December 7-8 Rain/Snow over Northeast...

By: Zachary Labe , 2:13 PM GMT on December 06, 2011

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Current teleconnections and wavelengths continue to support a very mild pattern for much of the eastern CONUS over the next 2-4 weeks at the minimum. But there are still remain chances of snow in some areas given the right setup. A cold front will move through the region as a shortwave develops and moves along the front. Cold air advection will transition some areas from rain to heavy snow, but it will be a close battle for temperatures as a 1-2 degree difference will make this a very difficult forecast.

Thoughts on December 7-8 Rain/Snow Event
A strong cold front is slowly moving across the Northeast with several shortwaves moving up along the boundary. The first wave is moving through Tuesday night through Wednesday morning with a swath of moderate to heavy rain. As the front slowly moves east, cold air advection will filter cooler air into the region as temperatures fall into the mid to upper 30s by dusk. Rainfall amounts will range from 0.5in to 1in for most areas south of interstate 80. It will be a race for timing as precipitation begins to move back into the region by Wednesday evening and colder air begins to filter in from the northwest. The strength of the cold air remains weak with the lack of anticyclone to the north and absent blocking upstream. H85s will drop below 0C as far south as the Pennsylvania turnpike by Wednesday evening along with 1000-500mb thicknesses around 538dm.

A heavy axis of deformation precipitation will wrap around the second shortwave Wednesday night with several convective indices indicating high QPF rates. Impressive dynamics with weak cold air advection should transition the rain to snow from the northwest to the southeast Wednesday night across Maryland and Pennsylvania up to New Jersey. Heavy snow will fall at the rate of 1-3in/hr for a short 2-3 hour period. 2m temperatures will remain marginal and hover possibly above freezing for a duration of the event. Given warm and wet ground conditions and marginal temperatures, accumulations will be limited at first until snow rates become higher. Snow ratios will be limited to 10:1 for most areas, but higher ratios are likely during heavier rates with excellent dendritic growth zones being indicated on SKEW T charts. Most of the accumulation for the Middle Atlantic will occur during the overnight hours after rush hour and before the morning commute. This will limit impacts.

It is possible given current wavelengths that this low pressure continues to trend northwest. Given this potential my forecast for snow accumulations has been given a slight warmer bias. It is possible my accumulations may be too heavy given the marginal temperatures. But if rates are heavy enough there is the possibility for higher amounts with localized locations nearing a foot of snow. This will likely be limited to the higher elevations above 1000ft which have the best chance at winter storm warning criteria. This is a very difficult forecast and is very similar to the October event for snow axis placement. Marginal temperatures continue to play the biggest factor in the forecat. There will be no sleet or freezing rain in this event given the extent of the boundary layer warmth.

Most I-95 locations will stay primarily rain through the event. For areas towards New England a similar progression of rain to heavy snow is likely Wednesday morning into early afternoon and will be very similar to the Middle Atlantic. Most of the snow will be west of the coastal plain especially towards the higher elevations of western Massachusetts and northwestern Connecticut.

If the cold air advection filters in time, it is likely the Lower Susquehanna Valley along and above the turnpike through the southern Poconos will receive the highest accumulations with the possibility for 4-9in of snow. But for now I will forecast lower amounts. See below.

Radar...
Radar...

Warnings...
Warnings...

NAO...
NAO...

This is my current rain/snow line...
Winchester, VA - Frederick, MD - Westminster, MD - Coatesville, PA - Pottstown, PA - Quakertown, PA - Hillsborough, NJ - Peekskill, NY - Hartford, CT - Worcester, MA - Manchester, NH - Augusta, ME - Bucksport, ME

*Along and to the north and west of this line will be the highest snow accumulations and predominately remain snow throughout the event. South of the line will feature a mix or plain rain with limited to no snow accumulation. Elevations above 1000ft along and northwest of this line will feature the highest snow totals.

Storm Reports...
None.

Storm Impacts...
1. Quick moving storm with impacts less than 12 hours during early morning hours with low societal impact.
2. Brief periods of heavy snow of 1-2in/hr.
3. Wet snow with high water content as snow ratios remain around 10:1.
4. Higher elevations above 1000ft may see upwards of 7in or more.
5. Snow may fall at temperatures of only 32-34F for duration of event.

Snow Map...

