Update Sat @ 5:55 PM — More models are on-board with very light snow Sunday evening. The NAM and NAM-NEST have about 0.5-1″ by 1 AM Monday. The location of anything more than a coating varies by model. The HREF is provided below which shows the spotty nature of the snow forecast—
Things still uncertain for Tuesday. The NAM continues as an outlier with several inches of snow. Other models have a coating to less than 1 inch. Stay tuned.
Update Sat @ 11:19 AM — Sunday looks to be mostly sunny in the morning, increasingly cloudy during the afternoon. The weak upper air disturbance and cold front coming through Sunday night looks to have only some snow flurries, maybe a coating far western areas.
The possible storm on Tuesday is only being forecast by the NAM model. Other models have little development. This is not one of those situations where my “never ignore the NAM” mantra necessarily applies. In this sort of thing, the NAM is often off-target. But I’ll keep an eye on it.
This weekend’s weather will be influenced by an upper air trough that has been with us for over a week. Very cold air is contained in this trough and our temperatures will be well below seasonal average through the weekend.
A low pressure system predicted to form off the coast Saturday will move off well to our south and east. Cape May NJ will be on the very northern fringe.
Originally Posted Fri 5:04 PM —
The next active period may be Tuesday into Wednesday, when a more potent disturbance in the southern stream (2) may phase with the northern stream(1) closer to the coast, but the models currently have it occurring further north and east of us—
Friday night’s models are suggesting a greater possibility of coastal storm development on Tuesday.
Originally Posted Fri 5:04 PM —
Sunny and continued very cold. Gusty winds in the morning, subsiding in the afternoon. High temp 28.6º ± 2.6º (NBM model Blue Bell)
An upper air trough axis moves through.
Some cloudiness in the early morning, especially north and west of the city. Then partly/mostly sunny. Continued cold. Gusty winds. Clouds move in late afternoon.
The GFS is forecasting an upper air disturbance possibly bringing a dusting/coating of snow late evening Sunday, towards midnight.The NAM has some light snow earlier in the evening.
Update Thu @ 7:25 PM — One more thought about the forecast gone wrong….. Most models got the onset timing correct and the end of the precipitation correct. They also got the general QPF (quantity of precip falling) correct. They went very wrong on the thermal profiles.
As for the weekend, as previously mentioned, the models have been in consistent agreement with the storm early Saturday staying to our south.
We’ll have very cold weather this weekend. High temps likely won’t exceed freezing. At times, there will be cloudiness due to an upper air trough moving through with additional weak cold fronts/upper air disturbances. Otherwise dry.
The next period of interest is Tuesday, where secondary low pressure may develop off of the coast. This period also has the potential to be a mixed-precipitation event with icy conditions or sleet. Just a possibility now.
Look for my regular, weekly “Weekend Weather Forecast” Friday afternoon or evening.
Thu 08:35 AM Forecast Review — So…most models got this forecast wrong. BUT there’s always something to be learned.
First, which models did the best here? As was the case a few weeks ago, the ICON and the new Canadian GEM model correctly predicted the temperatures warmer and more precipitation as rain.
Of our major models, the NAM and NAM-NEST were the closest.
The HRRR, RAP, GFS, SREF, HIRESW and HREF groups had the 1-3″. Of course, since the NBM is a composite or ensemble model of other models, it steered its snow totals towards the greater population of high snow accumulation models. Our new GFS v 16 model has been very disappointing.
Another thing to learn with the NBM: the 25 percentile snow totals seems to be surprisingly accurate lately here. I have to admit, yours truly ignored the low-end NBM percentile and I discounted the warmer Canadian GEM and ICON as total outliers last night.
(BTW, last night’s ICON and GEM had predicted most of our precip as rain with just a coating possible.)
I may have to add to my mantra: “never ignore the NAM, GEM and ICON”
Update Thu @ 7:41 AM — The latest HRRR 11z “Analysis” (the 0 starting hour for each hourly HRRR model run) just available shows why we’re still getting rain.
The latest real time RTMA analysis at 7:30 AM shows surface freezing temp still north of Allentown—
Update Thu @ 6:33 AM — Last night’s 00z and 06z Canadian GEM model runs joined the NAM with less than an inch of snow. Indeed these models had barely a coating for our area.
Last night’s ICON model was similar to the Canadian GEM.
Based on a comparison of actual conditions (the real time RTMA ) and other models, the NAM’s minimal snow forecast may be on the high side. So this morning, I’m leaning towards less than last night’s NAM model snow total graphic immediately below.
Update Wed @ 9:27 PM — Tonight’s early models have become available.
Rain about 3 AM with the arctic cold front passage will change to snow from northwest to southeast between 6 and 8 AM.
Snow continues until about noon.
Of interest is the trend of this afternoon’s NAM and the new NAM and NAM-NEST which just became available. It has less than an inch throughout the region. The reasons are:
The ground will start off wet.
