Sun 11:07 AM —Forecast Review — The models forecasts for today (and even yesterday) were way off.
Today is illustrative of the reason I include the the NBM standard deviations with high temperatures forecasts. When the temperature forecasts have an incredibly high standard deviation (as they had this weekend), it suggests major lack of agreement among the models and major uncertainty (and likely error) in the forecast.
Saturday, we hit a high of 60º in Philadelphia. The NBM model showed 52º but with an incredibly high sd of ± 7.3º. Even the best performing model had a high of 56-57º.
Sunday, the front came though before daybreak and the showers/flurries came through at and before daybreak. To give an idea of how far off the forecasts were, the latest NBM (13z) from this morning still shows showers at 10 AM, even though it’s sunny.
Speaking of sunny, the forecast was for the sun to break out early afternoon, not mid-morning Totally off.
I can’t confidently account for the model forecast errors and high uncertainty. The “pattern” may truly be undergoing a change with greater ridging in the East. Since some models depend upon ‘continuity’ for their initial conditions; with a change occurring it threw off the forecasts(?)
We’ll have to see if the uncertainty (errors) continue with the Tuesday’s expected snow->sleet->rain storm forecast.
Sunday Light Showers-> Snow Flurries
Posted Saturday 02/28/26 @ 11:28 AM — As forecast yesterday, a sharp cold front will past through Sunday morning. Ahead of the front, light rain showers will develop during the morning. As temperatures chill rapidly in the lower and mid levels of the atmosphere, the rain showers will transition to snow showers and depending upon the dynamics, some snow squalls. Little to no accumulation near the city, but upper Montco and Bucks counties may see a coating, especially on grassy surfaces.
This morning’s 12zRRFS Coloration is conditional probability of snow/rain/freezing rain/sleet. This is NOT a radar image of coverage; Conditional probability means that IF it’s precipitating in the colorized area, this is the precipitation type to expect. Contours represent isotherms – 32º at specific levels of the atmosphere. Notice that it may snow where the temperature near the ground is above 32º GREEN = RAIN, VIOLET = SNOW, RED = FREEZING RAIN, ORANGE = SLEET (Click on image for a larger view.)
The latest GFS shows temperatures near the ground falling rapidly to 32º; the RRFS is slower with the temperature descent. Clouds break between 1 and 3 PM.
Originally Posted Fri 7:57 PM —A southwesterly flow of milder air will allow temperatures on Saturday to rise to levels not seen in awhile, although the trend today is for highs to be lower than previously advertised. A strong cold front moves through on Sunday morning .
Saturday Forecast
Some clouds in the early morning will break for sunshine for most of the day. Winds will be light. There remains extremely large spreads in the forecast high temperatures.
NBM high temperatures: Blue Bell, PA 49º Philadelphia, PA 51º Unusually high uncertainty (based on standard deviation): ± 7º
Sunday Forecast
Cloudy in the morning. The RRFS and other models are forecasting showers moving through between 8 AM and noon, with the heaviest activity north of the immediate Philadelphia area. Clouds clear for increasing sunshine through the afternoon. Strong winds and gusts near 30 mph after the front.
Today’s 18z RRFS is the most robust with the showers Sunday morning. (Click on image for a larger view.)
Temperatures in the mid 40s during the morning will fall into the 30s by 5 PM.
An interesting system will affect our area late Monday through Wednesday. Updates over the weekend when things clarify.
Posted Friday 02/27/26 @ 9:15 AM — Today, Friday, expect plenty of sunshine and highs 46º-49º (again a large standard deviation.) The trend is for even more widespread warmer temperatures on Saturday and highs reaching 57º-58º.
A strong cold front moves through Sunday morning. Most models keep showers associated with this front to our north and northeast, but some models have some sprinkles just grazing the city during the morning hours. Temperatures start to drop through the 30s by early afternoon with gusty winds.
