Area Forecast Discussion
Issued by NWS Paducah, KY

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000
FXUS63 KPAH 201801
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
101 PM CDT Sun May 20 2018

.UPDATE...
Issued at 100 PM CDT Sun May 20 2018

Secondary update early this afternoon was made to reflect addition
of severe thunderstorm wording with the addition of Severe
Thunderstorm Watch #110 for most of the WFO PAH forecast area
until 8 pm CDT.

The impact of the gravity wave has diminished with mixing of the
boundary layer. Surface winds have shifted back to the south and
moisture advection is resuming ahead of the convective line and
will continue to aid in propagation of this line through the area.

With the exception of the higher shear near the earlier warm
front to the north, the overall shear is rather weak across most
of the WFO PAH forecast area early on, and is not forecast to
increase until late this afternoon. The current line of convection
may end up pulsing until an organized cold pool can develop with
the line. This may be hard to do through at least 20z, so severe
level winds/hail may be limited through parts of southeast
Missouri. As the storms move into better surface based CAPE and
heating, may see better storm organization into west Kentucky and
southern Illinois.

UPDATE Issued at 939 AM CDT Sun May 20 2018

Minor mid-morning update to convert to coverage terms for shower
and thunderstorm activity today and to re-work precipitation types
(add thunderstorm mention) for parts of southwest Indiana and
northwest Kentucky this morning.

Low level instability was visually evident with virga and
altocumulus castellanus development near daybreak. However, also
noted on 6.19 to 7.4 um water vapor imagery that gravity waves had
moved through the area from the convection over the Midwest and
south overnight. Most of the convection through the middle of the
morning in the WFO PAH forecast area may be elevated, but as
mixing goes on, cannot rule out some convective wind gusts above
30 mph reaching the ground.

There continues to be some disparity as to the timing and coverage
of any shower and thunderstorm activity today in the warm sector
covering the WFO PAH forecast area. The high resolution convection
allowing models (CAM`s), with the exception of the mid-morning
13km RAP guidance, has been slow to generate precipitation over
the area the last several days. The CMC (Canadian) guidance does
show some development. The warm frontal boundary remains weak and
undefined along the northern edge of the WFO PAH forecast area.

The 13z RAP soundings show decent CAPE developing over the area
between 2-4k j2/kg2 accumulating through the afternoon, but very
marginal shear (20 m2/s2 or less). Without MCS/MCV influence, it
will initially be difficult to get organized convection early
today. Most of the current MCS showers and thunderstorms are
staying close to the cold/warm frontal zones this morning.

Most of the guidance appears to be hinting at late
afternoon/evening thunderstorm development at this time.

UPDATE Updated aviation section for 12z taf issuance

&&

.SHORT TERM...(Today through Tuesday night)
Issued at 322 AM CDT Sun May 20 2018

The main forecast concern is the strength and timing of convection
over the next few days. Overall model performance is likely to be
poor given the small-scale convective features that have already
muddled up the synoptic pattern.

Over the past 12 hours, widespread convection has altered the
temperature and moisture profiles over the mid-section of the
nation. As mentioned in the 06z SPC day 1 outlook, the 00z model
initializations were poor at several raob sites. In addition, a
surface meso-high over the lower Ohio Valley was not initialized
by the guidance. This all reduces confidence in the 00z model
suite. This includes the hi-res cam models, which did not
accurately depict the convection near the Kansas/Missouri border
at 06z.

Today and tonight, a series of 500 mb shortwaves will track
northeast from the southern Plains into the Missouri Valley.
Virtually all model guidance generates convection over our region,
but the timing and intensity varies. Given the lack of confidence
in any one model solution, the forecast will contain broad-brush
chance pops through tonight. The highest hourly pops will be
shaded toward the late afternoon and early evening hours, when the
peak contribution from diurnal heating occurs. For what its worth,
the high-res cam models target western Kentucky for the greatest
convective potential later this afternoon and early this evening.

Despite the recent convective overturning, it appears ample
sunshine and soupy dew points near 70 will allow the atmosphere to
become quite unstable again this afternoon. This should again
facilitate a few strong convective cores, with isolated hail and
gusty winds.

As for the Monday and Monday night forecast, the synoptic pattern
favors a continuation of thunderstorms chances. A 500 mb shortwave
will track eastward from the upper Mississippi Valley. A surface
boundary is forecast to approach from the north. Once again,
mesoscale convective features will likely play havoc with the
timing of precip. The forecast will maintain the highest pops
Monday afternoon, but only around 50 percent due to the potential
for residual clouds or cooling from overnight or early morning
convection.

On Tuesday, weak upper-level height rises are forecast in the wake
of the Great Lakes shortwave. However, a residual surface cold
front or outflow boundary could stall near the lower Ohio Valley.
Weakening flow fields suggest that any convection Tuesday will be
less organized than previous days. A small chance pop will
continue in the forecast Tuesday. Dry conditions are finally
forecast Tuesday night as 500 mb heights rise and the low levels
stabilize.

.LONG TERM...(Wednesday through Saturday)
Issued at 322 AM CDT Sun May 20 2018

Models continue to show a frontal boundary lingering across the
middle Mississippi/lower Ohio valleys Wednesday through Friday. This
will result in mainly daytime convection each day.  ECMWF and GFS
show the front just south of or in southern portions of the PAH
forecast area Wednesday and Thursday, which will keep any convection
mainly focused in southern and western portions of our region.  By
Friday, the front lifts northward and convection chances will be
area wide Friday afternoon.

After a lull Friday evening, models show low pressure over the Great
Lakes dragging a cold front toward the middle Mississippi valley on
Saturday.  Scattered showers and thunderstorms will spread from west
to east across the PAH forecast area late Friday night into
Saturday.

Unseasonably warm and humid conditions will persist through the
extended.  Dew points will remain in the middle to upper 60s through
the period.  Highs will be in the middle top upper 80s with lows in
the middle to upper 60s.

&&

.AVIATION...
Issued at 100 PM CDT Sun May 20 2018

Due to uncertainty on coverage of storms today through Monday
morning, kept with conditional (TEMPO) groups of MVFR ceilings and
visibilities associated with thunderstorm activity. Most cloud
bases associated with the thunderstorm activity has remained abovr
2kft agl. Outside of thunderstorm activity, keep ceilings in VFR
category through the forecast period for all of the WFO PAH TAF
sites.

&&

.PAH WATCHES/WARNINGS/ADVISORIES...
IL...None.
MO...None.
IN...None.
KY...None.
&&

$$

UPDATE...Smith
SHORT TERM...MY
LONG TERM...RST
AVIATION...Smith



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