Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Wilmington Ohio
912 PM EST Sat Feb 28 2015

Synopsis...
a warm front will move through the Ohio Valley on Sunday...with
snow and rain expected across the region. A stronger weather
system will move through the area on Tuesday and
Wednesday...bringing a significant amount of rainfall. High
pressure will bring dry conditions for the end of the week.

&&

Near term /until 6 am Sunday morning/...
area of snow is already working NE across Indiana and will lift into
the western portions of the forecast area in the next hour or so. Backed off on
the beginning of the snow by a couple of hours based on the radar
trend. Mesoscale models are still suggesting that this initial
shot will lift a little farther north leaving northern Kentucky dry. Trended
the forecast in that direction...but allowed for the precipitation to fill
in across the south between 09-12z.

Temperatures have fallen a little this evening and may hold steady
for a few hours...but expect the temperatures to slowly beginning
to rise by midnight.

&&

Short term /6 am Sunday morning through Monday/...
a warm front is expected to move north through the iln forecast
area on Sunday morning. As this occurs...Theta-E advection will be
steady across the area...with the environment becoming fairly
moist. On Sunday morning and early afternoon...model soundings
indicate saturation or near-saturation from the surface all the
way through 10kft. Precipitable water values will range from 0.5
to 0.8 inches across the forecast area...and yet it is snow that
remains the biggest concern for the short term of the forecast.

Though the moisture content will be high...overall the forcing
involved with this weather system will be fairly weak and
unfocused. It is clear that the greatest precipitation amounts
will set up through (or just north of) the middle of the iln
forecast area...and that the eventual northernmost extent of the
rain-to-snow transition will also exist somewhere in this
vicinity. The model spread in this area of temperature transition
is actually not all that large...and the northernmost extremes
among the solutions are not as far north as might be expected when
considering other weather events of this nature. Thus...there is
confidence that mixed rain and snow will be able to make it no
further north than a line from about Franklin County in to Licking
County Ohio...and that anywhere near that line will experience
mainly snow. Thus...the highest snowfall accumulations are
forecast just north of that line...roughly from Richmond in to
Delaware Ohio...with a solid swath of 5 inches in the grids.
Although moisture is plentiful...and thus quantitative precipitation forecast amounts are somewhat
heavy given the weak forcing...snow ratios will be toward the low
end of the scale. In fact...guidance numbers from wpc (12:1 to
14:1) may well be too high. There will be little contribution from
the dendritic growth zone...which will be residing somewhere
around 15kft and above the majority of the moisture. What this
event has going for it is duration...with snow expected to fall
across a period of around 18 hours. In addition...there may be
some isotherm compaction occurring across the northern iln County Warning Area on
Sunday morning (especially in the 925mb-850mb layer)...with weak
frontogenetic signatures on a few of the models. Thus...some 6
inch amounts will be possible...but with a long enough duration
that hazards worthy of a warning do not seem likely.

The transition between snow and rain will undoubtedly lead to a
little less certainty in the forecast further to the south...where
snow forecast totals range from 4 inches to 1 inch across a
stretch of only 30 miles. Of additional concern is the potential
for ice during the morning hours. There is a clear signal for a
warm layer near 900mb pushing above the colder air at the
surface...generally across the southern third of the forecast area
between 07z and 13z. However...BUFKIT soundings show this layer to
be of a fairly small magnitude...and unsaturated. This is not
terribly supportive of a whole lot of full hydrometeor
melting...favoring a mix of freezing rain and sleet. This seems
most likely to occur in the southwestern part of the County Warning Area...where a
tier of counties was added to the Winter Weather Advisory. This
addition was based on a combination of snow amounts (around an
inch) and ice potential (perhaps up to five hundredths).5

Temperatures should peak out in the 18z-21z time frame...matching
the northernmost extent of the rain-snow line. After this...winds
will shift to the west-northwest and allow cold advection to take
over...quickly changing the precipitation type to snow across the
entire County Warning Area. Additional accumulations of a few tenths (with perhaps
another inch in central ohio) will be possible before snow comes
to an end on Sunday evening. Temperatures on Sunday night will
cool from northwest to southeast...as skies also begin to clear out. Ignoring
the raw GFS numbers...should range from the upper teens to upper
20s.

On Monday...dry high pressure will build into the region...with
light winds and mostly sunny conditions.

&&

Long term /Monday night through Saturday/...
high pressure will quickly move off to the east Monday night as
return flow develops on the back side. Warm...moist ascent will
increase to our west during the evening and will shift
east/northeast into the region late Monday night into Tuesday
morning. Cold air will be in place at the surface. Thus...as precipitation
initially begins and before saturation...snow/sleet/freezing rain
will occur. As saturation increases...and maximum temperatures aloft
increase...more and more of the region will transition over to a
period of freezing rain. Temperatures will continue to warm aloft
and in the low levels through Tuesday morning which should change
the wintry precipitation to rain from south to north. Preliminary amounts
look like a tenth of an inch or less of icing and less than an inch
of snow/sleet.

