Central PA Winter 25/26 Discussion and Obs

[pasnownut]

3k ditto.  Small improvment thermally.  Will it add up to much....dunno, but 700/850's are a touch better for both in NAM suite.  

Going out to get wood n chores done.  Hoping to come back to good news.  Reel this one in boys n girls

[MAG5035]

3k NAM does try to bring the snow line back down as the secondary starts to take over, which keeps places like JST and AOO pretty much mostly or all snow but it slots before it really drops back into the Sus Valley. At the same time this model is showing the best comma head over western and northern PA and is probably the snowiest run yet for the max snows, which is exceeding 20” on Kuchera in west central PA. 

Sleet is prolific in the far PA southern tier/LSV and well,  just about everywhere else south of there. This is the kind of setup I feel there’s going to be a zone of multiple inches of sleet somewhere given the depth of the arctic air mass. That widespread? We’ll see. But that’s 2-3” of sleet on top of the 5-7” of snow it puts out in the LSV. 

Snowfall

 

[Mount Joy Snowman]

What's going on with the Canadians?  Did they drink too much again?  On the eve of our biggest event in years and they are a no-show ha.

[Itstrainingtime]

79/72 here in Key West. Just arrived at my airbnb for 2 nights. Can't believe I'm actually here. Overseas Highway...wow. 

The arctic front arrives here tomorrow night with temps getting down to the mid 50s. That doesn't happen every winter.

[Mount Joy Snowman]

Well I'll be damned, the CMC got out of bed.  It's slightly more aggressive with the sleet line but throws out a map that is basically identical to 0z.

[AccuChris]

18z rapid refresh was a slight improvement and the 19z HRRR was a tick colder as well at H17/18. Definitely going to be a nowcast situation as it unfolds


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[canderson]

Roughly what time would MDT flip to sleet according to that NAM (which I think is pretty dialed in)?

Was realy wanting 12” snow but don’t think it’s happening. Oh well. 

[Superstorm]

ICON colder too


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[Blizzard of 93]

The short range 12z Canadians also finally ran & the RGEM & HRDPS look a lot like the Euro, HRRR & GFS.

Every freaking model vs. the NAMs…

[Blizzard of 93]

14 minutes ago, Superstorm said:

ICON colder too


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Good to see…

[mahantango#1]

Bernie Rayno said this morning that he thinks the sleet will have a tough time making inroads on these negative dewpoints. But he said he's been wrong and right before on similar situations. That's his personal opinion.

[Jns2183]

Lessons from Arkansas below. Later I'll do an update for round two and look at Kentucky/Missouri, Ohio

Trust the NBM for the most likely total.
* Use the HRRR to define exactly where the heavy bands will set up (the "where").
* Keep the Euro in the mix to ensure you aren't being "fooled" by a high-res model (like the NAM) that might be missing a major moisture plume.

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[WmsptWx]

I mean, if the NAM scores a win here, they really need to look at halting decommission and supplementing it to where it's more than a short range specialist.

[mahantango#1]

We're probably less than 12 hours from the onset of our first major snowstorm in years. I hope you all enjoy no matter the outcome 

[canderson]

1 minute ago, mahantango#1 said:

We're probably less than 12 hours from the onset of our first major snowstorm in years. I hope you all enjoy no matter the outcome 

Amen! And it’ll stick around a long while afterward too, which is pretty rare overall 

[pawatch]

7 minutes ago, mahantango#1 said:

We're probably less than 12 hours from the onset of our first major snowstorm in years. I hope you all enjoy no matter the outcome 

I just go out when the snow is over and deal with what I got. Tomorrow’s snow I will probably clear a couple times. 

[Superstorm]

GFS slightly better for border counties.


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[Jns2183]

That's the 3k nam, reg nam, observed

The NAM suite’s performance during the Arkansas snowfall event was a case study in systematic failure, characterized by a severe northwest displacement and a critical underestimation of moisture transport. Both the NAM 3km and Parent models shifted the entire storm track 95–110 miles too far northwest, which effectively "erased" the heavy 7.8-inch snow core from Central Arkansas by placing it in a modeled dry slot. While the models were predicting 0.0" for Little Rock and Conway, the actual atmosphere was tapping into a rich Gulf moisture plume that the NAM fundamentally failed to resolve, leading to a massive +7.8-inch error at the storm's most intense point.
The root of this "NAM Gap" was a flawed thermal profile that modeled the 850mb "warm nose" much too aggressively and far north of its verified position. By over-amplifying the ridge ahead of the storm, the NAM physics engine incorrectly predicted that dry air would "eat" the incoming precipitation shield, when in reality, the column stayed cold and saturated enough for high-impact snowfall. For those tracking the storm's progression into the Ohio Valley and Northeast, the takeaway is clear: the NAM’s known dry bias and tendency to over-pump warm air can lead to a 100% miss in the transition zone, making a weighted consensus of the HRRR, Euro, and NBM a far more robust strategy for defining the true snow line.

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[Blizzard of 93]

18z GFS brings the goods again!