Still uncertainty with the Monday through Wednesday time frame. The GFS has no snow for Monday, the ECMWF-AI has some very light snow developing. Most models have a transition to sleet freezing rain and then all rain through Tuesday into Wednesday. It does look like it all ends as all rain and perhaps a good soaking.
The latest (06z) ECMWF-AI forecast for Monday at 1 PM. Light snow. The GFS has this precipitation to our south. (Click on image for a larger view.)
Good news! The first 7-10 days of March look to be much milder, but perhaps with plenty of clouds and rain.
Updates later this afternoon in my regular “Weekend Weather Forecast”.
Friday and The Weekend Outlook
Posted Thursday 02/26/26 @ 8:00 PM —Enjoy the next two days because the period from Sunday into next Wednesday suggests colder weather and an uncertain range of storminess.
For Friday, highs reach the mid to upper 40s with plenty of sunshine. For Saturday, highs reach at least 50º with some models showing highs near 57º, again with plenty of sunshine and occasional clouds.
(It should be noted that the model blend (NBM) shows a high of 50º but with an incredibly large standard deviation of ± 7º. That standard deviation is as large a spread in temperature forecasts as anything I’ve seen, suggesting that various models show very different highs. )
That same large model spread in high temperatures heralds the large uncertainty and spread in model forecasts from Sunday through next Wednesday.
A strong cold front moves through Sunday morning. Some models have rain or snow showers accompanying that front, some have them falling apart as they approach Philadelphia. Temperatures drop during the day
As for the period next Monday through Wednesday, everything from several inches of snow to heavy rain is being forecast with little model agreement. It promises to be interesting.
18z ECMWF-AI forecast for Saturday Temps at 1 PM. Actual high temps may be greater at 3 PM. (Click on image for a larger view.)
Thursday Forecast Update & Apple Weather App
Here’s the NWS response to my inquiry about the radar artifacts affecting Apple Weather—
“There is an issue with the ground clutter suppression map for the radar. Our technicians need to get out to the radar to correct it. However, a tree fell across the access road during the blizzard on Monday and thus they have been unable to access it to correct the issue. We are hoping that we will be able to get out there today.”
Posted Thursday 02/26/26 @ 8:41 AM — The GFS has joined the other models in having the precipitation move to our south. The closest any model has to having some light snow or light rain is the Canadian High Resolution (HRDPS) which is also an outlier and likely too far north with its precipitation. So mostly cloudy today.
A complicated and inconsistent forecast is unfolding for the period Sunday through Wednesday with some models showing snow, some a mix, and each with very different timing. There’s an unusual lack of agreement in this forecast period. I’m keeping an eye on it.
Finally, for iPhone users, perhaps you noticed that early this morning, the Apple Weather app was reporting “heavy snow in the next hour”. The Philadelphia area radar (KDIX) located in Fort Dix, NJ, was inoperational about a week ago and has since had bouts of strange echos, being misinterpreted as heavy precipitation (snow today) . I’ve observed this several times in recent days.
These radar echos are NOT what they suggest and are being misinterpreted as “heavy snow” this morning in the Apple Weather app. (Click on image for a larger view.)
I’m sure the Mount Holly Weather Office is aware of this, but I’ve contacted them this morning.
Cancel That Snow for Philadelphia
Posted Wednesday 02/25/26 @ 4:13 PM — This morning and this afternoon’s early models overwhelmingly show the precipitation with Thursday’s low pressure system staying to our south, with little to any snow in our neck of the woods.
The NBM mean shows a scattered coating of snow, but the median shows zero. The NAM shows minimal near Philadelphia. The HRRR shows zero.
This morning’s GFS (posted earlier, below) still showed some snow accumulation. This afternoon’s GFS keeps the snow just south of the and outside the immediate Philadelphia area (too warm for snow) —
This afternoons’ 18z GFS calculated snow accumulation by 7 PM Thursday. Where it doesn’t snow, there may be some light rain. Line contours are 1 inch increments (labeled). (Click on image for a larger view.)
With this latest GFS, it remains an outlier, with minimal support from only the Canadian HRDPS. Many models show the precipitation (as rain) much further south.
