in about 9 – 11 AM from the south and west on Sunday. It will start light and become heavier by 2 PM. For the rest Sunday and Sunday night, rain heavy at times and very windy. Rain should end by 6 AM Monday for most of the area.
As is always the case, there are differences in the model forecasts regarding banding and the placement of the heaviest rain on Sunday. Examples—
Today’s 18z HRRR total rainfall. (Click on image for a larger view.)18z HRDPS total rainfall Sunday into early Monday. (Click on image for a larger view.)
It will be quite windy, but the wind forecasts are not as extreme as earlier forecasts—
Todays 18z NBM wind meteogram for location Wings Field, Blue Bell PA (Click on image for a larger view.)
Posted Saturday 12/09 @ 9:05 AM — The fog is expected to lift before noontime. There’s a range of model guidance regarding cloud cover in the afternoon. The best bet is for some breaks of sunshine through high cirrus clouds after noontime with an increase in cloudiness again around 4 PM.
High Temperatures: 51º Blue Bell 52º Philadelphia (uncertainty high due to cloud cover uncertainty ± 3.5-4.0º)
For Sunday: Rain moves in about 8 AM from the west. Some of the extreme wind speed forecasts have decreased in intensity to the 30-40 mph range. Still 2-3 inches of rain forecast, but with some shifts in the placement of the bands of heavier rain to NJ.
Latest GFS forecast total rainfall (06z) by Monday morning. (Click on image for a larger view.)
Update Fri 12/08 11:13 PM — Based on tonight’s model guidance, it’s unclear how much sun we’ll see on Saturday afternoon, after the fog mist and light drizzle lifts just before noon. Considerable cloudiness is more likely than sunshine.
A warm front will slowly move through early Saturday. Previous model runs just had clouds and fog with this front, but the latest NAM-NEST and HRDPS show light showers drizzle and fog Saturday morning.
Current satellite water vapor image (Friday afternoon) with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine white contours) with superimposed MRMS RADAR. A warm front will move north Saturday. (Click on image for a larger view.)
An approaching cold front will have a low pressure wave develop along the front on Sunday afternoon. Heavy rain and strong wind gusts are expected with this front and low pressure system Sunday afternoon into the overnight hours of early Monday. The system is approaching faster than previously forecast, but may bog down over our area with several areas of low pressure developing.
Here’s the GFS forecast for early Sunday afternoon—
Latest GFS forecast (18z Friday) showing cold area of low pressure development along this very sharp cold front early Sunday afternoon. (Click on image for a larger view.)
Saturday
Low clouds, fog, drizzle and showers Saturday morning. Some clearing and sunshine after 12 PM Saturday afternoon. Light winds.
High temperature 52º ( NBM model— location Blue Bell, PA) High temperature 51º ( NBM model— location Philadelphia, PA) larger than usual uncertainty with standard deviation ± 3.5º
Sunday
The latest shows showers moving in around daybreak and rain heavy at times during the day, becoming heavier towards evening. Rain lasts through about 4-5 AM Monday morning. Rain will be heavy—
Latest GFS total rainfall forecast for Sunday. (Click on image for a larger view.)
High temperature 62º ( NBM model— location Blue Bell, PA) High temperature 64º ( NBM model— location Philadelphia, PA) uncertainty ± 2.1º
High winds through much of Monday!
NBM wind meteogram for location KLOM Blue Bell, PA (Click on image for a larger view.)
Posted Saturday 12/02 @ 5:50 PM — Rain moves in after midnight tonight and tapers off during the early afternoon. It will remain cloudy with light winds. It should no longer be raining in Philadelphia at the start of the Eagles game. Light winds and temperatures about 52º at 4:30 PM.
Sat 5:36 PM Forecast Review — Today’s high temperatures showed a wide range, as expected. The skies did clear about 11 AM to noon, except near the airport, which was reporting fog into early afternoon. PHL also had a relatively low max temperature.
RTMA measured and interpolated actual temperatures at 3 PM today—
20Z RTMA – actual measured and interpolated temperatures at 3 PM today. (Click on image for a larger view.)
Saturday Update
Posted Sat 12/02 @ 10:47 AM — Right after posting my update this morning, the 12z HRDPS became available. It’s showing 60º and mostly sunny in the afternoon. We’ll see.
Posted Saturday 12/02 @ 9:58 AM — This morning’s models still show low fog breaking for some sunshine through mid and high clouds between 11 AM and 1PM. (The latest HRDPS has clouds lingering until about 1:30-2 PM)
Here’s the HRDPS at 1:00 PM showing low clouds hanging tight in the low valleys near the rivers—
06z HRDPS showing low clouds near the river valleys at 1 PM (Clouds are in grey. black) (Click on image for a larger view.)
I don’t think many areas are going to see the 60º forecast by last night’s TV weather entertainers. This morning’s 12z NBM still shows a high of 57º with a very high standard deviation (high uncertainty) of ± 4º
So, there still remain a wide range of forecasts in temperature and cloud cover. The warm front is getting hung up near our area, causing the uncertainty.
Updates
Update Fri 12/01 11:22 PM — Tonight’s models suggest that high temperatures on Saturday are likely to be anywhere from 54° to 59°. Still high uncertainty. Clouds and fog may linger until 11 AM before skies brighten.
Posted Friday 12/01 @ 5:56 PM — The latest HREF and Canadian RGEM have become available and they capture the extreme uncertainty in tomorrow’s high temperature forecast—
18z HREF—
18z Canadian RGEM—
Previously Posted Fri 6:19 PM —
Two disturbances ahead of a cold front will affect our weather this weekend. One is bringing the rain we’re getting this afternoon (Friday). The next one will bring rain primarily Saturday night into Sunday.
Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine white contours) and superimposed MRMS radar. I’ve drawn in the cold front and warm front. Area 1 is affecting us now. Disturbance 2 will bring rain on Sunday. (Click on image for a larger view.)
As posted last night, there’s an usually large range of model forecasts for the next few days; the most significant differences (and uncertainties) are around the the cloud cover/high temperatures on Saturday and the location of the secondary coastal low formation on Sunday.
Let’s tackle each one individually.
Saturday
A disturbance and associated warm front is moving through Friday evening and it is expected to exit by Saturday morning. There’s a wide range in the cloud cover forecast. The ECMWF forecasting sunshine breaking through has our high temperatures in the low 60s! Most of our models show areas and periods of considerable cloudiness and are in the 56º to 59º range. The NBM shows an incredibly high standard deviation of ± 5.0º, which captures the very high uncertainty of the forecast.
Here’s the summary—
Clouds break for periods of sunshine through high clouds with periods that are considerably cloudy. I’m leaning towards a compromise on temperatures. A below average confidence forecast. Saturday will be dry. Rain begins again about 11 PM Saturday night
High temperature 57º ( — location Blue Bell, PA) High temperature 59º ( — location Philadelphia , PA)
Sunday
As the low pressure in the Great Lakes and its associated cold front spawns a secondary coastal low, we’ll have periods of rain much of Sunday with a taper off during the afternoon. There are huge model differences in the placement of the low, the amount of rain, and when the rain ends. It appears that the cold front will move through Sunday evening. I’m going with a compromise.
The latest Canadian RGEM model captures the uncertainty for Sunday with several centers of low pressure in its forecast. (Click on image for a larger view.)
Rain ends about 4-7 PM.
High temperature 54º ( NBM model— location Blue Bell, PA) High temperature 56º ( NBM model— location Philadelphia, PA) uncertainty ± 3.5º (still high uncertainty)