[Jns2183]

The statistical distribution of the 18 model forecasts for Harrisburg reveals a tightly clustered consensus with a specific high-end skew, indicating that while a significant winter storm is certain, the exact "boom" potential remains tied to moisture efficiency. The ensemble mean of 12.72 inches serves as the mathematical anchor, but the standard deviation of 3.11 inches highlights a notable disagreement between model families. Specifically, regional models like the NAM and SREF are pulling the distribution toward the 90th percentile of 16.38 inches, whereas global models like the GDPS and ICON remain more conservative near the 11th-inch mark. This spread suggests that the primary forecast challenge is not the track of the storm, but the Snow-to-Liquid Ratio (SLR); models predicting lower totals generally expect a warmer, denser "heart-attack" snow, while the high-end outliers assume a colder, fluffier accumulation.
The implications of this distribution, particularly when viewed through the lens of the Z-score mapping, suggest that the "most likely" outcome is not a single number but a high-confidence corridor. The data shows that 68% of all model guidance (the +/- 1.0 standard deviation range) falls between 9.61 and 15.82 inches, creating a very stable target for infrastructure planning. The fact that the Mean Absolute Deviation (MAE) is a relatively low 2.36 inches indicates that the models are in better agreement than typical for a complex transition-zone event. However, the 100-mile Northwest bias observed in Arkansas—where models like the NAM missed the core entirely—suggests that the statistical "tail" of the distribution should not be ignored. If the storm over-performs its moisture transport as it did upstream, the 17.38-inch mark (+1.5 sigma) is a scientifically plausible outcome rather than just an outlier.
Based on this statistical synthesis, the final forecast range for Harrisburg is set at 11 to 16 inches, with a localized "boom" potential of 18 inches if the higher SLRs verify. This range captures the bulk of the probability density, spanning from the 25th percentile (11.35") to just below the 90th percentile (16.38"). The lower bound is protected by the consistent 1.00"+ QPF (liquid equivalent) seen across almost all guidance, which provides a high floor for accumulation. Conversely, the upper bound is capped by the warm-nose intrusion observed in the regional NAM runs, which currently prevents the consensus from shifting entirely into the 20-inch category.



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[Blizzard of 93]

@Jns2183 this is next level information! Thanks

[Superstorm]

Wish UKMET was considered a good model. Almost all snow even in Lancaster.


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[Blizzard of 93]

Current dew point st MDT is -7

Impressive air mass that should help us.

[Jns2183]

Model Type Combinations Min Snow (in) Max Snow (in) Mean Snow (in)
CAM (Convective) 36 6.20" 19.00" 11.45"
Regional 9 7.11" 20.00" 12.44"
Global 25 9.51" 15.44" 12.23"
Ensemble Mean 24 8.46" 18.09" 13.56"

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[mahantango#1]

3 minutes ago, Blizzard of 93 said:

Current dew point st MDT is -7

Impressive air mass that should help us.

My dp is -0.6

[Jns2183]

[mention=4667]Jns2183[/mention] this is next level information! Thanks

Thank you

The last stats I provided basically was for each model qpf I used all ratios within that type to create a 94'row sensitivity matrix. Basically trying to create an "ensemble" at the expensive of assuming qpf and ratios are independent.

Key Vulnerabilities Identified
CAM Extreme Variance: The CAM group shows the widest volatility, with a 12.8-inch spread. This is driven by the 3kmNAM and RRFS predicting heavy/wet ratios (6.2–6.5), while the HRRR and FV3 predict high ratios (13.4–14.5). If the FV3's QPF (1.31") meets the HRRR's SLR (14.5), Harrisburg would see 19.00".
Regional High-End Potential: The Regional models (NAM, RDPS, RAP) produced the highest theoretical mean of 12.44" and a maximum potential of 20.00" (NAM QPF × RAP SLR). This confirms that if the regional models resolve the moisture axis better than the globals, totals will likely exceed the current consensus.
Global Stability: The Global models are the most "stable," with a relatively narrow 5.9-inch spread. Their consensus (9.5" to 15.4") acts as a high-confidence "floor" for the event.
Ensemble Mean Consensus: The Ensemble Means (SREF, GEFS, etc.) show a heavy lean toward the 13.5" mark, suggesting that the statistical "most likely" outcome is currently hovering just above a foot.

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[Voyager]

1 hour ago, canderson said:

Amen! And it’ll stick around a long while afterward too, which is pretty rare overall 

This.

Many times, big snows come at the end of a cold pattern, and half (or more) is lost to the warmer regime that follows behind after a couple days.

[pawatch]

Definitely interesting high tech calculations.

DP -8°

 

[GrandmasterB]

Thanks everyone for all of the commentary on the 18Z runs. Great last minute trends!  It’s been fun tracking with you fellas! I hope everyone enjoys the storm. Going out to get wood set-up then pouring a drink!

[Mount Joy Snowman]

We have some friends over right now but I plan on being dialed in for the 0z HRRR, maybe my most anticipated model run in years.