AccuChris

12z NAM kuchera snow map .

12z NAM slightly colder through H60…keeps mix line around the Turnpike and south .

Excellent write up by NWS State College in their AFD: “All in all, the probability of snowfall amounts exceeding 12 inches has increased to 60-80 percent across much of central PA. The probability of more than 18 inches ranges from 15 to 40 percent. Amounts near 24 inches can`t be ruled out (10 percent chance) especially if the comma head sits over central PA Sunday night. Always prepare for the worst case scenario and continue to check the forecast as it evolves. Now is the time to prepare. Think snow blowers, re-arranging travel plans to avoid travel during the storm, what to do in case of a power outage, etc. Remember it will be very cold before, during and after the storm. Never run a generator indoors due to carbon monoxide. Now if you really enjoy meteorology and want to know *why* we are predicting the most significant accumulations in the past 5+ years, feel free to read on through the remainder of this KEY MESSAGE 2 discussion. We`ve had our fair share of cold snaps this winter season, and tomorrow (Friday) will be no exception after the next Arctic front pushes through. A large factor driving all this cold weather has been the Polar Vortex displaced from the North Pole, with a tendency for the vortex to find itself over Hudson Bay and northern Quebec. With displacement that far south, we`ve gotten plenty of cold air, but it`s also nudged the storm track south of our area for many of the recent events. If you`ve ever heard people say "it`s too cold to snow", it`s not that it`s physically too cold to snow, it usually just means that the storm track has shifted far enough to the south that we end up under dry high pressure while our temps are cold. One key difference this time is the influence of a Pacific storm, located off the coast of southern California this evening, and a strengthening subtropical jet to its south. Over the next 24 hours these features will cross the Baja Peninsula of Mexico and then into Texas. Southwesterly flow out ahead of this southern stream feature will pump warmer air into the Gulf Coast and Southeastern US Saturday night into Sunday. At the same time, a northern stream shortwave trough coming down from western Canada into the Great Plains will bring a reinforcing shot of cold air to the back side of the northern longwave trough. The increasing temperature gradients will result in an overall increase in wind speed aloft at jet stream altitude, and an amplification of the northern stream trough and ridge. By Sunday, the right entrance region of the northern jet and the left exit region of the southern jet will align to produce enhanced upward motion through the troposphere. Remaining uncertainty in the forecast largely depends on the degree of phasing of these two jets, the angle at which they align, and how much amplification there is of the ridge out ahead of the deepening trough. The very cold air in place ahead of this system also plays other crucial roles to increase impacts. At 00Z Sunday, model consensus depicts a 1040+ mb high over New York State, which only slowly moves northeastward across New England over the next 24 hours. The deep, cold blocking high to the northeast of a winter storm is a textbook feature that supplies a stream of cold air down the Alleghenies and provides a cold dome for warmer, less dense air parcels streaming in from the southwest to ascend over. That upward motion is key to production of hydrometeors - in this case snow. And then there is the key role that cold temperatures have on ice crystal habits. The so called "dendritic growth zone" exists where temperatures are between roughly 0F and 10F. Given low temperatures Saturday night in the single digits, some spots might start off with a DGZ that extends from the ground up 10,000 to 15,000 feet which is pretty remarkable for a large synoptic storm in this region. As long as the column saturates quickly enough, any snow that falls late Saturday night and into early Sunday morning could be very high ratio, fluffy, rapidly accumulating snow that consists of many pristine dendrites (SLRs 15-20:1, could be even higher if the column saturates faster but there is dry air at low levels to overcome at onset). So after a few hours of virga, accumulations have the potential to begin with a vengeance in the predawn hours Sunday, especially from I-80 south.” .

18z GFS stays firm on staying all snow north of the M/D line for PA and buries us all .

12z GFS kuchera zoomed in on Harrisburg area .

0z GFS kuchera is beautiful. Much more in line with the other models .

0z NAM had some changes to the southern vort energy but it looked just slower to me as a whole, as the northern branch was slow too. Regardless, a ton of QPF with this one and party is just starting for our area at the end of its run .

And finally for good measure, the 18z Euro Op Kuchera snow .

Here are also the latest new para (experimental NBM) snow outputs and regular NBM. Remember these are simply 10:1 maps so the true snowfall via proper ratios are not considered here .

18z Euro AI holds firm and the 18z GEFS AI was actually a tick better in QPF too .

From Eric Horst: .

12z Euro Op kuchera snow map still looks great .

12z Euro AI 10:1 snow map stays solid .

12z GFS says “I see this weekends storm and raise you a blizzard the following weekend” .

Lock it in lol .

12z GFS AI QPF output was slightly better for PA compared to 6z .

Remember it very well too! Hi Res NAM 48 hours out started spitting out stupid numbers for Lebanon ranging from 27”-35” at times leading up to it. Measured 32” when it was over .

12z NAM has it a-coming .

Long range 0z NAM is primed and ready with the southwest energy ejecting out to phase .

18z Euro was a hit .

Yeah this falls more in line with the other models. May still not be done correcting as it wasnt even a clean or full phase .

Eric Horst was a fabulous non-biased forecaster and a great mentor to me at Millersville when I was a met major .

Thats how you get 20:1 ratios and get a foot of snow with half the QPF .

Nice improvement from the ICON…below is 0z and now 12z 10:1 snowmap .

It appears the GFS is doing what it does best and is stringing out the energy and handing too much off to the east too fast. Though, it does have the main low farther west over the weekend and puts more QPF/snow farther north and west. Plenty of time to watch. Key is how fast does the SW energy eject out and can phase with the northern branch so it can attack the cold high .