Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
1105 am EDT sun Mar 16 2014
low pressure crossing south of the region will bring snow mainly
south of Pittsburgh Sunday night into early Monday. Moderating
temperatures prevail by midweek.
Near term /until 6 PM this evening/...
clouds will continue to thicken as moisture associated with the
surface low over Arkansas pushes northward. While some middle-level
moisture has provided for a band of thickened clouds north of
I-80...this has produced little in the way of precipitation this
morning. Expect maybe a few flurries could be possible but dry air
at the surface with building high pressure will likely keep it as
virga. With the northeasterly wind flow...expect temperatures to
warm very little today. Most of the temperatures are falling
currently as opposed to what model guidance has been indicating.
Precipitation should begin to overspread the southernmost counties
late this afternoon. The newer model guidance continues to stream
in but an overall trend in a slightly northern edge to the
snowfall may lead to some adjustments later.
Short term /6 PM this evening through Wednesday/...
considerable divergence still exists on the amount of northward
progress the layer of deformation and frontogenesis will make into
the forecast area overnight tonight. The consensus strongly favors
at least the southeastern most portions of the forecast area seeing
at least a period of 12-18 hours of moderate lift through the
dendritic layer...so at least moderate snow accumulations seem
like a foregone conclusion there. Elsewhere...the moisture looks
to get quickly leaner as your proceed northward.
The 00z GFS and European model (ecmwf) both spread quantitative precipitation forecast a bit farther north than
their earlier counterparts...while the 00z NAM/Canadian basically
towed the line of their earlier cycles. Given increasing
northeastern gradients that seem to be likely through the day due
to pressure falls to our southwest...and upstream dewpoints to the
northeast in the single digits at the moment...it seems more
plausible that the northward extent of the lift will have a bit of
difficulty saturating the boundary layer. As such...the more
southerly solutions were favored a bit over the northerly ones.
This resulted in less quantitative precipitation forecast/snow over the Zanesville to Pittsburgh
corridor than would be suggested by the GFS.
Where the models concur a bit better is over the southeastern County Warning Area.
Solid lift extends from the top of the boundary layer up through
the dendritic layer with good saturation throughout the column.
Additionally...warm advection aloft...while impressive...does not
seem to be so effective so as to get the any portion of the column
near freezing. This means an all snow event seem to be in store
after the initial wet bulb cooling effects at onset eradicate any
warmer air in the layer that may allow for an initial mix with
rain. Because of this...ratios on the order of 13-15:1 seem to be
favored via objective schemes in BUFKIT from both the NAM and
GFS...and were also favored in the forecast.
While the northward spread of significant quantitative precipitation forecast is still a bit
questionable...it seems at least the models favor around 0.25 to
0.45 inches over 18-21 hours over the southeastern ridges. Using
the aforementioned ratios this yields roughly 3-7 inches in those
locations. Due to the duration of the event...this is not
sufficient to favor upgrades of the winter storm watches to
warnings...however heavy handed advisory accumulations seem in
store...particularly in Tucker County. While the 6 inch warning
criteria will be likely in eastern Tucker County...the duration
seems likely to be longer than 12 hours to get there.
The system looks to depart eastward quickly on Monday with warmer
air wasting no time in returning to the region. Warm advection
looks to increase into the area through Wednesday as the system
associated with the second middle-level low tracks to our south and
east through Tuesday.
A deeper system looks to approach from the Central Plains into the
Great Lakes by Wednesday. While taking on an increasingly negative
tilt with warm advection increasing ahead of a frontal boundary
that slices eastward through Wednesday...and very impressive 0-6
km shear values...showers are likely to break out across the area
Wednesday. Whether the warm sector advances into/through the area
on Wednesday seems likely to determine whether or not we see
thunder chances. The GFS has been far more consistent than other
guidance with this system and continues to indicated strongly
negative Theta-E lapse rates in the low levels along the
significant middle-level drying in association with the incoming middle-
level jet and deepening tropopause undulation. As a result...lapse
rates seem likely to increase locally along the front...which
would likely result in some sort of convective line developing
along the front as it tracks eastward. While thunder still seems
quite possible...have opted not to re-add it to the forecast to
prevent a yo-yo of thunder in the forecast until the details get a
little more clear. Regardless...temperatures do seem to peak out
Wednesday ahead of the system...before likely falling a bit into
the extended forecast. Fries
Long term /Wednesday night through Saturday/...
after the passage of wednesday's system...confidence remains low
with any particular shortwave...as model divergence makes it
difficult to pinpoint any time of higher or lower probability of precipitation. One aspect
where there is agreement is a pattern that will promote normal to
above normal temperatures.
Aviation /15z Sunday through Thursday/...
VFR forecast today with increasing high then middle clouds. Surface
winds will strengthen later this morning with gusts of 20 to 25
kts possible through the afternoon.
Outlook.../Sunday night through Thursday/...
the next chance for restrictions will come from the northern edge of
low pressure that should cross from the Gulf Coast to the coast of
North Carolina. IFR conditions with snow are possible tonight
into Monday morning at mgw...with lesser chances at lbe/zzv/hlg/pit/
agc. VFR conditions are expected behind the system Monday through
Tuesday night. Another system should bring showers and possible
restrictions Wednesday...with a return to VFR Wednesday night and
Maryland...Winter Weather Advisory from 5 PM this afternoon to 2 PM EDT
Monday for mdz001.
WV...Winter Weather Advisory from 2 PM this afternoon to 2 PM EDT
Monday for wvz041.
Winter Weather Advisory from 5 PM this afternoon to 2 PM EDT
Monday for wvz023.