Explaining Away Saturday's Spectacular Storm
Even though I am using spectacular in a sarcastic manner, some areas did see some snowfall on Saturday from a strung out and messed out storm. While my forecast was a pretty bad bust in the suburbs and points north, there were some things that went right and didn't go so well with the forecast and hopefully I can explain it away here.
What went right? Well, the thought that Philadelphia would not be an all-snow event. Mixed precipitation did work into the city and while it did not completely go over to rain there was a reason for that (that's forthcoming). Mixed precipitation did work in as warmer air moved in aloft and overnight a mixed bag of light freezing rain and sleet fell across much of the immediate Philadelphia area. The placements of the rain/snow/slop lines were ok but not great. I also handled the timing aspect (morning wave, a lull, and an afternoon/evening wave) well although precipitation was much less than I though it would be.
Now, onto what went wrong. This storm was not handled well at all in computer guidance for a myriad of reasons. One, a progressive flow in the middle atmosphere sheered the storm out to an extent. Winds aloft at the 300mb level (about 20,000 feet up) were pegged at 150 mph and these fast winds did not help the storm organize itself and helped keep it weak, moving it along pretty quickly in a disorganized fashion. The weaker storm system had less organized precipitation with it (hence why Philadelphia only received 0.07") as precipitation was sheered out to the east of the primary low and was lighter and more scattered in nature. The primary low, as it weakened, traveled across Pennsylvania to our north, carrying one area of energy and precipitation while the sheered energy shuffled by to our south, organizing and bringing rain to South Jersey and Delaware while grazing Philadelphia with some wintry slop.
Another factor was the development of a weak secondary low pressure center to the east and southeast of the primary low. The sheered energy with the storm system coalesced into a weak low pressure center that developed over the Chesapeake. This low tracked east-northeast into the Atlantic, to Philadelphia's south. This weak low helped maintain a northeast surface wind component in Philadelphia and prevented temperatures from rising much above 32 in the city while it was in the 40's across South Jersey.
Which computer model won the battle? Well, the EURO handled the lack of precipitation rather well across our immediate area and points north and should be commended for its forecast. It had the low tracking farther north (the primary did do that while weakening) as well and downplayed the event for Philadelphia pretty well. The GFS and NAM had the secondary low correct but were treating the low as the main low. Both models struggled with the progressive upper level flow and the resultant shear that happened. Both the NAM and GFS were also much more moist across our region. All the computer models, along with the EURO, struggled with the energy transfer from the weakening primary low to the developing secondary low.
In the end, I did warn of the potential for a busted forecast in my primary discussion on Friday afternoon and wasn't confident in the snowfall totals I forecast to the north of the city. I over-forecast the amount of precipitation that would occur in those areas and that was what nailed me on snow to the north. I also didn't expect the storm to weaken due to shear. Unfortunately for the suburban snow starved, not much went their way in this storm.
What went right? Well, the thought that Philadelphia would not be an all-snow event. Mixed precipitation did work into the city and while it did not completely go over to rain there was a reason for that (that's forthcoming). Mixed precipitation did work in as warmer air moved in aloft and overnight a mixed bag of light freezing rain and sleet fell across much of the immediate Philadelphia area. The placements of the rain/snow/slop lines were ok but not great. I also handled the timing aspect (morning wave, a lull, and an afternoon/evening wave) well although precipitation was much less than I though it would be.
Now, onto what went wrong. This storm was not handled well at all in computer guidance for a myriad of reasons. One, a progressive flow in the middle atmosphere sheered the storm out to an extent. Winds aloft at the 300mb level (about 20,000 feet up) were pegged at 150 mph and these fast winds did not help the storm organize itself and helped keep it weak, moving it along pretty quickly in a disorganized fashion. The weaker storm system had less organized precipitation with it (hence why Philadelphia only received 0.07") as precipitation was sheered out to the east of the primary low and was lighter and more scattered in nature. The primary low, as it weakened, traveled across Pennsylvania to our north, carrying one area of energy and precipitation while the sheered energy shuffled by to our south, organizing and bringing rain to South Jersey and Delaware while grazing Philadelphia with some wintry slop.
Another factor was the development of a weak secondary low pressure center to the east and southeast of the primary low. The sheered energy with the storm system coalesced into a weak low pressure center that developed over the Chesapeake. This low tracked east-northeast into the Atlantic, to Philadelphia's south. This weak low helped maintain a northeast surface wind component in Philadelphia and prevented temperatures from rising much above 32 in the city while it was in the 40's across South Jersey.Which computer model won the battle? Well, the EURO handled the lack of precipitation rather well across our immediate area and points north and should be commended for its forecast. It had the low tracking farther north (the primary did do that while weakening) as well and downplayed the event for Philadelphia pretty well. The GFS and NAM had the secondary low correct but were treating the low as the main low. Both models struggled with the progressive upper level flow and the resultant shear that happened. Both the NAM and GFS were also much more moist across our region. All the computer models, along with the EURO, struggled with the energy transfer from the weakening primary low to the developing secondary low.
In the end, I did warn of the potential for a busted forecast in my primary discussion on Friday afternoon and wasn't confident in the snowfall totals I forecast to the north of the city. I over-forecast the amount of precipitation that would occur in those areas and that was what nailed me on snow to the north. I also didn't expect the storm to weaken due to shear. Unfortunately for the suburban snow starved, not much went their way in this storm.
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