Area Forecast Discussion
Issued by NWS Paducah, KY
FXUS63 KPAH 110914
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PADUCAH KY
314 AM CST Wed Dec 11 2013
.SHORT TERM...(Today and Tonight)
ISSUED AT 313 AM CST WED DEC 11 2013
This forecast package, as well as subsequent packages through
Friday, the impact of the aging snow pack will play a factor in
skin temperatures and mechanical mixing within the boundary layer.
On top of the microclimate/diabatic influences, the secondary
issue will be the timing and ultimate form of the current closed
low off the northern California Baja peninsula. The degree of
phasing with the positively tilted low amplitude northern U.S.
trough will determine the degree of lift through the layer and the
ultimate precipitation types in place for Friday night and very
early Saturday morning.
The warmer GFS/colder ECMWF show dramatic influences on
precipitation type with the system late Friday/Friday night. The
Canadian (GEM) loosely serves as rough blend between the GFS and
ECMWF. The aforementioned discussion of lift with the approach and
phasing of the closed low with the broad trough will also impact
the ageostrophic response and ultimately the degree of low level
warm advection in place. There continues to be a fair degree of
uncertainty at this time, but still considered the ECMWF too cold
overall and tried to lessen its influence on the precipitation
forecast for Friday night.
Since in some cases the snowpack has compressed down to 1/3 to 1/2
its original depth, this denser mass will be somewhat slow to
dislodge before Friday, given the intervening shot of cold air
with the front this afternoon. This cold air advection will
stabilize the snow pack at least through daybreak Thursday until
warm air advection start to influence the area by midday Thursday.
There may be insufficient time to eliminate the remaining snow
pack through Thursday evening. This, in turn, would minimize the
overall impact on significant temperature increases over the snow
pack, thus affecting the precipitation type on Friday.
With respect to PoP coverage, leaned closer to the GEM solution
since any distinct mesoscale increase in precipitation will be
limited to a 3-4 hour time period and closer to the remnant low aloft.
Finally, did some blended manual adjustments with temperatures and
dewpoints over the snowpack and for locations with zero to two
inches of snow cover. This was to account for energy
requirements/warm air advection/cloud cover influences necessary
either raise temperatures over the snow during the day or drop
them overnight due to radiational and/or cold air advection
.LONG TERM...(Saturday through Tuesday)
ISSUED AT 313 AM CST WED DEC 11 2013
The 00Z GFS, ECMWF, and GEM are in reasonable agreement for much of
the period, at least through the weekend. However, the details are
not as well aligned.
The system currently off of Baja California will be exiting the area
either Friday night or Saturday morning. The models still look too
wet with this system given its rapid weakening as it moves through
the area, and the fact that it will be outrunning its inverted
All of the models dive more energy down the Rockies and create more
troughing over southern Plains by Saturday morning. This effectively
holds up the surface trough over the western portions of our
forecast area for much of the day Saturday. The GEM and GFS have a
more coherent mid/upper level trough moving through the area
Saturday into Saturday evening, while the ECMWF is less organized
with the trough and slower to push the last of it through the region
The implications are that the less organized ECMWF is slower to push
the Baja system through the area, leaving some deeper lift across
the area Saturday morning. This could be deep enough for snow
development aloft, and its temperature profiles indicate that the
snow could make it to the surface across at least the northern half
of the area Saturday morning. Most of the area should be close to
freezing if not above freezing by daybreak Saturday, depending on
the impact of the remaining snow/ice cover. Opted to just go with a
rain/snow mix over the southwest and north Saturday morning.
Behind the Baja system, the moisture is very shallow and not deep
enough to consider snow or sleet. The only concern for Saturday
afternoon and night is where the surface freezing line is located.
The GFS and GEM indicate substantial northerly flow and cold
advection across the area Saturday afternoon and evening, as the
second trough aloft moves through the region. The ECMWF does not
show as much northerly flow or cool advection.
The GFS/GEM solution could result in some light freezing
rain/drizzle over northern portions of the area late in the
afternoon and through the evening, but the ECMWF would confine it to
Saturday evening. Will keep Saturday afternoon as all rain, but will
indicate freezing rain in the northeast Saturday evening. QPF is
incredibly light so will not have any ice or snow accumulation at
this time for Saturday or Saturday night.
Given that we were able to warm above freezing over much of the area
with an east/southeast wind this past Saturday afternoon and
evening, my gut feeling is that the southerly flow should allow the
entire area to warm above freezing this coming Friday night and
Saturday morning. Really do not feel that there is much potential
for freezing precipitation until possibly Saturday evening, if there
is any moisture left.
The models keep the area in cyclonic flow aloft Sunday through
Tuesday. Relatively minor disturbances may bring some enhanced cloud
cover to the area periodically through that period, but
precipitation is not expected. Temperatures will struggle to reach
freezing over much of the area Sunday and possibly in the north on
Monday. Southerly winds are forecast by Tuesday which should allow
much of the area to reach the 40s. Wow...remember last Wednesday
when we reached record highs? Seems like a long time ago, doesn`t it.
Looking beyond the period into the last half of next week, the ECMWF
and GFS both develop troughing aloft in the southwest and eventually
bring southwest flow aloft over our region. It certainly looks
warmer next Wednesday and Thursday.
ISSUED AT 1130 PM CST TUE DEC 10 2013
Main concern for the overnight hours is dealing with patches of
fog developing. Nearby observations sites are reporting some fog
but it is not widespread. Therefore, will opt for the BCFG mention
for now with a tempo at KCGI. By the end of the planning period, swlys
are picking up ahead of a reinforcing shot of cold air that will
be making its passage and shifting the winds strongly/abruptly to
nwly, with associated gusts approaching 20 kts. Model forecast
time/height cross sections suggest probably a lowering VFR deck of
sct-bkn clouds with its passage. However, moisture looks low
enough and thick enough at KEVV/KOWB to possibly produce a MVFR
cig. SREF probabilties also are suggestive of this.