Scientific Forecaster Discussion
fxus63 kpah 180523
Area forecast discussion
National Weather Service Paducah Kentucky
1123 PM CST sun Nov 17 2019
Short term...(tonight through Tuesday night)
issued at 330 PM CST sun Nov 17 2019
Two main pcpn chances in the short term comes come successively,
about 24 hours apart, this evening, and tmrw evening. Each is
coincident with some upper positive vorticity advection and its chance to squeeze some qpf
out as it rounds the base of its mean trof axis/making passage
across the forecast area. Despite the qg forcing, the columnar moisture is
impeded by dry air intrusions at certain layers. For example, we
remain Bone dry below about 600 mb until this evening, and then as
we have some saturation down below 600 mb, dry air intrudes
aloft, effectively cutting off precipitating hydrometeors. This is
not to rain absolutely won't occur, but we think it effectively
limits the window narrowly and qpf should be minuscule, so as to
preclude mention for most. We'll carry a collaboratively blended
slight chance mention in the north to account for this energy's
passage this evening.
Again tmrw evening, similarly, dry air intruding before (below 600
mb) and after column saturating down (drying aloft), pinches best
pcpn chances off to the north and west. We're leaning again
carrying any mention for that this package, but may end up doing
similarly and carrying a small chance mention. Either way, it
again looks to be less is more, insofar as qpf is concerned. Blend
qpf is zero for both shots of energy passage.
Nwly flow aloft continues with seasonally cool temps largely
residing in the 50s/30s for highs/lows.
Long term...(wednesday through sunday)
issued at 330 PM CST sun Nov 17 2019
At the beginning of the extended forecast period, the pah forecast
area will be under ridging aloft, with a low level moisture return
on the back side of a surface high pressure ridge. Thus, Wed will
have increasing clouds from the west with relatively warm temps and
rather light winds. By Wed night, mid level shortwave energy
ejecting out of the southwestern Continental U.S. Will combine with northern
stream energy, helping to sustain the strength of a surface low
moving from the Central Plains into the upper Midwest. This low will
have a trailing cold front to its south, which should sweep through
our region late Thu. Ahead of the front, rainfall is expected to
peak prior to daybreak Thu over southeastern MO/southern IL, and
later that morning over western Kentucky/southwestern in. There could be
as much as a half inch of rain, mainly across parts of southern Illinois
before the rain ends Thu night/early Fri. Another minor shortwave,
mainly from the northern stream, may hamper this cessation in the
rainfall during this time. The National blend at this time does have
the Fri daytime/evening time frame mostly dry for now, before
ramping up pops again early Sat.
By this time, the deterministic model solutions did begin to
increasingly differ, mainly with the movement and eventual evolution
of a large southwestern Continental U.S. Mid level low into an open wave. The
feature should move eastward into our vicinity by the first half of
the weekend. The European model (ecmwf)/CMC developed a vigorous surface low ahead of
this feature vs. The GFS with more of a surface trof. This could
enhance pcpn rates. Meanwhile, in the early hours of Sat and sun,
there is a low-confidence forecast of a brief wintry mix. Whatever
happens, it appears that sun (day 7) will probably be mostly dry.
Expect cooler temps after Thu.
issued at 1122 PM CST sun Nov 17 2019
With the 06z Monday weather forecast office pah taf issuance, added some MVFR ceilings
through 18z for the taf locations to reflect the lower ceilings
associated with the passage of the the mid-upper level trough/low
moving through the area overnight and through Monday morning.
Visibility restrictions may periodically move into upper MVFR
category, but for the most part kept visibilities in the VFR
category for most of the forecast period.
Did not mention vicinity or categorical rain overnight in the
tafs given the short temporal impact on aviation, given the
overall isolated coverage.