The ground will be warm, above freezing, to start
Snow-water ratios will be low until it gets colder
Compaction of wet snow will occur initially
Regulars here know my mantra:“Never Ignore the NAM”
The latest NBM model is below and it captures most of the models forecasts from today. —
Important:the first number listed in the graphic’s snow total range is the most likely amount. (The second number in the range is a possible high amount.)
(Also note that 25% of the models that comprise the NBM had less than 0.8 inches, similar to the NAM!)
My best guess is that snow totals will lie between the NAM and the lower first number of the NBM range.
Temperatures will drop into the 20s during the day and it will be windy. It will be cloudy until mid afternoon. What falls will freeze during the afternoon, especially areas not in the sun.
Update Wed @ 1:36 PM — This morning’s models continue with a similar forecast of a generalized 1-2 inches snowfall. The GFS has moved to a 1.5″ range for most of us. The next major model runs, the 00z runs, will start becoming available about about 9 PM. I’ll update around 9:30 PM. Stay tuned.
BTW, the models are currently in good agreement that the potential storm Friday into Saturday will move south of us, missing us here.
Update Wed @ 8:01 AM — The models run at 06z continue with a similar snow forecast for Thursday morning The differences in the model forecasts appear to be not based on the amount of precip falling (QPF), but rather how fast the temperatures drop and rain changes to snow after the frontal passage tomorrow. The Canadian GEM is the warmest, with snow just towards the end. It has the lowest snow totals, especially in the city, with just a coating to an 1/2”. The HRRR is the coldest and snowiest.
The GFS is predicting an overall 1” snowfall.
Since this will likely involve convective snow, the “convection-allowing models” referred to as CAM models may be the most accurate with this snowfall. The HRRR and NAM-NEST are CAM models.
The Model Blend (NBM) is designed to do all the heavy lifting for predicting snow totals from all the model forecasts. Here’s the latest NBM—
At this time, the storm for Friday night into Saturday looks like it will miss us and move to our south.
Update Tue @ 10:19 PM — A quick update. Tonight’s models continue with a forecast of snow for Thursday morning. The models differ over what time the temperature drops to support snow.
The NAM (below) does a good job in the middle range. Some models support the previous HRRR model which has more snow.
Update Tue @ 5:23 PM — As mentionedbelow, the upper air configuration this week has many ingredients ripe for storm development. The triggers for such development are beginning to show in the upper flow. We may be affected by storms Thursday and again late Friday into Saturday.
Still much uncertainty, but it appears that Thursday morning, we’ll be clipped by a low pressure system that develops on the arctic front that moves through in the early morning. (see my morning update below.) The latest HRRR and NAM have moved to a quick 1-2 inch snowfall, starting daybreak Thursday and ending by noon.
The latest (18z) GFS stays with about 0.5 inches throughout the area.
The HRRR below is on the high end of the current forecasts—
Below is the 18z Canadian Regional GEM model forecast, fairly close to the latest GFS and NBM—
Another system is expected to follow Friday night into Saturday. There is a wide range of model forecasts about this second storm; it may move off to our south (according to the European ECMWF).
Updated Tue 12:01 PM — A quick look at the morning’s models shows most with about 0.5-1.5” snow accumulating Thursday morning with the higher amount northwest of the city. The NAM is on the lower end. “Never ignore the NAM”. Very cold temperatures after the frontal passage early Thursday.
Updated Tue 8:07 AM — Last night’s GFS suggests slight additional strengthening of Thursday morning’s low pressure system with a coating to an inch of snow here. The NAM does not show much of anything. Stay tuned.
Originally Posted Mon 7:16 PM —
A broad upper air trough will remain established over the Central and Eastern US for the next week or so. Cold air pooling in eastern Canada and pushing southward will suppress the jet flow to our south.
We’re in the seasonally coldest portion of the winter. Average seasonal high temperatures are 40º (Blue Bell) and 41º (Philadelphia). With the exception of this Wednesday, temperatures will be below seasonal averages through next weekend.
With the cold air and upper cyclonic flow in place, many ingredients are in place for stormy weather. However, the current forecast is for the trough configuration to be positively tilted; there aren’t (yet) any identified upstream short wave triggers that might induce low pressure development.
That said, a cold front comes through later Wednesday and there’s some suggestion that low pressure may develop to our south early Thursday. The models have us on the northern fringe of precipitation on Thursday morning and we may see some light rain changing to light snow Thursday.
Another impulse in the Gulf of Mexico may also slide to our south on Saturday. Again, we appear to be on the northern fringe.
In both cases, the currently forecast jet configuration is positively tilted and significant low pressure development near the coast is not currently forecast.
I’ll keep an eye on things. Check back during the week.
