Scientific Forecaster Discussion
fxus63 klmk 262346
Area forecast discussion
National Weather Service Louisville Kentucky
746 PM EDT Wed Jul 26 2017
..updated aviation discussion...
issued at 745 PM EDT Wed Jul 26 2017
Quick update to expand coverage of convection the next couple hrs,
as anywhere west of I-65 could be fair game before it dissipates
with the loss of heating.
Short term (now through Thursday night)...
issued at 310 PM EDT Wed Jul 26 2017
Afternoon satellite imagery showed partly to mostly sunny skies
across the region. Temperatures ranged from the middle 80s across
the Bluegrass region to the lower 80s across the I-65 corridor and
into western Kentucky. Higher dewpoints have pushed back northeastward
across the region. Dewpoints were well into the mid 70s out across
western Kentucky with lower 70 dewpoints confined to areas along and west
of the US 27/127 corridor. A few isolated thunderstorms were
rumbling across our western areas. These should continue through
the afternoon hours, with the best chances remaining along and west
of the I-65 corridor. Afternoon highs should be met in the next
hour or two, with readings falling into the 80s this evening.
For tonight, a surface low pressure and attendant surface frontal
boundary will continue to move eastward and approach the Ohio Valley
from the west/northwest. Latest guidance suggests and uptick in
convection out across MO later this afternoon and tonight with the
convective activity moving eastward toward our area. We'll see an
increase in cloud cover with storm chances picking up after midnight
as convection moves in from the west. We'll solidly be in the warm
sector overnight and lows will only fall into the 70-75 degree range
in most areas.
For Thursday, convection is likely to be ongoing at the start of the
period across southern Indiana and portions of north central
Kentucky. This activity should be weakening a bit as it moves
through the region. However, the front will remain to our west and
we'll continue to see moisture pool out ahead of the front.
Numerical guidance suggests a high plume of moisture residing across
the region on Thursday, characterized by dewpoints in the low-mid
70s and precipitable water values possibly approaching record levels for upper air
soundings at kbna and Kiln. Convective evolution is rather
challenging due to combination of poor lapse rates indicated by
model proximity soundings, plentiful cloud cover, and rather meager
lapse rates. There is some indication that some boundary layer
heating may commence across southern Kentucky which would result in decent
instability and perhaps the shot at seeing some strong to severe
convection. Previous day two outlook had US in a slight and has
since been downgraded and based on the latest guidance and lack of
confidence, we do not see an issue with that. We could see some
isolated strong/severe storms across portions of the area on
Thursday. However, given the rich plume of moisture and high precipitable water
values, it appears that our hazardous weather will likely be
confined to torrential rainfall and localized flooding issues.
In terms of qpf, run to run model consistency with respect to the
axises of quantitative precipitation forecast have been rather poor. Initial early morning data had
the higher quantitative precipitation forecast axis to the north of the region, but majority of the
12z data has trended a bit more south. Widespread basin amounts of
1-2 inches look reasonable from late tonight through Thursday night.
Some isolated amounts of 2-4 inches will be possible in localized
areas. Latest guidance suggests the heaviest quantitative precipitation forecast will fall over
areas that have been dry of late (western/SW ky) where ffg values
are rather high. For this reason, we collaborated with surrounding
offices and will hold off on any headlines. This is due to the
expected very localized nature of flash flooding (not widespread
occurrences) and that expected quantitative precipitation forecast swath will affect areas that have
been drier over the last week or two. However, we will highlight
the potential for local hydrologic issues in an sps, our weather
story graphics, and the hazardous weather outlook.
Showers and thunderstorms will continue into Thursday night with the
bulk of the action affecting mainly southern Kentucky and into northern
Tennessee as the area of surface low pressure races eastward and the
associated surface frontal boundary settles southward. Highs
Thursday will likely exhibit a gradient with lower 80s across
southern in and northern KY, with mid-upper 80s down along the Kentucky/Tennessee
border region. Lows Thursday night will remain mild with readings
mainly in the lower 70s.
Long term (friday through wednesday)...
issued at 326 PM EDT Wed Jul 26 2017
As we get into the extended period, Friday looks to be a day of
transition from the heat/humidity to one of a drier/less humid
weather for the Ohio Valley. Some Post frontal cloud cover and
shower activity will be possible on Friday with drier conditions
emerging by the afternoon. Highs Friday will range from the upper
70s to around 80 across southern in/northern Kentucky to the lower 80s
across southern Kentucky. The front edge of the drier air will move into
the region Friday night with lows dropping into the upper 50s to the
High pressure will then build into the region for the weekend
resulting in a rather beautiful weekend with plentiful sunshine and
cool nights. Highs on Saturday will range from the upper 70s to the
very low 80s, with overnight lows in the mid-upper 50s. Similar
readings are expected on Sunday with highs in the upper 70s to the
lower 80s, with lows in the lower 60s.
Dry weather looks to continue into early next week. The models are
still struggling with the overall pattern and while temperatures and
humidity will likely moderate somewhat, some diurnally driven
convection will be possible by Tuesday and Wednesday and afternoons.
High temperatures look to warm back into the mid-upper 80s with
overnight lows in the 60s.
Aviation (00z taf update)...
issued at 705 PM EDT Wed Jul 26 2017
Expect prevailing VFR conditions through the night and at least
Thursday morning. Cluster of storms over southern Indiana is moving
into a less favorable environment and should weaken or even
dissipate before reaching sdf.
Low-confidence forecast for much of the day on Thursday as we could
see multiple rounds of showers and storms, especially at sdf and
Lex, but timing is more difficult to nail down. Best chances for
precip will be in the afternoon, and for now will keep rain showers/thunderstorms in the vicinity
mention until timing is more certain. Will keep cig/vis VFR, if only
just barely, but heavier showers could result in a brief drop to
MVFR, or possibly even IFR. Will continue the thunderstorms in the vicinity mention into the
planning period for sdf.