Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
929 PM EDT Sat Jul 4 2015
high pressure provides dry weather and seasonable temperatures
Sunday. Rain chances return Monday with an upper disturbance...and
continue into Wednesday with an approaching front.
Near term /through Sunday/...
drier air has made some inroads into the County Warning Area this
evening...however dewpoints along the ridges...in the southern
tier of the County Warning Area...and in most of Ohio failed to drop below 60f.
This could be problematic going into tonight as mostly clear skies
dominate the region and radiation fog starts to become possible.
There are a few problems with this theory...however...as afternoon
visible imagery revealed abundant high level smoke from upstream
wildfires making its way into the area. This smoke is generally
opaque to outgoing longwave radiation...so it may actually work to
hold temperatures up a bit more than would have otherwise been the
case. This would work against widespread fog formation. That
said...decoupling seems to have already occurred...and given our
recent propensity for fog and our recent rain allowing for
abundant soil moisture which will allow for upward moisture flux
overnight...it would seem some fog is possible. It has been added
to the grids...and will likely develop by sunrise and dissipate
fairly quickly after sunrise. Fries
Previous discussion follows...the high slowly slides east Sunday.
Most locations will enjoy a mostly sunny day. However...southeast
flow will promote a moisture increase near the ridges in the
southeast County Warning Area. Soundings show some modest instability during the
afternoon as well. Cannot rule out an isolated diurnal rain showers here
and have included this possibility in the higher elevations of WV
Dropped low temperatures a couple degrees tonight given the clearing
skies...light wind...and lowering dewpoints. Readings recover
towards seasonable levels Sunday afternoon with warmer h850 air
and slightly more efficient mixing.
Short term /Sunday night through Tuesday/...
dry...and warmer weather is then expected as high pressure builds
through Sunday. Sunday night and Monday another disturbance is
now projected to bring chance of precipitation to our southern
County Warning Area of WV and Maryland.
Will keep current forecast of increasing chance with the Tuesday
frontal system all areas and temperatures readings at or
slightly above seasonal averages.
Long term /Tuesday night through Saturday/...
a deamplified...flattened upper flow is prognosticated by model
ensembles through the long term period. Individual shortwaves
advancing through the flow across the upper Ohio Valley are
expected to bring periodic chances for showers and thunderstorms.
Used wpc ensemble based guidance to smooth out model
differences...with some adjustments to probability of precipitation and the prognosticated
seasonally warm temperatures.
Aviation /02z Sunday through Thursday/...
diurnal cumulus should diminish shortly with the loss of heating with
only some residual cloudiness along the ridges where a bit more
middle-low level moisture remains. High pressure will settle
overhead tonight which should allow for fog formation toward
sunrise as dewpoint depressions get smaller. Question is how
significant fog formation will be. Generally feel most confident
in IFR visibilities where low level moisture...I.E dewpoints...are
highest this evening. High clouds over mgw may limit radiational
cooling and fog formation but for now will carry IFR. Further away
from the ridges and toward Lake Erie confidence in widespread IFR
was lower...so MVFR visibilities were carried at pit/fkl for now.
Guidance does not suggest much fog anywhere...which seems to be
incorrect with the saturated ground and decoupling expected.
Conditions should return to VFR after sunrise Sunday.
Outlook.../Sunday night through Wednesday/...
restriction chances are expected to increase early next week with
several crossing disturbances.