Foot's Forecast | Weather Intelligence

THE WEEKEND KAHUNA:

UPDATED AND COMPLICATED

FORMATTING NOTE: In deference to readers from school systems, instead of images I have embedded links in the text if your servers have an "all-image block" feature for security.

SYNOPSIS

A highly complex and evolving winter storm is projected to impact a large area of the country from Texas to Maine in the period Saturday through Monday. The Mid-Atlantic may see heavy wet snow along and west of the Appalachians, with a wide variety of precipitation east of the mountains, including sleet, freezing rain and yes, possibly even hail where severe storms develop along and east of the I-95 corridor from Richmond to Philadelphia. Parts of interior Pennsylvania, New York and New England may receive a near crushing blow of heavy snow, strong winds and intermittent freezing rain Sunday into Monday, on top of accumulations expected Thursday into Friday. Sunday night features the arrival of a strong northerly flow from a second Arctic High that will produce a hard freeze of any standing water or snowpack, which may seriously affect commuters and students on Monday the 17th.

STORM EVOLUTION

A large area of surface low pressure in the southwest on Thursday will develop in the southern plains Friday and move into the Lower Mississippi Valley on Saturday. The regime of paralyzing ice events in the Mid-West will give way to mostly snow as High Pressure finally begins encroaching on the eastern two-thirds of the country. This will begin reducing the warm moist air mass covering the southern Mid-Atlantic since Monday that sent overnight temperatures Tuesday into the 70's in southwest PA, and in the upper 70's to 80 in southeast Virginia on Wednesday.

Concurrently, Tropical Storm/Depression Olga near southeast Cuba on Wednesday may move to the southern Gulf of Mexico by Saturday. It is becoming increasingly likely that tropical moisture will become incorporated into the trailing cold front and developing low pressure as it moves across the Southeast. If Olga restrengthens or is able to access more tropical moisture before absorption into the Low, this could make for an even more significant event.

ANALYSIS AND DISCUSSION

On-going difficulty of computer models in projecting the timing of Arctic air intrusion into the Northeast on Saturday and Sunday may result in a complex and rapidly changing forecast which may confound meteorologists and the public alike. Eventually, a clash of extremely cold air with very warm, moist air over a short period of time may even result in what I am calling a "convective winter weather event" Saturday night into Sunday morning in the Mid-Atlantic. Tropical moisture arriving with cold air could spawn explosive convection in Eastern North Carolina, Eastern Virginia, and the Del-Mar-Va for a few hours. Once the secondary low establishes and moves northeast, it will carry the remaining tropical moisture with it and possibly create an extremely wet heavy snowfall in interior southern New England that con converts to a drier more powdery snow as Arctic air takes over. Given that cold air may not be firmly in place at onset of the storm in states such as Virginia, Maryland, West Virginia..if precipitation becomes all snow, it may have a very low liquid to snow ratio of 8:1 or even 6:1.

One factor that sours my outlook for major snows along I-95 south of Philly is a poor location of the Canadian High. Olga also presents a unique complexity in that some areas in Eastern North Carolina, Southeast Virginia and the DelMarVa could see almost summer-like explosion of thunderstorms in the middle stages of this storm late Sat into Sunday. If tropical moisture gets incorporated into the developing Low pressure at the same time that cold Canadian air is being pulled in from the north, the combination of these two could spawn some highly unusual "convective winter weather" ... in that I mean thundersnow or even thundersleet. The problem for powderhounds is that the track of the primary low (into Ohio) and secondary low (Carolinas to New England) will not be conducive for an all-snow-event. Indications of the potential "kitchen sink" type storm are evident in this graph of precip-type and timing for BWI airport.

I have dubbed this event a "Tropical Snowstorm," because the infusion of this moisture at upper levels over extensive surface cold air just when a secondary low pressure is deepening off the Virginina / Carolina coast may create extremely heavy snows in Eastern New York and Interior New England and in Pennsylvania... north and west of I-81. in PA, VA. The unfortunate part for snow lovers is that areas along I-95 are likely to see more mixed precipitation, due to close proximity of the secondary surface low, especially if it track close to the coast. As you have experienced in previous storms, this setup creates a multi-precip type event that may cut down considerably on any one type of precip.

