NWS Louisville, KY wrote its last area forecast discussion before the first event at Feb 4, 2:32 PM EST, 1 hour 28 minutes earlier.
Area forecast discussion, Feb 4, 2:32 PM EST, 1 hour 28 minutes before
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
232 PM EST Tue Feb 4 2014
...Forecast Update...
Issued at 220 PM EST Tue Feb 4 2014
...Precip Shield Entering the Pennyrile Region...
Expansive precip shield has now spread northeast into the western
third of Kentucky, with light freezing rain and/or sleet already
reported in our westernmost counties. This precip will continue to
spread northeast, and our forecast of 20-23Z for precip onset
remains on track. Temps remain critical, and most of the legacy
models do not have a good handle on it. Looking at surface wetbulb
temps currently in the mid/upper 20s over the snowpack, only the
ECMWF seems to be in the ballpark, and its progs support temps cold
enough for freezing rain over our current Winter Storm Warning area.
Could just barely crack 32F later this evening, but confidence in
the temp forecast is not sufficient to go all rain north of the
Parkways.
However, will continue the trend of somewhat downplaying the ice
accumulations, due to a few limiting factors. First off,
temperatures will be just barely below freezing, and ice accretion
at 31F is a different animal from ice accretion in the mid/upper
20s. Also, precip rates will be a bit high for effective ice
accretion. Finally, the duration will be a limiting factor as a
sharp dry slot should cut in after 06-08Z, with precip likely
tapering off to freezing drizzle after that.
Winter Storm headlines will remain the same, even if warning
criteria may not truly be realized in Kentucky, as we still expect
impacts especially during rush hour.
Issued at 1105 AM EST Tue Feb 4 2014
...Wintry Precipitation Taking Shape to our Southwest...
Precip shield has blossomed this morning west of the Mississippi,
with an area of snow over Missouri and some respectable ice
accumulations already over Arkansas. High-resolution models actually
have a much better handle on this precip shield, which enhances our
confidence in a late afternoon arrival time for potential wintry
precip in central Kentucky and southern Indiana. It appears that
precip will rapidly overspread the area between 20-23Z, without a
whole lot of difference in timing from SW to NE. Of course this is
just in time for the afternoon/evening commute.
Temperatures are very tricky, especially with the snowpack holding
us back along and north of the Western Kentucky and Bluegrass
Parkways. Believe we will be just cold enough for at least a period
of freezing rain in these areas. Southern Indiana is still on target
for a snow event from Jasper to Salem, with perhaps a bit more sleet
or freezing rain cutting into the totals as we get over toward
Madison.
Confidence in actually meeting warning criteria is moderate at best
right now, given that the NAM continues to be a warm outlier, but
the GFS now hints that temps could bump just above freezing across
SDF and LEX. Also expect ice accumulation to be limited east of I-65
in Kentucky as precip rates could be too rapid for the most
efficient ice accrual. Still believe that the warned area will see
at least Advisory-level icing, so no modifications to headlines at
this time.
.SHORT TERM (Now through Wednesday)...
Issued at 327 AM EST Mon Feb 3 2014
...Heavy Snow and Significant Icing Expected Later Today/Tonight...
Another complex forecast is in store for the region later today and
into tonight. In coordination with surrounding offices, will lift a
Winter Storm Warning across the northern half of the CWA, with a
tier of counties south of the warning in a Winter Weather Advisory.
Expecting the main precipitation type to be freezing rain across
most of the warning and all of the advisory, with mainly heavy snow
and some sleet across our far northern tier of Indiana counties.
Lots of complexities and issues to talk about.
...Meteorological Setup...
A progressive upper trough is now over northern Mexico, and will
slide across the southern Plains to the mid Mississippi River Valley
by tonight. Meanwhile, surface cyclogenesis will take place over the
Gulf Coast states this afternoon. The system will ingest plenty of
gulf moisture as it organizes. Then, the surface low will deepen as
it rapidly moves northeast to the central Appalachian spine by dawn
on Wednesday. Basically, this is a low track south and east of the
CWA. At the surface, the area will be under a notoriously cool and
steady northeast flow all day, along with heavy cloud cover. As was
mentioned in the update around Midnight, have become very concerned
about temperatures today. Almost every temperature guidance you can
think of has temperatures warming into the low and mid 30s across
our proposed headline area. However, if you look at current obs
temperatures are running from the low teens to around 20 across our
snowpack. If you combine the snowpack, heavy cloud cover and the
steady NE surface winds its hard to think we will get into the mid
30s. Therefore, have extended the freezing rain threat further south
from the original watch.
