Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 051324
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
824 AM EST Wed Mar 5 2014
Issued 820 AM EST Wed Mar 5 2014
Judging by webcams, obs, and reports, the freezing fog (dense in
spots) is sticking around longer than previously anticipated.
Therefore, went ahead and issued a freezing fog advisory through mid
morning. Slick spots have been reported with visibilities less than
a half mile over the last hour still. Temps are in 20s as of 815am.
Temps and visibilities should improve by mid morning.
Issued 645 AM EST Wed Mar 5 2014
Stratus deck looking pretty stout south of the Western Kentucky and
Bluegrass Parkways, with freezing fog showing up in some of the
METARs under that deck. It still appears that the thickest of the
fog is fairly patchy, so will continue to ride with the SPS
highlighting locally reduced visibilities and slick spots on
That said, a quick look at NAM 975mb wind progs suggests that the
stratus may be slow to erode, and GFS LAMP guidance is even more
pessimistic. South-central Kentucky may not see clearing until
midday, and this will hold back temps sufficiently that we will
scale back this afternoon`s max. Updated products on the way.
.SHORT TERM (Now through Thursday)...
Issued at 300 AM EST Wed Mar 5 2014
First 36 hrs of the forecast has more than its share of uncertainty,
but fortunately is a fairly benign weather scenario. Will start off
with zonal flow aloft, but the southern stream will buckle on
Thursday as a disturbance digs into the Mississippi delta and closes
off by the end of the day.
First challenge is near-term fog/stratus issues, which will mainly
be confined to areas south of the Western Kentucky and Bluegrass
Parkways. Will carry patchy fog through roughly mid-morning in these
areas where the dewpoints have remained in the upper 20s.
Fortunately most of this moisture is being lifted into a stratus
deck, so we have not seen greatly reduced visibilities or freezing
fog issues. Expect the low clouds and any fog to dissipate fairly
quickly around 14-15Z as low-level flow picks up out of the NE and
draws some drier air southward.
Plenty of sunshine expected today as a northern stream disturbance
shears out over Illinois/Indiana, keeping the thicker clouds to our
north. Based on recent performance, max temp forecast today will
split the roughly 5 degree difference between the cooler NAM MOS and
the warmer GFS MOS.
Do not expect a repeat performance of the fog/stratus for tonight as
the gradient tightens just enough to keep NE winds up close to 10
mph. Thursday`s max temp forecast is particularly challenging even
though the precip associated with the Gulf coast low remains well to
our south, as an inverted surface trof sets up over eastern
Kentucky. Our low-level winds will back to NNE, which never bodes
well for a warmup, and models are suggesting a decent stratus deck
north of I-64. Based on that, will favor the cooler NAM MOS
guidance, which keeps max temps around 40 north of I-64, while there
will be a pocket over south central Kentucky that sees substantially
more sunshine and should punch into the lower 50s.
.LONG TERM (Thursday night through Tuesday)...
Issued at 245 AM EST Wed Mar 5 2014
An upper-level low will track across the southeastern CONUS Thursday
night. All the moisture and forcing associated with this system
should remain well south and east of the forecast area. The NAM is
now trending drier than yesterday`s 12Z run, with some light precip
just grazing our southeast counties. Believe it will continue to
trend dry, aligning with the rest of the guidance. Temperatures
Thursday night will range from middle 20s to low 30s. Much like we
are seeing this morning, there could be some patchy fog and low
stratus around Friday morning as the upper clouds from the southern
system clear the area, yet low-level moisture remains. Have included
patchy fog for now. Temperatures should reach the low to upper 50s
Friday afternoon under mostly sunny skies.
For the weekend, a cold front is scheduled to push southeastward
across the forecast area Saturday/Saturday night. Return flow from
the Gulf will be minimal, however, there should be just enough
moisture and forcing to squeeze out some light precip. Temperatures
during the day Saturday of upper 40s and 50s should give way to 20s
and 30s by Sunday morning. However, the ECMWF is a little slower
with this frontal passage, so temps may remain elevated a bit into
Sunday morning before slightly dropping behind the front. Precip
would also be delayed in this scenario, with Saturday night and
Sunday morning being the best time periods. So, the forecast for
this frontal passage is certainly subject to change. Either way, the
precip should mainly be rain. There may be a rain/snow mix on the
very tail end, but soundings favor a cold rain or even drizzle as
moisture aloft dissipates.
Sunday through Tuesday`s forecast will feature a large upper-level
low slowly pushing east through Texas and into the Gulf Coast
States. Model spread is also greatest from Monday onward, as the
timing of this low`s eastward migration is in question. Guidance has
trended much drier for Monday night and Tuesday. Have trimmed back
the PoPs to very low-end slight chance for just Tuesday, as the GFS
and ensembles swing a weak front through the area. May end up going
completely dry in future forecasts if this dry trend continues.
Better large-scale forcing for precip does not appear to reach our
area until maybe Wednesday, if not later. As for temperatures,
Sunday will be a few degrees cooler than Saturday, in the wake of
the frontal passage, but then we warm into the 50s and 60s Monday
.AVIATION (12Z TAF Update)...
Updated at 615 AM EST Wed Mar 5 2014
Stratus deck that has been locked into BWG overnight has finally
expanded into LEX within the last hour and a half, and built down
into VLIFR freezing fog. SDF still has sufficiently low dewpoints
and the occasional bit of NE wind, and remains VFR. Main challenge
will be when and how gradually (or not) this fog can
Synoptic models continue to struggle, with only the LAMP guidance
even having a decent handle in LEX. Believe that LEX should see some
improvement in visibility shortly after sunrise, and quickly improve
to VFR mid/late morning as the surface gradient tightens and NE
winds pick up. BWG is a little harder to get a handle on. Looks like
975mb winds gradually back as the morning progresses, but it will be
close to midday before there is enough wind and a NE trajectory to
bring enough dry air in. Therefore will take a much more gradual
approach to bringing BWG out of the pea soup, only going VFR after
Confidence is limited with tonight`s forecast as the models again
want to bring low ceilings in. However, wind fields are enough to
keep decent boundary layer mixing. Therefore, will just bring in a
scattered 800 ft deck after midnight as a heads-up, rather than
including a ceiling at this point.
KY...FREEZING FOG ADVISORY until 10 AM EST /9 AM CST/ this morning