Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington Ohio
108 PM EST Thursday Dec 19 2013
high pressure will remain centered over the southeastern states on
Thursday. Southerly flow will gradually build warmth and moisture
into the Ohio Valley through the weekend. Significant rainfall is
expected to occur from Friday through Sunday. Colder conditions will
move in behind a cold front for Sunday night and the first part of
Near term /until 6 PM this evening/...
cirrus will prevail through the rest of the day. Southerly flow
will continue to advect warmer air into the region. Based on
observed trends have bumped up highs a few degrees.
Short term /6 PM this evening through Sunday/...
the primary forecast focus will be on rainfall potential from
Friday through Sunday...with high confidence in several inches of
rain occurring within the iln County Warning Area.
Surface high pressure will be moving off the southeast coast by
Friday...becoming rather strong as it drifts slowly over the
Atlantic. Another surface high will move eastward across the
northern tier of states..eventually reaching the Great Lakes by
the weekend. Between these two highs...a long and convoluted
boundary will set up from the northeastern states to Texas. This
boundary will have several waves of low pressure moving along
it...with the most significant surface low moving into the Ohio
Valley on Sunday morning. The upper level flow pattern will become
quite amplified...with ridging extending into the southeast...and
a big trough bisecting the middle-section of the country.
This pattern will be setting up in a way that melds several
important ingredients together...creating a scenario in which
significant amounts of moisture are brought into the Ohio Valley
and subsequently precipitated in a concentrated and systematic
manner. The wavy upper jet will have the Ohio Valley in a region
of divergence aloft. Lower in the atmosphere...the unimpeded flow
of Gulf moisture will enter an area of strong convergence along
the surface boundary...and just off the surface on the nose of an
intense low-level jet. The north-to-south wind component will be
several Standard deviations above normal (per GFS/sref anomaly
forecasts). Moisture parameters are perhaps even more
impressive...with European model (ecmwf)/GFS agreement in precipitable water
values approaching or even exceeding the highest December values
in the Kiln/kday observational record. Even if this is slightly
overdone...precipitable water values of near one and a half inches
would be at or near 300 percent of normal for this time of year.
Cips analogs confirm the obvious...with averaged impact guidance
suggesting heavy rainfall potential extending from the southern
Mississippi Valley through the Ohio Valley.
An initial wave of precipitation on Friday appears
likely...affecting the northwestern County Warning Area first and foremost...but
the more significant push of moisture is forecast to move into the
southwestern County Warning Area on Saturday morning. From here...the exact axis
of the heaviest rainfall is still somewhat uncertain...although
models have been aligning a little closer with the last several
runs. The focus appears to be placed on an axis north of the Ohio
River through Indiana...extending into the Whitewater and Miami
Valley regions. Put another way...roughly on a line from
Evansville Indiana to Marion Ohio. The precipitation forecast gets
a little less certain in terms of exact timing and placement by
Sunday...as surface low pressure makes its way through the region.
Ahead of the low...moisture transport will continue to be
strong...with the placement of the boundary determining the
location of the heaviest rain. As the low moves away...cold air
will quickly begin to filter in behind the trailing cold
front...ending the threat for heavy rain by middle-day Sunday.
Flooding potential will continue to be mentioned in the severe weather potential statement...with
one and a half to three inches of rain appearing likely for the
iln County Warning Area (the lowest amounts will be in the southeast corner). The
wavy frontal boundary may allow for just enough motion in the
heaviest bands to keep the threat for any widespread significantly
higher totals from occurring...but local maxima are beyond the
scope of the modeling this far out. With flooding impacts still
not expected to begin until at least the 5th (and maybe the 6th)
period of the forecast...a headline is not warranted quite
yet...though confidence is high that one will be issued at some
The temperature forecast through the period is non-diurnal at
times...thanks to the strong advection...and confidence in the
exact maximum and min numbers is not especially high. In terms of
winds...surface flow will be modest...and the thermodynamics of
the scenario suggest that the strong winds just off the ground
will have a difficult time making it to the surface without being
significantly dampened. From a larger-scale perspective...the
concern for strong winds may be greatest behind the cold frontal
passage on Sunday (as winds shift to more of a westerly
direction)...perhaps the only time the atmosphere will be able to
mix strongly in the boundary layer. These gusts in the grids have
been increased to around 30 knots for Sunday because of this...but
it is Worth noting that the GFS BUFKIT momentum Transfer indicates
the potential for slightly higher values.
If there is any potential for strong to severe wind gusts...it
appears more likely for the southern half of the County Warning Area on Saturday
night and early Sunday. Thermodynamic parameters are not
particularly favorable...with moist adiabatic low-level profiles
in the very wet air mass...and extremely small cape values with
equilibrium levels that may not reach high enough for much (if
any) thunder. Low-level wind shear will be quite strong (though
not likely as strong as the GFS is indicating...with a 90-knot low level jet
that seems unrealistic even in these conditions). The sherb
parameter reaches values of about 1.5 on the GFS/CMC/ECMWF
models...but the ideal value for sherb is right around 1...
higher values indicating perhaps too strong a contribution from
shear. Confidence will need to be gained in any potential
development of semi-discrete convection out of the larger-scale
widespread heavy rain...before making more of a definitive call on
severe potential. Winds this strong will need to be respected and
monitored as these mesoscale implications get closer in the
100-probability of precipitation have been included in the grids for much of this
period...with a slight downturn in precipitation potential
County Warning Area-wide on Friday night...and a few breaks to chance/likely built
into the southeastern County Warning Area on Saturday and Sunday (as the boundary
Long term /Sunday night through Wednesday/...
strong cold air advection for Sunday night into Monday. Shortwave passing
through and perhaps some lake effect snow showers. Nothing
significant with low chance probability of precipitation appearing sufficient for now. Ridge
builds in for Tuesday and Wednesday with near seasonable cold
temperatures and dry conditions.
Aviation /18z Thursday through Tuesday/...
VFR conditions will prevail through at least the first half of the
taf period. Low level moisture will increase overnight with rain
developing after 06z. This will eventually lead to MVFR ceilings
and visibilities. South to southwest winds will generally remain
between 10 and 15 knots.
Outlook...MVFR conditions will deteriorate to IFR/LIFR Friday
night and persist Saturday into Sunday. Conditions will likely
improve to MVFR on Sunday with MVFR ceilings lingering Sunday
night into Monday.