Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 10/28/17
Area Forecast Discussion
National Weather Service Wilmington OH
750 PM EDT Fri Oct 27 2017
A cold front will move through the region today, with rain and
cooling temperatures coming in behind it. After the
precipitation ends on Saturday morning, much cooler conditions
will be in place over the weekend and into early next week.
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Adjusted temperatures downward by a bit this evening as readings
below 40 were eking into Indiana counties ahead of schedule by
several hours. This is likely due to the rain falling and
cooling the column in short order to a moist adiabatic profile.
The rain will take its time in moving through the CWA and get a
push in the late overnight period to clear out from west to east
by daybreak over much of the area. The cooler temperatures had
me change the mixed ptype timing as well, given that the
inclusion of snow as a mixed precip occurred at and below 37
degrees for this forecast. Previous in-depth discussion still
valid and speaks to the core of the development of the
The basics of the forecast are straightforward -- a gradual
spreading of PoPs from west to east, with 100-percent chances of
precipitation for every grid point in the forecast area. Winds
shift to the WNW (and may remain slightly gusty for a while)
along with a steep drop in temperatures that will continue into
the overnight hours. Though precipitation amounts of several
tenths of an inch (over a half inch in the SW ILN CWA) are
possible, heavy rain is not expected. One thing that is notable
is that the front is forecast to slow and lose some definition
as the night progresses, which is likely one reason why
precipitation amounts should be heavier further to the west.
Two wrinkles to the forecast involve the potential for wintry
precipitation. There have been several spots with enhanced
radar echoes over Indiana today, along with a few reports of
sleet mixing in with the rain. Looking at HRRR soundings, there
does appear to be a narrow cool layer at around 925mb that will
slowly advect west along with the greatest ascent. Though said
ascent may be gradually weakening with time, and thus this part
of the forecast is not of the highest confidence, there does
appear to be a chance for some sleet to mix in during the late
evening / early overnight hours, primarily in the western half
of the forecast area. Nothing beyond a nuisance accumulation is
The second concern is the possibility of a change to snow on the
back side of the band of precipitation. PoPs were adjusted
carefully based on 12Z WRF-ARW/WRF-NMM projections, in order to
keep the cut-off timing as sharp as possible. Nonetheless,
temperatures should be dropping into the 30s before this occurs,
at least where temperatures will be coldest in the western half
of the ILN CWA. This has forced the inclusion of a rain/snow mix
in the final hour or two in which precipitation will be
possible. The most likely scenario will be for most locations to
not receive any accumulations from this mix. However, even along
the back edge of the precipitation, there may be some slightly
heavier waves -- recent HRRR and RAP runs have indicated this.
Thus, it appears possible that there could be some isolated
tenth-or-two accumulations (if the warm antecedent conditions do
not melt everything that falls). Another cause for uncertainty,
though, is that sounding profiles may be somewhat isothermal --
which could keep hydrometeors just on the warm side of the
freezing mark as they fall.
.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
The surface cold front will be east of the ILN CWA by Saturday
morning, but precipitation (likely a little lighter than during
the overnight hours) will still be ongoing (possibly with a
brief mix with snow on the back edge, as discussed earlier). The
cut-off will continue moving west to east during the morning,
with virtually the entire CWA dry by noon or just after.
Cold advection will be the story for the rest of Saturday, with
enough cold air moving in to steepen lapse rates and develop low
stratocumulus clouds across much of the forecast area (though
there could be some breaks in the sky conditions in the first
few hours behind the precipitation). Highs will thus be unlikely
to get any higher than the mid 40s, and lows Saturday night will
be within a degree or two on either side of the freezing mark.
The area of the coldest air at 850mb will be moving over the ILN
CWA at around 12Z Sunday morning. As lapse rates continue to be
steep, even overnight, there will be a chance of flurries
(possibly some light rain or sprinkles) late in the overnight
and into Sunday morning. Only trace amounts are expected either
.LONG TERM /SUNDAY THROUGH FRIDAY/...
Deep H5 trof will located over the Ohio Valley at 12Z Sunday. H8 caa
and a flow off Lake Michigan should combine with the upper support
to produce a few sprinkles or flurries Sunday morning. Temperatures
will remain well below normal on Sunday with highs only in the mid
to upper 40s.
The upper low lifts and fills Sunday night. With the clouds
breaking, lows could fall to or below freezing. The H5 flow goes
zonal and the models continue to struggle to find consensus on the
forecast for Monday. All the extended models swing a H5 vort
through Monday, but their timing and placement are all different.
Will continue to go dry on Monday. Temperatures will moderate, with
highs reaching the lower to mid 50s.
A high pressure system builds in for Tuesday with high temperatures
remaining in the lower 50s, but then another H5 trof swings out of
the MS valley and brings a chance of rain for late Tuesday night
into Wednesday. Lows Wednesday morning will be around freezing in
Central Ohio, so there could be some mixed pcpn for the morning
The rain overspreads the region on Wednesday, then pcpn chances
linger for the rest of the forecast period, as broad sly flow at the
sfc keep moisture working north. Temperatures should be warm enuf
during the period for the pcpn to be all rain.
