January 21 Southwest Flow Event (SWFE) Timeline and Discussion…
Current surface observations across the Northeast indicate a relatively dense arctic air mass over the region. Dew points remain generally in the single digits across northern areas while temperatures will struggle to reach freezing as far south as Washington DC. A zonal flow accompanied by a shortwave dropping out of the Rockies will set the stage for the first widespread winter storm this winter season across the eastern United States. The progressive jet stream will maintain a fast progression of the shortwave as it advances eastward across the contiguous United States. A southeast ridge associated with increasing upper level heights will attempt to push an area of warmer air into the Northeast. But the passing of a cold front overnight Thursday has allowed for a dry and cold air mass to reach as far south as Virginia. This battleground for temperatures will set up a thermal gradient from west to east around a latitude line near Washington DC. This position will assist in the exact track of the shortwave.

Actual dynamics for the shortwave remain relatively weak with little amplification as it translates east. This is a classic southwest flow event with the precipitation primarily fueled by warm air advection acting as a lifting mechanism over the region.

Looking at past climatology several things can be noted in these types of setups. There will often be a widespread precipitation shield that stretches farther north than computer models are able to detect. Precipitation rates can be very significant closer to the track of the 850hPa low. Typically there are two belts of the heaviest snowfall. One belt exists 50-100mi due north of the track of the 850hPa low. Another heavy snow belt exists in the coldest sector of the storm where ratios in the arctic air mass account for ideal dendritic growth.

Many people in recent days have noted that these southwest flow events often shift northward as the event approaches. This is likely the case due to the underestimated strength of the southeast ridge an anomalous warm air advection. But in this case the shortwave is meager and therefore the dynamics are actually be squashed by the northern stream. This likely was the reason we saw a southerly shift in model guidance. Also many SWFE’s are accompanied by an underestimated warm tongue. Therefore while areas 25-50mi north of the H85 0C line are only receiving sleet instead of sleet; this is because there is a warm tongue in the mid levels of the atmosphere. I have accounted for this possibility in my forecast.

The cold air damming in this setup is quite impressive given a 1028mb anticyclone to the north accompanied by an arctic air mass. Dew points in the single digits to teens will allow surface temperatures and the entire thermal column to cool by evaporational cooling and wetbulbing during the first hour or two of the storm system. Given this possibility, I believe a 1-2 hour period of snow is even possible as far south as Washington DC.

Looking at mesoscale features of this event, these SWFE’s often produce a heavy burst of the snow on the front end typically accompanied by high snow rates. If the warm tongue can remain push backed by the density of the cold air mass, then some areas may see more snow than forecast. I would expect the development of several mesoscale bands including the formation of a possible CIS band across areas about 50mi north of the mix line. This band may affect areas around the Pennsylvania turnpike north to I-80. This may affect areas also such as NYC, Connecticut, and Rhode Island. The weak dynamics of this shortwave will prevent heavy QPF despite a relatively good flow from the Gulf of Mexico. QPF will likely be in the range .35-.6in range for most locations. Snow ratios in the coldest sector of the storm system around I-80 will be nearly 20:1. Soundings in this region indicate excellent dendritic snow growth in the -15C sector. Large flakes are possible during the height of the event.

The quick progressive nature of the jet will allow precipitation to only last in one location for no more than 6-8 hours and will be well out of the Northeast by Saturday afternoon and early evening. Low dew points should allow for some virga on the onset, but I am increasing my start time due to the past history of SWFE warm air advection precipitation to break out earlier than anticipated. Confidence in this forecast remains relatively high with only a few discrepancies noted for snow amounts towards the mix region.

7-10pm Friday: Stratus and cirrus decks over the Northeast will begin to thicken as the shortwave begins to approach the region. Warm air advection precipitation will begin to break out across northern Pennsylvania and southern New York State in the form of light snow. Visibilities will remain around 2-4mi with weak forcing and meager lift at this point.

10-12am Friday: This period will define the majority of the precipitation developing over the region in the form of light snow across the entire Northeast south to Washington DC. Snow rates will begin to increase across western Pennsylvania with 1in accumulation possible in this period. Light snow will move into NYC and southern New England in this period also.

