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fxus61 Kiln 211721 
afdiln

Area forecast discussion
National Weather Service Wilmington Ohio
121 PM EDT Tue Aug 21 2018

Synopsis...
a surface low pressure will track northeast from the mid
Mississippi Valley across the Great Lakes today. As this occurs,
a cold front will move east across the area. This will cause
scattered showers and thunderstorms. In the wake of the cold
front, a much cooler and more comfortable airmass will move into
the area, bringing dry weather through the remainder of the
work week.

&&

Near term /until 6 PM this evening/...
surface observations indicate that an area of low pressure is
centered over southeastern Michigan, with broadly confluent
(generally southwesterly) flow over Ohio and Indiana. A surface
cold front is somewhere further northwest toward Chicago, with
one of several 500mb shortwaves (within a broader trough) moving
forecast to move through the region early this afternoon. This
pattern supports bands of lift, with SW-to-NE oriented broken
lines of convection gradually moving through the region at times
during the day. Though close to a diurnal minimum, additional
development is expected by early afternoon, with SBCAPE
increasing to around 1000-1500 j/kg (though model soundings
suggest this will be narrow). With weak shear, any severe threat
should be limited, perhaps most likely (as Storm Prediction Center has indicated) in
central Ohio and further east where instability and shear may
both be slightly stronger than elsewhere. To note, westerly
deep-layer shear is actually forecast to increase by evening,
but convection is expected to be waning by then.

Pops were adjusted to increase the focus on the eastern third of
the County Warning Area today, but allowing for chance pops through the entire
forecast area during the peak of the diurnal cycle. As of now,
no changes appear necessary for the temperature forecast, with
highs in the upper 70s to around 80.

Previous discussion >
500 mb shortwave will swing east across the southern Great Lakes/
Ohio Valley today. In association with the S/W a sfc low and
cold front will also work east. Latest mesoscale models are
suggesting that the convection lifting north thru Kentucky will
expand a little as it lifts through early this morning, but that
behind it, a good chunk of the fa could be dry for the morning
hours. By afternoon, expect convection to redevelop as convective available potential energy
reach 1500-2000 j/kg. Went with the higher pops in the east for
today, this is also were there is a better chance of an isolated
strong to severe thunderstorms this afternoon.

Kept the skies cloudy today, this will keep highs in the upper
70s in the northwest and the lower 80s in the southeast.

&&

Short term /6 PM this evening through 6 PM Wednesday/...
by 00z Wednesday the axis of the 500 mb trof will be east of the
region. However, a secondary front/trof will pivot through the
area this evening. This will keep a chance of some showers and
perhaps a thunderstorm with the marginal instability over the
area. Drier air will begin to pushing in after midnight but the
clouds will be slow to erode. Expect lows to range from the
lower 60s in the northwest to the mid 60s southeast.

By Wednesday morning, the region will be under northwest flow
aloft. The region will just be begin to feel the affects of
surface high pressure which will be centered over the Central
Plains. A small chance of precipitation will be lingering in the
southeast counties, but it will only last for a few hours in
the morning. Clouds will be on the decrease through out the day
and temperatures will be below normal, ranging from the lower
70s in the north to the upper 70s across nrn Kentucky.

&&

Long term /Wednesday night through Monday/...
the long term portion of the forecast continues to depict a touch of
early autumn at the outset, returning to mid-Summer heat and
humidity by the end the upcoming weekend, and through much of next
of next week as a high confidence synoptic signal is present in the
long range deterministic and ensemble suite of data.

Starting 00z Thursday, anomalous longwave trough over the eastern
Continental U.S. Will be shifting slowly east with large pool of negative
925mb/850mb temp anomalies. With 1020mb surface high set to follow
on the backside of this trough, several days very un-Summer-like
temperatures and humidity are in store for the Ohio Valley,
particularly Wed night/thur/thur night where overnight minimums will
drop well into the 50s and daytime highs solidly in the low to mid
70s under ample sunshine. This is a very mid-late September weather
regime for the end of the work week.

