Updated Thursday 12/21 @ 4:58 PM — Today’s models have backed off considerably on the chance of showers late Saturday evening and Sunday morning. In fact, at this time, most models with the exception of the ECMWF and ICON-EPS have any showers staying well west and far north of the immediate PHL areaon Sunday. Considerable cloudiness is still expected.
Posted Thursday 12/21 @ 9:55 AM — A back door cold front will slip through later today with colder air for Friday. The front will return as a warm front on Saturday into Sunday. I’ve summarized the situation in the graphic caption below—
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 violet contours) and superimposed MRMS radar. A back door cold front will move through tonight. Colder weather for Friday. The front will return as a warm front through late Saturday into Sunday with scattered showers, mostly far north and west, but a shower here can’t be ruled out. Warmer weather for Sunday, but early morning showers are possible, along with a light shower later in the evening.
For much of Saturday and Sunday we’ll be in the transition zone between warmer and colder air masses and significant cloudiness is expected. Less certain is the amount of shower activity here.
Posted Wednesday 12/20 @ 8:25 AM — No, not what you think or want… it’s a warm front that will move through on Saturday. Past models had this frontal passage dry in our area, but the ECMWF and GFS are cranking out some light showers here Saturday night and Sunday morning into afternoon. Considerable cloudiness as well.
Wednesday’s 06z GFS shows light showers (3 hour precip ) at 1 PM on Sunday. Cloudy as well. Mild in the mid 40s. (Click on image for a larger view.)
Quiet and Cold
Posted Tuesday 12/19 @ 4:44 PM — A very quiet weather pattern will take over through Christmas day. We’ll be in an upper trough (cold) while much of the country will be under an upper ridge (warm). High pressure at the surface will dominate—
GEFS forecast for Saturday shows a huge upper level ridge with warmer than average air poking well into Canada. A trough over us will keep things somewhat on the cold side. While this upper ridge is expected to slide eastward towards us after this weekend, some of that strong warmer ridge will be flattened out as it makes it here. The low pressure system in southeastern Colorado is expected here next Tuesday into Wednesday. Colder and stormier the end of next week, but any snow still seems elusive. (Click on image for a larger view.)
So the this week is a good time for meteorologists in this area to take a vacation. Perhaps a few impulses will move through from the northwest, but nothing major forecast.
Light Snow Flurries Possible Tuesday Morning
Posted Monday 12/18 @ 7:47 PM — As low pressure continues to move away, a mid level disturbance will rotate through our area around daybreak Tuesday. Several models show scattered snow flurries but temperatures are expected to marginally at or just above freezing. The NBM puts the maximum probability at only 32% and while Canadian models (onboard for the last two flurry events) show nothing.
Here’s what the experimental RRFS is showing at 7 AM—
18z RRFS (experimental) – Precipitation type: lavendar (snow) green (rain) at 7 AM Tuesday. This does not show the amount, just the type and likely over-states the coverage. (Click on image for a larger view.)
Considerable low, instability cloudiness mid day and during the early afternoon.
High pressure builds in and fair skies and cold temperatures are expected for much of the rest of the week.
Posted Monday 12/18 @ 9:52 AM —This should be no news to anyone around here, but we had a generalized heavy 3″-4+” rainfall through this morning. It’s still raining, but I thought I’d post the current rainfall totals through 8 AM this morning—
MRMS rain gauge & radar-based estimate of rainfall received through 8AM today. Additional rainfall up to 0.10 inches possible. Color shading is in inches. Line contours are in mm (25.4 mm= 1 inch) (Click on image for a larger view.)
I don’t think we had the high wind gusts forecast, but another period of windy conditions is expected later this morning into this afternoon, as temperatures drop off.
Posted Sunday 12/17 @ 3:10 PM — It’s looking like 3-4 inches of rain (even 5″ in a few spotes just to our west) will fall over a large area, according to the majority of this morning’s models.
Here’s the 3 PM MRMS radar. The heaviest rain is expected between 1 and 4 AM —
MRMS radar at 3 PM Sunday.
Sunday Storm Update
Posted Sunday 12/17 @ 9:33 AM — After reviewing the latest model guidance, the forecast remains pretty much intact. Very light drizzle of increasing intensity is visible on radar moving up from the Cape May area. A gradual increase in dampness as the morning proceeds.
The current setup, via satellite water vapor imagery—
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 violet contours) and superimposed MRMS radar. The main storm is far to our south moving almost directly towards us. (Click on image for a larger view.)
