Area Forecast Discussion
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FXUS61 KILN 060845
Area Forecast Discussion
National Weather Service Wilmington OH
345 AM EST Tue Dec 6 2016
As an area of low pressure moves northeast through the Ohio
Valley, rain is expected for much of the morning and afternoon
today. Drier conditions will return on Tuesday, but temperatures
will drop for the second half of the week after a cold front moves
through on Wednesday.
.NEAR TERM /THROUGH TODAY/...
Current surface observations and MSAS analysis indicate the
presence of a roughly-1003mb surface low over the state of
Mississippi, with a low pressure trough that extends northeast and
abuts the appalachian mountains in eastern Kentucky and West
Virginia. This low is forecast to move northeast today, roughly
following the aforementioned trough, with a compact 500mb
shortwave trailing just behind. Both the shortwave and surface low
are expected to gradually weaken through the day today, but there
is plenty enough of a combination of features -- a feed of gulf
moisture, upper jet support, and mid-level vorticity advection --
to lead to the development of widespread rain. This current
expanse of radar echoes covers the Tennessee Valley, and is
moving northward into the Ohio Valley -- just beginning to enter
the ILN forecast area as of this writing. This rain will
overspread the entire CWA between 08Z and 14Z.
Surface observations across the ILN CWA are all reading 32 degrees
or higher, and with temperatures continuing a very slow rise
overnight, there should be no concern for freezing rain. With the
strength of warm layer (max T of +4C to +6C between 950mb-700mb)
the vast majority of hydrometeors should be well melted before
reaching the ground, so this update will continue the going trend
of having only rain as a precipitation type in the weather grids.
Because this layer is very dry, however, an extremely limited
chance for a brief period of ice pellets is possible -- limited
to the first band or two of precipitation. By 09Z at KCVG, a
plethora of model soundings indicate that even the wet bulb
temperatures will be at or above freezing through the lowest 10kft
of the atmosphere. Upstream reports will continue to be monitored,
but this will not be included in the current forecast.
PoP amounts have been increased to 100 percent for the entire
CWA. Though there may be a quick break or two in the rain shield
as it transposes itself northeastward with time, the overall
amount and duration of the expected rain warrants this, rather
than getting bogged in grid semantics and putting in lower numbers
for an hour or two. Precipitable water amounts are expected to
range from about 0.90" in the northwestern CWA to 1.20" in the
southeastern CWA, give or take a tenth or so depending on which
model is used. A blend of NAM/GFS/ECMWF numbers would suggest a
widespread rainfall of around 0.25" in the northwestern CWA to
around 0.75" in the southeastern CWA. WRF-ARW /WRF-NMM QPF fields
illustrate the potential for some isolated totals of around an
inch or slightly higher, particularly in the southeastern ILN CWA.
Overall model timing agreement is strong, and the grids will also
show a sharp cut-off to the rainfall on the back side of the
shortwave -- ending from 20Z-00Z from west to east. The slight
wrinkle with regards to the end time for precipitation is that a
shallow layer of moisture appears likely to persist behind the
deep moisture ahead of the shortwave. Model soundings suggest that
a well-mixed boundary layer will exist this evening, leading to a
low stratocumulus deck and the potential for some drizzle. Drizzle
has been included in the forecast for a few hours after the main
rain has ended. Current expectations suggest that the drizzle will
likely end before surface temperatures drop below freezing.
Aside from the rising temperatures early this morning,
temperatures through the rest of the day should generally
approximate a diurnal curve, though with a significant gradient
from NW (upper 30s) to SE (near 50).
.SHORT TERM /TONIGHT THROUGH FRIDAY/...
Aside from the low clouds, the rest of the air mass in place over
the Ohio Valley tonight will be quite dry. On the southeastern
periphery of a stacked low over northern Minnesota, a dry cold
front is expected to move through the ILN CWA early Wednesday
morning, with winds switching to the WNW. The surface reflection
of this front is not overly easy to pick out, and in fact, surface
winds are expected to back to the SW again by Wednesday afternoon.
The cold advection will do its best to stunt the diurnal cycle,
and much cooler conditions are expected going into Wednesday
By Thursday, the slowly-moving longwave pattern feature troughing
setting up across the western Great Lakes, with very quick WSW
flow aloft over the Ohio Valley and mid-Atlantic. Previous
forecasts have mentioned the possibility of snow on Thursday
morning, but models continue to back away from this proposition,
with both the 00Z GFS / 00Z ECMWF keeping conditions dry across
the ILN CWA. While a 20 PoP for a slight chance of light snow has
been maintained, a mention of flurries might eventually cover the
scenario. There are no signs of forcing in the low levels at all,
with only some weak/transient upper support on the south side of
the 300mb jet, and this no longer appears to be enough to generate
any reasonable chance of accumulating snow. The continued cold
advection just off the surface will keep the shallow boundary
layer well mixed through Thursday and Friday, which does present
the possibility of a persistent (though maybe broken)
stratocumulus deck. As the top of this mixed layer intersects the
lower end of the DGZ on Friday, some flurries may be possible
(particularly in the northern half of the ILN CWA). This has been
included in the forecast.
Until the advection pattern really starts to adjust itself over
the weekend, temperatures will only continue to cool each day this
week. Model blends for temperatures looked generally reasonable,
but NAM min temps appeared too high for the latter half of the
week, and were not used.
.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
With the upcoming weekend, a warm front will cross northeast
through the Ohio Valley and increase the threat for rain on
Sunday, possibly a rain/snow mix as the warm air overspreads the
cooler air in place. Models continue to disagree in how this
system evolves with the ECMWF notably drier and further west than
the more progressive and much wetter GFS. Modelers from WPC also
seem to have split the difference here noting the warm frontal
passage similar to the GFS solution on Sunday, then bows to the
ECMWF on Monday morning with the surface low east of Lake
Superior. GFS occludes the front off of the east coast on Monday
morning and pushes in drier air. Meanwhile the ECMWF continues to
keep a strong baroclinic zone in the midwest with continued threat
for precipitation over the Ohio Valley during this same time
Tried to trend lower on the threat for precipitation on day 6 and
7 and culminated the pops on the high end category Sunday
afternoon, in line with the GFS. Against better judgment and
nodding towards ECMWF the pops have been lingered into Monday and
Monday night to account for lower confidence and natural
variability of the models on day 7.
.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
TAFs are currently VFR as mid and high clouds stream n ahead of low
pressure in the deep south. The low will track ne up the wrn side
of the Appalachians today.
Mesoscale models continue to delay the onset of the rain ahead of
the system, now showing a 09Z-10Z time frame for it to reach the
srn tafs. The low is forecast to reach srn KY by 12Z, then it will
fill as it moves farther n. Despite this, there is plenty of
convergence at H9 and H8 across the region, so the the rain is
expected to overspread TAF sites from south to north quickly after
12Z. Ceilings are forecast to drop to IFR as the boundary layer
saturates due to the steady rain.
As the low tracks off to the east after 18Z, a cold front will
sweep through the TAF sites. Rain will end late around 20-21Z for
the wrn tafs and a few hours later for the ern tafs. Behind the
cdfnt, winds will turn to the w and will increase, with gusts of
20 kts possible. Cigs will try to lift to MVFR in the w behind the
front, but the should linger in the IFR range further e.
Some drizzle will be possible during the late afternoon and
evening hours of Tuesday as the forecast soundings are showing
drier air aloft coming in over the trapped low level moisture.
OUTLOOK...MVFR ceilings will persist into Wednesday morning. MVFR
ceilings are possible again on Friday.