On Friday/Friday night, several key changes will begin. A large Rex
block feature off the West Coast of the U.S. Will really become
entrenched...with high latitude and very strong mid/upper level
ridge over the northern Pacific. This will in turn force a weakness
and eventual deepening /trough/ in the height field over the wrn
Continental U.S. And in particular the Pacific northwest. In response to this, heights
over the eastern Continental U.S. Will begin to dramatically rise as
subtropical ridging becomes dominant across the southern 2/3rds of
the eastern U.S. A migratory shortwave trough spreading east
through the northern tier of the U.S. On Friday will begin to
deamplify as it runs into the burgeoning ridge in the east. Per the
latest 20.12z European model (ecmwf) ensemble and deterministic runs of the
GFS/ECMWF/CMC, this shortwave will bring a rapid and significant low
level moisture flux to the Ohio Valley in the wake of the departing
high, despite much of the larger scale forcing starting to glance
off to the north as the shortwave decays. Precipitable water and
boundary layer dewpoints will very rapidly return to above normal
values by Friday night into Saturday with increasing southwesterly
flow ahead of the wave. This should be enough to carry a shower or
thunderstorm complex or two into or through the iln County Warning Area sometime
later Friday night and/or into Saturday. Rain chances were raised a
little in this timeframe into the ~40 percent range.

Height rises will continue Saturday/Sunday/Monday as the very stable
longwave pattern becomes entrenched over the Pacific/North America,
featuring the Rex block in the Pacific and anomalous subtropical
ridge across the southeast half of the Continental U.S.. the iln County Warning Area will
eventually become centered under the core of 700mb/500mb
standardized height anomalies /+1.5 sigma/ by early next week, and
attendant to this will be +1.0 to +1.5 sigma 850mb temps - which
pretty much guarantees a long-duration period of sticky late Summer
heat with daily highs in the upper 80s to around 90, and overnight
lows in the lower 70s, especially considering precipitable water anomalies 1.5 to
2.0 sigma above climo. This pattern will extend well beyond Monday,
likely remaining entrenched into the middle/end of next week.

Once this pattern really takes hold /Sunday/ rain chances are never
really gone but synoptic forcing signals become very muted. The
20.12z cips analogs for the gefs pattern shows a likely mesoscale convective system corridor
on the southern fringe of the westerlies which are displaced well to
the north, and these mcss are likely to develop at the inflection of
the subtropical ridge and positive tilt nature of the longwave
western trough. This would be in the Minnesota/WI area and storms riding
southeast into Michigan or nrn Ohio dropping into the feed of
hot/unstable/high precipitable water air over the southern Great Lakes. So in
addition to probably dealing with some late August heat indices in
the mid 90s, other threats going into next week may a pattern of
repeat 'ridge rider' mesoscale convective system complexes into the northern County Warning Area which is
depicted very well by the cips analog threat plots.

&&

Aviation /17z Tuesday through Saturday/...
scattered to numerous showers have developed, and will
potentially impact the airports over the next 4-6 hours. It
appears the greatest chance for showers will be during the
18z-22z time frame, though some scattered activity will likely
continue until around 02z-04z. Though VFR conditions should be
prevailing outside of the showers, they may bring some brief
MVFR ceilings and visibilities. Thunder appears to be somewhat
unlikely, though a thunderstorms in the vicinity will be carried for a few hours at the
Columbus airports where it is a little bit more possible to
occur.

As winds shift to the northwest, VFR conditions will begin the
overnight period. However, MVFR ceilings are expected to develop
at all taf sites, and there is at least some potential for IFR
ceilings as well. This is more likely for the Columbus airports,
where a tempo for IFR ceilings will be used.

The ceilings will eventually lift during the mid to late morning
hours on Wednesday, leading to dry VFR conditions through the
rest of the day. Winds will settle out of the northwest, with
gusts in the 15-20 knot range.

Outlook...thunderstorms are possible on Friday night and
Saturday.

&&

Iln watches/warnings/advisories...
Ohio...none.
Kentucky...none.
In...none.

&&

$$
Synopsis...AR/sites

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