.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
Have seen a few IFR cigs and jumped the gun on this phenomenon
for the better part of the overnight period with this area of
rain. While cigs may drop this low, vsbys are hanging tough in
the VFR category early this evening. Tried to time the onset of
rain a little better in CMH/LCK and felt the need to linger pops
a little longer into the morning hours than previous forecast
had. Expect a scattering of cloud cover after the precip ends
later tonight, then to go bkn-ovc in the late morning given the
cold pool overtop of the region, right between 2-3kft.
Will maintain a watch on the progression of the area of rain and
hopefully clear the western edge a little earlier overnight,
even as the latest forecast delayed this feature given most
models had solid area of QPF still found along the I-75
corridor at 12z.
OUTLOOK...MVFR ceilings may continue through Sunday morning.
Area Forecast Discussion
National Weather Service Louisville KY
1200 AM EDT Sat Oct 28 2017
Issued at 1159 PM EDT Fri Oct 27 2017
The LVX radar tonight shows a very sharp back edge to the
precipitation with portions of southern IN and far west central KY
dry. However, regional radar does show another light area of
precipitation further back to the west. Pops were adjusted to taper
them more quickly across the northwest portion of the forecast area,
but have not removed them completely due to the returns showing up
to the west. No snow reports have been seen yet, but there is still
a brief window where we could see some flakes. Other than the pop
update, only minor tweaks were made to the forecast.
Issued at 855 PM EDT Fri Oct 27 2017
Surface cold front has just pushed through LEX, with mild temps
giving way to cold rain within the last hour. Temps along and west
of the I-65 corridor have already dropped into the upper 30s/lower
40s, and we expect that cold air to continue to overspread central
Kentucky through late evening.
Still a brief window overnight where the soundings show deep enough
saturation and low-level temps are just cool enough to get a few
snowflakes to the ground, but the soil temps are much too warm for
any accumulation other than on grassy or elevated surfaces. Previous
forecast has a good handle on these trends, so no update planned
beyond tweaking hourly details.
.Short Term...(This evening through Saturday night)
Issued at 310 PM EDT Fri Oct 27 2017
Surface cold front currently is bisecting the forecast area this
afternoon. The front was located just east of the I-65 corridor.
The edge of the colder airmass is not too far behind it, lagging
about 30-40 miles or so. Out ahead of the front, southerly winds
have allowed temperatures to spike into the upper 60s to the lower
70s. With cloud cover rapidly increasing across the region, high
temperatures in the east will be met shortly, while temperatures
across central and western KY will continue to fall throughout the
afternoon. Regional radars show an increase in shower activity.
Scattered showers continue to blossom across western TN and these
will move northeastward into the Bowling Green area in the next few
hours. Post frontal band of precipitation extends from near
Indianapolis southwestward to Memphis. This band should expand a
bit as it moves eastward into our region later this afternoon.
As we head into the evening and overnight hours, the cold front will
continue to plow on off to the east and colder air will advance into
the region from the west. Short term high resolution model data
continues to show increasing deep-layer forcing over the region.
This is in response to a jet streak rotating through the upper Ohio
Valley. The jet streak looks to stretch out and develop into to
distinct jets, one over the Great Lakes, and one over the northern
TN valley. This would place much of southern IN and central KY in
the left entrance region of the Great Lakes jet, while also in the
right exit region of the northern TN valley jet. With the jets
stretching out, mass evacuation looks to be fairly efficient aloft,
resulting in upward motion. The thermal gradient will also tighten
resulting in stronger frontogenetical forcing. The combination of
these forcing features support an increase in shower activity this
evening. Some moderate to heavy rainfall is likely as these
features push through and a solid 0.50-1.0 inch of rainfall looks
possible through early Saturday morning.
As we move into the overnight period, forecast becomes a bit more
complicated as influx of colder air pushes in from the west and
makes an attempt to catch up to the back edge of the precipitation
shield. The NAM solutions have been the most aggressive with this
over the past few days, while the more coarse GFS has been slightly
warmer with its profiles. This is still the case this afternoon
with the latest data, though the higher res 3km NAM and HRRR
solutions keep the colder air a little more confined to the north.
Current indications are that we`ll see the column cool overnight for
the back edge of the precipitation to mix with snow and possibly end
as a period of snow. The best chances for this to occur will be
along and west of the US 127 corridor, mainly over southern IN and
portions of north-central KY. The switchover looks to be late
tonight, probably after 2-3 AM and will linger through around
sunrise. Further east, toward the I-75 corridor, thermal profiles
have trended a little warmer during the overnight hours and really
do not cool sufficiently until after sunrise, when most of the
precipitation will be exiting the region.
With that said, our overall forecast has not changed all that much.
