Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
749 PM EDT Tuesday Sep 23 2014
dry weather can be expected for the remainder of the week and temperatures
will gradually warm.
Near term /through Wednesday/...
only changes needed for the early evening update were some downward tweaks
to overnight lows based on the latest dewpoint and guidance trends.
Otherwise...surface hipres centered in eastern Ohio will develop northeastward as an upper
trough over the coastal southeastern Continental U.S. Cuts off and builds northward slowly
through Wednesday night. At the same time...upper ridge over the northern plains will
Southeastern Continental U.S. Trough will move northward along the Appalachians on Wednesday as associated
coastal lopres develops toward the chesepeake Bay. Cirrus associated with this
trough will begin to spread nwwd across forecast area on Wednesday morning.
Precipitation is not expected to reach the forecast area as strong upper ridge to west
should be sufficient to prevent westward mvmt of upper low.
Rising heights/warm advcnt will spprt an increase in temperatures for Wednesday to slightly
above climatology amid deeper mixing and strong morning sunshine.
Short term /Wednesday night through Friday/...
model guidance is somewhat split on the track of a middle-level low
up the East Coast going into Thursday. Most domestic guidance
keeps the majority of the height falls and moisture to our
east...while the European model (ecmwf) wraps Atlantic moisture a tad farther
west...and the Canadian wraps it all the way back into our
forecast area. This is a manifestation of a weakening of the middle-
level ridge over the central Great Lakes from Wednesday afternoon
into Thursday on all the models...however a tracking of the system
up the coast directly into this weakening ridging seems a bit far-
fetched. As such...the domestic preponderance of guidance was
followed more closely...which maintains a dry forecast through
Thursday with only nominal increases in sky cover along the
eastern periphery of the forecast area. It would seem later model
cycles will be likely to focus in on a more well clustered
solution over the next few cycles...but for the moment...it would
seem the most prudent forecast is continuity.
By Thursday night and Friday...whatever track that middle-level
system takes quickly becomes irrelevant as the preponderance of
guidance has weakened it and absorbed it into the polar jet flow
well to our north. This allows for strong ridging to over-Arch a
weak middle-level low in the Central Plains...bringing a strong middle-
level ridge from The Four Corners to the Red River valley of
ND/MN...toward Lake Ontario. This means abundant warm air
aloft and strong stabilization. As such...the chance of rainfall
looks to be essentially zero...and even clouds will be difficult
to muster as the lapse rates will be so unfavorable in a very dry
column. Additionally...given that we will be on the downstream
side of the ridge...dewpoints seem unlikely to Jump Up too quickly
with very light east/northeast flow in the low levels likely to
continue. This means a gradual warm up with low humidity seems
While 500 mb height values will be impressive...a lack of vertical
mixing and strong stability will keep 850 mb temperature values
from being realized dry adiabatically at the surface...even with
the dry column. As such...while the +13 to +15c that can be noted
on most models by late in the week is impressive at 850 mb for
September...middle to a few upper 70s seem more likely than the 80s
that will be more likely over the upper Midwest on the upstream
side of the surface high...where mixing will be much more
Long term /Friday night through Tuesday/...
temperatures will hover between 5 to 10 degrees above normal into
early next week as a large ridge of high pressure transitions
across the region.
Aviation /00z Wednesday through Sunday/...
vf and light surface wind can be expected into the pre dawn hours...when
IFR visibility restrictions become possible...mainly river fog at the
same locations as yesterday morning. The shallow fog will lift by
Outlook.../Wednesday night through Sunday/...
high pressure will maintain genl VFR through the weekend.