Thursday Snow Outlook
Posted Wednesday 02/25/26 @ 9:48 AM — The most recent model runs continue with the GFS still predicting snow and the ECMWF AI having moved a bit north from its southern track. The regular ECMWF is still far south, only affecting Cape May with some rain.
So, like last week, the GFS has some snow for us, the ECMWF doesn’t. Since the time horizon is only about 40 hours, we can’t ignore the lack of consensus. Only the next model runs with clarify this.
Here’s the GFS worst case scenario for snow here—
GFS accumulated snow by Thursday evening. My own algorithm. Likely too high by 1 inch overall. (Click on image for a larger view.)
The above is likely too high by about 1 inch. We’ll need some additional model runs to clarify. Updates later.
Wednesday-Thursday Outlook
Posted Tuesday 02/24/26 @ 5:20 PM — There is a lack of model consensus in the forecasts for Wednesday and Thursday. Most models have no snow to a dusting of snow either before daybreak Wednesday or into the morning. Some show as much as an inch of snow. It’s not clear which is correct, but the latest NBM median is similar to the snow forecast shown below for the RRFS. (There is a low possibility that we could see as much as an inch by late morning. Again not likely.)
Things are even more uncertain as we get to the system expected to affect our area later Thursday. The European models (ECMWF-AI ECMWF) now show the low moving well to our south, missing us.
The latest GFS maintains the track of yesterday’s AI models with what appears to be light to moderate snow for us. (The NBM is showing little to no snowfall similar to the ECMWF.)
18z GFS forecast for Thursday- snow accumulation. (Click on image for a larger view.)
I’m presenting the GFS because it did so well with our recent ‘almost Blizzard’. Unfortunately, there’s too much spread in the models to be sure of a forecast at this time. Stay tuned.
Tuesday-Wednesday Outlook
Posted Tuesday 02/24/26 @ 9:45 AM — Low pressure will move to our north, and an associated weak warm front will bring the chance of very light snow to our area Wednesday morning.
Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. (Click on image for a larger view.)
Temperatures today, Tuesday, will range from 32º Blue Bell to 35º Philadelphia with sunshine and windy conditions.
For Wednesday very light snow in the morning. Cloudy much of the day, clearing late afternoon. Highs in the mid 40s.
RRFS snow totals for Wednesday’s weak system (Click on image for a larger view.)
Another stronger system moves in on Thursday. There are temperature differences between the GFS and ECMWF-AI forecasts which will affect the amount of snow (if any). Updates when things clarify.
Originally Posted Mon 8:27 PM —Another active week, weather-wise, as two systems will affect our area. The first moves through Tuesday night into Wednesday morning. Some light rain or wet snow is possible with this system. A relatively minor accumulation is expected if anything at all.
ECMWF forecast for Wednesday at 7 AM light rain/or snow (especially north of the city) (Click on image for a larger view.)
Another system moves up from the central US on Thursday. We’ll be on the snow-rain boundary with this system, but mostly rain from Philadelphia and south to possibly wet snow especially to the far north—
18z ECMWF-AI forecast for Thursday at 1 PM. Yellow, Red, Magenta lines show the boundary line for rain-snow. We’ll also be above freezing for much of the time. (Click on image for a larger view.)
Mon 5:00 PM —Forecast Review — That was quite a storm with heavy snow and accumulations, as always dependent on location. The NWS has a web page for what’s considered semi-official snow accumulation totals. Here’s a sample—
Was the snow storm really a ‘Blizzard” as defined by NOAA? A review of the NWS KPHL website shows only about 1 hour visibility of 0.25 mile or less, instead of the defined 3 hours. Wind gusts were not sustained at over 35 mph for 3 hours either. So, while the storm will likely be remembered as the “Blizzard of 2026”, it really wasn’t. (Areas near the shore likely qualified for the blizzard designation.)
A few things about the models. Timing the transition from rain to snow was very good. Perfect snow total predictions are never achieved, just as rainfall is never exactly as forecast.
