Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
1003 am EST sun Mar 1 2015
..updated near term discussion...
significant snow accumulations are expected across
the region today into this evening...with snow mixing with and
changing to rain for a period south of I-70. A second
storm...accompanied by milder temperatures...will bring mostly
rain to the region Tuesday and Tuesday night...possibly beginning
as a period of snow or freezing rain earlier in the day.
Temperatures will gradually trend warmer through
Wednesday...before colder air returns later in the week.
Near term /through this evening/...
Widespread precipitation envelope extends from the Pennsylvania
commonwealth all the way back to Missouri this morning...showing
the very large presence of this winter event. Widespread 2-3" snow
amounts have already been reported across many areas...giving ME
some concern. 12z kpbz sounding shows a saturated profile...with
the warmest layer (950-900mb) still a couple degrees celsius below
freezing. To that end...given the vast volume of precipitation on
regional radar imagery...most of the modest warm advection is
being realized as vertical ascent. See little reason to justify
the degree of middle level warming indicated in previous forecasts.
This should ultimately yield a nearly isothermal profile just
below freezing...with the exception of the boundary layer in the
So the question is just how much boundary layer warming can we
expect...and is there enough vertical depth greater than 0c to
mix/change to rain? It is quite possible that precipitation could
remain as snow and/or a mix up to maybe 35-36f surface
temperatures. Pittsburgh-Indiana seems like a rough demarcation
of the northern extent of this diurnal warming as it is March
1st...with the greatest chance for a complete temporary changeover
likely south of I-70 and west of the ridges.
If my hunch is correct...this also throws a wrench into the
Laurel ridges into WV/MD...where its certainly possible that
locations with elevations of 2-4kft mean sea level are actually /above/ the
warm layer and could remain snow. Have made upward snow adjustments
in these areas as well.
With that said...this will be an extremely heavy...wet snow that
will be tough to manage. Persons out shoveling need to exercise
extreme caution...taking frequent breaks. Additionally...with
pavement temperatures so cold from the previous Arctic
outbreak(s)...a glaze of surface ice will likely develop on
untreated...exposed surfaces where snow melt and/or rain makes
As for snow amounts...this is an event where proper measuring is
key. If one waits until the end of the event to stick a ruler into
the snow...you may only see 1/3 to 1/2 of what actually fell due to
the combination of rain over the south and the
compaction/settling/melting of heavy...wet snow across the entire
region. Given 2-3" of snowfall already...total amounts of 5-8"
seem reasonable across most areas north of I-70 as well as the
ridges. However...due to the long duration of the event...mixing
of rain...and settling there may never be a snow /depth/ greater
than 4-6". For these reasons...and the mainly light nature to
periodically moderate of the snow...will maintain the upper end advisory
headline at this time.
Short term /Monday and 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 /14z 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-