12am-4am Saturday: Precipitation will increase in intensity in this period across Pennsylvania and Maryland. We will also begin to see warm air aloft moving northward into southwestern Pennsylvania and parts of Maryland. Areas will begin to change to freezing rain and sleet as far north as a line from Pittsburgh, PA to Frederick, MD. Snow rates will be near .5in or 1in/hr especially over Pennsylvania. Light snow will continue to be falling in southern New England also. Areas towards central and southern Maryland will change to rain.

4-7am Saturday: The heaviest snows will pull out of Pennsylvania into southern New England. Warm air advection will allow mid levels to rise above freezing as far north as the Pennsylvania turnpike with sleet and freezing rain possible as far south as extreme northern Maryland to the northwest Philadelphia suburbs. Snow rates of 1-2in/hr are possible in southern New England down into New York City.

7-11am Saturday: The heaviest snows will pull into eastern New England with only light to moderate precipitation back eastward into New Jersey. Most precipitation will be pulling out of Pennsylvania. The mix line will reach the southern New England coastline with rain over Nantucket and Martha’s Vineyard.

Regional Radar…

(Courtesy of NOAA)

Regional Advisories…

(Courtesy of NOAA)

This is my current freezing rain/rain line…
Winchester, VA – Baltimore, MD – Wilmington, DE – Southampton, NJ – Middletown, NJ – Brentwood, NY – Chatham, MA

***Areas north of the line will stay primarily below freezing for the entire event with a mix of snow/sleet/freezing rain. Areas south of the line may see some wintry precipitation also, but will be primarily rain for the majority of the event.

This is my current mix/snow line…
Butler, PA – Indiana, PA – Altoona, MD – Harrisburg, PA – Reading, PA – Allentown, PA – Newark, NJ – New London, CT – Plymouth, MA

***Areas north of the line will stay primarily snow through the entire event. Areas south of the line will start as snow and eventually change to a mix at some point for the event. Advisory criteria snows are possible as far south as 50mi south of this line.

Storm Reports…
None.

Storm Impacts…
1. First widespread winter event of the season affecting the entire I-95 corridor.
2. Significant ice accretions in excess of .25in are possible for the higher elevations in southern Pennsylvania and parts of Maryland.
3. Quick thump of snow will allow for a period of 1-2in/hr rates.
4. Snow accumulations will be near warning criteria of 6in for some locations.
5. Cold temperatures in the teens and twenties will allow for easy accumulations of ice and snow.

Snow Maps…

***The areas in the mix region may also see accumulations of 1-2in of snow/sleet. The large region of 5-10in will likely be in reality a region of 4-8in amounts. I do not expect any 10in snow totals for any locations. If any changes to this map were to be made, it probably is to shift the snow totals a bit to the north across southern Pennsylvania given the warm mid layer may be underestimated. But for now no changes look necessary.

***The area of lowest confidence is along southern Pennsylvania where mixing concerns are a possibility especially in central and eastern Pennsylvania. The Laurel Highlands have a stronger confidence forecast given the high likelihood of freezing rain after some light snow accumulations.

Current Great Lakes Water Temperatures…

(Courtesy of NOAA)

Selected City Accumulations for the Northeast…
Hagerstown, MD- 1-3in of snow followed by .25in of freezing rain
Baltimore, MD- 1-2in of snow followed by .25in of freezing rain
Salisbury, MD- Up to 1in of snow followed by a mix then rain
Pittsburgh, PA- 2-4in of snow followed by freezing rain up to .25in
State College PA- 4-8in of snow
Williamsport, PA- 4-8in of snow
Altoona, PA- 3-7in of snow followed by .1in of freezing rain or sleet
Harrisburg, PA- 3-7in of snow followed by .1in of freezing rain or sleet
Lancaster, PA- 2-4in of snow followed by .25in of freezing rain
Philadelphia, PA- 1-3in of snow followed by .1in of freezing rain then rain
Allentown, PA- 4-6in of snow possibly mixing with sleet
Scranton, PA- 4-8in of snow
Washington, DC- Up to 1in of snow with a trace of freezing rain then rain
Wilmington, DE- 1-2in of snow then freezing rain and rain
Dover, DE- Mix of precipitation followed by rain
Trenton, NJ- 2-5in of snow followed by sleet and freezing rain with trace accumulations
New York City, NY- 3-6in of snow followed by a bit of sleet/freezing rain
Poughkeepsie, NY- 4-6in of snow
Binghamton, NY- 3-6in of snow
Ithaca, NY- 2-4in of snow
Albany, NY- 2-4in of snow
Hartford, CT- 4-7in of snow
Concord, NH- 1-3in of snow
Providence, RI- 3-7in of snow followed by a freezing rain/sleet mix
Worcester, MA- 2-4in of snow
Boston, MA- 2-4in of snow
Nantucket, MA- 1-4in of snow followed by plain rain
Hyannis, MA- 2-4in of snow followed by a mix then plain rain
Burlington, VT- 1-2in of snow
Portland, ME- Less than 1in of snow
Bangor, ME- Less than 1in of snow
“Subject to Change”