It should be remembered that the GFS model did much better than the ECMWF with this storm, for days in advance. The ECMWF-AI did much better than the regular ECMWF. The Canadian models were under-performers as was the German ICON model. The new RRFS, waiting to be released, was very disappointing, especially with its forecast at 60 hours and 84 hours in advance. However, the 24 hour snow forecast was very good.
The old NAM did much better than the RRFS at 60 to 84 hours. The NBM median was pretty good, maybe a tad high in some areas.
Snow Tapering Off
Posted Monday 02/23/26 @ 9:51 AM — The storm is well to our east and the vertical motion (Omega) near and west of Philadelphia has turned to a downward motion. Snow continues to rotate around the low into the city from the northeast along the 700 mb (~10,000 ft) wind direction but as it meets downward vertical motion, it should taper off. Additional scatted snow showers still possible into early afternoon near the city.
Radar and MRMS radar at 9:37 AM with superimposed RAP model 700 mb wind streams (green and green arrows) and Omega (yellow contours and yellow labels) Negative indicates downward motion.
Additional snowfall from the 13z NBM (8AM) run is minimal west of the city.
13z NBM Additional Median snowfall accumulation from 8 AM through 1 PM. Line contours are 1 inch increments (labeled). (Click on image for a larger view.)
I’ll do a wrap up and forecast review later today.
Additional Snowfall
Posted Sunday 02/22/26 @ 10:27 PM — Despite the afternoon’s models suggesting a more eastward track, the latest model blend (NBM) shows significant additional snow from 8 PM through 4 PM. (In the city, snow tapers to just snow showers/flurries by 12 PM)
01z NBM median snowfall (additional) from 8 PM to 4 PM Monday. Line contours are 1 inch increments (labeled). (Click on image for a larger view.)
If this holds, the NBM mean snowfall (appeared too high) did the best at predicting this storm’s snowfall.
Updates Monday morning.
Storm Update
Posted Sunday 02/22/26 @ 5:58 PM — Folks, I violated my own cardinal rule- “never look at the new model runs when the storm is in progress”.
The reason: it always changes and you never know if it’s a dramatic change in the forecast or if it’s a model “spin-up” error. Put another way, the latest models are not often not the best once a storm is in progress. I mentioned this aspect this this morning.
So, I looked at the latest GFS and RRFS models (18z) , and I shouldn’t have been surprised that the predicted snow amounts are significantly less. Is this a real change? Or a spin-up error?
(Models literally re-create the atmosphere mathematically; they often have a multi-hour “spin-up” time for the model to come close to matching the atmosphere.)
Anyhow, here’s the latest RRFS, which has cut back the snow totals. (The GFS has gone even lower to only 6 inches in Philly.)
18z RRFS has cut the totals significantly near the city and westward. Is this a real change in the forecast?? (Click on image for a larger view.)
Snowstorm Update
Posted Sunday 02/22/26 @ 3:51 PM — As forecast, it started snowing between 3 and 4 PM in many areas. Near ground temperatures are still above freezing, according to the RTMA, (however the hourly RAP model indicates that temperatures about 800 feet above ground are at or below 32º) —
3:30 PM RTMA analysis Most areas are having snow but are above freezing at this time. Line contours are 1 degree increments (labeled). (Click on image for a larger view.)
The 1 PM (18z) HRRR has chimed in with its snow forecast and it remains in the range I’ve been supporting—
18z HRRR snow accumulation -snow depth by 1 PM Monday. (Click on image for a larger view.)
Last but not least, the latest NBM run with updated precipitation (19z) is available and the median snowfall has increased from the last run. I’m still placing bets on the median snowfall. (hard to believe, but the mean is even greater)
19z NBM with updated precipitation stats. Median snowfall (Median = Half the models comprising this ensemble are showing more than these values, half are showing less.)
Sunday Storm Update
Posted Sunday 02/22/26 @ 11:50 AM — The latest RRFS finally caught up with the rest of the models regarding snow accumulation—
Today’s 12z RRFS – Snow accumulation. Calculated values. Line contours are 1 inch increments (labeled). (Click on image for a larger view.)
