Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington Ohio
107 am EST Monday Mar 2 2015
snow will end across the middle Ohio Valley this evening...as high
pressure builds in from the west...providing dry conditions for
Monday. A strong area of low pressure will move north of the
region on Tuesday through Tuesday night. This will bring a mix of
snow and freezing rain...turning to rain during the afternoon.
Additional rain and snow may persist through Wednesday night. Dry
high pressure will move into the region for the end of the week.
Near term /until 6 am this morning/...
still seeing some echoes on radar running from Chillicothe back
west through Wilmington and over towards Indianapolis. Observation arent
reporting too much so will carry a mention of flurries. From Owen
County Kentucky up to Pike County Ohio...the echoes look more
like drizzle. Gradually shifted these features east and out of the
forecast area over the next 6 hours or so.
Temperatures will fall overnight...ranging from the teens in the
northwest to the middle 20s for the southeast corner.
Short term /6 am this morning through Tuesday night/...
relatively quiet conditions are expected on Monday...as an area of
high pressure moves in from the west. This high will not be in
place for long...with its center forecast to be over Ohio on
Monday evening...and clear of the area quickly thereafter. This
should allow for more sun than clouds on Monday...though some
low-level clouds will still be scouring out in the eastern County Warning Area
during the morning...and some high clouds in advance of the next
system will already be moving in by afternoon. Temperatures will
get to near or just above the freezing mark in the southern third
of the County Warning Area...with cooler values further north (with fresh snow on
the ground). GFS raw model temperatures (especially at 00z) appear to be
significantly too cool...and were not used.
As high pressure moves away on Monday evening...attention will
turn to a multi-faceted incoming weather system...which will
affect the region through much of Tuesday and into Wednesday. Flow
on the back side of the high will switch to southeasterly at the
surface...and southwesterly aloft. Weak ridging over the Ohio
Valley and southeastern states will give way to a developing
trough in the plains...setting up a favorable and deep path for
moisture transport out of the Gulf of Mexico. There is high
confidence in a significant warming occurring aloft between 00z
Tuesday (monday evening) and 18z Tuesday (tuesday afternoon).
However...a shallow layer of Arctic air (influenced by snow cover)
will be slow to retreat...leading to a surface temperature
forecast that increases only slowly early Tuesday morning.
Isentropically-driven precipitation will develop and move into the
iln County Warning Area as early as 06z...though convergence ahead of the
strengthening low-level jet will be maximized between 12z and 21z.
The strength of the warm layer will be such that...outside of a
very brief period of possible sleet near the onset of
precipitation...freezing rain will be the favored precipitation
type for a period of several hours. As an example...a GFS BUFKIT
sounding for kday at 15z Tuesday has a surface temperature of
-2...with a saturated warm layer 5000 feet thick (and maxing out
at +5). Thus...the use of freezing rain in the weather grids was
significantly increased...with high-end probability of precipitation (70 to 100 percent)
to match the confidence. While this precipitation may be more
showery in nature than purely stratiform...liquid amounts are
likely to be one to two tenths of an inch through 18z. The end
result for an ice forecast is that the entire County Warning Area will be likely
to get a light glaze of a few hundredths...while the northwestern
half of the County Warning Area will have the greatest potential for higher
accumulations...perhaps a tenth or slightly higher.
However...warning criteria ice looks very unlikely.
Further complicating matters...the far northern sections of the
County Warning Area may get into the precipitation before the warm layer
arrives...allowing for a few hours of light snow. Accumulations
do not appear likely to exceed an inch.
The freezing rain threat will end from south to north as surface
temperatures eventually begin a quick rise...as the warm front
moves north through the forecast area. As this occurs...the axis
of heaviest precipitation will set up across the County Warning Area on a SW-to-NE
orientation. This axis will shift southeast with time...as a cold
front advances from the northwest...and pushing the heaviest
precipitation southeast of the County Warning Area by early Wednesday morning. As
this occurs...temperatures may become cold enough for a mix with
snow at the same time. However...there is a lot more spread in the
models at the end of the evolution of this weather system than at
the beginning...so the timing of this transition is unclear.
If there is going to be any potential for convective showers with
this system...it appears to be in the southeastern County Warning Area on Tuesday
evening. Elevated thunderstorms appear to be a fairly low
probability...though not impossible to occur. Near-surface
thermodynamics look rather unfavorable for any wind potential
during the same time period...but a stronger southerly push of
warm air could change that.
Some periods of heavy rain may be possible...as precipitable water
values easily climb over an inch. As has been covered in previous
discussions...the combination of warming temperatures / snow melt
/ rainfall will lead to a potential for flooding during the middle
of the week. Rainfall forecast amounts by themselves are not
overly concerning right now (an inch or two over a period of 24
hours)...but the snow complicates matters in this case. The
heaviest rain is expected to fall over the southern and
southeastern sections of the County Warning Area.
Temperatures will hit their highest values for the entire week on Tuesday
evening...before cold air begins moving in ahead of the front.
Confidence in the exact temperature values is not very
high...since this is a non-diurnal scenario driven by strong
advection...and a strong gradient will exist across the County Warning Area.
Long term /Wednesday through Sunday/...
models continue to struggle as to how fast precipitation will exit our area
on Wednesday...or whether it will redevelop well north of
southeastward moving cold frontal boundary. The GFS is the outlier
in that it pushes precipitation through the remainder of the region during
the morning. This seems to be driven by its solution of pushing a
broad middle level trough through the region quicker. This solution
also results in faster cold air advection into the region. The other operational
models...NAM...CMC and European model (ecmwf) indicate a slower middle level trough
propagation. The NAM looks strange at the surface...so have taken a
blend of the CMC/European model (ecmwf) solution. This solution allows for
redevelopment of precipitation north of the frontal boundary into our region.
The redevelopment seems to be mainly driven by upper level jet
dynamics resulting in an anafront configuration. Will keep probability of precipitation
going on Wednesday with likely or higher forecast for points mainly
south of I-70. Precipitation should transition to snow through the day as cold air advection
continues. However...there could be some mixed precipitation early on if cold air advection
is faster at the surface than warmer air aloft.
Precipitation should gradually exit the region Wednesday night as more Arctic
air filters in from the northwest.
The remainder of the extended should be relatively quiet considering
the recent active weather. Arctic high pressure will build into the
region on Thursday...moving east of the area Thursday night. Well
below normal temperatures are forecast with highs in the teens and
20s on Thursday...followed by single digit lows Thursday night.
Normal highs are in the middle and upper 40s and normal lows are in the
upper 20s to the lower 30s. For Friday into Sunday...an upper level
split flow pattern will develop over the Continental U.S. And southern Canada with
northwesterly flow persisting into the Great Lakes. The good news is
that any frontal systems will be weak with a modification of the
airmass expected. A weak front is forecast to shear out across the
Great Lakes Friday night into Saturday...followed by more high
pressure on Sunday.
Aviation /06z Monday through Friday/...
precipitation has moved out of the taf sites. Dry conditions are
expected for the taf period as high pressure builds into the
region. MVFR ceilings will gradually move out of the taf sites
overnight. VFR conditions are then expected for the remainder of
the taf period. A few wind gusts will be possible at the start of
the taf period however expect winds to gradually decrease with time.
Outlook...MVFR/IFR ceilings and visibilities are possible Tuesday
into Wednesday with rain and snow.