Models continue to indicate that a middle level shortwave will eject NE from
the Central Plains to the western Great Lakes Monday afternoon into
Monday evening. As this occurs...a fairly strong low level jet of 60 to 70
knots along with a deepening surface low will pump anomalously high
precipitable waters into the lower Great Lakes and the Ohio Valley. These values
are forecast to be between 1.20 to 1.40 inches...which is about 300
percent above normal for early March. This scenario will bring rain
to the region...moderate to locally heavy. There will be a concern
for flooding given moist conditions and snow pack/stored snow water
equivalent from sundays system. Although most streams/rivers are
running low at this time...snow melt and moderate/heavy rain will result in
rapid rises. In addition...the prolonged cold has allowed ice to
form on many rivers...creeks and streams...some thick in spots.
Water flowing into fairly ice covered basins will have a hard time
draining which may lead to flooding and/or ice jams. Another
concern...but with low confidence at this time...will be whether a low
topped qlcs (maybe a line of focused showers with little to no
thunder) gets going along the front in a regime of high shear/Low
Cape along with strong low level frontal convergence. To add to the
complexity...where snow cover can hold on...it may add a shallow
stable layer to any qlcs development. At this time...the best chance for a
potential qlcs to have an affect seems more possible along and south
of the Ohio River. The warmest temperatures of the week will occur
Tuesday night ahead of front as temperatures warm into the 40s and
50s. A few lower 60s may occur in our far southeast counties. Will continue
to mention the wintry and flood threats in the severe weather potential statement. Will hold off on
the mention of the qlcs potential at this time. Would like to see a few more
runs and some high resolution models.

A cold front in association with the middle level energy/surface low will
push southeast through the region late Tuesday night into Wednesday
morning. This will be followed eventually by an Arctic front. Models
are now hedging on whether or not additional energy moving through
the southwest flow aloft will allow for a secondary surge of
precipitation north into our region later on Wednesday. Have kept
some continuity with the previous forecast to allow for a higher
chance for precipitation for points along and south of the Ohio River. Precipitation
in the cold air advection will transition from rain to snow. Given low confidence
on secondary surge...pinpointing potential snow amounts at this time would not
be prudent.

The entire system finally gets shunted far enough east for
precipitation to end Wednesday night. High pressure will build in
Thursday and Friday bringing dry conditions but a return to below
normal temperatures. A weak front is forecast to drop through the
region on Saturday. This front appears moisture starved.
Temperatures will moderate some ahead of the front on Saturday but
will still average below normal.

&&

Aviation /02z Sunday through Thursday/...
latest runs of the hires models are indicating that the initial
shot of snow that is working up the Ohio Valley will only graze
cvg/luk...so adjusted the forecast for some MVFR ceilings and visibilities
from 04-06z due to the snow. Additional lift comes in towards 12z
causing precipitation to redevelop. Soundings are indicating that precipitation might
fall as freezing rain/pl by that time...before transitioning to rain after
sunrise.

Farther north into iln...the snow should last longer before the warmer
air begins to mix in after 12z. Kept a rain/snow mix for much of the
daylight hours after 14z as the rain/snow just seems to hang over iln.

Kday should remain all snow through the event...with the heaviest
snow 10-14z. A couple of the models are suggesting a brief
break/decrease in intensity in the precipitation around 18z...so tried to
show that in the tafs.

Colder air begin to work back in late Sunday afternoon gradually
changing the precipitation back to snow for all locations.



Outlook...MVFR/IFR ceilings likely into Sunday night. MVFR/IFR
ceilings and visibilities are possible Tuesday into Wednesday with
rain and snow.

&&

Iln watches/warnings/advisories...
Ohio...Winter Weather Advisory until 10 PM EST Sunday for ohz026-034-
035-042>046-051>056-060>062.
Winter Weather Advisory until 3 PM EST Sunday for ohz063>065-
070>072-077.
Winter Weather Advisory until 10 am EST Sunday for ohz078.
Kentucky...Winter Weather Advisory until 10 am EST Sunday for kyz089>093.
In...Winter Weather Advisory until 10 PM EST Sunday for inz050-058-
059.
Winter Weather Advisory until 3 PM EST Sunday for inz066-073-
074.
Winter Weather Advisory until 10 am EST Sunday for inz075-080.

&&

$$
Synopsis...hatzos
near term...
short term...hatzos
long term...Hickman
aviation...sites

National Weather Service Glossary of Abbreviations