Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
1158 am EST Friday Feb 27 2015
dry conditions and a warming trend in temperatures can be expected
as we head into the weekend before the next area of low pressure
results in rain and snow Sunday through Monday morning.
Near term /through Saturday/...
cut back on cloud cover this morning through midday where at least
mostly sunny conditions are expected. Despite the more potent late
February sunshine...temperatures will remain cold through the day as
cold air continues to advect in. Coldest 850mb temperatures near
-22c will be directly overhead this afternoon as high pressure
As the sun sets tonight...temperatures will fall quickly under
calm...dry and mostly clear conditions. This should optimize
radiational cooling for most locations...especially those
locations that received a fresh coating of snow. Surface high pressure
will be directly over the forecast area pre-dawn Saturday. Models
continue to show warming of the middle-levels to the tune of 6 to 8
degrees c above the 00z minimums. With a lack of flow between the
surface and 750mb...a near surface inversion will decouple the atmosphere
and drop temperatures below zero for most locations...well below
zero across the northern zones.
At this point...the only fly in the ointment for temperatures as
low as forecast is possibility of high clouds shielding the
outgoing longwave radiation. The 00z NAM is a little bit faster in
introducing high clouds in response to a weak upper level wave.
Ultimately tweaked Sat morning lows...which includes bumping up
temperatures across the southern 1/2 of the County Warning Area by a degree or two.
After daybreak Saturday...the near surface temperature inversion
should mix out and allow for a quick warm-up under mostly sunny conditions.
Short term /Saturday night through Monday/...
latest model runs show relatively good agreement for the next low
pressure system to impact the region...and overnight changes to
the forecast were based mainly on a blend of ec...GFS...and
ensemble guidance as the HPC preferred solutions...with less
weight put on the sref and NAM. This trends towards a more
amplified and colder solution. Precipitation will start out as all
snow...but begin changing over to rain over the course of the
morning and now anticipate snow slightly farther to the
south...and a rain and/or rain/snow mix mainly limited to
locations south of Interstate 80. Looking at critical thicknesses
there is still some uncertainty...largely south of Interstate 80
as low level warm air advection will be strong on Sunday which is
reflected in the 850-700mb values. Therefore...have held back
forecast snow amounts south of Interstate 80 for now as a
changeover to rain on Sunday afternoon or mixed precipitation will
impact accumulations. North of I-80 snow amounts were introduced
or increased as forecast confidence in all snow is higher
here...and based on the latest headlines may be introduced
soon...at least for northern locations. By Monday morning...any
ongoing precipitation will have changed back over to snow with
cold air advection on the backside of the system.
Long term /Monday night through Thursday/...
a warm up in temperatures expected early on in the extended as
troughing deepens over the western US and southwesterly flow sets
up across the eastern states. A closed off area of low pressure
ejecting out of the Desert Southwest early in the week will start
to transition east Monday and help drag in moisture off the socal
coast into the strong southwesterly flow...with additional
moisture pooling in from the Gulf. Ensemble members are showing
minimal spread in this setup...which will may result in a
significant amount of precipitation through the middle of the week
across the region. Forecast precipitable water values are around the 99th
percentile by Wednesday over the area and ivt values are showing
a bulls eye over the region. This combined with the additional snow
across the north and a more ripe snowpack to the south raises
flooding concerns and will be following this closely in upcoming
Aviation /18z Friday through Wednesday/...
snow and associated MVFR cloud cover is continuing to wane this
morning. The latest satellite imagery reveals clearing skies west
to east. MVFR ceilings may hold in mgw and lbe this morning for a
few more hours...but all sites are expected to be VFR with mostly
clear skies shortly after daybreak. High pressure will ensure VFR conditions
with weak surface flow through the balance of the taf period.
Outlook.../Friday night through Tuesday/...
restrictions are likely with crossing low pressure Sunday into
..first significant river rise of the season expected Tuesday and
Wednesday Mar 3 and 4...
given the fact that many of the area rivers... tributaries and
smaller streams and creeks are covered over with ice... a dramatic
increase in flow and stage levels will cause ice movement starting
Tuesday Mar 3. Currently forecast has a high level of uncertainty
about many factors including rain amount...temperature...snowmelt...ice
movement. Small streams and tributaries will be first affected
Tuesday Mar 3. Rises on the major rivers is projected to occur
starting Tuesday night and contine through Wednesday Mar 4.