In the upper levels, the right entrance region of an upper level jet
will be in place over the Ohio Valley, with mid level frontogenesis
occuring in the deformation zone northwest of the surface low. Forecast
soundings show saturation through the dendritic growth zone, and
with the synoptic setup supporting snow bands think that snowfall
rates could approach 1-2" per hour at times. Again, this only across
Dubois eastward to Jefferson county, IN.
...Amounts...
...Dubois County IN Eastward Through Jefferson County IN...
This area will is expected to be mostly snow, with some sleet mixing
in. Will go with a 4-6" range across these northern counties,
locally up to 7.
...Counties Along the I-64 Corridor...
Precipitation type will mainly be freezing rain in this area, with
ice accumulations expected to be around a quarter of an inch.
Locally, as much as a third of an inch will be possible. A swath
from the Louisville Metro Area to the Lexington Metro Area and into
the northern Bluegrass will be most likely to see the highest ice
amounts. A dusting of snow may also end the event late tonight.
...Rest of the Warning Across Central Kentucky...
This area is expected to have around a quarter of an inch of ice
accumulation. Please note that in the counties on the southern
fringes of the watch, only expect warning criteria ice in the
northern halves. The southern parts of the counties look to be
advisory criteria. Again, this is the area that will have potential
to bust if the forecast temperatures don't end up working out.
...South central Kentucky...
All rain. A dusting of snow possible by dawn on Wednesday.
...Advisory...
The advisory was placed mainly as a buffer area. Expect the northern
portions to receive around a tenth of an inch of ice accumulation.
Timing...
Precipitation will rapidly spread northeastward around 4 PM EST,
precipitating every by 7 PM EST. The bulk of the event will happen
between 7 PM and 3 AM EST.
...Confidence...
It is a bit unsettling to have one of your better short term models
being an outlier from the model consensus at this stage of the
forecast. The NAM does depict an overall warmer north and west track
from the multi model consensus. Will disregard it, mainly for the
reasons mentioned above with the snowpack keeping the near surface
temperatures cooler today. Feel moderate to high confidence in the
northern portions of the warning, basically along and north of I-64.
Will see a very sharp cutoff from heavy snow to freezing rain, and
this will occur only across our northern tier of counties. The rest
of the counties will see mostly freezing rain, with some sleet
mixing in.
South of the I-64 corridor, confidence is not as high in reaching
warning criteria ice accumulations mainly because it all hinges on
how the surface temperatures do across the snowpack. Decided to go
with a warning because if the scenario develops like anticipated it
would be high impact.
Also have to consider the fact that it will rain quite hard at times
and this makes deciding how much accumulation will occur very
difficult. The overall rain that will fall with a thermal profile
supportive of freezing rain is much higher than what forecast ice
accumulation will be for that reason.
...Other Concerns...
Potential also exists for this storm to overachieve, especially if
ice is able to accumulate better than expected. Keeping this in
mind, it is possible that an Ice Storm Warning could replace
portions of the Winter Storm Warning if impacts are expected to be
greater. Most likely area for this to occur would be between
Louisville and Lexington. Stay tuned.
Winds will be between 8 and 13 mph through much of the event (except
when the surface low is passing) and if ice is able accumulate it
could cause problems for power lines. Winds will continue to be
around 10 mph into the day on Wednesday.
Will also be a bit wary of some hydro concerns as up to 1.5" of rain
could fall. Heavier rain over the snowpack and frozen ground could
cause some ponding of water.
Light snow or freezing drizzle could continue into Wednesday with
lingering low level moisture and/or the lack of ice crystals.
.LONG TERM (Wednesday Night through Monday)...
Issued at 300 PM EST Tue Feb 4 2014
Wednesday...
By Wednesday morning, the surface low pressure system responsible
for tonight's mixed precipitation north of the Ohio will have moved
over southern Ohio. Accumulating precipitation will end well before
dawn as dry air at mid levels is expected to wrap around the base of
a weakening 500mb trough over Lake Michigan. With ample shallow
moisture still available however, some freezing drizzle or snow flurries
are still possible along and north of Interstate 64.
