Scientific Forecaster Discussion
fxus63 kpah 200846
Area forecast discussion
National Weather Service Paducah Kentucky
346 am CDT Tue Aug 20 2019
Short term...(today through Thursday night)
issued at 346 am CDT Tue Aug 20 2019
All indications point toward an extended period of relatively wet
weather through at least Thursday night. It will begin with a
potentially severe mesoscale convective system rolling through the area this afternoon,
and then continue with a weak cold front loitering over our region
from Wednesday night through Thursday night. In addition, heat
indices from 100 to 105 are likely over much of the region before
the mesoscale convective system arrives today, and throughout the area Wednesday.
All indications point toward the recently developed convection
over Iowa, organizing and diving southeast toward our region
through the day. Used the 00z href as a guide for timing, and it
should reach the Pinckneyville and Mt Vernon Illinois areas by
18z. However, it should be weakening by then, and should continue
to weaken as it moves through our region. Its low-level inflow
weakens and veers to the west, and the 0-3km bulk shear will not
be sufficient to offset the very strong cold pool expected. By
the time it reaches our region, or just after, the leading line
convection will take on an upshear look, tilting strongly over
the cold air.
The only questions are just how strong the winds will be as it
enters the area, and how quickly they will weaken. Figure there
could be a pretty good swath of winds well above 60 mph initially.
Would expect the severe-level winds to be more sporadic by the
time it reaches the Ohio River. The leading edge of the outflow
and leading line of convection should move through the area in the
18z-22z time frame.
The GFS and NAM soundings show significant overturning of the
atmosphere behind the line. It will take some time for the
atmosphere to recover, so convection tonight may be difficult to
get. However, there is some signal in much of the 00z guidance for
some development mainly over the northeast half of the area
overnight into Wednesday morning.
The cold front will remain to our north Wednesday, but there is
some evidence in the arw and nmm WRF for a less organized area of
convection to move southeast through the region Wednesday
afternoon. If this does not happen our 40-50% pops will be too
high, and the heat and humidity could be the bigger story. Already
have heat indices 100 to 105 over the entire area, and if we stay
dry and have more sunshine, temperatures may warm a few degrees
more than forecast. Wednesday could be a heat advisory day.
Confidence in the long-awaited cold frontal passage is diminishing
with each forecast cycle. It doesn't really arrive now until
Thursday and it basically just waffles around the area through
Thursday night. We will have good chance pops in the north
Wednesday night and then likely pops over much of the region
Thursday and Thursday night.
We could see some heavy rainfall with multiple rounds of convection
possible beginning this afternoon and continuing through at least
Thursday night. Storm total quantitative precipitation forecast through Thursday night ranges
from just over an inch in the southeast to 2.5" in the northwest.
Isolated areas could see significantly more, especially from
Wednesday night on. We will have to monitor trends for a potential
Flash Flood Watch if the boundary decides to stall out over the
High temperatures are forecast to drop below normal into the
middle 80s throughout the region Thursday due to widespread cloud
cover and precipitation moreso than the cold front. With the noted
trend toward a slower and less impactful frontal passage, this
cooling may be overdone, at least over southern portions of the
Long term...(friday through monday)
issued at 346 am CDT Tue Aug 20 2019
The weekend weather is highly dependent on the movement of the cold
front mentioned in the short term section above. There is a lot of
model variability regarding the movement of the front or lack
There is good model agreement on Friday, when the cold front is
expected to slowly drift south across the lower Ohio Valley and
southeast MO. Scattered showers and storms will accompany the front
in the very humid air mass. Clouds and precip should limit highs to
the lower and mid 80s.
Friday night into Saturday, the mid and upper level winds will
weaken in the wake of the departing trough. The models depict a weak
500 mb ridge over the Great Lakes on Saturday, but a weak trough
over the lower Mississippi Valley may undercut the ridge. Given the
weak steering flow, it is possible the front will stall near or over
the lower Ohio Valley (as indicated by the 00z ecmwf). The GFS and
its ensemble mean take the front well to our south. The forecast
will be based a bit more heavily on the ECMWF, which means at least
a slight chance of precip through most of the weekend. Humidity
levels will depend on the frontal movement. Temps will be held in
check either by clouds if the front stalls, or by cooler northeast
winds if the front continues southward.
Heading into early next week, the front will return north as a warm
front by Monday. The main effect will be on humidity, as dew points
rise into the lower 70s. Temps will be limited by abundant clouds
and possible thunderstorms.
issued at 1055 PM CDT Mon Aug 19 2019
Should be mainly quiet overnight, aside from maybe a stray shower
or thunderstorm. Could have some fog issues at kmvn due to storms
earlier. We continue to analyze the possibility of a complex of
storms diving southeast toward the terminals Tuesday afternoon.
Mid/high clouds should be on the increase as this complex
approaches and diurnally driven bases. Timing of these storms
continues to be problematic so a prob group will have to do for
now, until we can obtain a better pulse on how the convection will
evolve. While the chance does exist for convection to possibly
last beyond 00z, confidence is too low to include any mention.