January 21, 2005
A Winter Storm Watch has been issued for most of Pennsylvania, New Jersey, and parts of Delaware in advance of a major winter storm that is scheduled to affect our area over the weekend.
Friday will be "the calm before the storm" so-to-speak as much of the Mid-Atlantic and parts of the Northeast await what will be the largest snowstorm of the season thus far for many. Skies will be mostly sunny with a brisk Northwesterly wind and high temperatures in the low 20s. Wind chills will be in the low teens and single digits at times throughout the day. So the first ingredient in our winter storm, arctic temperatures, will certainly be in place as we head into the weekend.
Clouds will be on the increase Friday night as high cirrus approach from the North and West. Winds will diminish but it will be quite cold with lows around 12F. Then the fun begins:
Saturday will start off cloudy and snow will begin here around noon as low pressure moves through the Ohio Valley and spreads precipitation Eastward. Winds will increase during the day out of the North and Northeast at 10-20 mph. The catalyst for our storm system will be a phasing of the Polar jet stream and the Arctic jet stream in the Ohio Valley. The idea of an historic snowstorm that was prevailent early in the week on guidance has dwindled due to a massive piece of energy holding it's ground in the Baja Peninsula. However, don't be fooled . . . this storm means business and will have a major impact in our region! Originally forecast by many to eject Eastward and phase with the Polar and Arctic Jets, this gyre will remain virtually stationary, but will still provide a key role in the development of the storm system in the East on Saturday. Instead of joining forces with upper energy in the Plains, the closed upper low over the Southwest will actually pump up a ridge (Rex Block) to the North of the disturbance which in turn will help to amplify the entire PNA pattern over the continental 48. This actually allows the energy over the Ohio Valley to dig farther South, thus carving out a decent trough which will energize our developing storm system.
While the 50 / 50 low as discussed on Wednesday will not be a "true" 50 longitude / 50 latitude gyre, there will be a bombing storm system over the Canadian Maritimes which will allow for cold air to remain funneled into the Northeast and much of the Middle Atlantic (thus, serving a similair purpose as a textbook 50/50).
In terms of apparent weather here at the surface, snow will become moderate rather quickly once it begins on Saturday. As upper air dynamics take place and a screaming low level jet gets involved, snow will pick up in intensity later Saturday afternoon and snowfall rates in our area could reach 2-3" per hour for a period. As the upper energy phases and shifts Eastward over the area, a few rumbles of thunder may be heard (yes, thundersnow for those who couldn't decipher that one on your own!). High temperatures on Saturday in the mid 20s.
With all large developing storms, a "dry slot" develops which generally is to the South and East of the mid level disturbance. There is some agreement that this area will not be *too* far from us generally speaking, into part of MD, lower DE, and South Jersey. This is an area where precip basically cuts off and stops for a period while the storm system intensifies, develops a comma head signature on radar and satellite, and begins to wrap up (wraparound snows). Just something to keep your eyes on during Saturday as this could keep snowfall totals down for some folks. Dry slots are difficult to predict as this is generally the time when the storm is developing quickly and takes off.
Snow will continue Saturday night and will be heavy at times. Low temperature around 20F. Snow *may* mix with sleet and rain down in South Jersey for a period, but the jury is still out on this one, though I am hedging towards a changeover for a little while. Water temperatures in these parts are some 6-8 degrees above normal and with a low level jet roaring in from the East, it could warm the lower levels of the atmosphere just enough along the Cape to allow for a mix / changeover. Looks like all snow for Philadelphia at this time.
Snow will lighten and end around midday on Sunday. However, it will be very cold and blustery with highs around 20F and wind chills around 0. If you are headed to the game, dress appropriately and stay warm by cheering on our Birds!!
Total snowfall accumulations are looking as follows at the moment. Philadelphia and adjacent counties: 8-14", North and Western 'burbs': 6-12", Central and parts of Eastern NJ: 8-16", South Jersey: 6-12" with some mixing. If no mixing occurs at the shore, this total *could* potentially increase to 10-18", but will stick with 6-12" for now until further data allows for a change.
Relatively speaking, this will be a quick moving system as the phasing energy will struggle to go negative tilt aloft, close off, and stall, though there is a slight chance that this will happen and prolong the snowfall into Sunday evening (more on this Friday). Also, the fact that the upper low in the Southwest is not forecast to phase with Ohio Valley energy should keep the system moving. A kicker behind the storm should also help push it away by midday on Sunday.
So, while not an 'historic' snowstorm of epic proportions, this will be a significant storm that will probably be our big one for the year. Luckily, it is happening on a weekend and traffic should be light, so this will help with clearing roads. As always, if there is a shift in the storm track, it will greatly impact our forecasted snowfall amounts, though guidance is finally honing in on a reasonable solution.I will likely issue a final pre-storm update late Friday night. For now, take care and go Eagles!
S.B.
