December 23, 2002
With computer guidance hinting at a Christmas Eve/Christmas Day storm for over a week now, things are finally starting to become clearer as to the eventual outcome. I will be honest and admit that I have been skeptical about this potential storm sytem bringing much frozen precipitation to the immediate Philadelphia area, and I am still not sold on that fact that we are destined for an all snow event or even a modest snowfall. However, I am very confident that our area will have unsettled weather late Christmas Eve and into much of Christmas Day itself. For those of you Parade watchers and other Christmas Day outdoorists (is that even a word?), this may be a year to pass on the outdoor activities and cuddle with the loved ones in front of the fire place while exchanging holiday gifts. Either way you cut it, rain or snow, it will be raw. Let's just dive into the details.
As I type this, a very potent upper level shortwave is pushing into Arizona from the Baja Peninsula. This vortmax in the Southwest will be the one responsible for bringing the unsettled weather later Christmas Eve and into Christmas Day. However, guidance has been anything but consistent with this feature and it's timing over the next 48-84 hours. We have one model, GFS, that has been fairly consistent with itself, but that doesn't necessarily mean that is it right in it's solution.
Here's what we are looking at in terms of a forecast right now. A shortwave will drop out of Minnesota and race ESE over the next 12-24 hours. This will cool temperatures behind it just in time for Tuesday. Meanwhile, the Southwest energy will begin to head ENE across the country. Questions still arise as to how much energy ejects and timing. For now, looks like the energy will come out in two pieces. Early on Tuesday, an upper ridge on the West Coast will conitnue to build and will force the system to shear itself slightly as it crosses the Rockies. During the day Tuesday the two pieces of energy should meet up somewhere near Missouri. While all this is going on, a separate piece of Northern energy is racing Southeastward across Minnesota (note the trend here). These shortwaves look to phase very late on Tuesday or early Christmas morning. While it sounds exciting, the upper level maps are void of much in the way of dynamics. However, details at the surface are much more threatening in terms of apparent weather.
While all this is going on 'upstairs', things at the surface are looking very threatening on Tuesday. An area of low pressure (actually two separate areas) are evident near Louisiana and Mississippi. At the same time, tremendous thunderstorms are hammering part of the Gulf Coast early on Christmas Eve in response to what is going on in the upper atmosphere. As the day progresses on Tuesday, low pressure will be dominant in Northwest Georgia with precipitation extending as far North as the Maryland/Virginia border by midday.
As all this is happening, remember what is going on at the 500mb level. That piece of energy in Minnesota is getting ready to race Southeastward to catch up and join with what once was the Southwest shortwave.This is reflective at the surface by late Tuesday night as the primary low that was over Georgia begins to head North/Northeastward into Southern Appalachia and the Ohio Valley BUT a secondary area of low pressure begins to form off the North Carolina/South Carolina Coastline. By early Christmas morning, the secondary low is beginning to take over due to the upper level phase and precipitation is moving into Southwestern and Central PA. This is where things get very tricky for the immediate Philadelphia region.
As is usually the case with many coastal lows this time of year, precipitation type often becomes a tough call. The key factors will be timing of phasing, strength of the shortwaves, and of course temperatures both upstairs and at the surface.
The one predominant feature that has me skeptical to call for a major snowstorm just yet is the positioning of low pressure at 850mbs. With low pressure at that level forecasted to be to our West until the surface system is almost gone implies Easterly and Southeasterly winds at 850mbs. Simply stated, that spells warmer ocean air working into the system, thus warming the atmosphere enough to prevent much in the form of snowfall until the system is almost finished here. However, I am not convinced that this means no snow for us. The trend has been to crash the rain/snow line South and East as the 850 low races toward the coast in response to the developing system. Are you still with me here?
Anyway, with at leat 36 hours to go until things start to get interesting around here, there's not much point to go any farther about what may or may not be. In simple terms here is what we expect as on now, early Monday morning:
Monday will be a gorgeous December day with mostly sunny skies and temperatures in the mid 40s for highs. High cirrus clouds start to increase late Monday night with the low around freezing. Clouds will continue to thicken during the day Tuesday and will keep temperatures down with highs only reaching the upper 30s (remember that shot of colder air in response to the first shortwave racing Southeast from Minnesota?). Chance of rain or mixed precipitation beginning late Tuesday night with lows in the low to mid 30s. Christmas will be raw and windy with rain or mixed precip turning to all snow by noon. Highs around 35. Some accumulation likely and could be very significant depending on temps and speed of system. Things clear out Wednesday night with lows in the mid to upper 20s and partly cloudy skies. A snow shower or flurry possible. Mostly sunny Thursday with highs in the mid 30s.
Still time to fine tune this forecast and I am quite sure it will change as confidence is lower than normal for this system. I will update again Monday night and provide storm totals and any changes that need to be made. Again, this storm could provide significant measurable snowfall to the area (4"+) but temperatures are still questionable. At this time, leaning slightly towards more rain than snow for the area. Take care.
S.B.
