Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington Ohio
407 PM EST Sat Dec 20 2014
weak surface high pressure will gradually move east of the Ohio
Valley by Sunday. As southerly wind flow increases...temperatures
will begin to warm early in the week. Chances for rain will
increase by Tuesday...as a strong low pressure system moves into
the Great Lakes. Windy and cool conditions are expected after the
low moves away in the middle of the week.
Near term /until 6 am Sunday morning/...
the thick and extremely persistent clouds at around 2000 feet will
have one more forecast period to hang around. The 12z Kiln
sounding was very similar to previous observations...with moisture
trapped under a sharp inversion at around 875mb. The one
difference with the sounding this morning was a greater presence
of middle and high level moisture...a product of the shortwave
already moving out of the Ohio Valley. Behind this wave...the
higher clouds are clearing...and visible satellite is providing
an enlightening picture of the progression of the low-level cloud
shield. What has become evident today is that erosion of these
clouds (generally south and west of the iln forecast area) has
become a little more pronounced. With the lower Ohio Valley now on
the back side of an 850mb ridge...some slight warm advection has
begun...and this will start to affect the middle Ohio Valley by
early Sunday morning. Until then...the persistence forecast will
largely pay off. With a few breaks in the clouds expected...and
nearly calm winds...min temperatures may drop a couple degrees below
observed numbers from early Saturday morning.
With radar still showing a few light echoes in northern and
central Ohio...flurries were kept in the forecast in the Columbus
area for a few hours late this afternoon. Current observations are
coming in without mention of precipitation...so it seems likely
that most of the observed returns are not reaching the ground.
Short term /6 am Sunday morning through Monday night/...
a pattern change will begin on Sunday across the Ohio Valley.
Surface high pressure will be centered well east of the
region...and the axis of a low-amplitude middle-level ridge will be
shifting gradually to the east as well. This will set up a regime
of deep southerly flow by Sunday night...especially once the
gradients begin to tighten in advance of the troughing moving
through the plains states. During the day on Sunday...the impacts
from this will be relatively minor...with little to no change in
surface flow. However...with temperatures beginning to warm
aloft...significant breaks in the clouds are finally expected to
develop. With sun...temperatures will be able to rise 5-8 degrees
from Saturday to Sunday...especially in the southern County Warning Area (where
clouds should erode more quickly).
As Monday begins...strengthening south-southwest 850mb winds will begin to
feed moisture into the middle Ohio Valley. On the nose of the
moisture push...weak isentropic ascent will allow for the
possibility of some light showers. Following probability plots
from the 15z sref...it appears that anything that develops will
move northeast through the County Warning Area between 12z and 21z...and will not
be directly associated with the impending large-scale activity
still far off to the west. Coming out of another cool overnight
period...precipitation type at onset would be somewhat in
question...but any amounts are expected to be very light.
Only a 20-percent chance was added to the forecast.
A substantial warming is expected on Monday...as low-level flow
makes the switch to the south (ssw at 850mb and south-southeast at the
surface). A slight upward adjustment was made from the previous
forecast...with lower 50s now expected in the far southern County Warning Area.
Strengthening flow will keep temperatures from falling much on
Monday night...and non-diurnal temperatures were required for the
forecast grids. With a slowing in the model trends for the
progression for the next system...probability of precipitation were reduced for the Monday
night period...with the onset of widespread precipitation
appearing likely to hold off until Tuesday.
Long term /Tuesday through Saturday/...
have used a blend of the 12z European model (ecmwf) and wpc guidance for the long
term forecast. The European model (ecmwf) is faster...deeper...but more quick to
take strong low pressure from the Ohio Valley region Tuesday night
to southern Ontario by Thursday morning. The GFS is slower...not as
deep...and rotates more of a surface trough through the region on
Thursday/Christmas. The European model (ecmwf)/wpc blend is preferred at this time.
An upper trough is forecast to pivot and becomes negatively tilted
as it lifts through the Ohio Valley Tuesday night into Wednesday.
This scenario will eventually push surface low pressure and cold
front across the forecast area. Above normal temperatures are
expected ahead of the system with temperatures cooling off toward
more seasonal normals in cold air advection behind the cold front.
In terms of precipitation chances...they are expected to steadily increase as
forcing gets closer/stronger by Tuesday night into Wednesday
morning. Am expecting rain showers across the entire area Tuesday
into Tuesday night. Coverage will decrease some on Wednesday as the
strongest lift moves northeast. There is some evidence that some
embedded thunder may occur with this system...but have decided to
hold off the mention at this time. Colder air coming in on the back side of
the system will cause a change over to some snow showers later
Wednesday into Wednesday night. Snow showers will taper off by
Thursday morning/Christmas. Considering the warmer air ahead of the
system and temperatures bottoming out mainly in the 30s by Thursday
morning...am only expecting minor snow accumulations with the snow
shower activity. It appears the biggest impact with this weather
system will be the wind. However...with the European model (ecmwf) solution...the
strongest winds may remain north of our area Wednesday night.
Ridging develops rather quickly late Thursday and moves east
Thursday night. This will allow for modest warm air advection ahead
of the next low pressure system and cold front that will approach
the region on Friday. The cold front will slowly move through the
area Friday night and Saturday. Will continue with a chance of rain
shower ahead of the cold front with perhaps a brief mix with or
change over to some snow showers before tapering off on Saturday.
Temperatures will rebound again on Friday and then cool off toward
seasonal normals by Saturday.
Aviation /20z Saturday through Thursday/...
MVFR ceilings will prevail through the taf period though some
indications over southern tafs show that the high pressure might
possibly overcome the entrenched lower level moisture.
Given the trend this month...am hesitant to forecast a clearing of
MVFR ceilings even with the high parked right over County Warning Area and axis
extending SW over Ohio River. Models are in good agreement of the
light nature of the winds and the high positioning...but the lower
clouds have just not been able to get scoured out as of late.
Will change the overcast sky condition to a broken deck later tonight and
possibly scatter out kcvg at the end of the period.
Outlook...MVFR ceilings possible Monday. MVFR/IFR ceilings and
visibilities are possible Tuesday into Wednesday...along with
precipitation and windy conditions.