Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington Ohio
748 PM EDT Sat Sep 20 2014
a strong cold front will shift through the Great Lakes tonight and
Sunday with the threat for showers and thunderstorms. A mild night
is in store ahead of the front tonight...with lows in the 60s.
Cooler air will drop in quickly behind the front Sunday afternoon
and night...leading to several nights with lows in the 40s...with
perhaps some upper 30s in outlying areas. Much of the work week
will feature strong high pressure...which will bring mild
days...cool nights...and low humidity. Plenty of sun is expected
through the week.
Near term /until 6 am Sunday morning/...
update...sped up arrival of rain chances by an hour or two based
on time trends from radar on now well-developed line of convection
from northestern ind back to wcntl ind. Most concentrated north-S portion of
line with history of wind damage will pass north of our area
through early evening. Rest of line seems to be struggling to be
severe...and we/re watching portion of the line from Logansport
back to Illinois border as the portion that will affect US. Gust front
now kicking southward from segment of line from Columbia City back to
Logansport so this portion of line likely to weaken. Mesoa over
ind/Ohio shows instability decreasing with ewrd extent into iln County Warning Area and with
sunset approaching still think overall severe threat is marginal.
Did update severe weather potential statement to give heads up to wcntl Ohio/ecntl ind counties
that a gusty/weakening line of storms is likely 00z-02z. Reference
mcd#1741 that a new watch /if issued/ may grab a couple of our far
northwestern counties. Also pulled rain chances in northern Kentucky as cumulus/pvs
rain showers down there have largely dissipated.
Previous discussion -> several foci for precipitation now through
sunrise on Sunday morning. First...is in a relatively narrow axis
of higher Theta-E that runs northeast along the Ohio River into
the cvg area this afternoon. Boundary layer dewpoints have withstood
mixing to remain in the middle 60s yielding some marginal MLCAPE from
200-500 j/kg. Confluent flow/very weak convergence has led to this
extension of buoyancy and some isolated rain showers have developed in this
axis and will continue to do so through about 7-8 PM tied strongly
to the boundary layer flow/instby. Have placed a small rain chance
in parts of our northern Kentucky counties for this evening to account for
Larger concern is strong cold front which is shifting through
WI/IA/nrn Illinois at this time attendant to a couple of height fall
centers...one in WI this afternoon and another very compact one in eastern
ND. Low level forcing is strong with the front and associated pre-
frontal trough...and deep layer flow is increasing as the through
amplifies with time tonight.
Convection has recently erupted as of 19z just south of Chicago
and Storm Prediction Center watch #504 accompanies this increase in convection off to
our northwest. According to 20.12z awr-WRF...20.09z sref...and multiple
well-agreed upon hrrr runs...a rather robust line of storms will
develop along/ahead of the front over the next few hours and push
east/northeast. Orientation/magnitude /30+kts by tonight/ of 0-3km
shear vector is favorable for some duration/organization to line
segments which can Orient in more north-S fashion or some variation of
that. Not surprisingly...hrrr runs have demonstrated sporadic swaths
of damaging wind gusts > 50kts across northern ind potentially
involving northwestern/wcntl Ohio later this evening /after 10 PM or so/. Really
like the 5% wind contour into wcntl Ohio based on the hrrr/arw WRF
solutions showing a weakening organized line coming into our area
around 10 PM...perhaps with some 40-50 miles per hour winds. This fits
current mesoa which has minimum of instability in western Ohio due to mixed
dewpoints in the 50s...so line should weaken considerably as it
crosses into Ohio...despite bringing a modest increase in moisture
Struggled a lot with rain chances...feel confident enough for
likely rain chances with dying line into about the northwestern 1/2 of our
forecast area...but all cams/parametrized data showing activity
becoming more scattered with time as night wears on. So rain chances
overnight range from a low confidence 20% in far south/east up to
a 70% chance or so in wcntl Ohio. All will depend how quickly the
incoming line weakens.
Short term /6 am Sunday morning through Monday night/...
first height fall center mentioned above pulls east of the area
Sunday morning taking primary convergence/prefrontal trough with
it through the morning. Residual showers/storms to cross the area
in the morning and then scattered storms will begin to bubble along this
boundary late morning/early afternoon and likely in our far
south/east or just out of our area. Rather impressive/narrow dry
slot should wrap in late morning/early afternoon with some
sunshine before strong cold air advection sets in during the afternoon with the
passage of secondary front. With shallow instability developing...could
see some scattered rain showers develop with this feature which was already well
handled by pvs forecaster. This most likely across our north
closer to strong/compact shortwave digging across Northern Ohio. Don/T
expect much if any thunder after about 18z...as instability becomes too
shallow for charge separation. Will definitely a be a changeable
/autumn-like day despite maximum temperatures in the 70s. Winds will become
breezy/gusty in the afternoon as the cold advection arrives and
changeable sky with the dry slot to lower clouds along/behind the
secondary surface front.
As this action all clears the County Warning Area Sunday night and strong high
pressure builds in...a very chilly night in store with lows well
into the 40s in our Indiana/western Ohio counties as 925mb temperatures drop
to +5c in this area by sunrise on Monday. Should see a slow decrease
in low clouds through the night and lessening winds as
well...given the degree of cold air inbound...am a little worried
I am clearing the skies a little too quickly and stratocumulus may
be more widespread.
Monday brings 1024mb surface high pressure sprawling into the
Great Lakes and centered near Chicago by Monday evening. Should be
plenty of sunshine with dewpoints in the 40s. Continued weak cold air advection
should keep 925mb temperatures in the +8 to +10c range which per climatology
study suggests maximum temperatures in the low to middle 60s most
areas...despite plenty of sun. Monday night will be coldest of the
week and would not be surprised to see cold prone/outlying sites
touch the middle to upper 30s again like Delaware/Marysville et cetera did
last week. May even be a touch colder this time around. Low temperatures
this night may still be a degree or two too warm in going
forecast...especially with surface high moving right overhead at
12z and the very dry profile /pwats running 40% of normal/. An
ideal radiational cooling signal being seen in the data.
Long term /Tuesday through Saturday/...
middle/upper level ridge will remain in place across the region
through the end of the week. This will result in dry conditions and
fairly seasonable temperatures through the long term period. As the
airmass slowly modifies...high temperatures will gradually warm each
day...from the low to middle 70s middle week into the upper 70s to around
80 degrees by the weekend.
Aviation /00z Sunday through Thursday/...
line of storms developed to our west across Indiana in moderate
instability ahead of surface cold front and was beginning to push into Northwest
Ohio. Instability sharply drops off into marginal category across Ohio.
This line was showing a weakening trend and expected to continue
to weaken as it moves into less favorable thermal environment.
Have timed a period of vcsh to account for remnants of this
convection at all taf sites tonight. Expect a period of MVFR ceilings
late tonight prior to surface cold front sweeping through Sunday morning.
Expect some improvement to ceilings for a period Sunday afternoon when VFR
conditions are expected. Low level moisture associated with upper level
trough late afternoon into Sunday evening. Expect these ceilings to be VFR
between 4000 and 5000 feet but a period of MVFR ceilings can not be
SW winds will continue around 10 kts tonight and then shift west
with frontal passage on Sunday and then become northwest Sunday afternoon. Winds will
gust as high as 27 kts Sunday in wake of the front
Outlook...MVFR ceilings possible Sunday night.