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FXUS61 KPBZ 190229

Area Forecast Discussion
National Weather Service Pittsburgh PA
929 PM EST Fri Jan 18 2019

Strong low pressure will bring a wintry mix to the region
Saturday and Saturday night, with the potential for 
accumulating snow and ice. Very cold air will arrive in its wake
Sunday and Monday before moderating by mid-week.


No major changes needed in the late evening update. Previous 
discussion follows...

A relative steady state of clouds, no precipitation, light wind
and low temperature in the 20s is expected tonight ahead of 
strengthening low pressure in the Plains states.


A major winter storm remains in the offing for the Ohio Valley
Sat and Sun as low pressure in the lee of the Rocky Mountains
strengthens on its enewd trajectory across the CONUS.

Snow is expected to start Sat morning as warm air advection 
begins in earnest, but higher snow rates are not likely to occur
until Sat afternoon, when warm air advection strengthens and
H7 frontogenesis increases considerably, leading to strong
upward motion.

All models continue to show a plume of warm H9/H8 air spreading
newd Sat afternoon and evening, but still show some degree of
inconsistency in the nwd extent of this plume. The usually-
aggressive NAM is the most conservative with the nwd extent of
the warm air as a result of a swd trajectory for the system.
Most other models suggest a wintry mix will affect somewhere
along and swd of a line from ZZV-PIT-IDI.

Nwd from DUJ-UCP-PHD, cold air will remain firmly ensconced, 
with frontogenesis and an eventual deformation zone leading to 
banded snowfall enhancements. Winter storm conditions in these 
zones are a high-confidence forecast, with 8-12+ inches expected.

Freezing rain, especially in the ridge zone valleys, also may 
be not insignificant, where the warm air plume rides atop cool, 
low-level air resulting from prolonged sely upslope flow.

There remains, however, considerable forecast bust potential 
with this system as a result of several items:

1) The strength and nrn extent of the warm air plume differ
considerably among the various forecast models, leading to low 
confidence in the forecasted amounts in this transition zone.

2) The duration of any high precipitation rates with respect to
any transition to rain. In other words, will the high QPF occur
while it is cold enough for snow?

3) The potential for ice/sleet to interfere with snow totals.

As has been the case for several days, a gradient in snowfall
is expected to occur from near Zanesville to near Pittsburgh to
near Latrobe, and will lead to a sharp decrease in snow totals
from north to south across whichever counties experience this
transition zone.

***OVERALL MESSAGE: A shift by as little as 20 miles in the
 system track will drastically alter the location of this
 transition zone.

 In this zone, it is possible for up to a foot
 of snow. In this zone, it also is possible for 4-6 inches snow
 and some ice/sleet. In this zone, it also is possible for 
 virtually all rain during the height of the storm. And all of 
 that could occur in different sections of the same county.

 Anyone in this zone should be prepared for up to 12" of snow, 
 or as little as 1" snow, because that is the reality of this 
 system's event horizon.***

By late Sat night, cold air will advance sewd at the back side
of the upper-level low, leading to a transition back to snow in
all zones as the system departs.

Apart from far-ern sections, widespread snow should be winding 
down during the morning on Sun, and a transition to snow 
showers will ensue as lake enhancement increases.

Cold air Sun night and continued wind will lead to wind chill
values reaching advisory levels in many areas. This concern has
been highlighted in the HWO at this time, with headlines likely
once any wintry precipitation is over.


Mid-level troughing will linger through Monday along with much
below-normal temperatures. However, as surface high pressure 
approaches, a relaxing pressure gradient will lead to lighter 
wind and less-brutal wind chill values. May need to add 
flurries in to future issuances as some moisture may linger in 
the near-ground snow growth zone.

That high will provide dry weather Monday night and most of Tuesday. 
Temperature will moderate to normal levels as flat 500 mb 
ridging passes by, and even rise above normal by Wednesday.

A shortwave trough returns precipitation chances during the midweek 
period. At this distance, a bit more rain is likely with this 
system as opposed to snow, but timing and strength differences 
in the models may affect this balance. Colder air and snow 
showers will follow to close out the work week.


MVFR cigs should partially erode or lift to VFR in some areas
tonight under weak low level ridging, though moisture is still 
progged to persist under a temperature inversion. Deterioration 
to IFR is expected Saturday as low pressure tracks across the TN
Valley to the OH Valley region, spreading wintry precipitation 
over the region.

Mainly snow is expected N from around PIT N, with snow changing
to a mix of snow, freezing rain and rain near PIT and S. Ports S
of I 70 should see a change from snow to rain in warm advection
ahead of the approaching low. All areas should see precip change
to all snow Saturday night in cold advection on the wrn side of
the low as it tracks to the E Coast. NW wind should also become
gusty Saturday night with a tight pressure gradient on the wrn 
side of the exiting low. 

Cig restrictions and gusty winds are expected Sun in cold NW 
flow. Restrictions are likely with a Tue night/Wed cold front.


MD...Winter Storm Warning from 1 PM Saturday to 1 PM EST Sunday for 
OH...Winter Storm Warning from 10 AM Saturday to 10 AM EST Sunday 
     for OHZ039>041-048>050-057>059.
     Winter Weather Advisory from 10 AM Saturday to 1 PM EST Sunday 
     for OHZ068-069.
PA...Winter Storm Warning from 10 AM Saturday to 10 AM EST Sunday 
     for PAZ007>009-013>016-020>023-029-073-074.
     Winter Weather Advisory from 10 AM Saturday to 1 PM EST Sunday 
     for PAZ031-075-076.
WV...Winter Storm Warning from 10 AM Saturday to 10 AM EST Sunday 
     for WVZ001>003.
     Winter Weather Advisory from 10 AM Saturday to 1 PM EST Sunday 
     for WVZ004-012-021-509>514.