Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
257 am EDT Friday Aug 1 2014
temperatures will move to near average with daily chances for
showers and thunderstorms today through Sunday. Dry and mild
conditions to return by the beginning of next week.
Near term /through tonight/...
mostly clear skies and calm winds early this morning allowing
temperatures to radiate down to dewpoint temperatures giving rise to patchy
valley fog. High clouds beginning to amass south and east of the
area in response to a system that will ride north along the eastern
periphery of the broad trough this morning. As the middle-level wave
translates north-northeast along The Spine of the
Appalachians...low to middle level flow will be southeasterly pooling
copious amounts of Atlantic moisture along the eastern slopes. 850 mb-700 mb
flow remains primarily out of the south and east through midday
wringing out a vast majority of the moisture before it can cross
the higher terrain.
By about 18z the deeper moisture arrives along our ridges as noted by
the low 500 mb dewpoint depressions prognosticated by 00z runs of the
NAM/GFS. Have continued to follow this for timing of the likely
probability of precipitation entering the County Warning Area. Hi-res WRF models have consistently shown a
band of quantitative precipitation forecast moving southeast to northwest across the area this afternoon. While
difficult to pick this feature out of the broad models...the 00z
NAM middle-level fields show a weak boundary pushing away from the
ridges approaching the eastern Ohio near 00z. The northwestern 1/2 of the County Warning Area
should warm quickly this morning before the thicker clouds stream
in. 00z NAM/GFS showing a corridor of cape values near 1000j/kg
and NAM bufr soundings have lapse rates in the -8 to -10 c/km
range through 700 mb. Am expecting at least scattered rain showers/thunderstorms and rain along this
boundary pushing east to west...have bolstered probability of precipitation for this feature.
The middle-level wave pushes NE of the area quickly overnight ushered
out by a 50kt 500 mb jet. However...residual Atlantic moisture will
keep rain chances in heading into Sat as the mean trough remains in
Short term /Saturday through Sunday/...
southwest upper flow will continue with deep moisture
overspreading the region as weak trough axis shifts west while a
ridge builds into the western Atlantic. Will be tough to time any
significant upper impulse so have gone with a broad brush for
chance shower probability of precipitation with higher probability of precipitation in the ridges. A diurnal
trend for thunderstorm chances with afternoon heating. Little
change to temperatures to average a bit below normal.
Long term /Sunday night through Thursday/...
mainly dry and mild conditions can be expected through the first
half of the week as the long wave pattern becomes more zonal and dry
west/northwest flow sets up across the region. Operational models
vary greatly on the idea of showers/thunderstorms returning by
midweek so have included a slight chance on Wednesday...increasing
on Thursday based on ensemble guidance.
Aviation /07z Friday through Tuesday/...
mainly MVFR fog developing currently as temperatures continue to drop
under mostly clear skies. Some terminals could drop to IFR/LIFR
range briefly...decided to handle these with tempo groups through
12z. An increase in southerly moisture and instability should
result in some showers and thunderstorms scattered about the
region by Friday afternoon. Temporary restrictions may occur in
any stronger shower/thunderstorms...but mostly VFR will prevail
Outlook.../Friday night through Tuesday/...
a low pressure system could bring restrictions through the balance
of the weekend.