Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 210654
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
254 AM EDT Thu May 21 2015
.SHORT TERM (Now through Friday)...
Issued at 253 AM EDT Thu May 21 2015
The synoptic pattern this morning features zonal to northwest flow
across the Ohio Valley. This regime will continue through the short
term period, bringing continued seasonably cool conditions.
A weak surface low has pushed east into western North Carolina this
morning. In its wake, northerly surface winds have brought much
cooler temperatures along with copious cloud cover to the region.
Temperatures this morning currently range from the 50s to lower 60s,
with lows by sunrise expected to drop into the upper 40s and 50s.
For today, a PV anomaly will slide southeast through the northwest
flow aloft, bringing large scale height falls across northeastern
portions of the CWA. Many hi-res guidance members and ensembles
suggest this forcing may be enough to squeeze out some sprinkles or
a few light showers this afternoon in the northern Bluegrass
region. However, an analysis of forecast soundings this afternoon
show that the moisture profile will be rather shallow. While we
cannot rule out a few sprinkles or even a light shower this
afternoon east of I-65, think the threat is too low to put in the
forecast. Given the expected cloud cover and northerly surface
winds, temperatures today will be well below normal, with readings
expected to top out in the low and mid 60s.
For tonight, any lingering cloud cover should dissipate/push east by
the late evening hours. If this occurs as forecast, we should be in
for a chilly night as surface ridging slides into the Ohio Valley.
The best radiational cooling conditions will be found across western
and southern KY, where the ridge axis looks to set up. Temperatures
across the area will dip well into the 40s, with even some upper 30s
readings not out of the question in the typically cooler locations.
Some patchy fog will also be possible, mainly south of I-64 where
winds should be lightest. This threat will likely be dependent on
dewpoints this afternoon, and if we are able to see any breaks in
the cloud cover which may help to mix them out. The day shift will
monitor trends this afternoon and adjust as needed.
Friday is shaping up to be a rather nice day, but still seasonably
cool. A good amount of sunshine should help us mix to near 800mb,
but 850mb temps will only be around 7-8C. Therefore, highs will
only top out in the low to mid 70s, despite the sunshine and
expected deep mixing. A weak front will approach from the north
later in the day, which may spread a few clouds into southern IN and
northern KY by later in the afternoon/evening.
.LONG TERM (Friday Night through Wednesday)...
Issued at 250 AM EDT Thu May 21 2015
The long term will be dominated by upper ridging developing over the
southeast U.S., peaking Monday-Tuesday. At the surface a large dome
of high pressure centered over Michigan (but covering much of the
eastern half of the country) will slide to the ESE Friday night and
Saturday. It will then park off the Carolina coast as it meets up
with the westernmost extent of the Azores High.
The operational runs of the GFS, ECMWF, and CMC have all trended
significantly drier for Sunday and Sunday night in the Ohio and
Tennessee Valleys. Most ensemble plume traces of the GFS are flat,
but there is still enough uncertainty that we`ll need to keep at
least low chance PoPs in the forecast. The best chance for
convection will be in the afternoon as surface LI`s go slightly
negative and a weak wave comes up from the southwest.
For much of the rest of the long term we`ll be in deep, moist
southwesterly flow. As waves move through this atmosphere
convection can be expected. While the large scale models are likely
overdoing precipitation coverage Monday through Wednesday, chance
PoPs through that time period are well warranted. Forecast
refinements will be made as individual triggers become more apparent.
The best instability and upper dynamics are expected to remain just
to our west from the southern Plains to the western Great Lakes. As
a result, widespread severe weather outbreaks are not expected at
this time across southern Indiana or central Kentucky through
.AVIATION (06Z TAF Update)...
Updated at 104 AM EDT Thu May 21 2015
A weak surface low has pushed east of all terminals this evening,
leaving northerly winds and lingering low-level moisture in its
wake. KBWG is currently seeing IFR cigs and should remain within
IFR cigs through the remainder of the night into the mid-morning
hours. Confidence in cigs further north at KSDF and KLEX is a bit
lower, as these areas did not see measurable rainfall earlier thus
low-level moisture is lacking. Have gone with a more optimistic
forecast at these sites overnight into the morning hours given
upstream trends in obs, but still can`t rule out a period of
fuel-alternate cigs at KSDF or KLEX.
Otherwise, high-end MVFR cigs will persist into the late
morning/early afternoon at all sites today, before daytime mixing is
able to raise bases to VFR. Northwesterly winds around 10 knots
will continue through the day, before dying down after sunset.