We believe that we could see a slushy accumulation of snow in some
areas...mainly along and just east of the I-65 corridor. Combination
of warm ground temperatures and surface temperatures above freezing
look to result in little if any impacts. Lows tonight will range
from the lower 30s over SW In with middle 30s over our central and
For Saturday, precipitation will quickly move eastward during the
morning hours. Current thinking is that we`ll get dry slotted for a
while in the morning allowing some sunshine. However, steep lapse
rates and a little heating will quickly lead to a strato-cu field
developing during the afternoon. Highs of 40-45 look attainable
across SE IN down into the Bluegrass region. Highs in the 45-50
degree range look possible in areas west of I-65.
For Saturday night, cold cyclonic flow is expected across the
region. It appears that we`ll have some low cloudiness persist in
the NW flow across our northeastern half of the CWA. A few snow
flurries probably will fall out of those clouds. Further west,
clearing skies will allow temperatures to fall and patchy frost will
be possible out in areas west of I-65. Lows look to cool down into
the 32-37 degree range.
.Long Term...(Sunday through Friday)
Issued at 330 PM EDT Fri Oct 27 2017
Sunday through Tuesday Night...
Upper trough axis will start to pull east and lift away from the
area. Skies will start off partly to mostly cloudy and then
partially clear throughout the day. Temperatures will remain below
normal with highs in the mid-upper 40s across our eastern sections
with upper 40s to around 50 in the west. Dry conditions look likely
as we head into Sunday night. Though, some mid-high level
cloudiness may start to streak in from the west/northwest. In
addition, a light southerly wind looks to return and that may keep
temperatures warmer than previously forecast. As of now, we plan
on going with lows in the lower to middle 30s. Will probably see
some frost, and a few sheltered valleys could see a light freeze.
Heading into Monday, a large scale trough axis looks to remain
centered just to our west. The flow will be mainly zonal across our
region and a sheared out perturbation within the main flow will
cross the area. Moisture is just not all that impressive, but
enough will be around for some mid-high level cloudiness and perhaps
a few sprinkles here and there. This feature will shift eastward
Monday night with quiet weather continuing into Tuesday. Clouds
will increase Tuesday night however as the next weather system to
our south and west takes aim at the region. Highs Monday look to
warm into the lower-mid 50s in the north with upper 50s to around 60
in the south. Lows Monday night look to cool into the mid 30s.
Highs on Tuesday will warm into the lower 50s in the north with mid-
upper 50s across southern KY. Current forecast suggests that dry
conditions are expected for trick or treaters Tuesday evening.
Temperatures will likely be in the 40s across the region and
overnight lows will dip to around 40 or so before rising toward
Wednesday through Friday...
Broad upper level troughing is expected to be in place across the
central US on Wednesday as a large ridge builds across the central
Pacific into Alaska. A disturbance will move out of the southern
Plains and streak across the Ohio Valley on Wednesday bringing
rounds of showers. As we move into Thursday and Friday, mid-level
heights are expected to build across the southeastern US and SW
Atlantic. As this occurs, the ridge over the Pacific and into
Alaska looks to grow and eventually experience a wave break of
sorts. This will end up helping significant troughing off the
western US coast. For the Ohio Valley, a broad southwest flow will
remain in place aloft with several weak perturbations streaking
through. In general, the sensible weather pattern can be classified
as unsettled with chances of showers and possibly a few storms on
Thursday and into Friday. There is some signaling that the flow
may flatten out a bit by Friday allowing some cooler air to push
back into the region. Model cross sections show a bit shallow depth
cold air dropping southward, while low-level wind fields remain
southwest to west. So for now have backed off temps slightly for
Friday with highs mainly in the lower 60s in the north with middle
60s down south.
Beyond Friday and into Week Two...
Hemispheric pattern looks to remain fairly active as we go into the
first weekend of November. Upper air pattern is likely to see a
deep trough off the California coast with a general ridge pattern
over the southeastern US. A broad W/WSW flow will be seen across
the Ohio Valley with the main storm track located to our north. A
few perturbations within the flow will bring episodic bouts of
precipitation to the region. While temperatures look to remain
generally above seasonal averages, brief warm ups prior and cool
downs after perturbation passages will be seen. The cold looks to
be centered mostly across the northern High Plains with the warm
concentrated along the Gulf Coast and into the SE US.
Organic techniques along with updated signal analysis suggest the
hemispheric pattern will amplify a bit as we head into the second
weekend of November. There are hints of another pattern shift
around that time which could spell a period of volatile weather in
the central US spreading eastward into the Ohio Valley (roughly in
the 11/14-17 time frame).
.Aviation...(00Z TAF Issuance)
Issued at 705 PM EDT Fri Oct 27 2017
Strong cold front pushing through central Kentucky, and now just on
the doorstep of LEX. Will be a close call initializing wind there,
but other terminals are already around to NW or even NNW. Rain
shield now making slow progress, and by 01-02Z should have all
terminals into fuel-alternate ceilings with TEMPO IFR cig/vis
expected for much of the night. Could see a few snowflakes flying
after midnight, mainly at SDF and LEX, but probability is too low to
mention in the TAF.
Conditions will improve fairly quickly Sat morning as a dry slot
scours out some of the low clouds. Only exception is LEX, which will
stay in precip through midday, and not recover to VFR until mid/late
afternoon. Winds will be light out of the NW.