Scientific Forecaster Discussion
fxus63 kpah 122313
Area forecast discussion
National Weather Service Paducah Kentucky
515 PM CST Tue Dec 12 2017
updated aviation section for 00z taf issuance
Short term...(tonight through Thursday night)
issued at 246 PM CST Tue Dec 12 2017
The 12z model consensus keeps our area dry through Thursday night.
The main concern will be strong gusty southwest winds and the
resultant elevated fire danger for Wednesday.
The persistent upper-level trough over the Great Lakes and much of
the eastern half of the country will relax through Thursday
night, but not before one final upper-level impulse drags a cold
front through our area Wednesday night. The primary impact on our
region will be strong and gusty southwest winds ahead of the
front Wednesday. The consensus of 12z model guidance takes
sustained winds near 20kts with gusts just over 30kts.
We already have one notable wildfire in southeast Trigg County (shows
up well in khpx reflectivity data) which is definitely being
sustained by the gusty northwest winds this afternoon. While
temperatures will warm into the lower half of the 50s Wednesday
afternoon, dewpoints are expected to climb into the lower 30s
which yields minimum relative humidity in the 35%-45% range. With
the obviously dry vegetation across the region and the very strong
winds expected, we will issue a Special Weather Statement to
address the strong winds and resultant elevated fire danger.
Gusty northwest winds this afternoon will subside toward sunset
and become light and variable by 03z tonight. Dewpoints have
dropped into the single digits in many locations and they will be
slow to recover. Given the potential for strong radiational
cooling late this evening into the overnight hours, temperatures
are likely to drop through the 20s by midnight. Went on the low
side of guidance for lows tonight, but that may not be cold
enough. These lows will likely occur around 06z, as south winds
will be developing by 12z Wednesday. Also leaned toward the warm
side of guidance for highs Wednesday, with dry air in place,
southwest winds and lots of sunshine expected.
The surface low will be on more of a easterly track and that will
keep Post-frontal winds in check, generally under 10kts, Wednesday
night. Surface high pressure will settle over the region Thursday
and Thursday night as the flow aloft becomes zonal. Temperatures
Thursday will be similar to today, but with much less wind. An
upper-level trough over the Central Plains will spread thick mid
and high cloud over the region, and that will provide a decent
blanket for the region Thursday night. Current forecast lows in
the middle 20s may be too low, but if the clouds are not as solid
as currently expected, lows may drop into the lower 20s.
Long term...(friday through tuesday)
issued at 246 PM CST Tue Dec 12 2017
The medium range models were in reasonable agreement on the overall
longwave pattern aloft over most of the conus, primarily the
northern stream. The pattern is progged to eventually become more
active across the MS River Valley, as systems with more of a Pacific
origin develop and move across the country.
However, starting during the weekend, the latest deterministic European model (ecmwf)
solutions continued to differ significantly from the other model
solutions and their ensemble means, regarding phasing/non-phasing of
a mid level low over northern Mexico with substantial mid level
energy diving through The Rockies. All models suggest that the
Mexican low will be kicked eastward as an opening wave, with an
associated surface low passing by well to our southeast.
Early on, the European model (ecmwf) had the Mexican low farther to the northeast
than the other models, which suggested it may be more susceptible to
phasing with the northern stream shortwave in the model; this was
not necessarily the case with the other models. What all this means
is there is uncertainty as to the timing of the onset of pcpn for
the pah forecast area early sun, with the European model (ecmwf) being the earliest.
Pcpn duration is also in question, as the other models still have
the aformentioned, unphased shortwave moving through our region as
late as Tue, with limited pcpn chances.
For now, we will go with a mostly rain event, moving in from the
southwest sun, possibly transitioning to light snow in the northern
half Sun night, and diminishing during the day Mon as rain showers.
Little to no impact is anticipated, since temps should be able to
rebound into the upper 40s/lower 50s. Any pcpn Mon night/Tue appears
sketchy at best at this time.
Temps will be above seasonable averages day 5-7.
issued at 515 PM CST Tue Dec 12 2017
The forecasts are VFR with no cigs or vsby restrictions. Northwest
winds will become light and variable this evening. The pressure
gradient will tighten in a hurry early Wednesday as a strong surface
low descends from the upper Midwest toward the upper Ohio Valley.
South to southwest winds will develop before sunrise, and pick up by
mid-morning with gusts pushing 30kts possible by midday and through
the afternoon. As a cold front approaches during the late afternoon
hours, winds will veer more westerly.