February 21, 2001
Warm weather comes to an end Wednesday as all eyes turn to Thursday afternoon and a potential snow event. Wow, quite a roller coaster ride we have been on throughout the Delaware Valley the past few weeks. Hold on tight as the ride approaches another climax over the next 5 days. Temperatures topped off around 60 degrees across much of the region on Tuesday! Hard to believe that we are still in February based on those temps. However, a cold front has pressed through the region with very little fanfare, just a scattered shower or two in a few locales. The aforementioned cold front will be our return to close to normal temperatures on Wednesday with below normal readings for Thursday.
Wednesday will be partly to mostly sunny and breezy with highs in the lower 40s. Winds will die off and skies will become clear for Wednesday night. Ideal conditions for radiational cooling (no clouds and little or no wind allows surface warmth to escape quickly, thus cooling the air quite dramatically). Lows Wednesday night in the upper teens.
Clouds increase early on Thursday. This is where the fun begins (well, not really so fun if you are a forecaster). A cold area of high pressure will be positioned over the Northeast on Thursday. Meanwhile, warmer air from the South is trying to head North (Warm Air Advection) and counteract with the cold air. The warm air is forced up and over the dome of cold air and often leads to instability and precipitation. Hence, the term overunning precipitation. That will be true with the system for Thursday, but a weak shortwave riding along the jetstream will help to develop an area of low pressure in the Southeast or just off the Southeast Coast late morning/early afternoon on Thursday. Looking at upper level data for that time, the features are anything but impressive. Generally, at H5(500mb), the flow is quite progressive with very little amplification in the flow over the NE quadrant of the US. In order to amplify the flow, one would generally look west to see if another shortwave is digging to try and help sharpen the trough. There are hints of a shortwave present to the left of the weak trough, but the vort is having a difficult time moving Southward. That, coupled with a general West to East flow (ever so slightly WNW->ESE) at the jet stream level (300mb) implies the low heading out to sea. Regardless, a handful of guidance heads the system NE and off the coast of Cape Cod by Friday. The trends over the past few runs have been for a farther North system, but I'm still skeptical of the idea of a Nor'Easter as the ETA portrays based on the majority of guiance portrayal of upper level data. Will still stick with the idea of a coastal low developing as mentioned in Monday morning's discussion, but exact track is yet to be determined. Hedging with the farther East scenario at this time, but not before our area receives a bit of snowfall out of the storm.
As for apparent weather Thursday, will go with increasing clouds with light snow or snow showers developing midday and continuing through the afternoon and early evening. Snow may pick up in intensity for a period, but this should not be a big deal by any stretch of the imagination. Anywhere from a coating (mainly on grasy surfaces) up to 4 inches possible. If the ETA solution from 0Z were to verify, we'd be looking in the range of 4-8 inches, but the ETA seems overdone in this situation, especially given the general progressive flow of the pattern. The only thing that may cause such a scenario is if the high pressure area over the Northeast decides to move just offshore and blocks the system from heading East and/or the vortmax to the West decides to dig into the backside of trough to help amplify the flow a bit. Still some details to nail down, and the devil is in the details as usual. Highs Thursday in the lower 30s.
Things clear out on Friday with partly sunny skies. Highs around 40. Saturday will be mostly sunny early with increasing cloudiness in the afternoon with another round of precipitation moving in by evening. Precip may start off frozen, but should change to rain overnight Saturday. Highs in the upper 30s. Rain will continue on Sunday and may be heavy at times. There are hints that the system associated with the precipitation (a strong area of low pressure moving across the midwest) may try to redevelop off the East coast sometime on Sunday. Too early to be any more specific, but will say that we need to keep an eye to the sky for any tricks the system decides to play. This is the time of year when these systems often start misbehaving (who's really to be the judge on behavioral aspects anyway?) so will continue to monitor guidance the next several days. Highs Sunday in the low 50s.
So the roller coaster ride continues for our area with quite a variety of weather likely over the next 5 days. Wednesday will be mostly sunny with highs in the lower 40s. Thursday will feature increasing clouds early with light snow likely by midday. Snow may increase in intensity for a period Thursday afternoon with some accumulation expected (coating-4"?). Have a few more model runs to try and nail down totals, so that is just a temporary range for the time being. If it needs to be tweaked, I will do so either midday Wednesday, but more likely late Wednesday night. Things clear out Friday with the high around 40. Saturday will feature increasing clouds with another round of precip moving into the region by afternoon. Could start off as a mix but will turn to rain overnight. Highs Saturday in the upper 30s. Rain continues Sunday, possibly heavy at times with highs in the low 50s. That's it for now. Check back for updates.
11:40AM Update
Things on schedule for snow Thursday afternoon, just a few important features that need to be discussed, and an update to snowfall totals for the area. Glancing over satellite and water vapor data, one doesn't need a degree in nuclear physics to see that the shortwaves downwind of the Rockies are currenty a touch stronger than progged. Also of interest is a vortmax over Western Canada. That piece of energy is already stronger as well and is currently showing signs of digging South slightly. This was one of the fears in last night's disc as alluded to. This may help spin up a) a slightly stronger storm system and b) keep the system farther North than had been noted. As of the 12Z model suite, guidance has continued to increase QPF ever so slightly for our region (except the ETA which has basically held it's ground or lessened just a touch). Based on these trends, will go with 2-4" of snow during the afternoon and early nighttime hours on Thursday. Light snow should break out around noon and continue throughout the afternoon. Snow could become moderate for a period before tapering off to flurries after dark. Will add one more update late tonight, but 2-4" range looks reasonable. Check back later and have a great day.
