Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
558 PM EDT Sat Mar 28 2015
dry and cold through Sunday. Another cold front will bring light
rain or snow Sunday night.
Near term /through Sunday/...
early evening update for hourly temperature and dew point trends.
Previous discussion follows.
High pressure will be firmly planted over the area and surface
flow diminishes to calm. With clear skies and weak boundary layer
flow...there should be little trouble Delaware-coupling the lower levels
tonight. This means that despite warm air advection taking place
above 925mb...temperatures tonight will remain some 15 to 20
degrees below the average. Any remaining snow cover that survives
melting and sublimation today could locally drive some
temperatures down even further.
Axis of the surface high sifts east of the area Sunday...allowing
low-level flow to shift to the south-southwest. Sunshine and
southwesterly flow will bring warmer air back to the area as a
system approaches from the west. Copious dry air in place will
keep precipitation at Bay through much of Sunday...but rain
chances will increase quickly heading into the short-term.
Short term /Sunday night through Tuesday/...
isentropic ascent ahead of an incoming warm frontal boundary
increases rapidly during the evening hours on Sunday. While the
boundary layer starts out rather dry...fairly potent warm/moist
advection looks to quickly overcome this dry air with widespread
light precipitation. Model spread on precipitation coverage by late
Sunday evening is not very large...nor is the divergence of quantitative precipitation forecast
solutions. All models indicate a rather high pop/low quantitative precipitation forecast type of
event for Sunday evening and overnight. The real tough part of the
forecast is more likely to be precipitation type.
While thermal profiles Sunday evening and overnight do not vary
significantly from model to model...small boundary layer
differences as well as the handling of quickly eroding cold air
with strong advection and a lack of an inversion to inhibit mixing
all will work toward making precipitation all rain. However...very
wide dewpoint depressions as well as fairly cold initial 850 mb
and boundary layer temperatures will work toward more frozen
precipitation at onset. It does seem that at least initially
moisture and lift will be deep enough to partially bisect the
dendritic growth region...however deeper moisture quickly peels
off as as surface low moves through the region. This leaves a bit
leaner moist layer below the dendritic growth region with lift and
continued enhanced baroclinity. That said...probability of precipitation were tough to
rule out into Monday afternoon and night across the northeastern
zones and p-types do look to trend toward liquid for a time.
While most of the area will temporarily dry out with the passage
of the surface low on Monday...this will likely only be temporary.
Another slug of moisture and warm advection associated with the
next low to affect the region will slide down the frontal boundary
that will remain over the area. Deju vu looks to happen as well
with deeper moisture and lift again bisecting the dendritic growth
region and colder air to the north favoring at least some mix with
snow...however strong southwesterly flow will favor all liquid
across the southern half of the County Warning Area. Probability of precipitation were increased for this
system on Tuesday as model solutions really seem to have fallen in
line for favoring that time frame for wet weather. Fries
Long term /Tuesday night through Saturday/...
high pressure will briefly build into the region Tuesday night
through Wednesday night before next system approaches from the
west. Although GFS/European model (ecmwf) both show a front crossing the region
Thursday night...models quickly diverge by Friday. Have thrown out
the fast GFS outlier and stuck closer to European model (ecmwf) forecast which has
ensemble support indicating the front will likely hang up for
another day. This could allow for a prolonged period of rain
across the region. Temperatures will be near normal through the
period...with thursdays values rising to more than 10 degrees
Aviation /22z Saturday through Thursday/...
VFR conditions are expected through Sunday under high pressure. SW
winds should increase Sunday afternoon as the high shifts east and
the pressure gradient increases.
Outlook.../Sunday night through Thursday/...
restriction potential returns Sunday night/early Monday with an
occluded front...and again crossing low pressure Tuesday and