Lower Dewpoints = More Snow and Ice
www.DCstorms.com
- Dewpoints are running in the single digits. I will stress that the dry air will produce a much stronger evaporational cooling effect causing snow and sleet to hang on longer before a change over to freezing rain.
- Immediate Washington Area: 1 to 2″ of snow is likely across Northern Virginia, downtown Washington, & central Maryland before a change over to sleet and freezing rain Saturday morning. A glaze of ice accretion (freezing rain) is likely on top of the snow in the immediate Washington Area. Western Fairfax county, upper Prince William County and upper Montgomery county could see ice accretion of a quarter inch if the cold air hangs tough through mid morning Saturday (hence a Winter Storm Warning may be needed).
- Northern Maryland: I think snow will hang on longer… and upwards of 3 to 4″ will fall in Frederick and Baltimore. As you get closer to the Pennsylvania border, I see 4 to 5″ accumulating before a change over to sleet and freezing rain Saturday afternoon.
- Southern Pennsylvania: you will remain snow for 90% of this system before possibly mixing in with a bout of sleet towards the end… 5 to 7″ with sweet spots of 8″ is possible here.
- Areas in pink on the map (Fredericksburg, Quantico, Stafford, Culpeper, over to southern Maryland): Expect a period of snow (coating to 1″) that will switch over to sleet… and freezing rain early Saturday morning. A glaze of ice on untreated roads will make for extremely hazardous driving on Saturday morning. Temperatures should warm into the middle to maybe upper 30s by afternoon… I say should b/c the air is so damn dry… this will be a nail biter.
- DEWPOINTS are low… teens and single digits! This air is dry and a rapid warm up at the lower levels of the atmosphere is extremely unlikely as evaporational cooling slows mid-level warming. I think some folks need a Winter Storm Warning versus a “Winter Weather Advisory” due to the threat of Freezing Rain holding on in the Northern Shenandoah Valley and even across the Interstate 66 corridor of Northern Virginia and parts of Maryland.
- Don’t be surprised if you wake up to Winter Storm Warnings in the northern and western suburbs as the cold air hangs on tough.
Mid-Atlantic Mess Coming: DC, Baltimore & Philly
- While I would love to be blogging about a widespread 3 to 6 inches of snow for the Washington to Philadelphia corridor, it looks like the warm air will win out in the end with the system headed our way Friday night through Saturday.
- Precipitation will begin as snow for everyone Friday night before changing over to sleet and freezing rain Saturday morning.
Cold Air Damming: Cold air trapped at surface, warm air glides on top (www.DCstorms.com)
- Freezing rain may hold on throughout the event for portions of the Shenandoah Valley, northern Maryland, the Susquehanna River Valley and the northern and western suburbs of Philadelphia on Saturday.
Snow and Ice Accumulations by Saturday Evening (www.DCstorms.com)
- A dusting to one inch of snow can be expected in Northern Virginia, Baltimore, Philadelphia, & central and southern New Jersey before a change over to sleet and freezing rain occurs Saturday morning.
- Central Pennsylvania, the Lehigh Valley, northern NJ, and New York City can expect 3 to 6″ of snow. The Poconos could easily wind up with totals closer to 8″.
- The long-term looks much more promising for more winter weather as we head into late January and February. Some brutal arctic outbreaks are likely as the Arctic Oscillation tanks. The NAO (North Atlantic Oscillation) is trending neutral to negative, but until we actually see it go negative… I will remain cautiously optimistic that we can get a Bos-Wash (Boston to Washington… Northeast Megalopolis) Blizzard before this winter is out. Stay tuned!
Positive NAO Responsible for Snow Drought
Positive Phase of the North Atlantic Oscillation: Arctic Air Currently "Locked Up" in Alaska and Greenland
- Last time the NAO Index was neutral was back in late October when parts of the DC Area received one to four inches of snow.
- Since the late October snow storm the NAO index has been positive.
- Forecast ensemble models (see graph below depicting red lines) predict the NAO index may go neutral or negative towards the end of January.
- In order to get a major east coast snow storm, you must have blocking over Greenland (a massive area of High Pressure) found during a negative NAO oscillation.
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.shtml
Ski resorts are suffering in the Mid-Atlantic and New England (Northeastern United States):
Snow Drought Continues across portions of the Mid-Atlantic & southern Appalachians (Courtesy: NOAA)
Snow Drought continues across New England & New York State (Courtesy: NOAA)
Temperature Anomalies During a Positive NAO Index:
** Other factors contributing to the extremely mild start to winter has been the Arctic Oscillation (AO) index which has also been positive favoring strong westerlies over the high latitudes. The lack of an amplified jet stream (lack of high latitude blocking/strong westerlies) has kept all the arctic air locked up in Alaska, extreme northern Canada and Greenland.
** Notice that the AO is beginning to drop drastically over the next several days indicative of the arctic blast heading to the Midwest, Mid-Atlantic and New England Monday through Wednesday (the end of the black line in the chart below shows the current AO index; the red line depicts where the multiple forecast models are predicting the index to be in the future).
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.shtml
