Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
359 am EDT Sat Aug 29 2015
dry conditions will hold today as temperatures increase. Rain
chances return late tonight and Sunday with passage of a weak
Near term /through tonight/...
high clouds continue to skirt our northwestern zones...but skies
have remained generally clear area-wide. This and the vast amount of
low to middle-level dry air has resulted in temperatures trending
sharply cooler. Temperatures outside of urban corridors and mainstem
river valleys already pushing the 50 degree mark.
The lower temperatures combined with steady dewpoints in the low to
middle 50s means another morning of patchy fog is expected. Have
inserted the mention of fog in the pre-dawn hours...primarily
holding to the river valleys.
Temperatures should rebound fairly quickly today after sunrise as
the low-level flow veers to the west-southwest. Upper-level trough
axis will approach the area this afternoon/evening but copious dry
air and weakening dynamics will keep the bulk of Saturday dry. By
the time the weakening front reaches the area...conditions will be
unfavorable for continued convective growth. Have maintained slight
chance/isolated wording for probability of precipitation tonight.
Short term /Sunday through Tuesday/...
front will settle into the forecast area on Sunday. However...with
midlevel support weakening and pulling off to the east...and surface
high pressure in control...the front will stretch out and largely
dissipate. Enough moisture and instability will remain for
scattered showers and storms. Coverage will be influenced by
diurnal factors...with a peak during the afternoon hours. Going to
keep probability of precipitation in the chance range due to lack of large-scale
forcing...with values dropping to slight chance Sunday night. Precipitation
totals will likely remain under a quarter inch in general...and will
not do much to alleviate the overall dry antecedent conditions.
On Monday...a drier and more stable airmass will try to sink into
the County Warning Area...but will be hindered by weak midlevel energy moving
northwards across the central Appalachians. Do not think that this
fairly diffuse vorticity will make it to the Pittsburgh area...but
there will be enough support for some isolated rain showers/thunderstorms and rain mainly
south of I-80. So will maintain a slight chance pop during the day.
Loss of heating plus stretching/weakening of vorticity energy will allow
precipitation to cease by sunset. Drier air makes a little more
headway south on Tuesday...and think isolated diurnal convection
will be limited to south of pit.
Blended mosguide with going forecast to derive temperatures. Not
much change to values...with above normal temperatures throughout.
Long term /Tuesday night through Friday/...
h500 ridging pattern takes hold through the extended period...with
no organized weather systems foreseen through Saturday. Appears to
be just enough moisture for isolated diurnal convection into
Friday at least...and elected to add slight chance probability of precipitation each day.
However...the strength of middle- level capping will need to be
evaluated as time GOES on to determine the daily precipitation
chances...with Friday and Saturday particularly noteworthy dry
candidates. More certain is the likelihood of continued above
normal temperatures...and HPC guidance was used in combination
with the previous forecast for the values.
Aviation /08z Saturday through Wednesday/...
quickly falling temperatures and steady dewpoints will again
combine for patchy dense fog conditions...mainly in
climatologically favorable locations. Have included IFR fog tempo
groups for bvi/hlg/zzv/fkl/duj with this update. The only fly in
the ointment at this point is whether the high clouds can increase
through the pre-dawn hours and hold off the radiational cooling.
Outside of the morning fog chances...the area will remain
restriction free through the remainder of the taf period.
Expecting high clouds to give way to diurnal cumulus development later
this afternoon. Light surface winds this morning will increase to
5-10kts while veering to the west-southwest.
Outlook...Saturday night through Wednesday...
brief restrictions in scattered convection Sunday as a weakening
cold front crosses the region.