Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
538 PM EST sun Dec 21 2014
midwestern low pressure will bring warmer temperatures and rain to
the upper Ohio Valley region for the first half of the week. Snow
shower chances will come with passage of that systems cold front
Near term /through Monday/...
with 530pm update...tweaked sky coverage and hourly temperature
trends. As quickly as low clouds erode from the south to the
north...additional high clouds also move in from the south. Also
updated hourly temperature trends as partial clearing has allowed
a slightly quicker drop off in temperatures. Previous discussion
Warm advection begins in earnest overnight...bringing h850 temperatures
above zero for the southern County Warning Area by 12z Monday. Some lower clouds
could be realized prior to 12z along east-facing slopes in
Garrett/Tucker counties. Generally leaned towards the warmer
temperature guidance for lows.
On Monday...inverted trough sets up along the Carolina coast. With
departing high across New England...a cold air damming scenario
looks to be setting up for the east-facing ridges. For most
areas...it will simply be a mostly cloudy day. Better
moisture spins around the inverted trough towards the southeast
corner of the County Warning Area beginning later Monday morning. With the
overnight warm advection aloft and cold air expected to be trapped
in the valleys...a temperature structure ideal for freezing rain
will be present across Garrett/Preston/Tucker counties.
However...quantitative precipitation forecast will be the limiting factor as moisture and lift are
too shallow to support precipitation of any significance. Cannot
justify anything higher than chance probability of precipitation at present. Only a couple
hundredths of an inch of liquid are expected at most. Will not be
issuing any headlines at the present time. Special Weather Statement can be used in the
morning if the threat increases. Continued warm advection west of
the ridges will push Monday high temperatures at least several degrees
above normal...while temperatures remain in the 30s along the eastern County Warning Area
Short term /Monday night through Wednesday night/...
upper trough will amplify and slowly pivot eastward during this time
frame...becoming more negatively tilted with the help of a
strengthening upper level jet by late Wednesday. Early on...there
still remains some uncertainty in regard to the amount of precipitation
that we will see on Tuesday...as we will be parked in between the
better moisture from the weak coastal system...and the deepening
moisture associated with the cold front to our west. In
addition...S-se wind should help provide some downsloping and drying
just east of the ridges in PA/WV. With this in mind...opted to stay
with the chance probability of precipitation for the day Tuesday. Will increase probability of precipitation
overnight as the low level jet strengths and helps to pull moisture
from the Gulf northward. With the nearly northward progression of
the developing surface low...both the European model (ecmwf) and GFS suggest the bulk
of the heaviest rain will stay just west of the forecast area...with
the best chance for moderate rain to be as the cold front crosses
Wednesday night. This should limit any flood threat for the period.
Strong warm advection will be ongoing through the entire time
period. Even with cloud cover...high temperatures on both Tuesday
and Wednesday should be well above seasonal values with highs
likely to reach the middle 50's.
Long term /Thursday through Sunday/...
cold front passage will initiate snow shower chances on Christmas...but
lack of depth to the cold layer...dry advection in the middle levels on the
southern perimeter of northward digging upper low...and rapidly ensuing warm
advection will ensure a lack of snow cover for Christmas.
The remainder of the long term period is prognosticated to feature seasonal temperatures
with a late week cooling trend as models forecast a reamplifying flow
portion with eastern Continental U.S. Troffg.
Aviation /00z Monday through Friday/...
genl improvement to VFR for all taf sites can be expected for the
taf period as southerly flow aloft intensifies over linger surface high pressure.
Outlook.../Monday through Friday/...
midwrn low pressure will spread rain and restrictions over area ports on
Tuesday and Wednesday. Improvements are expected to be initiated
later on Thursday with dry shortwave ridging.