*I put this map together rather quickly, but I wanted to show a generic distributation of the snowfall totals to help make my forecast a bit more simple to understand. The 3-6in range will have the highest snow totals with locally higher amounts possible (especially above 1000ft)

Selected City Accumulations for the Northeast...
Hagerstown, MD- 2-4in of wet snow
Baltimore, MD- Up to 1in of wet snow in some areas
Salisbury, MD- Heavy rain
Pittsburgh, PA- Light snow up to 1in
State College PA- Light snow. Accumulations 2-6in
Williamsport, PA- Light snow. Accumulations of 2-6in
Altoona, PA- Light to moderate snow. Accumulations 3-6in
Harrisburg, PA- Rain changing to heavy snow. Accumulations 2-5in
Lancaster, PA- Rain changing to snow. Accumulations 1-3in
Philadelphia, PA- Rain mixing with snow. Little to no accumulation
Allentown, PA- Raing changing to heavy snow. Accumulations 2-5in
Scranton, PA- Moderate snow. 2-5in of accumulation
Washington, DC- Rain mixing with a few flakes at end. No accumulation
Wilmington, DE- Primarily rain mixing with a few flakes at end
Dover, DE- Heavy rain
Trenton, NJ- Mostly rain changing to rain/snow. Little to no accumulation.
New York City, NY- Mostly rain changing to rain/snow. Little to no accumulation.
Poughkeepsie, NY- Moderate snow. 3-6in of accumulation
Binghamton, NY- Light snow. Accumulations 2-4in
Ithaca, NY- Light snow. Accumulations 1-2in
Albany, NY- Light snow to moderate snow. Accumulations 2-6in
Hartford, CT- Rain changing to snow. 1-3in of accumulation
Concord, NH- Light snow. 1-4in of snow is possible
Providence, RI- Heavy rain
Worcester, MA- Rain to moderate snow. 1-4in of accumulation
Boston, MA- Mostly rain changing to rain/snow. Up to 1in is possible in some areas
Nantucket, MA- Heavy rain
Hyannis, MA- Heavy rain
Portland, ME- Rain to rain/snow. 1-2in of wet snow is possible
Bangor, ME- Rain to heavy snow. 2-5in of snow is possible
"Subject to Change"

Model Analysis
Current guidance has been edging the position of the low pressure farther northwest while weakening the first shortwave and strengthening the primary low. This trend will likely continue slightly northwest with limited blocking upstream enhancing uncertainty for many forecasts at the cusp of the rain/snow line. Current SREF and GEFS ensemble means suggest widespread 1-1.5in QPF amounts well inland towards I-81 with a sharp cutoff to the northwest. Ensembles are continuing to shift northwest in QPF and likely will continue to do so by about 25-50 miles. The NAM remains on the colder edge of the envelope suggesting very high snow amounts for parts of east-central Pennsylvania up through northwest New Jersey and up through central New England. Given my analysis this morning, it appears the NAM is already running a bit too cold given 2m temperatures in the low to mid 50s for most observing sites. GFS remains the farthest southeast of guidance and given the nw shift in the ensembles, it likely remains too far southeast and can be disgarded.

ECMWF is a mean between the GFS and NAM operational models and therefore has the highest concentration. Model QPF remains similar with 1in amounts as far northwest of Harrisburg, PA. Much of this QPF is progged as snow, although I believe guidance is running a bit too warm for boundary layer temperatures and there I am cutting snowfall QPF totals in half accounting for factors such as wet and warm ground temperatures, poor snow ratios, and above freezing boundary layer temperatures.

After the Storm
The continued +AO and +NAO will dominate the long range with little to no blocking in the northern Atlantic. Current MJO phases support an instilled mild pattern over the eastern contiguous United States with limited chances of wintry weather through the end of the month. As my December forecast alerted to, it is likely another very mild spell is possible mid to late month perhaps towards the holidays as the EPO begins to retreat to a positive anomaly. In the shorter term colder (seasonable to slighty below normal) temperatures are likely under a west-northwest flow. Given the lake of cyclonic flow, lake effect snows will remain unorganized for the most part with a snow drought continuing across the Great Lakes. Any chances of lake effect snow will be limited to directly after frontal passages.

A weak clipper will move across upstate New York and northern New England with the threat of light snow. Moisture will be lacking, but orographic lift may aid in the possibility for a coating to two inches of snow in some areas Friday night. A brief period of lake effect snow is likely over the weekend under a northwest flow. Given the reduced north Atlantic blocking and the return of the unfavorable Alaskan vortex, most storm systems will have a higher likelihood of moving up through the Great Lakes. This will favor a snowy pattern towards the Midwest. Enjoy the the taste of winter this system provides as chances will remain minimal ahead.

