Area Forecast Discussion
Issued by NWS Paducah, KY

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FXUS63 KPAH 031956
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
256 PM CDT TUE MAY 3 2016

.SHORT TERM...(Tonight through Thursday night)
Issued at 255 PM CDT Tue May 3 2016

Isolated shower activity over Southeast Missouri and into the
Purchase Area of West Kentucky (as well as extreme Southern
Illinois) should diminish markedly with the loss of insolation
(sunshine) and the passage of the southwest to northeast
elongated low to middle level trough later this afternoon and
this evening. There will be some gradual clearing during the
evening hours as upper level wind flow and thermal profiles modify
with weak ridging in advance of the sharp wave moving toward the
WFO PAH forecast area between midnight and 7 am CDT.

The high resolution short term guidance, as well as the medium
range guidance have been pushing up the onset time for
precipitation to move into the Interstate 64 corridor of Southern
Illinois by as much as 3 hours, bringing the first showers into
the area between 230 am and 330 am CDT. From Wednesday morning and
into the afternoon the low/middle lapse rates are quite
impressive (-8 to -10C/km), along with downdraft CAPEs above 1000
per the NAM-WRF guidance for the aforementioned time period. In
addition, with freezing levels between 5500-6300 ft AGL and -20
isotherm levels 15000-16500 ft AGL...small hail and gusty winds
should be a given, even with low level bulk shears in the 15 to 30
knot range. Don`t like to flip flop in the Hazardous Weather
Outlook, but will have to add a mention of hail and winds back to
that product.

With the fast and sharp passage of the trough axis through the
eastern 1/3rd of the WFO PAH forecast area during the afternoon,
rain should shut off very quickly from northwest to southeast.

This weather compact system will create a sharp gradient of
clouds and temperatures across the WFO PAH forecast area Wednesday
and Thursday...with the warmest temperatures expected to persist
along and West of the Mississippi River each of those days.

.LONG TERM...(Friday through Tuesday)
Issued at 255 PM CDT Tue May 3 2016

Lower confidence has set in regarding the weekend forecast. While
mid/upper level ridging is still expected to gradually slide east
toward the region, both the 12z operational GFS/ECMWF/Canadian
models now advertise a surface cold front pushing south into the
region (almost in back door fashion) Saturday night into Sunday.
This will likely force us to introduce at least some low POPs for
showers and even a few thunderstorms, especially over the north
half of the region, Saturday night. GFS is advertising 1000-700 mb
MU CAPES 500-1000 J/kg on the north side of the front. Thus
thunderstorms will definitely need a mention to boot.

A bigger question arises on Sunday/Sunday night. The GFS wants to
shift the upper ridge over top of the forecast area in that time
frame, which would result in the front stalling and lifting back
north as a warm front. On the other hand, ECMWF and even the 12z
Canadian hold the ridge off to our west, keeping us in nw flow aloft
longer. This would result in a much farther push south of the cold
front. Thus, the bust potential on max temps Sunday afternoon is
fairly large. Out of respect for the GFS, will play it closer to the
middle of the road since this is still several days out.

In any event, things looks to become more unsettled next week as the
upper ridge shifts farther east and allows the flow to turn more
swly with time, especially by day 7/Tuesday. In addition,
indications are that an extensive area of low pressure will be
pushing east toward the region. Thus, will increase POPs up into the
high chc category (50 percent or so) for showers and scattered
thunderstorms at that time.

&&

.AVIATION...
Issued at 1235 PM CDT Tue May 3 2016

Main challenges with the 18z Tuesday WFO PAH TAFs were duration of
diurnal cold air stratocumulus and the onset and coverage of
showers/thunderstorms and attendant ceilings on Wednesday.
With the transition of ceiling heights over a 1-2kft layer,
selected a mean ceiling height that represented the longest
duration within a time period.


&&

.PAH Watches/Warnings/Advisories...
IL...None.
MO...None.
IN...None.
KY...None.
&&

$$

SHORT TERM...Smith
LONG TERM...GM
AVIATION...Smith


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