Additionally, a newer run of the NBM (13z) showed somewhat higher median snow totals than what was posted earlier.
No real change in the timeline. Snow starts accumulating between 2 and 4 PM RRFS model—
Snow accumulation time line for Blue Bell PA (Click on image for a larger view.)
Winds
Wind Meteogram for Blue Bell PA Further east, winds and gusts approach 50 mph. (Click on image for a larger view.)
Sunday Storm Update
Posted Sunday 02/22/26 @ 8:28 AM — The models show little change from last night, perhaps a bit higher snow totals. The ECMWF’s low pressure position finally caught up with the position the GFS was forecasting all week. (Who says the ECMWF is the Mercedes and our GFS is a Chevy ?) Here’s the current position of the low—
Water Vapor image at ~ 8 AM with superimposed RAP model MSL pressure (black contours). and GFS & NAM potential vorticity contours (violet and yellow fine contours) When the purple and yellow lines (potential vorticity) meet up with the coastal low, deep intensification will occur. (Click on image for a larger view.)
Time line is similar to that posted yesterday. Current rain or rain/snow mix changes more to snow around noon or 1 PM. Then all snow by 3-4 PM in the city and westward. Accumulations start about 4 PM or so. The storm tapers and ends around 11 AM- noon Monday. Some additional snow showers possible in the afternoon.
Here are some of the latest snow accumulation totals—
NAM built-in algorithm— (Generally too low)
Today’s 06z NAM snow depth totals as of 11 AM Monday Line contours are 1 inch increments (labeled). (Click on image for a larger view.)
The NBM median and mean (implying higher certainty) are reasonably close together and I’m going with the median snow accumulation below (the mean values are even higher!) —
This morning’s 12z NBM median snow accumulation (calculated) Line contours are 1 inch increments (labeled). (Click on image for a larger view.)
The GFS built-in snow depth values are about 1 inch less than the the NAM shown above.
Here’s my “experimental” values calculated on GFS data of precipitation and temperatures at 4 levels and water-snow ratio; they are similar to the NBM median—
Experimental GFS calculated snow totals. I’m happy to see they’re not that much different than the NBM median values. Line contours are 1 inch increments (labeled). (Click on image for a larger view.)
At this point, with consistent model forecasts, I’m not inclined to change my forecast. Any newer models today may not have the proper ‘spin-up’ time to capture the atmosphere.
I know my snow forecast with this storm is different and higher than forecast by the TV forecasts and the NWS.
Interestingly, the RRFS and REFS (expected to become operational soon) are in the 8-10″ range for the city. I’m somewhat disappointed that both models are slow to pickup this storm at 60 and 84 hours and are perhaps forecasting snow totals. that are too low, especially the RRFS.
I guess we’ll see what happens.
I’ll update during the day as needed.
Final Storm Thoughts for this Evening
Updated Saturday 02/21/26 @ 11:40 PM —Based on tonight’s models, I’m thinking the TV forecasts of 8-12 inches of snow near the city may be too low. I’m staying with the NBM median and even my experimental forecast below.
Posted Saturday 02/21/26 @ 8:51 PM — The snow totals forecast I heard on TV this evening was 8-12 inches. A reasonable number, and depending on how much precipitation falls before 4 PM may make that the correct forecast. However, the latest NBM still shows higher median snow totals as shown earlier.
My GFS snow experimental forecast is much higher and may very well be wrong, especially if much precipitation falls before 4 PM. But here’s the latest in what I’m calling ‘experimental’ forecast based on GFS data —
GFS snow forecast (experimental) based on temperature at four levels and snow water ratio of 10:1 Line contours are 1 inch increments (labeled). (Click on image for a larger view.)
My forecast above isn’t very different than the latest NBM median—
Tonight’s 00z NBM Median snow total forecast. Line contours are 1 inch increments (labeled). (Click on image for a larger view.)