Please post storm reports in this blog from across the Northeast during the winter storm and please post location of observation in each report...

This blog is in progress. Check back soon...

Winter Forecast 2011-2012... Link

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Lower Susquehanna Valley Doppler...

(Courtesy of WGAL)

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Quoting PalmyraPunishment:
SNOW.
Yup. Very light snow mixing in now in Camp Hill
Member Since: August 29, 2006 Posts: 0 Comments: 169
SNOW.
Member Since: January 31, 2007 Posts: 9 Comments: 2250
still no snow that I can see. Down to 35f. Percip shield def seems to be shrinking for us as well
Member Since: August 29, 2006 Posts: 0 Comments: 169
Finally, something icy falling from the sky here in Harrisburg.
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Quoting wunderstorm87:

If you think about it in both storms it was unseasonably warm 24-36 hours before the event started. Such an extreme temperature difference is likely why thundersnow occurred in both of these events. The increased frequency of coastal storms is probably a key factor as well.

I'm sure there are some more reasons but those are just a couple ideas.


They've been very dynamic.

I can't help but think that if we had a block with this one, it woulda been another hugey!
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40F @ 8pm, -3F last hour
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Quoting MariettaMoon:


There needs to be studies done on the reasons for the incredible amounts of thundersnow with these systems lately. Thundersnow used to be a rarity.

If you think about it in both storms it was unseasonably warm 24-36 hours before the event started. Such an extreme temperature difference is likely why thundersnow occurred in both of these events. The increased frequency of coastal storms is probably a key factor as well.

I'm sure there are some more reasons but those are just a couple ideas.
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Hey Patch, thanks for mentioning WGAl, will check it out
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I've noticed the orange and reds on WGAL radar has moved due south the past hour.

39F and rain with a rate of 0.14"/hour.
Campbelltown, PA
Member Since: January 26, 2007 Posts: 72 Comments: 6256
Just to help me get me bearings, I will post this map. Hope it doesn't overwhelm any of the systems, if so, tell me and I'll delete it.

now let's see if I can do this.
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Quoting wunderstorm87:
Wow 2nd snowstorm this year and 2nd thundersnow event...Not to rub it in or anything Blizz (lol).

All snow here now- 35F


There needs to be studies done on the reasons for the incredible amounts of thundersnow with these systems lately. Thundersnow used to be a rarity.
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WOW, I am having so much fun listening to ya'll, as though I am there with ya.
Snow and thunder, oh my.
Got me so excited, I just went outside and I can report that it still is about 70 degrees here and I think the wind has subsided a bit.
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Quoting shipweather:
isn't the precip shield moving north?


Yeah, moving northeast, snow line moving southeast through it.

Palmyra, on close inspection, the western fringe of the precip shield is 20 to 40 miles east of what was expected at this time. Dry air eating it up very slightly. Still 3-5 hours of snow for the LSV I think. Cut the total hours in half and that's probably total snowfall accums.

You thinking we might see less of a window for accums?
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Wow 2nd snowstorm this year and 2nd thundersnow event...Not to rub it in or anything Blizz (lol).

All snow here now- 35F
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Ok, so we don't have the snow in Harrisburg, just the thunder and pouring rain.
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Quoting LivelySnow:

Currently 36 degrees. Even if this doesn't lay,..I can't tell you how happy this makes me!!!!! :) LOL!!


WOW!!!!!!!!! Just was outside!!! Thunder and lightening!!! Way too cool!!!!!!!!!!!!!!!!!!!!!!!
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Thundersnow!!
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lightening and thunder!!! Large flash followed immediately by thunder. Seemed to come from the West Shore. Wow. Just witnessed.
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Welp, just had some thunder and lightning...
Member Since: January 31, 2007 Posts: 9 Comments: 2250
Wow awesome blast of thunder just now
Member Since: August 29, 2006 Posts: 0 Comments: 169
Congratulations Lively. enjoy.
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Quoting LivelySnow:

Just north of Linglestown in Fishing Creek Valley!

