Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
509 am EST sun Mar 1 2015
snow and rain showers can be expected today and again on Tuesday.
Temperatures will trend warmer through Wednesday...before cold
returns late in the week.
Near term /until 6 PM this evening/...
Precipitation should be steady but relatively light the majority
of the day with persistent moisture advection and isentropic lift
on the front side of an approaching trough. Strong low level warm
air advection will result in a changeover to mixed precipitation
south of Interstate 80 and rain primarily south of Pittsburgh.
Although model quantitative precipitation forecast amounts continue to show large values...snow
accumulations will be hampered by the mixed precipitation and rain
across the aforementioned areas. Additionally...soundings show the
saturated layer up to approximately 10kft nearly isothermal with
temperatures above -10c which is not quite cold enough fro
large dendritic snow growth. Further...average mixing ratio
values of along 295k (which seems to be showing the best
isentropic lift between 700-750mb)look to be around 2.5-3g/kg from
this morning until Monday morning...which suggests the potential
snowfall through that time only to be around 5-6 inches.
Looking at upstream observations and snowfall rates thus far and the
duration the forecast area will be under the main lift and
moisture advection...these amounts seem pretty reasonable for
locations expected to see all snow. (Along and north of the i80
The associated cold front will pass through overnight changing any
remaining precipitation back over to snow...diminishing quickly as
high pressure builds back in across the region.
All things considered...the Winter Weather Advisory thus remains
appropriate and has been maintained for prolonged snow
accumulation...and for southern locations in the advisory...a
brief period of ice in the morning.
Short term /6 PM this evening through Tuesday/...
building surface high pressure will result in mainly dry
conditions on Monday...however...however...a south-southwesterly
850mb jet on the eastern flank of midwestern low pressure is
projected to strengthen on Monday night and fuel another round of
warm...moist advection with light precipitation again developing
over the region. That precipitation likely will begin as light
snow...but with rapid warming aloft...a period of freezing rain is
likely in the entrenched surface cold.
That period is expected to be relatively brief with the surface
low projected to pass northwest of the forecast area...leaving the
Ohio Valley well within the warm...hence the rain sector until
cold frontal passage on Wednesday.
As models tend to overdo quantitative precipitation forecast in warm air advection patterns...and
given that this projected system is an open wave...quantitative precipitation forecast on Tuesday
was toned down slightly from the more assertive GFS values.
Long term /Tuesday night through Saturday/...
cold front crosses the region Tuesday night...reaching the middle-
Atlantic region around dawn Wednesday. Rain will continue to fall
as the front passes through. Will decrease probability of precipitation from northwest to
southeast Wednesday with colder and drier air advecting in. Any
lingering precipiation Wednesday afternoon...will change to snow
showers as colder air rushes in from the northwest.
Surface high moves in Thursday...keeping the remainder of the long
term dry and cold.
Aviation /10z Sunday through Thursday/...
a quick deterioration in conditions overnight from west to east.
Large area of snow will overspread the region taking VFR conditions
quickly to MVFR. The main problem will be the long term reduction
in visibility...generally IFR...for much of the forecast period.
Still some uncertainty on how far north the warmer air will make
it...but it does appear a good bet to see an eventual change over
to rain at mgw by late morning. A mix is possible at all ports
except fkl and duj. Right now confidence is low...so kept all snow
for the forecast. Winds will remain light through the day.
Outlook.../Sunday night through Thursday/...
restrictions likely will persist through much of Monday. Low
pressure crossing the Great Lakes is likely to spawn widespread
restrictions once again by late Tuesday.
many of the area rivers and streams are ice covered. An increase
in flow and stage is expected to cause ice movement starting
Tuesday...beginning on the southernmost river basins. The current
forecast continues to have a high level of uncertainty about
precipitation phase and amounts as well as the degree of snow
Maryland...Winter Weather Advisory until 7 am EST Monday for mdz001.
Ohio...Winter Weather Advisory until 7 am EST Monday for ohz039>041-
PA...Winter Weather Advisory until 7 am EST Monday for paz007>009-
WV...Winter Weather Advisory until 7 am EST Monday for wvz001>003-