Area Forecast Discussion
Issued by NWS Paducah, KY

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522
FXUS63 KPAH 231027
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
527 AM CDT Tue Apr 23 2019

.SHORT TERM...(Today through Thursday night)
Issued at 310 AM CDT Tue Apr 23 2019

The split flow regime advertised last weekend is coming to
fruition now, with a northern stream shortwave shearing out just
to the north of the area and broad ridging to the south ahead of a
southwest U.S. low.

The impressed surface frontal boundary is just northwest of the
WFO PAH forecast area as of 0730z and will slowly slide southeast
and stall out between 16-18z Tuesday. This will set the stage for
the pooling of moisture and instability across the central and
southern sections of the area during the afternoon hours today.
Most of the thunderstorm development this afternoon will be
focused between 21z-02z, initially tied to minor vorticity maxes
ejected from the southwest U.S. low and into the broad west-
southwest and weakly anticyclonic flow aloft. Given the shear and
CAPE profiles (strongly influenced in the surface to 3 km layer),
anticipate isolated to scattered convective activity becoming
outflow dominated by early evening.

Very weak and broad lift further north in Missouri/Illinois
overnight will lead to intermittent showers along and north of the
weakened frontal boundary.

The high resolution and medium range model guidance support some
thermal/height ridging during the morning on Wednesday, as the
southwest U.S. low/trough moves into western Texas and briefly
phases with a trough axis over KS and northwest Missouri.
Anticipate a reduction in the coverage of rain, except closer to
the trough axis over southeast Missouri.

As the elongated trough axis from northern IL/MO to the TX low
moves closer to the area, diurnal instability and moisture along
the remnants of the effective frontal boundary will generate a new
round of showers and thunderstorms from late morning through mid-
evening on Wednesday. The loss of significant CAPE will return the
area to isolated to scattered shower activity Wednesday night into
Thursday.

On Thursday, the effective trough axis just to the north of the
WFO PAH forecast area will transform into a decent deformation
zone, as the Texas low moves quickly into Arkansas by midday
Thursday, then eastward into the lower Mississippi Valley. It is
at this time, the best rainfall amounts (QPF) will be expected
across the WFO PAH forecast area Thursday afternoon.

Although there is still some uncertainty in the timing and phasing
potential of the closed low in the lower Mississippi valley and
the northern stream trough, the expectation is that there will be
enough phasing to move the trough axis completely through the WFO
PAH forecast area by Thursday night, effectively drawing away lift
and moisture for precipitation.

Although an isolated thunderstorm may develop enough updraft shear
to produce some minor hail or gusty winds, the predominant
environment of low shear (25-30 knots below 3 km) and most
unstable CAPE (100-2200 j2/kg2 below 3km) would mainly lend to
isolated/multicell thunderstorm development, tied closely to the
thermal/moisture gradient offered by the decayed frontal boundary.
The loss of any focused upper level forcing would shift the
environment to outflow dominated storms, then just background
broad scale lift for showers. It is more likely that we will see
isolated heavy downpours, occasional lightning, and some gusty
outflow winds with this event.

Prior to and to the south of the convective activity today,
temperatures will be 5 to 10 degrees above normal. As the frontal
boundary settles in across the area through mid-week, temperatures
will hold near normal for this time of year.

.LONG TERM...(Friday through Tuesday)
Issued at 310 AM CDT Tue Apr 23 2019

Relatively fast west northwest flow aloft will prevail through the
weekend, and then it will back a little to more zonal for early next
week. Impulses in this flow will bring some chances of showers and a
few thunderstorms mainly across the northern half of the forecast
area beginning on Saturday. At the surface, high pressure will pass
through the region on Friday, and then southerly flow will return
early Saturday ahead of a cold front that will make passage Saturday
night. More high pressure is expected Sunday, but south winds will
increase significantly and lead to more summer-like conditions for
Monday and next Tuesday.

The 00Z GFS is the strongest and fastest with an upper-level impulse
that moves through the region Saturday night. It may be quite breezy
ahead of it on Saturday with gusts 25-30 mph possible. The ECMWF and
GFS agree that there will be a decent chance of warm advection
showers moving eastward mainly across the northern half of the
region on Saturday.

The models differ in the extent to which the cold front will make it
into or through the area. The GFS pushes it through the entire
region Saturday night, while the ECMWF stalls it out near the Ohio
River and the Canadian keeps it to our northwest. Impulses in nearly
zonal flow are generally not handled well by the models, so
confidence is not great, but this forecast will lean more toward the
GFS and push the front through the entire area Saturday night, and
then keep Sunday dry with surface high pressure building overhead.
Moisture return will be very limited ahead of the front, so will
limit PoPs and thunderstorm mention to southeast Missouri and
southern Illinois Saturday evening, and then dry it out overnight.

Following with the GFS solution, we will keep Sunday dry with
surface high pressure in place, but if the ECMWF and CMC models are
correct the boundary may be in play somewhere across our region on
Sunday. We will not be surprised if PoPs are eventually added to the
Sunday forecast.

With the zonal flow expected early next week, the surface boundary
may get strung out east to west somewhere over or most likely to the
north of our forecast area, as we await a southern stream system
that will be weakening as it approaches the mid-Mississippi valley
late Tuesday and Tuesday night. Confidence in timing and coverage of
any convection Monday and Tuesday is low, so we will be advertising
warm, breezy, and mostly dry conditions Monday and Tuesday. PoPs
should be on the increase heading into next Tuesday night.

Increasing low-level moisture will lead to an increase in potential
instability across the region Monday and Tuesday. If thunderstorms
can get going either afternoon, they could become rather strong.
Thankfully, wind fields will not be very stout at all, so updraft
organization may be lacking.

Temperatures will start out near normal through the weekend and then
climb well above normal Monday and Tuesday when highs in the lower
80s are expected over much of the region.

&&

.AVIATION...
Issued at 527 AM CDT Tue Apr 23 2019

As the cold front continues to travel southeast across the WFO PAH
TAF sites, ceilings will move from VFR to MVFR through the period.
The location and coverage of the rain will be quite variable, so
left vicinity showers where the best chance are expected in the
TAF sites.

&&

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

$$

SHORT TERM...Smith
LONG TERM...DRS
AVIATION...Smith



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