Currently 36 degrees. Even if this doesn't lay,..I can't tell you how happy this makes me!!!!! :) LOL!!
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Actually all the local stations on wunderground in mechanicsburg are showing some pretty quick drops in temperature. The main one I've been using just dropped another 1f and other ones are showing similar drops. Sweet. Gonna run out for a smoke and see if its snowing
Member Since: August 29, 2006 Posts: 0 Comments: 169
torrents of rain....
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Down to 38f in Mechanicsburg! Getting there though some of the other stations in the Camp Hill area are reporting higher :(
Member Since: August 29, 2006 Posts: 0 Comments: 169
Quoting MariettaMoon:


Are you in Linglestown?

Just north of Linglestown in Fishing Creek Valley!
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isn't the precip shield moving north?
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It is def pouring outside the windows right now. Thought this image of rain totals according to radar from Level3 was pretty neat
Member Since: August 29, 2006 Posts: 0 Comments: 169
Back edge of the precip is approaching State College. Might be running out of time...
Member Since: January 31, 2007 Posts: 9 Comments: 2250
Quoting LivelySnow:
Winds a howling and first wet flakes just started!!!! Wahoooooooo!!! (baking Christmas cookies and listening to Christmas music.......PERFECT!!!)


Are you in Linglestown?
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Winds a howling and first wet flakes just started!!!! Wahoooooooo!!! (baking Christmas cookies and listening to Christmas music.......PERFECT!!!)
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Thanks, Marietta. I always feel a little embarrassed if/when he knows some weather something that I have missed.

We are now under a severe thunderstorm watch and a special statement has been issued, quoted below, but I can tell you from real-time observation, it is howling out there.

... Gusty winds and falling temperatures this evening...A strong cold front will cross the area this evening. Areas of showers will precede the front with strong and gusty winds developing behind
it. South winds between 15 to 25 mph will shift into the northwest at 20 to 30 mph with gusts between 30 to 40 mph later this evening and through the overnight hours. Temperatures will quickly fall
through the 50s and into the 40s once the front comes through. The gusty winds may cause tree branches to break. Secure loose objects...
including trash cans and Holiday decorations.
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Airport is only showing .38in for today? Seems low.
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Moon that's some nice calculations. Here's to hoping the temp's just bomb out at some point.
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nasty and raw!! wind gust to 34mph heavy rain and 38f

no good
Member Since: November 5, 2008 Posts: 0 Comments: 1139
Quoting wunderstorm87:
Wet snowflakes just started mixing in. Temp- 37F


Well, there you go. Rain/Snow line starting to break out of the mountains. Looks like we change over at about 2m 37F. I have 6F to go. At a rate of -2F per hour, I'd be mixing at around 10pm. Palmyra, maybe about 9pm for you?

EDIT: Oh wait, you're in NORTHERN Dauphin County so I consider that the mountains. Snow line decidedly sweeping southeast now. South mountain area looks like it has changed over, which is just west of Gettysburg.
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Quoting hurigo:
Hubby has headed out to put more lines on the boat and batten down the hatches. He said it is suppose to get real windy and cold. He also said some places up north are expecting a foot of snow. Is that so?


Yeah, high elevations of West Virginia & interior New England.
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Wet snowflakes just started mixing in. Temp- 37F
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43F @ 7pm, -1F last hour
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Hubby has headed out to put more lines on the boat and batten down the hatches. He said it is suppose to get real windy and cold. He also said some places up north are expecting a foot of snow. Is that so?
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raining very hard here currently
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I'm down to 38F and have well over an inch of rain. The winds have picked up dramatically and are whipping the heavy rain against the windows. With the increase in wind the temperatures have started to drop at a much faster rate.
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Quoting Blizzard92:

I say let's shoot for 80in! Hahahah


That's not funny...you take that back. hahahaha
Member Since: January 26, 2007 Posts: 72 Comments: 6256
41 degrees here in Camp Hill. Wind has picked up, precip has picked up in the last half hour. It's 39 as far east as Greencastle, so we're probably an hour or two away from doing this thing.
Member Since: January 31, 2007 Posts: 9 Comments: 2250
Wow, look at the WGAL radar, verr heavy precip over Cumberland County. Mixing or just very heavy rain?!

Really hoping this snow starts soon or we're going to get snubbed.
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Text of the discussion:

MESOSCALE DISCUSSION 2373
NWS STORM PREDICTION CENTER NORMAN OK
0521 PM CST WED DEC 07 2011

AREAS AFFECTED...CNTRL/ERN PA...S CNTRL AND SERN NY...MUCH OF VT/NH

CONCERNING...HEAVY SNOW

VALID 072321Z - 080315Z

A TRANSITION TO SNOW WILL OCCUR OVER THE NEXT FEW HOURS. EXPECT
SNOWFALL RATES TO CONTINUE TO INCREASE TO NEAR 1 INCH/HR...WHILE
SPREADING EWD WITH TIME.