Update Sun @ 9:15 PM — Snow is changing over to rain. High winds possible after midnight. Temps wise to near 50 before falling back to the 30s. Rain ends before daybreak. Tonight’s models continue to forecast snow showers between 7AM and 11 AM Monday morning (mentioned in my 12:50 PM update today). Temperatures will be above freezing and little or no additional accumulation is expected.
Update Sun @ 7:15 PM — The current radar precipitation type shows that the transition to rain is lagging about an hour or so from the model forecasts. The latest HRRR and RAP models still show the transition in PHL about 8 PM, but the actual transition time may be later.
The next new major model run is the 00z run. The 00z HRRR, NAM, RAP and HIRESW will be available by 9:15. I’ll update if necessary.
Update Sun @ 12:50 PM — This morning’s high resolution models (HIRESW) show considerable snow shower activity Monday morning with a wet coating possible.
Update Sun @ 8:03 AM — Last night’s 06z models continue with a generally earlier start and transition to rain. Clouds move in between 11 AM and 1 PM. Some snow possible as early as 5 PM, very rapidly changing to a period of freezing rain and then rain by 8 PM in the city and shortly afterwards north and west as temperatures rise above freezing. With heavy rain (1.25-1.5”) late evening, temperatures climb to 47º-50º around midnight. Strong wind gusts about 1 AM approaching 50mph.
Rain ends about 4 AM. Cloudy Monday, windy, temps above freezing in the 30s until evening.
Update Sat @ 9:30 PM — The major model runs are are done at 00z and 12z ( 7 PM & 7 AM EST). Some of the 00z high resolution models have become available. The changes noted in my previous update, with an earlier change to rain, are still expected. Some freezing rain possible about 8 PM.
Temperatures below freezing all day Sunday, will rise into the mid 40s after midnight! About 1.5 inches of RAIN will fall before it ends before daybreak Monday.
Update Sat @ 5:05 PM — This afternoon’s 18z models have become available. The trend is for the system to approach earlier on Sunday. Clouds increase by late morning. Some snow develops as early as 5 PM.
The warm-up also appears to occur 2 hours earlier with a transition to sleet and rain in the city around 8 PM and moving quickly north. By 10 PM, the HREF has mostly rain for our area —
By Monday morning, little if any snow will be left after the heavy rain.
Another slight change. Non-accumulating snow flurries/showers likely Monday morning and again late afternoon.
It looks like the coming week will be colder than average with another chance of sleet or snow on Thursday.
Update Sat @ 10:01 AM — The forecast for Sunday evening’s storm is essentially unchanged from the GFS forecast from this past Wednesday. While we still have plenty of time before Sunday evening, the models have done usually well with this storm forecast so far. There weren’t the usual differences among models we often see with winter storms.
The Sunday night period is within forecast range now of the very high resolution models.
Snow starts at about 5-7 PM from south to north. Changes to rain most immediate suburbs by midnight. Here’s the latest HRRR with the snow-sleet transition line at 10 PM Sunday—
Originally Posted Fri 5:23 PM —
Very cold air will move in due to the flow around a low pressure system (that missed us) in the Atlantic today.
A storm that is has been forecast since Tuesday to form near Virginia early Sunday will move northeastward across our area. We’ll be on the warm side of the storm.
While the model forecasts for this storm have been remarkably consistent, a possibility exists that a more prolonged period of freezing rain will develop in the very far northwestern suburbs Sunday night due to cold air damming. It should all change to rain during the pre-dawn hours.
Embedded in the upper very cold flow is an upper air disturbance. Sunny early morning. Then we’ll have mid-level cloudiness from late morning through much of the afternoon. Very cold. High temperature will be 23.2º ± 2.1º (NBM model Blue Bell, PA). It will be very windy in the morning, but winds should subside in the afternoon.
Originally Posted Fri 5:23 PM —
It will be sunny and very cold Sunday morning. Clouds move in about 1 PM. Snow starts between 5 and 7 PM and quickly mixes with and changes to rain about 10 PM. Up to an inch is possible before changing to rain. (Details have not changed since my earlier forecast.) High temperatures in the 20s during the day will rise to 39º by midnight.
The total QPF (quantity of precipitation falling) is predicted to be on the order of 1.25″ water, but only about 0.2-.0.3″ will fall as snow.
Some areas in the far-western suburbs may have a more extended period of freezing rain.
In the Philadelphia area and immediate surrounding counties, it changes to all rain. If you were to go out and measure the snow at midnight, you’d find maybe 1 inch of snow in the immediate PHL area, but with most of the precipitation expected to fall as rain after midnight, there will be about 0- 1/2 inch of heavy, slushy mess by the morning. Rain ends before daybreak.
Some snow showers possible during the morning. Temperatures do not go below freezing until Monday evening.
Considerable cloudiness Monday. Some snow showers possible late Monday afternoon or evening. Gusty winds in the afternoon.