Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Pittsburgh PA
1248 PM 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/...
cold front currently lies over western Ohio...with an imminent
wind shift coming for Zanesville and New Philadelphia. Much of the
rain is in advance of the front itself...and areas north and west
of Pittsburgh has already seen the bulk of the rainfall for the
day. Updated pop grids to account for speed of frontal
band...along with some slight temperature modification to account
for falling temperatures behind the front. Locations south and east
of Pittsburgh could pick up upwards of a quarter inch of rain this
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 /18z Thursday through Tuesday/...
a cold front will move slowly across the area this afternoon/Erly evening. Generally MVFR
conds expd...with occasional IFR in heavier showers S and east of pit. Exptg a
decreases to IFR conds overnight into Thursday as waves of low pressure move NE along
the fnt spreading addnl precipitation across the region. Temperatures will cool as
well...and expg a wintry mix of precipitation across the area lt tonight into
Thursday. Mgw should stay warm enough for all rain however through Thursday.
Precipitation should change to all snow Thursday evening before ending.
Outlook.../Friday through Tuesday/...
rstrns are likely through Sat morning as low pressure exits the region. Rstrns
are likely again sun and Monday as low pressure brings a wintry mix to
the region. VFR should rtn Tuesday under building high pressure.
Thursday update: the change from Wednesday is an expansion of the
heavier rainfall a bit farther eastward over the cheat and
youghiogheny river basins. This will result in slightly higher
crests on Saturday. Crests are still expected to remain below
flood stages, but action levels are still projected to be reached
along the Monongahela and Ohio rivers.
Summary: a river rise of 5 to 9 feet is coming this Saturday (dec
7, 2013) on the Monongahela and Ohio rivers. And then a second
rise is likely Monday (dec 9, 2013). Current weather models have
been consistent for the past 3 days and continue to indicate a
forecast of 2 to 3 inches of rain over the Monongahela and upper
Ohio River basins Friday. That heavier rainfall area has been
expanded into the cheat and youghiogheny river basins.
A general 5 to 9 foot rise is expected Saturday on the
Monongahela and upper Ohio rivers. At this point projections are
to bring Grays Landing up 17.9 feet Saturday late morning and
Pittsburgh 20.2 feet Saturday afternoon. The Monday parking wharf
will be affected starting Saturday.
Still another rainstorm 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. The rainfall
amounts on Sunday are more uncertain at this time. Initial
estimates are projected to be 1 and 2 inches, but the ground will
be saturated and it will take less rain to the cause the same
River levels should recede Saturday night and Sunday then
a second rise is expected Monday that could crest at higher than
Saturday levels. The current projections do not include the
projected rainfall for Sunday at this point.
The precipitation over the Allegheny will be minimal and will
likely fall in the form of snow.
Rivers will recede Tuesday and for the rest of the week.