AS OF 23Z...RAINFALL RATES CONTINUE TO INCREASE ACROSS MUCH OF THE
NORTHEAST...WITH A TRANSITION TO SNOW NOW NOTED FROM ALTOONA PA NEWD
TO NEAR BINGHAMTON NY AND JUST WEST OF ALBANY. HEAVIER PRECIPITATION
RATES WILL LIKELY BE SUSTAINED AMIDST ANOMALOUSLY HIGH PWAT VALUES
/0.88 INCH PER 21Z ALBANY SOUNDING/ AND FAVORABLE VERTICAL MOTION
BETWEEN...

1. A WEAKENING UPPER TROUGH ROTATING EWD THROUGH THE HUDSON BAY AND
GREAT LAKES...AND
2. A STRONGER AND MORE POTENT UPPER TROUGH APPROACHING FROM THE
SOUTHWEST.

23Z VAD WIND PROFILE AT ALBANY REFLECTS IMPLIED COLD ADVECTION
BEGINNING...WHILE FARTHER EAST TOWARDS COASTAL REGIONS OF THE
NORTHEAST...WARM ADVECTION WILL CONTINUE TO SUPPORT A PREDOMINATE
PRECIPITATION TYPE OF RAIN. HOWEVER...THE 21Z ALBANY SOUNDING SHOWED
A MORE PROMINENT 850 MB WARM LAYER THAN NEARLY ALL 18Z DETERMINISTIC
MODEL FORECASTS...WITH COLDER SURFACE TEMPERATURES AS WELL. WITH
RAPID COLD ADVECTION OCCURRING AT THE SURFACE...A VERY BRIEF PERIOD
OF FREEZING RAIN IS POSSIBLE BEFORE A COMBINATION OF LOWER
TROPOSPHERIC COLD ADVECTION...DYNAMIC COOLING...AND DIURNAL COOLING
CONTRIBUTE TO A P-TYPE OF SNOW. GIVEN THE AFOREMENTIONED PWAT
VALUE...VERY WET HEAVY SNOW CAN BE ANTICIPATED. ALTHOUGH THE
SEASONABLY WARM WEATHER PRIOR TO THE ONSET OF THIS EVENT HAS
SUPPORTED WARM GROUND TEMPERATURES AND ACCUMULATIONS MAY INITIALLY
BE LIMITED...HEAVY SNOWFALL RATES NEAR 1 INCH/HR SHOULD SUPPORT
INCREASING ACCUMULATIONS WITH TIME.

..HURLBUT.. 12/07/2011


ATTN...WFO...GYX...BTV...ALY...PHI...BGM...CTP...

LAT...LON 39887670 39907824 40837775 42447574 43277412 44447238
44387152 43557203 42437366 39887670
Member Since: August 29, 2006 Posts: 0 Comments: 169
Member Since: January 31, 2007 Posts: 9 Comments: 2250
Mesoscale discussion for central PA. I can't post it from my iPhone, but someone else should (looking good!). Back end is moving east pretty quickly which is a concern of mine. Looks like a dud up here in Ithaca.
Member Since: December 14, 2007 Posts: 284 Comments: 15112
Right around 41f in Mechanicsburg now. When I was out carrying some groceries in from the gfs car it did sorta feel like some stuff was mixing in with the rain but could be my imagination as well
Member Since: August 29, 2006 Posts: 0 Comments: 169
Quoting TheRasberryPatch:
42F with 1.02" of rain for the storm

Campbelltown, PA just east of Hershey

Are we tired of all this rain, YET

I say let's shoot for 80in! Hahahah
Member Since: December 14, 2007 Posts: 284 Comments: 15112
I don't expect the changeover to come in till about 10pm southeast of Blue Mountain.
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About Blizzard92

Cornell University- Atmospheric Sciences Undergrad; Research Assist.- Onset of Spring Indices Toolbox; Interests- Small spatial scale climatolology

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Linglestown, PA
Elevation: 520 ft
Temperature: 24.2 °F
Dew Point: 15.2 °F
Humidity: 68%
Wind: Calm
Wind Gust: 9.0 mph
Updated: 10:37 AM EST on January 18, 2014

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