December 15-16, 2024 | Cold Air Damming (CAD) Event

[Penn State]

Over the past few days, a cold air damming (CAD) event has become more likely for Sunday into Monday. Stubborn cold will hold-on in parts of the Pennsylvania, Maryland, West Virginia, and Virginia, likely leading to that area’s first light to moderate snowfall. This event has drawn some attention in other threads, and this provides a home to document and discuss the potential. 

Happy Tracking 🤓

[JDClapper]

CTP with a good write up

.SHORT TERM /6 PM THIS EVENING THROUGH MONDAY/... Focus tonight into Sunday morning is on approaching 5h trough swinging into the high plains Saturday and middle to upper Mississippi River Valley tonight and to the lower central GLAKS on Sunday. Strong easterly ageostrophic flow around anomalous arctic high pressure over the Gulf of Maine and and weak sfc trough with approaching system on Sunday means that cold air from the retreating arctic high will be very difficult to dislodge. CAD entrenched along and to the east of the Allegheny Mtns will set the stage for overrunning mixed precip Sunday into Sunday night. Model trends indicate S/IP as most likely Ptype well east of the Alleghenies with a good signal for fzra/icing closer to the Alleghenies. Confidence was sufficient for near warning criteria fzra over the Laurel Highlands to hoist a Winter Storm Watch for .25"+ potential ice amounts, with Advisories stretching northward through Clearfield, Elk and Cameron Counties (coordinated with PBZ).

 

The depth of the cold air will be a major factor in determining the amount of frozen (snow and sleet) versus liquid (freezing rain and rain) precipitation. The bulk of the wintry mix and snow/ice accumulation should stay along and to the northwest of I81. As far as snow accumulations go, Advisory criteria snow/sleet is looking increasingly likely over parts of the Central Mountains, Ridge and Valley Region, and up towards the Endless Mountains region of northeast PA. It appears that hourly snowfall rates of .25" to .5" at times Sunday afternoon and evening bring the best chances for 2-4 inches of snow to an area stretching from State College to Lewisburg, Bloomsburg to Lock Haven and back to Philipsburg, with 1 to 2.5" amounts common across the remainder of central and north central PA (south of Rt 6). Amounts drop off south of I-81, and along the Alleghenies where ptypes are more likely to be freezing rain and sleet.

 

Max Wet Bulbs in the 1000-500 mb layer don`t get above 0C during the event to the east and north of a line from KELZ to KUNV and KSEG/KHZL, and this area is targeted/coinciding with the highest precip rates beneath the strongest upper level diffluence and left exit region of a 90+ kt upper jetlet on the south side of the compact shortwave moving just to our NW. As is typical in strong CAD regimes, models may be too bullish on transition to rain late Sunday night/early Monday, and some mix may survive into the daylight hours Monday before ending. Increasing WAA and southwest flow should warm the llvls sufficiently for rain to be the dominant ptype by Monday afternoon.

 

Expect somewhat of a lull in precip Monday afternoon into Monday night with low clouds, fog and drizzle. Steadier rain returns overnight Monday.

[JDClapper]

0z EPS all-in on accumulating snow in the Williamsport area.  CTP suggested we're in a good spot for 2-4".

 

[JDClapper]

[Jon Snow]

Thanks for the thread, its always nice to see the discussions here.  My area is under a WWA for the morning, snow/sleet/freezing rain to start. 

[JDClapper]

9z SREFs are in .. they have been leaning on the warmer side of things, VS EPS.

Some trending of 24 hr snow through 8am Mon, chance of 1"+ and 4"+

 

[geeter1]

If the GFS is to be believed, our area is still in the frozen area at 6:00 AM Monday morning. Could 

make for a slippery commute... 😎

[JDClapper]

1 hour ago, geeter1 said:

If the GFS is to be believed, our area is still in the frozen area at 6:00 AM Monday morning. Could 

make for a slippery commute... 😎

Yeah, the CAD is putting up a fight, according to 12z NAM/HRRR .. still 32 dp/temp up this way as late as like 4-6am Monday.  NAM does show a sneaky warmer layer at 925mb after midnight, so could see some sleet mixing in or grainy snow.  QPF amounts in the 0.3" range, so that 2-4" that CTP mentioned has support from NAM/HRRR at 12z.

[JDClapper]

3k NAM makes the precip field look like it could be showery, but HRRR (caution!) shows a better precip shield.

[geeter1]

Still frozen precip at 4:00 AM on Monday I-78 and north...