The real time weather analysis (RTMA) shows low pressure just beginning to form off the coast, as picked up by 3 hour barometric change—
8 PM (01z) RTMA shows negative barometric change occurring as forecast off the coast indicating the beginning of low pressure formation. (Click on image for a larger view.)
Final thought for tonight. Snow during this period in February melts quickly. Sun angle is high and even with clouds, radiant insolation is strong. It’s not expected to be frigid afterwards like after the last storm.
Sunday Snowstorm Update
Posted Saturday 02/21/26 @ 4:46 PM — Let me address a few things that were not given attention to.
Current water vapor shows the storm beginning to form near SC coast—
Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. (Click on image for a larger view.)
Precipitation starts as either light rain or light snow or a mix as early as 5 AM Sunday near the city.
Despite falling as snow by 1-3 PM, little accumulation occurs until 4 PM
Winds pick up about the same time. Winds 20 mph gusting to 40 mph.
Precipitation ends around 12-1 PM Monday, later somewhat in eastern NJ.
Snow accumulation Meteogram for Blue Bell PA. While little snow accumulates before 4 PM, it will be a rain snow mix as early as 4 AM. Snow ends about 1 PM
The latest NBM (19z) median snow totals are below. The mean totals are much higher. I’m still leaning towards these values.
19z NBM mean snow accumulation totals (calculated) (Click on image for a larger view.)
Where could these totals go wrong? The RRFS has a good portion of the precipitation falling during Sunday but not accumulating. Is it a model error or does it know something the other models don’t? Here’s the RRFS time line—
RRFS has a good portion of the precipitation falling before temperatures allow for accumulation. As a result, it’s prediction is for 5 inches of snow in Blue Bell. It is correct? (Click on image for a larger view.)
I’ll update this evening if things change.
Yet Another Storm Update
Posted Saturday 02/21/26 @ 11:41 AM — The latest GFS just became available. Here’s its built-in snow depth parameter (which I find to underestimate snow accumulations)—
12z GFS snow depth accumulation which I find to underestimate snowfalls. Line contours are 1 inch increments (labeled). (Click on image for a larger view.)
Here’s what I’m calling my “experimental” snow accumulation estimate, using my own algorithm calculations on GFS data—
12z GFS with my own snow algorithm. Maybe it’s not ready for prime-time, but it seems closer to the NAM and HRRR models of the same model run. Line contours are 1 inch increments (labeled). (Click on image for a larger view.)
Updates later this afternoon. Check back.
Winter Storm Update
Posted Saturday 02/21/26 @ 10:17 AM — Additional models have become available. A picture is worth a thousand words—
13z NBM median snow totals. Line contours are 1 inch increments (labeled). (Click on image for a larger view.)12z NAM snow depth forecast by Monday. Line contours are 1 inch increments (labeled). (Click on image for a larger view.)
Timing update: Light rain and or snow begins as early as 4 AM Sunday. By 9 AM, the latest RRFS shows this precipitation type:
12z RRFS with precipitation type and temperatures at 9 AM. (Click on image for a larger view.)
Precipitation falling as snow does not equate to accumulating snow. Temperatures above freezing with high sun angle and insolation through clouds in late February will likely reduce or eliminate snow accumulation before late afternoon or evening in many areas.
Updates later. Stay tuned.
Winter Storm Update
Posted Saturday 02/21/26 @ 9:24 AM — The models still show some differences in snow totals. What had looked like a uniform move to the higher end last night has splintered again into the lower totaling ECMWF/ECMWF-AI and the NOAA models GFS/NAM/HRRR which are forecasting higher amounts. (For reasons to be determined, the new still unreleased RRFS is in the low end of about 5-7″.) The latest NAM, NBM and others will be available over the next hour or two.
Much, but not all of the storm, now falls into the range of the high resolution HRRR.