As this low pulls away, sprawling high pressure centered over
Montana will begin to build across Missouri and Illinois. Winds will
become blustery by noon Wednesday and increase to 10 to 15 mph with
a few higher gusts. Lapse rates will steepen Wednesday afternoon,
which may support frequent snow flurries or even some brief snow
showers, especially across southern Indiana and the Bluegrass
Region. Temperatures Wednesday won't rise at all and may even fall a
degree or two towards 30 by late afternoon. Skies will likely stay
cloudy through Thursday morning. Colder air will filter in all day
Wednesday and thus expect lows by early Thursday in the teens.
Thursday through Friday...
This period will be dominated by arctic surface high pressure, which
will initially lie over Missouri, but will build eastwards right
over the Commonwealth through Friday afternoon. This period will
stay cold, with high Thursdays struggling to reach 20 across
southern Indiana, and maybe reaching the mid 20s near Tennessee. A
cold morning is anticipated early Friday. With some potential for
mostly clear skies and light winds, a good radiational cooling setup
may allow early Friday temperatures to fall into the single digits
across our north.
Late Friday through late Sunday...
Recent guidance has actually become less in agreement for this time
period with the two main extended models, the ECMWF and the GFS,
showing large differences as early as Friday. Differences begin with
both models' handling of a sharp 500mb shortwave diving south across
the Pacific Northwest. The ECMWF show phasing between this shortwave
and a northern stream trough in Canada, with a broad trough then
moving east towards the Ohio Valley by Sunday. The newest GFS shows
a different solution with this system, weakening it and moving it
across the Great Lakes early Sunday.
For the Lower Ohio Valley, an initial weak flat disturbance will
quickly scoot across southern Indiana early Saturday. Temperatures
are cold enough to support snow, but questions arise over whether
enough moisture is available to support any accumulations.
Forecasting the exact location of what will likely be a thin strip
of light snow will be challenging as well. Weak troughing will
continue over the Lower Ohio Valley Sunday, with the ECMWF showing
the potential development of a storm system across the southern
Appalachians or the southeastern states. The Commonwealth will lie
on the northwestern edge of any precipitation envelope (If it even
develops). Will continue with a chance of snow Sunday in a low
confidence forecast.
Monday and Tuesday...
High pressure will build south across the Missouri Valley late
Sunday in the wake of any potential storm that may develop off the
east coast. Cold temperatures, potentially quite cold, are likely
early next week. Expect some partial clearing late Sunday with highs
Monday only in the lower 20s, modifying into the low 30s Tuesday.
&&
.AVIATION (18Z TAF Update)...
Updated at 1210 PM EST Tue Feb 4 2014
IFR conditions at BWG and LEX with fog and stratus as a result of
abundant low-level moisture and the healthy snowpack near LEX. SDF
has broken out to VFR. Plenty of confidence in precip onset around
20Z at BWG, 21Z at SDF, and 22-23Z at LEX, but precip type is a
little more tricky. Could see a brief period of sleet even at BWG as
precip begins, but should be all rain there while SDF and LEX will
be just cold enough to start with FZRA. Precip could be moderate at
times through a decent chunk of the evening, which will have an
operational impact. IFR, or even LIFR conditions at times with winds
backing to NNE and cranking up to around 15 kt. Temps should warm
enough by late evening to at least briefly go to plain rain,
especially at LEX. Freezing rain potential is greatest at SDF from
21Z to perhaps 02-03Z, less at LEX and essentially nil at BWG.
Precip should taper off around 07-09Z, and we could see a period of
freezing drizzle before daybreak Wednesday. WNW winds should be
around 10 kt on Wednesday, but overly gusty. Low clouds will hang on
for much of the day, keeping all three terminals in fuel-alternate
past the end of the valid TAF.
&&
.LMK WATCHES/WARNINGS/ADVISORIES...
KY...WINTER STORM WARNING until 7 AM EST /6 AM CST/ Wednesday FOR
KYZ023>026-028>043-045.
WINTER WEATHER ADVISORY until 7 AM EST /6 AM CST/ Wednesday FOR
KYZ027-046>049-053.
IN...WINTER STORM WARNING until 7 AM EST /6 AM CST/ Wednesday FOR
INZ076>079-083-084-089>092.
&&
Update.........RAS
Short Term.....BJS
Long Term......MJ
Aviation.......RAS