[Penn State]

CTP Snowfall Forecast.. I-81 corridor, and points NW. Depends on what temperatures look like tomorrow, but if we can start as snow down here in Franklin County, we can hold it for a while in these CAD events. Certainly think 1-2” is possible. 

[geeter1]

Our local professional weather sites opinion for tomorrow night into Monday. @Penn State, looks like you

are close to cashing in... 😎

[JDClapper]

15z SREF is in.. honing in and tightening up.  Yep, pretty much a 1-3" dealeo in Central PA, spotty 4"'s.

Trends of chance of 4"+, 1"+ and mean snowfall.
 

[Penn State]

29 minutes ago, geeter1 said:

Our local professional weather sites opinion for tomorrow night into Monday. @Penn State, looks like you

are close to cashing in... 😎

I’m hoping for as early an onset as possible with good rates.. if we start at Noon or before.. and it’s relatively clear tonight.. with clouds rolling in during the AM.. temp should settle right there around 32. Then.. with rates.. should keep it for a bit. 

[JDClapper]

9 minutes ago, Penn State said:

I’m hoping for as early an onset as possible with good rates.. if we start at Noon or before.. and it’s relatively clear tonight.. with clouds rolling in during the AM.. temp should settle right there around 32. Then.. with rates.. should keep it for a bit. 

Per 18z HRRR.. cloud cover in full effect at 7am.. and temps look tasty.

Edited December 14, 2024 by JDClapper

[JDClapper]

CTP forecast hot off the press.

[Penn State]

Winter Weather Advisory just issued for us in Franklin County.

[JDClapper]

18z HRRR .. I like it.  Hope it's right.

[jbrumberg]

NWS/B-N Office discussion for this timeframe:

Spoiler

* Wintry mix late Sunday Night

More of the same to start Sunday. Large high pressure shifts
farther offshore, but still remains in control. The forecast for
late Sunday night is complicated, and the details remain
uncertain. These details will be crucial, as a subtle shift of a
single degree in temperature will make a difference in
precipitation type. Add in the timing errors for precipitation
at all, and the result is a complicated forecast. The latest
trend in the guidance is for a slower arrival timing, which
makes sense given the large high pressure that should be just to
our east. Even by daybreak Monday, surface MSLP should be 1035+
mb. Thus, it may be difficult to generate precipitation at all.
Will trend the forecast to a slower timing, but further timing
adjustments may be needed for Sunday night.

In general for this update, thinking snow is the most likely
type through about 2 AM, where it precipitates at all. Then
a warming trend begins higher up in the atmosphere. Differences
in the guidance, with the GFS holding onto a colder solution in
the idea of a weaker warm nose over a greater depth when
compared to the NAM. The NAM had a shallower warm nose, but it
was more potent. Both of these scenarios do depict the same
pattern, one of a transition from snow to rain, with a period
of freezing rain some time in between. Do not have enough
confidence for a Winter Weather Advisory at this time. However,
one may be issued with later forecasts as confidence in the
timing and location of freezing rain increases.

Increasing clouds during the afternoon, with even more clouds
Sunday night. Below normal temperatures Sunday, ten trending
closer to normal Sunday night.

 

And the related snow graphic🤣:

[JDClapper]

18z 3k NAM is decent too.  Brings the steadiest batch in earlier than HRRR.  Has that showery look after the initial band.

 

[RTC3-LAST CHANCE]

That you for opening guys, and Clap , looking like an inland track snow setup again, which favors Port area. Good for you. Snow begets snow thang. No jinxing, but a setup does tend to have a memory 👍

[Penn State]

18z GFS looking decent 

[losetoa6turbo]

Some short range guidance gives us along the m/d line a possible brief coating . I think the hrdps also did.

3knam 

Edited December 14, 2024 by losetoa6turbo

[losetoa6turbo]

Latest Hrdps

[losetoa6turbo]

Latest from NWS Sterling 

Predicting precip type for this system has been particularly
challenging, as warm air advection is fighting the CAD wedge
from the persistent, strong high pressure to the north. Precip
type is dependent on the surface temp in relation to a dynamic
warm nose aloft. Initially 850mb temps against the Blue Ridge
seem to linger just above freezing in the early morning, then
drops below freezing from CAA, before eventually returning to
above freezing as WAA finally beats out the CAA. At the surface,
temps east of the Blue Ridge will be allowed to rise above
freezing by mid-morning, but cold air will become trapped
between the Alleghenies and the Blue Ridge, retaining sub-
friezing temps.