Area Forecast Discussion
Issued by NWS Paducah, KY

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951
FXUS63 KPAH 211736
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
1136 AM CST Wed Feb 21 2018

.UPDATE...
Issued at 1136 AM CST Wed Feb 21 2018

Temperatures continue to linger right at the freezing mark north
and west of a line from Mt Vernon to Carbondale Illinois, and
onto to Jackson and Van Buren Missouri. Very light glazing on
trees has recently been reported in Pinckneyville. This area is
right on the edge of the persistent light rain, and temperatures
are not expected to change much at all this afternoon, so some
light icing of elevated objects may continue. Given the warm
temperatures the last few days, road surfaces are not expected to
freeze up this afternoon. Will keep the Special Weather Statement
going through the afternoon.

The AVIATION section has been updated for the 18Z TAF issuance.

&&

.SHORT TERM...(Today through Thursday night)
Issued at 317 AM CST Wed Feb 21 2018

Cold front at 09z is through all but the KHOP area. Initial batch of
rain produced 1 to 2 inches over the past 12 hours across SEMO,
into southern IL. Rainfall rates in this area have been kept in
check, and are lowering, not to mention antecedent conditions were
such that, well, the rain was welcome. No issues so far tonight
anywhere, other than maybe some puddling on roads. Expect similar
to slightly lower amounts into SW Indiana and west KY for the rest
of the night. RAP and NAMNest data have handled the winds and
temperature trends the best, and will continue with those
forecasts through today. Today, temps do get close to 32F from say
Marble Hill northeast to Pinckneyville. So some patchy icing,
mainly elevated cannot be ruled out completely in this corridor.
Not sure that it will be all that impactful. Will monitor.

Extended the Flood Watch through 12z Sunday. Higher confidence for
appreciable areal flooding will be from later Thursday on through
Saturday night which has been messaged for a few days. The long term
discussion will have more on the Friday through Saturday period.
Much lower confidence on the front end through tonight, in
particular for SEMO, southern IL and southwest IN, given lowering
rates, and an axis shift to west KY. Isolated low impact issues
cannot be ruled out I suppose. Will keep a close eye on west KY
through 12z Thursday. Expect a bit of a lull on Thursday before
another surge Thursday night as the boundary pushes back into the
area along with another mid level wave moving across the region.
Select river flood warnings have already been out for several days.
The extended watch expands upon that, once the rainfall really
tallies up.

.LONG TERM...(Friday through Tuesday)
Issued at 317 AM CST Wed Feb 21 2018

The primary concern in the long term continues to be the potential
for heavy rainfall and flooding Friday into Saturday night. While
finer details such as precipitation amounts and the axis of heaviest
rain still require some fine tuning, overall forecast confidence is
higher than average given generally good agreement among forecast
models.

An anomalously moist southwest flow pattern will persist into the
first half of the long term period. Perturbations in the strong
southwest flow aloft will provide the necessary lift to generate no
fewer than three precipitation events during the period. The first
round of rain is expected to carry over from Thursday night into
Friday as energy ripples along a stalled out frontal boundary draped
southwest to northeast across the area. The second round of rain
arrives Friday night into early Saturday as a warm front lifts north
across the area. The third and final round of activity accompanies
the approach and passage of a cold front late Saturday afternoon
into Saturday night.

Some of the rain that falls could be quite heavy with precipitable
water values forecast to be 2 to 3 standard deviations above normal.
The collaborated forecast has 2 to 4 inches of rain over much of the
area from Friday through Saturday night. That`s in addition to what
falls prior to Friday. Thunder will also be possible through much of
the period, which will serve to focus even heavier rainfall amounts
in some basins--leading to the potential for flash flooding.

The potential for severe weather will also need to be monitored--
mainly Saturday afternoon into Saturday night--as a deepening
surface low shifts northeast from the Plains into the Quad Cities
area by 00Z Sunday. Atmospheric dynamics including deep layer shear
appear favorable for thunderstorm organization. However, the quality
of surface-based instability will be the greatest variable given the
amount of sky cover and rain forecast. The 21/00Z GFS suggests that
instability will not be a problem, but it is something that we will
monitor as the period comes into the window of near term CAM model
guidance.

The precipitation should taper off abruptly from west to east
Saturday night as the cold front sweeps through. In its wake, dry
and slightly cooler conditions are forecast to start the new week as
high pressure builds in. After unseasonably warm conditions Friday
and Saturday, a return to temperatures still slightly above normal
is forecast Sunday through Tuesday. In fact, even low temperatures
may remain above freezing behind the front early next week.
Generally used a blend of available guidance for temperatures.

&&

.AVIATION...
Issued at 1136 AM CST Wed Feb 21 2018

Fairly high confidence forecast with IFR conditions and northeast
winds the general rule through the period. Light rains will
linger through the afternoon and then another round moderate to
heavy rainfall will move through the region in the 03Z-09Z time-
frame. MVFR visibilities are expected with the heavier rains, and
then fog will keep them going into Thursday morning. LIFR ceilings
will be a possibility at this time as well. Northeast winds may
gust at or below 20kts for awhile over west Kentucky during the
heavier rains late this evening into the early morning hours.

&&

.PAH WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch through late Saturday night for ILZ075>078-080>094.

MO...Flood Watch through late Saturday night for MOZ076-086-087-100-
     107>112-114.

IN...Flood Watch through late Saturday night for INZ081-082-085>088.

KY...Flood Watch through late Saturday night for KYZ001>022.

&&

$$

UPDATE...DRS
AVIATION...DRS



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