Here are the salient points regarding this storm—
light scattered drizzle, increasing late afternoon
heaviest rain 2-3 AM tonight
rain taper late Monday morning ends after 1-2 PM Monday
highest winds after midnight and especially 2-3 AM into Monday morning. Wind gusts 40 mph with some models showing 50mph.
Snow flurries, especially north and west daybreak Tuesday. Possibly a coating northwestern suburbs.
Here’s the hourly forecast rain rate in inches per hour (NBM model)—
NBM Precipitation rate (Blue Bell) in inches/hour. Greatest rainfall intensity (and winds) 2-3 AM (Click on image for a larger view.)
Total Rainfall expected—
Today’s 12z NBM total precipitation forecast. Some models show areas of 4″+ Click on image for a larger view.)
Wind gusts—
Sunday Forecast Update
Posted Saturday 12/16 @ 4:55 PM — The high cloud cover today was less than forecast and the NAM-NEST was one of the few models that got this right today (but discounted by me) .
The NAM-NEST continues to forecast showers and drizzle as early as the early to mid morning Sunday, and because of its accuracy today, I’m going with it for Sunday. It’s supported somewhat by the latest ECMWF and to some extent, the latest GFS, but the rainfall it’s predicting early is quite enhanced—
NAM-NEST shows showers and drizzle starting before daybreak and rainfall by noon Sunday is considerably more than several other models. Nonetheless, I’m going with it, based on its performance today. (Click on image for a larger view.)
The heavier rain starts between 5 PM and 7 PM Sunday evening. At that time, the main storm is still in South Carolina moving north. By midnight, wind gusts increase to 40 mph.
By 5 AM Monday morning, the low is an elongated two-center system right over our area—
Latest GFS (18z) shows a two center elongated low at 5 AM Monday. (Click on image for a larger view.)
The rain tapers off after noontime Monday. Total rainfall (GFS) —
Today’s latest GFS (18z) shows total rainfall expected. (Click on image for a larger view.)
Forecast Update
Posted Saturday 12/16 @ 9:18 AM — The forecast is on track for today, Saturday, with increasing high cloudiness, especially by late afternoon.
The general trend of last night’s models continue with the rain of the main low pressure reaching us during the evening hours, and with light scattered showers and drizzle during the day Sunday.
The NAM-NEST is particularly aggressive (maybe too aggressive?) with the light rain/drizzle during the day Sunday —
The latest NAM-NEST has light showers and drizzle, even Sunday morning. Possible, but maybe too soon. (Click on image for a larger view.)
One trend I’m watching (hinted only slightly by the GFS and ECMWF) is a more westward track of the low, with the heaviest rain west of the city into central PA.
I’m particularly interested in this trend, since the still experimental RRFS model (now at version 0.7.7) shows a considerable westward track of the storm. (It’s more than likely that the RRFS is not ready for prime time, but I’m curious to see if it provides any insights. You can see the ongoing development notes of the RRFS model here. )
I’ll update later this afternoon with the latest.
Previously Posted Fri 6:42 PM —
An impulse in the middle of the US will merge with another over Florida and move up towards us on Sunday, resulting in a powerful storm directly over us. Here’s the current water vapor image showing the setup—
Current satellite water vapor image (Friday afternoon) with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine violet contours) and superimposed MRMS radar. (Click on image for a larger view.)
A deep and significant storm will be the result. The NAEFS which combines the GEFS and Canadian GEPS shows the sequence—
NAEFS forecast Sat night–> Sunday night–> Monday night. (Click on image for a larger view.) By Tuesday, we’ll have a closed upper low with very cold air over our area. (Click on image for a larger view.)
High winds and heavy rains expected.
Saturday
Sunshine through high level cirrus cloudiness. The high level cloudiness thickens later in the day. Light winds.
High temperature 53º ( NBM model— location Blue Bell, PA) High temperature 54º ( NBM model— location Philadelphia, PA) average to low level of uncertainty ± 1.5º
Previously Posted Fri 6:42 PM —
Sunday
Cloudy. Scattered light rain in the morning, becomes more widespread and heavier by 2-3 PM. Light winds, becoming windy in the evening.
High temperature 56º ( NBM model— location Blue Bell, PA) High temperature 58º ( NBM model— location Philadelphia, PA) average level of uncertainty ± 2.5º
Rain, very heavy, Sunday night into Monday morning. VERY WINDY!
By late Monday, becoming quite cold and windy. Possibly some snow flurries.
Total Rainfall—
NBM model total rainfall through late Monday. (Click on image for a larger view.)
