Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
335 am EST Thursday Dec 5 2013
rain will spread over the region today as a cold front slides
toward the region. Rain will change to sleet and snow on Friday.
Near term /until 6 PM this evening/...
another mild day is in store for the upper Ohio Valley region as warm
air continues to flows northward on the eastern flank of deepening low pressure
digging north of the Great Lakes. Expect highs about 20 degrees above
the avgs...which were forecast using persistence and the bias
Rain chances will also be on the increases as moisture and sny forcing for
ascent increases undr eastward sliding jet with diffluent flow developing as
upper ridge axis over the East Coast also is eroded eastward. Bottom line is
that pop escalation to likely...then categorical numbers...has been
maintained although timing adjustments were included.
Short term /6 PM this evening through Saturday night/...
latest deterministic trends indicate a break in precipitation intensity
this evening as initial shortwave races northeastward...however...another
disturbance in the strong flow is expected to quickly refill the
precipitation field. Have not tried to account for the disparity within
this progressive regime...but have tempered evening...and Friday
morning quantitative precipitation forecast especially after study of various quantitative precipitation forecast guidance. This
trend combined with timing adjustments for later rain development today
has eased forecast event rain totals back undr the inch and a
half mark for the region.
Next concern is of precipitation type on Friday as front slides eastward. In
genl...previous forecast trends lake OK and minimal adjustments were
included. However...ovrall thermal profile and thickness prognosis
seems more indicative of a prolonged mix of rain...sleet...and
snow as warm advcnt above the boundary layer continues to thwart decisive
phase change. Nevertheless...cold advcnt and gradually increasing
snow to liquid ratios will spprt accumulating snow where temperatures fall
to freezing...so new accumulation forecast falls within advisory
criteria. Also of interest as far as snow limitations are
concerned is the more rapid advancement of the middle level dry
slot...especially as per the latest GFS which encroaches that
feature shortly aftr 00z Saturday.
The bottom line after collaboration with neighbor forecast offices
is that a third...to fourth period headline may not be prudent
given the continued evolution of the system. A hazardous weather
outlook mention will be continued.
Long term /Sunday through Wednesday/...
more active weather is expected early next week as portion of central
Continental U.S. Trofg and western Atlantic ridging maintains ovrall southwesterly flow
over the upper Ohio region. European model (ecmwf) and GFS again indicate rapid
emergence of low pressure from the lower MS valley to the Great Lakes on
Sunday and Monday.
As these models have consistently shown a track west of the
immediate area over the last cpl of days...have maintained the warmer
wpc solution as is typical of that winter path. Unfortunately...the
warm advection on the eastern flank of the low...combined with cold air
entrenchment reinforced via New England high pressure...may spprt
prolonged freezing rain for part of the area...namely counties
north and east of Pittsburgh. For now...have opted for a more
aggressive precipitation phase change for most zones from snow and freezing
rain to all rain as Sunday night progresses. A hazardous weather
outlook will also be included and monitoring will continue.
Otherwise...colder and drier air will be shunted over the upper Ohio
region in the wake of that system...with temperatures and probability of precipitation forecast
using a combo of wpc and MOS guidance.
Aviation /08z Thursday through Monday/...
general VFR overnight with stratocu coverage increasing.
Restrictions will move in from the north and west around dawn...
with a MVFR ceilings falling to IFR by late in the day. Rain will move
in from the southwest after dawn. Main concern through morning
will be low level wind shear at all ports.
Outlook.../tonight through Monday/...
rstrns are likely through Sat morning as a slow moving cold front...along with a
series of disturbances...cross the region. Rstrns are possible again sun
and Monday as low pressure moves across the area.
a river rise of 4 to 6 feet is expected Saturday on the
Monongahela and Ohio rivers. And then a second rise is likely
Monday. Initial quantitative precipitation forecast forecasts showed 1.5 to 2 inches of rainfall
over the Monongahela and upper Ohio River basins Friday. Then
another rain event is projected over the same area Sunday that
will continue to keep river levels and flows on the Monongahela
and Ohio rivers elevated through Tuesday.
A general 4 to 6 foot rise is expected Saturday on the
Monongahela and upper Ohio rivers that may affect the Monday parking
wharf. At this point projections are to bring Grays Landing (15
foot action stage) up 16.7 feet Saturday morning and Pittsburgh
(18 foot action stage) to 18.8 feet Saturday evening. These stage
projections would still be below flood stages. River levels should
recede on Sunday then a second rise is expected Monday that could
crest at nearly the same levels or perhaps 1 to 2 feet higher than
Saturday levels. The precipitation over the Allegheny basin will