So, CTP does look at the Nam:
.SHORT TERM /7 AM THIS MORNING THROUGH 6 PM TUESDAY/...
The forecast from all models/ensembles has been changing
drastically over the past 12 hrs. The result is a shift of
50-70 miles south for the expected stripe of heaviest snow. The
timing is similar to prev runs. The NAM is the most pronounced
with this shift.
The complimentary, strong (150KT) jet streaks over NY/New
England and the Carolinas will generate tremendous lift for a
4-6 hr period over a good portion of the CWA. While the precip
will initially be rain thanks to the milder-than-normal temps
we`ve gotten used to, it will turn to snow. Wet-bulbing/dynamic
cooling and the NErly wind typical with a Miller Type B Nor-
Easter will cool the column to a solid snow profile for most of
the area through the night. The SE will be the last to turn.
However, there is still some doubt/uncertainty as to the p-type
for the far SE (Adams, York, Lancaster) as the heaviest slug of
precip arrives in the early morning. If it falls hard enough,
it could be all snow as it won`t have time to warm up. However,
we could see another waggle back north, and this would keep the
temps just mild enough for a mix or just plain rain. SLRs
continue to look very low (sloppy, wet snow) for all of the
area. Extreme snowfall rates are possible. HREF progs support
high (90 pct chc) of 1-2"/hr snowfall rates over the central
zones, and even as far S as the Turnpike. That would make a big
mess for Harrisburg right when everyone is traveling. The
uncertainty of the band placement has kept us from converting
the Harrisburg metro into a warning at this point. We`ll ride
the watch.
Many models now keep the entire nrn tier (and almost certainly
the far NW) dry. However, we`ll leave accums there for the time
being, bringing them down, but holding the warning for
Potter/Tioga. This warning may get dropped by later shifts, but
the novelty/newness of the change begs for small steps to the
change in forecasts.
The biggest changes to warning/watch flags are the addition of
a watch area over the south-central mountains, and an expansion
of the warning area to cover a great portion of the CWA. SF
rates could cause a significant impact to travel Tuesday AM,
pretty much right when everyone is trying to get out the door to
work, school, Dr appointments and other activities. Again, as
with the changes across the nrn tier, the shift to the south is
so new that a watch is a good first step.
The snowfall should exit stage left mid-morning in the
west/central and around noon for the far eastern towns. Temps
rise thanks to some breaks in the clouds and a downslope in the
SE. The snow might start to melt nicely in the SE in the aftn.
But, we`ve also nudged temps down across the board for Tuesday,
adjusting to the expected snow cover. The WNW wind kicks in and
intensifies as the sfc low deepens and moves away, off the East
Coast through the day. The upslope flow for the Laurels and
cross-lake flow for the NW will generate sct SHSN there. But,
they should not extend past the Allegheny Front. The wet/heavy
character of the snow should preclude any blowing/drifting.
