Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 120706
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
306 AM EDT WED JUN 12 2013
.SHORT TERM (Today through Thursday)...
Issued at 302 AM EDT Wed Jun 12 2013
...Severe Weather Possible Tonight...
First, though, this morning a weak warm front is arcing around the
north and east sides of the LMK CWA. Models are suggesting some
convection could blossom along the boundary this morning, between
09Z and 15Z. Radar and satellite loops show nothing developing as
of 07Z, but will keep an eye on it.
Otherwise, today will be hot and very muggy. We`ll have quite a bit
of sunshine, and gusty southwest winds will allow temperatures to
rise into the lower and middle 90s with dew points in the upper 60s
and lower 70s. The heat index will peak just a few degrees shy of
the century mark this afternoon.
On to tonight. An upper shortwave trof sparking convection over the
Dakotas at 07Z will head east today and into the Great Lakes and
Ohio Valley tonight. A surface low over Nebraska will advance to
northern Illinois by this evening, and then deepen overnight as it
interacts with the sharpening upper trof and moves into Ohio by 12Z
Thursday. The surface cold front will sweep into the LMK CWA around
sunrise Thursday morning.
Thunderstorms will continue to develop well to our northwest from
Iowa to Indiana through the day today. This evening those storms
will congeal into an MCS, possibly a derecho, and head ESE. While
the main bulk of the MCS will remain to our north in a region of
much better moisture and dynamic forcing, a line of storms will
likely develop southwestward from the MCS into the Ohio Valley.
These storms are expected to enter our southern Indiana counties in
the 02Z-04Z time frame and then proceed across the river and through
central Kentucky overnight. A 50kt low level jet riding up the Ohio
Valley, strong theta-e ridging, and our position beneath the left
exit region of an upper jet streak support the idea of storms
building southwestward from the main MCS to our north.
There will be plenty of low level shear to support severe weather,
including enough 0-1km helicity to raise the possibility of
tornadoes. Damaging winds will also be possible, given the squall
line type mode of convection. Hail is of lesser concern but with
the healthy shear and wet bulb zero heights around 10K-11K feet it`s
However, there are still some question marks. One is that soundings
are showing a pretty good inversion tonight, which could help to
keep strong winds and rotation aloft. Also, moisture supply is not
as great as it could be. With high pressure strung out along the
Gulf Coast, most of the moisture associated with this system will be
of Pacific origin and will be found to our north in concert with the
surface low. Soundings don`t have a whole lot of moisture in them,
and precipitable water amounts of 1.50-1.75 inches are actually only
slightly above normal for mid June. Nevertheless, it is felt that
the strength of the system and soupy surface dew points in the upper
60s to around 70 will help to fuel storms.
The line will sink southward across Kentucky early Thursday morning,
slowly losing its strength as it does so. By late Thursday morning
the cold front and any remaining storms should be to our south.
Temperature-wise, tonight will be very uncomfortable with high
humidity and temperatures only falling into the middle 70s (outside
thunderstorms). High temperatures on Thursday will be in the lower
and middle 80s with afternoon dew points in the lower and middle 60s.
.LONG TERM (Thursday Night - Tuesday)...
Issued at 300 AM EDT Wed Jun 12 2013
Upper air pattern late Thursday will feature a potent, still
strengthening 500mb trough that is forecast to move across the
mid-Atlantic States. This upper air feature will be responsible for
an out-of-season coastal storm over New England as both the GFS and
the NAM forecast surface pressure deepening to 990mb early Friday
south of Long Island.
High pressure of Canadian origin will build southwards from the
western Great Lakes towards the Tennessee Valley Thursday through
late Saturday. This feature will bring a brief period of mostly
clear skies, low humidities and seasonably cool temperatures late
Thursday through Saturday morning. With this Canadian airmass,
dewpoints will fall towards the lower 50s by early Friday,
continuing through Saturday morning. Under light north winds, lows
may fall into the upper 50s early Friday morning and top out near 80
by afternoon. After one more cool morning, Saturday afternoon will
warm into the mid 80s.
High pressure will move east over the mid-Atlantic states by Sunday,
allowing light southerly winds to arrive by Saturday afternoon. Both
temperatures and humidities will begin to rise by Saturday
afternoon, ensuring that Sunday through Tuesday will become warm and
relatively humid for mid-June.
Both the 12z ECMWF and the new 00z GFS forecast a compact closed low
slowly moving across southern Canada over the weekend. It is
expected to arrive near Lake Superior by early Monday morning.
Southerly flow will bring Gulf moisture north along the Mississippi
River Valley during the weekend. Moisture will increase by early
Sunday across southern Illinois and Indiana ahead of a surface
trough expected to develop across the Midwest.
Low level winds will veer to the southwest Sunday through early
Tuesday as a slow moving frontal boundary becomes stretched from the
Great Lakes through Missouri by late Monday. PWATs will increase to
over 1.5 inches by late Sunday across the lower Ohio Valley. Rich
moisture combined with possible waves along a broadly cyclonic flow
around this Great Lakes 500mb closed low, may bring a chance of
convection to southern Indiana as early as Sunday morning. Will
continue our current forecast of at least a chance of convection
Sunday through Tuesday, with our best opportunity or rain arriving
Monday, when a weak surface trough approaches southern Indiana.
Highs Sunday through Tuesday will likely range from the mid to the
upper 80s, with lows in the mid to upper 60s.
.AVIATION (06Z TAF Update)...
Issued at 104 AM EDT Wed Jun 12 2013
The pre-dawn hours should generally be quiet, with a couple of
possible exceptions. One is that scattered showers and storms may
develop in an area of isentropic lift stretching roughly from
southern Wisconsin to the central Appalachians. Most of this
activity should remain to our north and east, but some convection
could approach the LEX terminal, especially between 09Z and 15Z.
The other possible concern early this morning is LLWS, but at this
time it appears borderline for inclusion in the TAFs so will leave
out for now.
Winds will be the main concern for the daylight hours, as they come
in from the southwest and gust into the 20 to 25 knot range.
Thunderstorms aren`t expected, other than some lingering morning
activity in the far east as mentioned before.
Tonight a strong upper wave, surface low, and cold front will
approach from the west. Moisture supply is in question, but
scattered storms should still accompany the system. Will introduce
VCTS to SDF for late tonight. LLWS may be a concern too as a
low level jet cranks up along the length of the Ohio Valley.
However, surface winds are also expected to remain brisk and there
won`t be much directional shear, so TAF-worthy LLWS looks
borderline. Since it`s so far out in the forecast period, will
leave it out of the SDF TAF for now.