Despite many differences among computer models, it is likely that following the storm, strong winds and very cold air up to 10 degrees below normal will pervade the Northeast region from Sunday through Tuesday, causing additional power outages problems.

ARRIVAL TIME (MID-ATLANTIC REGION ONLY)

Current indications are moisture will be arriving in the Ohio and Tennessee Valley on Saturday, reaching coastal population centers by late evening. I expect this to be a rain changing to snow situation, unless boundary layer temperatures on Saturday end up colder than expected. (Washington, Sat High 35 - Baltimore Sat High 36.. rain and snow currently forecasted by NWS, where as yesterday, their call was for all rain, no mention of snow.)

PRECIPITATION TYPE
Based on the dynamic nature of this potential "forcing" event, on Sunday I would not be surprised to see thundersnow or thundersleet in areas east of I-95 and even tornadoes in Eastern Virginia or Del-Mar-Va, as well as the possibility of hail. Anyone can agree that when Arctic air and Tropical moisture meet abruptly in a very short period of time over a relatively small distance (confined to the Eastern Mid-Atlantic), there is bound to be a variety of extreme weather and precipitation types. The precipitation type analysis created by the GFS and found at this site: continues to shows a very dynamic event.. totaling near 1.5" of liquid equivalent over the 2-day period for just the Eastern Mid-Atlantic.

Regarding areas to see all snow: I expect this to be along and west of a line from West Chester County, NY to Harrisburg, PA to Frederick, MD... total storm amounts could approach 12" given overrunning snow Saturday that is added onto by wraparound snow Sunday and then lake effect on Monday (for Western PA and Western MD, WV).

Overall, I cannot yet project specific point by point snowfall/ice amounts, but will at least keep you updated on the liquid equivalents and potential for changes in the precipitation types. For the Mid-Atlantic, I expect to provide my "storm grade amounts" Friday or Saturday. For New England, I will likely post some snowfall projections on Saturday night for Sunday.

CLIMATIC INDICATORS
The North Atlantic Oscillation, currently slightly positive, is forecasted to trend neutral to slightly negative. AccuWeather forecasters have already discussed extensively, this indicates that a large push of cold air may occur right at the storm's onset (in MD and VA) Saturday night. This tells me that even though major cities and their suburbs from Washington to Central New Jersey will see more rain than snow, it also indicates cold air is going to play a major role in the final track and outcome of this storm.

FACTORS NOT CONSIDERED IN ANALYSIS AS OF YET

-What effect will Gulf Stream temperatures will have on development of the secondary Low?
-Would a recent and refreshed snowpack in PA and NY be able to strengthen and reinforce the approaching High to a degree that more cold air filters in on Saturday, enhancing overrunning snowfall Saturday night?

-Would earlier entrainment of tropical moisture lead to more rapid development of secondary Low, thereby sapping energy from primary Low and allowing for cold air to invade more quickly along the Appalachians?

WHAT ABOUT SCHOOL AND WORK MONDAY?

Since this situation is so fluid and will be such until it ends, it may be hard to project exactly the impact to your Monday morning schedule. It does bear watching that some computer models show a period of freezing rain Sunday, followed by very cold overnight temperatures and strong winds. Were that to play out as shown right now, many schools in the "wetter/southerly" part of the storm (Central and Northern MD, Eastern and Northern VA) could face delays. What I've been telling people who ask (or happen to be unlucky enough to see me in the hall..) is: "Whatever falls Saturday or Sunday is going to freeze over hard Sunday night, lots of standing water with overnight temps around 20 in the major cities and teens in the suburbs.

As for the Heat Miser vs. Snow Miser contest... By Christmas Day, all this is distant memory south of Philly, and temperatures could reach 60 F at BWI by 12/25. So I believe Heat Miser wins Christmas this year. Sorry powderhounds.