Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington Ohio
405 PM EDT Wednesday Jul 23 2014
a cold front shifting southeast through the Ohio Valley this
afternoon will continue to bring a threat for showers and a few
thunderstorms along and ahead of it...mainly near and south of the
Ohio River tonight...before cool and dry high pressure builds in
for Thursday. This feature will Promise several days of
comfortable humidity and below normal temperatures before low
pressure and a developing warm front approach and pass through the
area Saturday and Sunday...with a return of a warm and humid
airmass...and the threat for showers and thunderstorms. Once this
system passes...yet another plunge of cooler air will return to
the Ohio Valley for the first part next week...with below normal
Near term /until 6 am Thursday morning/...
objective mesoanalysis at 23.19z indicates that outflow from
decaying mesoscale convective system from this morning had pushed through most if not all
of the weather forecast office iln forecast area...with west-northwesterly flow at most sites. This
has scoured deeper instability out of the area...though the cold
front at 18z was still analyzed from near Cincinnati to just south
of Columbus. Boundary layer moisture lagged a bit behind the
front...but dew points in northern Indiana/Ohio were falling into
the lower 60s on slightly stronger northerly flow. The best
surface-based instability at 19z resided south of the Ohio River where
MLCAPE from 1000 to 1500 j/kg remained. However...observation-
derived shear values in this modest instability region were weak...with
0-6km bulk shear < 20 kts...and very weak low level flow.
Thus...think the severe threat is pretty negligible given the
presence of middle/upper level cirrus that continues to temper the
heating potential...and inability for any renewed convective
development /cntl Indiana/ to maintain itself. With a couple of
very weak ripples of low pressure potentially developing along
front /one already analyzed near cvg and another in southeastern
MO/...will hang onto rain chances in the evening mainly
along/south of the Ohio River where the advancing cold front and
any of these weak ripples may induce a few updrafts or shower
areas through the night. This is supported by decent height falls
through the night and a more favorable middle/upper level jet
development over the upper Great Lakes as the night wears
on...suggesting at least a little redevelopment is possible this
evening or overnight in northern Kentucky/scntl Ohio near or behind the front.
Short term /6 am Thursday morning through Thursday night/...
a cool/dry Thursday is expected as high pressure builds in from
the north. 925mb temperatures during peak heating around 16c will support
mainly temperatures in the 70s during the day with light northeasterly flow as
high pressure will be situated over western Illinois. Precipitable water will
drop significantly through the day to 0.60" or so...and should see
a mix of clouds and sun with afternoon cumulus. Surface dewpoints
should mix out/fall into the Lower-Middle 50s during the afternoon.
This sets up another cool night /for late July standards/ across
the area with widespread Lower/Middle 50s on Thursday night.
Long term /Friday through Wednesday/...
at 12z Friday...surface high pressure will be centered directly over
the Ohio Valley...fully encompassing the iln County Warning Area with a fairly dry
air mass. Very tranquil conditions are expected for Friday...with
maximum temperatures forecast within a few degrees of 80.
One large middle-level trough over the eastern half of the country will
begin to move away by Friday night...as the pattern begins to
flatten across the Ohio Valley for most of the weekend. This will
actually put the region more into the stronger flow aloft...bringing
the possibility for convective systems to move east or southeast
through the area. There remains some uncertainty in timing of these
smaller features through the weekend...precluding using higher probability of precipitation
in the forecast quite yet. Several models allow for some light quantitative precipitation forecast
to move into the region Friday night...though relative humidity values in the low
levels are not very high. This suggests perhaps some sprinkles or
dissipating showers as warm advection begins over the region...with
a sharp increase in Theta-E noted aloft...and generally moving
WNW-to-ESE. Forcing on Saturday appears nebulous...so probability of precipitation have been
kept in the low chance category...though the air mass will become
hot and humid.
On Sunday...a more pronounced front will be moving eastward through
the region. Both the GFS and European model (ecmwf) have consistently developed one
or several convective complexes to the west of the iln County Warning Area on
Saturday night...driving them through the region in the westerly
flow...with fairly wet conditions expected through the day. Probability of precipitation for
Sunday have been increased slightly as a result of this
confidence...and it is likely that chances will have to be increased
further as soon as the exact timing of the convection becomes more
A rather sharp upper trough will be moving south through the Great
Lakes on Sunday and into Monday...driving the aforementioned cold
front...and a surface low through southern Michigan. This trough
will establish itself rather robustly across the eastern half of the
country...in a well-advertised pattern shift toward another round of
below-normal temperatures. 12z GFS/European model (ecmwf) projections show the
coolest air over the Ohio Valley at 12z Tuesday morning.
Precipitation chances seem more likely on Monday rather than later
in the week...though still on the lower end of the scale (and likely
Aviation /20z Wednesday through Monday/...
area of convection continues to push eastern through northern Kentucky/SW Ohio. Expect
redevelopment S and east of the tafs this afternoon. MVFR ceilings north of day
and cmh are expected to work into the northern tafs later this
afternoon...before high pressure to the northwest gradually scatters things
out from mw to southeast after 00z.
Cumulus development is expected during the day Thursday as the normal
diurnal cycle kicks in. Better coverage will be over the northern tafs
where broken ceilings are expected.
Outlook...thunderstorms possible Saturday and Sunday.