Current Northeast Snow Depth and Northeast Wind chills…

(Courtesy of Wunderground)

Model Analysis
The main model discrepancies exist in the position and strength of the 850hPa low. The latest NAM runs indicate a track across southern Pennsylvania. Given snow climatology rules, this would put the heaviest track of snow along and just north of I-80. The NAM closes of the 850hPa low, which allows for more significant mid level warming across southern Pennsylvania and New Jersey. Therefore in these runs ice would be a significantly higher threat with lower snow amounts. The GFS and ECMWF are running a tad colder and remain weaker with the strength of the 850hPa low. This allows for minimal mid level warming with the snow line hovering around the H85 0C line instead of 50mi north of it like the NAM. Given the density of the arctic air mass, I am expecting the NAM to be running a tad too warm. A few recent model runs of the GFS suggest a more dense warm tongue causing sleet in some areas that originally looked like snowfall. The most difficult forecast will be for areas along the Pennsylvania turnpike into central and northern New Jersey up through the New York City metro region. High resolution models including the 4km NMM keep thermals a bit cooler, but they also have a slight cold bias. This cold bias also exists with the latest GGEM and RGEM, which seem to be a bit too cold in this scenario. Also QPF remains in question for this event with the ECMWF running below .5in for all areas while the NAM has the highest totals. The weak dynamics of the shortwave will likely allow QPF totals to lean toward the ECMWF, although I think in the end it will verify too dry.

After the Storm
Latest guidance is suggesting some light QPF over central Pennsylvania up through the Northeast during the Sunday night period. Given cold air damming maintaining the cold sector over the region, it is likely pockets of sub 30F temperatures will exist over much of the region. While QPF looks to be at .2in or less, cloud crystal growth will remain minimal. This will allow for a light freezing rain potential Sunday night. Amounts will remain at advisory levels or lower, but could cause some travel headaches into the night. Given a fresh snow pack over the region, guidance is likely even underestimating 2m temperatures. Later on Monday a strong southwest flow will finally undermine the cold air allowing the warm sector to creep well into the Northeast with temperatures in the 50s as far north as the Mason-Dixon line. Plain rain is likely over main areas later in the day. As I suspected earlier, the torch week that was expected next week does not look likely at this point. Temperatures will only remain slightly above normal. Towards the end of the month global teleconnections are supporting another change in the global regime with a negative AO. Latest ECMWF ensembles continue to show also the development of a west-based negative NAO. While I am not biting yet on this major pattern change, I think it is likely that February will be the most winter-like this season. If a pattern change does occur, the lasting strength of the regime will be critical. Also as I mentioned earlier, we are nearing 90 days for a similar teleconnection pattern. Usually by this many days, the pattern begins to swing. We shall see and I will have more updates later next week.

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…

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Lower Susquehanna Valley Doppler…

(Courtesy of WGAL)

“10mi northeast of Harrisburg 2011-2012 winter statistics”
(Snow Stats)
Current Snow Cover- 0.0in
Monthly Total (October)- 5.5in
Monthly Total (November)- 0.0in
Monthly Total (December)- 0.4in
Monthly Total- (January)- 0.2in
Seasonal Total- 6.1in
Winter Weather Advisories- 1
Winter Storm Warnings- 1
Ice Storm Warnings- 0
Blizzard Warnings- 0
Freezing Rain Advisories- 0
Winter Storm Watches- 1

(Temperature Stats)
Lowest High Temperature- 29F
Lowest Low Temperature- 10F
Wind Chill Advisories- 0
Wind Chill Warnings- 0

(Snow Storms Stats)
Historic October Nor’easter – October 29 – 5.5in of wet snow
322 Lake Effect Snow Band – December 17 – 0.3in of wet snow
Weak Clipper – December 29 – 0.1in of snow
322 Lake Effect Snow Band – January 18 – 0.2in of snow