Scientific Forecaster Discussion
fxus61 kpbz 192255
Area forecast discussion
National Weather Service Pittsburgh PA
555 PM EST Fri Jan 19 2018
high pressure provides dry and milder weather through Saturday.
Light rain chances return by Sunday, with a period of steadier
rain possible by Monday night. Cooler weather returns for the
middle of next week.
Near term /through Saturday/...
mainly cirrus is expected through the night as high pressure
slides to the east and warm advection continues. Low
temperatures will end up above normal and could be achieved
fairly early during the night. Values may steady or rise
slightly overnight as the column remains mixed, although any
valleys that manage to decouple could cool more than forecast
given the remaining snowpack.
Warm advection resumes relatively unabated with gusty
southwesterly flow continuing on Saturday. Snowmelt should begin
in earnest as temperatures rocket into the 40s, however the
increase in boundary layer moisture should allow for stratus to
develop by the end of the day as the boundary layer moistens up
a bit. Fries
Short term /Saturday night through Sunday night/...
an upper trough will trudge eastward during this period, from
the Pacific coast and across The Rockies to the plains by Sunday
night. While flow aloft remains quasi-zonal across our region,
low-level southwest flow will continue the warming trend. High
temperatures in the 40s both Saturday and Sunday will help to
melt much of the snow pack.
Clouds will be on the increase Saturday, with a lower
stratocumulus deck possibly arriving by the afternoon. Enough
moisture should be advected in on weak low- level isentropic
lift to support drizzle or light rain starting Saturday night
and continuing into Sunday night. Lift and forcing remain weak
through this period, and minimal quantitative precipitation forecast of around or less than a
tenth of an inch is forecast in general. There is still a small
concern that freezing rain/drizzle may be possible mainly north
of Pittsburgh for a time Saturday night, depending on how
quickly the boundary layer can cool during the evening. Chances
of that remain low however, and do not merit even an severe weather potential statement mention
at this point.
Long term /Monday through Friday/...
as the vertically-stacked low crosses the Mississippi Valley and
the Great Lakes Monday and Tuesday, a crossing cold front will
provide the focus for the next chance of organized rainfall,
mainly Monday afternoon and night. The moisture plume ahead of
the front remains deep but fairly narrow, and the system is
still expected to be progressive as the upper trough takes on a
negative tilt. Models and ensembles have been fairly consistent
in keeping the quantitative precipitation forecast under control with this system, generally a
half-inch or less. Given the loss of much of the snow pack prior
to this, large-scale flooding issues are still not projected to
be a concern. However, localized issues due to ice jams will
remain quite possible, and this threat has been addressed in the
severe weather potential statement for now.
Behind the system, temperatures will drop back to seasonable
levels for Tuesday and Wednesday, with any lingering showers
turning over to snow by Tuesday night. A minor ridge
accumulation is possible during this period. High pressure
should allow Thursday and Friday to feature dry weather and
another warming trend to close out the week.
Aviation /00z Saturday through Wednesday/...
VFR will prevail for much of the period as high pressure remain
in place. Diurnal mixing will support a south-southwest with gusts to 20kts
both tomorrow afternoon. Model guidance suggest moisture,
trapped under building inversion aloft may lead to some
late afternoon and evening restrictions tomorrow. It would seem
this should take until after diurnal mixing starts to abate,
however, so it was generally restricted to the very end of the
period or beyond.
restriction potential returns with the new week and an
approaching frontal system.