The latest HRRR (whose forecasts only extend to 48 hours) includes a part of the storm up to 7AM Monday—
12z HRRR snow depth forecast by Monday at 7AM NOTE that the storm will last longer than covered by the current HRRR, meaning snow totals will be higher than shown. Line contours are 1 inch increments. (Click on image for a larger view.)
I’ll be making additional updates before noon. Stay tuned.
Storm Update
Posted Friday 02/20/26 @ 10:15 PM — A quick update. Tonight’s models have moved towards higher snow totals. The NBM median is almost as high as the mean.
01z NBM median snow totals through Monday. Line contours are 1 inch increments (Click on image for a larger view.)
The latest NAM is even higher. Updates Saturday morning.
I thought I’d share this cool, off the beaten path website with readers. From Iowa State University, it draws snow accumulation meteograms for Philadelphia from the latest GFS and NAM data. Includes algorithms for melting and compaction along with water snow ratios— https://www.meteor.iastate.edu/~ckarsten/bufkit/image_loader.phtml?site=kphl
Experimental
Posted Friday 02/20/26 @ 5:51 PM — With the soon to be retired NAM, I’ve been working on a snow algorithm with considerable help from Anthropic’s Claude AI that uses my snow forecasting technique derived from the old NAM FOUS data that served me so well years ago. I’ve applied it to the GFS model data. Probably not ready for prime time, but here’s the latest GFS model snow accumulation forecast based on this very experimental algorithm—
18z GFS data with my experimental algorithm for snow accumulation prediction. It might NOT be correct. We’ll find out. (Click on image for a larger view.)
Major Snowstorm Increasingly Likely
Posted Friday 02/20/26 @ 5:18 PM —The models have come and they are all moving towards a major snowstorm for the Delaware Valley and NJ late Sunday into Monday. Details about the timing are below.
First, as described earlier, I’ve resurrected my favorite model for snowstorms in Philadelphia. This will likely be its last snow forecast for our region, since the model will be retired sometime this year when the RRFS becomes operational. Here’s the NAM snow forecast—
18z NAM built in snow depth algorithm. As high as these values are, they might be too low! (Click on image for a larger view.)
I’ll get more into the details as the weekend progresses, but I’m going to cut to the chase and post the latest Model Blend Mean and Median foreasts—
19z NBM mean snow accumulation forecast by 1 PM Monday. Line contours are 1 inch increments (Click on image for a larger view.)
Here’s the NBM median forecast. I’ve been leaning towards the median forecast, but I think it may be too low. Somewhere between the median and mean sounds about right.
18z NBM median snow forecast through 2 PM Monday. Line contours are 1 inch increments (Click on image for a larger view.)
My regular Weekend Weather Forecast is below:
Major Snowstorm Increasingly Likely
Originally Posted Fri 4:55 PM — This weekend’s weather will start tranquil and will likely end much less so.
Here’s the setup as shown in current water vapor imagery—
Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. On Saturday, we’ll be in the mild orange area to our west. Low pressure in Colorado will move into an area along the southeast coast by Sunday and intensify as it moves up along the coast. At the same time, cold air will be plunging to our immediate west. (Click on image for a larger view.)
Saturday Forecast
Clouds early morning then breaking for sun late morning. Cloudiness returns early afternoon in most areas except southern Chester/Delaware counties. Breezy winds 8-16 mph will make temperatures feel closer to 40º.
NBM high temperatures: Blue Bell, PA 47º Philadelphia, PA 49º Higher than Average Uncertainty (based on standard deviation): ± 2.4º
Sunday Forecast
Major Snow Storm Likely for our Region
Light precipitation moves in during the morning hours. A mix of snow and rain possible and and where it snows, there will be little accumulation until evening. It will snow despite ground temperatures above freezing because just 1500 feet above ground, temperatures fall off rapidly below freezing.
Near ground temperatures drop towards evening as the storm picks up in intensity. Strong winds and heavy snow likely. Updates through the weekend. Snow tapers off late Monday morning.
NBM high temperatures: Blue Bell, PA 34º Philadelphia, PA 41º Average Uncertainty (based on standard deviation): ± 1.4º
